Major and Catastropic Storms and Floods in Texas

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1 Major and Catastropic Storms and Floods in Texas Source: h=ÿp.ÿ!= fl!flop_d.safet_.ÿl=.ÿeÿ_u SG Sd e m o/b a cl<ÿro u n d. ht m I Background Cÿ.,:,':i Geography Texas, bounded on the southeast by the Gulf of Mexico and on the west by arid and semiarid regions characteristic of the southwestern United States, is a land of climatic and geographic diversity. The terrain is equally diverse, ranging from the featureless coastal plains along the Gulf Coast to the spectacular features of West Texas, which include the Guadalupe Mountains, the canyons of Big Bend, and the Caprock escarpment of the High Plains (Figure 1). and subtropical Atlantic moisturÿ Figure 1: Sources of Moisture Influencing Texas Weather Sources of Moisture The principal sources of moisture for Texas are the Gulf of Mexico and, to a lesser extent, the eastern Pacific Ocean. Moisture from the Gulf of Mexico is carried into the State by low-level southerly and southeasterly winds. Moisture from the eastern Pacific is carried into the State from the southwest by tropical continental air masses. In addition to the oceans, important moisture sources include local and upwind land masses, as well as lakes and reservoirs, from which moisture evaporates to the atmosphere. Typically as a moisture-laden ocean air mass moves inland, it is combined with moisture that has been recycled through the land-vegetation-air interface. Precipitation The average annual precipitation differs little from north to south but greatly from west to east. Average annual precipitation in El Paso is less than 8 inches. More than 770 miles to thÿ past, average annual precipitation in the lower Sabine River valley of extreme eastern Texas exceeds 56 inches. The precipitation varies seasonally as well as geographically. Although spring and fall usually are the wettest seasons, intense rainfall can occur in late summer during the tropical storm and hurricane season. For most of the State, however, the average precipitation during summer is only slightly greater than that during the winter. Spring is the wet-test season in most of Texas, with April and May often the wettest months. Spring thunderstorms generally are caused by successive weak frontal systems that move through the State. These cool air masses from the north meet warm, moist air masses from the Gulf of Mexico. The warm, moist air is less dense than the cooler air and thus overtops the cool air mass. ( ÿ""ÿ':ÿ 2012 Rice University- All Rights Reserved 4

2 Major and Catastropic Storms and Floods in Texas Source: hÿp :/gfl_oodsafetv.org/t_e.xas Uÿd emo/backgrou nd. html Precipitation, continued The moist air then condenses and forms thunderstorms along the line of contact between the two systems. Summer usually is drier than spring in most of the State, but a secondary peak of rainfall often occurs in September and October. Tropical cyclones (hurricanes and tropical storms) originate in weather systems that begin in the Caribbean Sea or the Gulf of Mexico in late summer or early fall. Rainfall amounts that result from tropical cyclones can differ greatly because of the different conditions in each storm. Remnants of some hurricanes reaching landfall have produced large depths of rainfall over wide areas of the State. Figure 2: Some Major Storms along Balcones Escarpment in Central Texas iÿ:,ÿ (O 2012 Rice University- All Rights Reserved

3 HANDOUT Major and Catastropic Storms and Floods in Texas Source: h ÿp_..j!_f! o o d s af etv. o rg/texas/_u SGS_d e m o/b a cl<gro u n d, ht m I Balcones Escarpment The hills and associated elevation increases along the north side of the Balcones escarpment in central Texas assist in the uplifting of air masses and the formation of storms. Many large thunderstorms form along the escarpment, where they can stall and produce extreme precipitation depths during a few hours or few days. Many of the largest storms in the State have occurred in this area. Some of those occurring along the escarpment during about the past 80 years are indicated in Figure ooo ooo (y) o z _y,j L,L. z,< {SO ! I L I I... ] l t... L... I I ao 6 24 MINUTES HOURS DAYS DURATION Figure 3: Some of the Largest Storms in the World Largest Storms Many Texas storms represent some of the largest storms in the world. Figure 3 shows the largest precipitation depths in the world, for durations ranging from 1 minute to 24 months. Also shown are some of the largest known precipitation depths in Texas. As indicated, many of the largest storms with durations from about 1 hour to 48 hours have occurred in Texas. Examples of these storms include a 1921 storm in Thrall that produced 32 inches of rainfall in 12 hours and a 1935 storm in D'Hanis that produced 22 inches of rain in 2 hours and 45 minutes. co 2012 Rice University- All Rights Reserved ÿ:i

