HomeMy WebLinkAbout20073955.tiff OTTENJOHNSON
ROBINSON NEFF + RAGONETTI.
March 29,2007 KIMBERLY A. MARTIN
303.575.7552
KMARTINQOTTENJOHNSON.COM
BY FEDERAL EXPRESS
Kim Ogle
Planning Manager
Weld County Planning Services
918 10th Street
Greeley,Colorado 80631
Re: Carma Bayshore—Final Plat for St.Vrain Lakes Filing No. 1
Dear Mr. Ogle:
As you know from our prior conversation,this firm represents Carma with respect to the Bayshore project to be
located in Weld County(the"County").
In connection with the County's recording of the final plat for St.Vrain Lakes Filing No. 1 (the"Final Plat"),
enclosed please find(i)one fully executed and notarized original Declaration of Covenants, Conditions and
Restrictions for Bayshore,A Planned Community(the"Declaration"), (ii)two copies of the Declaration,
(iii)one fully executed and notarized CCIOA Map and Plat for Bayshore,A Planned Community(the"CCIOA
Map"), (iv)one original letter authored by Tyler Packard authorizing the completion by the County Clerk and
Recorder(the"Clerk")of certain recording information in the Declaration and CCIOA Map,(v)one original
letter authored by Tom Staab authorizing the Clerk's completion of certain recording information on the
CCIOA Map,and(vi)two checks in the total amount of$412.00 to the Clerk for recording fees for the
Declaration and the Map.
As we previously discussed,the Declaration and CCIOA Map must be recorded contemporaneously with the
Final Plat and in a specific order of recording. Also,there is recording information that must be inserted into
the Declaration and CCIOA Map and completed by the Clerk prior to the recordation of those documents. I
request that you provide to the Clerk(i)the specific information set forth in this letter with respect to recording
order and completion of the Declaration and CCIOA Map, and(ii)those items described in clauses(iv)through
(vi)in the first paragraph above.
950 SEVENTEENTH STREET SUITE 1800 DENVER COLORADO 80202 P 303 825 8400 F 303 825 8525 W OTTENJOHNSOON.COM
DENVER ASPEN STEAMBOAT SPRINGS x(/1- 395
PL Ill
Kim Ogle
March 29,2007
Page 2
Specifically,the Final Plat, Declaration and CCIOA Map must be recorded in the following order:
1. Final Plat— record first
2. Declaration—record after Final Plat(record second)
3. CCIOA Map—record after Declaration(record last)
In addition,the Declaration and CCIOA Map must be completed by the Clerk as follows:
1. After recording the Final Plat,but prior to recording the Declaration, insert the recordation date
and reception number of the Final Plat in Section 2.39(page 6)of the Declaration.
2. After recording the Declaration, but prior to recording the CCIOA Map, insert the recordation
date and reception number of the Declaration in the Declarant's Statement(sheet 1)of the CCIOA Map.
Finally, please provide my telephone contact information to the Clerk in the event the Clerk has any questions
with respect to the content of this letter.
Please do not hesitate to contact me with any questions or concerns. Thank you for your attention to this
matter.
Sincerely,
Kimberly A. Martin
for the Firm
KAM/mb
715853.1
Enclosures
cc: Tyler Packard-by E-Mail (w/o enclosures)
Irene Berest-by E-Mail (w/o enclosures)
Amy Hansen(w/o enclosures)
ico 1018
Carroll & Lange
Professional Engineers&Land Surveyors
Lakewood • Loveland • Winter Park
February 5, 2007
JN: 3075
Kim Ogle
Weld County Planning Department
4209 County Road 24.5
Longmont, CO 80504
Re: St. Vrain Lakes PUD (Final Plan) Comment Responses
Dear Kim:
Carroll & Lange, Inc. has reviewed the comments made in the memorandum by David
Bauer dated January 9, 2007. These comments have been addressed below, and we
offer our responses in bold.
Offsite Flows
1. Concerns regarding stormwater runoff from offsite areas to the north of the St.
Vrain Lakes PUD Filing I was noted in the October 19, 2006 comments from
Public Works. The revised drainage report has updated the calculations and
proposed drainage facilities for the western area of St. Vrain Lakes PUD Filing
(Basins OS1 and 0S2). However, the report text and the plans of the proposed
condition in the revised (November 2006) Filing 1 Drainage Report for the area
east of Weld County Road 9-1/2 show offsite future drainage facilities to be
"constructed by others". The previous submittal did not identify these
improvements as being constructed in the future "by others". The specific
capacities and locations of these un-constructed or proposed offsite drainage
management facilities by others were not provided in the revised report and
cannot be verified and therefore cannot be considered as operable when
evaluating the designs of the St. Vrain Lakes PUD project. Management and
safe conveyance of the offsite peak flows to and through the proposed Filing I is
requisite. As stated in the October 19, 2006 comments, the applicant is to
provide details of the designs and erosion control for all drainage facilities that
convey the un-detained offsite flows through or around the St. Vrain Lakes PUD
site.
Response: The offsite drainage requirements at Weld County Road 28 and
the Weld County Road 9.5 intersection involves a number of parties who
are collectively working toward a final design solution. Carroll & Lange,
Inc. will finalize drainage calculations/design once the parties have agreed
to a final design. This item will be discussed at the next Weld County Road
9.5 meeting, Thursday, February 8th.
165 South Union Blvd.,Suite 156•Lakewood,Colorado 80228•Main(303)980-0200•Fax(303)980-0917
3985 South Lincoln Avenue,Suite 250•Loveland,Colorado 80537•Main(970)292-5635•Fax(970)292-5639•Denver Dir. (303)865-5088
47 Cooper Creek Way,Suite 328•P.O.Box 3345•Winter Park,Colorado 80482-3345•Main(970) 726-8100•Fax(970) 726-9100-Denver Dir. (303)980-9600
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 2
2. Of particular concern are Offsite Basins OS5 and OS7 draining approximately
three-quarters of a square mile (475 acres) of currently undeveloped land which
are shown (Sheet DR-1) discharging runoff across the proposed road WCR 9.5
and proposed road WCR 28. The 146 cfs flowing across WCR 28 from Offsite
Basin OS6 is also of concern (See item #8 below).
Response: See response to comment#1.
3. We have received no information regarding the Richie Brothers detention pond
(i.e. area served, release rate, detention, etc.). Under current conditions, 186 cfs
flow undetained from offsite Basin OS7 into St. Vrain Lakes PUD Filing I. The
proposed plans provide no information regarding the dimensions and design
specifics of how these flows are safely conveyed across WCR 9.5. Please
provide a description and designs for safe management of these offsite flows to
and through the St. Vrain Lakes Filing I. Please show on the plans the location
and sizing of the conveyance. Please describe who will construct these facilities.
Prior to filing the Final Plat, please provide evidence that the proposed drainage
facilities are in dedicated drainage easements or public Right-of-Way.
Response: As discussed with Todd Borger and Dave Bauer in a meeting on
December 27, 2006, the Ritchie Brothers detention pond has been built and
will be as-built soon. Weld County has indicated that they will allow the
detained peak release from the pond to determine the total offsite flow
rates. Based on this methodology, the 100-year offsite flow from Ritchie
Brothers is approximately 116 cfs.
4. The proposed plans (Sheet DR-1) in the November 2006 Filing 1 Drainage
Report show a proposed drainage swale along the north side of WCR 28
supposedly to be constructed by others. The current condition, in the absence of
this WCR 28 roadside swale, would be that the stormwater runoff from Basin
OS5 would overtop WCR 28 and flow to proposed St. Vrain Lakes PUD home
sites in proposed basins Al , A2, B9, A22, B5 and others. As stated above and in
the October 19, 2006 comments, the applicant must provide adequate drainage
facilities to handle all current condition flows, both on-site and off-site. The
specific capacities and locations of these un-constructed or proposed offsite
drainage management facilities by others were not provided in the revised report
and cannot be verified and therefore cannot be considered as operable when
evaluating the designs of the St. Vrain Lakes PUD project without further details
and commitments. Under current conditions, this WCR 28 swale would need to
convey the 186 cfs undetained peak flow from offsite Basin OS7 and the 261 cfs
undetained peak flow from offsite Basin OS5. The proposed plans in the
November 2006 Filing 1 Drainage Report do not give the dimensions and design
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 3
specifics for the proposed this WCR 28 drainage swale. Please describe who
will construct these facilities and when they will be constructed. Please describe
how is this swale to be constructed given the gas line and utility easement
conflicts in the same proposed alignment shown in the November 2006 Filing 1
Drainage Report and shown on the plans. Please provide evidence that the
proposed drainage facilities are in dedicated drainage easements or public Right-
of-Way.
Response: See response to comment #1.
5. Please describe the correct captured flows and capacity of the proposed
drainage swale along the north side of WCR 28. According to the Developed
Case SWMM model in the November 2006 Filing 1 Drainage Report, the peak
flow at design point 201 is 585 cfs. However the printout of the HEC-RAS model
output provided in the November 2006 Filing 1 Drainage Report gives 403 cfs as
the upstream most flow rate for the Basin B4 wetland channel. This peak rate
does not match the discharge rate used to model the Basin B4 wetland channel
in the electronic version of the HEC-RAS model provided to the County for
review. See below for additional comment on the HEC-RAS model of this
channel.
Response: The CUHP/SWMM analysis in Appendix D was prepared to size
the regional detention pond and does not account for the existing
detention pond located in the Richie Brothers Auction Park development.
Appendix G contains the CUHP/SWMM model that was utilized for the HEC-
RAS modeling of the wetland channel. This SWMM model has been
revised to show only a portion of what was previously Basin C1. This will
flow to the most upstream cross-section of the wetland channel at design
point 201. The remainder of runoff from the original Basin C1 (now labeled
as OS8) is added into the HEC-RAS model at section 2521. A SWMM
diagram is included in Appendix G showing the revised basins to help with
flow distribution for the HEC-RAS model. The revised peak flow at design
point 201 is 218 cfs, which represents the area to the north and the east of
the Ritchie Brothers development. The detained release from the Ritchie
Brothers development of 116 cfs was directly added rather than routed.
Appendix G provides two 11" x 17" plans identifying runoff amounts and
locations relative to the HEC-RAS model for the wetland channel.
6. The plans of the proposed condition in the November 2006 Filing 1 Drainage
Report show the location of a RC Box Culvert to convey combined flows from
Basin OS7 and OS5 under WCR 28. The Developed Case SWMM model in the
November 2006 Filing 1 Drainage Report indicates that this pipe is to be 5 feet in
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 4
diameter. Please clarify which geometry (box or pipe) is correct. Please clarify
the timing of construction of this culvert and who will be performing the
construction. The November 2006 Filing 1 Drainage Report does not give the
dimensions and design specifics for the proposed this RCBC / storm pipe.
Please provide these details, pipe inverts and HY-8 or equivalent evaluation of
the RCBC / storm pipe capacity, entrance type, evaluation of inlet control, etc. If
the proposed 5-foot diameter pipe does not convey the full peak flow that is
directed to this design point, please provide an evaluation and water depths of
WCR 28 overtopping. Details of the erosion protection design at the RCBC /
storm pipe inlet and discharge point are also requested.
Response: See response to comment #1. Once a final drainage design is
determined for the Weld County Road 9.5 project (north of WCR 28), a peak
runoff rate will be determined for Carroll & Lange, Inc.'s use in sizing the
proposed culvert. Details of the box culvert design and associated
downstream erosion protection will be provided upon determination of the
offsite drainage condition. The Weld County Road 28 improvements will be
completed by Carma Colorado, Inc. These improvements are required
prior to the 88th building permit being issued in the St. Vrain Lakes Filing
No. 1 development.
