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Why Understanding Stream Channel Bankfull Width Matters for Flood Reduction

Authors: David Azinheira, P.E., CFM and Holden Sparacino

Culvert and small bridge replacement designs that consider natural stream characteristics improve ecological functions and flood resilience.  One key characteristic used in design is the bankfull width of the stream. This article will explain this characteristic of streams, and how designs that consider bankfull width can improve stream functions, reduce flood risks, and improve infrastructure resilience.

What is bankfull width?

You may already know that culvert and small bridge replacements meeting the Massachusetts Stream Crossing Standards) are required to have spans of at minimum 1.2 times the field measured bankfull width, but what is bankfull width and how is it measured?

The water level of a stream fluctuates constantly, influenced by factors such as rain, snow, and drought. Stream channels are sculpted by the downhill movement of water, with their shapes being determined by the historical patterns of stream flows at a particular site. The banks of a stream are usually dry and are typically overtopped every year or two. The width of the channel when it is full, just before it spills over into the floodplain, is referred to as the bankfull width. Good indicators of areas within the bankfull width include wet areas with stream bed material, unstable or eroding banks, and/or dry areas next to the streambed without perennial plants or trees (especially in summer or drought conditions). Good indicators of areas outside of the bankfull width include stable areas with perennial plans or trees. The figure below shows the bankfull width over a photograph. 

Stream scientists call the stream flow when the stream level is at the bankfull width the ‘channel forming flow,’ as this is when the most significant channel erosion and sediment transport occurs within the channel. Bankfull width flows typically occur at about the 1.5-year frequency storm, or during storm events with a two-thirds chance of occurring in any single year.  During even larger floods, the higher velocities, debris loads, and heavy erosion are more likely to be concentrated within the deeper, well-defined channel.

The Division of Ecological Restoration (DER) Stream Continuity program recommends determining a representative bankfull width by averaging three (3) bankfull width measurements taken upstream of the culvert (or small bridge) and three (3) downstream of the culvert (or small bridge). Typically, measurements approximately 10 to 20 bankfull widths away from anthropogenic features (for example dams, bridges, and culverts) are recommended for determining a representative bankfull width.

How does the bankfull width impact flooding and ecological functions?

Historically, most culverts and small bridges were installed with spans that are significantly smaller than the streams’ bankfull widths. Undersized culverts act as partial barriers to stream functions and limit hydraulic capacity, increasing the risk of flooding, and can result in significantly altered water velocity and sediment movement, as well as increased debris accumulation during heavy storm events. Undersized structures also can significantly disrupt stream connectivity and fish and wildlife passage – often reducing the number of species and population sizes thereby disrupting the natural function of a stream. The figure below illustrates a simplified stream cross-section before and after replacing an undersized culvert. 

A road stream crossing structure with a span of at least 1.2 times the stream’s measured bankfull width ensures the structure is built outside of the stream channel. This facilitates natural channel dynamics, provides sufficient hydraulic capacity during storm events, ensures greater fish passage, and provides dry passage for wildlife (including turtles) to travel along the stream corridor without crossing over the road. Structures that meet the MA Stream Crossing Standards (including having a span of 1.2 times the stream’s bankfull width) are also protected from structural stress caused by repetitive wet/dry cycles, heavy erosion, and debris blockages that are common for undersized culverts. This can reduce maintenance costs, increase structure lifespan, and generally reduce the probability of flooding due to either structure failure or overtopping.

Culverts and bridge crossings designed to meet the Massachusetts Stream Crossing Standards increase flood resiliency while also providing passage for water based and land based wildlife.  The DER Stream Continuity Program promotes the replacement of undersized culverts and bridges with crossings meeting the Massachusetts Stream Crossing Standards through our Grant Programs.  Our stream continuity grants typically open in February or March – if you would like to receive announcement on upcoming grant opportunities you can sign-up for the Culvert Connection newsletters.

About the authors: David is a Stream Crossing Specialist within the DER Stream Continuity Program and Holden Sparacino is the DER Stream Continuity Program Manager. This article incorporates materials developed in coordination with Carrie Banks (DER Capacity Building Branch Manager), the Massachusetts Stream Crossing Standards, and the USDA Stream Simulation manual. 

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