P.T. Weiss, A.J. Erickson, J.S. Gulliver, R.M. Hozalski
Some sedimentation practices may be too large for synthetic runoff testing (i.e. a sufficient water supply is not available) and therefore may require monitoring (level 4) to achieve the assessment goals. To successfully monitor a stormwater treatment practice, it is necessary to follow appropriate procedures for Water Budget Measurement, Sampling Methods, and Analysis of Water and Soils. In addition, there are some monitoring considerations specific to dry ponds, wet ponds, and underground sedimentation practices provided in the following sections.
By monitoring dry ponds, one can assess the peak flow reduction and pollutant removal efficiency. Measuring and comparing inflow and outflow hydrographs for a dry pond can give an estimate of the reduction in peak flow for a given storm event and, therefore, an estimate of the hydraulic effectiveness of the stormwater treatment practice. Results from sampling and analyzing stormwater samples from the inflow and outflow can be used to estimate the pollutant removal effectiveness. See Data Analysis for guidance on analyzing data collected from monitoring studies.
Monitoring of wet ponds (also known as wet detention basins) is well documented (Wu et al. 1996, Comings et al. 2000, Koob 2002, Mallin et al. 2002). Short-circuiting within a wet pond can be estimated by monitoring the movement of a naturally occurring conservative tracer, such as chloride, as it moves through a wet pond if a sufficient pulse in concentration has occurred at the inlet. Plotting the inflow and outflow tracer concentrations as a function of time and comparing the two curves can determine if, and to what extent, short-circuiting may be occurring (see Example 10.2).
Underground sedimentation devices
Monitoring wet vaults and proprietary devices for hydraulic performance or water quality treatment is not recommended because wet vaults and proprietary devices are typically designed for small sub-watersheds and are located underground with limited access. Monitoring these systems can be costly, labor-intensive, and result in little, if any, conclusive data.
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