P.T. Weiss, A.J. Erickson, J.S. Gulliver, R.M. Hozalski
Capacity testing (level 2) can be applied to sedimentation practices to estimate sediment storage capacity. All sedimentation practices can be assessed with sediment retention tests if adequate access is available. If repeat measurements are made over time, the sediment accumulation rate can be estimated. Dry ponds can also be assessed with hydraulic conductivity tests.
Hydraulic conductivity testing of dry ponds is used to estimate the rate at which stored water infiltrates into the soil. This value can then be used to estimate the runoff volume reduction by infiltration. A single point measurement with a Modified Philip-Dunne infiltrometer can take between 30 seconds and several hours, depending on the soil characteristics of the dry pond. Conductivity tests should be performed shortly after construction to establish a baseline for future tests and to investigate or identify construction impacts on infiltration capacity.
Sediment retention tests are used to estimate the depth and, subsequently, volume of sediment retained in a stormwater treatment practice. Surface elevations in a dry pond are measured either with a level and level rod or a total station (i.e., surveying equipment), and the corresponding longitude and latitude are recorded either with GPS or with a total station. Using the basin topography and the original topography (from as-built plans or design drawings), the amount of sediment retained in the stormwater treatment practice can be estimated. The amount of retained sediment can be compared to the design capacity to determine the available sediment retention capacity and to estimate when the pond will require maintenance (i.e., sediment cleanout). One to three days are typically required to perform sediment retention assessment for each dry pond.
Sediment retention tests can be performed on a wet pond to estimate the depth and subsequently volume of sediment retained. Bottom elevations in a wet pond are measured either with a level and level rod (from a boat) or with a sonar depth measurement device. The water surface can be used as a local elevation standard if a staff gauge has been installed in the pond to measure water surface elevation. Sonar depth measurements can be made in the winter when the wet pond is covered with sufficient ice to traverse or in the summer from a boat or while using waders. Corresponding longitude and latitude are recorded either with GPS or with a total station. Using the basin topography and the original topography (from as-built plans or design drawings), the amount of sediment retained in the pond can be estimated. The volume of retained sediment can be compared to the design capacity to determine the available sediment retention capacity and to estimate when the pond will require maintenance (i.e., sediment cleanout). As with dry ponds, it is recommended that these tests be performed soon after construction is complete to develop a benchmark for future assessment.
Underground sedimentation devices
If the sediment collection area can be accessed, sediment retention testing can be performed by utilizing staff gauges or visual benchmarks and as-built plans to determine the volume of sediment collected. These measurements can be used with estimates or measurements of sediment inflow rates to develop a maintenance or cleanout schedule. When the collected solids volume meets or exceeds the solids storage capacity of a wet vault or proprietary device, solids will no longer be removed at desired levels. Furthermore, re-suspension of retained solids can result in negative pollutant removal efficiencies.
Continue to Synthetic Runoff Testing.