Summary Analysis:

The 1941 cross section is not an accurate depiction of the river bed. The straight line does not represent the river bed but must, instead, represent a water surface. The evidence indicates that a combination of factors have contributed to bed scour at the bridge footings. The extent of scour is not that stated in the PEIR (approximately 27 feet).

Discussion:

The PEIR indicates that since 1960 the average bed (across the entire channel width) has lowered about 8 feet. Maximum lowering is 14 feet. It also indicates that CALTRANS surveys of the 299 bridge indicate a maximum bed lowering since 1941 of 22 feet. Comparison of 1941 bridge cross-sections, 1948 photos provided by CALTRANS, the 1960 CALTRANS cross-section, and the 1966 photo provided by CALTRANS indicate several things. First, if you were to believe the CALTRANS information, between 1941 and 1948 approximately 7 to 8 feet of bed degradation occurred which would indicate that degradation was caused by the bridge impacting the river.; However, CALTRANS has indicated that the earlier cross-sections that show a flat channel are inaccurate and misrepresentative of river conditions. Again, looking at the CALTRANS 1960 cross-section and the 1966 photograph suggests that 6 to 7 feet of degradation occurred. Again, this occurred shortly after construction of the new bridge.

The impacts from the rock weir immediately above the Highway 299 bridge, which was placed for the USGS Gaging Station, was not discussed in the PEIR. Though subject to minor changes , the HBMWD has indicated, in correspondence to the County in 1994, that rocks upstream of Highway 299 have remained visible since 1948, indicating again, no degradation since then, other than local pier scour. Observations during high flows indicate that this weir directs flows to the middle bridge piers which have been the subject of concern regarding degradation. Note that the CALTRANS cross-sections per 1941, 1960, and 1972 all show a flat bottom channel which we know does not occur around bridge pier structures. These cross-sections also show very little change occurring between 1974 and 1991. The removal of the rock weir in 1997 in a generally mild winter has resulted in a maximum bed aggradation of 8 feet in some places. Construction activity last year left the deep hole excavated during construction adjacent to the pier empty - this has not yet filled in. This indicates that flows against this pier tend to be erosive and not depositional. Upstream configurations cause bridge scour and the site is not prone to recovery. Attached is a photograph taken upstream of the Highway 299 bridge which shows where the bottom of the channel would be if the CALTRANS cross-sections were accurate. A bridge would not be constructed with such little capacity under it. These factors were brought up during preparation of the final PEIR and have not yet been addressed by the County. Obviously the PEIR analysis is incorrect.