hazards


FLOODS

Through the years, Woodland Hills has experienced—as have all other regions of California—flooding that have resulted in loss of life, caused tremendous damage to infrastructure and economies, and degraded native ecosystems (Department of Water Resources, 2013). But not all flooding is injurious; floods recharge water tables, replenish top soil, and support floodplains for endangered species (Department of Water Resources, 2013).

In general, flooding is caused by:

* alluvial flooding at the base of hillsides
* fast-moving flash floods caused by heavy rains
* slow-rise deep flooding on valley floors
* tsunamis in coastal areas



flood risk is a function of:

  • the frequency and magnitude of flooding
  • performance of flood management measures
  • the relationship between the flood hazard (rising or flowing water) and its effect on life loss, property, and/or environmental resources
  • consequences


Flooding remains a risk in parts of Woodland Hills. The sedimentary nature of the clay and silt in low-lying parts Woodland Hills inhibits the absorption of rain water, though storm drains installed in recent years serve to mitigate that risk.


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LIQUEFACTION



liquefaction   \ˌli-kwə-ˈfak-shən\ 

a process by which water-saturated sediment temporarily loses strength and acts as a fluid, like when you wiggle your toes in the wet sand near the water at the beach. This effect can be caused by earthquake shaking. 




Liquefaction requires both strong shaking and a high ground-water table. Other factors that control liquefaction-induced ground failure are the extent, depth, density, and thickness of liquefiable materials, depth to ground water, rate of drainage, slope gradient, and intensity and duration of ground shaking (Department of Conservation, 1997).



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Figure 1. Areas where flooding and liquefaction may occur during a strong earthquake.



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References

California Nevada River Forecast Center. (2010). Heavy Precipitation Event-California and Nevada. (General summary). Retrieved from http://www.cnrfc.noaa.gov/storm_summaries/dec2010storms.php 

Department of Water Resources. (2013). History of California Flooding. [PDF]. Retrieved from http://www.water.ca.gov/sfmp/resources/PRD_AttachC_History_4-3-13.pdf

Department of Conservation. (1997). Seismic Hazard Zone Report for the Canoga Park 7.5-minute Quadrangle, Los Angeles County, California. [PDF]. Retrieved from http://gmw.consrv.ca.gov/shmp/download/quad/CANOGA_PARK/reports/canpk_eval.pdf

Floods in California. (n.d.). In Wikipedia. Retrieved from https://en.wikipedia.org/wiki/Floods_in_California

Sources used for Figures

Figure 1. Areas where flooding and liquefaction may occur during a strong earthquake.

California Geological Survey. (1998). Seismic Hazard  Zones-Canoga Park Quadrangle. [PDF]. Retrieved from http://gmw.consrv.ca.gov/shmp/download/quad/CANOGA_PARK/maps/ozn_canpk.pdf

Leggatt, J. (2015). [Image based on seismic hazards map.] [Image rendered by author.]




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