Whittier Narrows Dam
| Whittier Narrows Dam | |
|---|---|
Aerial view of Whittier Narrows Dam from downstream looking upstream through the gap.
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| Location | Los Angeles County, California, United States |
| Coordinates | 34°01′12″N 118°04′58″W / 34.02000°N 118.08278°WCoordinates: 34°01′12″N 118°04′58″W / 34.02000°N 118.08278°W |
| Opening date | 1956 |
| Dam and spillways | |
| Impounds |
Rio Hondo San Gabriel River |
| Height (foundation) | 56 feet (17 m) |
| Length | 16,960 feet (5,170 m) |
| Elevation at crest | 236 feet (72 m) |
| Width (crest) | 32 feet (9.8 m) |
| Dam volume | 12,166 cu yd (9,302 m3) |
| Spillway type | Automated spillway overflow |
| Parapet width | 16 ft (4.9 m) |
| Hydraulic head | 182 ft (55 m) |
Whittier Narrows Dam is a 56-foot (17 m) tall earth dam on the San Gabriel River located, as its name implies, at the Whittier Narrows in Montebello, California. It provides water conservation storage and is also the central element of the Los Angeles County Drainage Area (LACDA) flood control system. Its reservoir has a capacity of 67,060 acre·ft (82,720,000 m3).
The Whittier Narrows are a natural gap in the hills that form the southern boundary of the San Gabriel Valley of Southern California. The Rio Hondo and the San Gabriel River flow through this gap and are impounded by the reservoir. The Pomona Freeway (CA-60) passes through the reservoir flood control basin and the San Gabriel River Freeway (I-605) passes along the eastern boundary of the basin.
Authorization for the project construction is contained in the Flood Control Act of 18 August 1941 (PL 77-228) and the initial funds for construction were provided in the 1949 Appropriations Bill. The U.S. Army Corps of Engineers completed construction of the dam in 1957.
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.
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 .5 ft (15 cm) 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 201.6 ft (61.4 m) NGVD29, 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.
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