Flood and Spillway

The modern criteria for spillway design, as advocated by ICOLD, are based upon two floods: the traditional “Design Flood” chosen most often with a yearly probability between 1/100 and 1/1,000. The reservoir level is kept well under the dam crest (freeboard).The second is the “Check Flood” of much lower probability (or chosen as the Probable Maximum Flood) for which the reservoir level is close to embankments crest and possibly over concrete dams crest. Damages are acceptable but not the dam failure.

The failure of dams by flood is caused by a small overtopping of embankment dams and a huge overtoping of high concrete dams.
For most existing and many new dams the overall design has been based upon a “Design Flood” for which the reservoir level is kept well under the dam crest level.

Present methods for designing spillways are generally based on traditional criteria, which are not well adapted to present knowledge of actual risks. A review of facts and costs suggests that better adapted criteria and methods of design could reduce risks and costs.

As everywhere else in the world also in China reservoirs have made a tremendous contribution to multipurpose utilization of water resources thus creating benefits and mitigating disasters.

Most reports of failures were taken from ICOLD Bulletin 99 “Dam Failures ­Statistical Analysis”. However, about twenty of the failures mentioned in the Bulletin were not cIassified, through lack of data or because they were not due to the Irerein classified causes, and 15 of the failures in this Appendix do not appear in Bulletin 99 (mark X).

For several reasons the design of spillways represents a significant opportunity for cost saving. Spillways not only represent a significant direct costs, by virtue of their design and possibly by their impact on the overall project layout, but also on the potential cost of lost storage in the case of ungated spillways.