received planning division
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Water Quantity
The proposed improvements add approximately 652 square feet of impervious area. Using the City of
Beaverton Engineering Design Manual rainfall values and the SBUH runoff method. Table 3 shows the
following flow rates and volumes generated from the additional impervious area
Table 2: Flow rates and Volumes
Storm Event Flow rate (cfs) Volume (cf) Volume (acre-ft)
2-year, 24hr 0.005 66.6 0.0015
10-year, 24hr 0.007 95.8 0.0022
25-year, 24hr 0.008 110 0.0025
100-year, 24hr 0.009 125 0.0028
The original Otak report showed hydrographs and available storage for each storm event. Table 3
summarizes the findings from their report and our additional storm volume.
Table 3: Capacities
Storm Event Original Storm
Volume* (acre-ft)
Additional
Volume (acre-ft)
Total Storm
Volume (acre-ft)
Available Storage*
(acre-ft)
2-year, 24hr 0.281 0.0015 0.2825 0.568
10-year, 24hr 0.367 0.0022 0.3692 0.568
25-year, 24hr 0.414 0.0025 0.4165 0.568
100-year, 24hr 0.436 0.0028 0.4388 0.568
*Based on the Otak Storm Report Hydrographs – See Appendix 3
The flow rates and volumes shown above are negligible and the existing facility can handle the
increased flow rates and volumes. The Otak report also explains the pond was modeled with a bottom
of pond elevation set at 207 instead of as-built elevation of 206.6. See Appendix 2 for Hydrographs.
Hydromodification
Redevelopment code section 4.03.2 of CWS R&O 19-22 - CWS section 4.03.2a states that if the project
results in the addition and/or modification of less than 12,000 square feet of impervious surface, a
Hydromodification assessment is not required. The new impervious area of 652 square feet and the
3,148 square feet of modified impervious area gives us a total area of 3,800 square feet.
Table 4-2 from CWS R&O 19-22 lists the Approach Project Categories for Hydromodification. The
proposed project falls under Category 1 because the project is less than 12,000 square feet. Under this
Category, a Hydromodification Fee-In-Lieu payment can be made at $1.00 per square foot of
unmanaged impervious area.
Conclusion
The proposed improvements do not warrant new water quality or water quantity requirements. The
existing extended dry basin facility designed by Otak can handle the new impervious area. For
Hydromodification, the new 652 square feet of impervious area shall be paid-in-lieu.
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5/7/2021
5/7/2021
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DISTURBED AREAS
MODIFIED IMPERVIOUS (SF) 3,148
NEW IMPERVIOUS (SF)
652
WATER QUALITY FLOW (CFS)
0.0013
WATER QUALITY TREATMENT
VOLUME (CF)
19
LEGEND
NEW IMPERVIOUS AREA
MODIFIED IMPERVIOUS AREA
NOTE: ALL MODIFIED IMPERVIOUS WAS ORIGINALLY
TREATED IN THE BASIN 2 POND DESIGN. SEE OTAK
TECHNICAL MEMO CALCULATIONS.
Hydrograph Report
Hydraflow Hydrographs Extension for AutoCAD® Civil 3D® 2018 by Autodesk, Inc. v2018.3 Friday, 05 / 7 / 2021
Hyd. No. 1
New Impervious
Hydrograph type = SBUH Runoff Peak discharge = 0.005 cfsStorm frequency = 2 yrs Time to peak = 7.90 hrsTime interval = 2 min Hyd. volume = 67 cuftDrainage area = 0.008 ac Curve number = 98Basin Slope = 0.0 % Hydraulic length = 0 ftTc method = User Time of conc. (Tc) = 5.00 minTotal precip. = 2.50 in Distribution = Type IAStorm duration = 24 hrs Shape factor = n/a
1
0 2 4 6 8 10 12 14 16 18 20 22 24 26
Q (cfs)
0.00 0.00
0.01 0.01
0.02 0.02
0.03 0.03
0.04 0.04
0.05 0.05
0.06 0.06
0.07 0.07
0.08 0.08
0.09 0.09
0.10 0.10
Q (cfs)
Time (hrs)
New Impervious
Hyd. No. 1 -- 2 Year
Hyd No. 1
Hydrograph Report
Hydraflow Hydrographs Extension for AutoCAD® Civil 3D® 2018 by Autodesk, Inc. v2018.3 Friday, 05 / 7 / 2021
Hyd. No. 1
New Impervious
Hydrograph type = SBUH Runoff Peak discharge = 0.006 cfsStorm frequency = 5 yrs Time to peak = 7.90 hrsTime interval = 2 min Hyd. volume = 84 cuftDrainage area = 0.008 ac Curve number = 98Basin Slope = 0.0 % Hydraulic length = 0 ftTc method = User Time of conc. (Tc) = 5.00 minTotal precip. = 3.10 in Distribution = Type IAStorm duration = 24 hrs Shape factor = n/a
2
0 2 4 6 8 10 12 14 16 18 20 22 24 26
Q (cfs)
0.00 0.00
0.01 0.01
0.02 0.02
