SPILLWAY CAPACITY ANALYSES FOR NEW YORK CITY DAMS IN THE CATSKILL AND DELAWARE WATERSHEDS

摘要

GZA GeoEnvironmental, Inc. of New York (GZA) is currently providing engineering consulting services to New York City for their Catskill and Delaware System Dams. The focus of the work is the performance of detailed safety inspections of six of the City's largest water supply reservoirs which provide 90 percent of New York City's 1.3 billion gallons per day demand. The goal of the project is to provide the City with recommendations for modifications and repairs needed to upgrade these critical water supply reservoirs to first-class working condition. A major aspect of the project (and the topic of this paper) deals with the evaluation of current spillway capacity for these large, high hazard dams. The main objective of the hydrologic studies is to re-evaluate and, in some cases, re-confirm the hydraulic capacity of existing principal spillway structures at each of the six dams under Probable Maximum Flood (PMF) conditions. It was determined that the spillway capacity of the dams required re-assessment, primarily because: (a) The unit hydrographs, formerly used to estimate rainfall/runoff relationships were generally developed from streamflow records through the late 1970s/early 1980s and did not include more recent severe storms; and (b) in most cases, the estimation of the Probable Maximum Precipitation (PMP) was based on now outdated HMR-33 methods. Thus, it was crucial for City officials to identify potential risks of overtopping, using refined hydrologic/hydraulic methods, for these key water supply structures and, as necessary, reduce risks by implementing spillway improvements. GZA engineers built upon the body of existing hydrologic information within the river basins (from previous Corps of Engineers hydrology reports) to evaluate whether these dams could safely pass extreme floods under current, state-of-the-practice conditions and methods. Detailed unit hydrograph theory was employed for the gaged and ungaged streams within these very large contributing watersheds (ranging in size from 90 to 456 mi~2). The general sequence of hydrologic analysis involved review of river basin model methodology, input data, and supporting calculations as well as obtaining records from USGS streamflow gages and National Weather Service precipitation stations within the study areas for periods subsequent to the publication of the Corps river basin reports. The original COE HEC-1 runoff models were "re-created" using historic 1950's-era floods and further refined using updated information on key hydrologic characteristics