A precipitation-runoff model for part of the Ninemile Creek watershed near Camillus, Onondaga County, New York

A precipitation-runoff model, HSPF (Hydrologic Simulation Program Fortran), of a 41.7 square mile part of the Ninemile Creek watershed near Camillus, in central New York, was developed and calibrated to predict the hydrological effects of future suburban development on streamflow, and the effects of stormwater detention on flooding of Ninemile Creek at Camillus. Development was represented in the model in two ways: (1) as a pervious area (open and residential land) that simulates the hydrologic response from mixed pervious and impervious areas that drain to pervious areas, or (2) as an impervious area that drains to channels. Simulations indicate that peak discharges for 30 non-winter storms in 1995-96 would increase by an average of 10 to 37 percent in response to a 10- to 100-percent buildup of developable land represented as open/residential land and by 40 to 68 percent in response to 10 to 100 percent buildup of developable area represented as impervious area. A 10 to 100 percent buildup of developable area represents an impervious area of about 1 to 7 percent of the watershed. A log Pearson Type-III analysis of peak annual discharge for October 1989 through September 1996 for simulations with full development represented as impervious area indicates that stormflows that formerly occurred once every 2 years on average will occur once every 1.5 years, and stormflows that formerly occurred once every 5 years will occur once every 3.3 years.

Simulations of a hypothetical 147-acre residential development in the lower part of the watershed with and without stormwater detention indicate that detention basins could cause either increase or decrease downstream flooding of Ninemile Creek at Camillus, depending on the basin.s available storage relative to its inflows and, hence, the timing of its peak outflow in relation to that of the peak discharge in Ninemile Creek; and the degree of flow retention by wetlands and other channel storage that affect the timing of peak discharges. Design and management of detention basins in the watershed will require analysis of each basin.s hydraulic characteristics and location relative to Ninemile Creek to predict their effect on downstream flooding. The runoff model described herein can be used to evaluate alternative detention basin designs and locations.