Downstream Effects Resulting from the Operation of the Pompton Lake Dam Floodgates

摘要

The Governor's Passaic River Basin Flood Advisory Commission charged the New Jersey Department of Environmental Protection (NJDEP) to evaluate the operational impacts of the Pompton Lakes Dam Floodgate Facility. NJDEP retained AECOM to develop a hydraulic computer model to simulate the downstream effects resulting from the operation of the Floodgate Facility. The floodgate facility provides an unconventional approach to flood mitigation. Unlike most flood control projects involving dams, the project only provides flood reduction benefits to upstream residences. The project was designed to have no benefits or negative impacts to downstream floodplain communities. The study included approximately 5.50 miles of the Ramapo River (including Pompton Lakes) and 0.17 miles of the Pompton River. The study focused on the floodway/floodplain area downstream of the Pompton Lakes Dam, which is within the Borough of Pompton Lakes and Wayne Township. The analysis began at the downstream face of the Route 287 Bridge (Ramapo River crossing) and continued downstream and terminated at the Jackson Avenue Bridge (Pompton River crossing), or approximately 11,500 feet downstream of the Pompton Lakes Dam. Tributaries to the Ramapo that may be affected by backwater were also studied; these included the Wanaque and Pequannock Rivers A hydraulic model, the unsteady flow model within the Hydrologic Engineering Centers River Analysis System (HEC-RAS), was used to determine downstream water surface elevations and velocities during various flood events. The existing Operating Rule Curve controlling the floodgate openings and operations of the Pompton Lakes Dam was incorporated into the HEC- RAS computer model. The HEC-RAS unsteady flow model is a widely accepted hydraulic model and it was chosen for this study due to its ability to model the unsteady flow (flow rates that vary with time) discharging from the flood gates and predict both the stages (water surface elevations) and discharges (flow rates and velocities) downstream of the dam. Precipitation for four recent floods; March 13-15, 2010, March 6-8, 2011, March 10-12, 2011, and Hurricane Irene, August 2011, were entered into a hydrologic model, the Hydrologic Engineering Centers Hydrologic Modeling System, HEC-HMS, to develop inflow hydrographs for input into the unsteady flow model. Applied Weather Associates, (AWA), used the Storm Precipitation Analysis System (SPAS) model in conjunction with the Next Generation Radar (NEXRAD) weather radar data to analyze the rainfall for these four storms over the Pompton Lakes drainage basin. The rainfall amounts were provided in a 1 km grid with 5 minute time intervals for each of the basin/sub-basin boundaries. The hydrologic model was calibrated to several USGS stream gaging stations throughout the basin. The hydraulic model was also calibrated in a similar manner but was also verified with several high water marks for Hurricane Irene. The four storms were significant flood events. The three March storms were somewhere between a 10 year and 25 year return period (10% to 4% probability of occurrence). Based on flow records and rainfall, Hurricane Irene was somewhere between a 500-year event in the upper portion of the watershed and about a 100-year event in the lower portion. The four storms occurred within a short time span of one another, March 2010 to August 2011. All four storms caused significant flooding. The Western Regional Climate Center Desert Research Institute in Reno Nevada produced a map of the United States showing the 72-month accumulated precipitation departure from normal through the end of February 2012. Northern New Jersey has had 50-to 65 additional inches of precipitation over that 72-month period. This same trend is shown in the peak stream flow data taken from the USGS Stream gages for the Ramapo River near Mahwah NJ. The average of the peak stream flow data for the past decade, 2000 to 2011, is about 55% higher than the average of decades for the past 60 years. Besides analyzing the Floodgate Facility for the four recent flood events, a range of flows from low to high were also analyzed/modeled. All of the flows investigated the Operating Rule Curve by modeling the system as pre-project condition (without gates) and post-project condition (with gates). The results of the analyses found that the gates function as intended with no significant downstream impacts during any of the flow events. The impacts were considered at four locations. Three sites downstream from the Pompton Lakes Dam and one site upstream in Oakland, NJ.