Urbanization of the Strawberry Creek watershed has resulted in greater peak flows and higher flood stages in the downstream reaches. Channel culverting and confinement as well as stormwater routing have also promoted higher peak flows and flood stages. In addition, buildings and other structures have been constructed in close proximity to the creek in numerous locations, posing a potential for extensive property damage.
Figure 8 shows the 100-year floodplain on the central campus as delineated on the FEMA contractor's work maps for Berkeley (1978). The 100-year flood is not necessarily a flood with a recurrence interval of 100 years. It is a statistical extrapolation based on existing records of a storm event that has a 1% probability of occurring in any given year. The accuracy of this extrapolation is dependent on many variables so it must be considered as only an estimate.
Two flood hazard zones are defined on Figure 8. The two zones are based on flood hazard factors which are derived by taking the difference of the weighted average water surface levels of the 10 and 100 year floods. The lower the difference, the higher the flood hazard. Zone A2 on the map is a higher flood hazard area than A1. Most of the Strawberry Creek areas are designated Zone C which indicates that minimal flooding will occur during a 100-year storm event.
Overtopping of the confined creek banks on the central campus normally occurs in several areas each winter. The North Fork overflows its low rock walls onto the adjacent lawn area north of LSB. Likewise, the South Fork overtops its southern bank onto the lawn opposite Dwinelle Annex. The North Fork also overflows into the Eucalyptus Grove near its confluence with the South Fork. Finally, the Main Branch has overflowed its banks in the past at the entrance to the city culvert at Oxford Street because of debris clogging the trash rack in the channel and subsequently diverting flows. Previous overtopping has caused erosion of the northern bank at this location. None of these areas of frequent bank overtopping pose a significant hazard at this time.
In the canyon areas and portions of the North Fork upstream of Haviland Hall, the creek has become deeply incised, resulting in a high terrace. Streambank erosion is more pronounced in these reaches because the steep banks become undercut and eventually topple into the channels, causing sedimentation downstream. Another significant source of sediment is the sand and gravel which tends to accumulate upstream of the retention basin on the South Fork. During peak flow periods this bed material is scoured out and transported downstream through the Big Inch bypass culvert and subsequently deposited in the South Fork channel on the central campus. The sand and gravel accumulates in existing pool areas and along the margins of the stream channel, resulting in a more homogeneous riffle regime which degrades aquatic habitat.
Wet weather streamflows presented in Table 15 were measured in conjunction with the storm sampling round on October 27, 1987. All flow measurements were performed within an hour of each other and it was found that the flow in fork increased about sixold between the canyon sites and the Eucalyptus Grove. Disharge in the South Fork ranged from 2.3 cfs in the Grove, Discharge in the South Fork ranged from 2.3 cfs in Strawberry Canyon to 14.6 cfs in the Grove, whereas streamflow in the North Fork ranged from 7.9 cfs in Blackberry Canyon to 45.0 cfs in the Grove. The North Fork exhibited higher flows than the South Fork, especially further downstream. This is due to the highly urbanized nature of the North Fork subwatershed and extensive stormwater routing in these areas. The peak flows resulting from this storm event were not determined but at the time of observation about 60 cfs was entering the city culvert at Oxford Street.