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TECH GUIDE

Site Preparation: Hydrologic Controls: Sedimentation Basins/Ponds

Drainage ways, ditches, chutes, and, downpipes commonly direct water towards sedimentation basins. Surface water, captured by diversion structures, often carries with it significant concentrations of suspended solids which are picked up as the water flows over land. The suspended solids may be clean erosive sediments or they may be waste materials, such as tailings, which can pose an environmental threat if carried off-site. The purpose of installing a sedimentation basin is to slow down the velocity of surface run-off carrying solids by impounding water, thus allowing sufficient time for the suspended solids to settle. When water is brought to a velocity near zero, suspended solids tend to drop (settle) out of solution. Larger, denser particles will settle out first and smaller, less dense particles will settle out last. Sedimentation basins are designed to retain water for a residence time long enough to allow for settlement of the small particles such as clays and silts. Water from a diversion structure flows into a sedimentation basin through an inlet structure and flows out of the basin through an outlet structure that controls the outlet flow rate in order to maintain a desired water level in the basin. When inflows are above average, such as during a storm event, the water level in the basin will be above the average level. An emergency spillway is installed to divert flow downstream when the water level in the basin reaches a level that may threaten to overtop the banks. Emergency spillways, commonly made of pipe or conduit, allow for the safe conveyance of large flows past earth embankments.

The size of a sedimentation basin is determined from characteristics of the flow such as the inflow concentration of suspended solids, the desired effluent concentration of suspended solids (this value is usually regulated by local and/or Federal government authorities), the particle-size distribution of the suspended solids, and the peak influent flow rate to the pond. The peak influent flow rate is determined using a design storm event. As an example, EPA has chosen the 10-year, 24-hr precipitation event as a design criteria for the overflow rate determination. The overflow rate represents the peak influent flow rate that the basin can handle before the water level in the basin rises above the emergency spillway. The efficiency of sedimentation basins varies considerably as a function of the overflow rate. Basins perform poorly during periods of heavy rains and cannot be expected to remove the fine-grained suspended solids (i.e. clays and silts). If the sedimentation basin is expected to remove sediments that may have contain waste materials, consideration should be given to improving removal efficiencies by enlarging the basin, thereby giving the basin a larger residence time to allow suspended solids removal during periods of heavy rain.

The quantity of material to be stored is also an important design parameter that needs consideration before construction of a sedimentation basin takes place. The volume of settling occurring over time can be calculated by multiplying the total area disturbed by a constant sediment yield rate. The storage capacity of the sedimentation basin is determined by the settling volume over a given time period. The storage capacity of the basin should equal the volume of sediment expected to be trapped at the site during the planned useful life of the basin. If it is determined that periodic removal of sediment will be practicable, the capacity may be proportionally reduced and removal will be necessary on a reduced time interval (i.e. once a year).

For more information on sedimentation basins, see the following website:

 

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