Water Resources Research Act Program

Details for Project ID 2014NY212B

Development of Regional Unit Hydrograph Parameters for Application to Ungaged NYS Watersheds

Institute: New York
Year Established: 2014 Start Date: 2014-05-01 End Date: 2015-12-31
Total Federal Funds: $10,000 Total Non-Federal Funds: $14,216

Principal Investigators: James Kilduff

Abstract: The DEC administers the dam safety program for the State of New York. Recent revisions to the dam safety regulations increased responsibilities for owners of Intermediate Hazard (Class B) and High Hazard (Class C) dams. New York has about 400 High Hazard Dams and almost 800 Intermediate Hazard Dams. The increased responsibilities include the completion of comprehensive Engineering Assessments, which were/are due in 2012 for Large-High Hazard Dams, 2014 for Small-High Hazard Dams, and 2015 for Intermediate Hazard Dams. A key component of the Engineering Assessments is the evaluation of spillway capacity. As the majority of these were designed and constructed well before the establishment of the state dam safety program, very few can meet the spillway capacity requirement. The procedure to evaluate spillway capacity involves the development of a hydrologic model to generate and route the inflow design flood hydrograph through the reservoir to determine whether there is sufficient spillway capacity. Detailed hydrologic models simulate two processes during the rainfall-runoff process. The first process predicts the “hydrologic losses” and includes infiltration and rainfall abstraction based on vegetation, soil types, and impervious surface storage. Rainfall that reaches impervious surfaces or exceeds the infiltration rate of pervious surfaces is termed “excess rainfall”. The second process is the transformation of this excess rainfall into surface water runoff and the translation of surface water runoff to the point of interest. This translation is typically accomplished through application of a synthetic unit hydrograph. A unit hydrograph is a numerical representation of how a watershed responds to a unit of excess rainfall (1 inch) and is highly dependent on the topography, drainage network structure, shape of the basin, and surface storage. There are several unit hydrograph methodologies available though the one developed by the Soil Conservation Service (SCS and now known as NRCS) is the most widely used unit hydrograph methodology because of its ease of application. The SCS unit hydrograph methodology utilizes a single hydrologic parameter, lag time, which is readily computed with use of topographic mapping and aerial imagery. Other unit hydrograph methodologies such as the Clark Unit Hydrograph and Snyder Unit Hydrograph require two or more hydrologic parameters, which are often not physically based and need to be based on regional studies or determined through model calibration. As such, these methodologies can be difficult to utilize for projects within un-gauged watersheds, which comprise the majority of locations in NYS, for dams or otherwise. Many previous studies have found that the SCS unit hydrograph methodology is very conservative, and often over-predicts of peak flows by a factor of two to four. Therefore, spillway adequacy may be significantly underestimated, resulting in over-designed and costly spillway rehabilitation projects. In some cases this additional cost may represent a financial hardship for the dam owner, and may preclude the owner’s ability to finance a dam rehabilitation project. In such cases, the goals of the dam safety program, including public safety, are not met. In its application during detailed FEMA Flood Insurance Studies, the use of the SCS unit hydrograph methodology can result in an over-prediction of 100-year flood flows, which translate to inaccurate flood insurance maps and the associated issues.