Local Hazard Mitigation Plan.pdf - Sonoma County Water Agency ...

1 February 18, 1986 48.56
2 January 9-10, 1995 48.01
3 December 23, 1955 46.95
4 December 23, 1964 46.85
5 January 1, 1997 45.10
6 January 5, 1966 42.53
7 March 10, 1995 42.24
8 January 27, 1983 41.63
9 January 24, 1970 41.20
10 February 1, 1963 40.95
Flood level at Hacienda Bridge = 34 feet
Figure 11 shows the flood hazard within the Agency’s service area prepared by the Federal
Emergency Management Agency (FEMA). These maps use computed or estimated water
surface elevations combined with topographic mapping data to represent the flood hazard. The
100-year flood represents a compromise between minor floods and the greatest flood likely to
occur in a given area. In most cases the 100 year flood is less than the flood of record and has
been widely adopted as the common design and regulatory standard in the US. It was formally
established as a standard for use by Federal agencies in 1977 and later confirmed by FEMA in
1982.
Figure 11 shows the Agency’s system overlain on the FEMA flood maps. The figure also shows
the locations where the pipelines cross creeks. These locations together with areas of high flood
hazard are at the highest risk of damage due to channel scour. Debris flow in streams and the
potential for bottom scour and the resulting pipeline damage is also a potential hazard. As
shown in the figure, the Agency facilities with the highest risk of flooding include those
located in the Mirabel and Wohler area, Ely booster station, the Sebastopol and Todd
Road wells, significant sections of the Russian River-Cotati Intertie, Forestville aqueduct
and Wohler Intertie and some portions of the Santa Rosa aqueduct. Stream crossing
locations of the transmission lines located in the areas of high flood hazard are most
vulnerable to damage due to flood related scour. In addition, there are three locations
where a pipeline is suspended from a bridge at the stream crossing location. At these
locations, the pipeline is vulnerable to damage by impact from floating debris.
18 Local Hazard Mitigation Plan 1-18-08.doc