HomeMy WebLinkAbout20042509.tiff December 2003
PRELIMINARY
DRAINAGE STUDY
FOR
_ DALMAR ESTATES
MINOR SUBDIVISION
PREPARED FOR
Dallas and Marjorie Schneider
520 Weld County Road 18
Longmont,Colorado 80504
(303)776-7916
PREPARED BY:
Pickett Engineering,Inc
808 8th Street
Greeley,Colorado 80631
(970)356-6362
2004-2509
ENGINEER'S CERTIFICATION
CHANGE OF ZONE
This Preliminary Drainage Report for DalMar SUBMITTAL FOR PUD Weld County,
Colorado, has been prepared under my direct supervision upon request of the property
owners, expressly for their use.
R. Clayton Harrison Date
Colorado Registered P.E. #35620
•
PRELIMINARY DRAINAGE REPORT
CHANGE OF ZONE
DALMAR ESTATES 1SUBMITT
WELD COUNTI, ULU AHO FOR PUD
1. General Legal Description
DalMar Estates Minor Subdivision will be located on the Northeast Quarter of Section 30,
Township 2 North, Range 68 West of the 6t' Principal Meridian, Weld County, Colorado.
2. General Location with Respect to Public/Private Roads
The proposed subdivision is bordered on the north, east, and west by agricultural and
residential property. The southern property line is bordered primarily by agriculture, with
the exception of approximately 330 feet bordered by WCR 16'/z.
3. Names and Descriptions of the Surrounding Developments (One-half Mile)
Helen Bryant lives south of the proposed subdivision. The land type is currently
agricultural; there are oil and gas facilities located on this property.
Richard and Christine Foster live to the north. The land type is residential, with one
residential building and one outbuilding currently on the property.
Cindy Hewitt has two properties to the northwest of the proposed subdivision. Both
properties are currently residential—one has no buildings and the other has a residential
building and an outbuilding.
Dallas and Marjorie Schneider own the properties to the west and to the southwest of the
proposed subdivision. Both land types are agricultural and currently, each has one
residential building.
The William's family farm is east and south of the proposed subdivision. The land type is
agricultural and there are currently a few outbuildings on the property. They are currently
looking into designing a residential development on their property.
Most of the properties surrounding the DalMar subdivision are residential properties. The
agricultural lands to the east and south of the property are currently being re-zoned for
residential property. Changing the land from agricultural to residential will be compatible
with the surrounding areas. In addition, the owners have contacted and consulted with all
of the property owners about the development of the minor subdivision.
4. General Description of Proposed Subdivision Property
a. Area
The property is approximately 55 acres in size. Thirty acres will be used for residential
lots, twenty for open space, and five for a road right-of-way. There are nine lots
averaging 3.3 acres in area; the largest lot is 4.1 acres and the smallest is 2.6 acres.
Pickett Engineering, Inc. DalMar Estates Minor Subdivision
— Preliminary Drainage Report December 3,2003
Page 1
�-. b. Ground Cover
Currently the ground is vegetated with weeds and grasses.
c. General Topography
The majority of the property is sloped to the southeast. The high point of the site is
approximately at elevation 4990 at the northwest corner, falling to the southeast
property corner that has an elevation near 4930. Surface drainage follows this path and
leaves the site along the eastern and southeastern property lines. The existing
topography of the site is shown on Map C-2.1.
d. General Soil Conditions
Aquolls and Aquepts, Gravelly Substratum: This soil is located in the southwest corner
of the site. It encompasses a small portion of the site and is located in Outlot 3. No
lots will be platted on this soil type.
Aquolls and Aquepts, Flooded: This soil is located in the southeast corner of the site.
Located in Outlot 2, it encompasses a small portion of the site and contains the tank
batteries. No lots will be platted on this soil type.
Nunn Loam, I% to 3% Slopes: This soil is located along both sides of the ditch. The
back edges of Lots 1 and 2 of Blocks 1, and Lots 5, 6, and 7 of Block 2, will be located
,.� on this soil type.
Otero Sandy Loam, 5% to 9% Slopes: The majority of the property contains this soil
type. All nine lots will be platted on this soil type.
e. Irrigation Ditches or Laterals
The Gooding, Dailey & Plumb Ditch flows north along the southern portion of the
property.
f. Drainage Ways
The Gooding, Dailey & Plumb Ditch flows along the southern portion of the proposed
site with the majority of the runoff from the site currently draining into the ditch.
5. Description of the Drainage Basin and Sub-Basins
a. Any Major Drainage Way Planning Studies
N/A
Pickett Engineering, Inc. DalMar Estates Minor Subdivision
Preliminary Drainage Report December 3,2003
Page 2
b. Major Drainage Characteristics
The majority of the stormwater discharge from this site, in its existing condition, flows
into the Gooding, Dailey & Plumb Ditch, and overtops the banks, continuing southeast.
The remainder of the discharge from the site south of the ditch enters the County Road
16'1 right-of-way or flows onto the adjoining properties to the east.
1. Existing Major Basins
The site is currently located in two major drainage basins. The first basin, B-1,
encompasses approximately 11 acres of the southern portion of the site and
approximately 60 acres west of the site. This basin drains southeast to the
Gooding, Dailey& Plumb Ditch or to WCR 16'/2.
The second basin (B-2) encompasses approximately 43 acres of the northern
portion of the site as well as approximately 38 acres northwest of the site. This
basin also drains southeast to the ditch. Please refer to Page 8 and the attached
— calculations.
2. Existing Sub-Basins
There are three sub-basins associated with the site. The first sub-basin (SB-1) is
_ located to the west of the property. The flows from this basin will continue in the
historical pattern through Outlot 3.
Sub-basin 3 (SB-3) is also located to the west of the site. The majority of the flow
from this basin will either flow within a roadside ditch west of DalMar Road or
flow south along the west property line to WCR 16%2. In either case, the flow will
discharge unattenuated into the north roadside ditch.
The last sub-basin (SB-2) is located west and north of the site. The stormwater
from this basin will travel along the historic path and enter the site at the rear of
Lots 3 and 4. The area of inundation during the 100-year event is shown on the
Developed Drainage Basin Map.
Please refer to the Historic and Developed Drainage maps, located in the Appendix.
c. Nearby Irrigation Ditches/Laterals that will be Affected
With the exception of Basins SB-2, P-1, and P-3, the historic patterns will be
maintained.
The flows from these basins are not designed to flow into the irrigation ditch, but rather
to Outlot 1, a grassed field, or to Weld County Road 16%2. Basins P-2, P-5, and P-4
will follow the historic drainage patterns and discharge into the Gooding, Dailey &
Plumb Ditch.
Pickett Engineering, Inc. DalMar Estates Minor Subdivision
Preliminary Drainage Report December 3,2003
Page 3
.-� d. The Historic Drainage Flow Pattern of Property
Currently the property drains the Gooding, Dailey & Plumb Ditch and overtops the
banks, continuing south.
e. Off-Site Drainage Flow Patterns and Impact on Proposed Subdivision
Currently, two sub-basins impact the proposed subdivision. The first sub-basin (SB-2)
is located to the north of the site and drains approximately 0.5 cfs per 5-year storm and
21 cfs per 100-year into an existing natural channel through Lot 3. This water will then
be intercepted by DalMar Road's west roadside ditch. This flow—along with other
flows—will be collected in two 24"pipes and conveyed under DalMar Road. A flume
over the Gooding, Dailey & Plumb Ditch will convey the stormwater to Outlot 1.
The second sub-basin (SB-3) is located to the west of the site and drains approximately
0.2 cfs of water during the 5-year storm and 9 cfs during the 100-year storm into the
west roadside ditch of DalMar Road. This water, along with developed flows from
Basin P-3, is then routed south of the Gooding, Dailey & Plumb Ditch to the roadside
ditch along WCR 16'/2.
6. Drainage Facility Design Concepts and Details for the Proposed Subdivision
a. Compliance with Off-Site Runoff Considerations
The majority of the runoff from the surrounding site should not greatly impact the
subdivision. A concern regarding the storm drainage is the water flowing into the
subdivision from Sub-basin 2 (SB-2). The area of inundation through Lot 3 has been
identified on the Developed Drainage Map and the building envelope will be located
outside this area.
