Santa Cruz County History - Disasters & Calamities



Floods and Flood Control on the San Lorenzo River in the City of Santa Cruz
by Daniel McMahon

Characteristics of Floods on the San Lorenzo River

The San Lorenzo River drains a watershed of approximately 138 square miles, and drops from an elevation of 2900 ft. to sea level in its 22 mile length. (The first 2000 ft. drop occurs in the first three miles.) To quote from an Army Corps of Engineers report of 1982, "Historically, the San Lorenzo River has frequently flooded and caused substantial damages. Peak flows occur when a short-duration, intense storm follows a longer period of heavy rainfall which saturates the soil." These peak flows do not last very long, generally no longer than 18 to 36 hours. Damage from floods is caused as much by the force of moving water and debris as by inundation, as is seen clearly in the photograph below of the 1955 flood on Pacific Ave.1

Dec. 1995 floodwater on Pacific Ave.
Floodwater on Pacific Ave. near Soquel Ave.
Dec. 23, 1955

 

 

[This photograph shows the flood tide running down Pacific Ave. on Dec. 22, 1955. Photo taken by Ed Webber. Provided courtesy of Daniel McMahon. (Available at Covello and Covello Photography.)]

Most of downtown Santa Cruz, and many neighborhoods on the east side of the river lay in the floodplain, generally below the 20 ft. elevation point. These areas are largely surrounded by bluffs, which rise to the top of an upraised marine terrace that is from 60 to 100 ft. in elevation, and which is thought to be about 100,000 years old. Beach Hill is in the center of the floodplain, at the terminus of the river, and is approx. 55 ft. in elevation at its highest. The river only runs against the bluffs in two places today, at the back of Beach Hill (under Laurel St. Extension) and at the east side of the river mouth. There is some evidence that the river has taken different routes across the floodplain in geological time than the route it has been known to (generally) follow since 1769. To quote Margaret Koch,

When the Santa Cruz Civic Auditorium was under construction in 1941, test borings revealed that the San Lorenzo River originally had run around the foot of Mission Hill instead of its present course, and Neary's Lagoon is probably a remnant of the river's ancient bed.2

Flood Control: The 19th Century through the 1950's

Beginning with the bulkhead erected north of the downtown in the 1860's, Santa Cruzans have sought some measure of protection from high river levels, especially when several floods occurred within a few years (1889 and 1890, 1938, 1940 and 1941). As far back as 1871, editorials in one local paper were calling for a bulkhead that would run the length of the river, and that could limit erosion and prevent the river from overflowing its banks. Funding for a flood control project was provided by the U.S. Congress in the mid 1950's, and while this project was being planned, the flood of December 1955 occurred. The project was scaled up as a result, and construction began in 1958. The San Lorenzo River's levees were designed by the U.S. Army Corps of Engineers, and constructed by the Granite Construction Company.

Flood of 1958 Photo
The Flood of April, 1958 Reaches Front St.
Photo taken by Ed Webber. Provided courtesy of Daniel McMahon.

One more flood occurred before completion of the levees, in April 1958, but many of the buildings that would have been damaged had been torn down to make space for the levees. A redevelopment area was also created, to reclaim the "wrecked areas" from 1955, and this would ultimately include the elimination of several streets (such as Garfield and Eagle Streets), the rerouting of other streets, and the building of San Lorenzo Park, the shopping center on Front St. (Longs, etc.), and the banks and parking structures near the Veteran's Memorial building. A new county government center was added to the area set aside for San Lorenzo Park as well. But the major part of the project was the construction of the levees. All homes and trees adjacent to the river in the city were removed, including Santa Cruz's last Chinatown, adjacent to Front St. A channel from 150 feet to 200 feet wide, and theoretically 25 feet deep was constructed, and this channel was designed to contain a "100 year" flood.

The floodwaters of April 1958 reached Front St., behind the St. George Hotel. Many of the buildings that would have been flooded had been removed in preparation for the construction of levees, as is the case with the flooded foundations of buildings on the now-gone Eagle St., behind the Post Office. The recent experience of the flood of 1955 had also left citizens, merchants and public officials well prepared for dealing with river flooding in 1958.

Problems with the Flood Control Project: 1970's and 1980's.

By the 1970's, it became apparent that a tremendous amount of silt was accumulating in the channel, and the cost of removing this silt was higher than the city of Santa Cruz felt it could afford, especially as a huge quantity would need to be removed just to reach a level where maintenance of the river's capacity could take place. Dire predictions were made about the capacity of the silt-filled channel, but the storm of 1982 brought a welcome surprise. Much of the silt and sediment in the channel was moved out of the river with the floodwaters, and the levees were able to contain an estimated 33,000 c.f.s of water in the city. (The figure for 1955, the highest known flood, was 39,000 c.f.s.) Only this "scour effect" saved the levees from failing and inundating the city, but the level of protection offered was still not equivalent to the 100-year level, which is mandated for both continued development within the floodplain, and for insurance coverage.

