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2

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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APPENDIX 1 Project Areas

SDx SDx SDx SDx SDx

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SD SD SD SD SD SD SD SD

COMPUTER

LAB

G114

NEW

CLASSROOM

G102

BOILER AND

MECHANICAL

ROOM

F112

NEW

CLASSROOM

G104

NEW

CLASSROOM

G108

NEW

CLASSROOM

G106

WORK ROOM

G112

CIRCULATION

G100

LIBRARY

G110

SHEET NUMBER

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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.

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APPENDIX 2 Hydraflow Hydrographs

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


Hyd. No. 1

New Impervious


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

Hydrograph Report


Hyd. No. 1

New Impervious


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

Hydrograph Report


Hyd. No. 1

New Impervious


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

Hydrograph Report


Hyd. No. 1

New Impervious


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

Hydraflow Rainfall Report6


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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APPENDIX 3 Otak Report

Documents

Received Planning Division