The additional drainage created by the subdivision should be minimal. The developed
flow from the proposed subdivision should not significantly impact the surrounding
land. According to the Weld County Public Works Department, if a site has a percent
impervious less than 10, on-site stormwater detention is not necessary. For each lot,
the proposed improvements include a house, a gravel driveway from the road to the
garage, and the options for an outbuilding, sidewalks, and a patio. These
improvements will slightly increase the stormwater runoff from the site, but will not
negatively impact the ditch or adjacent property owners.
b. Anticipated and Proposed Drainage Patterns
As previously mentioned, the unattenuated, off-site stormwater from Basin SB-2 will
travel along the same historic channel to DalMar Road's west roadside ditch. The
developed flows from Basin P-1 will also be collected in the same roadside ditch. At
the southeast corner of Lot 2, the off-site and developed flows merge, and two 24"
CMP will direct the flows under DalMar Road to 6' wide concrete flume. The flume
will convey the stormwater over the Gooding, Dailey & Plumb Ditch. Stormwater will
then flow southeast through Outlot 1 over a grassed field. The grassed field will have a
Pickett Engineering,Inc. DalMar Estates Minor Subdivision
Preliminary Drainage Report December 3,2003
Page 4
slope less than 1%, allowing the velocity of flow to spread out and slow, providing
water quality. From the grassed field, the stormwater will be discharged off-site,
unattenuated, to the adjoining property.
The storm drainage flows from Basin P-2, P-4, and P-5 will flow to the Gooding,
Dailey& Plumb Ditch.
The storm drainage flows from SB-3 and P-3 will be collected in the west roadside
ditch of DalMar Road, directing the flow south of the Gooding, Dailey & Plumb Ditch
to the north roadside ditch in WCR 161/4, where it will be met by the flows from SB-1.
c. Contents of Tables, Charts, Figures, Plates, or Drawings in Report
1. Appendix A
Historic Drainage Basin Map
Developed Drainage Basin Map
2. Appendix B
Table 1: Weighted "C" Coefficient Calculations
Table 2: Basin Characteristics
Table 3: Time of Concentration
Table 4: Storm Drainage Runoff Calculations
Table 5: 5-year and 100-year Sub-basin Routing
3. Appendix C
Technical information used in the support of the drainage facility design concept.
d. Presentation of Proposed and Existing Hydrologic Conditions
Please see enclosed documents.
e. Approach to Accommodate Drainage Facilities
The two, 24"-diameter culverts and the 6'-wide flume were designed for the 100-year
storm event. The 15" culvert under DalMar Road, near the intersection with WCR
161/4, is the minimum size given the available cover. No improvements are proposed
for off-site drainage facilities.
f. Maintenance
The swales, flume, and storm pipes will be maintained by the homeowners association.
r
Pickett Engineering, Inc. DalMar Estates Minor Subdivision
Preliminary Drainage Report December 3,2003
Page 5
7. Technical Information
All technical information used in the support of the drainage facility designed concept is
referenced and in the Appendix. The Rational Method was used to obtain the values for
the flows of each basin.
8. Map Showing General Drainage Patterns
Enclosed
9. Drainage Plan Map
Enclosed
Pickett Engineering,Inc. DalMar Estates Minor Subdivision
Preliminary Drainage Report December 3,2003
Page 6
Appendix A
Historic Drainage Basin Map
Developed Drainage Basin Map
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• N �\ / I / N1\' ae•mP / I BASIN CONTRIBUTING 5-YR HISTORIC 100-YR HISTORIC
• \ \ •• / / / D ---' POINT AREA RUNOFF RUNOFF
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(ACRESI (CFS) (CFS)
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Appendix B
Table 1: Weighted "C" Coefficient Calculations
Table 2: Basin Characteristics
Table 3: Time of Concentration
Table 4: Storm Drainage Runoff Calculations
Table 5: Five-year and 100-year Sub-basin Routing
—
DALMAR ESTATES 12/1/2003
Weld County, CO PEI#01-034
-
,-. TABLE 1: WEIGHTED "C" COEFFICIENT CALCULATIONS
HISTORICAL CONDITIONS
-
NRCS Area % Impervious Runoff Coefficients
Land Use I Soil Group (acres) 2-Year 5-Year 10-Year 100-Year
Grass A 33.067 2 0.00 0.01 0.07 0.22
PROPOSED CONDITIONS
- BASINS P-1, P-2, P-4, & P-5
The percent impervious calculation is based on the smallest sized lot and half of the roadway frontage.
Lot Area 2.560 ac NRCS Runoff Coefficients
Soil Group % Imperv. 2-Year 5-Year 10-Year 100-Year
_ Avg. House Size 2500 sf 0.057 ac A 90 0.69 0.71 0.73 0.79
Roadway Frontage 6000 sf 0.138 ac A 40 0.19 0.25 0.30 0.41
Gravel Driveway 3600 sf 0.083 ac A 40 0.19 0.25 0.30 0.41
Out Buildings 2500 sf 0.057 ac A 90 0.69 0.71 0.73 0.79
- Sidewalks/Patio 1000 sf 0.023 ac A 90 0.69 0.71 0.73 0.79
Agricultural 95914 sf 2.202 ac A 2 0.00 0.01 0.07 0.22
Weighted%Imperviousness 10
NRCS % Imperv. 2-Year 5-Year 10-Year 100-Year
Land Use I Soil Group
Large Lot Residential A 10 0.05 0.07 0.13 0.27
-
BASIN P-3
NRCS Area
Land Use Soil Group (acres) % Imperv. 2-Year 5-Year 10-Year 100-Year
Gravel Road A 0.418 40 0.19 0.25 0.30 0.41
— Undeveloped A 0.769 2 0.00 0.01 0.07 0.22
Total Area 1.187 acres
—
Percent Impervious 15
2-Year 5-Year 10-Year 100-Year
Composite Runoff, C 0.07 0.09 0.15 0.29
-
I CALCS v3.XLS Pickett Engineering, Inc. 11
I I I I I I I I I I I I I I I I I))
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DALMAR ES i TES
Weld County.CO 12/1/2003
PEI#01-034
TABLE 2 : BASIN CHARACTERISTICS
Basin Runoff Coefficient Overland Flow Gutter Flow
1 2 3 4 5 6 Weighted Total
Number Area Description C2 C5 C10 C100 L1 S1 L2 S2 V2 L2 S2 V2 L2 S2 V2 L2 S2 V2 L2 S2 V2 Slope Length
(acres) (ft) (%) (ft) (%) Ws) Oft (%) (f/s) (ft) (%) (f/s) (ft) (%) (Us) (II) (%) WOExisting Condition
B-1 71.839 Tillage/Field 0.00 0.01 0.07 0.22 200 2.5 460 1.7 0.7 1753 3.6 0.9 240 1.6 0.6 0 0.0 0.0 0 0.0 0.0 2.9 2653
B-2 81.408 Tillage/Field 0.00 0.01 0.07 0.22 200 1.0 533 3.8 1.0 1360 3.7 1.0 830 1.0 0.5 0 0.0 0.0 0 0.0 0.0 2.5 2923
SB-1 45.164 Tillage/Field 0.00 0.01 0.07 0.22 200 2.5 460 1.7 0.7 638 3.6 0.9 0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 2.6 1298
S8-2 38.348 Tillage/Field 0.00 0.01 0.07 0.22 200 1.0 533 3.8 1.0 1360 3.7 1.0 220 1.0 0.5 0 0.0 0.0 0 0.0 0.0 3.0 2313
SB-3 14.711 Tillage/Field 0.00 0.01 0.07 0.22 200 0.8 653 4.8 1.1 580 3.7 1.0 584 2.6 0.8 0 0.0 0.0 0 0.0 0.0 3.2 2017
Proposed Conditions
P-1 16.083 Tillage/Field 0.05 0.07 0.13 0.27 200 8.1 58 8.1 2.8 788 3.0 0.9 198 3.5 1.9 0 0.0 0.0 0 0.0 0.0 3.9 1244
P-2 9.980 Tillage/Field 0.05 0.07 0.13 0.27 200 6.8 620 6.8 2.6 0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 6.8 820
P-3 1.871 Gravel Road 0.07 0.09 0.15 0.29 200 7.8 58 7.8 2.8 250 0.7 0.8 550 6.0 2.4 630 1.4 1.2 0 0.0 0.0 2.9 1688
P-4 5.133 Tillage/Field 0.05 0.07 0.13 0.27 105 3.8 260 10.0 3.2 0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 7.7 365
P-5 4.012 Tillage/Field 0.05 0.07 0.13 0.27 130 3.8 150 10.0 3.2 0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 6.6 280
N I CALCS v3.%LS Pickett Engineering.Inc.
