Determining Design Criteria for Land and Flow Characteristics That Produce Non-Erosive Sheet Flow

About the report:

If a design engineer can show that a project site will produce non-erosive sheet flow, the cost and complexity of stormwater control measures that must be built for that site can be significantly reduced.  However, the criteria for establishing non-erosive sheet flow are not well defined in the Virginia Department of Transportation (VDOT) Drainage Manual.  This lack of a clear definition can result in uncertainty for projects when establishing non-erosive sheet flow through natural grading at sites or using stormwater control measures such as level spreaders.

To address this issue, this study conducted a series of computer modeling simulations to understand how key properties of a hillslope affect sediment export.  The properties investigated were slope, hillslope length, soil hydraulic conductivity, and surface roughness.  The Kinematic Runoff and Erosion Model, Version 2, developed by the U.S. Department of Agriculture, was used for the simulations.  Simulations were conducted for 24-hour design storms with total rainfall depths from 2 to 7 in.  These design storms represent 2-year to10-year return period storms for counties and cities across Virginia.  To validate the modeling results and relate them to real-world hillslopes, 18 sites proposed by VDOT engineers were investigated to measure their properties and to observe the presence or absence of erosive flow at the sites. 

The results of the study documented how slope, hillslope length, soil hydraulic conductivity, and surface roughness affect sediment transport from a computer-simulated hillslope.  Slope and hillslope length were the most important variables, each having a linear relationship with total sediment yield and peak sediment discharge.  Hydraulic conductivity and surface roughness, measured using Manning’s roughness, showed a negative correlation with total sediment yield and peak sediment discharge. A regression analysis resulted in a simple equation to estimate peak sediment discharge based on the properties of a hillslope and the total amount of rainfall received over the 24-hour design storm.  Applying the regression model to the field sites showed that the model generally matched what was found in the field, although each site had unique complexities that had to be considered.  The study concluded that it is possible to use a regression equation with only a few easily obtained hillslope characteristics to estimate peak sediment discharge.  Further, a peak sediment value of 5 g/s per width of hillslope for a 2-year, 24-hour design storm is a reasonable threshold for determining if a hillslope is at risk of producing erosive flows.

The study recommends that VDOT disseminate the outcomes of this study to designers so that they can better understand when hillslopes will generate erosive sheet flow.  Further, VDOT should continue to identify and record locations in the field where efforts to establish sheet flow resulted in erosive flows so that the peak sediment threshold values proposed in this study can be further tested and refined.  If VDOT implements these recommendations, it will allow designers to better ensure that hillslopes will result in non-erosive sheet flow, thereby avoiding the need for more expensive stormwater control measures while at the same time protecting the environment and water quality from harmful erosion.

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