Gibraltar Dam
Gibraltar Dam is located on the Santa Ynez River, in southeastern Santa Barbara County, California, in the United States. Forming Gibraltar Reservoir, the dam is owned by the city of Santa Barbara. Originally constructed in 1920 and expanded in 1948, the dam and reservoir are located in a remote part of the Los Padres National Forest.
The main purpose of Gibraltar Dam is domestic water supply. It provides about of water to Santa Barbara each year, supplying almost 30% of the city's needs. Water diverted from the dam also powers a small hydroelectric plant. Due to having lost a massive portion of its capacity to sediment build-up, the lake can often fill and spill after a single storm, while drying up completely in some years. As of February 2019, sedimentation has reduced the reservoir's capacity to, only 19% of its designed capacity.
The dam is built in a part of the Santa Ynez River called the "Gibraltar Narrows" that gave its name to the Gibraltar mercury mine, which operated next to what is now Gibraltar Reservoir between the 1870s and 1990s.
Description
Gibraltar Dam is a constant radius concrete arch dam high and long. The dam is located about from the mouth of the Santa Ynez River and just above the confluence of Devils Canyon with the river. The dam controls runoff from a drainage basin of with an annual inflow of. The Santa Ynez basin upstream is mostly wilderness and inflows are unregulated with the exception of the smaller Juncal Dam. Downstream of Gibraltar, the Santa Ynez flows into the much larger Lake Cachuma, owned by the U.S. Bureau of Reclamation.The concrete spillway extends southward from the main dam and is controlled by four manually operated radial gates. Flashboards can be installed atop the gates during the non-flood season to increase the reservoir capacity. The spillway capacity is about. The spillway is designed so that overflow water cascades down the natural sandstone face adjacent to the dam, creating a high waterfall.
The original design capacity of Gibraltar Reservoir was. After its 1948 expansion the gross capacity was increased to, though so much sediment had already accumulated that the usable capacity was still only about. Since then, the dam has not been raised nor any sediment dredged from the reservoir, and it has gradually declined to a third of its original capacity. As of February 2019, sedimentation has reduced the reservoir's capacity to, or only 19% of its designed capacity.
Water is drawn from the reservoir via an intake structure near the south end of the dam into the Mission Tunnel, which transports water beneath the Santa Ynez Mountains to a small regulation basin, Lauro Reservoir. The water is either treated at the Cater Water Treatment Plant before entering the city's distribution system, or released into Mission Creek for groundwater recharge. The Mission Tunnel has a diversion capacity of.
At the end of the Mission Tunnel is the 820 kilowatt Lauro hydroelectric plant, which was initially constructed in 1985 but was idled in 1998 due to increasing costs. The city repaired and recommissioned the plant in 2015 after the cost of federal licensing decreased, but drought prevented the plant from actually running until early 2017.
Climate
Background and construction
In the early 1900s the City of Santa Barbara had exhausted its local water supplies, and looked to the drainage basin of the nearby Santa Ynez River, which is much larger than any of the city's local streams. In 1904 construction began on the Mission Tunnel which was bored under Mission Canyon in the Santa Ynez Mountains, to tap surface and ground water in the Santa Ynez basin. At that time, it was the longest water tunnel in the world. The city sought to build a dam somewhere on the Santa Ynez River in order to increase the water supply available for Mission Tunnel. Between 1913 and 1919, revenue bonds totaling $820,000 were issued for the construction of the dam and water system.Several dam sites were considered including the Mono Creek, Juncal and Main River sites, but all were dropped in favor of Gibraltar, which offered the largest storage capacity and highest available run-off as well as the most geologically favorable dam site. Rock-fill, masonry and concrete designs were all studied. The Mission Tunnel was extended to the Gibraltar dam site, which would ultimately allow water to flow by gravity to Santa Barbara. Site preparation work began in 1913 with the placement of foundations in the riverbed, for which a $40,000 contract was awarded to Arthur S. Bent Construction Co. In 1917 a concrete "thrust block" was poured on the south side of the canyon to take the weight of the future dam, as the rock on this side was not high enough to support the structure. The contract for the dam itself was let to Bent Brothers and W.A. Kraner on July 8, 1918.
