Lessons from Hurricane Ike

Ch4: Flood Prediction and Flood Warning Systems

Introduction Floods and flash floods are among the leading causes of weather related deaths in the United States, resulting in 136 deaths per year and over $4.0 billion in property damage. With heavy rains and the continual threat of severe storms, the Gulf Coast region is particularly susceptible to flooding. Far from a declining hazard, population growth has caused expansion of residential and commercial areas deep within floodplains yielding ever greater property loss and more frequent damages (fig. 4.1). Urbanization alters the hydrological setting of watersheds resulting in faster watershed response times and higher peak flows. Flood and flash flood forecasting skills have seen much less improvement when compared to tornado or hurricane forecasting. however, significant efforts were made to improve forecasting skills after the devastating flooding following hurricane Floyd (1999) and the recent onslaught of inland flood damage from tropical cyclones making landfall on the Gulf Coast. Urban flash flood forecasting is the most difficult of all flood forecasting efforts due to the rapid response of urban watersheds and limited computer modeling ability, leading to untimely data when compared to flood forecasting for major river systems. Customized flood alert systems using radar, however, have seen some advance in texas (Bedient et al. 2008). Hydrological Ingredients for a Flash Flood Event the Gulf Coast endures frequent flooding events tied to many meteorological scenarios such as the landfall of tropical cyclones, the passage of frontal systems, and mesoscale phenomena, such as the sea-breeze front and outflow boundaries. Just as the moisture available in the atmosphere that producesheavyrainfallisimportantinde termining a flood event, so too is what happens once the rainfall contacts the ground.Rainfallratesanddurationare Flood Prediction and Flood Warning Systems Jeffrey Lindner, Dave C. Schwertz, Philip B. Bedient, and Nick Fang 4 lindner, schwertz, bedient, and fang 39 key ingredients for a flash flood especially in an urban setting, but the type of ground the rain falls on, the time of year, and the moisture content of the soil all play an important role in determininghowhydrologicalbasinswillre spond to the rainfall. the types of watersheds that exist along the Gulf Coast are varied and the effects of excessive rain may be felt far downstream. Generally, the elevation rise along the Gulf Coast is considered flat with some exception over south-central texas where the terrain becomes slightly hillier. the flat terrain along the Gulf of Mexico causes slow run-off and large areas of ponding , which usually yield a slow rise on bayous and creeks that are low-lying and interface with the marshlands. in the hills of Central texas more rapid run-off occurs because soils are rockier , leading to flash flooding in normally dry creek beds. the corridor along i-35 from del Rio northward through northern texas is known as “flash flood alley” and is the site of multiple high water rescues and flooding deaths each year (figs. 4.2 and 4.3a-e.) Urbanization around the large city centers along the Gulf Coast yields Figure 4.1. Floodwaters caused by tropical Storm Allison inundate trucks and cars trapped on a freeway. tropical Storm Allison dumped over 24 inches of rain on houston in 9 hours. Copyright© 2001 dan Wallach. Used with permission. flood prediction and flood warning systems 40 another interesting hydrological setting . Most of the larger urban centers are nestled in the flat coastal plains where drainage is already slow, due to a lack of elevation rise when compared to that of Central texas. in addition, urban sprawl consisting of concrete freeways, supermarkets, and housing developments results in miles of impermeable ground cover. thus, where storm water was historically allowed to collect and pond in marshes, there are now subdivisions. When excessive rainfall occurs in an urban setting, the first lines of defense are the street gutters and roadside ditches that collect roadway storm water and discharge into area streams and watersheds. these systems have varying design capacities and are frequently overwhelmed during the warm-season months when short-term, excessive rainfall from daily sea-breeze can rapidly produce 2–3 inches of rain in less than an hour. Once the primary drainage system is at full capacity, storm water begins to collect and pond in low-lying areas. Figure 4.2. the Central texas hill Country denoted in the white band is the most flash flood prone area in the United States. Rains originating in the hills (red area) quickly flow down...