SR/SSD 98-2

January 15, 1998

Technical Attachment

A PRIMER ON COASTAL FLOOD FORECASTING

Daniel A. Sobien and Charles H. Paxton

NWSO Tampa Bay Area, Florida

Introduction

Coastal NWSOs will assume coastal flood watch/warning responsibilities for their county warning areas during the next several months. Unfortunately, as meteorologists and weather forecasters, few of us have much background in oceanography principles that are required to make accurate coastal flood predictions. This Technical Attachment provides a short, qualitative description of factors that cause coastal flooding.

Two meteorological parameters that are closely associated with coastal flooding are atmospheric pressure and surface wind, and of course the two are not mutually exclusive. Other parameters such as coastline geography and topography also affect water levels along the coast. These are briefly discussed below.

The Role of Atmospheric Pressure

Atmospheric pressure exerted on the surface of the ocean is balanced by the upward force exerted by the ocean. Therefore, as atmospheric pressure decreases in a localized area, the force exerted by the ocean must respond by an increase in water levels in that area. As a general rule of thumb, water rises about a foot for every 30 mb decrease in surface pressure (Fig. 1). Accordingly, for a tropical storm with a central pressure of about 1000 mb which intensifies to a Category 1 hurricane with a 970 mb central pressure, the associated surge will increase about one foot due to the decrease in surface pressure alone. This dome of water in a tropical system will be localized near the eye, which is in part the reason why the largest surges are just to the right of where a hurricane makes landfall. In most winter storms pressures are generally not low enough to cause significant rises in water levels, and any increase in water levels that occurs is spread over a broader area because of the size of the storm.

Atmospheric pressure also plays a role in larger scale seasonal variations in water levels. In the eastern Gulf of Mexico water levels tend to be about a foot higher in the summer than in the winter. There are at least three reasons for this:

1) In the summer, shelf water warms to near 90F, resulting in a decrease in density and expansion of the water;

2) Wind direction generally shifts to southerly, which, as will be explained later, causes water to accumulate along the West Florida continental shelf; and

3) Atmospheric pressure tends to be lower in summer than in winter.

Other areas have similar responses. On the east coast of Florida and southern Texas, water temperatures are warmer in summer and atmospheric pressure is lower, both of which help to increase water levels, but the prevailing southeasterly winds actually work to lower water levels near the coast.

The Role of Wind

Of all the meteorological parameters that cause an oceanographic response, wind plays the greatest role. How surface winds affect coastal flooding may be surprising at first. To understand how wind changes water levels, one must understand currents and, particularly, the Ekman spiral.

The Ekman effect. First, consider a calm area of ocean, from the surface to some depth, which has the same water elevation at all points. When wind begins to blow over this surface, drag induces a geostrophic current to form. This current develops downward, but the deeper from the wind forcing at the ocean surface, the lighter the induced current is. With time, the Coriolis force affects this flow and turns it to the right. This sets up a logarithmic spiral known as an Ekman spiral. The current turns further to the right of the surface flow as depth increases, but decreases in strength with depth.

At some depth, the induced current decreases to zero and then an equal and opposite flow sets up below that point, with currents moving to the left of the surface flow (Fig. 2). This opposite flow keeps water levels at a relative equilibrium over deep water, but as the wind blows over shelf waters, the deep water flow is disrupted. At this point, the only portion of the flow affected by the Ekman spiral is to the right of the surface winds with no counter current below. This allows a net build-up of water to the right of the surface wind over the continental shelf (Fig. 3). Because this is a Coriolis-induced phenomenon, it takes a certain amount of time and ocean surface to develop fully.

Because of the Ekman effect, with a synoptic scale disturbance the wind component blowing parallel to shore has the most influence on coastal flooding. Although the onshore wind component has some influence, as a rule of thumb it is only about one-half the impact of the along shore wind component. Since water builds to the right of the surface winds, southerly winds accumulate water on west-facing beaches, and northerly winds drain water from west-facing beaches (Figs. 4a, 4c). Similarly, a southerly wind would force water away from east-facing beaches, while northerly winds would accumulate water along the same beaches (Figs. 4b, 4d). The stronger the winds, the stronger the induced current and the Ekman effect, so the greater the buildup of water. In addition, cold, relatively dense air blowing over warmer waters has a greater effect on currents than does less dense warmer air, and this in turn may affect resulting coastal flooding.

Effect of storms. With smaller, relatively fast moving systems such as a hurricane, the Coriolis effect does not have time to develop as much, so during such storms the onshore wind component has a much greater impact on coastal flooding than does the along shore wind component. This is the other reason why the surge is highest just to the right of where the eye makes landfall, because winds are strongest in that quadrant (and are combined with forward motion of the storm as well). Late or early season "hybrid" type tropical storms may have strong winds in outer bands far from the center of circulation. For example, Tropical Storm Josephine not only caused coastal flooding near the eye, but the along shore wind component associated with outer bands caused significant coastal flooding up to 150 nm away from where the storm's center made landfall.

Geography, shape of the coastline and other factors. Generally, as water accumulates on a continental shelf a slope develops with higher water toward shore and lower water away from shore. Accordingly, the wider the continental shelf, the worse the coastal flooding, as this allows more area for water to accumulate. A 30 kt wind produces significantly less flooding along the narrow shelf of Puerto Rico's north coast than a similar wind over the wide shelf of Appalachee Bay in the northeast Gulf of Mexico. Local geography also has a significant influence on water levels. For example, concave features such as bays, lagoons and inlets may be conducive to the piling up of water which is blown into the feature and is unable to escape.

Astronomical tides are also a consideration, since a very high tide coupled with a surge of water can escalate the effects of flooding. Astronomical tides must be considered when monitoring tide gauges for coastal flooding. For instance, the gauge may show a steady tide, but if the astronomical tide is ebbing then flooding may be a concern when the next high tide returns. Tide tables should be kept easily accessible for ready reference.

Strong along shore winds blowing over shelf waters can cause a run-out of water as well as a run-up of water. For instance, a strong north wind blowing along west-facing beaches will cause abnormally low water levels and upwelling along the coast. Abnormally low water levels can be a hazard to shipping and other coastal interests.

Summary

Coastal flooding can cause widespread destruction. Although this paper may be helpful as a quick refresher on coastal flooding, several points need to be examined locally. A better understanding of the effects of various wind directions and speeds on differing coastlines requires local study. Historical accounts of coastal flooding should be assimilated into a nomogram to help with prediction of events. The SLOSH model may be of use for tropical scenarios, but it may not be accurate for extratropical systems.

During episodes of coastal flooding, tides must be monitored frequently to determine trends. If automated monitoring is not available, sites that are accessible during flood conditions should be surveyed and equipped with a measuring staff. Coastal areas should be surveyed and areas affected by certain incremental increases in water level should be noted for specific mention in coastal flood products. Although coastal flooding may occur only infrequently, preparedness can make the difference between life and death.

Key points to remember in forecasting are the following.

1) Pressure drops can cause a rise in sea level; a drop of 30 mb will cause about a one foot rise in water levels.

2) With a large scale system, given enough time for the Coriolis effect to come into play, the along shore winds have about twice the impact on increasing near shore water levels than does the onshore wind component.

3) For smaller systems such as tropical systems, the onshore wind component is much more important.

4) In general, the wider the continental shelf, the worse the flooding will be.

5) Local geographic features of the coastline may exacerbate flooding in localized areas.

6) Astronomical tides may increase or decrease the effects of coastal flooding.