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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:
Problem | Compliance
| Health & Safety | Sampling
| Analytical | Data
Quality
Site Assessment | Prediction
| Construction | GIS
| Monitoring & Assessment
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