Considerations for Developing Rainfall-Runoff Models for ...
Risk Analysis Division Risk MAP Considerations for Developing Rainfall-Runoff Models for Large Watersheds Passaic River Watershed, New Jersey Presented by Paul Weberg, P.E., Senior Engineer Z. John Licsko, P.E., CFM, Dewberry Dinakar, Nimmala, CFM,Dewberry Outline Risk Analysis Division Risk MAP Study Scope Watershed Description Flood Characteristics
Model Approach Modeling Challenges Results Conclusions 2 Study Scope - Location Risk Analysis Division Risk MAP New study - Motivation Effective FEMA Study 1970s New discharge data Land use changes Study Reach 41.2 miles total 18.2 miles unsteady 23 miles steady
NJ Counties Morris, Essex, Passaic Bergen, Hudson 3 Study Scope - Watershed Risk Analysis Division Risk MAP Passaic River Length = 86 miles (study includes 41.2 miles) Drainage Area = 937 Sq.Miles 84% area in NJ, 16% area in NY 4 Major Tributaries Whippany River Rockaway River Pompton River Ramapo Wanaque Pequannock
Saddle River Numerous Storage Features (Natural Wetlands and Reservoirs) 4 Watershed Characteristics Risk Analysis Division Risk MAP Highland Areas Forested Upland Areas Natural lakes and large reservoirs. Central Basin and Lower Valley Highly Urbanized Large natural wetland Valley constriction below Pompton/ Passaic Confluence
Highly variable difference in hydrologic response of different watersheds. 5 Flooding Characteristics Risk Analysis Division Risk MAP Pompton and its tributaries peak 24-48 hrs earlier than Passaic Flow reversal of Passaic at confluence with Pompton Natural storage (Great Piece Meadows, Hatfield Swamp, etc.) Water Supply Storage (Wanaque, Charlottesburg, etc) Looped rating curves Double discharge peaks for most events, below Little Falls
100 Year SWM Criteria in Urban Areas 6 Initial Modeling Approach Risk Analysis Division Risk MAP Hydrology Single HEC-HMS Model SCS CN and Lag (Unit Hydrograph) Reach Routing (Hydrologic and Hydraulic) Hydrologic Muskingum-Cunge Modified-Puls (steady state HEC-RAS) Hydraulic (unsteady state HEC-RAS, detailed and approximate) Reservoir Routing Hydraulics Detailed Unsteady State HEC-RAS above Little Falls
Detailed Steady State HEC-RAS below Little Falls 7 Modeling Challenges Risk Analysis Division Risk MAP Applicability of SCS Hydrology Required significant increases in lag times to match gages Rainfall Distribution/Duration Rainfall Distribution Methodology Required Four day 100 event required Routing Methods Muskingum Cunge, Modified Puls Routing Hydrologic Routing and Unsteady HEC-RAS Hydraulic Routing Significant Levels of Storage Reservoirs and Wetlands
8 Final Modeling Approach Risk Analysis Division Risk MAP Hydrology/Hydraulics Modeling Groups 7 HEC-HMS Basin Models Average subbasin area = 10 Sq.miles 7 Approximate Steady State HEC-RAS models (Modified Puls) 4 Approximate Unsteady State HEC-RAS models (Tributary Storage) 1 Coupled HEC-HMS/ Detailed Unsteady State HECRAS Model 1 Detailed Steady State HEC-RAS HEC-DSS Data Management Requirements 9 Central Passaic Model Group
Risk Analysis Division Risk MAP 10 Central HEC-HMS Basin Upstream of Little Falls Risk Analysis Division Risk MAP Effective Modeling Hydrology- Gage Transfer Hydraulics - Steady State HEC-2 Proposed Revision HEC-HMS hydrology with approximate unsteady
state routing and a detailed unsteady HEC-RAS model Corrects Gage Transfer Hydrology Steady State Assumption 11 Lower Passaic Model Group Risk Analysis Division Risk MAP 12 Risk Analysis Division Risk MAP HEC-HMS Calibration 100 Year Frequency Storm 24 Hour
Basin Gage Peak (cfs) Drainage Area (square miles) Model Obs. (LPIII) Diff. (Mod/Obs) Pompton Pompton River at Pompton Plains, NJ (01388500) 355
24,401 25,480* -4.4 % Whippany Rockaway near Pine Brook, NJ (01381800) 68.5 1,788 1,870** -4.0% Rockaway River below Reservoir at Boonton, NJ (USGS 01381000)
119.0 6,196 5,595* +10.0% Passaic River near Millington, NJ (USGS 01379000) 55.4 2,959 3,365 -12.0%* Saddle River At Lodi, NJ (01391500)*
Results Risk Analysis Division Risk MAP Upstream of Little Falls preliminary 1% annual chance, water surface elevations indicate increases from 0.07 to 2.70 feet in the Effective Profile Primarily due to corrections in Gage Transfer Analysis. Floodway extent about Little Falls extends outward and is similar in extent to floodplain. Encroachment into storage areas in the unsteady model without increasing water surface by more than 0.2 feet is difficult. Downstream of Little Falls (steady State) 1% annual chance elevations are similar to or lower than the effective. 21
Risk Analysis Division Risk MAP Results Below Little Falls Discharges (cfs) Location Above confluence with Second 1 River Upstream of Beatties Dam Drainage 2 Area (mi ) FIS 1%
New %Diff FIS 0.2% New %Diff 906 30,200 30,772 2% 46,200 43,185 -7% 777.2 21,700 21,469 -1% 30,200 30,008 -1% 22 100-Yr Hydrograph on Passaic River above Second River Risk Analysis Division Risk MAP
23 Proposed Mapping Above Little Falls Risk Analysis Division Risk MAP 24 Conclusions Risk Analysis Division Risk MAP Gage transfer procedures for large watersheds need to be used carefully. Steady State Assumptions for Flood Studies need to be better supported particularly when related to storage and possible flow
reversals during 100 year events. Procedures for developing rainfall distributions for large watersheds is limited Data management requirements for large studies could benefit from familiarity with tools, such as HEC-WAT 1.0, for linking HECHMS and Unsteady HEC-RAS Models Approximate 2-D Unsteady State Modeling needed to validate the 1-D Analysis Assumptions 25 Acknowledgements Risk Analysis Division Risk MAP Alicia F. Gould & Roy Messaros (USACE, New York) USACE 1995 General Design Memorandum which included a detailed hydrologic and hydraulic analysis of the Passaic River Basin, including an digital version of the UNET hydraulic model for the Passaic River and Tributaries
Patti Wnek & Joe Ostrowkski (National Weather Service/NOAA/Middle Atlantic River Forecast Center) Alice Yeh (US EPA) Joe Ruggeri (NJ Department of Environmental Protection) Bob (Robert) Schopp, Kara Watson (NJ USGS) 26
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