HomeMy WebLinkAbout20211152.tiffThornton Water Project
Weld County
Use by Special Review for
Pipeline — Domestic Water
Second Supplemental Permit
Application — Drainage Narrative
Water Tank Exception to
Stormwater Detention
Volume 4 of 4
lir Thornton
WATER PROJECT
Submitted to:
Weld County Department of Planning Services
1555 N. 17th Avenue
Greeley, CO 80631
Dated: January 29, 2021
Prepared By:
CH2M HILL.
9191 South Jamaica Street
CH2MHILL
� Englewood, CO 80112
es City of Thornton Thornton, CO 80229
li'
.JJVV Civic Center VI
Prepared For:
City of Thornton
9500 Civic Center Drive
DRAINAGE NARRATIVE
Thornton Water Project
Water Tank Exception to Stormwater
Detention
Prepared for
Weld County
on behalf of City of Thornton
Weld County Case Number USR18-0130
April 2020
CH2M Hill Engineers, Inc.
9191 South Jamaica Street
Englewood, CO 80112
Digitally
signed by
AVaughan
Date:
2021.01.26
17:42:04-07' OC
TWP USR SUPPLEMENT 2 SECTION 16 WATER TANK DRAINAGE NARRATIVE WT -1
THORNTON WATER PROJECT
DRAINAGE !NARRATIVE - WATER TANK EXCEPTION TO STORMWATER DETENTION
Existing and Proposed Improvements to the Property
This section addresses WCC Section -11-40 L i .
Thornton is proposing to locate and construct the Thornton Water Project (TWP), which is a
water delivery system that will convey high quality domestic water Thornton purchased in
the mid -1980's from the water Supply and Storage Company (WSSC) system, located in
Larimer County, to Thornton. The TWP includes an approximate `1 -million -gallon water tank
located south of the intersection of County Road 13 and County Road 92 on the east side of
County Road 13 (Section 19, Township 8 North, Range 67 West).
The existing condition of the water tank site is predominately undeveloped with native
vegetation but includes a portion of the unpaved County Road 13. The soils on the site are
hydrologic group B, as shown in the attached soil map from Natural Resources Conservation
Service (NRCS) (Attachment C). An existing water tank site is adjacent to the south side of the
site. The water tank site is a non -urbanizing area and there are no known previous drainage
problems with the property.
The proposed improvements on the property are an approximate 84 -foot diameter, 24 -foot
tall steel water tank encircled by 25 feet of gravel for access and parking.
The Applicable Exception
This section addresses WCC Section 8-11-40t2 and USE? Questionnaire, Public Works
Questions 3.a.
The water tank site is excepted from the stormwater detention requirements per WCC Section
8-11-40 L The exception that applies is:
Exception 6: "Non -Urbanizing areas where the total pre-existing and post development
impervious area produces stormwater runoff of less than, or equal to, five (5) cfs for the 1
hour, 100 -year, storm event. This exception shall be supported by calculations signed and
stamped by a Colorado Licensed Professional Engineer.
See Additional Pertinent Information, below, for documentation of the required analysis.
Where the Water Originates If It Mows onto the Property from an Off -Site
Source
This section addresses WCC Section 8-11- 0 13 and USE? Questionnaire, Public Works
Questions 3.b.
Surface stormwater runoff originates from the adjacent water tank site and undeveloped
land west of County Road 13 and the majority of that land is agricuLturaL use. Land drains
from southwest to northeast. Ditches on either side of the roadway may divert offsite flow to
the north and away from the water tank site but it is assumed that they do not capture all the
offsite flows as they are not reflected in the contour data.
TWP USR SUPPLEMENT 2 SECTION 16 WATER TANK DRAINAGE NARRATIVE WT -2
THORNTON WATER PROJECT
DRAINAGE NARRATIVE -WATER TANK EXCEPTION TO STORMWATER DETENTION
Where the Water Flows to as it Leaves the Property
This section addresses WCC Section 8-11-40 L4 and USR Questionnaire, Public Works
Questions 3.c.
Stormwater sheet flows from the water tank site property to the adjacent property north and
east of the site in both the existing and proposed condition toward an unnamed swale that
appears to be tributary to Black Hollow Creek.
The Direction of Flow Across the Property
This section addresses WCC Section 8-11-40 LS and US!? Questionnaire, Public Works
Questions 3.d.
