2
Flash Floods

A flash flood is a flood that rises and falls quite rapidly with little or no advance warning, usually the result of intense rainfall over a relatively small area.1

Most flash floods in the United States are caused by intense rainfall from slow-moving thunderstorms, thunderstorms that repeatedly move over the same location, or excessive rainfall from hurricanes or other tropical systems (Junker, 1992; Davis, 2001; Kelsch, 2001). Along the West Coast of the United States, in contrast, flash floods frequently are caused by orographic precipitation—that is, precipitation influenced by mountainous terrain (Maddox et al., 1980; Cotton and Anthes, 1989; R. Smith et al., 1997). There, flash floods are typically associated with land-falling extratropical cyclones and fronts during the winter months, rather than summer thunderstorms. 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. For example, various combinations of rainfall intensity and duration may lead to flash flooding, depending on the hydrologic and hydraulic factors of a watershed. Flash floods result from a complex interaction among hydrometeorological, hydrological, and hydraulic processes across various spatial and temporal scales.

1  

Definition of a flash flood from the American Meteorological Society Glossary of Meteorology (AMS, 2000).



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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California 2 Flash Floods A flash flood is a flood that rises and falls quite rapidly with little or no advance warning, usually the result of intense rainfall over a relatively small area.1 Most flash floods in the United States are caused by intense rainfall from slow-moving thunderstorms, thunderstorms that repeatedly move over the same location, or excessive rainfall from hurricanes or other tropical systems (Junker, 1992; Davis, 2001; Kelsch, 2001). Along the West Coast of the United States, in contrast, flash floods frequently are caused by orographic precipitation—that is, precipitation influenced by mountainous terrain (Maddox et al., 1980; Cotton and Anthes, 1989; R. Smith et al., 1997). There, flash floods are typically associated with land-falling extratropical cyclones and fronts during the winter months, rather than summer thunderstorms. 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. For example, various combinations of rainfall intensity and duration may lead to flash flooding, depending on the hydrologic and hydraulic factors of a watershed. Flash floods result from a complex interaction among hydrometeorological, hydrological, and hydraulic processes across various spatial and temporal scales. 1   Definition of a flash flood from the American Meteorological Society Glossary of Meteorology (AMS, 2000).

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California THE PHYSICAL PROCESSES OF FLASH FLOODS The National Weather Service (NWS) Hydrologic Glossary (NWS, 2004c) defines flooding as the inundation of a normally dry area caused by high flow or overflow of water in an established water channel (e.g., river, stream, drainage ditch) or the ponding of water at or near the location where substantial rain fell. Flooding occurs whenever a drainage system receives more water than it can handle. As noted at the beginning of this chapter, a flash flood is a flood that follows the causative event in a short period of time and often is characterized by a sudden increase in level and velocity of a flowing water body. 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). Thus, flash floods are localized phenomena that occur in watersheds with maximum response times of a few hours—that is, at spatial scales of approximately 10,000 km2 or less, depending on the catchment characteristics (Hirschboeck, 1987; Gruntfest and Huber, 1991; O’Conner and Costa, 2004). Most flash floods occur in streams and small river basins with a drainage area of a few hundred square kilometers or less (Kelsch, 2001). Such basins respond rapidly to intense rainfall rates because of steep slopes and impermeable surfaces, saturated soils, or because of human- (i.e., urbanization) or fire-induced alterations to the natural drainage. Causative events may be either excessive rainfall in a natural drainage basin or human-altered catchment or the sudden release of water impounded by a natural jam (i.e., formed by ice or rock, mud, and wood debris) or human-made dam or levee. This report focuses on flash flood events associated with heavy rainfall. Extraordinary unit discharges (i.e., the rate of water flowing past a stream gauging station divided by the drainage area) from a watershed are related to specific topographic and climatologic conditions (Kelsch, 2001; O’Conner and Costa, 2004). In general, basins producing high unit discharges correspond to areas in which regional climatic patterns can produce extraordinary precipitation, such as within and flanking the Appalachian Mountains along the Atlantic seaboard, the western or southwestern flanks of mountain ranges near the Pacific Coast, and a broad northeast-trending zone in the southern Midwest extending from southwest Texas to southeast Kansas and southern Missouri. Such extraordinary rain-

