HEC-RAS MODELING OF SPLASH DAM DISCHARGE IN THE UPPER HUDSON RIVER WATERSHED, NEW YORK

From the mid-1800s to the mid-1900s the Hudson River was used to transport logs from the Adirondack Mountains to the lowland mills. In order to have enough water to transport the logs, splash or sluice dams were built to store and release water during the spring snow melt. Thus, the normally high spring floods were increased by an unknown amount. Despite the regionalized use of this technique, both the locations and size of these splash dams have not been well archived. Some of the dams no longer exist, while others have been rebuilt for recreational purposes.

In order to more accurately estimate the magnitude of splash dam discharges, we used the Army Corp of Engineers’ Hydrological Engineering Centers River Analysis System (HEC-RAS) model to quantify the dam release of five streams in the Adirondack Mountains. For each stream we used 10 - 20 topographic cross sections, estimated Manning’s roughness coefficients, measured high water marks, and calculated reservoir areas. While 4 of the 5 streams have reconstructed dams, the dams may not have same dimensions as when they were used for logging thus, we estimated a range of sluice geometries based on historical photographs.

We compared our model results with published magnitude/frequency curves for the Adirondack region (Lumia et. al., 2006). Preliminary results suggest some streams experienced high discharges (return intervals from 25 to >100-years), several times per year. These results compare favorably to discharges estimated for these same streams using paleoflood field relations (high water marks and topographic cross sections) and Manning's roughness equation (Loehr et al, 2010). The discharges estimated by Manning's equation suggest return periods between 2 and 10-years for two streams and 15 to >100-years for the other three streams. By comparison, estimates of peak discharge using only the height of dams (Q_max=10.5h^1.87, h = dam height; Costa 1985) estimate longer return intervals compared to the HEC-RAS models or Manning's equation.

While our data suggests that splash dams released up to 100-year floods several times per year, we still do not understand the impact of such practices on stream morphology. Future work will compare the morphology of dammed and undammed streams.