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

Risk Analysis Division — Risk MAP 2 Outline  Study Scope  Watershed Description  Flood Characteristics  Model Approach  Modeling Challenges  Results  Conclusions

Risk Analysis Division — Risk MAP 3 Study Scope - Location  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

Risk Analysis Division — Risk MAP 4 Study Scope - Watershed 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)

Risk Analysis Division — Risk MAP 5 Watershed Characteristics 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.

Risk Analysis Division — Risk MAP 6 Flooding Characteristics Pompton and its tributaries peak 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

Risk Analysis Division — Risk MAP 7 Initial Modeling Approach 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

Risk Analysis Division — Risk MAP 8 Modeling Challenges  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

Risk Analysis Division — Risk MAP 9 Final Modeling Approach 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 HEC- RAS Model  1 Detailed Steady State HEC-RAS  HEC-DSS Data Management Requirements

Risk Analysis Division — Risk MAP 10 Central Passaic Model Group

Risk Analysis Division — Risk MAP 11 Central HEC-HMS Basin Upstream of Little Falls 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

Risk Analysis Division — Risk MAP 12 Lower Passaic Model Group

Risk Analysis Division — Risk MAP 13 HEC-HMS Calibration 100 Year Frequency Storm – 24 Hour BasinGage Drainage Area (square miles) Peak (cfs) Diff. (Mod/Obs) Model Obs. (LPIII) PomptonPompton River at Pompton Plains, NJ ( ) 35524,401 25,480*-4.4 % WhippanyRockaway near Pine Brook, NJ ( ) ,7881,870** -4.0% RockawayRockaway River below Reservoir at Boonton, NJ (USGS ) ,196 5,595* +10.0% Upper PassaicPassaic River near Millington, NJ (USGS ) ,959 3, %* Saddle RiverSaddle River At Lodi, NJ ( )* ,2155, %* * Gage effected by regulation ** Looped Rating Curve – Backwater from Passaic (Hatfield Swamp)

Risk Analysis Division — Risk MAP 14 Flow Reversal – Upstream Pompton/Passaic Confluence

Risk Analysis Division — Risk MAP 15 USGS Gage – Above Beatties Dam at Little Falls

Risk Analysis Division — Risk MAP 16 Flow Reversal – Upstream Pompton/Passaic Confluence

Risk Analysis Division — Risk MAP 17 USGS Gage – Twin Bridges

Risk Analysis Division — Risk MAP 18 USGS Gage at Pine Brook

Risk Analysis Division — Risk MAP 19 Central Passaic Model Group Final Calibration

Risk Analysis Division — Risk MAP 20 Central Passaic Model Group Final Calibration Rainfall Areal Correction Factors (NOAA Atlas 14 – 4 day 100 year rainfall)

Risk Analysis Division — Risk MAP 21 Results  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.

Risk Analysis Division — Risk MAP 22 Results Below Little Falls

Risk Analysis Division — Risk MAP Yr Hydrograph on Passaic River above Second River

Risk Analysis Division — Risk MAP 24 Proposed Mapping Above Little Falls

Risk Analysis Division — Risk MAP 25 Conclusions  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 HEC- HMS and Unsteady HEC-RAS Models  Approximate 2-D Unsteady State Modeling needed to validate the 1-D Analysis Assumptions

Risk Analysis Division — Risk MAP 26 Acknowledgements  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)