Case Study - Sedimentation at LEWAS Site

Walter McDonald

The LEWAS site has noticed a significant increase in the amount of sediment that is being transported through the upstream stormwater network and depositing at the site. This sedimentation has caused a change in channel shape, flow characteristics, and interfered with monitoring equipment. In order for the LEWAS lab to deliver valid data sedimentation should be addressed and limited in such a way that it does not significantly affect the accuracy of flow data. This report contains the observations of sedimentation over the past year and investigations into what might be causing the sedimentation as well as recommendations for mitigation and cleanup.

Significant amounts of sedimentation have been observed entering the stream from the upstream stormwater network during the past year. Figure 1 illustrates the sediment deposition in the upstream culvert near a weir located at the outlet on June 3rd, 2013. Significant sediment deposits have occurred both behind and in front of the weir structure. An island of sediment can be observed during base flow as illustrated in Figure 1; the depth of this location was not measured directly but was believed to be between 1.5 – 2 feet in height. Previous measurements of sediment height in the culvert during April 2013 found sediment depositions as much as 1.15 feet. Members of the lab walked back into the culvert approximately 60 feet and observed that the sediment continues to extend far back into the culvert.

Figure 1. Sedimentation in Upstream Culvert June 3rd, 2013

The sedimentation in the culvert changes over time due to high flows during storm events. Figure 2 illustrates the sedimentation in the culvert on June 1st, 2013. Comparing the area behind the weir it is clear that the island of sediment that was there just a month before is completely gone and the bottom of the culvert is exposed.

Figure 2. Sedimentation in Upstream Culvert July 1st, 2013

The Webb branch of Stroubles Creek is a highly urbanized watershed with high level of construction activity. Road, parking lot and driveway gravel as well as construction runoff could all contribute to sedimentation at the site during runoff events. To investigate these sources members of the lab toured the watershed during storm events to attempt to identify sources of runoff. There are two construction sites on Virginia Tech campus that contribute to the duck pond as illustrated in Figure 3.

Figure 3: LEWAS Watershed and Virginia Tech Construction

During a rain event on June 10th, 2013 a member of the lab took pictures of runoff from the new arts building construction site, given as construction site 2 in Figure 3. The pictures in Figure 4 illustrate how the silt fences meant to reduce sediment runoff have been compromised. Sediment is clearly running off of the construction site and into the stormwater network. It should be noted that this side of the construction site does not runoff into the stormwater network at the LEWAS site, however it is entering Stroubles creek and the duck pond via the stormwater network under the drill field.

Figure 4. June 10th, 2013 Construction Site Runoff

To investigate possible sources of sediment runoff into Webb branch members of the team toured the watershed on June 18th, 2013. All major roads in the watershed were explored and pictures of areas with high levels of sedimentation or where large amounts of sediment could clearly be seen entering the stormwater network were documented. Figure 5 demonstrates a sediment trap in front of a stormwater inlet that had been washed into the inlet and was no longer functioning. The image on the far right of Figure 5 shows sediment that had been washed into the stormwater inlet and on its way eventually to the LEWAS site and the duck pond.

Figure 5. Failed sediment trap

Figure 6. Sediment from construction runoff

Sediment from construction site 2 is illustrated in Figure 6. Even though there is a silt fence surrounding the construction site there is clearly sediment that has a high potential to runoff the site outside of the silt fence.

In addition to construction sites on Virginia Tech campus, sediment runoff occurs from areas where gravel is used to surface the ground. Figure 7 is a good example of a gravel surface on Virginia Tech campus that is contributing a large amount of sediment and gravel to the stormwater network.

Figure 7. Gravel runoff on Virginia Tech campus

Major sources of sediment in the watershed from within the town of Blacksburg appeared to be from gravel parking lots, roads, and driveways. Pictures taken from North Main, Giles Road, Patrick Henry Drive, and Progress Street indicate that sediment and gravel from these sources do runoff into the stormwater network.

Figure 8. Sediment sources in watershed within town of Blacksburg