4 HANDOUT Major and Catastropic Storms and Floods in Texas Source: _bÿlÿ'!! flo_q_d sÿÿtj_sgs d e m_o/b a ckgro u nd. h t m I Flooding Flooding from large storms has affected Texas throughout its history, causing many deaths and much economic loss and hardship. Floods occur regularly in Texas, and destructive floods occur somewhere in the State every year. Many of these floods are destructive because they often occur in areas where extreme flooding had not occurred for many years. These floods often are perceived as unexpected or even unprecedented because their peak water-surface elevations (stages) can greatly exceed those of past floods. For example, a recent report by the U.S. Geological Survey identified, for sites throughout the State, maximum known peak stream discharges that greatly exceed peak discharges for 100-year floods. The maximum known discharges typically range from about 1.5 to about 3 times greater than 100-year discharges in the western and eastern parts of the State, but documented discharges for some sites along the Balcones escarpment have been as much as 4 or S times greater than 100-year peak discharges. Such peaks usually are devastating because structures and development typically exist outside the 100-year floodplain but often are within floodplains for maximum floods. ÿ?ÿ" ÿ Rice University - All Rights Reserved 7

Central Texas Flood, November 15, 2001 Source: httbttjÿ::!/floodsafetw.o rg!_texas/usgsdemo/2001to 1975.

5 Central Texas Flood, November 15, 2001 Source: httbttjÿ::!/floodsafetw.o rg!_texas/usgsdemo/2001to htm I The storm of November 15, 2001, began with mild to moderate rainfall over the night of November 14th and through the morning of November 15th. At about 3:30 p.m. on November 15th, a large thunderstorm cell began to approach the Hays-Travis County line, with tornadoes and intense rainfall. The tornadoes touched at various locations along the 1-35 corridor starting near Buda and the Ben White Blvd interchange and continuing north and east through Travis County. The heaviest rainfall began about 3:30 p.m. over south-central Austin in the middle of the Slaughter Creel< ÿ.ÿ Basin and upper South Boggy Creek Basin. The intense thunderstorm cells continued to track rapidly and generally north-northeast over Barton Creek at Loop 360, West Bouldin Creel<, Johnson Creek, and Shoal Creel<. The area of the most intense rainfall decreased as it tracked farther north over the upper Little Walnut and Walnut Creek watersheds. Although the storm gradually decreased in intensity over the next 6 hours, it continued to produce intense rainfall Widespread rainfall totals typically ranged from 5 to 8 in., with individual reports of 10 in. and more. Much of this rain fell within about 6 hours. Generally, the storm intensities and flood levels were higher on the south and west sides of Austin. Rainfall intensities exceeded the estimated 100-year rainfall rates in some locations and caused widespread but isolated flood damage where the drainage capacity of streets and storm drains was exceeded by localized rainfall. The flow measured in area creeks does not match the statistical significance of individual maximum rainfall measurements because the intense rainfall was not evenly over all of the watersheds. For example, the USGS estimated that the flood peak in Onion Creek at U.S. Highway 183 was approximately 93,200 cubic feet per second, which corresponds to about a 50-year peal<. Other watersheds on the south and west (for example, Slaughter, Williamson, South Boggy, West and East Bouldin, Barton, Bull, Johnson, and Shoal Creeks) experienced flood peaks that correspond to a return period of 5 to 20veers. The USGS gage on Shoal Creel< at W. 12th Street recorded a peak corresponding to approximately a 10-year return period. Watersheds on the east side of town (for example, Waller, Boggy, Tannehill, Fort, and Buttermilk Creel<s) were less affected and experienced flood peaks corresponding to return periods of less than 10 years. Other watersheds that flow from northwest to east (for example, Little Walnut and Walnut Creeks) experienced peaks corresponding to return periods of up to 10 years. Deaths and Damages:: 968 homes were flooded and cost of damages to public property was 11.3 million Max Precipitation: 14 inches Severity: Major Storm (ÿ) = 2012 Rice University- All Rights Reserved

6 TEACHER PRINTOUT 7.8A: Catastrophic Events FLOOD Information DischarKe (or streamflow[; measured volume of water that moves past a point in the river in a given amount of time. Discharge is usually expressed in cubic feet per second. The Mississippi River at Baton Rouge has a maximum annual discharge average of 300,000 cubic ft/sec (cfs). Cubic feet per second (ft3/s, cfs)- The rate of discharge representing a volume of I cubic foot passing a given point during I second and equivalent to 7.48 gallons per second or gallons per minute. River Stagÿ: elevation of river above its bed, i.e. water depth. Peak Level (or StaFÿJ: highest level or elevation reached by a river in flood stage. River Velocity: rate of water movement in the river. Note that the velocity doesn't specify how much (volume of) water is moving. The volume rate is needed to determine flooding. Streamflow Gaging Station: stream gauging is performed to accurately determine the volume of water moving past a given point per time. This information is crucial for flood planning and prevention. The UoS. Geological Survey maintains a number of real-time stream gauging sites in the U.S, for these purposes. A map of Texas gaging stations can be found at ÿterdata.usÿ Flood: any relatively high stream flow that overtops the natural or artificial banks of a river. Floodplain: relatively flat lowland that borders a river, usually dry but subject to flooding. Floodplain soils actually are former flood deposits. Recurrence Interval (also called a return period): average number of years between floods of a certain intensity or size. Record Peak Discharge: highest amount of water recorded in a specific river location. For specific data from gaging stations all across Texas to go: ÿ.fl.qÿqdÿ.:p._r.g/texas/_u.ÿsgsdemo/jntro Mapl.htm. Maximum Precipitation: total amount of precipitation recorded in a weather event. Storm Center: geographic center or location of a storm. Federal Disaster Area designation: Four types of disaster designations are made: 1) Presidential major disaster declaration; 2) USDA Secretarial disaster designation; 3) Farm Service Agency (FSA) Administrator's Physical Loss Notification and, 4) Quarantine designation. Presidential major disaster declarations, which must be requested by a governor to the President, are administered through the Federal Emergency Management Agency (FEMA) Rice University - All Rights Reserved