The existing drainage patterns on the site are from southwest to northeast toward an
unnamed swale that appears to be tributary to Black Hollow Creek. The unnamed tributary
generally flows from northwest to southeast. Offsite drainage sheet flows through the site
from the adjacent water tank site and undeveloped area west of the County Road 13. In the
proposed condition the drainage pattern will remain the same. The water tank will be
constructed with an unpaved access road around the entire water tank. The grade will be
raised slightly to level the east side of the site and stormwater will drain overland from
southwest to northeast.
Previous Drainage Problems with the Property, if Any
This section addresses WCC Section 8-11-40 L6 and USR Questionnaire, Public Works
Questions 3.f
No previous drainage problems are known. There are no regulatory floodplains on or
adjacent to the site. The FEMA flood map for the vicinity is provided in Attachment B.
The Location of Any Irrigation Facilities Adjacent to or Near the Property
This section addresses WCC Section 8-11-40 1.7 and USR Questionnaire, Public Works
Questions 3.e.
There are no irrigation ditches on or adjacent to the water tank site. Irrigation ditches are
located approximately one third of a mile west of site, but the site does not drain toward
them. The water tank site and general flow patterns are shown on the Vicinity Map in
Attachment A. The site is part of a larger parcel leased to Anheuser Busch for land
application of treated effluent from its wellington brewery operation. That land application is
located southeast of the water tank site and the water tank site does not drain toward that
land.
Additional Pertinent Information
This section addresses WCC Section 8-11-40 L8 and US! Questionnaire, Public Works
Questions 3.a, supporting documentation.
TWP USR SUPPLEMENT 2 SECTION 16 WATER TANK DRAINAGE NARRATIVE WT -3
THORNTON WATER PROJECT
DRAINAGE !NARRATIVE - WATER TANK EXCEPTION TO STORMWATER DETENTION
I lydrology and HydraulicAnalysis
The existing and proposed conditions were analyzed using UD Rational version 1.02a (2005)
as required by Weld County to determine the stormwater runoff produced from the pre-
existing and post development impervious areas. Rainfall depths were taken from NOAA
Atlas 14 and are shown on Attachment D.
In the existing condition the impervious area is the unpaved County Road 13, which is 4,685
square feet (0.11 acres) and 40% impervious per the Urban Drainage and Flood Control
District Recommended Percentage imperviousness Values table (Attachment E). The
construction of the water tank and gravel area surrounding the water tank will increase the
imperviousness area to approximately 21,085 square feet (0.48 acres) that has a composite
imperviousness of 51,%. The runoff from the impervious areas during a 1 -hour, 100 -year
storm is 0.40 cfs in the existing condition and 2.04 cfs in the proposed conditions. All
imperviousness and rational calculations are provided in Attachment F.
Conclusion
The proposed water tank site meets the criteria for an exception to the requirement for
stormwater detention per Weld County Code Section 8-1 1-40 I on the basis of impervious
areas producing stormwater runoff of 5 cfs or Less as described above. Attachments have
been provided that show the location of the site, the floodplain and soils maps for the area,
and supporting calculations.
Attachments
Attachment A: Vicinity Map
Attachment B. FEMA Floodplain Map
Attachment C: NRCS Soils Map and Description
Attachment D: NOAA 14 Precipitation Data
Attachment E: UDFCD Recommended Percentage Imperviousness Values
Attachment F: Imperviousness and Rational Calculations
Attachment G: Tank Site Drainage Plan
TRIP USR SUPPLEMENT.2 SECTION 16 WATER TANK DRAINAGE NARRATIVE WT -4
Attachment
Vicinity Map
(Not to Scale)
Pid
z
d
Cobb
Lake
ri
a
0
tez
M1
Summ.at{
I
5446 ft
Tank Site
Flow direction on site
southwest to northeast,
overall valley slope
northwest to southeast
E iC R - 7"
a
z
I
County Road 92
County FloarL 90
733
T
N
National Flood Hazard Layer FIRMette
FEMA
Legere!
Attachment B
104`56'54.35'W
40'3918.33"N
0
250
500
T8N R68W S13
000690190F
F
12/19/2006
Not Primed
TeN R681,411/ 324
1,000
1,500
TBN R67W 18
M IN F LOOD., H AZAR.D
08123(O900 E
1/20/2016
Not Printed
TBN R67W S19
National Map: Orthoirna ery. Data refreshed April, 2019_
Feet 1:6,000
2,000
40038'51.03"N
SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT
SPECIAL FLOOD
HAZARD AREAS
Without Base Flood Elevation (BFE)
Zone A, 11, 499
With BFE or Depth Zone AL AO, AN, tIE, AR
Regulatory Floodway
OTHER AREAS OF
FLOOD HAZARD
t/i669 9 F 19 k'0 1.
