32 Geomorphic Impacts of Tropical Storm Irene

Tuesday, 19 March 2013
8:00 AM - 12:00 PM
Adams Room

RAINFALL, FLOOD MAGNITUDE, AND GEOMORPHIC IMPACTS OF TROPICAL STORM IRENE ON THE WHITE RIVER WATERSHED, EAST-CENTRAL VERMONT

Paper number: 32-2
Program: Theme Sessions
Time: 8:10 AM – 8:25 AM

Authors

Abstract

The White River watershed (WRW) in east-central Vermont (VT) was heavily impacted by Tropical Storm Irene (TSI) on August 28-29, 2011. The flooding is considered the second most severe event in the last 100 years, surpassed only by the flood of 1927. The White River (WR) drains 1,840 km2. The WRW is 84% forest, 7% farmland, and 5% developed, with settlement concentrated along the river valleys. During TSI, most of WRW received between 125 and 175 mm of rain. Rainfall at Randolph totaled 140 mm and fell with a peak intensity of about 18 mm/hr. There, the rain started around midnight on the 27th and ended at around 11 p.m. on the 28th, with most of the rain falling between 8 a.m. and 4 p.m. on the 28th. The WR at West Hartford (1,790 km2) peaked at 2,552 m3/s on the 29th at 1:30 a.m. The stage rose quickly and then receded to <50% of the peak height within 24 hours. The peak flow at the West Hartford streamflow gage had an estimated annual recurrence interval of 0.2 percent (500-year storm).

The headwater tributary reaches are transport-dominated, cascade or step/pool channels. During TSI, woody debris and sediments were mobilized from these tributaries and gullying, streambank erosion, landslides, headcutting of channels, and debris jams were widespread. Roads, bridges, culverts, and homes in the narrow tributary valleys were heavily damaged and in many cases, destroyed.

Along the mainstem above Bethel the valley bottom is relatively broad, but historic channel incision and channel and floodplain alterations had reduced floodplain access. In unconfined reaches, extensive areas of the floodplain surface and low stream terraces were overtopped. Vast sheets of sand and gravel were left behind on these surfaces. Woody debris was deposited as individual pieces stranded in the channel and on the floodplains and terraces, as debris jams at obstructions, and as woody debris ramparts up against the trees at the edges of floodplains and low stream terraces. Scour was severe at valley pinch points, but even in reaches with floodplain access the channel was scoured and widened. Large landslides occurred along the mainstem, many of which appear to have been reactivated.

The Lower WR from Bethel to Hartford has a relatively confined channel, which resulted in very high flood stages. Channel widening there was limited due to extensive bedrock exposures.