0.03 0.03
0.04 0.04
0.05 0.05
0.06 0.06
0.07 0.07
0.08 0.08
0.09 0.09
0.10 0.10
Q (cfs)
Time (hrs)
New Impervious
Hyd. No. 1 -- 5 Year
Hyd No. 1
Hydrograph Report
Hydraflow Hydrographs Extension for AutoCAD® Civil 3D® 2018 by Autodesk, Inc. v2018.3 Friday, 05 / 7 / 2021
Hyd. No. 1
New Impervious
Hydrograph type = SBUH Runoff Peak discharge = 0.007 cfsStorm frequency = 10 yrs Time to peak = 7.90 hrsTime interval = 2 min Hyd. volume = 96 cuftDrainage area = 0.008 ac Curve number = 98Basin Slope = 0.0 % Hydraulic length = 0 ftTc method = User Time of conc. (Tc) = 5.00 minTotal precip. = 3.50 in Distribution = Type IAStorm duration = 24 hrs Shape factor = n/a
3
0 2 4 6 8 10 12 14 16 18 20 22 24 26
Q (cfs)
0.00 0.00
0.01 0.01
0.02 0.02
0.03 0.03
0.04 0.04
0.05 0.05
0.06 0.06
0.07 0.07
0.08 0.08
0.09 0.09
0.10 0.10
Q (cfs)
Time (hrs)
New Impervious
Hyd. No. 1 -- 10 Year
Hyd No. 1
Hydrograph Report
Hydraflow Hydrographs Extension for AutoCAD® Civil 3D® 2018 by Autodesk, Inc. v2018.3 Friday, 05 / 7 / 2021
Hyd. No. 1
New Impervious
Hydrograph type = SBUH Runoff Peak discharge = 0.008 cfsStorm frequency = 25 yrs Time to peak = 7.90 hrsTime interval = 2 min Hyd. volume = 110 cuftDrainage area = 0.008 ac Curve number = 98Basin Slope = 0.0 % Hydraulic length = 0 ftTc method = User Time of conc. (Tc) = 5.00 minTotal precip. = 4.00 in Distribution = Type IAStorm duration = 24 hrs Shape factor = n/a
4
0 2 4 6 8 10 12 14 16 18 20 22 24 26
Q (cfs)
0.00 0.00
0.01 0.01
0.02 0.02
0.03 0.03
0.04 0.04
0.05 0.05
0.06 0.06
0.07 0.07
0.08 0.08
0.09 0.09
0.10 0.10
Q (cfs)
Time (hrs)
New Impervious
Hyd. No. 1 -- 25 Year
Hyd No. 1
Hydrograph Report
Hydraflow Hydrographs Extension for AutoCAD® Civil 3D® 2018 by Autodesk, Inc. v2018.3 Friday, 05 / 7 / 2021
Hyd. No. 1
New Impervious
Hydrograph type = SBUH Runoff Peak discharge = 0.009 cfsStorm frequency = 100 yrs Time to peak = 7.90 hrsTime interval = 2 min Hyd. volume = 125 cuftDrainage area = 0.008 ac Curve number = 98Basin Slope = 0.0 % Hydraulic length = 0 ftTc method = User Time of conc. (Tc) = 5.00 minTotal precip. = 4.50 in Distribution = Type IAStorm duration = 24 hrs Shape factor = n/a
5
0 2 4 6 8 10 12 14 16 18 20 22 24 26
Q (cfs)
0.00 0.00
0.01 0.01
0.02 0.02
0.03 0.03
0.04 0.04
0.05 0.05
0.06 0.06
0.07 0.07
0.08 0.08
0.09 0.09
0.10 0.10
Q (cfs)
Time (hrs)
New Impervious
Hyd. No. 1 -- 100 Year
Hyd No. 1
Hydraflow Rainfall Report6
Hydraflow Hydrographs Extension for AutoCAD® Civil 3D® 2018 by Autodesk, Inc. v2018.3 Friday, 05 / 7 / 2021
Return Intensity-Duration-Frequency Equation Coefficients (FHA)Period
(Yrs) B D E (N/A)
1 0.0000 0.0000 0.0000 --------
2 11.9593 9.9000 0.7942 --------
3 0.0000 0.0000 0.0000 --------
5 27.1103 13.8000 0.9192 --------
10 25.8093 12.4000 0.8784 --------
25 20.9855 9.6000 0.8015 --------
50 24.5728 9.9000 0.8123 --------
100 33.6610 11.5000 0.8535 --------
File name: portland.IDF
Intensity = B / (Tc + D)^E
Return Intensity Values (in/hr)Period
(Yrs) 5 min 10 15 20 25 30 35 40 45 50 55 60
1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
2 1.40 1.11 0.93 0.80 0.71 0.64 0.58 0.54 0.50 0.46 0.43 0.41
3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
5 1.83 1.47 1.23 1.07 0.94 0.84 0.76 0.70 0.64 0.59 0.55 0.52
10 2.10 1.68 1.41 1.22 1.07 0.96 0.87 0.80 0.74 0.68 0.64 0.60
25 2.45 1.93 1.61 1.39 1.23 1.10 1.00 0.92 0.85 0.79 0.74 0.70
50 2.74 2.16 1.80 1.55 1.37 1.23 1.12 1.03 0.95 0.88 0.83 0.78
100 3.08 2.45 2.05 1.77 1.56 1.40 1.27 1.16 1.08 1.00 0.94 0.88
Tc = time in minutes. Values may exceed 60.
Rainfall Precipitation Table (in)
Precip. file name: portland.pcp
StormDistribution 1-yr 2-yr 3-yr 5-yr 10-yr 25-yr 50-yr 100-yr
SCS 24-hour 0.00 2.50 0.00 3.10 3.50 4.00 0.00 4.50
SCS 6-Hr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Huff-1st 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Huff-2nd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Huff-3rd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Huff-4th 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Huff-Indy 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Custom 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
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