Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams11.4.4 Removal of the control structure of the spillway (concrete spillway)The concrete control structure/weir of an existing spillway can be partially or fullyremoved to facilitate the lowering of the reservoir in the event of an emergency. Theconcrete control structure is typically used in earth cut spillways as a grade fill. Ifblasting is employed to remove the concrete, caution should be taken to ensure thatthe blasting does not cause additional damage to the embankment dam orfoundation. Controlled blasting techniques with minimum particle velocities can beused effectively to remove the concrete structure. Figure 155 shows a concretecontrol structure being partially breached to allow for lowering of the reservoir.11.4.5 Excavation of a trench through an earthcut spillwayFor embankment dams with an earthcut spillway channel or emergency spillway atone of the abutments, a trench can be excavated through the discharge channel todeepen and/or widen the existing spillway discharge channel. Care should beemployed to ensure that water being released through the new channel does notcause erosional headcutting of the channel, resulting in an uncontrolled larger-thanplannedreservoir release. The trench should be excavated down to or into erosionresistantmaterials, if possible. If not, the excavated channel should be protected byplacing erosion-resistant materials in the channel, such as rock riprap, concreterubble (if temporary), sandbagging, plastic sheeting, or geotextiles. The materialsshould be properly placed in the channel and stabilized to prevent the materials frombeing washed away. If sandbagging is used in high velocity flows, the sandbagsshould be placed beginning near the edge of the flow, where the velocities are low,and working toward the high velocity area. The largest sandbag possible should beused, and the ends of the bags should be securely fastened so that material is notwashed out.11.4.6 Excavation of a spillway through the embankment dam abutmentSimilarly to excavating a channel through an existing earthcut spillway, a trench canbe excavated through the abutment of the embankment dam (figure 156) to provideemergency release of the reservoir, if required. The trench should be properlylocated to ensure that uncontrolled releases of the reservoir through the channel donot encroach upon the embankment dam and cause an unanticipated breaching ofthe embankment dam. Care should also be employed to ensure that the excavatedchannel does not cause larger-than-anticipated flooding downstream of theembankment dam. Construction and cautions similar to those mentioned in theprevious section for the excavation of a trench through an existing earthcut spillwayshould be employed.284