In addition, the 1958-59 flood control levees had transformed the river from a tree-lined and very scenic part of town, to a sterile drainage ditch. The siltation of the channel and the lack of deep pools of water, coupled with low summer river flows and a lack of shade on the water (once provided by trees on the banks) had decimated fish populations in the river. Fishing in the San Lorenzo had been incredibly popular until the 1960's, and trout and salmon were routinely caught in the city and in the San Lorenzo Valley. In contrast, the river contained by the high levees was barren of most wildlife, the fish populations declined, and the levees separated the two sides of town visually as well.

Aerial Photo of Santa Cruz 1967
San Lorenzo River Flood Control Project, circa 1967.
Photo taken by Ed Webber, and provided courtesy of Daniel McMahon.

[This aerial view shows the sterility of the river channel within the city, before many bushes or grasses had returned to the channel. The new County Government Center is under construction, as is the Laurel/Broadway St. Bridge.]

Present Flood Control Plans And The Future Of The River

New studies by the Army Corps of Engineers undertaken in the 1980's agreed with local earth scientists that the flood protection was inadequate, and that constant dredging of the riverbed was an expensive and impractical solution. But in light of the high degree of scour of sediment shown in the 1982 flood, the situation was not as bad as had been feared in the late 1970's. The Corps found that the major impediment to water flow in the river was certain bridges, specifically the upper part of the Water St. Bridge, and the Riverside Ave. bridge. (Both of these have since been replaced, the Water St. bridge only being finished the week that this article is being written.) And additional flood protection can be gained by constructing short walls atop the levees, from 1 to 3 feet high, and these would still allow for some vegetation to grow within the river channel, which will benefit wildlife, fish populations, and the river's scenic and recreational attributes.

In 1987, the City of Santa Cruz issued the San Lorenzo River Design Concept Plan, which contained the elements previously described to enhance the flood protection and environmental quality of the river, and to enhance the visual and recreational value of the river in town. Work has begun with the rebuilding of the Water St. and Riverside Ave. bridges, and hopefully it will continue in the next few years to achieve the twin goals of ensuring adequate flood protection for a 100-year event, and restoring some of the aesthetic qualities the river possessed before 1959. The scenic and recreational parts of the plan involve the construction of amphitheaters, the improvement of access from town, walkways along the river, and the encouragement of recreation-oriented businesses in the proximity of the river. Plans also call for the planting of trees both atop and within the levees, in a design that will enhance the beauty of the river and provide the shade necessary for vigorous fish life, while not causing problems at times of peak water flow.

If better flood protection and simultaneous restoration of the river's ecology and recreational potential seem like difficult or unobtainable goals, it is good to remember that citizens of Santa Cruz have been calling for and working toward protection from floods since the 1860's. The historic record of floods in Santa Cruz before 1959 contrasts markedly with the lack of floods since. Only the storm of Jan. 4, 1982 came close (very close) to topping the levees, and the quantity of water in the river was comparable to the quantity of water that caused the "Christmas Flood" of December, 1955. (See the Table of Floods.)

A reading of the history of a town developing in a floodplain, and struggling to cope with the floods of 120 years suggests that there is a relationship between the river and the city, and that this has always been a changing relationship. Some balance can hopefully be found between the protection of the City of Santa Cruz from the San Lorenzo River, and the protection of the natural aspects of the river from the city.

Citations:

  1. Information in this paragraph is from The San Lorenzo River Watershed Management Plan, p. 93, Report on the Floods of 4-6 January 1982 in the San Francisco and Monterey Bay Areas, p. 10, and Flood Control Failure: San Lorenzo River, California, p. 407.
  2. This intriguing report is from Parade of the Past, p. 204. More information on the geology of the downtown basin would be welcome. A test boring in the early 1990's at the Buick/Toyota/Kinko's building on Laurel St. at Pacific Ave. showed sandy soil, deposited by the river, to a depth of over 50 feet, when the drilling stopped. It would be interesting to know the depth and extent of river-deposited soil, and what other paths the river has taken through the downtown basin in the last 100,000 years.

For more information on the history of floods and more complete information on the sources used for this article, see:


© 1997 Daniel McMahon. Reproduced by permission of the author.

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floods, San Lorenzo River

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