Agenda - 08-23-1993 - C 2 and C 3 attch

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1',_ '3 to development. However, above the pond development will almost certainly increase the volume and rate of runoff. How a Wet Detention Pond Works The basic mechanism of pollutant removal is that suspended pollutant particles in runoff are captured by gravity settling as runoff enters the relatively still water in the pond (the same process that creates a delta). In the stream channel the energy of flowing water suspends the particles, but this churning action is lost as runoff enters the pond. This causes the particles to settle toward the bottom, the larger, heavier ones settling out first while very small ones may take days or weeks. Over time pollutants are trapped in the sediment that accumulates on the bottom. There may be some infiltration into the soil, but this is probably minimal in Orange County because of the tight soils and low infiltration rates. Particles, especially larger ones, are relatively easy to trap, but soluble nutrients have minimal settling velocities, so biological uptake is necessary. Soluble nutrients are removed by the biological activity of plants, alae, and other organisms that reside in the permanent pool. Plants convert soluble nutrients into biomass which can settle to the bottom where they can be consumed by bacteria. The ability of resident organisms to treat soluble nutrients is why wet detention ponds are preferred over dry ponds. Creation of the Desirable Environment A certain pond depth and volume must be established for these pollutant trapping and treatment processes to function properly. Consideration must be given to the particular pollutants to be captured, the size of the developed area the pond is built for, and the total drainage area to the pond, even if the offsite drainage area does not contribute significant pollution. The depth, volume, surface area, and shape of the pond must be such that inflow and wave action do not resuspend accumulated sediment and pollutants. Otherwise, pollutants will be washed through the pond and its purpose adversely affected. This is especially important at the shallow upper end where most pollutants accumulate as runoff enters the still water in the pond. Specifics from studies done on the efficiency of wet ponds can be found in the references listed in the bibliography. Designing the Pond To take advantage of the potential for pollutant removal, the pond must be designed to create conditions favorable for capturing and retaining pollutants and for sustaining the desired biological activity necessary to treat them. Such considerations as sufficient drainage area, adequate soils, runoff calculations, engineering, inspection during construction, and periodic inspection, maintenance, and repair must be considered. Who Designs, Inspects, and Certifies the Pond? A qualified registered professional engineer or landscape architect must design the pond, certify construction in writing, and provide as-built plans. DRAFT 4 June 1993