I I I I I I I I I I I I I I I I I 1 I
DALMAR ES fATES 12/1/2003
Weld County,CO PEI#01-034
TABLE 3 : TIME OF CONCENTRATION CALCULATION
INIT. INIT. INIT TM 1 TRVL 2 TRVL 3 TRVL 4 TRVL 5 TRVL 6 Tc= Total Tc= TOTAL
BASIN L1 S1 C5 Ti L2 V T2 L2 V T2 L2 V T2 L2 V T2 L2 V T2 Sum Ti L 10+(L/180) Tc
(ft) (%) (min) (ft) (Vs) (min) (ft) (f/s) (min) (ft) (f/s) (min) (ft) (f/s) Amin) (0) (f/s) (min) (min) (feet) (min) (min)
Existing Condition
B-1 200 2.5 0.01 20.5 460 0.7 11.8 1753 0.9 30.8 240 0.6 6.3 0 0.0 0.0 0.0 0.0 69 2653
B-2 200 1.0 0.01 27.7 533 1.0 9.1 1360 1.0 23.6 830 0.5 27/ 0 0.0 0.0 0.0 0.0 88 2923
SB-1 200 2.5 0.01 20.5 460 0.7 11.8 638 0.9 11.2 0 0.0 0.0 0 0.0 0.0 0.0 0.0 43 1298
S8-2 200 1.0 0.01 27.7 533 1.0 9.1 1360 1.0 ' 23.6 220 0.5 7.3 0 0.0 0.0 0.0 0.0 68 2313
SB-3 200 0.8 0.01 29.9 653 1.1 9.9 580 1.0 10.1 584 0.8 12.1 0 0.0 0.0 0.0 0.0 62 2017
Proposed Conditions
P-1 200 8.1 0.07 13.1 58 2.8 0.3 788 0.9 15.2 198 1.9 1.8 0 0.0 0.0 0.0 0.0 30 1244 <15%Imperv,Tc=30
P-2 200 6.8 0.07 13.9 620 2.6 4.0 0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 0.0 0.0 18 820 <15%Imperv,Tc=18
P-3 200 7.8 0.09 12.9 58 2.8 0.3 250 0.8 5.0 550 2.4 3.7 630 1.2 8.9 0.0 0.0 31 1688 <15%Imperv,To=31
P-4 105 3.8 0.07 12.2 260 3.2 1.4 0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 0.0 0.0 14 365 <15%Impel/.Tc=14
P-5 130 3.8 0.07 13.6 150 3.2 0.8 0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 0.0 0.0 14 280 <15%Imperv,To=14
la I CALCS v3.XLS Pickett Engineering,Inc.
I I I I I I I I I I I I I I I I I l/ I I) )DALMAR ESTATES 12/1/2003
Weld County PEI#01-034
•
TABLE 4 : STORM DRAINAGE RUNOFF CALCULATIONS
Basin Characteristics Intensities Sub-basin
Type AREA C2 C5 C10 C100 Tc 12 I5 110 1100 Q Q Q Q
BASIN 2-yr 5-yr 10-yr 100-yr
(acres) (min) (in/hr) (in/hr) (in/hr) (in/hr) (cfs) (cfs) (cfs) (cfs)
Existing Condition
B-1 Tillage/Field 71.839 0.00 0.01 0.07 0.22 69 0.89 1.28 1.55 2.47 0.0 0.9 7.8 39.1
B-2 Tillage/Field 81.408 0.00 0.01 0.07 0.22 88 0.75 1.09 1.31 2.09 0.0 0.9 7.5 37.5
SB-1 Tillage/Field 45.164 0.00 0.01 0.07 0.22 43 1.21 1.75 2.11 3.37 0.0 0.8 6.7 33.5
SB-2 Tillage/Field 38.348 0.00 0.01 0.07 0.22 68 0.90 1.30 1.57 2.51 0.0 0.5 4.2 21.2
SB-3 Tillage/Field 14.711 0.00 0.01 0.07 0.22 62 0.96 1.38 1.67 2.67 0.0 0.2 1.7 8.6
Proposed Conditions
P-1 Tillage/Field 16.083 0.05 0.07 0.13 0.27 30 1.51 2.18 2.63 4.21 1.3 2.4 5.3 18.1
P-2 Tillage/Field 9.980 0.05 0.07 0.13 0.27 18 2.02 2.92 3.53 5.63 1.1 2.0 4.4 15.0
P-3 Gravel Road 1.871 0.07 0.09 0.15 0.29 31 1.50 2.16 2.61 4.17 0.2 0.4 0.7 2.2
P4 Tillage/Field 5.133 0.05 0.07 0.13 0.27 14 2.31 3.33 4.02 6.42 0.6 1.2 2.6 8.8
P-5 Tillage/Field 4.012 0.05 0.07 0.13 0.27 14 2.25 3.24 3.92 6.25 0.5 0.9 2.0 6.7
2-year, 1 Hour Rainfall 0.97
5-year, 1 Hour Rainfall 1.40
10-year, 1 Hour Rainfall 1.69
100-year, 1 Hour Rainfall 2.70
i-- I CALCS v3.XLS Pickett Engineering, Inc.
)
DALMAR ES iATES
Weld County 12/1/2003
TABLE 5: 5-YEAR AND 100-YEAR SUB-BASIN ROUTING
Flow Time Channel Pipe
I
E TY u U
c c
c u
E E e 4
Design Point Basin _ 0 0. R o � ,- th 2 � 2 d n o 2 ca 0 > H in 0 0 3.
-J .n -, Remarks
1 SB-2 68.0 0.01 0.22 1.30 2.51 38348 0.6 21.1
From Ito II 986 9.1 3.10 21.1 1.80
II 5B-2,P-1 71.1 0.03 0.23 1.19 2.30 54.431 1.9 28.8 Discharge into a 2-24"dia.CMP
From B to III 70 0.2 0.50 28.8 4.80
III SB-2,P-1 77.4 0.03 0.23 1.19 2.29 54.431 1.9 28.7 Discharge into a 6'Wide Concrete Flume
From Ill to IV 100 0.3 0.50 28.7 6.00
IV SB-2,P-1 77.7 0.03 0.23 1.19 2.29 54.431 11 28.6
From Nte V 425 5.1 2.00 28.6 1.40
V SB-2,P-1 82.7 0.03 0.23 1.13 2.19 54.431 1.6 27.4 Outlot 1
VI SB-3 61.9 0.01 0.22 1.38 2.67 14.711 0.2 8.6
VII 58-1 43.0 0.01 0.22 1.76 3.40 45.164 0.8 33.7
From VII to VIII
VIII SE-1,38 P-3 43.0 0.01 0.22 1.76 3.40 61.746 1.1 46.1 Discharge to County Road 1612 roadside ditch
IX P-4 14.0 0.05 0.27 3.28 6.33 5.133 0.8 8.8 Discharge to inigabon ditch
X P-2 18.0 0.05 0.27 2.91 5.61 9.980 1.6 16.1 Discharge to irrigation ditch
XI P-5 14.0 0.05 0.27 3.28 6.33 4.012 0.7 6.9 Discharge to inigation ditch
•
1.... I CALCS v3.XLS Pickett Englneedng,Inc.
Ui
_
Appendix C
Technical information used in the support of the
drainage facility design concept.
DRAINAGE CRITERIA MANUAL(V. 1) RUNOFF
2.0 RATIONAL METHOD
For urban catchments that are not complex and are generally 160 acres or less in size, it is acceptable
that the design storm runoff be analyzed by the Rational Method. This method was introduced in 1889
and is still being used in most engineering offices in the United States. Even though this method has
frequently come under academic criticism for its simplicity, no other practical drainage design method has
evolved to such a level of general acceptance by the practicing engineer. The Rational Mecnoc properly
understood and applied can produce satisfactory results for urban storm sewer and small on-site
detention design.
2.1 Rational Formula
The Rational Method is based on the Rational Formula:
Q= CIA (RO-1)
in which:
Q=the maximum rate of runoff(cfs)
C=a runoff coefficient that is the ratio between the runoff volume from an area and the average
rate of rainfall depth over a given duration for that area
I=average intensity of rainfall in inches per hour for a duration equal to the time of concentration,
rc
A =area(acres)
Actually, Q has units of inches per hour per acre(in/hr/ac); however, since this rate of in/hr/ac differs from
cubic feet per second (cfs) by less than one percent, the more common units of cfs are used. The time of
concentration is typically defined as the time required for water to flow from the most remote point of the
area to the point being investigated. The time of concentration should be based upon a flow length and
path that results in a time of concentration for only a portion of the area if that portion of the catchment
produces a higher rate of runoff.
The general procedure for Rational Method calculations for a single catchment is as follows:
1. Delineate the catchment boundary. Measure its area.
2. Define the flow path from the upper-most portion of the catchment to the design point. This flow
path should be divided into reaches of similar flow type (e.g., overland flow, shallow swale flow,
gutter flow, etc.). The length and slope of each reach should be measured.
r
3. Determine the time of concentration, tc,for the catchment.
06/2001 17
Urban Drainage and Flood Control District
DRAINAGE CRITERIA MANUAL(V. 1) RUNOFF
4. Find the rainfall intensity,I, for the design storm using the calculated r,and the rainfall intensity-
duration-frequency curve. (See Section 4.0 of the RAINFALL chapter.)