7. This proposed 5-foot diameter / RCBC is shown with a proposed riprap apron
that discharges to the existing abandoned stock pond (Basin B4 on Sheet DR1).
The proposed riprap apron does not extend to the bottom of the wetland channel.
Please address headward erosion of these features to insure that WCR 28 is not
impacted. Please provide riprap sizing supported with calculations.
Response: Riprap will be extended to the wetland channel. The
calculations for the riprap sizing have been included in Appendix C.
However, the detailing will be added to the box culvert plan upon final
determination of the offsite flow rate.
8. Basin OS6 does not have a functioning detention pond or discharge pipe. Under
current conditions, 146 cfs flow undetained from offsite Basin OS6 into St. Vrain
Lakes PUD Filing I. The proposed plans provide no information regarding the
dimensions and design specifics of how these flows are to be safely conveyed
across WCR 28. As explained to the applicant in previous reviews, the un-
detained offsite peak flows must be managed through or around the St. Vrain
Lakes site and un-constructed or proposed offsite drainage management
facilities (by others) cannot be considered in design and approval of the St. Vrain
Lakes PUD project. Please provide a description and designs for safe
management of these offsite flows to and through the St. Vrain Lakes Filing I.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 5
Please show on the plans the location and sizing of the conveyance(s). Please
describe who will construct these facilities. Please clarify the timing of
construction. Prior to filing the Final Plat, please provide evidence that the
proposed drainage facilities are in dedicated drainage easements or public Right-
of-Way.
Response: See response to comment #1.
Grading
1. The St. Vrain Lakes PUD Filing I grading and drainage plans show proposed
breaching of the existing berm that forms the southeast margin of the existing
stock pond immediately south of WCR 28 (Basin B4 on Sheet DR1). Given the
existing steep slopes and that the applicant currently proposes to flow an
approximately 400 cfs peak discharge from offsite Basins OS5 and OS7 through
the breached berm, please provide an evaluation of the backwater and erosional
stability of this feature. Grade control and riprap will likely be needed; please
provide riprap sizing supported with calculations.
Response: This berm will be removed to the extent that wetlands are not
disturbed. The proposed grading will maintain approximately the same
slope as that of the existing wetland. The St. Vrain Metro District will
maintain the entire wetland channel area. The proposed grading will
eliminate backwater effect in this area, as the constricted area will be
removed.
2. Carma has begun grading the site under a grading permit issued after the
Change of Zone approval. In some locations in excess of 10 feet of fill have
been placed. Weld County requires that these areas of cut and fill be re-
evaluated for geotechnical stability for the existing conditions (i.e. post cut and
fill). Weld County requests copies of updated geotechnical reports giving data
from these re-graded areas and areas of cut and fill. In some locations Weld
County will require that new geotechnical borings be collected to verify the
current soil conditions of areas proposed for homes and other infrastructure.
Response: The geotechnical engineers, Terracon, are testing the areas of
cut and fill and taking daily boring soil samples from the site. The test
result data is attached along with this response letter.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 6
General Hydraulic Comments
1. Please add the following statement to the General Notes sheet of the
construction plans and the text of the Final Drainage Report; "Weld County shall
not be responsible for the maintenance of drainage related facilities."
Response: The plans have been revised to include this note on the general
notes sheet, note number 30. The final drainage report has been revised to
include this note on page 1, paragraph 1, sentence 4.
2. All culvert inlets shall include debris racks to prevent clogging due to debris
accumulation and to protect the public safety. Debris racks shall be sloped at
3H:1V or flatter per UD&FCD research. Please provide a detail drawing in the
plan set to illustrate an FES with debris rack.
Response: A detail of a trash rack has been included in the construction
drawings on sheet DT6 and is referred to at all upstream (inlet) FES
locations.
Global Comments for Rock Riprap
1. According to the USDCM, Chapter 7, Section 4.4.2.3 (2001), riprap thickness for
channel linings shall be no less than twice the median stone size (≥ 2.0D50) in
sandy soils for added safety factor against erosion damage.
Response: Plans have been revised to represent this change.
2. All proposed rock riprap protection in the subdivision must be supported by
engineering design calculations consistent with UD&FCD guidelines in the
USDCM (2001). As an alternative, FHWA guidelines in HEC-11 (1989), HEC-23
(2001), and HDS-6 (2001) may also be used for rock riprap design. All
calculations must be included in the Final Drainage Report.
Response: All riprap protection calculations are included at the end of
Appendix C.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 7
3. All rock riprap protection designs must include a termination detail consistent
with guidelines published in the USDCM, Chapter 4.4.2.4 (UD&FCD 2001). If
desired. the guidelines published in HEC-11, Chapter 4, Figures 20 and 22
(FHWA 1989) may be used, which may reduce the total volume of rock required
for terminations in the subdivision.
Response: All riprap terminations have been revised to include this detail.
4. It is highly recommended that the UD&FCD methods published in Chapters 7
and 8 of the USDCM (2001) be double-checked against other methods for the
benefit of erosion protection and public safety and welfare. Other acceptable
methods may include the Factor of Safety Method (HDS-6, FHWA 2001), the
HEC-23 Method (HEC-23, FHWA 2001).
Response: Comment Noted. We are investigating the use of the ArmorFlex
and Contech erosion protection systems and will be using this product in
specified areas in place of rock riprap. The low-tail water basins have been
designed per UD&FCD criteria. The HEC-15 maximum shear stress
methodology will be utilized for riprap sizing in the 100-year swales.
5. An appropriate geotextile filter fabric must be designed and identified by product
name or engineer-approved equal on all construction plan sheets. The choice of
geotextile must be accompanied by engineering design calculations and included
in the Final Drainage report.
Response: As discussed with Terracon (geotechnical engineers), they
approved using Mirafi 170N filter fabric under riprap protection. A
specifications sheet is included in the report.
Storm Sewer Pipe Design — General Comments
1. The pipes designations shown on the plan and profile sheets (PP), storm plan
and profile sheets (STMPP), and all drainage sheets (DR) do not match the
unique names provided as input to StormCAD. (These unique names are
published in Exhibit C, Appendix C (Hydraulic Calculations) of the Final Drainage
Report). This inconsistency complicates review. Please utilize the same naming
convention on all plans and in the reports and StormCAD printouts or provide a
correlation table.
Response: We have tried to accommodate this request although it is very
difficult to match the names together. We have revised the report to match
the StormCAD output to the drainage report design point numbers for
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 8
convenience of review. The storm manhole and inlet calls on the
construction plans go in sequential order to help the contractors install the
manhole correctly. Changing the names on the construction plans would
create more issues and would not be beneficial during construction.
2. Use of the computer models HEC-RAS and UDSWMM 2000 are identified in the
text of the Final Drainage Report; mention of these programs need to be added
to the list of references for completeness.
Response: The references have been updated to include these programs.
3. For completeness, please provide a digital copy of all hydraulic and hydrologic
models referenced in the text of the Final Drainage Report and reproduced in the
report appendix.
Response: A digital copy of all hydraulic and hydrologic models used in
the Final Drainage Report has been provided on a CD with the submittal.
Storm Sewer Pipe Design — Specific Comments
Following Public Works' October, 2006 comments, the revised St. Vrain Lakes PUD
Filing I Drainage report (November, 2006) included upsized proposed pipes on the
steeper streets. However, it is apparent that many of the proposed storm pipes with
steep slopes will have supercritical flow and many will operate under inlet control. An
excerpt from HDS-5 (FHWA 2005) defines inlet control as follows: "Inlet control occurs
when the culvert barrel is capable of conveying more flow than the inlet will accept. The
control section of a culvert operating under inlet control is located just inside the
entrance . . . . Hydraulic characteristics downstream of the inlet control section do not
affect the culvert capacity. The upstream water surface elevation and the inlet
geometry represent the major flow controls." -- FHWA 2005, Page 7, Paragraph 2(a).
Cross reference to USDCM, Chapter 9, Section 2.1.2 and Section 3.1.
1. In the November 2006 Filing 1 Drainage Report, the 100-year Pipe Report from
StormCAD does not clarify or evaluate which pipes will operate under inlet
control conditions and which operate under outlet control conditions. As
commented on in the October review of the Filing I Drainage report, proposed
manholes and storm pipes on the steeper streets will be induced into hydraulic
jump conditions by the addition of flow from side street inlets. The hydraulic
jump in or near a manhole will cause inlet control in the pipe leaving that
manhole. The current StormCAD analysis routines, which utilize HEC-22 energy
grade line evaluation procedures, do not compute inlet control condition. As a
result, many of the pipes proposed throughout the subdivision will not be capable
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 9
of capturing the design flows predicted by StormCAD. Weld County's recent
conversations with drainage engineers working for other jurisdictions confirms
this limitation of StormCAD. The experience in these nearby jurisdictions
included inlet controlled pipes that surcharged at the upstream pipe entrance
creating a backwater through the manhole and adjacent inlets and causing a
flooding condition on streets in the subdivision. Weld County contends that the
proposed inlets will not collect the flows indicated in the StormCAD output.
Please review all proposed storm sewer pipes to check for inlet or outlet control
and provide revised associated inlet capacity calculations.
Response: All pipes have been evaluated for inlet control. At the
January 25th meeting with Public Works staff, we agreed that the inlet
control EGL could be at a maximum of 1.5 feet above the finished grade,
and we would keep the outlet control EGL 0.5 feet below the finished grade
as stated in the Weld County addendum to the USDCM manuals. This
provides for the design to not be overly conservative while insuring that
inlet control is accounted for. A spreadsheet analyzing inlet control on all
pipes is included in Appendix C, subsection StormCAD Storm Sewer
Profiles and Calculations, located just after the StormCAD output report. It
is important to note that portions of the storm sewer system are designed
for minor storm flows, where the street carries the major storm. Inlet
control/pipe hydraulics in these minor storm pipes is not held to the
criteria during the major storm event. Storm sewers designed to convey
the major storm were analyzed for inlet control under the major storm
event.
a. Example: An HY-8 analysis of Pipe P-49 indicates the 60" RCP proposed
from Inlet I-A53 to Pond 106 will flow under inlet control conditions at Qioo =
284 cfs. HY-8 shows the pipe has a headwater depth (HW1oo) of 11.59 ft, or
WSEL,00 = 4834.13 ft. The StormCAD output shows the pipe has an
upstream EGL1oo = 4830.66 ft, which is 3.47 ft lower than the WSEL
computed by HY-8. Since this pipe is operating under inlet control
conditions, the StormCAD-calculated EGL upstream of P-49 is incorrect.
The rim elevation of Inlet A-53 on WCR 9.5 is 4830.9 ft, and by inlet control
calculations, there will be at least 3.23 ft of flooding in the travel lane of
WCR 9.5. Hand calculations of this situation provide similar results. This is
an unacceptable condition.
Response: This pipe has been increased in size to a 66-inch RCP. The
berm to the south of this location has been lowered to ensure that in
an emergency condition (e.g., a clogged inlet), water does not pond up
more than 1.5 feet.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 10
b. Proposed storm sewer lines operating under inlet condition need to be
recalculated to prevent detrimental street flooding in the subdivision. Storm
sewer lines upstream of these inlet-controlled pipes must be recalculated
because the associated downstream tailwater conditions will change.
Response: Please see response to comment #1 under this section.
2. Please incorporate curved vane inlets on-grade and/or in sumps along the gutter
flowline of all streets with slopes exceeding 2% longitudinally. This will prevent
skipping and splashing that will occur over grate inlets in gutters flowing at high
velocities and high Froude numbers. Our discussions with other municipalities
suggests vane inlets will be the most effective structures for intercepting high-
velocity flows on steeper (>3%) street surfaces. The appropriate grate size,
number, and configuration can be determined with UD-Inlet (UD&FCD 2006), or,
as a reasonable alternative, with HEC-22, Chapter 4, Section 4.4.3.3 (FHWA
2001).
