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Monitoring Hurricane Rita Inland Storm Surge

By Benton D. McGee, Roland W. Tollett, and Burl B. Goree

Pressure transducers (sensors) are accurate, reliable, and cost-effective tools to measure and record the magnitude, extent, and timing of hurricane storm surge. Sensors record storm-surge peaks more accurately and reliably than do high-water marks. Data collected by sensors may be used in storm-surge models to estimate when, where, and to what degree stormsurge flooding will occur during future storm-surge events and to calibrate and verify stormsurge models, resulting in a better understanding of the dynamics of storm surge.

Introduction

Storm surge associated with hurricanes occurs when winds push water up onto the shoreline. Storm surge can be intensified by sustained winds, low barometric pressure, excessive rainfall, and high tides (http://geology. com/articles/storm-surge. shtml, accessed September 11, 2006). Historically, the magnitude of hurricane storm surge has been measured by using water lines,

called highwater marks, left behind by flood waters. Identifying and qualifying high-water marks and determining how well these marks represent the peak are often subjective. The quality of the high-water mark depends upon the type of mark, such as debris, seed, mud, or stain, and on whether the mark was created in a protected environment, such as the interior wall of a building, or in an unprotected environment, such as an exposed bridge piling or fence post. High-water marks do not record the date and time of their creation nor do they record the duration of the storm-surge event.

Methods

In the days prior to the landfall of Hurricane Rita (fig. 1), an experimental waterlevel and barometric pressure sensor network (fig. 2) was deployed to record the magnitude, extent, and timing of inland storm surge and coastal flooding in the monitored area along the Louisiana and Texas Gulf Coast. East of Rita's storm path, sensors recorded storm-surge water levels over 14 ft (4.3

258

Science and the Storms: the USGS Response to the Hurricanes of 2005

Arkansas Louisiana

Sept. 24, 2005 10 a.m.

Mississippi Alabama

Monitored area

Florida

Texas

Sept. 23, 2005 10 p.m.

Gulf of Mexico

Sept. 23, 2005 1 a.m.

N

Sept. 22, 2005 4 a.m.

Figure 1. Map showing path of Hurricane Rita and study area (Hurricane Rita satellite imagery obtained from the National Aeronautics and Space Administration, 2006).

Monitoring Hurricane Rita Inland Storm Surge

259

94º00'

27 96

93º00'

49

LOUISIANA TEXAS

Beaumont 30º10' B1 Orange B19b B20

Sa bin eL ak e

10

LC4

LC3

LC2b (8.03) LC2a (8.93) LC6b LC6a

Ca lca La sieu ke

165

LA3 (10.84) LA8

10

Lake Charles

LA2 (4.49)

14

90

Lafayette

167

Port Arthur 29 50'

º

B10 B12

B19a

(5.04)

27

LC5 (6.93)

(3.86)

27

LA7 (4.21)

Abbeville

N

0 29º30' 0 10 10 20 Kilometers

Gulf of Mexico

LA10 (8.70)

20 Miles

Land-surface elevation, in feet above the North America Vertical Datum of 1988 (NAVD 88) Greater than 30

Data-collection site and identifier, by sensor type

LC6b L C5 L C7

Barometric pressure Water level--Maximum surge elevation, in feet above NAVD 88, shown in parentheses Barometric pressure and water level--Maximum surge elevation, in feet above NAVD 88, shown in parentheses Sensor lost to hurricane

0

LC10

Figure 2. Map showing locations of storm-surge sensors in southwestern Louisiana and southeastern Texas. Maximum storm-surge elevation measurements are shown in yellow.

Ve rm La ilio ke n

LC7 (11.15) LC12 LC9 (7.52) LC11 B15b (13.82) 82 (14.90) LC13 Sabine Pass LC10 (9.35) (10.62)

LC8b (7.38)

82

Cameron

Grand Lake

LF3 (9.52) LF5

(10.07)