OTHER AREAS
GENERAL
STRUCTURES
OTHER
FEATURES
MAP PANELS
NO SCREEN
II II III
0.2% Annual Chance Flood Hazard, Areas
of 1% annual chance flood with average
depth less than one foot or with drainage
areas of less than one square mile
Future Conditions 1% Annual
Chance Flood Hazard zone x
Area with Reduced Flood Risk due to
Levee. See Notes. zone x
Area with Flood Risk due to Leveezone o
Area of Minimal Flood Hazard zonex
Effective LOMRs
Area of Undetermined Flood Hazard Zone a
Channel, Culvert, or Storm Sewer
Levee, Dike, or Floodwall
20.2 Cross Sections with 1% Annual Chance
17.5 Water Surface Elevation
r4 Coastal Transect
Base Flood Elevation Line (BFE)
Limit of Study
Jurisdiction Boundary
--- --- Coastal Transect Baseline
Profile Baseline
Hydrographic Feature
Digital Data Available
No Digital Data Available
Unmapped
The pin displayed on the map is an approximate
point selected by the user and does not represent
an authoritative property location.
This map complies with FEMA's standards for the use of
digital flood maps if it is not void as described below.
The basemap shown complies with FEMA's basemap
accuracy standards
The flood hazard information is derived directly from the
authoritative NFHL web services provided by FEMA. This map
was exported on 2/25/2020 at 5:10:00 PM and does not
reflect changes or amendments subsequent to this date and
time. The NFHL and effective information may change or
become superseded by new data over time.
This map image is void if the one or more of the following map
elements do not appear basemap imagery, flood zone labels,
legend. scale bar, map creation date, community identifiers,
FIRM panel number, and FIRM effective date. Map images for
unmapped and unmodernized areas cannot be used for
regulatory purposes.
Attachment C
40° 39'9"N
40° 39'0" N
1
504540
I
a
504540
504580
Hydrologic Soil Group Larimer County Area, Colorado; and Weld County, Colorado, Northern Part
(Tank Site)
504620
564660
Soil Map may not be valid at this scale.
A
E&F580
5C1-700
504740
504660
504700
Map Scat: 1:1,890 if printed on A landscape (11" x 8.5") sheet.
Meters
0 25 50 100 150
Feet
0 50 100 200 300
Map projation: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WG584
504740
rt
504780
504780
504860
504900
504860
504900
1040 56' 29' VQ
504940
1
50&0
via
400 39' 9° N
40° 39' 0° N
USDA Natural Resources
a Conservation Service National Cooperative Soil Survey
Web Soil Survey
3/20/2020
Page 1of5
Attachment C
Hydrologic Soil Group Larimer County Area, Colorado; and Weld County, Colorado, Northern Part
(Tank Site)
MAP LEGEND
Area of Interest (AOl) O C
Area of Interest (AOl)
Soi is
Soil Rating Polygons
n A
I
A/D
B
B/D
C
CID
D
Not rated or not available
Soil Rating Lines
A
A/D
B
BID
r 0 C
,�• CID
D
Fr • Not rated or not available
Soil Rating Points
0
0
■
■
A
AID
B
B/D
0 CiD
D
0 Not rated or not available
Water Features
Streams and Canals
Transportation
. Rails
sarao
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
MAP INFORMATION
The soil surveys that comprise your AOlwere mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: Larimer County Area, Colorado
Survey Area Data: Version 14, Sep 13, 2019
Soil Survey Area: Weld County, Colorado, Northern Part
Survey Area Data: Version 14, Sep 13, 2019
Your area of interest (AOl) includes more than one soil survey
area. These survey areas may have been mapped at different
scales, with a different land use in mind, at different times, or at
different levels of detail. This may result in map unit symbols, soil
properties, and interpretations that do not completely agree
across soil survey area boundaries..
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Sep 20, 2015 —Oct
21, 2017
USDA Natural Resources Web Soil Survey
a Conservation Service National Cooperative Soil Survey
3/20/2020
Page 2 of 5
Attachment C
Hydrologic Soil Group Larimer County Area, Colorado; and Weld County, Colorado, Northern Part.
(Tank Site)
MAP LEGEND MAP INFORMATION
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor
shifting of map unit boundaries may be evident.
