Flash Flood Forecasting Over Complex Terrain With an Assessment of the Sulphur Mountain NEXRAD in Southern California (2005) / Chapter Skim
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2 Flash Floods
2 Flash Floods
Pages 12-25
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From page 12... ...
This leads to somewhat different forecasting considerations and poses challenges in measuring storm rainfall along the West Coast, as discussed throughout this report. Moreover, flash flooding is determined not only by meteorological factors but also by hydrological factors -- such as terrain slope, land use, vegetation and soil types, and soil moisture -- and by hydraulic processes related to the character of stream or river channels subject to flooding.
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From page 13... ...
The term "flash" reflects a rapid response to the causative event, with rising water levels in the drainage network reaching a crest within minutes to a few hours of the onset of the event, leaving extremely short time for warning. A threshold of approximately 6 hours often is employed to distinguish a flash flood from a slow-rising flood (Mogil et al., 1978; Georgakakos, 1986a; Gruntfest and Huber, 1991; Polger et al., 1994; NWS, 2004c)
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From page 14... ...
The following sections cover aspects of the hydrometeorology, hy drology, and hydraulics of flash floods, complemented by a discussion of the human impacts of urbanization. Hydrometeorological Processes Flash floods typically result from intense rainfall rates that occur in individual thunderstorms, or lines or clusters of thunderstorms, or in con junction with bands of heavy rain and showers in tropical or extratropical cyclones (Schwarz, 1970; Maddox et al., 1978, 1979; Caracena et al., 1979; Chappell, 1986; J
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From page 15... ...
The greatest threat of excessive rainfall and significant flash flooding tends to occur when and where thunderstorms are slow-moving or stationary, continually reform over the same area, or repeatedly move over the same location. For example, if new convection develops on the storm flank opposite the direction of thunderstorm motion, then a quasi-stationary storm complex may evolve (J.
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From page 16... ...
By far the dominant precipitation forming process in the tropics, coalescence also plays a role in midlatitude clouds whose tops may extend to subfreezing temperatures. Cloud droplets are formed through a process called nucleation, when water vapor condenses onto small hydrophilic aerosol particles.
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From page 17... ...
Sometimes this deep plume of moisture originates from the subtropics near Hawaii; thus, the term "pineapple express" was coined. Heavy rainfall in California and the Pacific Northwest is not always confined to the immediate west slopes of the coastal mountains.
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From page 18... ...
. These events usually involve relatively shallow bands of rain and showers associated with land-falling extratropical cyclones and fronts in the winter months.
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From page 19... ...
region affecting the windward slopes of coastal mountains ahead of land-falling cold fronts within extratropical cyclones can be a critical contribution to orographic precipitation. In a seminal paper on flash flooding along the California coastal range, Neiman et al.
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From page 20... ...
. Hydrological Processes Steep orography can funnel the excessive runoff from heavy rains into river basins, increasing the chance for flooding conditions and underscoring the fact that flash floods are not caused solely by meteorological phenom ena.
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From page 21... ...
. tion, rainfall rate, and duration of high rainfall rates in the drainage basin.
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From page 22... ...
. The resulting drastically increased ratio of precipi tation runoff to infiltration exacerbated the high rainfall amounts, leading to a deadly and destructive flash flood.
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From page 23... ...
, which can be calculated from historic stream gauging records of annual peak discharge and mean daily discharge values. The lag time -- defined as the time difference between the time centroid of rainfall and peak discharge -- provides a useful time scale for the analysis of space-time variability of rainfall over a catchment.
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From page 24... ...
. When the peak discharge exceeds river capacity and the floodplain is inundated, a channel-dominated flow transitions into a valley bottom dominated flow, which tends to attenuate the flood wave propagation and thus increase lag time, depending on floodplain topography and surface roughness.
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From page 25... ...
The hydrologic response to urbanization typically is characterized by increasing flood peak magnitudes, decreasing lag time, and increasing runoff volumes (Leopold, 1968; J
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