Response: The addition of inlets on streets with slopes that exceed 2.0% is
not economical. In accordance with urban site design, the streets serve as
open channels for flow conveyance during the 100-year event with safety
factors applied to reduce the maximum `theoretical' street capacity to an
`allowable' street capacity as established under the Weld County (UDFCD)
criteria. We are proposing Type R inlets in the St. Vrain Lakes
development as they have a lower clogging factor. These inlets have been
placed on side streets to effectively intercept storm runoff prior to its
intersection onto steeper streets.
3. The proposed design longitudinal slope of all streets and pipes at intersections
along Eagle River Road exceeds 4% and the surface runoff in adjacent gutters
will experience supercritical flow. The momentum of this supercritical flow will be
high and will not round corners at the proposed Eagle River Road intersections
to proposed sidestreet inlets (ex: DP-A15.1 , A6.2). The high velocity Eagle River
Road gutter flow will create unsafe driving conditions for residents exiting or
entering Eagle River Road from these side streets. To correct this problem,
please add on-grade curved vane inlets to Eagle River Road. These inlets will
be needed on both sides of all intersections downstream of STMH A-9, including
Canadian River Road, Elk River Road, Castle Creek Way, and Colorado River
Road.
Response: As previously stated, we have followed the Weld County criteria
and have not exceeded the allowable depth in the streets. Inlets have been
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 11
located along the side streets to capture flows so the flow does not have to
turn the corner. Placing inlets, even vane inlets, along Eagle River Road
will not pick up much flow relative to cost/maintenance factors (i.e., very
high cost to benefit factor). We have performed additional analysis of the
depth of flow in the street in the event a 25-feet inlet was added to Eagle
River Road and the flow regime remains virtually unchanged. As agreed
upon in the January 25th meeting with Public Works staff, inlets will not be
added to Eagle River Road as the flow in the street is within the criteria.
The following is the view of UDFCD on street drainage: "It should be noted
that proper drainage design utilizes the full allowable capacity of the street
gutter in order to limit the cost of inlets and storm sewers" (USDCM
volume 1, chapter 6, page ST-6, paragraph 1, sentence 4).
4. Proposed storm sewer lines operating under high velocities or under full-flow /
pressure flow conditions shall be constructed of pressure-sealed concrete pipe
rated for these conditions.
Response: All proposed storm sewer lines that will be used on the St. Vrain
Lakes site will conform with ASTM standard C 443. This specification
states that the storm sewer joints show no sign of leakage during pressure
tests up to 13 psi (30 feet). The pressure flow conditions on the St. Vrain
Lakes site are all well below a pressure head of 25 feet and therefore the
pressure flow conditions have been adequately accounted for. A copy of
the ASTM C443 has been attached with this letter.
5. The sizing's of proposed storm sewer pipes analyzed in StormCAD and reported
in Exhibit C, Appendix C (Hydraulic Calculations) of the Final Drainage Report do
not match the sizing's on the construction plan sheets.
a. Example: Construction Sheet DR5 of the Final Drainage Report shows
Pipe P-49 as a 60 in. diameter RCP flowing into Pond 106. Exhibit C,
Appendix C of the Final Drainage Report shows Pipe P-49 as L = 204 ft,
diameter = 60 in., Sppe = 0.042 ft/ft, Inv. U/S = 4822.55 ft, and Inv. D/S =
4814.00. The same pipe on Sheet PP3 of the WCR 9.5 Construction Plans
shows L = 210 ft, diameter = 54 in., Spipe = 0.039 ft/ft, Inv. U/S = 4823.10 ft,
and Inv. D/S = 4815.00. The WCR 9.5 construction plans also show a
WSEL1oo = 4819.00 ft, but has a scaled dimension of 4.0 ft (48 in.) at the
outlet end of Pipe P-49. Please cross-check the report text with the Final
Construction Drawings.
Response: All StormCAD data in the appendices have been updated to
match the Final Drainage Report and the construction plans.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 12
6. If the headwater-to-diameter (HW/D) ratio on proposed storm sewer pipes and
culverts exceeds 1.5, pipe collars will be required to reduce the potential for the
piping of fill material around proposed structures. The HW/D ratio is discussed in
the USDCM, Vol. II, Chapter 9 — Culverts (UD&FCD 2001). Pipe collars and
their design are detailed in Design of Small Canal Structures, Chapter VIII,
Section 8-10, Pages 362-364 (USBuRec 1978). Alternatively, the HW/D ratio
may be reduced by adjusting the size and configuration of proposed storm sewer
pipes and culverts.
Response: The storm sewer pipe size at DPC18 has been increased from a
48-inch RCP to a 54-inch RCP to lower the HW/D ratio to 1.6 and a berm has
been added to contain the flow so they do not overtop the street.
Comments for the Wetland Channel HEC-RAS Model
1. The text of the Final Drainage Report discusses the use of HEC-RAS to
determine hydraulic conditions in the wetland channel. The appendix of the
report includes a CAD drawing of cross section locations and a printout of
Standard Table 1 from HEC-RAS. For the wetland channel evaluation, please
provide the full HEC-RAS-generated report, including all input, output, and
intermediate calculations.
Response: A full HEC-RAS report is included with this submittal.
2. Please provide a digital copy of all hydraulic and hydrologic models referenced in
the text of the Final Drainage Report and/or reproduced in a report appendix.
Response: A digital copy is included with this submittal.
3. The HEC-RAS results printed as Standard Table 1 in the Final Drainage Report
appendix do not match those of the digital model submitted to Public Works on
12/12/2006. Please explain why the hydrology and computed hydraulics for the
wetland channel are different between the report appendix and the submitted
digital model. Please reconcile and provide the final model and tie the results to
the proposed channel facilities.
Response: The final model and the printed results in the report have been
checked for consistency and the design of the proposed wetland channel
facilities correspond with these results.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 13
4. Please discuss channel stability under computed hydraulic conditions that
include velocities in excess of 5.0 fps and shear stresses in excess of 3.0 psf.
More robust erosion protection will be required.
Response: The HEC-RAS model represents the existing condition of the
wetland channel. Velocities were calculated at slightly above five feet per
second (fps) and sheer stresses were determined in excess of 3.0 pounds
per square foot (psf). According to UDFCD, erosion protection should be
installed when velocities are greater than five fps for erosive soils and
seven fps for non-erosive soils. Therefore, erosion check structures have
been designed within the channel for areas where the velocities exceed the
allowable, and have been spaced to limit the channel degradation to what
is expected to be the final stable longitudinal slope. The final stabilized
slope was determined in accordance to the recommendation of the UDFCD
criteria.
5. Please justify Manning-n values of 0.200 used in cross-sections 860 through
820, 0.550 at cross-section 816, and 0.045 at cross-sections 812 through 784.9.
These values are inconsistent with the rest of the model, which uses 0.060 for
the main channel and overbanks. Please note cross-section 816 is internally
inconsistent, as the left overbank has an n-value of 0.055, and the main channel
and right overbank uses 0.550. Is this a typographical or input error?
Response: This is a typographical error and has been corrected.
a. When using HEC-RAS, all Manning-n values should be consistent with
UD&FCD and the HEC-RAS Hydraulic Reference Manual (HEC 2002),
Table 3.1, as adapted from Chow (1959). Other n-values may be justified
from appropriate hydraulic references, which must be referenced in text and
copied in the appendix of the Final Drainage Report.
Response: Upon further review of the channel vegetation and the
recommended n-values from the HEC-RAS Hydraulic Reference
Manual (HEC 2002), Table 3.1, the Manning's n-value is set at 0.060
due to the weedy vegetation. Photos of the vegetation in the wetland
channel have been attached to this letter and included in Appendix G.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 14
b. An appropriate n-value for the channel grade control structure. If the
anticipated flow depth exceeds 5.0 ft for the majority of each cross section,
the USDCM, Chapter 8, Table HS-2 may be utilized. Since most of the
study reach has maximum depths below 5.0 ft, it is more appropriate to
utilize values published in HEC-15, Chapter 2, Table 2.2 (FHWA 2005).
Response: The Grouted Sloping Boulder (GSB) drop will be replaced
by an ArmorFlex system as discussed with Weld County during the
January 25th meeting. Therefore, the GSB has been removed from the
HEC-RAS model and the downstream boundary condition is assumed
as critical depth at the top of the proposed ArmorFlex mat.
Calculations across the ArmorFlex mat will be provided under
separate cover as provided by the manufacturer.
6. The HEC-RAS model computed severely unstable hydraulic conditions on the
4H:1V grade control structure. Velocities will exceed 10.0 fps and shear stresses
will exceed 18.0 psf. More robust erosion protection will be required for County
approval of this structure.
Response: The GSB will be replaced by an ArmorFlex system. ArmorFlex
is currently working on the design of this system and will have shop
drawings to provide to Weld County for approval before any construction
takes place on the ArmorFlex system.
a. Residential lots are proposed on the right bank (looking downstream)
immediately adjacent to the grade control structure. The applicant must
show stable flow conditions on the structure to prevent channel erosion in
the channel and on the adjacent overbank near the structure.
Response: ArmorFlex will be provided/designed to prevent overbank
erosion. A detailed design is being prepared by ArmorFlex and will be
presented to the County for review.
b. The proposed grade structure must be protected with an appropriate lining
that is supported approved with engineering calculations. If grouted
boulders will be used, the applicant must prove they will be stable under
guidelines published in the UDSCM, Chapter 8, Section 2.4.3 (2001).
Response: The ArmorFlex system will use a dual liner system and the
details of this will be provided to Weld County with all the shop
drawings for approval.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 15
7. The 100-year proposed SWMM model shows a Manning-n value of 0.005 for the
wetlands channel. This is inconsistent with HEC-RAS model input by a factor of
10. Please correct this error, re-run the program, and re-evaluate the stability
and identify necessary and appropriate erosion protection.
Response: The previous value of 0.005 was the invert slope of the
conveyance element that was the wetland channel, not the Manning's
n-value. The Manning's n-value for this conveyance element was 0.035.
This was value was revised to 0.060 to match the HEC-RAS model. The
invert slope of the wetland channel conveyance element was also revised
to 0.0173 ft/ft, which is the existing slope, instead of 0.005; this is a more
conservative assumption because the runoff from the upper basin will
reach the pond faster and will overlap more of the hydrograph with the
downstream basin C2. The SWMM input file has the columns labeled to
show what each column represents because reading the columns can be
very difficult.
8. For completeness, please provide calculations to support the size, orientation,
and erosion protection provided by all proposed riprap basins and rundowns. All
rundowns must be shown to be stable under proposed conditions hydraulics.
Response: Calculations for the riprap spillways and swales are included in
the end of Appendix C.
9. A toe-down and key are required for all riprap rundowns terminating in ponds to
prevent undermining and failure of the structure at the toe.
Response: ArmorFlex has also been contacted for support on the spillway
out of Pond 104, the spillway out of the large lake (pond 105), the three
lateral spillways draining into the lakes from the St. Vrain River, and the
spillway between the two lakes. Where we will be utilizing riprap, we are
following the trapezoidal detail from Figure 20, HEC circular No. 11.
10. Filter fabric is required under all riprap, and should be supported with description
of manufacturer specifications and called out on the construction plans.
Response: As discussed with Terracon (geotechnical engineers), they
approved using Mirafi 170N filter fabric under riprap protection. A
specification sheet has been included in Appendix C, sub-section Swale
and Riprap Calculations.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 16
11. All utility conflicts must be resolved prior to recording the Final Plat.
Response: The water and sanitary districts have reviewed and approved
the St. Vrain Lakes Filing No. 1 civil construction plans.
12. Please add inlet and manhole rim elevations as a column to StormCAD output
file.