5. Determine the runoff coefficient, C.
6. Calculate the peak flow rate from the watershed using Equation RO-1.
2.2 Assumptions
The basic assumptions that are often made when the Rational Method is applied are:
1. The computed maximum rate of runoff to the design point is a function of the average rainfall rate
during the time of concentration to that point.
2. The depth of rainfall used is one that occurs from the start of the storm to the time of
concentration, and the design rainfall depth during that time period is converted to the average
rainfall intensity for that period.
3. The maximum runoff rate occurs when the entire area is contributing flow. However, this
assumption has to be modified when a more intensely developed portion of the catchment with a
shorter time of concentration produces a higher rate of maximum runoff than the entire catchment
with a longer time of concentration.
2.3 Limitations
The Rational Method is an adequate method for approximating the peak rate and total volume of runoff
from a design rainstorm in a given catchment. The greatest drawback to the Rational Method is that it
normally provides only one point on the runoff hydrograph. When the areas become complex and where
sub-catchments come together, the Rational Method will tend to overestimate the actual flow, which
results in oversizing of drainage facilities. The Rational Method provides no direct information needed to
route hydrographs through the drainage facilities. One reason the Rational Method is limited to small
areas is that good design practice requires the routing of hydrographs for larger catchments to achieve an
economic design.
Another disadvantage of the Rational Method is that with typical design procedures one normally
assumes that all of the design flow is collected at the design point and that there is no water running
_ overland to the next design point. However, this is not the fault of the Rational Method but of the design
procedure. The Rational Method must be modified, or another type of analysis must be used, when
analyzing an existing system that is under-designed or when analyzing the effects of a major storm on a
system designed for the minor storm.
06/2001 18
Urban Drainage and Flood Control District
DRAINAGE CRITERIA MANUAL (V. 1) RUNOFF
2.4 Time of Concentration
One of the basic assumptions underlying the Rational Method is that runoff is a function of the average
rainfall rate during the time required for water to flow from the most remote part of the drainage area
under consideration to the design point. However, in practice,the time of concentration can be an
empirical value that results in reasonable and acceptable peak flow calculations. The time of
concentration relationships recommended in this Manual are based in part on the rainfall-runoff data
_ collected in the Denver metropolitan area and are designed to work with the runoff coefficients also
recommended in this Manual. As a result, these recommendations need to be used with a great deal of
caution whenever working in areas that may differ significantly from the climate or topography found in
the Denver region.
For urban areas, the time of concentration, tt, consists of an initial time or overland flow time, t,, plus the
travel time, t, in the storm sewer, paved gutter, roadside drainage ditch, or drainage channel. For non-
urban areas, the time of concentration consists of an overland flow time, q, plus the time of travel in a
defined form, such as a swale, channel, or drainageway. The travel portion, t„ of the time of
concentration can be estimated from the hydraulic properties of the storm sewer, gutter, swale, ditch, or
drainageway. Initial time, on the other hand, will vary with surface slope, depression storage, surface
cover, antecedent rainfall, and infiltration capacity of the soil, as well as distance of surface flow. The
time of concentration is represented by Equation RO-2 for both urban and non-urban areas:
•
tc =t +t, (RO-2)
in which:
=time of concentration (minutes)
G= initial or overland flow time (minutes)
t,=travel time in the ditch, channel, gutter, storm sewer,etc. (minutes)
2.4.1 Initial Flow Time. The initial or overland flow time, t;, may be calculated using equation RO-3:
- 0.395(1.1—C5 WE
t — So3s (RO-3)
in which:
t;= initial or overland flow time (minutes)
C5 = runoff coefficient for 5-year frequency(from Table RO-5)
06/2001 19
Urban Drainage and Flood Control District
DRAINAGE CRITERIA MANUAL(V. 1) RUNOFF
L= length of overland flow(500 ft maximum for non-urban land uses, 300 ft maximum for urban
land uses)
S=average basin slope(ft/ft)
Equation RO-3 is adequate for distances up to 500 feet. Note that, in some urban watersheds, the
overland flow time may be very small because flows quickly channelize.
2.4.2 Overland Travel Time. For catchments with overland and channelized flow, the time of
concentration needs to be considered in combination with the overland travel time, t„which is calculated
using the hydraulic properties of the swale, ditch, or channel. For preliminary work, the overland travel
time, t„ can be estimated with the help of Figure RO-1 or the following equation (Guo 1999):
V= C Swos (RO-4)
in which:
V=velocity(fUsec)
C =conveyance coefficient(from Table RO-2)
Sw=watercourse slope (ft/ft)
TABLE RO-2
Conveyance Coefficient, C.
Type of Land Surface Conveyance Coefficient, C
Heavy meadow 2.5
Tillage/field 5
Short pasture and lawns 7
Nearly bare ground 10
Grassed waterway 15
Paved areas and shallow paved swales 20
The time of concentration,tt, is then the sum of the initial flow time, t;, and the travel time,t„ as per
Equation RO-2.
2.4.3 First Design Point Time of Concentration in Urban Catchments. Using this procedure,the time
of concentration at the first design point(i.e., initial flow time, t;)in an urbanized catchment should not
exceed the time of concentration calculated using Equation RO-5.
tc = 8 +10 (RO-5)
10
r
20
06/2001
Urban Drainage and Flood Control District
DRAINAGE CRITERIA MANUAL(V. 1) RUNOFF
in which:
= maximum time of concentration at the first design point in an urban watershed (minutes)
L =waterway length (ft)
— Equation RO-5 was developed using the rainfall-runoff data collected in the Denver region and, in
essence, represents regional "calibration"of the Rational Method.
The first design point is the point where runoff first enters the storm sewer system. An example of
definition of first design point is provided in Figure RO-2.
Normally, Equation RO-5 will result in a lesser time of concentration at the first design point and will
govern in an urbanized watershed. For subsequent design points, the time of concentration is calculated
by accumulating the travel times in downstream drainageway reaches.
2.4.4 Minimum Time of Concentration. Should the calculations result in a 4 of less than 10 minutes, it
_ is recommended that a minimum value of 10 minutes be used for non-urban watersheds. The minimum r�
recommended for urbanized areas should not be less than 5 minutes and if calculations indicate a lesser
value, use 5 minutes instead.
2.4.5 Common Errors in Calculating Time of Concentration. A common mistake in urbanized areas
is to assume travel velocities that are too slow. Another common error is to not check the runoff peak
resulting from only part of the catchment. Sometimes a lower portion of the catchment or a highly
impervious area produces a larger peak than that computed for the whole catchment. This error is most
often encountered when the catchment is long or the upper portion contains grassy parkland and the
lower portion is developed urban land.
2.5 Intensity
The rainfall intensity,L is the average rainfall rate in inches per hour for the period of maximum rainfall of
a given recurrence frequency having a duration equal to the time of concentration.
After the design storm's recurrence frequency has been selected, a graph should be made showing
rainfall intensity versus time. The procedure for obtaining the local data and drawing such a graph is
explained and illustrated in Section 4 of the RAINFALL chapter of this Manual. The intensity for a design
point is taken from the graph or through the use of Equation RA-3 using the calculated 4.
2.6 Watershed Imperviousness
All parts of a watershed can be considered either pervious or impervious. The pervious part is that area
where water can readily infiltrate into the ground. The impervious part is the area that does not readily
allow water to infiltrate into the ground, such as areas that are paved or covered with buildings and
06/2001 21
Urban Drainage and Flood Control District
DRAINAGE CRITERIA MANUAL(V. 1) RUNOFF
sidewalks or compacted unvegetated soils. In urban hydrology, the percentage of pervious and
impervious land is important. As urbanization occurs, the percentage of impervious area increases and
the rainfall-runoff relation changes significantly. The total amount of runoff volume normally increases,
the time to the runoff peak rate decreases, and the peak runoff rates increase as the area urbanizes.
]a y !�.. g •fi.� •a};a 3SA Y ,41 'i'-k
y 6Y }s [l�` tl' 3r 'vplW`,..::ct'C.�,.v R..t
'.S
I
Photograph RO-2
Urbanization (impervious area) increases runoff volumes, peak discharges, frequency of
runoff, and receiving stream degradation.
When analyzing a watershed for design purposes, the probable future percent of impervious area must
be estimated. A complete tabulation of recommended values of the total percent of imperviousness is
provided in Table RO-3 and Figures RO-3 through RO-5, the latter developed by the District after the
evolution of residential growth patterns since 1990.
