Highway embankments can provide an ideal location for integrating low impact development (LID) stormwater best management practices (BMPs) into a highway setting, specifically sheet flow BMPs such as vegetated filter strips (VFS) and dispersion. However, the design criterion at Washington State Department of Transportation (WSDOT) limits the embankment slope to 15% maximum. A possible justification for this limit is steeper slopes could concentrate flow, reducing the effectiveness of the BMP, and encouraging channelized flow that can erode the embankment.
This design criterion can present a challenge to WSDOT since highway embankments can be constructed, without vehicle safety barriers, on slopes as steep as 33%. When VFS or dispersion are required on slopes greater than 15%, embankments need to be flattened and the roadway footprint expanded. This may result in additional right of way acquisition as well as additional construction and ecological costs from the expanded embankment.
While studies have shown that VFS and dispersion areas can successfully meet runoff treatment and flow control goals on 33% slopes, limited research was found that evaluated the site characteristics that support stable embankments (without erosion) located at steeper slopes.
In an effort to evaluate design criteria, for both VFS and dispersion, that aligns with the 33% highway embankment limit, 45 sites in Eastern Washington were inventoried to determine the specific site characteristics that contribute to concentration of highway runoff on slopes steeper than 15%. Based on statistical analysis the embankment slope alone was not considered statistically significant to erosion severity compared to other site characteristics. Instead, low vegetation coverage and a high percentage of sand had the strongest correlation to erosion severity. In addition, empirical observations at the sites noted conditions at the edge of pavement that can also encourage channelized flows.
Based on these findings, modified design criteria for VFS and dispersion BMPs are recommended allowing embankment slopes of up to 33% when vegetation can be established, taking into consideration the soil characteristics. In addition, level spreaders may allow for dispersed flows regardless of the conditions at the edge of pavement.