Orange County Branch Newsletter
History and Heritage
Villa Park Dam – 50th Anniversary
By: Carl Nelson, P.E.
The Santa Ana Mountains of Orange County constitute a northwesterly extension of the Peninsular Range of Southern California trending southeasterly from the Chino Hills in Orange County to northern San Diego County. Santiago (5,687 ft.), Modjeska (5,496 ft.) and Pleasants (4,007 ft.) peaks form the ridgeline between Orange and Riverside Counties and drain westerly to Santiago Canyon, there melding with the alluvial plain of the Santa Ana River. The watershed of Santiago Creek runs westerly from Santiago Peak through the Trabuco District of the Cleveland National Forest (established by President Theodore Roosevelt in 1908) before crossing the northerly portion of Irvine Ranch. Principal tributaries are Silverado, Black Star, Limestone and Fremont Canyons.
Most of the watershed is rough, covered with chaparral.
Long-term records kept by Orange County Flood Control District at Santiago Peak, indicate an average annual rainfall of 34 inches. In contrast, the driest year recorded at the peak was 2006-07 with a total of 8 inches, and 1997-98, the wettest total was 105 inches. Over the lower elevations of the valley, annual rainfall averages about 13 inches. Important to agriculturists, the median annual rainfall (half the years are lower and half the years higher) is only 11.85 inches.
Early settlers found water for domestic purposes only in spring-fed streams at the mouths of canyons; for instance, the narrows of Santiago Creek at Villa Park. The Carpenter and Serrano Irrigation districts, established around 1878, constructed a clay barrier across the shallow alluvium of the creekbed, acquired riparian rights, and developed an irrigation system for orchards along the banks of lower Santiago Creek easterly of the new city of Orange.
Early occupants of the floodplain lands bordering the creek would be deceived by long periods of drought between damaging floods. By the late 19th century rural and urban encroachments within the new cities of Orange and Santa Ana would be overwhelmed when the wide, sandy bed of Santiago Creek would become a raging torrent for a day or two following torrential rainstorms. The flood threat would be partially improved with the development of Santiago Dam and Reservoir upstream from the original “Orange County Park”.
During the 1920s James Irvine II and the Carpenter and Serrano water interests resolved a water rights dispute with a cooperative agreement to construct Santiago Dam and Reservoir for conservation of flood water. Conserved water would be shared equally, and Irvine’s share would be diverted to irrigation of the Irvine’s land on the Tustin Plain.
When a water supply reservoir is filled before the advent of a large flood, the emergency spillway safely passes the excess flood waters, but with only a small reduction of the peak flow. The cities of Orange and Santa Ana experienced damaging floods even after construction of the Santiago Dam and water supply reservoir (better known as Irvine Lake). A federal flood control program in1936 had authorized a flood control dam on Santiago Creek at the Villa Park narrows; however after two decades without federal appropriations, funding for the dam was included in Orange County Flood Control District’s successful 1956 bond election.
Designed by the local firm of Harrison & Woolley, construction of the dam was completed in 1963. Resident Engineer for the construction was John Huntsman PE, assisted by district staff members including Ken Osborne PE, son of the district’s Chief Engineer H. George Osborne. Sequential floods during January and February of 1969 filled both Santiago and Villa Park dams to capacity as actual runoff volumes exceeded design estimates for the theoretical 100 year storm.
Whereas the dam had been designed for a gated discharge of 3,500 cubic feet per second for the 100 year storm, uncontrolled spillway discharge had peaked at 4,500 CFS in February. For several days after the February storm residual flow from the dam damaged several highway bridges and commercial gravel pit operations. In locations where little or no bank protection existed, residential properties experienced severe damages from lateral erosion.
In view of the 1969 damages experienced in Villa Park, Orange and Santa Ana, the Corps of engineers consented to inclusion of lower Santiago Creek improvements within the Congressional legislation that authorized Santa Ana River Mainstem project. Construction anticipated upon completion of Prado Dam enlargement by the Corps of Engineers will be modification of the Orange County Water District’s Bond Street groundwater replenishment basin providing increased flood storage, outlet gates at Prospect Street and downstream protection for storm runoff exceeding the 100 year Villa Park Dam design.