Development in the Strawberry Creek watershed has resulted in extensive alterations to the natural hydrologic regime of the creek as previously discussed. A complicated storm drainage system has been constructed to handle the increased peak flows and volume of storm runoff from the watershed (Figure 6). Bypass culverts have been installed on both the North and South Forks to carry this storm flow under developed areas.
An earthen retention basin has been constructed on the South Fork of the creek at the entrance to the lower fire trail to control downstream floooing. The retention basin has a flood storage capacity of 1,500,000 cubic feet (11,220,000 gallons). The dam outlet serves to control the flow of water into the "Big Inch" bypass culvert. The outlet mechanism is a 48" x 42" hydraulically operated slide gate which is controlled from a station located at the entrance to the lower fire trail just above the earthen dam. Operation of the gate is supervised by the DOFM Grounds Manager.
Potential problems at the retention basin include siltation and debris collection at the outlet which could clog the entrance to the bypass culvert. The entire canyon storm drain system is highly susceptible to debris or mudflows which could clog inlets and consequently divert flows out of normal channels, resulting in extensive damage. The location of the retention basin is less than ideal because it receives runoff from predominantly undeveloped portions of the watershed. The basin would have been more effective further downstream because it could have helped mitigate the rapid peak storm flows from the LBL complex. However, due to financial and other constraints at the time of its construction, the basin was situated further up into the canyon.
The retention basin outlet is the only means available at this time for controlling peak flows on the central campus. Therefore, flows into the bypass system from the basin should be regulated as much as possible. Lennert (1972) estimated that the maximum peak flow rate potentially entering the bypass system was 1650 cfs based on a stomf'event with a 25 year recurrence interval. It was further estimated that by limiting the flow into the bypass system to 850 cfs, damage to the central campus could be avoided. The remaining runoff could be held in the retention basin until it reached its capacity. Additional runoff would overflow onto Centennial Drive.
A reinforced concrete wall was constructed on the south side of Centennial Drive to a point just west of the Haas Recreation complex to divert overflow water past the clubhouse and pool areas. Wooden gates providing access to the Haas area must be closed for the system to operate correctly. The diverted water will then spread across Strawberry Field and follow Ritnway Drive down to the Piedmont Avenue area. Lennert estimated that this would happen about once every ten years. In order to avoid this to the extent possible, the dam outlet gate should not be excessively closed, nor should debris be allowed to accumulate in the retention basin. At the present time, there are no formal DOFM standard operating procedures for the gate, although standing policy has been to leave the gate open about 10 inches. This would allow about 100 cfs.to enter the Big Inch bypass culvert at this point if the retention basin was filled to capacity.
The Big Inch (6011-72") bypass culvert begins at the retention basin dam and travels approximately 4300 feet underground to its outlet adjacent to the Faculty Club on the central campus. Chicken Creek and two other unnamed tributaries that drain the LBL complex are pipep. directly into the Big Inch bypass culvert. The Stadium Hill area, Kleeberger Field, and the Greek Theater also drain directly into this culvert. The Big Inch bypass culvert was built in 1951 due to the possibility of structural failure of the older "Little Inch" bypass culvert.
Extensive channel work was performed on Chicken Creek.in the 1960's and 1970's. The accelerated peak runoff from the LBL complex caused substantial erosion and incision in this channel because of the steep slope and unstable soils. The channel bottom and sides were artificially hardened with concrete. In addition, rock and concrete groins and check dams were installed. However, the creek channel has since broken out of its artificial confinement and has continued to erode its banks and incise. At the bottom of the Chicken Creek channel adjacent to the Poultry Research building, a selfcleaning inlet structure was designed and constructed to accept peak flows. The inlet leads to a 54" RCP which connects directly to the 72" Big Inch bypass culvert near the Haas Clubhouse. The capacity of this inlet is approximately 500 cfs.
