Whittier Narrows Dam - Flood Control

Flood Control

The purpose of the project is to collect runoff from the uncontrolled drainage areas upstream along with releases into the San Gabriel River from the Santa Fe Dam. If the inflow to the reservoir exceeds the groundwater recharge capacity of the spreading grounds along the Rio Hondo or the bed of the San Gabriel River downstream, this water is stored temporarily in a water conservation pool. The Rio Hondo and San Gabriel sides of the reservoir each have their own water conservation pools. If the water conservation pool on the Rio Hondo side is exceeded, flows are released into the Rio Hondo at a rate which not exceed the downstream channel capacity of either the Rio Hondo or the Los Angeles River. If the water conservation pool on either side of the reservoir is exceeded a release of approximately 142 m³/s (5000 ft³/s) can be made into the San Gabriel River. If the pool in the reservoir exceeds flood control storage, the gates on the San Gabriel River outlet begin to open automatically and emergency releases are made into the San Gabriel River.

The Rio Hondo outlet has four main outlet passages plus a small diversion passage. The San Gabriel outlet has nine large gates installed on top of a spillway. Dimensions are furnished in the table below.

The "stand-by" position of the gates on the Rio Hondo outlet is wide open. On the San Gabriel side one gate is normally open about 15 cm (0.5 ft) with the remaining gates closed. The reservoir is normally empty and a "crossover weir" within the reservoir keeps the flows from the Rio Hondo and the San Gabriel River separated. The natural flow to each river therefore normally passes through the dam unhindered.

During the initial stages of a flood event, the gates on the Rio Hondo side are partially closed to build a water conservation pool. As long as the pool on the Rio Hondo side of the reservoir is below elevation 61.4 m (201.6 ft) NGVD, releases are made to accommodate the capacity of the spreading grounds downstream along the Rio Hondo. The spreading grounds are operated by the Los Angeles County Department of Public Works to recharge the groundwater basin. Flow reaches the spreading grounds either directly by way of the diversion passage or from a diversion structure in the Rio Hondo downstream of the dam. Both the diversion passage direct from the dam and the diversion structure in the Rio Hondo are operated by the county.

When the water conservation pool on the Rio Hondo side of the reservoir is exceeded, the releases to the Rio Hondo are increased to match inflow until either the capacity of the Rio Hondo or the Los Angeles River downstream are reached. If the water conservation pool on either side of the reservoir is exceeded, discharges on the San Gabriel side can be increased to approximately 142 m³/s (5000 ft³/s).

The San Gabriel outlet has automatic spillway gates. When the pool in the reservoir exceeds flood control storage these gates will begin to open automatically. The top of the flood control storage pool is at elevation 69.6 m (228.5 ft) NGVD.

The capacity of the Rio Hondo downstream from Whittier Narrows Dam is approximately 1,034 m³/s (36,500 ft³/s). The capacity of the Los Angeles River downstream of its confluence with the Rio Hondo is approximately 3,596 m³/s (127,000 ft³/s), and the capacity of the San Gabriel River downstream of the dam is approximately 371 m³/s (13,100 ft³/s).