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California fall situations feed off moisture sources originating from the Gulf of Mexico and the Pacific or Atlantic Oceans. In addition to the natural causes of flash floods, however, the increasing influence of urbanization cannot be ignored. The following sections cover aspects of the hydrometeorology, hydrology, 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 conjunction 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. A. Smith et al., 1996; Landel et al., 1999; Hjelmfelt, 1999; J. A. Smith et al., 2000; Sturdevant-Rees et al., 2001). Most extreme precipitation events are associated with mesoscale weather systems—that is, systems with horizontal scales of 10 to 1000 km. These include individual thunderstorms on scales of a few kilometers to tens of kilometers and clusters of thunderstorms (sometimes called mesoscale convective systems, or MCSs) on scales of a few hundred kilometers. Precipitation often varies significantly on spatial scales of a few kilometers, especially during extreme events. Moreover, complex topography can exaggerate the spatial variability of rainfall. There are several common meteorological scenarios conducive to a prolonged duration of localized heavy rainfall (Maddox et al., 1979; Doswell et al., 1996; Kelsch, 2001; Lin et al., 2001). The key ingredients of flash flood-producing storms are (1) ample and persistent supply of water vapor, (2) a mechanism to facilitate uplift of air in which the moisture condenses and precipitation forms, and (3) a focusing mechanism (or combination of focusing mechanisms) that causes precipitation to occur continuously or repeatedly over the same area. The physical processes at work are of dynamical, thermodynamical, and microphysical nature. The dynamic and thermodynamic conditions of the atmosphere determine the microphysical processes and the associated efficiency of precipitation formation and growth. Precipitation processes, however, have a significant effect on the dynamic and thermodynamic structure also. These feedback mechanisms of moist processes are only starting to be understood (Barros and Kuligowski, 1998; Rotunno and Ferretti, 2001; Colle, 2004). The uplift of air may result from convective instability (i.e., the presence of warm, moist air at low levels and cooler air aloft), but often it is aug-

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California mented or triggered by a focusing mechanism such as a front or mesoscale boundary (i.e., a smaller-scale front associated with, for example, the cool outflow from a previous or nearby thunderstorm or along a shoreline). The uplift may also be due to orography (e.g., moist air ascending a hillside). Large-scale ascent and bands of heavy showers and thunderstorms can also occur in association with extratropical or tropical storms. 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. A. Smith et al., 1996, 2000). On the other hand, storm movement may limit the duration of extreme rainfall rates at any given point and yield only moderate local rainfall accumulations. Regional- and drainage basin-scale differences in flash flood-producing events across the United States are summarized by O’Conner and Costa (2004). For basins with areas greater than about 500 km2, the highest unit discharges occur where large storms produce sustained rainfall for multiple days over broad areas, such as tropical storms in the eastern United States (Sturdevant-Rees et al., 2001) and Pacific cyclonic systems on the West Coast (Colle and Mass, 2000; Neiman et al., 2004). The highest unit discharges for smaller basins occur when convective systems deliver intense rainfall for periods of minutes to several hours. Such events are most common in Texas, the southeastern United States, and along the flanks of the Appalachians, where thunderstorms and mesoscale convective systems have produced near-world-record precipitation rates for durations up to 4 hours (J. A. Smith et al., 1996, 2000, 2001). Complex Terrain Special circumstances exist in complex terrain where a synoptically forced flow toward and over a topographic barrier may interact with the storm dynamics. This may lead to persistent (slow-moving or quasi-stationary) and orographically enhanced storm systems that produce heavy rainfall through both increased rain rates and increased time raining over a given area (R. B. Smith, 1979; Bruintjes et al., 1994; Buzzi et al., 1998; Rotunno and Ferretti, 2001; Medina and Houze, 2003). Often such precipitation systems are fed by a boundary layer jet (influenced by a stalled frontal boundary) that pumps near-saturated air into the storm, thereby facilitating an efficient formation of precipitation through predominantly warm rain

