Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams• Water requirements.—Providing for diversion and downstream waterrequirements (irrigation, etc.).• Traffic control.—Traffic control measures (lighting, signs, etc), road closures,construction of detours (such as detouring dam crest traffic).• Disturbance.—Larger disturbance areas and potential environmental issues.• Draining or drawing down of the reservoir.—Caution is required if the reservoir isdrained, since the existence of a heavy bed load could move and block theintake structure.13.7 Construction of a cofferdam or temporary diversion channel.A cofferdam (figure 187) may be needed to act as a temporary barrier to protect theconstruction area from flooding (Reclamation, 1987a, pp. 499-500). If constructionfor the removal and replacement of the existing conduit can be performed during thelow water season, the use of a cofferdam may be minimal. However, where thereservoir inflow characteristics are such that construction cannot be done during alow water season, the cofferdam must be designed for safety and for optimal heightto accommodate the full range of expected inflows during the construction period.Often the flood selected for sizing diversion requirements is based on the projectedlength of the construction period. For instance, if the projected length of theconstruction period is 1 year, the cofferdam would need to be able to accommodatea flood with a return interval of 5 years. If the projected length of the constructionperiod is 2 years, a 10-year diversion flood would be used.At some sites, a diversion channel in lieu of a cofferdam may be more practical.Diversions or cofferdams may not be justified for smaller embankment dam projectswith limited working room for the removal and replacement of conduits. For theseprojects, certain hydrologic events during construction may impede progress andcause delays resulting in added costs for clean up of debris caused by flooding. Forthis situation, construction should be scheduled during periods when the lowestrainfall is probable to avoid these problems. Figure 188 shows an embankment damsite where rainfall during construction has partially flooded the exposed excavationand conduit removal. This flooding resulted in a delay and cleanup beforeconstruction could resume.13.8 Design of entrance and terminal structuresThe removal and replacement of an existing conduit may require partial or fullremoval and replacement of certain structures to improve release capabilities or to336