The National Academies

NCHRP Synthesis 20-05/Topic 09-10 [Final (Synthesis)]

Design of Sedimentation Basins
[ NCHRP 20-05 (Synthesis of Information Related to Highway Practices) ]

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Sedimentation basins are useful for minimizing the effects of highway construction runoff on the environment and are often used in conjunction with other sediment-control practices. A sedimentation basin protects streams, lakes, recreation areas, and other areas that cannot tolerate sediment deposition. There are three types of sedimentation basins--expedient, temporary, and permanent. Expedient basins are quite 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 are used to intercept sediment during construction but remain after construction for other uses such as recreation, scenic enhancement, floodwater detention, or groundwater recharge. There are two design philosophies for sedimentation basins. According to one, a basin shall not discharge during small, frequent runoffs but shall be allowed to discharge during major storms. These basins are designed to trap all sediment (except from major storms) and are cleaned out often, probably after each storm. Adherents to the second philosophy hold that a basin operates as a detention reservoir while sediment is deposited by flow moving slowly through. Calculation of basin size requires information about the drainage area that includes erosion characteristics, surface cover and condition, and length and steepness of slopes. For the flow- detention philosophy, the designer will also need to estimate the sediment volume to be stored (using a rule of thumb or the universal soil loss equation), determine the percentage of eroded volume that will reach the basin, and estimate the efficiency of the basin in trapping sediment. Then the height of the dam can be calculated and the principal and emergency spillways can be designed.   The report for this topic can be purchased at http://www.trb.org/news/blurb_detail.asp?id=3487

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