Response: The inlet and manhole rim elevation columns have been added
to the StormCAD output file.
13. The 100-year EGL and HGL must be shown on all pipe profile drawings in the
construction plan set (previously requested).
Response: The 100-year EGL has been added to the profiles. The 100-year
HGL was shown on the previous plan set. Please note that County criteria
only require that design storm HGL information be provided on the plans.
14. Please show how water from Pipe P-114 (at Outlet 0-7) will fall into the Wetlands
Channel without eroding the right overbank of the channel.
Response: A low tail-water basin has been designed according to UDFCD
at this location. A riprap-lined channel has also been proposed to convey
the flow downstream of the low tailwater basin to the wetland channel. The
low tailwater basin calculations and riprap calculations for the swale are in
Appendix C.
15. Please consider replacing all sheet pile drop structures with grouted sloping
boulder drop structures, as per guidelines published in the USDCM, Chapter 8,
Section 2.2, Table HS-1 (2001).
Response: Due to the sensitivity of the wetlands it will not be possible to
add GSBD within the wetland channel. The sheet pile grade control
structures have much less disturbance to the wetlands and will provide the
required stability.
16. All 3-ft vertical drops require substantial toe protection to prevent drop scour, and
the potential undermining and buckling of sheet piles.
Response: Terracon is checking the buckling of the sheet piles in the
future condition taking into account the drop scour. The sheet piles will be
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 17
installed into the ground at existing grade without any vertical drop. As
time progresses and the channel erodes to a stable slope, vertical drop will
slowly occur. The 20-foot wide construction area that is allowed to be
disturbed, in accordance with the 404 permit, limits the amount of riprap
that can be placed at the time of construction of the check control
structures. The Metro District will place additional riprap if needed as the
channel attains a stable channel slope.
17. Please provide engineering calculations to show the sheet pile in vertical drop
structures will have sufficient burial to prevent buckling and failure, and protect
the public safety and welfare, especially during scouring storm events.
Response: Terracon is checking the buckling of the sheet piles in the
future condition. The calculations and findings will be sent to the County
once completed. The buried sheet pile depth has also been increased from
4.5 feet to seven feet as advised by UDFCD, which will add significant
buckling resistance.
18. Please provide engineering calculations of the drop scour anticipated at all
vertical drops.
Response: The drop scour calculations are provided in Appendix G as
calculated by following the HEC 23 Hydraulic Engineering Manual
equation 11.1.
19. Please provide engineering calculations to support the proposed channel bed
slope of 0.40%.
Response: The UDFCD states that check structures in wetland channels,
"...should be used as needed to limit slopes to about 0.50%..." (USDCM
vol. 2, chapter 8, page HS-39, paragraph 3, sentence 3). A normal depth
analysis shows that the flow characteristics in the channel will maintain
velocities in the channel that will be non-erosive and below five fps at a
channel slope of 0.50%. Therefore, we are being conservative to design
the future channel at a 0.40%. UDFCD was contacted and they suggested
also using the spreadsheet titled "Analysis of Trapezoidal Grass-Lined
Channel," which incorporates the SCS vegetal retardance factors. The
results of this analysis and the results of the normal flow depth analysis
have been included in Appendix G.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 18
20. A copy of all applicable wetlands permits from the U.S. Army Corps of Engineers
must be provided in the drainage report before any grading/construction activities
can be initiated in wetland areas.
Response: A copy of the 404 Nationwide permit from the Department of the
Army was included in Appendix I with the previous submittal.
Updated Comments for the HEC-RAS Model
1 . Cross sections on the grouted sloping boulder (GSB) drop structure must
represent all changes in grade, and all changes in material type for numerical
accuracy.
Response: The GSB has been removed from the wetland channel HEC-RAS
model, since it will be replaced by ArmorFlex erosion control products.
The local distributor will provide the hydraulics and erosion protection
values for the ArmorFlex product.
2. Contraction and expansion coefficients on the face of the 4H:1V slope on the
GSB drop structure must be reduced to zero in the model to account for the
supercritical nature of flow on this portion of the structure.
Response: The GSB will be replaced by ArmorFlex products.
3. All proposed vertical drop structures must be represented in the HEC-RAS cross
section geometry. HEC-RAS is a program designed to compute gradually-varied
flow routines, and vertical drops are rapidly varied flow features. Rapidly varied
flow structures require more cross sections for accurate hydraulic model results.
Cross sections must be located at the top of slope and toe of slope, with a
maximum distance of 0.1 ft between sections. Sections must also be included at
least a 1/2 channel width upstream and a % channel width downstream of the
vertical face, and 1 full channel width upstream and downstream of the vertical
face. These excess cross sections allow HEC-RAS the opportunity to calculate
the appropriate M3 profile leading up to the drop, and also provide adequate
spatially-distributed numerical iteration locations downstream to identify and
represent the anticipated hydraulic jump.
Response: As discussed with Weld County on January 25, 2007, the plans
have been revised to show the maximum amount of riprap that can be
placed at the time of construction due to the 404 permit. The St. Vrain
Lakes Metro district will maintain the check structures and place additional
riprap if needed to dissipate the energy in the hydraulic jump.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN:: 3075
Page 19
Comments for the Vertical Drop Structures
1. The vertical drop structures proposed for the Wetlands Channel will create a
submerged hydraulic jump in the channel that will present a hazard to public
safety. According to the USDCM, Chapter 8, Section 2.4.4, first paragraph, ". .
the vertical hard basin drop is to be avoided where practical due to impingement
energy, related maintenance, and turbulent hydraulic potential under some flow
conditions." Please respond by exploring other grade control options and
discussing the viability of such options.
Response: A vertical hard basin drop is not currently proposed on the
wetland channel. USDCM volume 2, chapter 8, Section 2.9 recommends
use of the grade control structures in wetland channels, "...to provide
control points and establish stable bed slopes within the base flow
channel...."
2. If vertical drop structures must be utilized in the Wetlands Channel, they shall be
constructed according to guidelines published in the USDCM, Chapter 8, Figure
HS-9.
Response: Vertical drop structures are not being proposed.
3. Downstream of (below) the vertical drops, the channel must be protected with
substantial permanent erosion control to prevent undermining of the vertical drop
structure. Flow over the structure will scour the channel at the toe and
downstream channel due to impact forces and a hydraulic jump in the channel.
Please utilize guidelines published in the USDCM, Chapter 8, Section 2.4.4 to
determine the characteristics of the hydraulic jump, and to prepare adequate
downstream erosion protection. This requirement applies to all proposed vertical
drop structures in the proposed subdivision.
Response: The depth of scour downstream of the check structures has
been calculated and is provided in Appendix G. The metropolitan district
will be responsible for the maintenance of the check structures and will
place additional riprap as needed to dissipate the energy in the hydraulic
jump.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 20
4. All vertical drop structures must be designed with a low-flow or trickle-flow notch
for the safe conveyance of non-storm flows in the Wetlands Channel, as per the
USDCM, Chapter 8, Figure HS-13 (2001).
Response: A low-flow notch has been added to the check structures and
the calculations are provided in Appendix G. This low-flow notch will
ensure that the channel does not laterally migrate at the locations of the
check structures. This low-flow notch has been designed to convey 3.0%
of the 100-year runoff as stated per USDCM, chapter 8, Figure HS-13 (2001).
Comments for Detention Pond 104 Hydraulic Structures
1. Please consider the potential use of articulating concrete blocks (ACBs), turf
reinforcement mats (TRMs), and other permanent erosion control measures for
the final drop structure at the bottom of the Wetlands Channel into Pond 104.
Other measures that provide erosion protection, prevent headcutting upstream
through the main channel, and maintain public safety adjacent to the Wetlands
Channel may prove more economical for construction. Many of these measures
can provide a "green" alternative to the proposed grouted sloping boulder (GSB)
drop structure that provides adequate erosion control protection and is more
visually consistent with surrounding conditions. Please respond by exploring
other grade control options and discussing the viability of such options.
Response: The GSB will be replaced by an articulated concrete block
system as suggested. ArmorFlex is currently working with Carroll &
Lange, Inc. to design a more cost effective and "green" alternative.
2. The dimensions, slopes, and hydraulic performance of the proposed GSB drop
must be supported with engineering design calculations consistent with
guidelines published in the USDCM, Chapter 8.
Response: The GSB will be replaced by an ArmorFlex erosion protection
system.
3. A fully arrayed system of weep drains must be constructed beneath the GSB
drop structure as per UD&FCD standards. The weep drain array must be
supported with engineering design calculations, as per guidelines published in
the USDCM, Chapter 8, Figures HS-7a1 , HS-7b1, or HS-8.
Response: The ArmorFlex open cellblocks that Carroll & Lange, Inc. is
investigating will not require a weep drain system because the ground
water can drain directly through the articulated concrete block mats.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 21
4. The emergency spillway design between Pond 104 and Saint Vrain Creek must
be supported with engineering design calculations.
a. The spillway flow depth shall not exceed 6 inches, as per Weld County
Storm Drainage Criteria.
Response: A variance is being requested by Carroll & Lange, Inc. to
exceed this flow depth.
b. All proposed riprap sizes must be supported with engineering design
calculations consistent with guidelines published in the USDCM.
Response: All proposed riprap sizes for the spillway are supported
with engineering calculations in Appendix E.
c. The upstream and downstream termination (keys) of proposed riprap on the
spillway structure must be designed in accordance with UD&FCD standards
published in the USDCM, Chapter 7 (2001). If desired, the guidelines
published in HEC-11, Chapter 4, Figures 20 and 22 (FHWA 1989) may be
used, which may reduce the total volume of rock required for riprap
terminations in the river. It is critical to design an appropriate riprap tie-in at
Saint Vrain Creek to prevent scouring at the toe of the revetment, and
eventual failure of the spillway.
Response: The upstream and downstream termination points of the
spillway have been designed with termination keys as shown in
HEC-11, chapter 4, Figure 20. This detail has been added to PD01.
5. Please show and label elevation contours in the existing wetland delineated in
Pond 104.
Response: The contours in the existing wetland have been shown and
labeled.
6. Please support the proposed outlet pipe design configuration with hydraulic
calculations. All hydraulic calculations must be added to the Appendix of the
Final Drainage Report.
Response: The StormCAD model used to size the outlet pipe in regards to
outlet control is included in Appendix C will all the StormCAD outputs. All
other hydraulic calculations for the outlet pipe including the inlet control
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 22
spreadsheet, the orifice calculation, and the low tailwater basin calculation
have been added to Appendix E.
7. The low tailwater riprap basin design must be supported by engineering design
calculations consistent with guidelines published in the USDCM, Chapter 8.
Response: The engineering calculations for the low tailwater riprap basin
for detention pond 104 has been provided on the last sheet in Appendix E,
sub-section "Detention Pond 104."
8. Please provide engineering design calculations consistent with UD&FCD
guidelines to prove the proposed 30" RCP draining Pond 104 will not back up
water under outlet control conditions and submerge the proposed orifice plate.
Response: The StormCAD model for the 30-inch RCP draining pond 104
operates under outlet control. This hydraulic condition produces a flow
depth of 2.04 feet. Therefore, the orifice is not submerged and will operate
as an orifice.
Comments for Detention Pond 105 Hydraulic Structures
1 . Weld County has been provided Sheet PD3 of PD4, and PD04 of PD04 — please
use a consistent formatting for sheet number, and provide sheets PD01 and
PD02 if they are currently part of the Pond 105 design. It appears from the Title
Sheet that PD01 and PD02 are specific to the design of Pond 104, and should be
clearly labeled as "PD01 of PD04" and "PD02 of PD04" if this is the case. Please
update the third sheet to read "PD03 of PD04" for consistency with the rest of the
Construction Drawings.