White Lake

(14.83) (14.68) (13.34) LC8a LA12 LA11

LA9b LA9 (10.73) (6.62) Intracoastal City

260

Science and the Storms: the USGS Response to the Hurricanes of 2005

m) above the North American Vertical Datum of 1988 (NAVD 88) along the Louisiana Gulf Coast and rates of water-level rise in excess of 5 ft (1.5 m) per hour. Comparisons between high-water marks and water-level data from nearby inundated sensors indicated that the sensors recorded storm-surge peaks more accurately and reliably than did high-water marks. Data collected by the storm-surge sensors may be used in computer models (1) to estimate when, where, and to what degree storm-surge flooding will occur in future events and (2) to calibrate and verify storm-surge models, resulting in a better understanding of the dynamics of storm surge. The study area was situated east of Rita's storm path (fig. 1), in the right front quadrant of the storm, where the maximum storm surge was expected to occur. The monitored area covered approximately 4,000 mi2 (10,360 km2) and was generally bounded to the north by Interstate 10 (I-10), to the south by the Louisiana and Texas coastlines, to the east by the Vermilion River, and to the west by the cities of Beaumont, Orange, and Port Arthur, Tex. A total of 47 sensors (34 water-level sensors and 13 barometric pressure sensors) were deployed at 33 sites during September 22­23, 2005, prior to the landfall of Rita (fig. 2). Sensors were deployed along and inland of the Louisiana and Texas coasts, usually along waterways, from Sabine Pass, Tex., through Abbeville, La. The sensors were unvented pressure transducers capable of measuring and recording three parameters: absolute pressure, temperature, and internal battery voltage. Absolute pressure is the force exerted by air (barometric pressure) or water on the sensor. The force, or weight of water is converted to the height of water (water level) overlying the sensor. Inundated sensors were used to record stormsurge water levels, and noninundated sensors were used to record barometric pressure. Water-level data from inundated sensors were corrected for changes in barometric pressure by using data from a colocated barometric pressure sensor. If a barometric pressure sensor was not colocated with a stormsurge sensor, the data from the nearest barometric pressure sensor were used to correct the storm-surge data. The sensors recorded temperature and pressure every 30 seconds during the storm and for several days afterwards. The dimensions of the sensors were approximately 6 inches (15.2 cm) in length and 1 inch (2.5 cm) in diameter (fig. 3) and were encased in 1.5 inch (3.8 cm) by 18 inch (45.7 cm) metal pipes and strapped to permanent objects, such as piers and power poles (fig. 4). Storm-surge sensors were deployed on permanent structures at elevations that would likely be inundated by storm surge, and barometric pressure sensors were deployed on permanent structures at elevations that would not likely be inundated by storm surge. Water-level data from inundated sensors also were corrected for salinity because salinity content increases the density, and therefore the weight, of the water. Corrections for salinity were based upon the location of the sensor in proximity to the coast. In general, sensors located in the southern part of the study area were categorized as measuring

Figure 3. Photograph showing sensor used to record water level and barometric pressure.

Figure 4. Photograph showing metal pipe containing a waterlevel sensor or barometric pressure sensor strapped to a power pole.

Monitoring Hurricane Rita Inland Storm Surge

261

salt water, sensors located in the northern part of the study area were categorized as measuring freshwater, and sensors located in the middle were categorized as measuring brackish water. Elevation surveys were conducted to relate all stormsurge data, including water-level data from inundated sensors, high-water marks, and water-surface measurements, to sea level (sea level defined by the NAVD 88) (http://www.ngs. noaa.gov/PUBS_LIB/NAVD88/navd88report.htm, accessed September 11, 2006). Elevation surveys were conducted by using the Global Positioning System and differential levels. Quality assurance measures included (1) placing multiple sensors at a site, (2) placing a sensor near an existing U.S. Geological Survey (USGS) stream gage, and (3) manually measuring water-surface levels from reference marks. Multiple storm-surge sensors were deployed at two sites (three sensors at site LF3 and two sensors at site LA7) to assess the

variability between individual sensors. Sensors were deployed at two existing USGS stream gages (one each near sites LA2 and LA8) to assess the variability of water levels measured at sensors and USGS stream gages. Water-level data collected by multiple sensors agreed closely with each other (fig. 5), and sensors agreed closely with water-level data from USGS stream gages (fig. 6), as evidenced by the overlying plots of the sensor and USGS stream gage data during periods of sensor inundation. Water-surface levels, commonly referred to as "tape downs," were determined by measuring the distance from a reference mark, such as a bridge railing or pier surface, to the water surface at a specific date and time. Tape downs were made at seven sites, namely LA2 (fig. 6), LA3, LA9, LC2a, LF3 (fig. 5), LC5, and LC8b, and all agreed closely with sensor data.

15.0 Barometric pressure 14.8 14.6 14.4 14.2 14.0 Tape-down elevation Water-level elevation--Water-level sensor placed near USGS coastal streamgage 08012150 track together during surge inundation 13.8 13.6 USGS coastal streamgage Sensor Lowest recordable elevation 13.4 13.2 13.0 Barometric pressure, in pounds per square inch

16 15 14 13 12 Barometric pressure

15.0 14.8 14.6 Barometric pressure, in pounds per square inch 14.4 Tape-down elevation Water-level elevation--3 water-level sensors placed at different elevations track together during surge inundation Sensor 1 Lowest recordable elevation 14.2 14.0 13.8 13.6 13.4 Sensor 3 Sensor 2

05 Se 2 pt. 05 Se 2 pt. 05 Se 2 pt. 05 Se 2 pt. 05 Se 2 pt. 05 Se 2 pt. 05

16 15 14 13 12 Water-level elevation, in feet above NAVD 88 11 10 9 8 7 6 5 4 3 2 1 0

11 Water-level elevation, in feet above NAVD 88 10 9 8 7 6 5 4 3 2 1 0 -1

Se 2 pt. 3, 20

13.2 13.0

-1

4,

20

5,

20

6,

20

7,

20

8,

20

9,

20

S

t. ep

23

,2

00

5 S t. ep 24

,2

00

5 Se 2 pt.

5,

20

05 Se 2 pt.

6,

20

05 Se 2 pt.

7,

20

05 Se 2 pt.

8,

20

05 Se 2 pt.

9,

20

05

Figure 5. Hurricane Rita storm-surge data at site LF3a. The graph shows water-level elevations above North American Vertical Datum of 1988 (NAVD 88) from multiple storm-surge sensors and tape-down measurement.

