The National Resources Conservation Service (NRCS) and the Santa Clara Valley Water District (water district) completed a comprehensive restudy of the Llagas Creek floodplain 1982. At that time, an environmental impact statement and report were completed and construction began. The project was sponsored locally by the water district and the Loma Prieta Resource Conservation District. The NRCS provided assistance under the authority of the Watershed Protection and Flood Prevention Act.
At the time of the 1982 report, an estimated 1,123 residential buildings, 64 mobile homes, 463 commercial establishments, and 24 industrial buildings were located in the flood-prone area. At that time, damages from a 1-percent or 100-year flood were estimated to be $8.5 million. Current value of damages would be far greater.
Encompassing a watershed of 104 square miles in southern Santa Clara County, when completed, the Llagas Creek Flood Protection Project will protect residential, commercial, and agricultural areas in San Martin, and the cities of Morgan Hill and Gilroy.
Floods in 1937, 1955, 1958, 1962, 1963, 1969, 1982, 1986, 1996, 1997, 1998, 2002, 2008, 2009 and 2011 damaged existing homes and businesses. The largest recorded flood, estimated to be a 33-year event, occurred in December 1955.
Beginning in 1993, the City of Morgan Hill and the water district undertook an effort to preserve the natural creek on Reach 8, extending through downtown Morgan Hill. New alternatives have been investigated, focusing on environmental and aesthetic features while maintaining public health and safety. A restudy of Reach 8 including the investigation of a storm water retention system and underground bypass culverts were included in the planning and design effort. As a final design alternative, an underground tunnel (approximately 2,100 feet in length) is to be constructed within downtown City of Morgan Hill, from approximately West Main Street, beneath Nob Hill, to approximately West Dunne Avenue. The tunnel alternative will allow existing West Little Llagas Creek between West Main Street and Ciolino Way to remain "as-is", thus completely avoiding impacts from the Project.
In 2004, the City of Morgan Hill and the water district entered into a cost-sharing agreement to fund essential investigations to keep the project on schedule. The agreement funded a geotechnical investigation, hazardous materials investigation and a tree and riparian corridor assessment. The $680,000 cost will be equally shared by the City of Morgan Hill and the water district. At a later date, the parties expect to recover the monies spent from the federal government.
On January 4, 2008, many residential and commercial areas of Morgan Hill experienced flooding depths ranging from a half foot up to three feet. In a 25-hour continuous period, Morgan Hill received 4.25 inches of rainfall. According to preliminary estimates, the rainfall intensity varied up to a 60-year event (return period). Flood damages have not been finalized yet.
In 2009, the City of Morgan Hill and the water district entered into another cost-sharing agreement to fund and prepare project designs, Environmental Impact Report/Environmental Impact Statement and the U.S. Army Corps' design documentation report. The $9.7 million cost will be committed by the water district with the City of Morgan Hill to reimburse the water district for 50 percent of the consultant contract cost up to $3 million.