Sedimentation Basin Design          

Sedimentation basins are structures, typically found around highways and construction sites, to capture coarse sediment from storm flows.

The purpose of the basins is to:

• Capture course sediments
• Temporarily detain stormwater so that the velocity of storm flows is reduced and the sediment falls.

Typically, a large proportion of nutrients in stormwater will be bound to the suspended sediment particles. Therefore, since sedimentation basins are designed to facilitate the settling of sediment, they are also effective at removing a large proportion of particulate-based nutrients from the water. The rate at which a sedimentation basin removes sediment from the water is affected by the sediment particle size, the velocity of the water, and detention time of the stormwater.

Three Types of Sedimentation Basins

Expedient Basins
Small, exist for only a short time (possibly only one night), and their sites are determined by the engineer during grading operations.

Temporary Basins
Remain in place for the entire duration of a construction project or until the need for them clearly has passed. Their locations are usually shown on the plans, and they are built to a higher standard than expedient basins.

Permanent Basins
Used to intercept sediment during construction but remain after construction for other uses such as recreation, scenic enhancement, floodwater detention, or groundwater recharge.

Design Philosophies
There are two design philosophies for sedimentation basins.

   Design Philosophy #1:
   A basin shall only discharge during major storms and not during small,     
   frequent runoffs. These basins are designed to trap all sediment
   (except from major storms) and are cleaned out often, probably after
   each storm.

   Design Philosophy #2:
   A basin shall operate as a detention reservoir while sediment is deposited 
   by flow moving slowly through it. Calculation of basin size requires
   information about the drainage area that includes erosion characteristics,
   surface cover and condition, and length and steepness of slopes.

• Estimate the sediment volume to be stored
• Determine the percentage of eroded volume that will reach the basin
• Estimate the efficiency of the basin in trapping sediment