What is Storm Surge?

1 INTRODUCTION TO < strong >Stormstrong > SURGE. Introduction to < strong >Stormstrong > Surge BOLIVAR PENINSULA IN TEXAS AFTER HURRICANE IKE (2008). What is < strong >Stormstrong > Surge? Inland Extent < strong >Stormstrong > surge can penetrate well inland from the coastline. During Hurricane Ike, the surge moved inland nearly 30 miles in some locations in southeastern Texas and southwestern Louisiana. < strong >Stormstrong > surge is an abnormal < strong >Stormstrong > tide is the water level rise of water generated by a rise during a < strong >Stormstrong > due to < strong >Stormstrong > , over and above the the combination of < strong >Stormstrong > predicted astronomical tide. surge and the astronomical It's the change in the water level tide. that is due to the presence of the Since < strong >Stormstrong > tide is the < strong >Stormstrong > combination of surge and tide, it Since < strong >Stormstrong > surge is a difference does require a reference level Vulnerability between water levels, it does not A 15 ft.

2 < strong >Stormstrong > surge on top of a All locations along the East and Gulf coasts have a reference level are vulnerable to < strong >Stormstrong > surge. This figure shows high tide that is 2 ft. above mean the areas that could be inundated by water in any sea level produces a 17 ft. < strong >Stormstrong > given category 4 hurricane. tide. INTRODUCTION TO < strong >Stormstrong > SURGE 2. What causes < strong >Stormstrong > Surge? < strong >Stormstrong > surge is caused primarily by the strong winds in a hurricane or tropical < strong >Stormstrong > . The low pressure of the < strong >Stormstrong > has minimal contribution! The wind circulation around the eye of a Once the hurricane reaches shallower hurricane (left above) blows on the waters near the coast, the vertical ocean surface and produces a vertical circulation in the ocean becomes In general, < strong >Stormstrong > surge occurs where disrupted by the ocean bottom.

3 The winds are blowing onshore. The highest circulation in the ocean (right above). In water can no longer go down, so it has surge tends to occur near the radius of deep water, there is nothing to disturb nowhere else to go but up and inland. maximum winds, or where the this circulation and there is very little indication of < strong >Stormstrong > surge. strongest winds of the hurricane occur. The Saffir-Simpson Hurricane Wind Scale: Surge Not Included Hurricane Charley, a category 4. hurricane at landfall in Florida, produced a < strong >Stormstrong > surge of 6 to 8. ft. Too many exceptions to fit the Hurricane Irene, a category 1. scale: Hurricane Ike, a category 2 at hurricane at landfall in North landfall in Texas, also produced Carolina, produced extensive Hurricane Katrina, a category 3 damage with an 8 to 11 ft.

4 < strong >Stormstrong > at landfall in Louisiana, produced catastrophic damage with a 20- ft. < strong >Stormstrong > surge. surge. catastrophic damage with a 28- ft. < strong >Stormstrong > surge. INTRODUCTION TO < strong >Stormstrong > SURGE 3. < strong >Stormstrong > SURGE: BEFORE AND AFTER. Hurricane Ike made landfall at Galveston, Texas, but it still produced significant coastal flooding on the north side of Lake Pontchartrain near Mandeville, Louisiana. Remarkably, the highest sustained wind reported at the lake was only 43 mph! Before Hurricane Katrina Total Water Level In reality, < strong >Stormstrong > surge only makes up a part back out to sea. The water level therefore of what causes water levels to rise along the rises as a hurricane approaches, especially coast during a hurricane.

5 Here are the since the waves become larger and more others: water is pushed onshore. Wave setup caused water levels to rise along the Texas coast Tides nearly a day before Ike made landfall (below). Water levels rise and fall along the coast every day due to the gravitational pull of the moon and sun. This is the tide. In general, areas along the Gulf of Mexico, except After Hurricane Katrina Florida, experience one high and one low tide per day (diurnal tide). Elsewhere along the Images courtesy of David and Kimberly King, East Coast experience two high and two Waveland, Mississippi. low tides per day (semi-diurnal tide). FAST FACTS When the tide is combined with the < strong >Stormstrong > surge, it is called the < strong >Stormstrong > tide.

