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Delaware River Basin Releases Results of Flood Analysis Model

The Delaware River Basin Commission this week reported that its review of the 2004, 2005, and 2006 storm events utilizing the new Delaware River Basin Flood Analysis Model demonstrate that widespread river flooding would have occurred in each instance regardless of the pre-event storage condition in the upper basin reservoirs.

The findings were announced by DRBC staff at a public meeting of the Delaware River Basin Interstate Flood Mitigation Task Force held in Flemington, N.J.

“The results of the flood analysis computer model developed by a federal interagency team for the commission, as well as a review of inundation mapping and structural surveys prepared by the U.S. Army Corps of Engineers, indicate that operational changes to reservoirs alone will not substantially reduce flooding if we experience storms similar to the three major events in September 2004, April 2005, and June 2006,” DRBC Executive Director Carol R. Collier said. “We believe the results support the earlier conclusion of the Interstate Flood Mitigation Task Force that no single approach will eliminate flooding along the Delaware River and that we must continue to focus efforts on implementing a combination of flood loss reduction strategies.”
Of the 13 reservoirs simulated in the model upstream of Trenton, N.J., five did not spill during any of the three storm events and thus can provide no further flood mitigation with additional pre-event void space.

For the eight reservoirs that did spill, the model results indicate that larger pre-event voids could have reduced flood crests, but much of the main stem would still have remained in the National Weather Service designated moderate or major flood stage.

The amount of reduction in the flood crest that could potentially be achieved with pre-event voids was found to depend upon the characteristics of the storm event (path, precipitation intensity, duration, timing, and antecedent soil saturation) as well as on proximity to the reservoir, stream channel characteristics, and local topography.

The flood analysis model was used by DRBC staff to predict river stages for six hypothetical pre-event reservoir conditions for each of the three storms. In each scenario, river stages were simulated at nine NWS Advanced Hydrologic Prediction Service flood forecast points.

The model predicted that with the three New York City reservoirs hypothetically empty preceding each of the three storm events, river elevations would still have reached or exceeded flood stage at all but three of the AHPS forecast locations where flooding actually occurred.

These three locations were on tributaries a short distance below the reservoirs: Hale Eddy, N.Y in 2005, Harvard, N.Y. in 2004/2005, and Bridgeville, N.Y. in 2006. The model predicted that with partial voids, river elevations would still have reached or exceeded flood stage at all but two of the AHPS forecast locations that experienced flooding – again, at Harvard with a pre-event void of 10 or 20 percent in 2004, and at Bridgeville with a pre-event void of 20 percent in 2006.

The model results predicted that river elevations would have exceeded flood stage at all other AHPS forecast points regardless of pre-event reservoir levels. However, partial voids did result in flood crest reductions.

The largest simulated potential reduction in river elevation on the main stem Delaware under the 20 percent reservoir void scenario was approximately 4.5 feet at Easton, Pa. in connection with the 2006 storm event. Despite this predicted reduction, the stage at Easton would still have exceeded the NWS major flood stage. Inundation mapping and structural surveys prepared by the U.S. Army Corps of Engineers indicate that in this scenario 34 of 60 surveyed properties would still have been inundated.

The number of units spared under a hypothetical pre-event void scenario is less favorable when the analysis is extended to multiple sites. Thus, when Easton is considered together with 11 other basin locations included in the USACE structural damage survey, 1,191 out of 1,484 properties would still have been flooded with a simulated 20 percent pre-event reservoir void in connection with the 2006 event.

“Property analysis such as this reminds us that persons living in a flood hazard area have a high level of risk for experiencing flooding,” Collier said. “Considering operational changes to existing reservoirs as the only flood mitigation strategy would unfortunately provide a false sense of security for people who own or occupy the vast majority of structures currently located in or proposed to be located in flood prone areas. I believe it is clear that we need to work with all levels of government and the public, including our floodplain residents, to move forward with implementing all 45 recommendations of the Delaware River Basin Interstate Flood Mitigation Task Force which call for a comprehensive approach to flood loss reduction.”

DRBC will continue to work with basin reservoir operators to evolve spill mitigation programs that shift spilled water to managed water without diminishing the security of regional water supplies.

Eleven million people in New York, New Jersey, and Pennsylvania depend upon NYC’s three Delaware Basin reservoirs for drinking water – either directly via an out-of-basin diversion or through releases to augment river flows downstream. Other actions being pursued by the DRBC for flood damage mitigation include:

-- continuing to implement non-reservoir related task force recommendations, such as flood warning system upgrades and basinwide stormwater management with retrofits in developed areas;
-- strengthening flood plain management; and
-- continuing to explore basinwide water resource strategies that may include additional multi-purpose storage.

Development of the flood analysis computer model was among the 45 recommendations identified by the Delaware River Basin Interstate Flood Mitigation Task Force, formed at the request of the four governors, in its July 2007 action agenda for a more proactive, sustainable, and systematic approach to flood damage reduction.

The interagency team that developed the flood analysis model included staff from the USACE Hydrologic Engineering Center, NWS, U.S. Geological Survey and DRBC. Work on the flood analysis model began in August 2007 with $500,000 provided by the four basin state governors.

Additional funds and in-kind services from USACE, NWS, and USGS have totaled $285,000. The development of the model proved to be a more complicated undertaking than originally expected, delaying its release.

The DRBC expects to be able to publish these products in January 2010. DRBC also plans to have an independent technical review of the flood analysis model performed.

For more information, visit the DRBC Flood Analysis Model webpage.


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