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

Abstract

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.