2.7 Runoff Coefficient
The runoff coefficient, C, represents the integrated effects of infiltration, evaporation, retention, and
interception, all of which affect the volume of runoff. The determination of C requires judgment and
understanding on the part of the engineer.
06/2001 22
Urban Drainage and Flood Control District
DRAINAGE CRITERIA MANUAL(V. 1) RUNOFF
TABLE RO-3
Recommended Percentage Imperviousness Values
Land Use or Percentage
Surface Characteristics Imperviousness
Business:
Commercial areas 95
Neighborhood areas 85
Residential:
Single-family
Multi-unit(detached) 60
Multi-unit(attached) 75
Half-acre lot or larger
Apartments 80
Industrial:
Light areas 80
Heavy areas 90 _
Parks, cemeteries 5
Playgrounds 10
Schools 50
Railroad yard areas 15 —
Undeveloped Areas:
Historic flow analysis 2
Greenbelts, agricultural 2
,., Off-site flow analysis 45
(when land use not defined)
Streets:
• Paved 100
Gravel (packed) 40
Drive and walks 90
Roofs 90
Lawns, sandy soil 0
Lawns, clayey soil 0
See Figures RO-3 through RO-5 for percentage imperviousness.
Based in part on the data collected by the District since 1969, an empirical relationship between C and
the percentage imperviousness for various storm return periods was developed. Thus,values for C can
be determined using the following equations(Urbonas, Guo and Tucker 1990).
CA =KA + (1.31i' —1.4412 +1.135i—0.12)for CA ≥0, otherwise CA =0 (RO-6)
CCD =KCD + (0.858i3 -0.786i2 +0.774i+ 0.04) (RO-7)
Cs =(CA + CCD)l2
in which:
/= % imperviousness/100 expressed as a decimal (see Table RO-3)
23
06/2001
Urban Drainage and Flood Control District
DRAINAGE CRITERIA MANUAL(V. 1) RUNOFF
CA= Runoff coefficient for Natural Resources Conservation Service (NRCS)Type A soils
CB=Runoff coefficient for NRCS Type B soils
Cap=Runoff coefficient for NRCS Type C and D soils
— K,, =Correction factor for Type A soils defined in Table RO-4
K�o=Correction factor for Type C and D soils defined in Table RO-4
TABLE RO-4
Correction Factors KA and Kw for Use With Equations RO-6 and RO-7
Storm Return Period
NRCS Soil Type 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year
C and D 0 -0.101+0.11 -0.181+0.21 -0.281+0.33 -0.331+0.40 -0.391+0.46
A 0 -0.081+0.09 -0.141+0.17 -0.191+0.24 -0.221+0.28 -0.251+0.32
The values for various catchment imperviousnesses and storm return periods are presented graphically in
Figures RO-6 through RO-8, and are tabulated in Table RO-5. These coefficients were developed for the
Denver region to work in conjunction with the time of concentration recommendations in Section 2.4. Use
of these coefficients and this procedure outside of the semi-arid climate found in the Denver region may
not be valid.
See Examples 7.1 and 7.2 that illustrate the Rational method. The use of the Rational method in storm
sewer design is illustrated in Example 6.13 of the STREETS/INLETS/STORM SEWERS chapter.
06/2001 24
Urban Drainage and Flood Control District
- DRAINAGE CRITERIA MANUAL (V. 1) RUNOFF
TABLE RO-5
Runoff Coefficients, c
—
Percentage
Imperviousness Type C and D NRCS Hydrologic Soil Groups
2-yr 5-yr 10-yr 25-yr 50-yr 100-yr
-
0% 0.04 0.15 0.25 0.37 0.44 0.50
5% 0.08 0.18 0.28 0.39 0.46 0.52
10% 0.11 0.21 0.30 0.41 0.47 0.53
- 15% 0.14 0.24 0.32 0.43 0.49 0.54
20% 0.17 0.26 0.34 0.44 0.50 0.55
25% 0.20 0.28 , 0.36 0.46 0.51 0.56
- 30% 0.22 0.30 0.38 0.47 0.52 0.57
35% 0.25 0.33 0.40 0.48 , 0.53 0.57
40% 0.28 , 0.35 0.42 0.50 0.54 0.58
45% 0.31 0.37 , 0.44 0.51 , 0.55 0.59
-
50% 0.34 0.40 0.46 0.53 0.57 0.60
55% 0.37 0.43 0.48 0.55 0.58 0.62
60% 0.41 0.46 0.51 0.57 0.60 0.63
- 65% 0.45 0.49 0.54 0.59 0.62 0.65
70% 0.49 0.53 0.57 0.62 0.65 0.68
75% 0.54 0.58 0.62 0.66 0.68 0.71
80% 0.60 0.63 0.66 0.70 0.72 0.74
-
85% 0.66 0.68 0.71 0.75 0.77 0.79
90% 0.73 0.75 0.77 0.80 0.82 0.83
95% 0.80 0.82 0.84 0.87 0.88 0.89
- ' 100% 0.89 0.90 0.92 0.94 '0.95 0.96
Type B NRCS Hydrologic Soils Group
0% 0.02 0.08 0.15 0.25 0.30 0.35
- 5% 0.04 0.10 0.19 0.28 0.33 0.38
10% 0.06 0.14 0.22 0.31 0.36 0.40
15% 0.08 0.17 0.25 0.33 0.38 0.42
20% 0.12 0.20 0.27 0.35 0.40 0.44
- 25% 0.15 0.22 0.30 0.37 0.41 0.46
30% 0.18 0.25 0.32 0.39 0.43 0.47
35% 0.20 0.27 0.34 0.41 0.44 0.48
- 40% 0.23 0.30 0.36 0.42 0.46 0.50
- - 45% 0.26 0.32 0.38 0.44 0.48 0.51
50% 0.29 0.35 0.40 0.46 0.49 0.52
55% 0.33 0.38 0.43 0.48 0.51 0.54
-
60% 0.37 0.41 0.46 0.51 0.54 0.56
65% 0.41 0.45 0.49 0.54 0.57 0.59
70% 0.45 0.49 0.53 0.58 0.60 0.62
- 75% 0.51 0.54 0.58 0.62 0.64 0.66
80% 0.57 0.59 0.63 0.66 0.68 0.70
85% 0.63 0.66 0.69 _ 0.72 0.73 0.75
90% 0.71 0.73 0.75 0.78 0.80 0.81
-
95% 0.79 0.81 0.83 0.85 0.87 0.88
100% 0.89 0.90 0.92 0.94 0.95 0.96
25
06/2001
— Urban Drainage and Flood Control District
DRAINAGE CRITERIA MANUAL(V. 1) RUNOFF
TABLE RO-5 (CONTINUED)
Runoff Coefficients, C
—
Percentage
Imperviousness Type A NRCS Hydrologic Soils Group _
2-yr 5-yr 10-yr 25-yr 50-yr 100-yr
-
0% 0.00 0.00 0.05 , 0.12 0.16 0.20
5% 0.00 0.02 0.10 0.16 0.20 0.24
10% 0.00 0.06 0.14 0.20 0.24 0.28
15% 0.02 0.10 0.17 0.23 0.27 0.30
20% 0.06 0.13 0.20 , 0.26 0.30 0.33
25% 0.09 0.16 0.23 0.29 0.32 0.35
30% 0.13 0.19 0.25 0.31 0.34 0.37
35% 0.16 0.22 0.28 0.33 , 0.36 0.39
40% 0.19 0.25 0.30 0.35 0.38 0.41
45% 0.22 0.27 0.33 0.37 0.40 0.43
-
50% 0.25 0.30 0.35 0.40 0.42 0.45
55% 0.29 0.33 0.38 0.42 , 0.45 0.47
60% 0.33 0.37 0.41 0.45 0.47 0.50
- 65% 0.37 0.41 0.45 0.49 0.51 0.53
70% 0.42 0.45 0.49 0.53 0.54 0.56
75% 0.47 0.50 0.54 0.57 0.59 0.61
80% 0.54 0.56 0.60 0.63 _ 0.64 0.66
-
85% 0.61 0.63 0.66 0.69 0.70 0.72
90% 0.69 0.71 0.73 0.76 0.77 0.79
95% 0.78 0.80 0.82 , 0.84 0.85 0.86
- 100% 0.89 • 0.90 0.92 0.94 0.95 0.96 •
06/2001 26
Urban Drainage and Flood Control Disbict
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— DRAINAGE CRITERIA MANUAL(V. 1) RUNOFF
90
15.000 sq.It homesf to
80 I (4.000 sq.R homes)
/ • �
....