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California processes (Box 2.1) at relatively low levels in the storm (Maddox et al., 1978; Caracena et al., 1979; Reinking and Boatman, 1986; J. A. Smith et al., 1996; Petersen et al., 1999; J. A. Smith et al., 2000; Kelsch, 2001). This low-level maximum in storm intensity poses a real challenge for radar detection in complex terrain or at farther ranges, where the lowest radar beam may be blocked or overshooting the low-echo-centroid signature (Wilson and Pollock, 1974; Westrick et al., 1999; White et al., 2003). In addition, melting snow layers concurrent with intense or persistent rainfall in complex terrain may substantially increase the threat of flooding (Barros and Kuligowski, 1998). BOX 2.1 Warm Rain Processes According to Rogers (1979), most of the world’s precipitation falls to the ground as rain, much of which is produced by clouds whose tops do not extend to temperatures less than 0°C. The mechanism responsible for precipitation in these “warm” clouds is coalescence among cloud droplets. 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. These tiny cloud droplets grow in size as additional water vapor is deposited onto them, a process called diffusional growth. Very small cloud particles have negligible fall velocities; however, as they grow in size their fall speeds increase as well. Differences in fall speed lead to collisions among the cloud particles, and some of these collisions result in combined, larger particles. This process of collisions among and coalescence of cloud droplets is a much more effective growth process than growth through deposition of water vapor; thus, raindrop-sized precipitation particles may rapidly develop. Moreover, the larger raindrops interact with cloud droplets as well as other raindrops, and this “warm rain process” of collision and coalescence continues. Growing particles may ultimately reach a size at which the surface tension is no longer able to keep the body of water together, and a large raindrop may break up into a number of smaller ones. Breakup of large raindrops may also be caused by collisions with other particles. Other factors limiting the development of rain are the supply of water vapor and the number of available hydrophilic aerosol particles for cloud droplet nucleation. Regions with an ample supply of moisture from large water bodies, whether seasonally or throughout the year, may exhibit a higher fraction of surface rainfall produced by the warm rain process (as contrasted with the “cold rain” process, which involves evolution from ice particles in the clouds). If this supply of water vapor is combined with orographic uplift (e.g., along the California coastal mountains), this may lead to very effective precipitation production and flash flooding situations.

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California The coastal mountains, extending along virtually the entire length of western North America, represent a significant barrier to the lower tropospheric flow (Bond et al., 1997). These mountains can provide lift of moisture-laden air even under stably stratified conditions (Maddox et al., 1980). Indeed, substantial rainfall often follows when sustained strong winds associated with extratropical cyclones force high-humidity air up the mountains of California and the Pacific Northwest. Sometimes this deep plume of moisture originates from the subtropics near Hawaii; thus, the term “pine-apple express” was coined. Heavy rainfall in California and the Pacific Northwest is not always confined to the immediate west slopes of the coastal mountains. Often, a surface layer of cooler, stable air may exist upstream of the mountains prior to the onset of a precipitation event. In these situations, a strong southeasterly flow of air, called a “barrier jet,” may form parallel to and west of the Experimental Studies of Orographic Precipitation The past few decades have seen a number of concerted, experimental efforts to advance a conceptual and quantitative understanding of the effects of terrain on the flow of moist air past it and the generation of precipitation on the upstream side. Examples along the mountain ranges of the U.S. Pacific northwest coast are the Cyclonic Extratropical Storms Project (CYCLES; Hobbs et al., 1975; Houze et al., 1976; Matejka et al., 1980; Hobbs and Persson, 1982); the Coastal Observation and Simulation with Topography Experiment (COAST; Colle and Mass, 1996; Bond et al., 1997; Braun et al., 1997); and most recently the Improvement of Microphysical Parameterization Through Observational Verification Experiment (IMPROVE; Stoelinga et al., 2003). Details about IMPROVE may be obtained from http://improve.atmos.washington.edu; moreover, a special issue of the Journal of the Atmospheric Sciences is in preparation. Further to the south, the Sierra Cooperative Pilot Project (SCPP; Marwitz, 1983; Reynolds and Dennis, 1986; Reynolds and Kuciauskas, 1988; Pandey et al., 1999) was conducted over the Sierra Nevada Mountains. In addition, California has seen numerous recent efforts to study orographic precipitation by means of the California Landfalling Jets Experiment (CALJET; Neiman et al., 2002, 2004; Ralph et al., 2003, 2004; White et al., 2003), the Pacific Jets Experiment (PACJET), and the National Oceanic and Atmospheric Administration Hydrometeorological Testbed (HMT). Particulars about these recent and ongoing projects may be found at http://www.etl.noaa.gov/programs. Another notable, internationally coordinated experimental effort, the Mesoscale Alpine Program (MAP; Bougeault et al., 2001), was focused on the European Alps. Information about this project may be retrieved from http://www.map.ethz.ch and from the many articles compiled in a special issue of the Quarterly Journal of the Royal Meteorological Society (QJRMS, 2003).

