MODULAR ENERGY DISSIPATORS

Unless checked by energy dissipators, water exiting from culverts may cause considerable damage. However, the rock fills normally used for this purpose are not permanent, as repeated flooding moves the rocks. The University of Akron has designed a permanent and low cost structure, the modular energy dissipator (MED), consisting of precast reinforced concrete components that could be adapted to local conditions and assembled at the site with considerable ease. To develop reliable design specifications for modular energy dissipators, model studies were conducted at the Hydraulic Laboratory at the University of Akron. A wide variety of basin shapes were studied both for culvert outlets at grade and for cantilevered outlets. Each of the selected arrangements was analyzed in terms of basin stability and in terms of scour formation inside and outside of the basin. The basin stability and the resulting scour patterns were found to depend on flow properties existing at the culvert outlet, the bed material, the dimensions and shape of the MED, and the dimensions of the downstream channel. Results indicated that undermining was the most critical type of failure of modular dissipators. It depended on the stone size as well as the dimensions and location of the structure. It was shown that placement of riprap inside the basin contributed significantly to its stability, as long as the stone size allowed the formation of a scour hole within the basin. Within the flow ranges tested, the best performance obtained was at a length-to-diameter ratio of 9.0 and at a side flare of 3 to 1 for cantilevered culverts. With culvert outlets at grade, modular units set 1 diameter below the culvert's invert provided stable performance. For small tailwater elevations the height of the back panels may be set at 0.25 times the diameter of the culvert, and for high tailwaters the back panels may be set slightly above the tailwater elevation. Also, for culverts at grade, the back panel may be set at 2 to 3.5 times the culvert diameter.