Reservoir Simulations for the
Delaware River Basin Flood
of June 2006

Middle Atlantic River Forecast Center
State College, PA
August 2007 (Revised 9/13/07)

Introduction:
In the late spring of 2005, the Delaware River Basin Commission (DRBC) in West Trenton, NJ asked the National Weather Service (NWS) Middle Atlantic River Forecast Center (MARFC) to perform some model simulations using their operational forecast system to examine the effects of spilling reservoirs during the April 2-4, 2005 major flood on the Delaware River. Initial results were presented at a public officials meeting on May 25, 2005 and demonstrated that even though the reservoirs spilled during this event, they reduced downstream flood crests.

During the fall of 2005, DRBC asked MARFC if it could run additional hypothetical simulations to examine the impacts various voids in the New York City water supply reservoirs would have had on April 2-4 flood crests on the Upper Delaware. Results from those simulations shed some light on the impact those reservoirs had at downstream locations during the flood event, or would have had given varying amounts of storage.

In late June 2006, another major flood event affected the Upper Delaware Basin as a nearly stationary frontal boundary interacted with deep tropical moisture over a several day period. The result was episodes of heavy rainfall which led to moderate to major flooding throughout the Upper Delaware Basin on June 26th and June 27th. With another major flood event in just over a year’s time from the April 2005 event, MARFC decided to see what impacts the Cannonsville and Pepacton water supply reservoirs had on this event, and to see how these results compared with those from the April 2005 event. This exercise was done primarily as a learning experience.

During the June 26-27 event, major flooding was observed at many locations throughout the basin. This report addresses in detail the effects of two large dams affecting the watershed: Cannonsville and Pepacton. MARFC currently models inflows to these two reservoirs as well as the Neversink reservoir. Other reservoirs in the basin, such as Rio and Wallenpaupack are not currently modeled by MARFC.


Methods:

For Cannonsville and Pepacton, MARFC ran five hypothetical model simulations for the cases of no reservoir, three different reservoir voids, and no spill, to show the effects of these hypothetical scenarios on river levels at ten downstream NWS forecast points. Summaries of the simulation results are shown in the tables in this report. The numbers in the tables indicate what effect the particular case had on the simulated crest/flow in comparison to the actual crest/flow at the given forecast point.

All model simulations done for this report begin at 8am June 22nd, prior to the heavy rain that caused the June 26-27 major flood.

To run these cases, MARFC first set up its forecast model with all initial conditions that were present at 8am June 22, prior to the flood event. The original “actual event” case was run first to coordinate all relevant information including maximum pool levels at Cannonsville and Pepacton and crests at the ten NWS forecast points (Hale Eddy, Fishs

Eddy, Callicoon, Barryville, Port Jervis, Montague, Tocks Island, Belvidere, Riegelsville and Trenton) which will be used for these scenarios. Once this information had been verified, the model was then altered or modified to fit each of the case scenarios described below and re-run. These scenario case runs were then compared with the “actual event” case.

For Case 1, both dams were virtually removed from the model. All modeled inflow into the Cannonsville and Pepacton pools was merely passed as outflow with no lag. Comparing this case to what actually happened provides useful information on how much the dams actually reduced the downstream flow and flood crests during this event despite being full.

For Cases 2, 3 and 4, MARFC set the 8am June 22nd model pool elevations to generate hypothetical voids of about 2.5, 5, and 10 billion gallons at each of the two reservoirs (Table 1). The main effect from these void cases is the delayed timing of reservoir spillage which allows a portion of the unregulated crests for these simulations to pass downstream forecast points before reservoir contributions arrive.

For Case 5, MARFC set outflows for Cannonsville and Pepacton to zero, which in effect held back all contributions to the downstream crest. While this no spill scenario is totally unrealistic for this event, for smaller storms or when reservoir levels are lower, these reservoirs often do hold back all the runoff. This case was run so that the hypothetical intermediate void cases 2, 3, and 4 could be compared against both theoretical extremes, a “no dams” scenario (Case 1) and “no spill” scenario (Case 5).

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