Using baffles to improve sediment basins

SoilFacts Using Baffles to Improve Sediment Basins ■ Sediment traps and basins at construction sites, agricultural operations, and other unsettled areas provide temporary pools for runoff that allow sediment to settle before the water is discharged into a river, stream, or landscape. They prevent erosion and trap sediment and other coarse material. They are most effective in sandy soils and less effective in clayey soils. Unfortuna tely, these traps and basins are not efficient when the swift, turbulent water moves along a straight-line flow that takes runoff quickly to the basin s outlet. This short-circuits interaction with the entire basin. Using baffles to slow, calm, and distribute the water can help solve this problem. Baffles can lengthen the flow path and distribute the flow more widely. They significantly increase the amount of sediment captured and also trap much smaller particles than open basins. The flow path Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914, North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, or disability. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating. The typical flow in an open basin receiving peak-period, high-speed runoff is illustrated in Figure 1. The majority of the water entering the basin rushes down the middle from inlet to outlet and does not interact with the entire basin area. Turbulence resulting from the high flow rate gives rise to backflow currents and side eddies, which interfere with the set¬ tling process and reduce settling efficiency. Installing either solid or porous baffles can improve the basin’s efficiency. Solid baffles are the simplest to install, but porous baf¬ fles are more efficient. Solid baffles A simple way to lengthen the flow path is to install solid baffles that force the water to move from side to side in the basin (Figure 2). These may be made of sturdy plywood or similar materials, usually with a notch or weir cut into the top at opposite ends to create a long, back-and-forth flow path. Silt fencing (or geotextile) has also been used because the material is sturdy, inexpensive, Figure. 1 Flow pattern of water in an open sediment trap during peak runoff. Note the high velocity down the middle, as swift water bypasses much of the basin’s breadth. There are significant back flows near the outlet and smaller side eddies in the middle and near the inlet, all of which reduce settling efficiency. STATE UNIVERSITY A&T STATE UNIVERSITY COOPERATIVE EXTENSION Helping People Put Knowledge to Work