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California mountains. Thus, the approaching weather system with its strong west or southwest winds encounters this trapped layer near the coastline, forcing the moist air to rise offshore. Consequently, the heaviest precipitation may fall west of the mountain slopes, above the cold layer of blocked or trapped flow (Neiman et al., 2002). The presence of the barrier jet complicates the forecast and warning process, because it can erode (i.e., evaporate) rainfall otherwise headed toward the mountain slopes. In some parts of the country, including the Rocky and the Appalachian Mountains, flooding due to orographic precipitation is from deep convection, as thunderstorms remain stationary or continually form in air lifted over the sloping terrain. In many of these areas, moist winds blow in at low levels from Gulf of Mexico and Atlantic Ocean moisture sources to the east or southeast, and the dangers are greatest in the warm season. Sometimes the moist air in these instances has its origin behind, or north of, a weak cold front. Although thunderstorms in which ice processes tend to dominate the development of precipitation have been involved in the worst storms in these areas, warm rain processes have been found to make important contributions to rainfall generation (Maddox et al., 1978; Reinking and Boatman, 1986). Southern California California’s coastal mountain ranges are typically less than 100 km long and extend vertically only about 500–1500 m above mean sea level, but they can generate significant, orographically induced floods (Neiman et al., 2002). These events usually involve relatively shallow bands of rain and showers associated with land-falling extratropical cyclones and fronts in the winter months. White et al. (2003) show that the radar echoes often top around 3 km or less above mean sea level. In these cases, a strong south or southwesterly moist low-level flow from the Pacific Ocean impinges on the upward-sloping terrain. Despite a low-lying freezing level (around 2 km) during the winter months, the coastal convergence and orographic uplift may still result in significant concentrations of liquid water within the warm cloud layer. Thus, these storms can be associated with greater than 200 mm (8 in.) of rain in 24 hours and strong surface winds in excess of 50 m s–1. For example, Davis (2001) noted that on January 3–5, 1982, upslope winds produced widespread rainfall near Santa Cruz, California, with rainfall amounts of 610 mm (24 in.) accumulated over a period of 28 to 30 hours in the higher elevations (National Climatic Data Center [NCDC], 1982). Although much of the precipitation can be attributed to orographically enhanced

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California widespread (stratiform) rainfall, potential instability may be present and realized when air parcels are pushed up the terrain (Lowndes, 1968; Browning et al., 1974; Neiman et al., 2002). This allows embedded showers, and sometimes thunderstorms, to develop and locally increase rainfall rates. Because of the high population density, flooding along California’s coastal mountains may produce millions of dollars in property damage and fatalities (NCDC, 1995, 1998). White et al. (2003) suggest that nearly one-quarter of the total rainfall at a California coastal site may be attributed to warm rain processes. Although these events tend to be associated with light rain (less than approximately 5 mm h–1 [0.2 in. h–1]), they can reach rain rates of 12–18 mm h–1 (0.5–0.7 in. h–1). Such values would not normally cause flooding in most other locations across the country; for the Los Angeles area, however, rain rates of 12 mm h–1 (0.5 in. h–1) or more surpass the rule of thumb used by local forecasters for guidance in issuing flood statements. A number of modeling studies have shown the dependence of orographic precipitation on the upslope flow (Collier, 1975; Bell, 1978; Sinclair, 1994). Browning et al. (1975) and Neiman et al. (2004) show that the low-level jet (LLJ) 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. (2002) quantitatively show that the amount of precipitation that falls on the coastal ranges of California is related to the magnitude of the upslope flow. They calculated vertical profiles of the linear correlation coefficient of upslope flow versus rain rate for all cases containing an LLJ in the winter season 1997–1998. These correlation coefficient profiles show a direct relationship between the magnitude of the upslope component of the LLJ flow affecting the coast and the rain rate in the downstream coastal mountains. Maximum correlation coefficients were as large as 0.94 in some individual cases and 0.70 for the entire winter season. Neiman et al. (2002) also found that the layer of upslope flow that optimally modulates orographic rainfall is near the mountaintop, approximately 1 km above mean sea level for California’s coastal range. This height also corresponds to the mean altitude of the landfalling LLJs. Although the most common flash flood scenario for the Los Angeles area is based on winter orographic precipitation involving southerly or southwesterly surface winds, as described above, flooding problems occasionally arise in other contexts as well. For example, after the passage of winter storm cold fronts, elevated cold air moving in from the northwest may result in sufficient instability to trigger severe thunderstorms with hail,