Response: The naming convention has been revised for consistency.
2. The emergency spillway design between Pond 105 and Saint Vrain Creek must
be supported with engineering design calculations.
a. The spillway flow depth shall not exceed 6 inches, as per Weld County
Storm Drainage Criteria.
Response: A variance is requested by Carroll & Lange, Inc. to exceed
the six-inch spillway flow depth criteria. This variance is requested
because a portion of the flow that must be conveyed over the spillway
for detention pond 105 comes from the overtopping of the berm
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 23
between the St. Vrain River and the St. Vrain Lakes per the St. Vrain
Lakes CLOMR 2006 during the 100-year flood.
b. All proposed riprap sizes must be supported with engineering design
calculations consistent with guidelines published in the USDCM.
Response: The spillway for detention pond 105 will utilize an
ArmorFlex erosion protection system on the downstream face of the
spillway. A high performance turf reinforcement mat will be utilized on
the top of the spillway to prevent erosion. The calculations and details
of these products will be provided to Weld County for approval prior to
any construction on the spillway.
c. The upstream and downstream termination (keys) of proposed riprap on the
spillway structure must be designed in accordance with UD&FCD standards
published in the USDCM, Chapter 7 (2001), or FHWA standards published
in HEC-11 (1989). It is critical to design an appropriate riprap tie-in at Saint
Vrain Creek to prevent scouring at the toe of the revetment, and eventual
failure of the spillway.
Response: Carroll & Lange, Inc. and ArmorFlex are currently working
on the details of the termination of the articulated concrete blocks to
prevent scouring from the St. Vrain River at the toe of the spillway.
d. The tie-in shown on Sheet GR15 is not consistent with the tie-in detailed on
Sheet PD3 of PD4. Please correct the design, add calculations to the Final
Drainage Report, and update all pertinent sheets for accuracy and
completeness.
Response: The tie-in will be finalized upon completion of the
termination detail by ArmorFlex. This will be provided to Weld County
for approval prior to construction of the spillway.
3. The low tailwater riprap basin design must be supported with engineering design
calculations consistent with guidelines published in the USDCM, Chapter 8
(2001).
Response: The engineering calculations for the low tailwater basin have
been added to Appendix E, sub-section "Detention Pond 105."
4. Please support the proposed outlet pipe design configuration with hydraulic
calculations consistent with standards published the USDCM, Chapter 9 (2001).
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 24
A complete set of hydraulic calculations must be added to the Final Drainage
Report. Hydraulic calculations must show that the proposed RCP draining Pond
105 will not back up water under outlet control conditions and submerge the
proposed sharp crested weir structure.
a. Please provide a justification for and discussion of the operation of the
proposed sharp crested weir.
Response: The proposed sharp crested weir was added to the outlet
structure as requested from the Little Thompson Water District
(LTWD). The sharp crested weir would be used approximately once a
year to release storm flows. The proposed 12" x 12" slide gate would
release flows into the outlet structure when the outlet structure is not
receiving flows from a storm event and the St. Vrain River is flowing at
normal discharge. The sharp crested weir has been added so that the
released amount of water is quantifiable. The Little Thompson Water
District has approved the design of the sharp crested weir.
b. The sharp crested weir will not operate under weir flow conditions if the
structure is submerged by tailwater created by backwater from the outlet
pipe. If the proposed 30" RCP is flowing under outlet control conditions, this
feature will not be as efficient as a typical weir, and the WSEL in the pond
will be higher than predicted by the equation Q = 3.15*L*H1 5
Response: A hydraulic analysis of the outflow pipe operating under
outlet control conditions with 14.0 cfs release rate shows a flow depth
of 1.0-feet during. The weir crest is proposed at an elevation of 4,814.5
feet, and the outlet pipe invert is proposed at 4,813.5 feet, providing
for 1.0 feet of flow depth in the outlet pipe. 14.0 cfs is the maximum
release that LTWD anticipates and therefore the tailwater created by
backwater from the outlet pipe is not a concern. A printout of the
hydraulic analysis performed using the FlowMaster 2005 software is
included in Appendix E, sub-section "Detention Pond 105."
c. The nomograph provided on Sheet PD03 of PD04 is labeled incorrectly.
The design values do not match those indicated on the rest of the sheet.
Please update all hydraulic information on the sheet to be internally
consistent, and consistent with published information in the Final Drainage
Report.
Response: The nomograph on sheet PD03 is to provide LTWD with the
necessary information to determine the release rate out of the slide
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 25
gate depending on the water surface in the lake. The slide gate and
the sharp crested weir will measure the release rate of the water when
so desired.
5. Please clarify and provide a rationale for the proposed slide gate at the front of
the proposed outlet structure. The proposed design appears to enable the pond
to be drained at any time, and bypass the water quality capture features of the
detention pond.
Response: The sharp crested weir has been added as requested by LTWD.
The slide gate will also provide the opportunity to drain the lake in the
future if needed, due to maintenance, considering the size of the lake.
Comments for Detention Pond at WCR 9.5 Hydraulic Structures
1. An emergency spillway must be included as part of the detention pond design for
WCR 9.5. The spillway must have appropriate permanent erosion control
measures supported with design calculations consistent with guidelines
published in the USDCM, Chapter 8.
Response: The water quality pond located north of WCR 9.5 is currently
designed so that the entire inflow to the pond can be released through the
Type D outlet structure. In the event that this outlet structure plugs, the
flow will back-up the 24-inch RCP inflow pipe and surcharge Inlet A-59 and
flow to Inlet A-67, which will capture and convey the runoff to the lake.
2. Please support the proposed outlet pipe design configuration with hydraulic
calculations consistent with standards published in the USDCM, Chapter 9
(2001). A complete set of hydraulic calculations must be added to the Appendix
of the Final Drainage Report.
Response: The StormCAD analysis of the WCR 9.5 water quality pond
storm sewer system has been added to Appendix C with all of the
StormCAD output files. A complete set of the hydraulic calculations for the
water quality pond is in Appendix E, subsections "Water Quality Pond
north of WCR 9.5."
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 26
Underdrain Design Comments
1. All bends must have two (2) cleanouts for proper maintenance of the underdrain
system. Each cleanout shall be oriented in the direction of the flowline to allow
for periodic cleaning with pressurized water.
Response: The underdrain system is a private system that will be
maintained by the homeowners association. Cleanouts will be located
adjacent to each sanitary sewer manhole. Based on the number of
manholes/cleanouts, a single one-way cleanout will be provided.
2. Cleanouts in the proposed underdrain system shall have a maximum spacing not
to exceed 750 ft.
Response: We have provided a maximum cleanout spacing of 400 feet,
since the cleanouts will be one-way only.
3. Proposed underdrain pipes shall have a minimum diameter of 8 inches.
Response: Underdrain pipes are recommended and sized by the project
geotechnical engineer. The minimum underdrain will be four inches in
diameter. Underdrains are provided as a means to capture subsurface
drainage around foundations.
4. The underdrain system shall not intercept any surface stormwater runoff.
Response: The underdrain system will not intercept any surface
stormwater runoff.
November 17, 2006 Response by Carroll & Lange to General Comments
#7 — The EGL is reportedly 6-inches below the proposed finished grade. However,
EGLs have not been plotted on any of the pipe profiles. Please show the 100-year EGL
on all proposed storm sewers pipes and culverts in the subdivision.
Response: We have added the 100-year EGLs to all storm sewer pipes in the
Filing No. 1 subdivision plans. However, current County code only requires that
100-year HGL elevations be plotted.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 27
November 17, 2006 Response by Carroll & Lange to Specific Drainage Comments:
#15 —Carroll & Lange are strongly encouraged to contact Public Works staff and
request clarification and assistance when they cannot locate specific comments
identified during the review process. It is not acceptable to discount staff comments
simply because the engineer(s) could not find where those comments apply.
Response: Carroll & Lange, Inc. appreciates Public Works comments and feels
that the clarification received from Public Works during the January meetings has
been very helpful. We feel that we have addressed all comments from the second
submittal and would like to set-up a meeting to quickly go through the comments
with Weld County to help expedite the third review.
#21 — Erosion protection calculations for proposed swales were not found in
Appendix C, or in Exhibit C, or any other part of the Final Drainage Report. Please
provide this information to Public Works for a complete review.
Response: The erosion protection calculations for the proposed swales are now
provided in Appendix C, sub-section "Swale and Riprap Calculations."
#27(a) — For completeness of the report and review, please provide calculations for
structural losses through manholes and inlet boxes.
Response: The StormCAD model utilizes the FHWA HEC-22 energy loss
methodology to calculate the head losses through structures. The StormCAD
node output file has columns identifying the Energy Grade Elevation in and out of
the structure.
#27(b) — Since Carroll & Lange has proposed to lower the 100-year EGL below
proposed grade, bolt-down lids on manholes will no longer be necessary. However,
venting will be required on all proposed storm sewers with steep slopes, with
supercritical flow, or with inlet-controlled flow conditions. Hydraulic conditions inside
these pipes will entrain air at hydraulic jumps and pressurize the inside of the manhole.
Without venting, this pressurization will lead to separation of the individual manhole
riser sections, thus compromising the structural integrity of the pipes, and causing
leakage around all section gaskets. Weld County's recent conversations with drainage
engineers working for other jurisdictions confirms their experience with similar situations
and they have found that air-entrainment conditions will be worst during smaller storm
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 28
events (5-year and less), and may present an expensive maintenance problem for the
future residents of St. Vrain Lakes PUD.
Response: All storm manholes will be vented as discussed with Weld County
Public Works during the January 25th meeting.
#28 — Public Works does not accept this statement on the grounds of technical
accuracy. Please refer to the response #27(b) above, and the inlet control discussion
under Specific Drainage Comments.
Response: The drainage report has been revised to take inlet control into account
as previously discussed. With the revisions to the plans, velocities in the storm
sewer pipes at storm manholes do not exceed 15 fps. This is less than the
UDFCD maximum velocity criteria of 18 fps utilized in their storm sewer design
software NeoUDSewer.
#36 — Please refer to response #27(b) above.
Response: The storm sewer system will be installed and compacted in
accordance with the criteria and specifications provided by Weld County. Storm
sewer trenches and areas surrounding manholes will be appropriately compacted
and tested. The storm sewer system will include watertight joints (R-4) in
accordance with ASTM C 443 standards and manholes will be vented.
#40 — The only way to prove the 7.5 cfs may turn the corner into the side-street is to
provide a momentum calculation of the flow around the specified corner. This comment
applies to all areas where flow is assumed to wrap around corners, especially where
proposed flows entering the corner are supercritical.
Response: The surface runoff at design point (DP) B15 of 7.5 cfs is assumed to
fully turn the corner and drain to the sump inlets at DP B3. To be conservative,
Carroll & Lange, Inc. has also assumed that 2/3 of that flow is conveyed to the
sump inlet at DP A19. We analyzed the other location of concern identified by
Weld County at the intersection of Elk River Road and Cimmaron River Road.
The runoff in the east gutter of Cimmaron River Road is 2.6 cfs. We have
provided an open channel analysis on this flow and found that the depth of flow
at this location is 0.28 feet. A portion of this small amount of runoff will be
conveyed in the curb and gutter to the east, the remainder of the flow will sheet
flow to the south gutter of Elk River Road, which will convey the flow to the same
inlet at DP A6.1. A print out of this hydraulic analysis is located after the major
storm inlet calculation for Basin A6.
Kim Ogle
Weld County Planning Department
February 5, 2007
JN: 3075
Page 29
#51 — Please add the specified comment to the text of the drainage report, and to the
final plat for the subdivision.
Response: The requested note has been added to note number 30 on the general
notes sheet of the construction plan set and sheet 1, paragraph 1, sentence 4 of
the final drainage report.