USDA Natural Resources Web Soil Survey
a Conservation Service National Cooperative Soil Survey
3/20/2020
Page 3 of 5
Attachment C
Hydrologic Soil Group—Larimer County Area, Colorado; and Weld County, Colorado, Northern Tank Site
Part
Hydrologic Soil Group
Map unit symbol
Map
unit name
Rating
Acres in AOI
Percent of AOI
8
Ascalon
to 5
sandy loam,
percent slopes
3
B
3.4
22.1%
78
Otero
sandy loam,
percent slopes
3 to 5
A
0.3
2.0%
80
Otero
loams,
slopes
-Nelson
3 to
sandy
25
percent
A
4.2
26.8%
108
Thedalund
loam,
percent slopes
3 to 9
C
1.7
11.1%
Subtotals for Soil Survey Area
9.6
62.0%
Totals for Area of Interest
15.5
100.0%
Map
unit symbol
Map unit name
Rating
Acres in AOI
Percent of AOI
4
Ascalon
loam,
slopes
fine sandy
0 to 6
percent
B
3.2
20.6%
65
Terry
sandy loam,
percent slopes
3 to
9
B
0.8
4.9%
67
Thedalund-Keota
3 to 9
percent
loams,
slopes
C
1.9
12.5%
Subtotals for Soil Survey Area
5.9
38.0%
Totals for Area of Interest
15.5
100.0%
USDA Natural Resources Web Soil Survey
r Conservation Service National Cooperative Soil Survey
3/20/2020
Page 4 of 5
Attachment C
Hydrologic Soil Group—Larimer County Area, Colorado; and Weld County, Colorado, Northern Tank Site
Part
Description
Hydrologic soil groups are based on estimates of runoff potential. Soils are
assigned to one of four groups according to the rate of water infiltration when the
soils are not protected by vegetation, are thoroughly wet, and receive
precipitation from long -duration storms.
The soils in the United States are assigned to four groups (A, B, C, and D) and
three dual classes (AID, BID, and CID). The groups are defined as follows:
Group A. Soils having a high infiltration rate (low runoff potential) when
thoroughly wet. These consist mainly of deep, well drained to excessively
drained sands or gravelly sands. These soils have a high rate of water
transmission.
Group B. Soils having a moderate infiltration rate when thoroughly wet. These
consist chiefly of moderately deep or deep, moderately well drained or well
drained soils that have moderately fine texture to moderately coarse texture.
These soils have a moderate rate of water transmission.
Group C. Soils having a slow infiltration rate when thoroughly wet. These consist
chiefly of soils having a layer that impedes the downward movement of water or
soils of moderately fine texture or fine texture. These soils have a slow rate of
water transmission.
Group D. Soils having a very slow infiltration rate (high runoff potential) when
thoroughly wet. These consist chiefly of clays that have a high shrink -swell
potential, soils that have a high water table, soils that have a claypan or clay
layer at or near the surface, and soils that are shallow over nearly impervious
material. These soils have a very slow rate of water transmission.
If a soil is assigned to a dual hydrologic group (AID, BID, or CID), the first letter is
for drained areas and the second is for undrained areas. Only the soils that in
their natural condition are in group D are assigned to dual classes.
Rating Options
Aggregation Method: Dominant Condition
Component Percent Cutoff: None Specified
Tie -break Rule: Higher
