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<br />3-2.3 Inlet Control. <br /> <br />Under inlet control the discharge from the culvert is controlled at the <br />entrance of the culvert by headwater depth and inlet geometry. Inlet geometry <br />includes the cross-sectional area, shape, and type of inlet edge. Inlet <br />control governs the discharge as long as water can flow out of the culvert <br />faster than it can enter the culvert. <br /> <br />Most culverts, except those in flat terrain, are designed to operate under <br />inlet control during peak flows. Since the entrance characteristics govern, <br />minor modifications at the culvert inlet can significantly effect hydraulic <br />capacity. For example, change in the approach alignment of the stream may <br />reduce capacity, while the improvement of the inlet edge condition, or <br />addition of properly designed headwalls and wingwa11s, may increase the <br />capacity. <br /> <br />3-2.4 Outlet Control. <br /> <br />Under outlet control water can enter the culvert faster than water can flow <br />through the culvert. The discharge is influenced by the same factors as inlet <br />control plus the tai1water depth and barrel characteristics (slope, length, <br />and roughness). Culverts operating with outlet control usually lie on flat <br />slopes or have high tailwater. <br /> <br />When culverts are operating with outlet control, changes in barrel <br />characteristics or tailwater depth may effect capacity. For example, <br />increased tailwater depth or debris in the culvert barrel may reduce the <br />capacity. <br /> <br />3-2.5 Special Hydraulic Considerations. <br /> <br />a. Inlet and Outlet Protection--The inlets and outlets of culverts may <br />require protection to withstand the hydraulic forces exerted during peak <br />flows. Inlet ends of flexible pipe culverts which are not adequately <br />protected or anchored may be subject to entrance failures due to buoyant <br />forces. The outlet may require energy dissipators to control erosion and <br />scour and to protect downstream properties. High outlet velocities may cause <br />scour which undermines the endwall, wingwalls, and culvert barrel. This <br />erosion can cause end-section drop-off in rigid sectional pipe culverts. <br /> <br />b. Protection Against Piping--Seepage along the outside of the culvert <br />barrel may remove supporting material. This process is referred to as piping <br />since a hollow similar to a pipe is often formed. Piping can also occur <br />through open joints. Piping is controlled by reducing the amount and velocity <br />of water seeping along the outside of the culvert barrel. This may require <br />watertight joints and in some cases anti-seep collars. Good backfill material <br />and adequate compaction of that material are also important. <br /> <br />29 <br />