•
70 /• •
/ • 13,000 sq.ft homes r
— • -
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d/ / • • • 12.000 sq.!L homes k
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— • •
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Single Family Dwelling Units per Acre
—
FIGURE RO-3
Watershed Imperviousness, Single-Family Residential Ranch Style Houses
06/2001 28
— Urban Drainage and Flood Control District
DRAINAGE CRITERIA MANUAL(V. 1) RAINFALL
R 71 W 0. 5 R 70 W R 69 W R 68 W R 67 W R 66 W R 65 W R 64 W R 63 W
0.8 0.9 1.0 1.0 0.95
0.75 .87 1.0L 1.04 A.95, 1.0
—
/ / LONCM NT
• 0.95 -
z I ry
N Gr
•
91111t1
m
I WELD IL)BRICHTO ADAMS —
w ( J
Q
•rte, m 4
0 DER
E ERSON J
I, O N
a DENVER
,— / Li
V �- X0.97
_ ADAMS
ID.•VE I ARAPAHOE
A.
e �` 0.99 -
• L v X
o
RGRE N
N
- � / -
A''PAHOE ARAPAHOE
D• GLAS LDERT
CONIFER •.7 • 1.0
Lc
• PAR ER
III I i 43
0 m n
w
in
•
imm
°1 SEDALIA
-- 1.05 0.95 X 0.99 -
0. 5 0.8 ' 1.0 0.95 FRANKTw•
1.0
0.85 0.95
R 71 W R 70 W R 69 W 0.9 R 68 W R 67 W I R 66 W R 65 W R 64 W R 63 W
FIGURE RA-1
—
Rainfall Depth-Duration-Frequency: 2-Year, 1-Hour Rainfall
r
06/2001 29
Urban Drainage and Flood Control District
DRAINAGE CRITERIA MANUAL (V. 1) RAINFALL
R 71 w R 70 W R 69 W R 68 JN- R 67 W R 66 W R 65 W R 64 W R 63 W
1.3 1 41.45 1.45 1.4 1.35 1.35 1.4
I / I 1 z
LONGMONT I
-
I •
I I IC
Y 1 NIWOT 1l'
I / 3I
_ � 0
1
CO
_
ó
wE BRIGHT."' ADA S
I
. DENVER F
I S �
1.1 ` 1.37 - •
....
/ 1.39 ADAMS
I /
D' VER ARAPAHOE
o
e +
— 1.0 in 1—r
J V4 JEN 4
A' PAHOE ARAPAHOE
D. CLAS ELfiERT
IFER JPARERSEDAL X 1.39
._1.0 1.1 1.2 1.. FRAN KT.W
1.4 1.4
R 71 W R 70 W R 69 W R 68 W R 67 W R 66 W I R 65 W R 64 W R 63 W
—. FIGURE RA-2
Rainfall Depth-Duration-Frequency: 5-Year, 1-Hour Rainfall
n
06/2001 30
Urban Drainage and Flood Control District
-
DRAINAGE CRITERIA MANUAL(V. 1) RAINFALL
R 71 W R 70 W R 69 W 6V 68 W R 67 W R 66 W R 65 W R 64 W R 63 W
1.5 1.6 1.7 1.75 1.75 1.65 1.6 1.6 1.65
LONGMONF I \
r \
IIwoT I
1
= o � i
5 I -
1\ li W541.4 .' I BRICHIC AOA s N I--
;a —
R0i DER
EF EPSON 1 I J 1 ij
H. 1 eN
a
. DENVER
N 1 i ��. o
/ I
_ \ • ADAMS
D ,VER ARAPAHOE
1.3 \ �J \ \ F
EVE BEEN / � 1.65
1 55 F
— PAHOE / ARAPAHOE
0• GLA5 ELBERT
H 1�V
• PAR ER
o , �o • �a
�
— /
S
Gi
m -
N.._.' •
6
SE^DALIA` \ m
F \ 1.3 1.4 1.5 I / ..N \
1.65 1.7 • 1.65
1.7
R 71 W R 70 w R 69 W R 68 W R 67 W R 66 W R 65 W R 64 W R 63 W
FIGURE RA-3
Rainfall Depth-Duration-Frequency: 10-Year, 1-Hour Rainfall
06/2001 31
— Urban Drainage and Flood Control District
DRAINAGE CRITERIA MANUAL(V. 1) RAINFALL
e-...
R 71 W1.95 R 70 W R 69 W R 65 W R 67 W R 66 W R 65 W R 64 W R 63 W
1.7 1.92.
1.8 I 2.05 2.1 2.1 2.05 2.0 2.0 2 05 2.1
— t
z LONGMO NT
I ) •
♦ I
cv NI
�i �I 0 -
1 r
I
1' m / z1.
D
B RICHTO ' AEI
0
0<
0m6
B0.'DER
J- E`'ON J 1 `7\\
ii
N J I cm
ry
,
I\ \
' n JJ
/W a DENY R
4?2
r
6 I
,' \ Lin
.
_ I \ ADAM
] D VER • ARAPAH
En
• `l �'�,� 2.05
EVERGR: N
1.6 \ uo
m
LI ~
.-
w g
APAHOE ARAPAHOE
D• OS ELBERT
En I NIFER '•
w • PAR ER
mm
I ow
1 •
.... O1 SEDALiA in
\ FRANKTIWN
•
1.6 1.7 / 1.95 I 2.0
1.8 1.9
R 71 W R 70 W R 69 W R 66 W R 67 w R 66 W R 65 w R 64 w R 63 W
FIGURE RA-4
Rainfall Depth-Duration-Frequency: 25-Year, 1-Hour Rainfall
r"�
06/2001 32
Urban Drainage and Flood Control District
DRAINAGE CRITERIA MANUAL(V. 1) RAINFALL
-
R 71 W R 70 W R 69 W R 68 W R 67 W R 66 W R 65 W R 64 W R 63 W
2.2 2.4 2.42 2.4 2. 2.3 2.4
X I
- 2 � 235
2 \• n
\ 2.4
z 1 ' z
NIWDT I•
� �2.42 o
= ' II, Z
2.25
m
2.1 1_ WELD
V SRI TO A0 5 Icc
r.
oc m
' 2.2— - 0
-
cc
BO LDE
EF ERS
Sim ZN2.42 ' ¢a / DENVER I ..
x /
2.0 I
in
n 1
ADAMS
•
. I 'AVER ARAPAHOE
ma. \ in
1.9 d
1.8 �1ANOE
—
E CREE HI n
22.35
/ ARAPAHOE
J D GUS ELBERT
WFER 1
1 PAR ER in F
o o e
X � U w
2.28 2.29 o Lu
m
_
�^ r sEDALIA ul
2.�
r.3
~ 2 25 A 2.3 FRANKT.WN ~
1.0 '.0 2.0 2.1 2.2 •
I
R 71 W R 70 W R 69 W R 68 W R 67 W ) R 66 W R 65 W R 64 W R 63 W
mum
FIGURE RA-5
Rainfall Depth-Duration-Frequency: 50-Year, 1-Hour Rainfall
06/2001 33
Urban Drainage and Flood Control District
— DRAINAGE CRITERIA MANUAL(V. 1) RAINFALL
R 71 w R 70 W R 69 W R 66 W R 67 W R 66 W R 65 W R 64 W R 63 W
2.4 2.5 2.65 2.7 2.7 2.6 2.65 2.7
~ 2 6 LONGMONT \ \ z
1 ( r � \
44 I NIWOT 1 I z
�I!'
TN
O
m / WELD
2'a ¢ BRiGHiO • ,DAMS F
— I in
I 2<Om<
I 0, DER /
JFEPONN I
Np 4, N
N- ENVER `
I
X2.2
ADAMS
: O' VERI ARAPAHOE .
cn cm
e 2.15 \ I e
F 2 \ ir I 2.7
—
l F EVE EEN \
in Lei
` 2.65
— N
A PAHOE ARAPAHOE
I \
2.05 0 CL45 ELSERT
`CONIFER
vl
0 • tin viER N
I Oto
) • V La
K
N. H26
/2.05SED ANK2.5
R71W t R70W 15 2R269W3 24R 6B W R6] w R66 R 65 W R 64 W R 63 w
FIGURE RA-6
Rainfall Depth-Duration-Frequency: 100-Year, 1-Hour Rainfall
e".
06/2001 34
— Urban Drainage and Flood Control District
�- Schneider Minor Subdivision
SOILS CLASSIFICATION
A review of Sheet 28 of the United States Department of Agriculture, Soil Conservation
Service, Soil Survey of Weld County, Colorado, Southern Part, shows the following soils
types:
SCS
MAP SYMBOL SOIL NAME AND SLOPE
3 Aquolls and Aquents, Gravelly Substratum
4 Aquolls and Aquepts,Flooded
40 Nunn Loam, 1% to 3% Slopes
53 Otero Sandy Loam, 5% to 9% Slopes
Please see the attached map.
