Sediment wave-induced channel evolution following the 2006 avulsion of the Suncook River in Epsom, New Hampshire

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2008.Includes bibliographical references (p. 83-91).Large volumes of sediment can be released into a river when an avulsion carves a new channel in the landscape. Gilbert (1917) described the evolution of a similar pulse of material from mining along the Sacramento River, California as a sediment wave. Sediment waves are transient accumulations of sand and gravel that locally increase the elevation of the bed and reduce the transport capacity of the channel, and diffuse and translate down the channel over time. The Suncook River in Epsom, New Hampshire, avulsed in May 2006. This event created a new channel and mobilized approximately 100,000 m3 of sand into the river in a period of 12 to 24 hours (Perignon, 2007; Wittkop et al., 2007). In April 2007, a new channel formed through a meander bend downstream of the site of the first avulsion, where sediments mobilized the year before had increased the bed elevation by one meter. We propose that the material released in 2006 is traveling down the channel as a sediment wave, increasing the elevation of the bed and driving avulsions. The purpose of this study is to model the evolution of a sediment wave in the Suncook River in order to understand how it can increase the risk of floods and avulsions in the system over time. We developed a mathematical model using the equations of Lisle et al. (1997, 2001) to observe the evolution of the sediment wave under bankfull conditions. We found that the wave evolved mostly through diffusion and showed minimal translation downstream. These findings suggest that the risk of avulsions will be contained near the center of mass of the sediment wave, which we place near the site of the 2007 meander cutoff. Likewise, the diffusive nature of the wave implies that the river could reach a new equilibrium profile with no restoration work and without significantly affecting populated areas downstream.by Mariela C. Perignon.S.M

Analysis of the May 2006 Suncook River avulsion in Epsom, New Hampshire

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2007.Cataloged from PDF version of thesis.Includes bibliographical references (pages 60-66).Avulsions, or rapid changes in the location of a river, usually occur in environments such as deltas, floodplains, and alluvial fans where net-deposition can raise the bed of the river above its floodplain. Avulsions are less frequent in transient landscapes, such as New England, where topography and hydrography are still responding to recent glaciation. One of these rare avulsions occurred during a 100-year flood on the Suncook River, Epsom, NH, between May 14 and 15,2006. We studied the Suncook River event to develop a model for the drivers of avulsions in transient landscapes. We suggest that a strong substrate in the parent channel, such as bedrock or immobile boulders, can facilitate an avulsion by preventing incision and driving water overbank. Easily erodible substrates in the path of the new channel can also contribute to avulsions by allowing a knickpoint to migrate quickly upstream and create a channel with a more favorable slope during a single flood. Based on Slingerland and Smith's (2004) model, we also propose that a low water-surface slope in the parent channel could be a direct driver for avulsions. In the Suncook River, this low water-surface slope was created in the backwater of a small mill dam in the parent channel. A 200-year flood that occurred in the Suncook River in 1936 did not create an avulsion. We suggest that ice floats could have damaged the dam and increased the water-surface slope of the parent channel, making an avulsion less favorable and reducing the depth of water flowing overbank. The topography in the path of the 2006 avulsion, which was lowered by activity in a sand pit starting in the 1960s, probably prevented water from finding a new path. We believe that these anthropogenic modifications directly contributed to the occurrence of the May 2006 avulsion in the Suncook River. These conditions are common throughout New England, and could increase the risk of avulsions in the region.by Mariela C. Perignon.S.B