USDA Natural Resources Web Soil Survey
r Conservation Service National Cooperative Soil Survey
3/20/2020
Page 5 of 5
215/2020
Precipitation Frequency Data Server Attachment D
NOAA Atlas 14, Volume 8, Version 2
Location name: Fort Collins, Colorado, USA*
Latitude: 40.6515°, Longitude: -104.9428°
Elevation: 5336.35 ft**
* source: ESRI Maps
** source: USGS
POINT PRECIPITATION FREQUENCY ESTIMATES
Sanja Perica, Deborah Martin, Sandra Pavlovic, Ishani Roy, Michael St. Laurent, Carl Trypaluk, Dale
Unruh, Michael Yekta, Geoffery Bonnin
NOAA, National Weather Service, Silver Spring, Maryland
PF tabular PF graphical I Maps & aerials
PF tabular
PDS-based point
precipitation
frequency estimates
with
90% confidence
intervals
(in inches)1
i
Average recurrence interval (years)
Duration
7
L 2
5
I 10 I
-
25
50
100
200
500
1000
5 -min
0.245
(0.196-0.303)
0.293
(0.235-0.363)
0.389
(0.310-0.482)
0.484
(0.383-0.602)
0.637
(0.496-0.847)
0.774
(0.582-1.03)
0.927
(0.669-1.26)
1.10
(0.756-1.53)
1.35
(0.888-1.92)
1.56
(0.988-2.21)
10 -min
0.359
(0.287-0.444)
0.430
(0.343-0.532)
0.570
(0.453-0.706)
0.708
(0.560-0.8821
0.933
(0.726-1.24)
1.13
(0.852-1.51)
1.36
(0.979-1.84)
1.61
(1.11-2.23)
1.98
(1.30-2.81)
2.28
(1.45424)
15 -min
0.438
(0.350-0.541)
0.524
(0.419-0.648)
0.695
(0.553-0.861)
0.864
(0.683-1.08)
1.14
(0.886-1.51)
1.38
(1.04-1.84)
1.66
(1.19-2.25)
1.96
(1.35-2.72)
2.41
(1.59-3.42)
2.78
(1.76-3.95)
30 -min
0.588
(0.470-0.727)
0.702
(0.561-0.869)
0.929
(0.740.1.15)
1.16
(0.914-1.44)
1.52
(1.19-2.03)
1.85
(1.39-2.47)
2.22
(1.60-3.02)
2.64
(1.81-3.66)
3.24
(2.134.61)
3.75
(2.38-5.32)
60-i n
0.718
(0.574-0.887)
0.863
(0.689-1.07)
1.15
(0.918-1.43)
1.45 i
(1.14-1.80)
1.92
(1.50-2.56)
2.35
(1.77-3.13)
2.82 I
(2.04-3.84)
3.36
(2.31-4.67)
4.15
(2.73-5.90)
4.81
(3.05-6.83)
1 2 -hr
0.$ 7
(0.682-1.04)
1.38
(1.10-1.69)
1.73
(1.38-2.14)
2.32
(1.82-3.06)
2.84
(2.15-3.76)
3.43 l
(2.50-4.63)
4.09
(2.84-5.64)
5.06
(3.36-7.14)
5.88
(3.76-8.27)
3 -hr
0.919
(0.742-1.12)
'1.1'1
(0.8917-1.36)1
1.50
(1.2,1-1.84) j
1.89
(1...51.2.33) i
2.54
(2.00-3.34) I
3.12
(2.374.11) j
3.77
(2.76-5.07)
4.50
(3.14-6.18)
5.58
(3.73-7.84)
6.48
(4.17-9.09)
6 -hr
1.06
(0.859-1.28)
1.29
(1.05-1.56)
1.74
(1.41-2.11) I
2.19
(1.76-2.66)
2.90
(2.29-3.76)
3.52
(2.70-4.59)
4.22
(3.11-5.61)
5.00
(3.52-6.80)
6.13
(4.14-8.53)
7.07
(4.60-9.84)
12
-hr
1.27
(1.04-1.52)
1.52
(1.24-1.82)
1.99
(1.62-2.40)
2.45
(1.98-2.96)
3.17
(2.52-4.06) I
3.80
(2.92-4.89)
4.49
(3.33-5.91)
5.26
(3.73-7.08)
6.37
(4.34-8.79_)
7.29
(4.80-10..1)
24 -hr,
1.53
(1.26-1.82)
1.79
(1.47-2.12) i
2.27
(1.85-2.70)
2.73
(2.22-3.27)
3.46
(2.77-4.38)
4.10
(3.18-5.23)
4.81
(3.60-6.27)
5.59
(4.01-7.47)
6.74
(4.63-9.21)
7.68
(5.10-10.5)
2 -day
7.74
(1.44-2.05)
2.07
(1.71-2.43)
2.65 iF
(2.18-3.13)
3.17
(2.60-3.78)
3.96
I 3.16-4.91) I
4.61
(3.59-5.79)
5.31
(3.99-6.82)
6.07
(4.37-7.98)
7.13
(4.94-9.63)
7.99
(5.37-10.9)
3 -day
1.89
(1.57-2.21)
2.24
(1.85-2.62_)
2.84
(2.35-3.34)
3.38
(2.78-3.99)
4.19
(3.35-5.16)
4.86
(3.79-6.05)
5.56
(4.19-7.09)
6.32
(4.57-8.27)
7.39
(5.14-9.92)
8.24
(5.57-11.2)
4 -day
2.01
(1.67-2.35)
2.37
(1.97-2.77)
3.00
(2.48-3.51)
3.56
(2.93-4.18) I
4.38
(3.51-5.37)
5.06
(3.96-6.27)
5.78
(4.37.7.34)
6.55 !