November 17, 2006 Response by Carroll & Lange to Erosion and Sediment Control
#3 — The D50 calculations for the riprap spillway were not found in Appendix D, or in
Exhibit D, or any other part of the Final Drainage Report. Please provide this
information to Public Works for a complete review.
Response: The calculations for the proposed riprap are included in Appendix C,
sub-section "Swale and Riprap Calculations."
#4 — The D50 calculations for culvert outlet protection were not found in Appendix C, or
in Exhibit C, or any other part of the Final Drainage Report. Please provide this
information to Public Works for a complete review.
Response: The calculations for the proposed riprap are included in Appendix C,
sub-section "Swale and Riprap Calculations."
We have included two (2) copies of the drainage report and drainage drawings for your
review. Should you have any questions or additional comments, please feel free to
contact us at any time.
Sincerely,
CARROLL & LANGE, INC.
go,,Lct-,c
Fred G. Tafoya III, PE Kevin Jennings, El
Engineering Manager Engineer II
cl
Attachments
cc: E / R
Weld County Planning Department
GREELEY OFFICE
JANE 1 0 2007, ---;\,( MEMORANDUM
1 � EIVED
TO: Kim Ogle, Planning Services ATE: 09 January 2007
Q FROM: David T Bauer,P.E.,CFM,Public Works Department, and v06
Brian P.E.,CFM,Public Works Department
COLORADO
SUBJECT: PF-1078 St.Vrain Lakes PUD(Final Plan)
Weld County Public Works Department has reviewed this Final plan request. Comments made during this
phase of the subdivision process may not be all-inclusive, as other concerns or issues may arise during the
remaining application process.
Drainage Comments
❑ Public Works received a Final Drainage Report for Saint Vrain Lakes PUD, Filing No. 1 (PF-1099)on
November 20,2006. The report was submitted by Katherine Strozinski(PE#37307),of Carroll&Lange,
Inc.,and is dated November 7,2006.
❑ The final drainage report is stamped, signed, and dated by a registered P.E. licensed to practice in the state of
Colorado. Most of the construction plan sheets are also stamped, signed, and dated. All construction drawing
sheets must be stamped, signed,and dated by a registered P.E. licensed to practice in the State of Colorado.
This follow-up set of comments on the proposed St. Vrain Lakes PUD Filing I drainage design have been
categorized based on general types of drainage issues. Concerns that arose during the October review included
management of offsite stormwater, grading, storm pipe design, swale design, inlet design, erosion control and
erosional and hydraulic. A detailed review identifying specific issues and concerns at specific locations in the
proposed St. Vrain Lakes PUD site was provided in the October 19, 2006 comments from Public Works. The
comments below reflect review of the revisions submitted November 20, 2006.
Offsite Flows:
1. Concerns regarding stormwater runoff from offsite areas to the north of the St. Vrain Lakes PUD Filing I
were noted in the October 19, 2006 comments from Public Works. The revised drainage report has
updated the calculations and proposed drainage facilities for the western area of St. Vrain Lakes PUD
Filing I (Basins OS! and OS2). However, the report text and the plans of the proposed condition in the
revised (November 2006) Filing 1 Drainage Report for the area east of Weld County Road 9-1/2 show
offsite future drainage facilities to be "constructed by others". The previous submittal did not identify
these improvements as being constructed in the future "by others". The specific capacities and locations
of these un-constructed or proposed offsite drainage management facilities by others were not provided in
the revised report and cannot be verified and therefore cannot be considered as operable when evaluating
the designs of the St. Vrain Lakes PUD project. Management and safe conveyance of the offsite peak
flows to and through the proposed Filing I is requisite. As stated in the October 19, 2006 comments, the
applicant is to provide details of the designs and erosion control for all drainage facilities that convey the
un-detained offsite flows through or around the St.Vrain Lakes PUD site.
2. Of particular concern are Offsite Basins OS5 and OS7 draining approximately three-quarters of a square
mile (475 acres) of currently undeveloped land which are shown (Sheet DR-1) discharging runoff across
the proposed road WCR 9.5 and proposed road WCR 28. The 146 cfs flowing across WCR 28 from
Offsite Basin OS6 is also of concern(See item#8 below).
Page 1 of 11
3. We have received no information regarding the Richie Brothers detention pond (i.e. area served, release
rate, detention, etc.). Under current conditions, 186 cfs flow undetained from offsite Basin OS7 into St.
Vrain Lakes PUD Filing I. The proposed plans provide no information regarding the dimensions and
design specifics of how these flows are safely conveyed across WCR 9.5. Please provide a description
and designs for safe management of these offsite flows to and through the St. Vrain Lakes Filing I.
Please show on the plans the location and sizing of the conveyance. Please describe who will construct
these facilities. Prior to filing the Final Plat,please provide evidence that the proposed drainage facilities
are in dedicated drainage easements or public Right-of-Way.
4. The proposed plans (Sheet DR-1) in the November 2006 Filing 1 Drainage Report show a proposed
drainage swale along the north side of WCR 28 supposedly to be constructed by others. The current
condition,in the absence of this WCR 28 roadside swale,would be that the stormwater runoff from Basin
OS5 would overtop WCR 28 and flow to proposed St. Vrain Lakes PUD homesites in proposed basins
Al, A2, B9, A22, B5 and others. As stated above and in the October 19, 2006 comments, the applicant
must provide adequate drainage facilities to handle all current condition flows, both on-site and off-site.
The specific capacities and locations of these un-constructed or proposed offsite drainage management
facilities by others were not provided in the revised report and cannot be verified and therefore cannot be
considered as operable when evaluating the designs of the St. Vrain Lakes PUD project without further
details and commitments. Under current conditions, this WCR 28 swale would need to convey the 186
cfs undetained peak flow from offsite Basin OS7 and the 261 cfs undetained peak flow from offsite
Basin OS5. The proposed plans in the November 2006 Filing 1 Drainage Report do not give the
dimensions and design specifics for the proposed this WCR 28 drainage swale. Please describe who will
construct these facilities and when they will be constructed. Please describe how is this swale to be
constructed given the gas line and utility easement conflicts in the same proposed alignment shown in the
November 2006 Filing 1 Drainage Report and shown on the plans. Please provide evidence that the
proposed drainage facilities are in dedicated drainage easements or public Right-of-Way.
5. Please describe the correct captured flows and capacity of the proposed drainage swale along the north
side of WCR 28. According to the Developed Case SWMM model in the November 2006 Filing 1
Drainage Report, the peak flow at design point 201 is 585 cfs. However the printout of the HEC-RAS
model output provided in the November 2006 Filing 1 Drainage Report gives 403 cfs as the upstream
most flow rate for the Basin B4 wetland channel. This peak rate does not match the discharge rate used to
model the Basin B4 wetland channel in the electronic version of the HEC-RAS model provided to the
County for review. See below for additional comment on the HEC-RAS model of this channel.
6. The plans of the proposed condition in the November 2006 Filing 1 Drainage Report show the location of
a RC Box Culvert to convey combined flows from Basin OS7 and OS5 under WCR 28. The Developed
Case SWMM model in the November 2006 Filing 1 Drainage Report indicates that this pipe is to be 5
feet in diameter. Please clarify which geometry (box or pipe) is correct. Please clarify the timing of
construction of this culvert and who will be performing the construction. The November 2006 Filing 1
Drainage Report does not give the dimensions and design specifics for the proposed this RCBC / storm
pipe. Please provide these details, pipe inverts and HY-8 or equivalent evaluation of the RCBC / storm
pipe capacity,entrance type,evaluation of inlet control,etc. If the proposed 5-foot diameter pipe does not
convey the full peak flow that is directed to this design point, please provide an evaluation and water
depths of WCR 28 overtopping. Details of the erosion protection design at the RCBC / storm pipe inlet
and discharge point are also requested.
7. This proposed 5-foot diameter / RCBC is shown with a proposed riprap apron that discharges to the
existing abandoned stock pond (Basin B4 on Sheet DR1). The proposed riprap apron does not extend to
the bottom of the wetland channel. Please address headward erosion of these features to insure that WCR
28 is not impacted. Please provide riprap sizing supported with calculations.
Page 2 of 11 .,�_
8. Basin OS6 does not have a functioning detention pond or discharge pipe. Under current conditions, 146
cfs flow undetained from offsite Basin OS6 into St. Vrain Lakes PUD Filing I. The proposed plans
provide no information regarding the dimensions and design specifics of how these flows are to be safely
conveyed across WCR 28. As explained to the applicant in previous reviews,the un-detained offsite peak
flows must be managed through or around the St. Vrain Lakes site and un-constructed or proposed offsite
drainage management facilities (by others) cannot be considered in design and approval of the St. Vrain
Lakes PUD project. Please provide a description and designs for safe management of these offsite flows
to and through the St. Vrain Lakes Filing I. Please show on the plans the location and sizing of the
conveyance(s). Please describe who will construct these facilities. Please clarify the timing of
construction. Prior to filing the Final Plat, please provide evidence that the proposed drainage facilities
are in dedicated drainage easements or public Right-of-Way.
Grading:
1. The St. Vrain Lakes PUD Filing I grading and drainage plans show proposed breaching of the existing
berm that forms the southeast margin of the existing stock pond immediately south of WCR 28 (Basin B4
on Sheet DR1). Given the existing steep slopes and that the applicant currently proposes to flow an
approximately 400 cfs peak discharge from offsite Basins OS5 and OS7 through the breached berm,
please provide an evaluation of the backwater and erosional stability of this feature. Grade control and
riprap will likely be needed;please provide riprap sizing supported with calculations.
2. Carma has begun grading the site under a grading permit issued after the Change of Zone approval. In
some locations in excess of 10 feet of fill have been placed. Weld County requires that these areas of cut
and fill be re-evaluated for geotechnical stability for the existing conditions (i.e. post cut and fill). Weld
County requests copies of updated geotechnical reports giving data from these re-graded areas and areas
of cut and fill. In some locations Weld County will require that new geotechnical borings be collected to
verify the current soil conditions of areas proposed for homes and other infrastructure.
General Hydraulic Comments:
1. Please add the following statement to the General Notes sheet of the construction plans and the text of the
Final Drainage Report; "Weld County shall not be responsible for the maintenance of drainage related
facilities."
2. All culvert inlets shall include debris racks to prevent clogging due to debris accumulation and to protect
the public safety. Debris racks shall be sloped at 3H:1V or flatter per UD&FCD research. Please provide
a detail drawing in the plan set to illustrate an FES with debris rack.
Global Comments for Rock Riprap:
1. According to the USDCM, Chapter 7, Section 4.4.2.3 (2001),riprap thickness for channel linings shall be
no less than twice the median stone size (> 2.0D50) in sandy soils for added safety factor against
erosion damage.
2. All proposed rock riprap protection in the subdivision must be supported by engineering design
calculations consistent with UD&FCD guidelines in the USDCM (2001). As an alternative, FHWA
guidelines in HEC-11 (1989), HEC-23 (2001), and HDS-6 (2001) may also be used for rock riprap
design. All calculations must be included in the Final Drainage Report.
3. All rock riprap protection designs must include a termination detail consistent with guidelines published
in the USDCM, Chapter 4.4.2.4 (UD&FCD 2001). If desired, the guidelines published in HEC-11,
Page 3 of 11 ----
Chapter 4, Figures 20 and 22 (FHWA 1989) may be used, which may reduce the total volume of rock
required for terminations in the subdivision.
4. It is highly recommended that the UD&FCD methods published in Chapters 7 and 8 of the USDCM
(2001) be double-checked against other methods for the benefit of erosion protection and public safety
and welfare. Other acceptable methods may include the Factor of Safety Method(HDS-6,FHWA 2001),
the HEC-23 Method(HEC-23,FHWA 2001).
