What caused the avulsion of the Suncook River in Epsom, New Hampshire (15-16 May 2006)?
Avulsions are common geologic processes on deltas, alluvial fans, and floodplains but are rare outside of these settings. Some of the largest losses of human life in recorded history have been due to avulsions. Nonetheless, their causes remain incompletely understood. The Suncook River did not avulse in one of the aforementioned settings. Rather, the avulsion occurred outside of the 100-year floodplain, crossed a small wetland, and incised a low glacial ridge. The avulsion created a new channel 0.7 km long, abandoned 3.2 km of former channel, and eroded an estimated 115,000 cubic meters of sediment. As a result of the avulsion, 298 homes downstream are currently at greater risk of flooding and 63 acres of farmland, prime habitat for trout and brook mussels, and use of a sand quarry were lost. Bridge stability has also potentially been affected. This study, in collaboration with Mariela Perignon (graduate student, MIT) and Will Ouimet (Penn State), is designed to determine the nature and cause of the avulsion, including possible anthropogenic factors (quarry adjacent to the river; possible aggradation and backwater slope effect due to mill dam just downstream of avulsion site). By combining eyewitness accounts of the flood, hydrographic data, and pre-existing topographic maps with surveys taken soon afterwards, we have so far estimated conditions at the time of the avulsion. Preliminary results indicate that erosional avulsions, such as this one, require fast knickpoint retreat rates in the new channel in order to occur (in this case, they were ~1 cm/s). The mechanics of knickpoint migration, which are poorly understood, are critical remaining pieces of the puzzle. Our work also suggests that the mill dam may have reduced the river’s backwater surface slope enough to make alternate flow paths, such as the current one, energetically favorable [Perignon and Miller, 2007, NE GSA meeting]. Our hope is that this knowledge may aid in predicting flood hazards elsewhere in the heavily populated East Coast where rivers and floodplains have, in some areas, been severely modified by human action.
Site of avulsion on the Suncook River. Flow is from left to right. Sandy bed of abandoned channel segment is visible to the right of downed and deposited trees (right-hand side of photo).
New channel of the Suncook River cut through a sand pit (broad surface visible left of the river).