(4.75-8.53)
7.62
(5.33-10.2)
8.49
(5.76-11.5)
I
I
7 -day
2.29
(1.92-2.65)
2.72
(2.27-3.15)
I
4.97
(3.99-6.01)
I
6.44
(4.89-8.09)
8.30
(5.84-11.0)
9.15
(6.27-12.3)
,10 -dad
L 2.55
(2..13-2.93)
3.02
(2.53-3.48)
3.81
(3.18-4.41)
4.49
,
5.43
(4.38-6.53)
6.18
(4.87-7.53)
6.95
(5.30-8.67)
7.74
(5.68-9.93)
8.82
(6.23-11.6)
9.65
(6.65-12.9)
20 -day
3.31
(2.79-3.78)
3.85
(3.24-4.40) V
4.74
(3.98-5.43)
5.47
(4..57-6.30)
6.49
(5.25-7.70)
V
8.07
(6.19-9.95)
8.87
(6.55-11.2)
10.7
(7.48-14.2)
ibTn.50
5.46
(4.60-6.22)
6.25
(5.24-7.15)
7.33
(5.95-8.62)
I
8.96
(6.91-11.0)
10.8
(7.77-14.0)
11.6
(8.16-154)
45 -day
4.60
(3.90-5.19)
5.28
(4.48-5.97)
6.37
(5.39-7.22)
7.25
(6.10-8.26)ai
8.43
(6.86-9.85)
9.32
(7.44-11.1)
10.2
(7.88-12.4)
11.0
(8.22-13.8)
12.1
(8.72-15.6)
12.9
(9.10-17.0)
5.14
(4.37-5.78)
5.92
(5.03-6.66)
7.16
(6.07-8.07)
8.14
(6.87-9.23)
10.4
(8.31-12.33)
11.3
(8.78-13.7)
12.2
(9.12-15.2)
13.3
(9.61-17.0)
14.1
(9.99-18.5)
_
1 Precipitation frequency (PF) estimates in this table are based on frequency analysis of partial duration series (PDS).
Numbers in parenthesis are PF estimates at lower and upper bounds of the 90% confidence interval. The probability that precipitation frequency estimates
(for a given duration and average recurrence interval) will be greater than the upper bound (or less than the lower bound) is 5%. Estimates at upper
bounds are not checked against probable maximum precipitation (PMP) estimates and may be higher than currently valid PMP values.
Please refer to NOAA Atlas 14 document for more information.
Back to Top
PF graphical
https://hdscanws.noaa.govihdscipfds/pfds_printpage.html?lat=40.65158don=-104.942881data=depth&units=english&series=pds 1/4
215/2020 Precipitation Frequency Data Server
Attachment D
12
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PDS-based depth -duration -frequency (DDF) curves
Latitude: 40.6515°, Longitude: -104..94280
.s,
E
in
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C
5 10
25 50 100 200
NOAA Atlas 14, Volume 6, Version 2
C
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Duration
(13 rC rw ra
IV V ".",
deJrJ 4 h4. 6 6 6 in O-
H 61 f Tt D
500 1000
Average reLurrertce interval (years)
Created (GMT): Wed Feb 5 i&04:45 2020
Back to Top
Maps & aerials
Small scale terrain
tua.:P rage recurrence
interval
(years)
1
2
5
10
25
50
100
200
500
1000
DJurationn.
10Lmin
15 -min
30 -min
6 0-imn
2-trr
3 -hr
6 -ht
12. -hr
24 -hr
t -day.
3 -ay
4 -day
7 -day
10 -day.
