HADLOCK POND DAM FAILURE: RECONSTRUCTION OF PEAK DISCHARGE AND COMPARISON TO REGIONAL FLOOD FREQUENCY

On July 2, 2005 the Hadlock Pond dam breached in Fort Ann, New York and allowed approximately 2 to 3 x 10⁶ m³ of water to discharge through the outlet stream. Several houses were dislodged from their foundations and three bridges were washed away. Such a large discharge was unexpected below a dam that had regulated discharge for more than a century. Our goal for this project was to reconstruct the peak discharge and compare it to flood frequency analyses of similar sized basins in New York and Vermont. The Hadlock Pond watershed is 20.5 km² and the surficial geology is dominated by clay rich till and bedrock outcrops. We used a differential GPS unit to map the pond's bathymetry and estimated a total volume of ~3 x 10⁶ m³, of which about 2/3 discharged from the pond. Just below the dam break, data from 36 pebble counts show a rapid decrease in grain size as flood waters filled the valley and flow depth and basal shear stresses decreased. We surveyed five cross-sections of maximum depth based on high water sand deposits and reconstruction of high water lines from photographs. We estimated Manning's roughness coefficients using Arcement and Scheider (1989). Peak discharge calculations based on Manning's equation, for all five sites, range between 2000 and 3500 cfs. USGS stream gauge data from similar sized basins in New York and Vermont show that discharges of >2000 cfs have occurred several times within the last century. Flood frequency analyses suggest that the peak discharge from Hadlock Pond dam break is equivalent to recurrence intervals that range from 10 years at the low end to 100 to 300 years at the high end. Therefore, we found that the Hadlock Pond dam break discharge, although destructive, was not of an unnatural size for its basin size and surficial geology.