35
,
" m
rII
x -
Aft
i
"
c.•
s
I
osul
A.
�� w
Jt S a
— I .r
o o
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tai a{
tea' '.i ilk
i
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r 3r
pr-Ag
142*
O '+� -�'lc
g .
P�_ * .x y r rZ
Sr
.Y. i
r4,..,:„;-,,;'-'-',.;-.-1,-..,,,t V y tir
r 'W
1
ila,n z 1/20 it ,]:,A t 41/1,-; aka 3
" gr
�eM' ° "� M &N.:- em �; ,yy'gqh�.yy.. tG•.;, .
1 kX k.''") �,ks#-'-la'.s .'S-v. k. 4d iaS' '; y
CwiJTools
CivilTools ROUND CULVERT DESIGN
Calculate the headwater depth required for a round drainage culvert to pass the design flow
Headwater Depth Calculation for a Round Pipe Culvert WLvErT -DEStr:,N) UPIi0 ie 1)ACwl42 izoc
INPUT DATA
Pipe Diameter(D):24 Inches
Pipe Length (L):155 feet �' ' 504A
Pipe Slope:0.50%
Design Q (Qd):15.00 cfs
Pipe and Entrance Type: on red to select
Pipe and Entrance Type:CMP mitred to conform to fill slope
Manning's 'n':0.024
Tailwater Depth (TW):2 feet
OUTPUT SUMMARY
Culvert is flowing:Full
Headwater Depth (Hw) is:4.15 feet
Culvert will operate under Outlet control
Normal Flow Depth (Dn) is:24 Inches
Flow velocity in culvert is:4.77 ft./s
Flow velocity at outlet is:4.77 ft./s
V
Date: 8/5/2003 Time: 4:41
P E Client Job# D -03'�
Protect D�+'-,^^-✓'-Q oz-c.1 P-- Calculatlons for C,RG'P
Made by r"L 1- Date Checked by Date Sheet I of S
PICKETT
C K E T T
ENGINEERING, INC.
LOW TAiLWAi'; 121Pa'A° at5iN: OUTLe.T OF (,' 141Oe fwav,!F pJEC
30 d)
ft, _ _.. .
5O PE ' ! L614.
Vyth'Nwt, 0,01S
I. Uk 4 GuwT4t
is c4„, Z 5 &Ps
A
3. Q/ 50 A 245 =.',P, i.Z rr+ - 0.22
niLL
1.4_,(34,14±5= 3p` 7�t�L5 ` 5 dlµ ' o,.1Z
S . ur s64e - (,.22 A744.w ozz
6,, A - n•/ri j4LL a Nf.Ntl 0..rz ( I •le) -
V o/A r .30/4.32 =, 1'ps
CA.cK,a,E Pd
USE AeoLIPc,Q ptPa•r-d Ti Pe NS (Dyz° /8"J
U. ry N T e t S T? 115 (IW)= 31,5
_
9 LEN a L= 4 ,Al 4 r oa L.: ( I1/z) . (a'1/z) ` II, 35P.'.. IZi
10. w,orrr , -co + 4 PI &� + I 10 1,15E ( foa.Z3 latPQM-? (o l8 )
ri ice-Jose, = 31,5"
D,w,QNS,JNS - i2 L r ID >�
39
1 I 1 I 1 I 1 I I 1 I 1 I I I I 1 1 I
CivilTools
lJ \ )
CivilTools TRAPEZOIDAL CHANNEL HYDRAULICS 1 1
Use section a,b,or c,depending on what you want to calculate
CALCULATIONS FOR A TRAPEZOIDAL CHANNEL
Cpnic ere FLUME Des,ean4
a) Solve for Q and V given channel dimensions and depth Base Width(B):6 ft. Q = 3a rats
Side Slopes(SS):0 H:1V
Mannings'n':0.015
Bed Slope(S):0.500%
Flow Depth(d):1 ft.
Flow Area(A):6.00 sq.ft.
Wetted perimeter(P):8.00 ft.
Flow Velocity(V):5.78 ft.ls
Flow Rate(Q):34.70 cfs
Flow is:Supercritical
Critical Depth(Yc):1.01 ft.
Sediment transport size(D75):0.5 inches,approx.
b) Solve for depth given Q and channel dimensions
Base Width(B):0 ft.
Side Slopes(SS):0 H:1V
Mannings'n':0
Bed Slope(5):0.000%
Flow Rate(Q):0 cfs
Flow Depth(d):ERR ft.
Flow Area(A):ERR sq.ft.
Wetted perimeter(P):ERR ft.
Flow Velocity(V):ERR fL/s
Flow is:ERR
Critical Depth(Yc):ERR ft.
Sediment transport size(D75):ERR ERR
c) Solve for base width(B)given depth,O,and channel dimensions
Side Slopes(SS):0 H:1V
Mannings'n':0
Bed Slope(S):0.000%
Flow Rate(Q):0 cfs
Flow Depth(d):0 ft.
Base Width(B):ERR ERR •
Flow Area(A):ERR sq.ft.
Wetted perimeter(P):ERR ft.
Flow Velocity(V):ERR ftJs
Flow is:ERR
Critical Depth(Yc):ERR ft.
Sediment transport size(D75):ERR ERR
Co
.te:8/5/2003 Time:4:45
IS r
,_ . - - Design of Low Tailwater Riprap Basins Srlc.ar z a,- s
for Storm Sewer Pipe Outlets
....
by
Michael A.Stevens,Consultant
Ben Urbonas,Urban Drainage&Flood Control District
Introduction Finding Flow Depth and Velocity at 1.49 2 Ii
Storm Sewer Pipe Outlets Oful! = —' Afix •(Rjon) •So
The Major Drainage chapter of The first step in the design of a n
— Volume 2 of the Urban Storm Drainage scour protection basin at the outlet of a
Criteria Manual. (USDCM)of the storm sewer is to find the depth and in which: 010=Pipe full discharge at
Urban Drainage and Flood Control velocity of flow at the outlet. Pipe-full its slope, in cubic feet per second;
— District provides guidance for the flow can be found using Manning's n=Manning's n for the pipe full depth;
design of scour protection downstream equation and the pipe-full velocity can elfin=Cross-sectional area of the pipe,
of culvert outlets. This guidance was be found using the Continuity equation. in square feet; S,=Longitudinal slope
intended for culvert crossings of major Namely, of the pipe. in feet per foot;
drainageway channels and assumed that R=Hydraulic Radius of the pipe
the culvert is in line with the channel. flowing full, in feet with
It also assumed there was significant
— tailwater that partially inundated the
culvert's outlet. This guidance does not
work for storm sewer pipes discharging ►--
— into open drainageways when the flow
depth in the drainageway provides a
low tailwater at the pipe's outlet. Thus,
when tailwater is low, scour protection I
—
at the outlet of a storm sewer has to be
designed differently than for culverts :
crossing major drainageways. For
see pole
— storm sewer outlets, low tailwater is w ,r p-— —-—-----— —-— —- — w
defined when:
D H
yr ≤ 3 or y` ≤ 3
— in which: yr=depth of tailwater at the
time the pipe is discharging its design
flow, in feet; D=the diameter of a I Plan
— circular pipe, in feet; H=the height of 9" layer of
a rectangular pipe, in feet. "see note granular type
r� �" I 2 bedding
H Oro id SWS._ —
Stevens(1969) reported on a I — yt
— series of studies describing the scour B • �' 0.5D or 0.5H
geometry in riprap located at pipe I T s - T
outlets. Quantifiable relationships were Z-
- found between flow depth and velocity
Perforated underdroln
at the outlet, the scour hole geometry, L
o daylight (optional)
and the rock size. Figure 1 describes Profile
the geometry of a pre-shaped scour hole —
---
downstreamofapipefittedwitha Note: For rectangular conduits use a standard design for a headwall
flared end section. Refer to this figure with wingwalls, paved bottom between the wingwolls, with an end
when reading the rest of this article. cutoff wall extending to a minimum depth equal to B
Figure 1 : Low tailwater basin at pipe outlets
40
11
-
R furl = D/4 for circular pipes, Finding the Appropriate Riprap Size at Srtser 3 or: 5
Use Figure 4 to find the size and •
RAH = Afuil/(2H+2w) for type of the riprap to use in the scour T= 1.75•D50
rectangular pipes. where w=width of a protection basin downstream of the pipe
_ rectangular conduit all in feet. Then. outlet [i.e., HG(grouted H).H.M or L]. in which: Dso=the median size of the
First calculate the riprap sizing design riprap(see Table 1).
A
hn parameter.Pd, namely.