20 -day
30 -day
45 -day
60 -day,
https://hdsc nws.noaa.govlhdsclpfds/pfds_printpage.html?Iat=40.6515&Ian=-104.9428&data=depth&units=english&series=pds 2/4
215/2020
Precipitation Frequency Data Server Attachment D
I
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Large scale aerial
https://hdscsnws.noaa.govlhdsclpfds/pfds_printpage.html?Iat=40.6515&Ian=-104.9428&data=depth&units=english&series=pds 3/4
2/5/2020 Precipitation. Frequency Data Server
Attachment D
Back to Top
US Department of Commerce
National Oceanic and Atmospheric Administration
National Weather Service
National Water Center
1325 East West Highway
Silver Spring, MD 20910
Questions?: HDSC.Questionsnoaa.gov
Disclaimer
https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_printpage.html?lat-40.0515&lon=-104.942$&data-depth&units=english&series=pds 4/4
Attachment E
Table 6-3►. Recommended percentage imperviousness values
Land Use or
Surface Characteristics
Percentage Imperviousness
(%)
Business:
Downtown Areas
95
Suburban Areas
75
Residential
lots (lot area only):
Single-family
2.5 acres or larger
12
0.75 —
2.5 acres
20
0.25 —
0.75 acres
30
0.25 acres or less
45
Apartments
75
Industrial:
Light areas
80
Heavy areas
90
Parks, cemeteries
10
Playgrounds
25
Schools
55
Railroad yard areas
50
Undeveloped Areas:
Historic flow
analysis
2
Greenbelts, agricultural
2
Off -site flow analysis (when land use not
defined)
45
Streets:
Paved
100
Gravel (packed)
40
Drive and walks
90
Roofs
90
Lawns, sandy soil
2
Lawns, clayey
soil
2
6-8 Urban Drainage and Flood Control District
Urban Storm Drainage Criteria Manual Volume 1
August 2018
Attachment F
Impervious Areas and Runoff Coefficients at the Tank Site
April 27, 2020
Area (square feet)
Type
Unpaved
Road
Gravel
Roof
Total
squre
Area
feet
Total
acres
Area
Composite
%impervious
%Impervious
40
40
90
Existing
4685
0
0
4685
0.11
40.0
Proposed
,
15273
0
5542
20815
0.48
53.3
Runoff
Soil
Coefficients
Group
B
% of Total
Existing
Impervious
Proposed
Area
40%
Unpaved
Impervious
Road
2-yr
Syr
100-yr
100%
73%
0.23
0.30
0.50
40%
Impervious
Gravel
2-yr
Syr
100-yr
0%
0%
0.23
0.30
0.50
90% Impervious
Roof
2-yr
5yr
100-yr
0%
27%
0.71
0.73
0.81
Compostie
Runoff
Coefficient
2 -year
0.23
0.36
5 -year
0.30
0.41
100
-year
0.50
0.58
Attachment F
Area -Weighting for Runoff Coefficient Calculation
Project Title:
Catchment ID:
TWP Tank Site
Impervious Areas - 100 yr
Illustration
Instructions: For each catchment subarea, enter values for A and C.
Subarea
Area
Runoff
Product
ID
acres
Coeff.
A
C*
1 CA
input
input
input
output
Exiting
0.11
0.50
0.06
Sum:
0.11
Sum:
0.06
Area -Weighted Runoff Coefficient (sum CA/sum A) =
0.50
LEGEND:
Flow Direction
Cep lent
Boundary
*See sheet "Design Info" for inperviousness-based runoff coefficient values.
2020.04.27_TankRational v1.02a, Weighted C_Existing 412712020, 1:22 PM
Attachment F
CALCULATION OF A PEAK RUNOFF USING RATIONAL METHOD
Project Title:
Catchment ID:
TWP Tank Site
Impervious Areas - 100 yr
I. Catchment Hydrologic Data
Catchment. ID =
Area =
Percent Imperviousness =
NRCS Soil Type =
Existing
0.11
40.00
B
II. Rainfall Information
Design Storm Return Period, Tr =
Cl =
C2=
C3=
P1=
Acres
�!4
A, B, C,orD
I (inch/hr) = Cl * P1 /(C2 + Td}"C3
100
28.50
10.00
0.786
2.82
years (input return period for design storm)
(input the value of Cl)
(input the value of C2)
(input the value of C3)
inches (input one -hr precipitation --see Sheet "Design Info")
III. Analysis of Flow Time (Time of Concentration) for a Catchment
Runoff Coefficient, C =
Overide Runoff Coefficient, C =
5-yr. Runoff Coefficient, C-5 =
Overide 5-yr. Runoff Coefficient, C =
0.50
0.30
(enter an overide C value if desired, or leave blank to accept calculated C.)
(enter an overide C-5 value if desired, or leave blank to accept calculated C-5.)
Illustration
LEGEND
O Brig
Flew Direction
Catchment
Boundary
NRCS Land
Type
Conveyance
Heavy
Meadow
2.5
Tillage/
Field
5
Short
Pasture/
Lawns
7
Nearly
Bare
Ground
10
Grassed
Swales/
Waterways
15
Paved Areas &
Shallow Paved Swales
(Sheet Flow)
20
Calculations:
Reach
ID
Overland
Slope
S
ftfft
input
Length
L
ft
input
5-yr
Runoff
Coeff
C-5
output
NRCS
Convey-
ance
input
Flow
Velocity
V
fps
output
Flow
Time
Tf
minutes
output
0.0430 200 0.30
N/A
0.26
12.67
1
2
3
4
Sum 200
IV. Peak Runoff Prediction
Rainfall Intensity at Computed Tc, I =
Rainfall Intensity at Regional Tc, I =
Rainfall Intensity at User -Defined Tc, I =
6.91
7.31
7.31
inch/hr
inch/hr
inch/hr
Computed Tc =
Regional Tc =
User -Entered Tc =
Peak Flowrate, Op =
Peak Flowrate, Op =
Peak Flowrate, Op =
12.67
0.38
0.40
0.40
cfs
cfs
cfs
2020.04.27_TankRational vi .02a, Tc and PeakO_Existing 4/27/2020, 1:23 PM
Attachment F
Area -Weighting for Runoff Coefficient Calculation
Project Title:
Catchment ID:
TWP Tank Site
Impervious Areas - 100 yr
Illustration
Instructions: For each catchment subarea, enter values for A and C.
