Forest Roads and Sediment Project at the Reynolds Homestead: Field Work Update, Summer 2012

We are nearly two-thirds of the way finished with rainfall simulation experiments to measure sediment delivery from forest road approaches to stream crossings at the Reynolds Homestead. We have been doing three rainfall simulation experiments at intensities ranging from 20 to 60 mm/hr at each of six road approaches with the following treatments: 1) fresh bulldozer traffic to maximize bare soil, 2) a 32-ft. gravel section closest to the stream, and 3) a 64-ft. gravel section closest to the stream. In the end, we’ll have 54 rainfall simulation experiments. Currently, we have completed 30.

Below are some photos of the process:

Clay Sawyers uses a dump truck to apply gravel to the road approach surface.

We apply the rainfall, measure surface runoff volume, rainfall amount and intensity, and use an ISCO automatic stormwater sampler to collect runoff samples for analysis of total suspended solids (TSS).

Road surface runoff during a simulation experiment on June 8, 2012. The 32-ft. gravel section makes up about 40% of the road approach surface area in this case.

Surface runoff is captured by an open-top box culvert and redistributed through a cutthroat flume and finally, into a sediment collection bag (true product name =Dirtbag).

Water level is monitored during rainfall simulation experiments in the stilling well pictured in this 1″ X 18″ cutthroat flume in order to calculate runoff volumes. Stormwater samples are collected for laboratory analysis of TSS concentration. TSS concentration (g/L) X runoff volume (L) = storm event sediment load

In addition to the rainfall simulation experiments, we are also measuring sediment delivery from forest road approaches to stream crossings through the use of sediment traps and monthly/large rain event monitoring of sediment deposition through the use of a total station and the process of differential leveling.

On June 19, 2012 Brian Morris and I performed the differential leveling process at the nine silt fence sites at the Reynolds Homestead for the 11th time since this project began in August 2011. We found that five of the silt fences had been damaged by surface runoff that likely occurred June 11-12, 2012, when nearly 4 inches of rain fell over the Homestead study sites.

To minimize the loss of trapped sediment and to avoid missing any storm events, I returned on June 22 to repair the silt fences.

I was startled/amazed to find a buddy hanging out on one of my silt fences..

I finished repairing a few of the sites by noon and then a thunderstorm rolled in and dumped an inch of rain on the sites. I went back to see the silt fences in action.

Overall, I was pleased with the amount of the runoff and sediment that the fences were trapping. We’re losing some of the fine sediment when the storage created by the silt fences is overwhelmed, but I feel that some loss is inevitable. For example, it’s taken three silt fences to trap the sediment delivered at this 427-ft. bare road approach:

In 10 months of monitoring, we’ve collected over 10 tons of sediment at this site! I think this site emphasizes the importance of adequately spacing water bars and minimizing bare soil on forest roads.

We’ve trapped a little more than one ton at one other site, but the rest of the sites range from 0.05 to 0.40 tons over the duration of this study.

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