1999-06-14 Regular Meeting and Public Hearing

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i CITY OF LA PORTE, TX CRS ACTIVITY 510 FLOODPLAIN MANAGEMENT PLAN Assess the Hazard Topography The City of La Porte has several major watersheds running through it, and is adjacent to Galveston Bay. Hurricanes The major threats to the City of La Porte are coastal storms and hurricanes. These events can produce three types of hazards: high winds, storm surge, and torrential rain. The following descriptions of the hazards has been taken from the Weather Channel's website, "Effects of a Hurricane," at http://www.weather.com/breaking weather/encyclopedia/tropicaVeffect.html: Rainfall Induced Flooding The heavy rains associated with a tropical weather system are responsible not only for major flooding in areas where the storm initially strikes, but can also affect areas hundreds of miles from where the storm originally made landfall. During landfall, it is not uncommon for 5 to 10 inches to fall. If it is large and moving slowly, rainfall could be even more excessive. As the storm moves inland, and is down graded to a tropical depression, the continued circulation, tropical moisture, and topography can contribute to copious amounts of rainfall. Intense flooding can also occur from tropical depressions and storms that do not reach the intensity levels associated with hurricanes. Storm Suroe The storm surge is perhaps the most dangerous and destructive part of a hurricane. The storm surge is a rapid rise in the level of the water that moves onto land as the eye of the storm makes landfall. Generally speaking, the stronger the hurricane, the greater the storm surge. As a hurricane approaches the coast, its winds drive water toward the shore. Once the edge of the storm reaches the shallow waters of the continental shelf, the water piles up. Winds of hurricane strength force the water onto the shore. At first, the water level climbs slowly, but as the eye of the storm approaches, water rises rapidly. Wave after wave hits the coast as tons of moving water hammer away at any structure built on the coastline. "A cubic yard of water weighs about 1700 pounds and it's almost incompressible," says John Hope, Tropical Coordinator at The Weather Channel. "You might just as well be hit with a solid object as to have this water smashing against a structure on a beach. " The storm surge is greater if a hurricane's track is perpendicular to the coastline, allowing the surge to build higher. The storm surge is also greater if the storm affects a bay or if it makes landfall at high tide. The gfeatest storm surge occurs to the right of where the eye makes landfall. The Winds The winds of a hurricane range from 74 miles per hour (65 knots) in a minimal storm to greater than 155 miles per hour (136 knots) in a catastrophic one. City of La Porte Page 5 of 26 Planning Department, rmh June 10, 19"