1. ODOT 2015 Geo-Environmental Conference
“Design of Storm Water Quantity Facilities Using Flow Duration Matching”
Kevin Timmins, PE
Joe Brascher
Presented to the:
ODOT 2015 Geo-Environmental Conference
Eugene, OR
April 22, 2015

2. Outline Existing Flow Duration Standards
Review Design Storm Methodology
Continuous Simulation Terminology
Review Continuous Simulation Methodology
Benefits of a Flow Duration Standard
Currently Available Tools in Oregon

3. Existing Flow Duration Standards

4. FAHP Stormwater Requirements
22. Stormwater Management
g. All stormwater flow control treatment practices and facilities must be designed to maintain the frequency and duration of flows generated by storms within the following end-points:
i. Lower discharge endpoint, by USGS flood frequency zone:
Western Region = 42% of 2-year event
2. Eastern Region
a. Southeast, Northeast, North Central = 48% of 2-year event
b. Eastern Cascade = 56% of 2-year event
ii. Upper discharge endpoint
1. Entrenchment ratio <2.2 = 10-year event, 24-hour storm
2. Entrenchment ratio >2.2 = bank overtopping event

5. SLOPES V Transportation
36. Actions Requiring Stormwater Management f.
viii. All stormwater flow control treatment practices and facilities will be designed to maintain the frequency and duration of instream flows generated by storms within the following end-points:
1. Lower discharge endpoint, by USGS flood frequency zone:
a. Western Region = 42% of 2-year event
b. Eastern Region
i. Southeast, Northeast, North Central = 48% of 2-year event
ii. Eastern Cascade = 56% of 2-year event
2. Upper discharge endpoint
1. Entrenchment ratio <2.2 = 10-year event, 24-hour storm
2. Entrenchment ratio >2.2 = bank overtopping event

6. Review Design Storm Methodology

7. Design Storm Based Design
Preliminary
Site
Layout
Characterize Site
& Assemble
Input Data
for Calculations
Design Storm
Calculations to
Calculate Storage
Preliminary
Grading
Land Use
Approval
Process
Update Calculations
to Check Design
Final
Grading
Update Calculations
to Check Design
Construction
Final
Design

8. Design Storm Based Design
Preliminary
Site
Layout
Characterize Site
& Assemble
Input Data
for Calculations
Design Storm
Calculations to
Calculate Storage
Preliminary
Grading
Land Use
Approval
Process
Update Calculations
to Check Design
Final
Grading
Update Calculations
to Check Design
Construction
Final
Design

9. Design Storm Calculations
INPUT
Drainage Basin Area
Total 24-hour Precipitation
Curve Number
Time of Concentration
DESIGN STORM
CALCULATIONS
SBUH
SCS
Rational
OUTPUT
Runoff Volume
Peak Flow Rate
Storage Volume
Flow Control Structure Design
These two steps are the only part that changes.

10. Continuous Simulation Terminology

11. Erosive Flow Range Transports the Most Sediment Causes Erosion
Discharge “terminology”
Channel Shaping
Channel Forming
Effective
Dominant
Threshold
Stream Specific
Between 1-year and 5-year Return Period

12. Duration of Erosive Flows
What is Flow Duration Based Design?
Duration Implies an Understanding of Time
Long-term Steam Gage Data would be Ideal

13. Timeseries
Definition - "A series of chronologically ordered values giving a discrete representation of the variation in time of a given quantity."  from HSPF manual.
PRECIPITATION
FLOW

14. Peak Flows
Peak Flows Based on Flow Frequency Analysis Annual Peak Flow Values
Log Pearson Type III Distribution (following Bulletin 17B)
Largest Flow Value During the Water Year (10/1 thru 9/30)
Peaks Precipitation Events in Design Storm Methodology
Pre-Developed Flow Frequency Results Establish the Lower and Upper Limits for the Flow Duration Analysis

