Dam Safety: Sediments and Debris Problems

Sedimentation of reservoirs has its negative impacts on dams, first by reducinguseful storage, altering the benefit/cost ratio originally calculated for the dam, andsecond by reducing the dams’ capacity for flood routing; increasing floodinghazards on the dam itself and for the downstream. More problems can be created bysediments and floating debris during floods on outlet structures by clogging themand thus creating dangerous situations, or damage trash screens leading to evenmore problems. If these debris and coarse sediments are allowed in, then they maydamage dam structures such as gates, spillways intakes in addition to chutes, stillingbasins and power penstocks by the mechanical abrasion impacts of such sedimentson them. Frequent inspections, especially after floods must be made to ensureproper functioning of such structure and take actions for reducing the damage. Insmall reservoirs, dredging; although it adds to maintenance cost, may ease theproblem, but in very large reservoirs, this may prove unpractical. Designers,therefore, have a duty to consider sedimentation problem seriously in the initialstages of design by: checking the anticipated accumulation of sediments, allowingenough storage free from siltation, foreseeing their negative impacts on intakes andoutlet structures and taking design measures to reduce these impacts. At the sametime, dam stability calculations shall have to provision for the anticipated newconditions of silting up at the face of the dam. Operators of dams, on the other hand,shall have to keep open eyes for all the negative issues created by sediments andfloating debris, repairing damages caused by them and take measures to reduce theirimpacts in the future.