Introduction to Continuous Simulation Hydrologic Modeling Kevin Timmins, PE Presented to the: APWA Oregon - Water Resource Committee Portland, OR May 29, 2015 Existing Flow Duration Standards Review Design Storm Methodology Limitations with Design Storm Methodology Continuous Simulation Terminology Review Continuous Simulation Methodology Benefits of a Flow Duration Standard Currently Available Tools in Oregon Building T.R.U.S.T. Outline Existing Flow Duration Standards 1
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: 1. 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 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 City of Tigard (River Terrace Area) Water Quantity Design Standard. Stormwater discharges shall maintain the duration of high flows at their pre-development levels for all flows greater than one-half of the 2- year peak flow to the 10-year peak flow and maintain the 25year peak flow rate to the predevelopment 25-year peak flow rate. 2
City of Wilsonville 10. Flow Control Requirement: The duration of peak flow rates from postdevelopment conditions shall be less than or equal to the duration of peak flow rates from pre-development conditions for all peak flows between 42% of the 2- year storm peak flow rate1 up to the 25-year peak flow rate, based on a 2- through 25-year, 24-hour return storm. Specifically, the 2-year postdevelopment runoff rates shall not exceed 42% of the 2-year predevelopment runoff rates; the 10- and 25-year post-development runoff rates shall not exceed the respective 10- and 25- year predevelopment runoff rates. A hydrologic/hydraulic analytical model capable of performing a continuous simulation of peak flow rates based on local rainfall data as presented in the Design Storm Distribution Chart (Table 3.3) shall be used to determine the peak flow rates, recurrence intervals and durations. The BMP Sizing Tool addresses flow control requirements for 42% of the 2-year to 10-year storm events to size stormwater management facilities. For the 25-year storm event the design engineer shall refer to Section 301.1.05.h, Stormwater Systems Design Criteria Review Design Storm Methodology Design Storm Based Design Preliminary Site Layout Characterize Site & Assemble Input Data for Calculations Design Storm Calculations to Calculate Storage Final Grading Land Use Approval Process Update Calculations to Check Design Preliminary Grading Update Calculations to Check Design Final Design Construction 3
Design Storm Based Design Preliminary Site Layout Characterize Site & Assemble Input Data for Calculations Design Storm Calculations to Calculate Storage Final Grading Land Use Approval Process Update Calculations to Check Design Preliminary Grading Update Calculations to Check Design Final Design Construction Design Storm Calculations INPUT Drainage Basin Area Total 24-hour Precipitation Curve Number Time of Concentration OUTPUT Runoff Volume Peak Flow Rate Storage Volume Flow Control Structure Design DESIGN STORM CALCULATIONS SBUH SCS Rational Limitations with Design Storms 4
Observations So Many Small Ponds - Difficult to Maintain Ponds Were Overtopping Too Frequently What is the antecedent moisture condition? How full is the pond when the design storm hits? Downstream Flooding Downstream Erosion Increased Sediment Transport 1) Point Discharge Concentrates Runoff Problems Inherent in Design Storm Methodology 2) Clear water is discharged from the pond 3) Guess-timating Initial Conditions a. What is the Antecedent Moisture Condition? b. How full is the Pond? 4) Target Discharges Over Predicted 5) Erosive Flows are sustained for a longer duration Erosive Flows Sustained An erosive peak flow rate that is sustained for a longer duration will result in more erosion 5
1) Point Discharge Concentrates Runoff Problems Inherent in Design Storm Methodology 2) Clear water is discharged from the pond 3) Guess-timating Initial Conditions a. What is the Antecedent Moisture Condition? b. How full is the Pond? 4) Target Discharges Over Predicted 5) Erosive Flows are sustained for a longer duration Continuous Simulation Terminology 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 Erosive Flow Range 6
What is Flow Duration Based Design? Duration Implies an Understanding of Time Long-term Steam Gauge Data would be Ideal Duration of Erosive Flows 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 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) Peak Precipitation Events in Design Storm Methodology Pre-Developed Flow Frequency Results Establish the Lower and Upper Limits for the Flow Duration Analysis Peak Flows 7
Flow Duration Analysis The Percent of Time Flow Exceeds a Specific Flow Value Timeseries Flow Duration Curve Review Continuous Simulation Methodology Flow Duration Based Design Preliminary Site Layout Characterize Site & Assemble Input Data for Calculations Continuous Simulation Calculations to Calculate Storage Final Grading Land Use Approval Process Update Calculations to Check Design Preliminary Grading Update Calculations to Check Design Final Design Construction 8
Continuous Simulation Calculations INPUT Drainage Basin Area Long-term Precipitation Long-term Evaporation Land Segment Data HSPF Calibration Coefficients OUTPUT Runoff Timeseries Peak Flow Rate Flow Duration Storage Volume Flow Control Structure Design CONTINUOUS SIMULATION CALCULATIONS (HSPF) Benefits of a Flow Duration Standard 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 9
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 River Terrace Example Existing wetland provides storage w/ Flow through LIDA Pond Volumes generally 150% to 250% larger Inundation of the facility Design of Vegetation Where to Allow Amenity Features Return Period Stage Stage Duration 2-year 10
2-year 3% 12% Currently Available Tools in Oregon 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 11
The WES BMP Sizing Tool Supports a Design Emphasis on use of LID Facilities Models LID Facilities Designed Per Local Standards 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 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 license is limited to use in Clean Water Services district boundary TRUST Simplifies Continuous Simulation INPUT Precipitation Evaporation Drainage Area Land Segments BMP s TRUST HSPF Compare Flow Duration Adjust Inputs OUTPUT Design Results TRUST 12
Building 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 Start a TRUST Model TRUST model schematic Each Element is Visually Represented on the Schematic Screen with all of the Linkages Between Elements Shown 13
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) Land Use Elements 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 Routing & LIDA Elements TRUST is good Having 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 Having TRUST helps the agencies: Review water quantity facility design Learn a single source and format of modeling output TRUST Results In River Terrace, Pond Volumes generally 150% to 250% larger One pond in an existing wetland eliminated with Flow through LIDA 14
TRUST is good Having 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 Having TRUST helps the agencies: Review water quantity facility design Learn a single source and format of modeling output TRUST Results In River Terrace, It s Time for Continuous Simulation Kevin Timmins, PE Principal / Water Resource Engineer Otak, Inc. kevin.timmins@otak.com 503.415.2340 www.otak.com 15