Labyrinth Weirs – A Cost Effective Spillway Alternative

Abstract Only - As the emphasis in dam safety has increased with time, many spillways have been identified as having inadequate capacity to convey the regulatory flood event. A typical problem for existing dams is how to enlarge spillways cost effectively when the width of the approach channel or the downstream chute cannot be widened. A cost effective approach is to increase the spillway capacity with a labyrinth weir. Viewed from above, a labyrinth weir presents a striking picture, with its zigzag plan forming a pattern a little like a Navajo rug. The use of labyrinth weirs is also becoming more popular for new dam construction. The spillway cost for a new dam can be as high as 30 percent or even higher of the overall construction cost. Therefore, it is very important to minimize the spillway size as much as possible to save cost, but still meet dam safety standards. The use of a labyrinth weir in concert with a site-specific rainfall study can greatly reduce spillway requirements even more. The overall construction cost of a project can also be greatly reduced if a labyrinth weir is constructed in conjunction with a stepped-chute spillway. The labyrinth weir will reduce the overall width of the spillway while the stepped-chute will reduce the amount of energy to be dissipated within a stilling basin. Boyle is presently preparing several labyrinth weir designs for existing and new dams. The designs include different combinations of site-specific rainfall studies and stepped-chute spillways to meet project requirements. The spillways range in flow capacities from 3,000 cfs to over 40,000 cfs; they will be constructed in urban and rural settings; they consist of two-cycles to over fifty-cycles in length; and are founded on native rock, embankment fill, and roller-compacted-concrete. A labyrinth weir is being considered as a possible alternative to increase the spillway capacity for the Lake Roberts Dam located in southwestern New Mexico. This project and others will be presented with respect to design considerations and the cost savings with respect to using a labyrinth weir compared with a linear weir. The discussion on the design considerations will include the history and an overview of the hydraulic and structural design requirements for labyrinth weirs.