16 ENVIRONMENTAL CONNECTION / THIRD QUARTER 2025 / IECA.ORG Promote Sedimentation: Porous Baffle Typical flow in a basin without baffles would see high-speed runoff enter the basin and move at a high velocity down the middle of the basin toward the outlet, which increases overall turbulence while ignoring using the sides of the basin for sediment settling. Porous baffles serve to improve the efficiency of sediment basins by spreading flow across the entire width of the sediment basin and slowing flow velocity to facilitate gravitational settling. Initial flow into the basin will generally be turbulent, and when there is turbulence, there inhibits the ability of sediment to settle to the bottom of the basin easily. Water that makes its way through a porous baffle will see its turbulent flow converted to laminar flow, and with laminar flow comes smooth sheets of water flowing on top of each other with little to no turbulence, creating conditions ideal for settling to occur. >> A minimum of three porous baffles should be installed perpendicularly to the flow of water between the inlet and the outlet to ensure the entire surface area of the basin is used for settling (Figure 2). One effective material for baffles is coir fiber matting sized between 700 to 900g/m2 (21 to 27 oz/yard3). Baffles should be installed so they are fully extended into the side slopes and the bottom of the basin and at a height that matches the depth of the flow over the auxiliary spillway. Flow should not be allowed under, over or around the baffles. Part Three: Primary Dewatering Outlet In a basin, as water settles to the bottom of the pond, the water at the surface will always have the lowest concentration of sediment. Therefore, basins must be dewatered from the top of the water column. The preferred method to accomplish dewatering from the surface is with a surface skimmer. Skimmers float on the surface of the water. They rely on gravity to remove water from the surface and make it flow through their plumbing and eventually through the outlet of the basin. The skimmer rises and falls as the basin fills and drains. The size of the orifice of the skimmer controls the rate at which the basin is dewatered. The design dewatering time of a basin is two to six days. Dividing the stormwater storage of the basin by this dewatering time will determine the necessary flow rate for the skimmer. It is important to list the manufacturer of the skimmer used as the basis for design on project plans. This ensures that the skimmer with the appropriate flow rate is installed in the basin. Skimmers should be installed near the outlet of the basin to allow for maximum flow time for water from inflow to outflow (Figure 3). Giving the skimmer something to rest on, like a bed of riprap or a dewatering pad, will keep the skimmer from getting mired in sediment when it is at the bottom of the basin. EROSION CONTROL PEER REVIEW
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