Hurricanes and Storm Surge

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1 Hurricanes and Storm Surge High-level winds fl ow outward Strong surface winds spiral inward EYE Calm, sometimes cloudfree, sinking air Air sinks in calm area at the storm s eye THUNDERSTORMS Bands of storms spiral around the center Strong upcurrents STORM SURGE Gradual rise in sea level prior to and during hurricane landfall. Deep Column of swirling water forms below the center of the storm Anatomy of a Hurricane What is Storm Surge? Storm Surge is the unusual rise in sea level caused by low pressure, high winds, and waves associated with hurricanes and tropical storms prior to and during landfall. Unlike seismic tidal waves, which occur on the ocean floor, storm surge is caused primarily by high winds pushing on the ocean s surface, causing the water to pile up higher than the ordinary sea level. Low pressure at the center of a weather system also affects surge levels, and when the storm nears the shore, the decreasing coastal depth also contributes to the overall height and consequential damage of the surge. This combined effect of low pressure and persistent wind over a shallow water body is the most common cause of storm surge flooding problems. Further inland, the rising and falling of the storm surge moves through structures resulting in significant water damage to contents and the structure itself. Real-time coastal water level gages have measured water level increases of 1-3 feet per hour, with the fastest rate of rise and peak occurrence when the eye is nearest the coast. Rates of rise up to one foot per 15 minutes were noted in the upper reaches of Trinity Bay as the eye of Hurricane Ike made landfall. Residents living in coastal areas threatened by tropical storms and hurricanes should understand the dangers storm surge and waves pose to their family and property. When a tropical storm or hurricane threatens the area, the National Weather Service will forecast expected heights of storm surge and wave action. Residents should use these forecasts, the elevation of their house and property, and advice from local emergency management officials to determine when to evacuate. (see the evacuation ZIP code zones map on the back page of this brochure)

2 The Factors of Storm Surge Various factors affect storm surge height and inland penetration, such as coastal topography, offshore coastal shelf, storm intensity, angle of approach, and storm speed and direction. The most important variable is the extent (size) of the storm s wind fi eld. Large diameter storms are capable of producing much higher surges than small diameter storms, as experienced during Hurricane Ike. Additionally, locations at the heads of bays and the mouths of rivers, creeks, bayous, and inlets can have surge values several feet higher than those experienced along the open coast due to the funneling of the surge into a smaller and shallower area (e.g. Clear Lake and the Houston Ship Channel). Wave height is determined by the depth of the water, the distance and speed of the wind, and the wind duration. Estimated wave heights on top of a 16 foot storm surge for Galveston Bay are on the order of 8-9 feet. Long period swells generated over the open ocean which can reach heights of feet will be forced to break well before they reach the coastline with heights at the coastline averaging 8-12 feet on top of the storm surge. Deep Water In open water, storm surge poses no threat to mainland property. Landfall When landfall is imminent, the danger from storm surge increases signifi cantly. Barrier islands can break waves, but regeneration and constant pounding of waves and debris can still devastate homes inundated by the storm surge.

3 Harris County Coastal Ground Elevations Use this map to estimate your property s fl ood risk from the National Weather Service s forecasted storm surge elevations. Storm Surge Both long period ocean swells and short period wind waves ride on top of the storm surge and result in signifi cant damage one to four blocks inland of the coastline. The illustration above shows the potential for destruction from storm surge. The photos to the right show the reality of it: September 2008 Hurricane Ike storm surge, Crystal Beach, TX September 9, 2008: Pre-Ike September 15, 2008: Post-Ike Coastal Residents Should Know the elevation of your property and the land elevation surrounding your property. Be prepared to evacuate if your property is at an elevation subject to surge fl ooding. Have fl ood insurance. No matter where you live, you could fl ood anytime. Have a family preparedness plan. Be familiar with evacuation routes and drive the routes, occasionally checking for changes in the lanes and construction. Understand the dangers of storm surge and wave action, and the potential for damage to your property. Listen to National Weather Service Hurricane Local Statements, which will include storm surge and wave height forecast. If told to evacuate by local offi cials, do so promptly. Useful Websites Harris County Flood Control District National Hurricane Center NWS Houston/Galveston Harris County Offi ce of Homeland Security & Emergency Management City of Houston Offi ce of Emergency Management Galveston County Offi ce of Emergency Management

4 Categories & Evacuation The Saffir-Simpson hurricane scale is a rating system used to estimate the potential property damage expected along a coast from a hurricane landfall. Evacuation ZIP code zones have been set for Harris, Brazoria, Galveston and Chambers Counties. The ZIP Zones are identified in the map on the back page of this brochure. 1 Category 1 Winds of 74 to 95 mph ZONE A Category 2 Winds of 96 to 110 mph 2 ZONE A Category 3 Winds of 111 to 130 mph 3 ZONE B Category 4 Winds of 131 to 155 mph 4 ZONE C Category 5 Winds greater than 155 mph 5 ZONE C

5 Hurricane Evacuation Zones For more information about tropical weather in Harris County, log on at: Write or call the District at: Harris County Flood Control District 9900 Northwest Freeway Houston, TX us on the web: This paper contains recycled content March 2009