Notes from the Field: Understanding Connecticut River's Floodplains

Friday, July 1, 2016
Dan Miller calibrating the sidescan sonar. Photo: A Ericson

Dan Miller calibrating the sidescan sonar. Photo: A Ericson

NE CSC Graduate Fellows Abigail Ericson and Daniel Miller, accompanied by Bogumila Backiel (graduate student in the Department of Environmental Conservation at UMass Amherst) and Joseph Pellegrino (intern at The Nature Conservancy), recently traveled to a northern section of the Connecticut River as part of a survey for the NE CSC project, Reconnecting Floodplains and Restoring Green Space as a Management Strategy to Minimize Risk and Increase Resilience in the Context of Climate and Landscape Change.

The team worked in two groups to collect bathymetry for the section of the river and to document ecologically important sandbars within the river. Abigail is building a Hydrologic Engineering Center (a branch of the US Army Corps of Engineers) - River Analysis System (Hec-RAS) model of the Connecticut River.  The Hec-RAS software allows the user to perform one and two-dimensional hydraulic calculations using the field-collected bathymetric cross sections, detailed LiDAR and orthogonal imagery, and additional USACE collected bathymetry. The model will quantify the floodplain storage capacity during storm events, as well as make projections on alternative flows based on land cover changes and floodplain loss. 

Floodplains, with little to no human development, allow space for storm waters to go during a high flow event (i.e. a big rain event), so water will overtop the river's normal banks and flow out onto the land. This means a lot of the water travels laterally through woods and marshy areas rather than flowing immediately downstream and the water level of the river stays relatively low and slow. When people build over floodplains and levees are constructed, water in the river is forced to stay in the channel rather than flow out onto the land. When a high-flow event happens in this situation, there is more water within the river banks (higher water level) and less to obstruct the flow (faster water).

Downstream inhabitants want less and slower water. So, a well preserved floodplain that is allowed to flood can 'hold', or store, excess water and slow the peak flows down by giving the water someplace to go other than directly downstream. While Abigail is working to quantify the capacity of CT River floodplains with the Hec-RAS model, Bogumila is investigating the ecological importance of naturally functioning floodplains and sandbars. Floodplains support tree species like willows, sycamores, silver maples and cottonwoods. Sandbars (which are associated with floodplains) are early successional habitats that support pioneer plants species like the rare sandbar willows. The threatened puritan tiger beetle, endangered wood turtle and endangered dwarf wedgemussell use sandbars as habitat. Stressors like channelization straighten and dredge rivers thus limiting erosion which supplies the sediment needed for the development of sandbars. In addition, impoundments such as dams, capture and store sediment limiting the formation of bars.
 
Together the results from these grad students’ investigations will help inform city planners, managers, and local conservation groups on how to most effectively use resources to preserve and maintain floodplains to protect both human and ecological interests.

    

Contributed by Abigail Ericson, NE CSC Graduate Fellow.