15. Flow Duration Analysis
The Percent of Time Flow Exceeds a Specific Flow Value
Timeseries
Flow Duration Curve

16. Review Continuous Simulation Methodology

17. Flow Duration Based Design
Preliminary
Site
Layout
Characterize Site
& Assemble
Input Data
for Calculations
Continuous Simulation
Calculations to
Calculate Storage
Preliminary
Grading
Land Use
Approval
Process
Update Calculations
to Check Design
Final
Grading
These two steps are the only part that changes.
Update Calculations
to Check Design
Construction
Final
Design

18. Continuous Simulation Calculations
INPUT
Drainage Basin Area
Long-term Precipitation
Long-term Evaporation
Land Segment Data
HSPF Calibration Coefficients
CONTINUOUS
SIMULATION
CALCULATIONS
(HSPF)
OUTPUT
Runoff Timeseries
Peak Flow Rate
Flow Duration
Storage Volume
Flow Control Structure Design
These two steps are the only part that changes.

19. Benefits of a Flow Duration Standard

20. Benefits Computes the Entire Hydrologic Cycle for Multiple Years
Model Changes in Moisture Conditions
Consider More Than Individual (hypothetical) Events
Model Calculates Pond Filling and Draining
Surface Runoff + Interflow
The alternative and state of the practice is continuous simulation and flow duration based design for water quantity management

21. Benefits
Multiple-year Hourly Historic Rainfall Used to Generate Multiple-year Hourly Runoff Data
Timeseries Results to Perform:
Flood Frequency Analysis
Flow Duration Analysis
Drawdown Analysis
Low Flow Periods

22. Currently Available Tools in Oregon

23. Two Existing Tools WES BMP Sizing Tool
Clackamas County Water Environment Services
City of Wilsonville
City of Oregon City
Tualatin River Urban Stormwater Tool (T.R.U.S.T.)
Clean Water Services District Boundary
City of Tigard, River Terrace Area
Both are Based on Hydrologic Simulation Program Fortran (HSPF) Model

24. The WES BMP Sizing Tool Intended for Design of LID Facilities
Programmed w/ Pre-run Results from a Continuous Simulation HSPF Model (does not run HSPF)
Water Quantity Detention Facility for Stormwater Not Managed by LID Facilities
Detention Facility Sized to Match Flow Duration Curves Between 42% of 2-year through 10-year
Only Models LID Facilities Designed Per Local Standards

25. Tualatin River Urban Stormwater Tool (T.R.U.S.T)
Similar to the WWHM, BAHM, SDHM, CCHM
TRUST Helps the User:
Build and run a continuous simulation model using HSPF
Model the water quantity benefits of water quality facilities
Design water quantity facilities to achieve a flow duration standard (upper and lower threshold can be changed)
Free Software, but Limited to Use in Clean Water Services District Boundary

26. TRUST Simplifies Continuous Simulation
INPUT
Precipitation
Evaporation
Drainage Area
Land Segments
BMP’s
TRUST
Compare Flow Duration
Adjust Inputs
HSPF
These two steps are the only part that changes.
OUTPUT
Design Results
TRUST

27. Build a TRUST Model User Locates Project on Tualatin Basin Map
Predevelopment and Mitigated Flows are Compared at Point of Compliance (POC)
Mitigated Flows are Not Allowed to Exceed Flow Control Flow Duration Standard

28. Build a TRUST model
Each Element is Visually Represented on the Schematic Screen with all of the Linkages Between Elements Shown

29. Build a TRUST model
Drainage Areas Divided into Impervious & Pervious Land Segments
Pervious Land Segments Based on Combination:
Vegetation (Forest, Pasture, Lawn)
Land Slope (Flat, Moderate, Steep)
Hydrologic Soil Group (A,B,C,D)

30. Build a TRUST Model User Inputs Proposed Mitigation Measure(s)
Pervious and Impervious Segments are Linked to Mitigation Measures
Traditional Stormwater Facilities
LID Stormwater Facilities
Elements Available for the User to Model Almost any Type of BMP

31. It’s Time for Continuous Simulation

32. Contact Info
Kevin Timmins, PE Principal / Water Resource Engineer Otak, Inc. 808 SW Third Avenue, Suite 300 Portland, OR
Joe Brascher President/CEO Clear Creek Solutions Inc