Laserfiche WebLink
<br /> <br /> <br /> <br />The following factors contribute to the frequency and severity of riverine flooding: <br /> Rainfall intensity and duration 
 <br /> Antecedent moisture conditions 
 <br /> Watershed conditions, including steepness of terrain, soil types, amount, and type of <br />vegetation, and density of development 
 <br /> The existence of attenuating features in the watershed, including natural features <br />such as swamps and lakes and human‐built features such as dams 
 <br /> The existence of flood control features, such as levees and flood control channels 
 <br /> Velocity of flow 
 <br /> Availability of sediment for transport, and the erodibility of the bed and banks of the <br />watercourse <br /> <br />5.6.3.2 Future Flooding <br />In the Bay Area, the potential for new or prolonged flooding as sea level rises will not be <br />confined to the shoreline. Sea level rise will increase the likelihood of major flood events <br />around the Bay Area because higher water levels in tidal creeks and flood control channels <br />will reduce capacity to discharge rainfall runoff. While some creeks already flood when <br />rainstorms coincide with high tides, rising sea levels will cause flooding during smaller, <br />more frequent rainfall events. <br /> <br />Sea level rise inundation maps (Figure 12) help to visually assess under what conditions <br />assets may be impacted by sea level rise and storm events and how far reaching the <br />consequences may be if they are impacted. To understand these factors it is helpful to <br />evaluate a range of possible future sea level rise scenarios. The “total water level” approach <br />presented below simplifies this process and reduces the number of maps needed. In this <br />approach each inundation map represents a number of different unique combinations of <br />sea level rise and extreme tide (storm surge) conditions.28 <br /> <br />A total water level of 36 inches above mean higher high water (MHHW)29 can represent a <br />new “daily” high tide with 36 inches of sea level rise. This amount of sea level rise, which is <br /> <br />28 Extreme tides are the maximum high tide level that has occurred over a specific return period (recurrence interval) <br />that correlates to a specific occurrence probability. For example a 100-year extreme tide has a return period of 100 <br />years, and therefore a one percent chance of occurring in any given year. <br />29 Mean higher high water (MHHW) is calculated as the average of the higher of the two daily high tides over a 19- <br />year tidal epoch.