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California damaging winds, and occasionally brief torrential rains sufficient to result in at least urban flooding. Alternatively, moisture flowing northward from tropical weather systems west of Mexico may also lead to flooding. Moisture flowing northwestward from the Gulf of California or during the Southwest monsoon in late summer and fall can lead to flooding from slow-moving thunderstorms in the eastern parts of the Los Angeles-Oxnard forecast area of responsibility (i.e., over east slopes and to the east of the coastal mountains). Global-scale climatic anomalies may exert a strong influence on the occurrence of severe regional winter floods in the southwestern United States. For example, Andrews et al. (2004) investigated the influence of the El Niño-Southern Oscillation (ENSO) on flood frequency and magnitude along the California coast. They showed that floods in coastal basins of Southern California (south of 35° N) are significantly larger during El Niño than La Niña periods (Figure 2.1). This is a consequence of increased precipitation during El Niño-like conditions, which is tied to a frequent southward displacement and slight rotation of the wintertime polar jet to result in more southerly flow along California (see also Mitchell and Blier, 1997; Cayan et al., 1999; Masutani and Leetmaa, 1999). Such systematic shifts in wind direction relative to the coastal mountain orientation have an impact on the upslope flow and the corresponding “rain shadows” (Ralph et al., 2003). Analyses of California rainfall (Monteverdi and Null, 1997) reveal that El Niño and La Niña are not the only factors that influence annual variations in rainfall and flooding in California, however. Although every Type I El Niño since 1949 has resulted in above-average winter precipitation at the Los Angeles Civic Center, not every El Niño episode results in unusual flooding in Southern California (Monteverdi and Null, 1997). Moreover, many flood events occur without the presence of El Niño. Of California’s ten costliest floods since 1949, only three occurred during El Niño episodes (Null, 2001). 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 phenomena. Heavy rainfall is required, but flooding also depends on the hydrologic characteristics of the watershed in which rainfall accumulates. The amount of runoff and the magnitude of streamflow depend on the rainfall distribu-

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California FIGURE 2.1 Comparison of observed El Niño and non-El Niño (i.e., neutral and La Niña) annual peak floods with fitted three-parameter log gamma distribution computed for San Juan Creek (U.S. Geological Survey station number 11046530) at San Juan Capistrano draining an area of 282 km2. SOURCE: Figure 5 of Andrews et al. (2004). tion, rainfall rate, and duration of high rainfall rates in the drainage basin. Physical characteristics of the basin (e.g., soil saturation and permeability, geography or slope, urbanization, vegetation) further modulate the runoff potential (Davis, 2001). Hydrologic processes determine what happens to water reaching the land surface. Raindrops falling toward the ground may be intercepted by vegetation such as trees, bushes, or other plants. Alternatively, raindrops may fall on open water bodies (e.g., lakes, rivers), impermeable surfaces (e.g., rooftops of buildings, roads, parking lots), or reach a land surface that absorbs them. The land surface may be regarded as a “hydrologic switch” that either directs rainwater into the soil layers (i.e., infiltration) or lets it run off the surface (i.e., overland flow). Ponding and surface runoff will occur

