Nebraska LTAP


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Two types of energy dissipation devices at circular culvert outlets were investigated: full-length weirs and staggered weirs. Related literature was reviewed; a model broken-back circular culvert and dissipation basin was built; instrumentation was installed to measure discharge, piezometric head, and velocities; and four sizes of full-length and staggered weirs were tested over a range of discharges and tailwaters. Weir heights ranged from D/8 to 4D/8, in which D was the culvert diameter. The two weir types were subjected to two styles of tests: (1) tests unaffected by tailwater and (2) tailwater-influenced tests. For tall full-length weirs (3D/8 and 4D/8), basin outlet depths could be reasonably predicted with a simple weir equation, general assumptions about the upstream flow, and the energy equation with no head losses. For shorter weirs (D/8 and 2D/8), the flow skimmed the weir and the weir equation was invalid, especially for high discharges. In these cases, the weirs were not effective energy dissipators. For the tallest weirs, the ratio of outlet energy and the critical depth was roughly constant. The outlet specific energy was about 3.2 and 2.9 times the critical depth for weir heights of 4D/8 and 3D/8, respectively. Similar results were found for the staggered weir, but specific energy was found to be 2.7 and 2.9 times the critical depth for weir heights of 4D/8 and 3D/8, respectively. Results can be used to determine dissipation basin outlet velocities for incoming runout Froude numbers in the range of 3.8 to 4.6 for full- length and staggered weirs with heights ranging from D/8 to 4D/8.