5. An appropriate geotextile filter fabric must be designed and identified by product name or engineer-
approved equal on all construction plan sheets. The choice of geotextile must be accompanied by
engineering design calculations and included in the Final Drainage report.
Storm Sewer Pipe Design-General Comments
1. The pipes designations shown on the plan and profile sheets(PP), storm plan and profile sheets(STMPP),
and all drainage sheets (DR) do not match the unique names provided as input to StormCAD. (These
unique names are published in Exhibit C, Appendix C (Hydraulic Calculations) of the Final Drainage
Report). This inconsistency complicates review. Please utilize the same naming convention on all plans
and in the reports and StormCAD printouts or provide a correlation table.
2. Use of the computer models HEC-RAS and UDSWMM 2000 are identified in the text of the Final
Drainage Report;mention of these programs need to be added to the list of references for completeness.
3. For completeness,please provide a digital copy of all hydraulic and hydrologic models referenced in the
text of the Final Drainage Report and reproduced in the report appendix.
Storm Sewer Pipe Design—Specific Comments
Following Public Works' October, 2006 comments,the revised St. Vrain Lakes PUD Filing I Drainage report
(November, 2006) included upsized proposed pipes on the steeper streets. However, it is apparent that many
of the proposed storm pipes with steep slopes will have supercritical flow and many will operate under inlet
control. An excerpt from HDS-5 (FHWA 2005)defines inlet control as follows: "Inlet control occurs when
the culvert barrel is capable of conveying more flow than the inlet will accept. The control section of a
culvert operating under inlet control is located just inside the entrance . . . . Hydraulic characteristics
downstream of the inlet control section do not affect the culvert capacity. The upstream water surface
elevation and the inlet geometry represent the major flow controls. " --FHWA 2005,Page 7, Paragraph 2(a).
Cross reference to USDCM,Chapter 9, Section 2.1.2 and Section 3.1.
1. In the November 2006 Filing 1 Drainage Report, the 100-year Pipe Report from StormCAD does not
clarify or evaluate which pipes will operate under inlet control conditions and which operate under outlet
control conditions. As commented on in the October review of the Filing I Drainage report, proposed
manholes and storm pipes on the steeper streets will be induced into hydraulic jump conditions by the
addition of flow from side street inlets. The hydraulic jump in or near a manhole will cause inlet control
in the pipe leaving that manhole. The current StormCAD analysis routines,which utilize HEC-22 energy
grade line evaluation procedures, do not compute inlet control condition. As a result, many of the pipes
proposed throughout the subdivision will not be capable of capturing the design flows predicted by
StormCAD. Weld County's recent conversations with drainage engineers working for other jurisdictions
confirms this limitation of StormCAD. The experience in these nearby jurisdictions included inlet
controlled pipes that surcharged at the upstream pipe entrance creating a backwater through the manhole
and adjacent inlets and causing a flooding condition on streets in the subdivision. Weld County contends
that the proposed inlets will not collect the flows indicated in the StormCAD output. Please review all
proposed storm sewer pipes to check for inlet or outlet control and provide revised associated inlet
Page 4 of 11
capacity calculations.
a. Example: An HY-8 analysis of Pipe P-49 indicates the 60" RCP proposed from Inlet I-A53 to
Pond 106 will flow under inlet control conditions at Q100 = 284 cfs. HY-8 shows the pipe has a
headwater depth(HW,00)of 11.59 ft, or WSEL100=4834.13 ft. The StormCAD output shows the
pipe has an upstream EGL100 = 4830.66 ft, which is 3.47 ft lower than the WSEL computed by
HY-8. Since this pipe is operating under inlet control conditions,the StormCAD-calculated EGL
upstream of P-49 is incorrect. The rim elevation of Inlet A-53 on WCR 9.5 is 4830.9 ft, and by
inlet control calculations, there will be at least 3.23 ft of flooding in the travel lane of WCR 9.5.
Hand calculations of this situation provide similar results. This is an unacceptable condition.
b. Proposed storm sewer lines operating under inlet condition need to be recalculated to prevent
detrimental street flooding in the subdivision. Storm sewer lines upstream of these inlet-
controlled pipes must be recalculated because the associated downstream tailwater conditions will
change.
2. Please incorporate curved vane inlets on-grade and/or in sumps along the gutter flowline of all streets
with slopes exceeding 2% longitudinally. This will prevent skipping and splashing that will occur over
grate inlets in gutters flowing at high velocities and high Froude numbers. Our discussions with other
municipalities suggests vane inlets will be the most effective structures for intercepting high-velocity
flows on steeper (>3%) street surfaces. The appropriate grate size, number, and configuration can be
determined with UD-Inlet (UD&FCD 2006), or, as a reasonable alternative, with HEC-22, Chapter 4,
Section 4.4.3.3 (FHWA 2001).
3. The proposed design longitudinal slope of all streets and pipes at intersections along Eagle River Road
exceeds 4% and the surface runoff in adjacent gutters will experience supercritical flow. The momentum
of this supercritical flow will be high and will not round corners at the proposed Eagle River Road
intersections to proposed sidestreet inlets (ex: DP-A15.1, A6.2). The high velocity Eagle River Road
gutter flow will create unsafe driving conditions for residents exiting or entering Eagle River Road from
these side streets. To correct this problem, please add on-grade curved vane inlets to Eagle River Road.
These inlets will be needed on both sides of all intersections downstream of STMH A-9, including
Canadian River Road,Elk River Road,Castle Creek Way,and Colorado River Road.
4. Proposed storm sewer lines operating under high velocities or under full-flow/pressure flow conditions
shall be constructed of pressure-sealed concrete pipe rated for these conditions.
5. The sizings of proposed storm sewer pipes analyzed in StormCAD and reported in Exhibit C,Appendix C
(Hydraulic Calculations) of the Final Drainage Report do not match the sizings on the construction plan
sheets.
a. Example: Construction Sheet DR5 of the Final Drainage Report shows Pipe P-49 as a 60 in.
diameter RCP flowing into Pond 106. Exhibit C, Appendix C of the Final Drainage Report
shows Pipe P-49 as L = 204 ft, diameter = 60 in., Spipe = 0.042 ft/ft, Inv. U/S = 4822.55 ft, and
Inv. D/S =4814.00. The same pipe on Sheet PP3 of the WCR 9.5 Construction Plans shows L=
210 ft, diameter= 54 in., Sop, = 0.039 ft/ft, Inv. U/S =4823.10 ft, and Inv. D/S = 4815.00. The
WCR 9.5 construction plans also show a WSEL100=4819.00 ft,but has a scaled dimension of 4.0
ft (48 in.) at the outlet end of Pipe P-49. Please cross-check the report text with the Final
Construction Drawings.
6. If the headwater-to-diameter(HW/D)ratio on proposed storm sewer pipes and culverts exceeds 1.5, pipe
collars will be required to reduce the potential for the piping of fill material around proposed structures.
The HW/D ratio is discussed in the USDCM,Vol.II,Chapter 9—Culverts(UD&FCD 2001). Pipe collars
and their design are detailed in Design of Small Canal Structures, Chapter VIII, Section 8-10,Pages 362-
364 (USBuRec 1978). Alternatively, the HW/D ratio may be reduced by adjusting the size and
configuration of proposed storm sewer pipes and culverts.
... . . . ._.. _.-�a . _..�- Page 5 of 11 »�
Comments for the Wetland Channel HEC-RAS model:
1. The text of the Final Drainage Report discusses the use of HEC-RAS to determine hydraulic conditions in
the wetland channel. The appendix of the report includes a CAD drawing of cross section locations and a
printout of Standard Table 1 from HEC-RAS. For the wetland channel evaluation,please provide the full
HEC-RAS-generated report, including all input,output,and intermediate calculations.
2. Please provide a digital copy of all hydraulic and hydrologic models referenced in the text of the Final
Drainage Report and/or reproduced in a report appendix.
3. The HEC-RAS results printed as Standard Table 1 in the Final Drainage Report appendix do not match
those of the digital model submitted to Public Works on 12/12/2006. Please explain why the hydrology
and computed hydraulics for the wetland channel are different between the report appendix and the
submitted digital model. Please reconcile and provide the final model and tie the results to the proposed
channel facilities.
4. Please discuss channel stability under computed hydraulic conditions that include velocities in excess of
5.0 fps and shear stresses in excess of 3.0 psf. More robust erosion protection will be required.
5. Please justify Manning-n values of 0.200 used in cross-sections 860 through 820, 0.550 at cross-section
816, and 0.045 at cross-sections 812 through 784.9. These values are inconsistent with the rest of the
model, which uses 0.060 for the main channel and overbanks. Please note cross-section 816 is internally
inconsistent, as the left overbank has an n-value of 0.055, and the main channel and right overbank uses
0.550. Is this a typographical or input error?
a. When using HEC-RAS, all Manning-n values should be consistent with UD&FCD and the HEC-
RAS Hydraulic Reference Manual (HEC 2002),Table 3.1, as adapted from Chow (1959). Other
n-values may be justified from appropriate hydraulic references,which must be referenced in text
and copied in the appendix of the Final Drainage Report.
b. An appropriate n-value for the channel grade control structure. If the anticipated flow depth
exceeds 5.0 ft for the majority of each cross section, the USDCM, Chapter 8, Table HS-2 may be
utilized. Since most of the study reach has maximum depths below 5.0 ft, it is more appropriate
to utilize values published in HEC-15, Chapter 2,Table 2.2(FHWA 2005).
6. The HEC-RAS model computed severely unstable hydraulic conditions on the 411:1 V grade control
structure. Velocities will exceed 10.0 fps and shear stresses will exceed 18.0 psf. More robust erosion
protection will be required for County approval of this structure.
a. Residential lots are proposed on the right bank(looking downstream)immediately adjacent to the
grade control structure. The applicant must show stable flow conditions on the structure to
prevent channel erosion in the channel and on the adjacent overbank near the structure.
b. The proposed grade structure must be protected with an appropriate lining that is supported
approved with engineering calculations. If grouted boulders will be used, the applicant must
prove they will be stable under guidelines published in the UDSCM, Chapter 8, Section 2.4.3
(2001).
7. The 100-year proposed SWMM model shows a Manning-n value of 0.005 for the wetlands channel. This
is inconsistent with HEC-RAS model input by a factor of 10. Please correct this error, re-run the
program,and re-evaluate the stability and identify necessary and appropriate erosion protection.
8. For completeness, please provide calculations to support the size, orientation, and erosion protection
provided by all proposed riprap basins and rundowns. All rundowns must be shown to be stable under
proposed conditions hydraulics.
Page 6 of 11 __
9. A toe-down and key are required for all riprap rundowns terminating in ponds to prevent undermining and
failure of the structure at the toe.
10. Filter fabric is required under all riprap, and should be supported with description of manufacturer
specifications and called out on the construction plans.
11. All utility conflicts must be resolved prior to recording the Final Plat.
12. Please add inlet and manhole rim elevations as a column to StormCAD output file.
13. The 100-year EGL and HGL must be shown on all pipe profile drawings in the construction plan set
(previously requested).
14. Please show how water from Pipe P-114 (at Outlet O-7) will fall into the Wetlands Channel without
eroding the right overbank of the channel.
15. Please consider replacing all sheet pile drop structures with grouted sloping boulder drop structures, as
per guidelines published in the USDCM,Chapter 8, Section 2.2,Table HS-1 (2001).
16. All 3-ft vertical drops require substantial toe protection to prevent drop scour, and the potential
undermining and buckling of sheet piles.
17. Please provide engineering calculations to show the sheet pile in vertical drop structures will have
sufficient burial to prevent buckling and failure, and protect the public safety and welfare, especially
during scouring storm events.