The original Strawberry Creek channel continues downstream of the retention dam and enters the Little Inch (30") bypass culvert at a drop inlet located just above the Haas upper pool. Storm flow in this channel is confined to local runoff from the hillsides. The Little Inch culvert was the first major diversion of Strawberry Creek and was necessitated by the construction of Memorial Stadium in 1923. The Little Inch culvert travels approximately 3500 feet and empties next to the Women's Faculty Club on the central campus. A small open channel then carries this water past Senior Hall and the Faculty Club and joins the South Fork at the outlet of the Big Inch bypass culvert. Most of the Haas Recreation Area as well as Memorial Stadium and the Cowell Hospital area drain directly into this culvert. The capacity of this culvert is about 180 cfs.
The North Fork drainage system has been significantly altered by development and stormwater routing. The upper reaches of the North Fork were obliterated by construction of the LBL complex. Drainage in the upper portion of the North Fork watershed is therefore culverted completely underground. A 48" CMP/RCP now extends from Grizzly Peak Boulevard and the Lawrence Hall of Science to the LBL outlet structure and another 30" CMP drains the northern hill area. The 48" RCP has been lined with steel cylinder pipe and an energy dissipation chamber has been constructed at the common outfall of the 48" RCP and 30" CMP in Blackberry Canyon. Three concrete energy dissipation structures were also installed in the 48" culvert between Lawrence Hall of Science and LBL. All inlets to the extensive underground drainage system in the North Fork watershed are protected with bar grates. Inlet structures in the main stream channels are protected by primary screens (trash racks) located upstream of the inlets.
The North Fork channel above Highland Avenue was regraded and reconstructed in the 1960's. In addition, five check dams were installed. This channel between the LBL fence and outlet structure has experienced both erosion and incisionjnto highly erodible colluvium and soils. A drop inlet structure to the city drainage system is located just above Highland A venue at the end of Le Conte A venue. During periods of low flow, drainage travels underground along Le Conte Avenue and resurfaces in an open channel south of Le Conte A venue between La Loma and Le Roy A venues. This narrow confined channel continues a short distance west of Le Roy Avenue before disappearing back into the city storm drainage system.
In 1966, a high flow storm drain bypass system was installed to relieve the flooding threat caused by development at the LBL complex. During storm events, peak flows pour into a 48"- 60" RCP at the drop inlet above Highland A venue and is routed from Le Conte A venue to La Loma A venue to Ridge Road to Euclid Avenue and empties into the North Fork open channel at North Gate (Figure 11). Lennert estimated that the maximum theoretical peak flow into the drop inlet above Highland A venue would be 500 cfs based on a 25-year storm event. It appears that present peak flows could approach this threshold. Major storm drain lines along La Loma Avenue and Euclid Avenue also empty into the North Fork at North Gate.
Kuntz (1980) calculated that the lag time in the North Fork at the entrance to the city culvert above Highland Avenue was only 15 minutes. The lag time for the South Fork at the retention basin was similarly calculated to be 25 minutes. Data collected by Dr. Luna Leopold in 1977 also indicated that the lag time of the North Fork at Haviland Hall was less than 15 minutes. Data presented for lag times in rural areas of the San Francisco Bay region (Dunne and Leopold, 1978) indicate that a drainage area the size of the North Fork subcatchment above Highland Avenue (388.1 ac. or 0.22 rru2) would have a lag time of about 45 minutes if it was not urbanized. Likewise, a drainage basin comparable in size to the subcatchment upstream of the retention basin (485 ac. or 0.75 mi2) would have a lag time of over an hour if it was not developed. Lag times on both forks of Strawberry Creek in the lower canyons are about three times faster than they would be under natural conditions and are even shorter further downstream. The impact of development on the hydrologic regime of the creek is evidenced by these drastically reduced lag times.
In conjunction with this study, the DOFM storm sewer maps were updated for the central campus and outlying areas. Drainage subcatchment areas were delineated for all major tributaries and culverts in the watershed through the use of DOFM, LBL, and city storm drain maps as well as extensive field inspections. Figure 9 and Table 6 define these subcatchment areas. These subcatchments may be used to identify possible sources of pollution, or to perform extensive hydrologic calculations.