= Q I Su
— t,z Table 1. Median(Dm)Rock Size of
in which: G j„u=Flow velocity of the Pd =(V2 +g•d) Urban Drainage District Riprap.
pipe flowing full, in feet per second Riprap Type Median Size(Inches)
—
The normal depth of flow,d, and in which: g=acceleration due to L 9
the velocity at that depth in a conduit gravity, 32.2 feet per second per second. M 12
can be found with the aid of Figure 2. When the riprap sizing design H—
&HG 18
Using the known design discharge, 0, Parameter indicates conditions that
and the calculated pipe-full discharge, place the design above the Type H
riprap line in Figure 4,use HG,or Finding the Basin Length
Om,enter Figure 2 with the value of S gt
larger,grouted rock. An alternative to The minimum length of the basin.
O/Of„n and find d/D for a circular pipe gt
or d/H for a rectangular a grouted or loose riprap basin is to use L in Figure 1, is defined as being the
the standard Bureau of Reclamation
Compare the value of this d/D(or greater of the following lengths:
- d/H) with that obtained from Figure 3 Basin VI, a reinforced concrete impact
using the Fronde parameter, namely, structure, to dissipate the energy in the For circular pipe,
flow at the outlet of the pipe.
z.5 1 After the riprap size has been t , v
•
- O/D or O/(w•H13) selected,the minimum thickness of the L = 4 D or L=(D)'z —
riprap layer. Tin feet in the basin is set 2
Choose the smaller of the two d/D(or
— dill)ratios to calculate the flow depth at 1 .2
"—"the end of the pipe, namely,
1.1 - : . . , ; : . , 1
d = D•(h) 1.0 -
or
Vic.,
0.8 NH
Again Figure enter 2usin the 0 . ". ."', Crcular
. , .
— smaller d/D(or d/H)ratio to find the Q .7 . / • !! : : : :
— y/l ' '
A/4 fix ratio. Use this to calculate the
area of flow at the end of the pipe, _ , ; • •
_ namely, a 0.5 - ' ; ; ' ' , , t- /-_-- Q/Q -
y `I to --
0.4 - a/a ` (Rectangular
t� full I-�- -;
— A = (� ) • Afurt 0.3 - Rectangular i.,-,--/ . , . .
0.2 %'/4. /..:„' i , , , , .
in which : A=Area of the design flow ' a `_i Q/Q,,,u
0.1 ,r--
in the end of the pipe, in square feet. � ;Circular (__ I ,
Finally, 0.0—
V= Q/ 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
r^ d/D or d/H
- u which: V=Design flow velocity at
the pipe outlet, in feet per second. Figure 2: Discharge and Flow Area Relationships for Circular and Rectangular
Pipes(Ratios for flow based on Manning's n varying with depth)
41
12
SH-eitr 4 o C
Step 3. Using the O/0,.,,,=0.88 ratio, =(12.12+32.2 • 2.28)1/2 = 14.8;
— For rectangular pipe. Figure 2 gives d/D=0.82 for a circular
Pipe. Use Type L Riprap
,.—' L V
— L = 4 H or L= (H)' 2 •- Step 4. Calculate: O/D=•'=2.81. Use Step 8. Calculate the minimum
2 this in Figure 3 to find d/D=0.57. thickness of the riprap layer for Dsa=9
inches:
Finding the Basin Width Step S. Since the smaller of the two
— The minimum width. W.of the d/D ratios is 0.57, use it to calculate T= 1.75 • 9.0= 15.75 inches
basin downstream of the pipe's flared depth, d, at the outlet and then in
end section is set at: Figure 2 to find the ratio for A/A
hu= Use T=16 inches.
— 0.59.
For circular pipes: Step 9. Find the length of the basin.
W = 4D d =(d/D) •D = 0.57. 4.0 = 2.28 feet namely the greater of the following two
— lengths:
Step 6. Using ihe.4/Af,r,=0.59 ratio,
For rectangular pipe: calculate flow area and velocity at the L=(D r") •(V/2)=4 I/2•(12.1 /2)=
end of the pipe: 12.1 feet
- W= w+4H
w+4H
.4 = (A/Ah,rr)'Amrr =(0.59) •Or•2.02) L=(d/D) •D =4.4=16 feet
Other Design Requirements =7.41 square feet (Greater of the two: use this value.)
— • All slopes in the preshaped
riprapped basin are 2H to 1V. V = (O/A)=(90)/(7.41)= 12.1 feet Step 10. Find the width of the riprap
• Provide pipe joint fasteners and a per second basin:
— structural concrete cutoff wall at
the end of the flared end section for Step 7. Calculate the riprap sizing W=4 •D=4 •4=16 feet
a circular pipe. or a headwall with design parameter,Pd.and use it in
wing walls and a paved bottom Figure 4 to find the appropriate riprap
between the walls,both with a size:
cutoff wall that extends down to a i%
depth of Pd =(V2 +g•d) z
— D • H '
B=—+ T or B = —+ T 1.0 -2 2• The riprap must be extended up 0.9 - I I I , / I/
the outlet embankment's slope to I I I I / III I
the mid-pipe level. 0.8 - I I 'd p --I I I
4, !„ I
— Examples 0.7 - I I ,I
Example 1 -Circular pipe on a j % I
relatively flat slope. = 0.6
6 I
Given: p 0.5 - I
Design flow,O= 90 cfs; /
— Tailwater depth,y,= 1.0 feet a 0 4 41
Pipe Diameter D=4.0 feet; 0.3 --H-1-j-/ d/H
Slope S=0:005 ft/ft
Manning's n=0.013 0.2 - 1'
Step 1. Determine if method is 0 1 C
applicable: y, < D/3; namely, low
— tailwater. 0.0 -, '
Step 2. Calculate the capacity of the 0.0 2.0 4.0 6.0 8.0
- pe flowing full: Oh,r, = 102 cfs is Q/D 2.5 or W W H1.5
found using the Manning's Equation.
Figure 3: Brink Depth for Horizontal Pipe Outlets
— 42
13
of 5
Example 2-Rectangular pipe on a Adams County Engineering
fair/v steep slope. Step 9. Find the length of the basin, Department for their review and
namely the greater of the following two suggestions; to Bryan for the
e" Given: lengths: preparation of the design examples:and
- Design flow O=300 cfs: to Ken McKenzie and Vince Vigil, the
Tailwater depth y,= 1.0 feet L=4 •H= 16 feet District's student interns for the
Box Height If=4.0 feet. preparation of the graphics.
Width w=5.0 feet L =(H v2) • (V/2)=4°' (20.5/2)= References
Slope S=0.05 fUft: 20.5 feet (use this length) The information on circular pipes
Manning's n=0.013 in Table 2 was taken from: Chow, Ven
Step 10. Find the width of the riprap Te(1959). Open-channel Hydraulics.
— Step 1. Determine if method is basin: McGraw-Hill Book Company, Inc..
applicable: y, < 11/3; namely, low New York page 135.
tailwater. W=w +4H=5+4 4=21 feet The information on brink depth
for mild slopes and size of riprap is
Step 2. Calculate the capacity of the Acknowledgments taken from: Stevens,M.A., (1969).
pipe flowing fill: The authors express their Scour In Riprap At Culvert Outlets.
appreciation to Bill DeGroot and Bryan Ph.D. dissertation, Civil Engineering
Oh,rt =426 cfs is found using the Kohlenberg, Urban Drainage and Flood Department, Colorado State University,
Manning's Equation. Control District and Besharah Najjar. Ft. Collins. Colorado.
— Step 3. Using the 0/Of u=0.70 ratio,
Figure 2 gives the ratio d/H=0.73.
30
- Step 4. Calculate OiwHts=7.50 and m Riprap Type =
use this in Figure 3 to find d/H=0.94.
_-
25 Step 5. Since the smaller of the two
.- .. ^d/H ratios is 0.73,use it to find the
depth,d,at the outlet and in Figure 2 to
find the ratio of A/A n=0.73. •
d=0.73 .4.0=2.92 feet— Step 6 Using.4/Aj,n=0.73 ratio, 0 15
calculate the flow area and velocity at tl
the end of the pipe:
A =A/Ahm'A/m =(0.73) • (4 . 5)= '0.)-
j 10
14.6 square feet O
V=0/A =(300)/(14.6)=20.5 feet per 5
second 1 2 3 4 5 6 7 8
- Step 7. Calculate the riprap sizing Storm Sewer Diameter, D, or Height, H, in ft.
design parameter,Pd,and use it in
Figure 4 find the appropriate riprap
_ size:
Pd = (20.52+32.2 . 2.92) un =221; Figure 4: Riprap selection chart for
UseTypeMRiprap low tailwater basin at pipe outlets
Step 8. Calculate the minimum
"-a-thickness of the riprap layer for D50=
.2 inches:
T= 1.75 • 12.0=21 inches.
43
14
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