Valuing the benefits that floodplains provide to people

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New paper published presents a method that combines field and geospatial datasets to estimate the amount of sediment eroded from stream banks and deposited on floodplains.

Floodplains trap sediment and slow flood waters during runoff events.

Floodplains trap sediment and slow flood waters during runoff events.

(Public domain.)

Issue: Floodplains provide numerous benefits to people by reducing downstream flooding and trapping sediments and nutrients. Information on the capacity of floodplains to retain sediment and nutrients is important to understanding the value that floodplain areas provide by reducing the transport of pollutants to downstream rivers and estuaries such as the Chesapeake Bay. Floodplains can improve downstream water-quality by intercepting upland sources of sediment and by trapping sediment during overbank flood events. However, there are few tools available to predict the water quality benefits that floodplains provide to people and how the value of those benefits vary within a watershed.

 

 

 

Stream bank erosion mobilizes sediment that can be transported downstream.

Stream bank erosion mobilizes sediment that can be transported downstream.

(Public domain.)

The Study: This study provides the first estimate of contributions of sediment and nutrients from the upland, stream banks, and floodplain for each stream segment in the Difficult Run watershed, an area of mostly developed land use in the Virginia Piedmont. A whole-watershed sediment balance was constructed to determine the net export or retention of sediment, nitrogen, and phosphorus in the watershed. The work combined the synthesis of field datasets to estimate the net flux of sediment and nutrients, lidar mapping and the application of a new GIS tool for characterizing stream channel geometry, and modeling to extrapolate to the more than 2,000 stream reaches in the watershed. The team then used economic methods to estimate the annual value of sediment and nutrient retention services that floodplains provide in the study area.

Key Findings:

  • Floodplains within the Difficult Run watershed provide appreciable ecosystem service benefits by trapping sediment, nitrogen, and phosphorus and meaningfully reducing their downstream transport.
  • Streams shifted from net erosional to net depositional at fifth order streams for sediment, second order streams for nitrogen, and fourth order streams for phosphorus.
  • Nitrogen trapping in Difficult Run floodplains had an estimated value of roughly $727,000 per year.

Implications:

Fine sediment deposited on a floodplain.

Fine sediment deposited on a floodplain.

(Public domain.)

  • The findings of this work can be used to assess economic tradeoffs between development and floodplain conservation and restoration. Results for this type of analysis can also be to identify streams with substantial bank erosion and floodplain storage to help with targeting of best management practices.
  • The methods presented in this article are scalable and transferable to other areas, if appropriate datasets are available for validation. The team is currently working to apply these methods to value the ecosystem services provided by floodplains in the entire Chesapeake Bay and Delaware River watersheds.
  • This approach and the results can help improve simulation models of the transport and delivery of nutrients and sediments through watersheds in response to management actions.

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