BEAVERS OR THE RAILROAD: WHAT CAUSED THE LARGEST FLASH FLOOD IN SARATOGA SPRINGS, NEW YORK, IN OVER A CENTURY?

On May 18, 2006, local media reported that a “massive” beaver dam had breached in Greenfield, NY, causing significant damage to local roads and flooding residential property. The instantaneous release of water also overtopped three small downstream reservoirs, called the Three Sisters Ponds, and breached the earthen dams upon which century old historic carriage trails were constructed. The goal of this study was to determine the cause of flooding.

To determine the volume of accumulated water prior to failure, we conducted a detailed topographic survey using a Sokkia SET610 Electronic Total Station accurate to ±3 mm, with measurements taken at density of 0.11 points/m². We reconstructed maximum water levels using high water debris deposits, first hand accounts, photographic documentation, vegetation levels, and remnant beaver dams. Elevation measurements were imported into ArcGIS and converted to a raster with 0.1 m cell size by way of a tin. Accumulation of flood waters began as beaver activity and/or slumping of sediment clogged a small culvert beneath an ~5 m high earthen railway embankment, impeding the flow of water from Putnam Brook. Though water level had remained static behind the structure for some time, it is likely that the rising water level from above average precipitation during the months of April and May caused the embankment to become unstable. At the time of failure, Greenfield experienced the 6^th wettest spring (165% of average precipitation) based on a 51-year record for April and May.

Based on a maximum water height of 1.22 m below the railroad embankment and 3.44 m above the beaver dam, we calculated that ~149,000 m³ of water was catastrophically released after failure. We surveyed four cross sections of maximum water height downstream, based on debris and sand deposits, in order to determine peak velocity and peak discharge. Values derived from Manning's equation suggest a peak velocity of 3.1 ± 0.5 m/s and a peak discharge of ~340 ± 80 m³/s. Based upon this study, we conclude that the lack of culvert maintenance, not a beaver dam breach, led to the failure of the railroad embankment which released ~149,000 m³ of water, breached historic dams, and caused considerable destruction to both public and private property.