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California whenever the land surface receives more rainfall than it can absorb, either naturally or due to man-made interference such as sealed surfaces. There are two types of natural processes that may generate excess rainfall runoff: infiltration excess and saturation excess (Beven, 2001). Infiltration excess is driven by the rainfall rate, while saturation excess tends to result from high rain volume. These processes are highly nonlinear and depend heavily on soil properties (e.g., saturated hydraulic conductivity, moisture deficit), which are quite variable in space and time, depending on land use and land cover conditions. Across a catchment the saturated hydraulic conductivity may vary from essentially zero for impervious surfaces to approximately 600 mm h–1 (24 in. h–1) for sandy soils (Clapp and Hornberger, 1978). Common soil types, such as silty clay loam soils, may absorb rainfall rates on the order of 5–10 mm h–1 (0.2–0.4 in h–1) or less. The actual infiltration capacity of a soil, however, depends also on the degree of saturation of the soil layers with depth and the underlying water table (Sturdevant-Rees et al., 2001). Extended periods of rainfall over a given area may decrease the soil’s infiltration capacity to absorb water and thus cause saturation of the upper soil layers, which increases the effective impervious area of a drainage basin. Similarly, frozen soil conditions or layers of snow covering the land surface may also contribute to essentially impervious areas. In addition, wildfires may dramatically alter portions of a natural watershed’s surface through the production of water-repellent soil conditions that result from the combustion of vegetation and other organic matter, thus increasing the effectively impervious area. For example, the flood of July 13, 1996, that occurred in Buffalo Creek, Colorado, was due partly to a wildfire that had burned a significant portion of the watershed only 2 months earlier (Warner et al., 2000; Chen et al., 2001). The resulting drastically increased ratio of precipitation runoff to infiltration exacerbated the high rainfall amounts, leading to a deadly and destructive flash flood. The burning of trees also affects the precipitation interception capacity. Both effects can significantly increase the runoff generation in the burned areas of a watershed (Johnson, 2000; Yates et al., 2001). The temporal and spatial variability of rainfall appears to matter more for infiltration excess than for saturation excess runoff production (Winchel et al., 1998). For extreme events such as the Fort Collins, Colorado, flash flood on July 28, 1997, Ogden et al. (2000) found that uncertainty in the space-time distribution of rainfall had a much more profound effect on runoff predictions than uncertainty in watershed characteristics. The most sensitive land-surface parameters were the soil-saturated hydraulic conduc-

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California tivity, the fraction of impervious area, and the retention depth (i.e., the capacity of the land surface to hold water in microtopographic features). Hydraulic Processes Rainwater that is not absorbed by the land surface starts running off hill slopes (at slow flow velocity) and eventually reaches a channel (attaining fast flow velocity), where it gets routed through the drainage network to the catchment outlet. The role of the network structure in controlling a basinscale flood response can be evaluated through the geomorphological instantaneous unit hydrograph (GIUH; Rodriguez-Iturbe and Rinaldo, 1997), which represents the basin response at time t to a unit mass input of rainfall at time t = 0 uniformly distributed over the catchment. The velocity of water flowing through a channel plays a fundamental role in determining the timing of flood response to rainfall input. A basin’s response time to rainfall input may be estimated by the runoff volume-to-peak discharge ratio (Potter, 1991; Bradley and Potter, 1992; J. A. Smith et al., 2000), 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. The lag time for a drainage basin can be viewed as an upper bound on the time scale of rainfall distribution that is relevant to flood magnitudes at the basin outlet (J. A. Smith et al., 2001). For warning purposes, the prediction of peak discharge, especially the time to peak, and of discharge volume is crucial. The peak discharge reveals how much the water level may rise above riverbanks or man-made drainage system confines, while the time from the onset of rising water levels to its peak indicates how much time might be available for evacuation and other protective actions. In addition, the discharge volume provides a measure of how long it will take for the water level to drop off to a normal stage again. The temporal and spatial organization of rainfall plays a fundamental role in flash flood hydrologic behavior. The rainfall properties that are important for a given drainage basin depend on the network properties of the drainage basin and the velocity of the water within the channel. Storm structure and motion interact with drainage network geometry to control flash flood response (J. A. Smith et al., 1996, 2000). An important factor determining flood response is the spatial and temporal extent to which a basin is covered by heavy rainfall. A large fractional coverage of heavy rainfall for durations corresponding to the basin response time results in