6 Freshwater Input Unfortunately, we can't time the arrival of a Heavy rainfall ahead of a hurricane can cause < strong >Stormstrong > within the tidal cycle, so it's safer to 32%. river levels to rise well inland from the coast. assume high tide when making decisions. Once all this water flows downriver and reaches the coast, local water levels Waves especially near deltas and in bays will rise. Population density increase in coastal counties along the Breaking waves contribute to the water level Gulf of Mexico coastline from 1990 to 2008. rise through wave runup and wave setup. Wave runup occurs when a wave breaks and the water is propelled onto the beach, as in 27%.

7 The picture below. Percent of major roads in the Gulf region that are at or below 4 ft. elevation. DID YOU KNOW? On average, a major hurricane makes landfall in the Gulf Coast region once every 2 years. Total Water Level =. Galveston, TX: every 18 years < strong >Stormstrong > Surge +. New Orleans, LA: every 19 years Mobile, AL: every 23 years Tides +. Tampa, FL: every 23 years Wave setup occurs when waves continually Waves +. break onshore and the water from the runup Freshwater Input piles up along the coast because it can't get INTRODUCTION TO < strong >Stormstrong > SURGE 4. The Many Factors that Influence < strong >Stormstrong > Surge < strong >Stormstrong > Forward Speed There are several factors that contribute to On the open coast, a faster < strong >Stormstrong > will Width and Slope of the the amount of surge a given < strong >Stormstrong > produces produce a higher surge.

8 However, a higher Ocean Bottom at a given location: surge is produced in bays, sounds, and Higher < strong >Stormstrong > surge occurs with wide, other enclosed bodies of water with a gently sloping continental shelves, while slower < strong >Stormstrong > . The two images below show lower < strong >Stormstrong > surge occurs with narrow, Central Pressure the surge generated by two hypothetical steeply sloping shelves. Areas along the Lower pressure will produce a higher surge. hurricanes hitting the Florida Panhandle Gulf Coast, especially Louisiana and However, as was shown on page 2, the one moving 5 mph and the other 25 mph. Mississippi, are particularly vulnerable to central pressure is a minimal contributor < strong >Stormstrong > surge because the ocean floor compared to the other factors.

9 Gradually deepens offshore. Conversely, areas such as the east coast of Florida have a steeper shelf, and < strong >Stormstrong > surge is not as high. < strong >Stormstrong > Intensity Stronger winds will produce a higher surge. The two images below show how much < strong >Stormstrong > surge is produced by two different storms, the second one having maximum winds that are 15 mph stronger than the first. All other variable are equal. Local Features < strong >Stormstrong > surge is highly dependent on local Angle of Approach to Coast features and barriers that will affect the flow The angle at which a < strong >Stormstrong > approaches a of water. A good example is the coast of coastline can affect how much surge is North Carolina, which has the complexities of generated.

10 A < strong >Stormstrong > that moves onshore such features as barrier islands, inlets, perpendicular to the coast is more likely to sounds, bays, and rivers. produce a higher < strong >Stormstrong > surge than a < strong >Stormstrong > that moves parallel to the coast or moves inland at an oblique angle. Shape of the Coastline Size < strong >Stormstrong > surge will be higher when a hurricane A larger < strong >Stormstrong > will produce higher surge. makes landfall on a concave coastline There are two reasons for this. First, the (curved inward, such as Apalachee Bay in winds in a larger < strong >Stormstrong > are pushing on a Florida) as opposed to a convex coastline larger area of the ocean. Second, the strong (curved outward, such as the Outer Banks of winds in a larger < strong >Stormstrong > will tend to affect an North Carolina).