124. Embankment Spillways

This article is the third of a series sharing practical information and insights about civil deisgn and engineering. The very name “Embankment Spillways” will seem to many dam engineers like a contradiction in terms. Spillways are almost always sited on natural ground adjacent to any embankment, sometimes at considerable cost. There are perhaps two reasons for this. First is the matter of crossing design disciplines. Embankment dams tend to be designed by geotechnical engineers, while spillways tend to be designed by hydraulic structures engineers. It is one thing to put a clear interface between the two, such as a lateral retaining wall. It is completely different to incorporate the two into a single concept. Second, embankments settle with time and are inherently erodible. Most dam engineers prefer a spillway on an entirely separate foundation, where settlement is limited or non-existent and where any leakage cannot directly affect the main dam embankment. Therefore, it is surprising to many to see how many embankments have been designed to accommodate either temporary or permanent flood discharge. In some situations, flows are passed over moderateheight earth embankments, but such cases are generally limited to low-unit discharges over cohesive soil slopes protected with grass, geomembrane, and/or concrete block “reinforced grass” protection. Where more significant discharges are concerned, typical options are: reinforced rockfill, buttressing rockfill downstream, or permanent concrete spillways. A review of these three options, with examples of where they have been successfully used, provides insight into the concept of “Embankment Spillways”.