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California maximum peak discharges (J. A. Smith et al., 2002). On the other hand, storm motion relative to a catchment may either amplify or dampen a flood wave. Movement of a storm down the basin results in anomalously large peak discharges for a given rainfall and runoff. For example, another factor in the aforementioned Buffalo Creek, Colorado, flash flood was that the focusing and timing of the rainfall and storm movement coincided with the movement of the flood wave along the valley (Landel et al., 1999). In essence, storm movement acted in concert with flood wave propagation to amplify peak discharges. If the peak discharge exceeds the riverbank’s holding capacity, then water will spread out into the floodplain. Floodplain inundation is a major environmental hazard that is not well understood and lacks consensus with regard to the level of model and data complexity required to achieve a useful prediction of inundation extent, water surface elevation, and sediment deposition (Hughes, 1980; Nicholas and Walling, 1997; Horritt and Bates, 2001; Romanowicz and Beven, 2003; Jothityangkoon and Sivapalan, 2003). 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. Flood peak attenuation is largely a result of storage (or greatly reduced velocity) of a portion of the runoff on overbank surfaces (Woltemade and Potter, 1994). This storage and the later release of a portion of the total flood volume produce flood hydrographs that are low and broad compared to those of similar watersheds that lack floodplain storage, such as gullies or mountain streams. Attenuation can play a major role in regional flood hydrologic behavior (Shiono et al., 1999; Zhang et al., 2001; Turner-Gillespie et al., 2003). Natural attenuation of flood waves results primarily from geologically controlled variations in the longitudinal profile of the river channel network and the valley bottom width (Wolff and Burges, 1994; Woltemade and Potter, 1994; J. A. Smith et al., 2002; Turner-Gillespie et al., 2003). Typically, flood wave attenuation increases with increasing valley bottom width and decreasing stream slope. Detailed analyses by Woltemade and Potter (1994) reveal that moderate-magnitude floods (5- to 50-year recurrence interval) with relatively high peak-to-volume ratios are attenuated most, since the storage of a relatively small volume of water can significantly reduce the peak discharge. In contrast, both small and large floods are attenuated relatively little.

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California Impact of Urbanization Urbanization significantly affects hydrometeorological, hydrological, and hydraulic processes. For example, pollution and urban heat island effects may alter the dynamics, thermodynamics, and microphysics of precipitation formation; rapidly expanding impermeable surfaces affect hydrologic processes at the land surface; and stormwater management and drainage network construction modify the hydraulics of water flowing in natural and man-made channels. Urban flooding is becoming an increasingly serious problem because of the removal of vegetation, placement of debris in channels, construction of culverts and bridges that constrict flood flows, paving and other replacement of ground cover by impermeable surfaces that increase runoff, and construction of drainage systems that accelerate runoff (Gruntfest and Huber, 1991). The flood response for a catchment can be altered significantly through urbanization, as demonstrated by J. A. Smith et al. (2002) for the Charlotte, North Carolina, metropolitan area. The hydrologic response to urbanization typically is characterized by increasing flood peak magnitudes, decreasing lag time, and increasing runoff volumes (Leopold, 1968; J. A. Smith et al., 2002). The timing and magnitude of flood peaks can be very sensitive to alteration of the drainage network, which increases the drainage density of the basin and the hydraulic efficiency of the drainage system (Graf, 1977; Hollis, 1988). Stormwater control structures, such as detention basins, have a significant impact on the flood discharge downstream. They typically are designed to shave off the peak of extreme discharges to keep the water within the natural or man-made channel network confines, but they may result in high-water-stage levels for a prolonged period of time. Urban sprawl results in rapidly changing hydrologic and hydraulic conditions, making it difficult to assess the relative timing of local and upstream contributions to flood response and, consequently, to determine the cumulative flood response of a basin. Moreover, overflowing sewer and drainage systems and street flooding increasingly threaten the population living in urban areas. Issuing timely warnings for flash floods, therefore, remains a challenging task more so than ever before.