Subarea
Area
Runoff
Product
ID
acres
Coeff.
A
C*
1 CA
input
input
input
output
Proposed
0.35
0.50
0.18
Proposed
0.13
0.81
0.11
Sum:
0.48
Sum:
0.28
Area -Weighted Runoff Coefficient (sum CA/sum A) =
0.58
LEGEND:
Flow Direction
Ce tikm. lent
Boundary
*See sheet "Design Info" for inperviousness-based runoff coefficient values.
2020.04.27_TankRational v1.02a, Weighted C_Proposed 412712020, 1:23 PM
Attachment F
CALCULATION OF A PEAK RUNOFF USING RATIONAL METHOD
Project Title:
Catchment ID:
TWP Tank Site
Impervious Areas - 100 yr
I. Catchment Hydrologic Data
Catchment. ID =
Area =
Percent Imperviousness =
NRCS Soil Type =
Proposed
0.48
53.10
B
II. Rainfall Information
Design Storm Return Period, Tr =
Cl =
C2=
C3=
P1=
Acres
4�Q
A, B, C,orD
I (inch/hr) = Cl * P1 /(C2 + Td}"C3
100
28.50
10.00
0.786
2.82
years (input return period for design storm)
(input the value of Cl)
(input the value of C2)
(input the value of C3)
inches (input one -hr precipitation --see Sheet "Design Info")
III. Analysis of Flow Time (Time of Concentration) for a Catchment
Runoff Coefficient, C =
Overide Runoff Coefficient, C =
5-yr. Runoff Coefficient, C-5 =
Overide 5-yr. Runoff Coefficient, C =
0.58
0.41
(enter an overide C value if desired, or leave blank to accept calculated C.)
(enter an overide C-5 value if desired, or leave blank to accept calculated C-5.)
Illustration
LEGEND
O Brig
Flew Direction
Catchment
Boundary
NRCS Land
Type
Conveyance
Heavy
Meadow
2.5
Tillage/
Field
5
Short
Pasture/
Lawns
7
Nearly
Bare
Ground
10
Grassed
Swales/
Waterways
15
Paved Areas &
Shallow Paved Swales
(Sheet Flow)
20
Calculations:
Reach
ID
Overland
Slope
S
ft/ft
input
Length
L
ft
input
5-yr
Runoff
Coeff
C-5
output
NRCS
Convey-
ance
input
Flow
Velocity
V
fps
output
Flow
Time
Tf
minutes
output
0.0130 200 0.41
N/A
0.21
16A6
1
2
3
4
Sum 200
IV. Peak Runoff Prediction
Rainfall Intensity at Computed Tc, I =
Rainfall Intensity at Regional Tc, I =
Rainfall Intensity at User -Defined Tc, I =
6.18
7.31
7.31
inch/hr
inch/hr
inch/hr
Computed Tc =
Regional Tc =
User -Entered Tc =
Peak Flowrate, Op =
Peak Flowrate, Op =
Peak Flowrate, Op =
16
6
1.72
2.04
2.04
cfs
cfs
cfs
2020.04.27_TankRational v1.02a, Tc and PeakO_Proposed 4/27/2020, 1:23 PM
1
2
4
6
A
B
C
D
WATER TANK PROPERTY
EXISTING C0NDITIGN:
0.11 ACRES OF IMPERVIOUS ROADWAY
4C1 Q100=0.4 CFS
PROPOSED CONDITIONS:
0.43 ACRES OF IMPERVIOUS ROADWAYS
AND STORAGE TANK ROOF, 53%; O100=2.04 CFS
GRAVEL ROADWAY
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FILENAME: SP-C-2200.dgn
PLOT DATE: 2020\04\28
PLOT TIME: 9:33:05 AM
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