A PRE- AND POST- TROPICAL STORM IRENE COMPARATIVE ASSESSMENT OF CHANNEL GEOMORPHOLOGY ALONG THE DOG RIVER, CENTRAL VERMONT

A unique opportunity presents itself on the Dog River, central VT, where a Phase II stream geomorphic assessment was conducted in 2008. Tropical Storm Irene strongly impacted the watershed, with total precipitation exceeding 6 in and most rainfall concentrated in a 12 hr period. Peak intensity of rainfall was ~0.8 in/hr. A USGS gauging station at the mid-point in the watershed recorded a stage height of 17.26 ft and a peak discharge of 23,500 cfs. The Dog River flows from ~2300 ft at its headwaters to its confluence with the Winooski River at ~510 ft. The watershed area is ~93 mi², most of which is forested, though sections parallel with VT Rt. 12 and 12A are urbanized or agricultural.

The 2008 pre-Irene geomorphic assessment was conducted by Bear Creek Environmental, LLC. All main stem reaches were assessed using criteria in the Vermont Agency of Natural Resources Phase II Handbook for Rapid Stream Assessment. These included bank full stage cross-sectional area, bottom characteristics and sinuosity. In Fall 2012 all cross-stream profiles were reoccupied and data were collected following the same methodology as in 2008.

The effects of Irene were dramatic and basin-wide. In the headwaters, channel cross-sectional areas increased significantly, up to ~100%. Here the stream has limited floodplain access due to stabilization for railroad and road beds. Engineered features in close proximity to the channel appear to have exerted a significant control on stream response to flooding. In central reaches the response of the stream was more variable but overall cross-sectional areas increased at all sites. In some instances, widening was as great as ~200% and deepening was as great as ~100%. The reaches near the mouth, those with a wider floodplain than upstream, show inconsistent change in cross-sectional area. In some reaches, cross-sectional area increased up to ~20% and in others it decreased up to ~10% due to aggradation. Bedload is pebble and cobble gravels and no obvious trends were observed between 2008 and 2012. However, many stretches of the river exhibit freshly exposed bedrock in the channel. This comparative study of geomorphic data reveals that Irene-related erosion impacted all reaches of the main stem. The cross-sectional profiles can be reoccupied annually to continue to quantify post-Irene channel evolution.