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Considerations for Developing RainfallRunoff 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
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 Study Scope
 Watershed Description
 Flood Characteristics
 Model Approach
 Modeling Challenges
 Results
 Conclusions
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Study Scope - Location
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 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
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Study Scope - Watershed
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• 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)
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Watershed Characteristics
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• 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.
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Flooding Characteristics
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• 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
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Initial Modeling Approach
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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
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Modeling Challenges
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 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
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Final Modeling Approach
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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
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Central Passaic Model Group
Risk Analysis
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Central HEC-HMS Basin
Upstream of Little Falls
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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
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Lower Passaic Model Group
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HEC-HMS Calibration
100 Year Frequency Storm – 24 Hour
Basin
Pompton
Whippany
Rockaway
Upper Passaic
Saddle River
Gage
Peak (cfs)
Drainage Area
(square miles)
Model
Obs.
(LPIII)
Diff.
(Mod/Obs)
Pompton River at
Pompton Plains, NJ
(01388500)
355
24,401
25,480*
-4.4 %
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)*
54.6
5,215
5,680
-8.0%*
* Gage effected by regulation
** Looped Rating Curve – Backwater from Passaic (Hatfield Swamp)
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Flow Reversal – Upstream
Pompton/Passaic Confluence
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USGS Gage – Above Beatties
Dam at Little Falls
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Flow Reversal – Upstream
Pompton/Passaic Confluence
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Reverse Flow
(HEC-RAS River Station 141998.2)
20000
15000
Modeled Flow
10000
Flow (cfs)
5000
0
01Jan2011 0100
05Jan2011 0100
09Jan2011 0100
13Jan2011 0100
-5000
-10000
-15000
-20000
-25000
-30000
Time
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USGS Gage – Twin Bridges
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USGS Gage at Pine Brook
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Central Passaic Model Group
Final Calibration
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Central Passaic Model Group
Final Calibration
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Rainfall Areal Correction Factors (NOAA
Atlas 14 – 4 day 100 year rainfall)
Basin
Pompton Basin
Upper Passaic Basin
Whippany Basin
Rockaway Basin
Saddle Basin
Central Passaic Basin
Lower Passaic Basin
D.A. (sq. mls.)
355
99
70
137
60
103
114
10
-5%
19%
-17%
-10%
11%
0%
0%
50
-11%
9%
-29%
-18%
2%
0%
0%
100
-13%
5%
-34%
-21%
-3%
0%
0%
500
-16%
-2%
-43%
-28%
-12%
0%
0%
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Results
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 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.
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Risk Analysis
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Results Below Little Falls
Discharges (cfs)
Location
Above
confluence
with Second
1
River
Upstream of
Beatties Dam
Drainage
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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%
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100-Yr Hydrograph on Passaic
River above Second River
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Proposed Mapping Above
Little Falls
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Conclusions
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 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
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Acknowledgements
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 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)
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