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National Weather Service Reservoir Model Simulations For The June 2006 Delaware River Basin Flood

The National Weather Service (NWS) originally presented the results of its reservoir model simulations for the June 2006 flood at the August 22, 2007 DRBC Flood Advisory Committee meeting. In this modeling study, NWS ran the "no reservoir" case downstream to Trenton, N.J.; the other partial void cases were run downstream to Barryville, N.Y.

Following the August committee meeting, DRBC staff reached out to NWS staff to inquire whether it would be possible to model the simulated flows from the partial void cases further downstream. After considering the limitations on the modeling and the ability to provide good estimates for the partial reservoir void scenarios downstream to Trenton for the June 2006 flood, the model simulations were re-run by NWS staff and their findings are presented here as a report as well as a powerpoint presentation.

The NWS identified the following limits on application of results:

  1. "The results are hypothetical cases based on hydrometeorological conditions prior to and during the June 2006 event."
  2. "This modeling effort is strictly hypothetical in that, among other things, the void conditions analyzed do not take into consideration either New York City's water supply needs or the water supply needs of the lower basin parties who may prefer to have water stored in the reservoirs for releases at a later point in time."
  3. "In addition, the scenarios modeled do not reflect the City's release obligations under the 1954 Supreme Court Decree governing operations of the reservoirs."

The NWS concluded:

  1. "For both events [April 2005 and June 2006 floods], the Upper Delaware basin crest reductions due to the presence of the Cannonsville and Pepacton reservoirs ranged from 0.9 to 2.2 feet while lower basin crest reductions ranged from 0.5 to 1.5 feet. These reservoirs attenuated flood peaks downstream even though they spilled, so their mere presence was beneficial despite having any additional storage capacity."
  2. "A 'No Spill' scenario for the June 2006 event would have required a massive pre-storm drawdown of pool levels of 41.93 ft (53.9 billion gallons) at Cannonsville reservoir and 19.95 ft (34.5 billion gallons) at Pepacton reservoir. Such an unrealistic drawdown would hypothetically yield a crest reduction of 2.0 to 10.3 ft on the Upper Delaware, and 1.6 to 3.1 ft on the Lower Delaware."
  3. "For both events, the magnitude of the flood mitigation provided by the dams (even when they spilled) was greater than or equal to the additional benefit that would have been provided by voids of 5 bg or less."
  4. "Voids, if possible, would have provided some additional attenuation of downstream flood peaks."
  5. "Comparing the June 2006 and April 2005 events shows that voids up to 5 billion gallons in each reservoir would have provided a similar reduction in downstream crests. Voids of 10 billion gallons or voids large enough to prevent the reservoirs from spilling at all would provide differing degrees of downstream peak reduction, based on the characteristics of the specific hydrometeorological event. Using specific reservoir void targets thus would not yield the same level of flood mitigation for every event."
  6. "The case study results presented here, while demonstrating the potential benefits of reservoir voids, are insufficient for optimizing flood mitigation plans for reservoirs in the Delaware basin. A detailed modeling analysis is needed that takes into account all large reservoirs; their release capabilities; limitations due to their hydropower, water supply, and other obligations; and the full range of historical and potential future hydrometerological conditions."

View NWS Report (pdf 145 KB)

View NWS Powerpoint Presentation (pdf 973 KB)

Development of the detailed model referred to by the NWS in its last conclusion, and recommended by the Interstate Flood Mitigation Task Force, is now underway.

Note: Usable storage capacities for Cannonsville and Pepacton reservoirs are approximately 96 billion gallons and 140 billion gallons, respectively. Maximum possible release rates with full reservoirs are 1.40 billion gallons per day for Cannonsville and 0.45 billion gallons per day for Pepacton (data provided by New York City Department of Environmental Protection).