Because there was no road to the site, workers and supplies were transported via an gauge, 220-volt electric railway inside the Mission Tunnel, which was only high and wide. This also limited the size of the equipment used at the site, such as a steam shovel, concrete mixers and rock crushers, which had to be disassembled and moved through the tunnel piece by piece. Up to three trains were operated at once, with a passing siding in the middle of the tunnel allowing for one trainload of supplies to be delivered every half-hour. Hazardous materials such as dynamite were not allowed inside the tunnel and had to be packed over the Santa Ynez Mountains on a primitive trail. The work was made even more difficult by the constant leakage of groundwater into the tunnel which formed "a continuous downpour from the roof in many places" and special care had to be taken to prevent contamination, since this water ultimately flowed into Santa Barbara's municipal supply.
After the numerous logistics problems were worked out, construction of the dam proceeded at a rapid pace with poured in the first three months of work. To allow normal river flows to pass through the dam during construction, a hole was left in the bottom of the dam. A total of 270,000 sacks of cement and 400,000 board feet of lumber were hauled through the tunnel during construction. Aggregate used for concrete making was mined from the river bed and processed at a crushing/screening plant located about upstream from the dam. The mixed concrete was then placed on the dam via a cableway system anchored by a steel tower. During the winter of 1918-1919 construction had to halt for three months due to the risk of flooding in the river-bottom gravel mining areas. The dam was initially built to a height of above the stream bed, and above bedrock.
When completed on January 23, 1920, Gibraltar became the first dam to impound the Santa Ynez River. Due to the extremely inaccessible location of the dam, construction ended up costing nearly $2,000,000. According to the Engineering News-Record "at no place on the dam is variation from true line greater than one inch." The hole in the base of the dam was closed by a temporary valve before being concreted in from downstream, allowing the reservoir to begin filling. Although the dam itself was now complete, there was a lack of funds to finish the spillway. The winter of 1920-21 was dry and the new reservoir failed to fill, forcing the city to use limited groundwater supplies. Heavy rains in January and February 1922 filled and overflowed the reservoir for the first time. Flooding heavily damaged the temporary spillway, and erosion along a previously unknown fault zone beneath the spillway nearly led to its collapse. The structure was rebuilt at a cost of $90,000.
''Gin Chow v. Santa Barbara''
In 1928 Gin Chow, who owned a farm on the Santa Ynez River, sued the city of Santa Barbara challenging its right to divert water from Gibraltar Reservoir, and soon 39 other farmers in the Santa Ynez Valley signed on to the case. Chow's demand was based upon the doctrine of riparian water rights in which existing landowners along a river have the right to the full, unimpaired flow of the stream through their property regardless of how or whether they use it. Riparian rights had been challenged by a controversial state amendment passed that year, which provided that "the water resources of the State be put to beneficial use" and that "the right to water... does not and shall not extend to the waste or unreasonable use... of water" and the Gin Chow case was the first serious legal test of the amendment.Five years later Gin S. Chow v. Santa Barbara was settled in the California Supreme Court in favor of the city. The court determined that "the waters to be impounded and taken by the defendants are extraordinary storm waters of the river and not a part of the usual and customary flow of the stream" – essentially, the diversion did not have a negative effect on downstream landowners since these floodwaters would have flowed into the ocean anyway. The Gin Chow decision allowed the city to divert up to per year from Gibraltar Dam as a "prescriptive right", but also required the city to release at least during the late summer and fall when the natural river flow is lowest. In addition, the city is allowed to take "an additional amount of storm runoff resulting from torrential rains." These rules continue to govern the operation of the dam today.
Gin Chow is considered a landmark case in the development of California water law, as it upheld the 1928 amendment, reducing riparian landowners' legal power over the state's water resources. This enabled more parties, such as the city of Santa Barbara, to file claims on surface water as long as the concept of "reasonable use" was observed. The court also emphasized that "what is such reasonable use is a question of fact, and depends upon the circumstances appearing in each particular case" and that "it requires no extraordinary foresight to envision the great and increasing population of the state... dependent upon stored water – water that is now wasted into the sea and lost to any beneficial use. This is considered to have set a legal precedent for the state of California to build more dams.