7 TEACHER PRMNTOUT 7.8A: Catastrophic Events 100-year Flood (or Return Period): an abbreviated way of describing the magnitude of a rainfall and resulting flood event that has a one percent chance of occurring. It does not mean that a flood of that magnitude can only occur once in 100 years. In fact the same statistical chance of 1/100 (1% probability of an occurrence in any one year) applies for any storm at any time and any given year. 50-year Flood(or Return Period): means that there is a 2% probability of an occurrence in any one year. lo-vear Flood (or Return Period): means that there is a 10% probability of an occurrence in any one year. Flash Flood: develop quickly, sometimes in just a few minutes and without any visible signs of rain because the water enters the river channel far upstream. Flash floods often have a dangerous wall of roaring water that carries rocks, mud, and other debris and can sweep away most things in its path. Special Flood Hazard Area (SFHA): If you live in a Special Flood Hazard Area (SFHA) or high-risk area and have a Federally backed mortgage, your mortgage lender requires you to have flood insurance. (To find your flood risk, fill out the Flood Risk Profile to the left.) http :// a rt.ÿ._rtzpages/flood f a_.cÿp_ Watershed (or Drainage basin): The part of the surface of the Earth that is occupied by a drainage system with a common outlet for its surface runoff, consisting of a trunk stream and all the tributaries that flow into it. Runoff: The part of the precipitation that appears in surface streams. Streamflow: The discharge that occurs in a natural channel Rice University - All Rights Reserved ÿ

8 TEACHER PRINTOUT 7.8A: Catastrophic Events FLOOD SAFETY: (source: During a Flood If a flood is likely in your area, you should: Listen to the radio or television for information. o Be aware that flash flooding can occur. If there is any possibility of a flash flood, move immediatelvto higher ground. Do not wait for instructions to move. o Be aware of streams, drainage channels, canyons, and other areas known to flood suddenly. Flash floods can occur in these areas with or without such typical warnings as rain clouds or heavy rain. If you must prepare to evacuate, you should do the following: o Secure your home. If you have time, bring in outdoor furniture. Move essential items to an upper floor. o Turn off utilities at the main switches. Disconnect electrical appliances. Do not touch electrical equipment if you are wet or standing in water. o If you have to leave your home, remember these evacuation tips: o Do not walk through moving water. Six inches of moving water can make you fall. If you have to walk in water, wall( where the water is not moving. Use a stick to check the firmness of the ground in front of you, o Do not drive into flooded areas. If floodwaters rise around your car, abandon the car and move to higher ground if you can do so safely. You and the vehicle can be quickly swept away. Driving Flood Facts The following are important points to remember when driving in flood conditions: o Six inches of water will reach the bottom of most passenger cars causing loss of control and possible stalling. o A foot of water will float many vehicles. o Two feet of rushing water can carry away most vehicles including sport utility vehicles (SUV's) and pickups. After a Flood The following are guidelines for the period following a flood: o Listen for news reports for safety of community's drinking water supply. o Avoid floodwaters; water may be contaminated by oil, gasoline, or raw sewage. Water may also be electrically charged from underground or downed power lines. o Avoid moving water. o Be aware of areas where floodwaters have receded. Roads may have weakpnpd and could collapse under the weight of a car. o Stay away from downed power lines, and report them to the power company. Return home only when authorities indicate it is safe. o Stay out of any building if it is surrounded by floodwaters. o Use extreme caution when entering buildings; there may be hidden damage, particularly in foundations. o Service damaged septic tanks, cesspools, pits, and leaching systems as soon as possible. Damaged e sewage systems are serious health hazards. Clean and disinfect everything that got wet. Mud left from floodwater can contain sewage and chemicals. iÿ!lÿ (c_-), Rice University-All Rishts Reserved

Ecological Context - Urban settlements are part of their surrounding ecosystem. Austin

Ecological Context - Urban settlements are part of their surrounding ecosystem Austin Ecotone Division between West and East Flyway Uplift River Balcones Escarpment Central Texas Climate The principal