Reconnaissance study of erosion and deposition produced by the flood of August 1955 in Connecticut

Abstract

A large area in the valley bottoms in Connecticut was inundated by the flood of August 1955. Relative to the total area flooded that part permanently modified by the flow was surprisingly small. Although great in some places, the distribution of these permanent modifications of channel and flood plain was spotty. Erosion of the channel and valley bottom appears to have been most severe in narrow, steep valleys. Environments of deposition were diverse. They appeared to be related to rate and direction of flow, quantity and size of sediments locally available, and in some cases to the presence of vegetation. Most of the coarse sediment deposited in the valley appears to have been derived from local sources such as valley walls and terraces composed of glacial outwash and till and flood plains containing considerable gravel. Fine sand predominated in most of the sediment deposited. Considering the magnitude of the runoff, the quantity of fine sediment transported or deposited by the flood was small. The maximum observed concentration was 473 parts per million in Scantic River at Broad Brook. Newly deposited fine sediments are thin or absent on the flood plains of many valleys which were beneath 20 ft of slow‐moving flood waters. The paucity of deposition in such ideal depositional environments also indicates that the flow did not have a high concentration of fine sediments. A number of boulders five to seven feet in diameter were moved by flood waters in reaches in which smaller gravels were undisturbed. Severe erosion in the uplands was minor. In this reconnaissance we saw little evidence of newly formed gullies and no areas of severe sheet erosion. This was the case in both woodland and pasture land. Large amounts of subsurface flow and relatively unerodible ground are presumed to be responsible for the absence of erosion.