18. Please provide engineering calculations of the drop scour anticipated at all vertical drops.
19. Please provide engineering calculations to support the proposed channel bed slope of 0.40%.
20. A copy of all applicable wetlands permits from the U.S. Army Corps of Engineers must be provided in
the drainage report before any grading/construction activities can be initiated in wetland areas.
Updated Comments for the HEC-RAS model:
1. Cross sections on the grouted sloping boulder (GSB) drop structure must represent all changes in grade,
and all changes in material type for numerical accuracy.
2. Contraction and expansion coefficients on the face of the 411:1 V slope on the GSB drop structure must be
reduced to zero in the model to account for the supercritical nature of flow on this portion of the structure.
3. All proposed vertical drop structures must be represented in the HEC-RAS cross section geometry. HEC-
RAS is a program designed to compute gradually-varied flow routines, and vertical drops are rapidly
varied flow features. Rapidly varied flow structures require more cross sections for accurate hydraulic
model results. Cross sections must be located at the top of slope and toe of slope, with a maximum
distance of 0.1 ft between sections. Sections must also be included at least a 1/2 channel width upstream
and a 1/2 channel width downstream of the vertical face, and 1 full channel width upstream and
downstream of the vertical face. These excess cross sections allow HEC-RAS the opportunity to
calculate the appropriate M3 profile leading up to the drop, and also provide adequate spatially-
distributed numerical iteration locations downstream to identify and represent the anticipated hydraulic
jump.
— Page 7 of 11 .�m
Comments for the Vertical Drop Structures:
1. The vertical drop structures proposed for the Wetlands Channel will create a submerged hydraulic jump
in the channel that will present a hazard to public safety. According to the USDCM, Chapter 8, Section
2.4.4, first paragraph, ". . . the vertical hard basin drop is to be avoided where practical due to
impingement energy, related maintenance, and turbulent hydraulic potential under some flow
conditions." Please respond by exploring other grade control options and discussing the viability of such
options.
2. If vertical drop structures must be utilized in the Wetlands Channel,they shall be constructed according to
guidelines published in the USDCM, Chapter 8,Figure HS-9.
3. Downstream of (below) the vertical drops, the channel must be protected with substantial permanent
erosion control to prevent undermining of the vertical drop structure. Flow over the structure will scour
the channel at the toe and downstream channel due to impact forces and a hydraulic jump in the channel.
Please utilize guidelines published in the USDCM, Chapter 8, Section 2.4.4 to determine the
characteristics of the hydraulic jump, and to prepare adequate downstream erosion protection. This
requirement applies to all proposed vertical drop structures in the proposed subdivision.
4. All vertical drop structures must be designed with a low-flow or trickle-flow notch for the safe
conveyance of non-storm flows in the Wetlands Channel, as per the USDCM, Chapter 8, Figure HS-13
(2001).
Comments for Detention Pond 104 Hydraulic Structures:
1. Please consider the potential use of articulating concrete blocks(ACBs), turf reinforcement mats(TRMs),
and other permanent erosion control measures for the final drop structure at the bottom of the Wetlands
Channel into Pond 104. Other measures that provide erosion protection, prevent headcutting upstream
through the main channel, and maintain public safety adjacent to the Wetlands Channel may prove more
economical for construction. Many of these measures can provide a "green" alternative to the proposed
grouted sloping boulder (GSB) drop structure that provides adequate erosion control protection and is
more visually consistent with surrounding conditions. Please respond by exploring other grade control
options and discussing the viability of such options.
2. The dimensions, slopes, and hydraulic performance of the proposed GSB drop must be supported with
engineering design calculations consistent with guidelines published in the USDCM,Chapter 8.
3. A fully arrayed system of weep drains must be constructed beneath the GSB drop structure as per
UD&FCD standards. The weep drain array must be supported with engineering design calculations, as
per guidelines published in the USDCM,Chapter 8,Figures HS-7a1,HS-7b1, or HS-8.
4. The emergency spillway design between Pond 104 and Saint Vrain Creek must be supported with
engineering design calculations.
a. The spillway flow depth shall not exceed 6 inches,as per Weld County Storm Drainage Criteria.
b. All proposed riprap sizes must be supported with engineering design calculations consistent with
guidelines published in the USDCM.
c. The upstream and downstream termination (keys) of proposed riprap on the spillway structure
must be designed in accordance with UD&FCD standards published in the USDCM, Chapter 7
(2001). If desired, the guidelines published in HEC-11, Chapter 4, Figures 20 and 22 (FHWA
1989)may be used,which may reduce the total volume of rock required for riprap terminations in
the river. It is critical to design an appropriate riprap tie-in at Saint Vrain Creek to prevent
scouring at the toe of the revetment,and eventual failure of the spillway.
Page 8 of 11
5. Please show and label elevation contours in the existing wetland delineated in Pond 104.
6. Please support the proposed outlet pipe design configuration with hydraulic calculations. All hydraulic
calculations must be added to the Appendix of the Final Drainage Report.
7. The low tailwater riprap basin design must be supported by engineering design calculations consistent
with guidelines published in the USDCM,Chapter 8.
8. Please provide engineering design calculations consistent with UD&FCD guidelines to prove the
proposed 30" RCP draining Pond 104 will not back up water under outlet control conditions and
submerge the proposed orifice plate.
Comments for Detention Pond 105 Hydraulic Structures:
1. Weld County has been provided Sheet PD3 of PD4, and PD04 of PD04 — please use a consistent
formatting for sheet number,and provide sheets PD01 and PD02 if they are currently part of the Pond 105
design. It appears from the Title Sheet that PD01 and PD02 are specific to the design of Pond 104, and
should be clearly labeled as "PDOI of PD04" and "PD02 of PD04" if this is the case. Please update the
third sheet to read"PD03 of PD04"for consistency with the rest of the Construction Drawings.
2. The emergency spillway design between Pond 105 and Saint Vrain Creek must be supported with
engineering design calculations.
a. The spillway flow depth shall not exceed 6 inches,as per Weld County Storm Drainage Criteria.
b. All proposed riprap sizes must be supported with engineering design calculations consistent with
guidelines published in the USDCM.
c. The upstream and downstream termination (keys) of proposed riprap on the spillway structure
must be designed in accordance with UD&FCD standards published in the USDCM, Chapter 7
(2001), or FHWA standards published in HEC-11 (1989). It is critical to design an appropriate
riprap tie-in at Saint Vrain Creek to prevent scouring at the toe of the revetment, and eventual
failure of the spillway.
d. The tie-in shown on Sheet GR15 is not consistent with the tie-in detailed on Sheet PD3 of PD4.
Please correct the design, add calculations to the Final Drainage Report, and update all pertinent
sheets for accuracy and completeness.
3. The low tailwater riprap basin design must be supported with engineering design calculations consistent
with guidelines published in the USDCM,Chapter 8(2001).
4. Please support the proposed outlet pipe design configuration with hydraulic calculations consistent with
standards published the USDCM, Chapter 9 (2001). A complete set of hydraulic calculations must be
added to the Final Drainage Report. Hydraulic calculations must show that the proposed RCP draining
Pond 105 will not back up water under outlet control conditions and submerge the proposed sharp crested
weir structure.
a. Please provide a justification for and discussion of the operation of the proposed sharp crested
weir.
b. The sharp crested weir will not operate under weir flow conditions if the structure is submerged
by tailwater created by backwater from the outlet pipe. If the proposed 30"RCP is flowing under
outlet control conditions, this feature will not be as efficient as a typical weir, and the WSEL in
the pond will be higher than predicted by the equation Q=3.15*L*H'•s
c. The nomograph provided on Sheet PD03 of PD04 is labeled incorrectly. The design values do
not match those indicated on the rest of the sheet. Please update all hydraulic information on the
sheet to be internally consistent, and consistent with published information in the Final Drainage
Report.
Page 9of11
5. Please clarify and provide a rationale for the proposed slide gate at the front of the proposed outlet
structure. The proposed design appears to enable the pond to be drained at any time,and bypass the water
quality capture features of the detention pond.
Comments for Detention Pond at WCR 9.5 Hydraulic Structures:
1. An emergency spillway must be included as part of the detention pond design for WCR 9.5. The spillway
must have appropriate permanent erosion control measures supported with design calculations consistent
with guidelines published in the USDCM,Chapter 8.
2. Please support the proposed outlet pipe design configuration with hydraulic calculations consistent with
standards published in the USDCM, Chapter 9 (2001). A complete set of hydraulic calculations must be
added to the Appendix of the Final Drainage Report.
Underdrain Design Comments:
1. All bends must have two (2) cleanouts for proper maintenance of the underdrain system. Each cleanout
shall be oriented in the direction of the flowline to allow for periodic cleaning with pressurized water.
2. Cleanouts in the proposed underdrain system shall have a maximum spacing not to exceed 750 ft.
3. Proposed underdrain pipes shall have a minimum diameter of 8 inches.
4. The underdrain system shall not intercept any surface stormwater runoff.
November 17,2006 Response by Carroll&Lange to General Comments:
#7 —The EGL is reportedly 6-inches below the proposed finished grade. However, EGLs have not been plotted
on any of the pipe profiles. Please show the 100-year EGL on all proposed storm sewers pipes and culverts in the
subdivision.
November 17,2006 Response by Carroll&Lange to Specific Drainage Comments:
#15 —Carroll & Lange are strongly encouraged to contact Public Works staff and request clarification and
assistance when they cannot locate specific comments identified during the review process. It is not acceptable to
discount staff comments simply because the engineer(s)could not find where those comments apply.
#21 —Erosion protection calculations for proposed swales were not found in Appendix C, or in Exhibit C, or any
other part of the Final Drainage Report. Please provide this information to Public Works for a complete review.
#27(a) — For completeness of the report and review, please provide calculations for structural losses through
manholes and inlet boxes.
#27(b) — Since Carroll & Lange has proposed to lower the 100-year EGL below proposed grade, bolt-down lids
on manholes will no longer be necessary. However, venting will be required on all proposed storm sewers with
steep slopes, with supercritical flow, or with inlet-controlled flow conditions. Hydraulic conditions inside these
pipes will entrain air at hydraulic jumps and pressurize the inside of the manhole. Without venting, this
pressurization will lead to separation of the individual manhole riser sections, thus compromising the structural
integrity of the pipes, and causing leakage around all section gaskets. Weld County's recent conversations with
drainage engineers working for other jurisdictions confirms their experience with similar situations and they have
found that air-entrainment conditions will be worst during smaller storm events (5-year and less),and may present
Page 10 of 11 --
an expensive maintenance problem for the future residents of St. Vrain Lakes PUD.
#28 — Public Works does not accept this statement on the grounds of technical accuracy. Please refer to the
response#27(b) above,and the inlet control discussion under Specific Drainage Comments.
#36—Please refer to response#27(b)above.
#40 — The only way to prove the 7.5 cfs may turn the corner into the side-street is to provide a momentum
calculation of the flow around the specified corner. This comment applies to all areas where flow is assumed to
wrap around corners,especially where proposed flows entering the corner are supercritical.
#51 —Please add the specified comment to the text of the drainage report,and to the final plat for the subdivision.
November 17,2006 Response by Carroll& Lange to Erosion and Sediment Control
#3 —The D50 calculations for the riprap spillway were not found in Appendix D, or in Exhibit D,or any other part
of the Final Drainage Report. Please provide this information to Public Works for a complete review.
#4—The D50 calculations for culvert outlet protection were not found in Appendix C, or in Exhibit C,or any other
part of the Final Drainage Report. Please provide this information to Public Works for a complete review.
The applicant shall address and resolve the comments listed above. The review process will continue only
when all appropriate elements have been submitted. Any issues of concern must be resolved with the
Public Works Department prior to recording the final plats.
Nue 1 1 of 1 1
Hello