Whittier Narrows Dam is a 56-foot (17 m) tall

earth dam An embankment dam is a large artificial dam. It is typically created by the placement and compaction of a complex semi-plastic mound of various compositions of soil or rock. It has a semi-pervious waterproof natural covering for its surface an ...

on the San Gabriel River and the smaller, parallel Rio Hondo. The dam is located, as the name implies, at the

Whittier Narrows The Whittier Narrows is a narrows or water gap in the San Gabriel Valley of Los Angeles County, California, United States, between the Puente Hills to the east and the Montebello Hills to the west. The gap is located at the southern boundary of th ...

. It provides water conservation storage and is also the central element of the Los Angeles County Drainage Area (LACDA)

flood control Flood control methods are used to reduce or prevent the detrimental effects of flood waters."Flood Control", MSN Encarta, 2008 (see below: Further reading). Flood relief methods are used to reduce the effects of flood waters or high water level ...

system. Its reservoir has a capacity of . The Whittier Narrows are a natural gap in the hills that form the southern boundary of the

San Gabriel Valley The San Gabriel Valley ( es, Valle de San Gabriel) is one of the principal valleys of Southern California, lying immediately to the east of the eastern city limits of the city of Los Angeles, and occupying the vast majority of the eastern part ...

Southern California Southern California (commonly shortened to SoCal) is a geographic and Cultural area, cultural region that generally comprises the southern portion of the U.S. state of California. It includes the Los Angeles metropolitan area, the second most po ...

. Both the Rio Hondo, a tributary of the

Los Angeles River , name_etymology = , image = File:Los Angeles River from Fletcher Drive Bridge 2019.jpg , image_caption = L.A. River from Fletcher Drive Bridge , image_size = 300 , map = LARmap.jpg , map_size ...

, 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. In September 2017, the

United States Army Corps of Engineers , colors = , anniversaries = 16 June (Organization Day) , battles = , battles_label = Wars , website = , commander1 = ...

officials warned local residents that the dam no longer met the agency’s 'tolerable-risk' guidelines and could fail in the event of a very large, very rare storm, similar to exceptionally intense California storms which occurred between December 1861 and January 1862, a so-called

ARkStorm An ARkStorm (for Atmospheric River 1,000) is a "megastorm" proposed scenario based on repeated historical occurrences of atmospheric rivers and other major rain events first developed and published by the Multi-Hazards Demonstration Project (MHDP ...

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 , colors = , anniversaries = 16 June (Organization Day) , battles = , battles_label = Wars , website = , commander1 = ...

completed construction of the dam in 1957.

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 Santa Fe Dam is a flood-control dam on the San Gabriel River located in Irwindale in Los Angeles County, California, United States. For most of the year, the -high dam and its reservoir lie empty, but can hold more than of water during maj ...

. 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 The National Geodetic Vertical Datum of 1929 is the official name since 1973 of the vertical datum established for vertical control surveying in the United States, United States of America by the General Adjustment of 1929. Originally known as S ...

, releases are made to accommodate the capacity of the

spreading ground A spreading ground is a water conservation facility that retains surface water long enough for it to percolate into the soil. Spreading grounds must be located where underlying soils are permeable and connected to a target aquifer. Locating them ...

s downstream along the Rio Hondo. The spreading grounds are operated by the

Los Angeles County Department of Public Works The Los Angeles County Department of Public Works (LACDPW) is responsible for the construction and operation of Los Angeles County, California, Los Angeles County's roads, building code, building safety, Sewage collection and disposal, sewerage, a ...

to recharge the

groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...

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 5000 ft³/s (142 m³/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 228.5 ft (69.6 m) NGVD. The capacity of the Rio Hondo downstream from Whittier Narrows Dam is approximately 36,500 ft³/s (1,034 m³/s). The capacity of the Los Angeles River downstream of its confluence with the Rio Hondo is approximately 127,000 ft³/s (3,596 m³/s), and the capacity of the San Gabriel River downstream of the dam is approximately 13,000 ft³/s (371 m³/s).

References

External links

* {{Authority control Dams in Los Angeles County, California Geography of the San Gabriel Valley San Gabriel River (California) Montebello, California United States Army Corps of Engineers dams Los Angeles County Department of Public Works Dams completed in 1957 1957 establishments in California

Flood control

See also

References

External links