Figure 6. Hurricane Rita storm-surge data at site LA2. The graph shows water-level elevations above North American Vertical Datum of 1988 (NAVD 88) from storm-surge sensor and U.S. Geological Survey stream gage and tape-down measurement.

262

Science and the Storms: the USGS Response to the Hurricanes of 2005

Results

High-water marks were identified and surveyed at seven sites, namely LA9b (fig. 7), LA12, LC2b, LC5, LC7, LC8a (fig. 8), and LF5, and were compared with nearby sensor data. Comparisons between high-water marks and storm-surge peaks from inundated sensors varied. In general, high-water marks of high quality agreed closely with the storm-surge peaks from the sensors, while high-water marks of lesser quality were consistently lower than the storm-surge peaks from the sensors. For example, the high-water mark near site LA9b (fig. 7) rated as "excellent" was approximately 0.2 ft (6.1 cm) lower than the storm-surge peak from the nearby storm-surge sensor, the high-water mark near site LC8a (fig. 8) rated as "good" was approximately 1 ft (30.5 cm) lower than the storm-surge peak from the nearby storm-surge sensor, and the high-water mark near site LF5 rated as "poor" was approximately 1.9 ft (57.9 cm) lower than the storm-surge peak from the nearby storm-surge sensor.

Conclusion

Sensors recorded storm-surge water levels over 14 ft above NAVD 88 at Constance Beach (LC11), Creole (LA12), and Grand Chenier (LA11), La., about 20 mi (32 km), 48 mi (77 km), and 54 mi (87 km), respectively, east of Sabine Pass, Tex., at approximately 2 a.m., September 24, 2005 (fig. 2). In general, storm-surge water levels increased eastward from the Sabine River into southwest Louisiana. The magnitude of the storm surge was greatest near the coast and decreased inland through the approximate latitude of I-10, about 35 mi (56 km) inland from the coast (fig. 2). Sensors reported rates of water-level rise during the storm-surge event in excess of 5 ft (1.5 m) per hour at sites LC8b, LC9, and LC13. By using data from the sensors and digital land-surface elevation data, a computer-generated map of storm-surge depth (fig. 9) in the monitored area was created and indicated over 13 ft (3.9 m) of water depth at 3 a.m. on September 24, 2005, along parts of the Gulf Coast of southwestern Louisiana (Dean Gesch, U.S. Geological Survey, written commun., 2006).

16 15 14 Water-level elevation, in feet above NAVD 88 13 12 11 10 9 8 7 6 5 4 3 2 1 0 -1

Se p 3 t. 2 ,2 00 5 Se p 4 t. 2 ,2 00 5 Se p 5 t. 2 ,2 00 5 Se p 6 t. 2 ,2 00 5 Se p 7 t. 2 ,2 00 5 Se p 8 t. 2 ,2 00 5 Se p 9 t. 2 ,2 00 5

15.0 Barometric pressure 14.8 Barometric pressure, in pounds per square inch 14.6

16 15 14 13 12

Water-level elevation, in feet above NAVD 88

15.0 Barometric pressure High-water-mark elevation 14.8

High-water mark

14.6 14.4 14.2 14.0

Barometric pressure, in pounds per square inch

High-water mark

High-water-mark elevation 14.4 14.2 Water-level elevation 14.0 13.8 Lowest recordable elevation 13.6 13.4 13.2 13.0

11 10 9 8 7 6 5 4 3 2 1 0 -1

Se 2 pt. 3, 20 05 Se 2 pt. 20 05 Se 2 pt. 20 05 Se 2 pt. 20 05 Se 2 pt. 20 05 Se 2 pt. 20 05 Se 2 pt. 20 05

Water-level elevation

13.8 13.6 Lowest recordable elevation 13.4 13.2 13.0

4,

5,

6,

7,

8,

9,

Figure 7. Hurricane Rita storm-surge data at site LA9b. The graph shows water-level elevations above North American Vertical Datum of 1988 (NAVD 88) from storm-surge sensor and high-water mark of excellent quality.

Figure 8. Hurricane Rita storm-surge data at site LC8a. The graph shows water-level elevations above North American Vertical Datum of 1988 (NAVD 88) from storm-surge sensor and high-water mark of good quality.

Monitoring Hurricane Rita Inland Storm Surge

263

94º00'

27 96

93º00'

49

LOUISIANA TEXAS

Beaumont 30º10' B1 B10 B12 B15b Sabine Pass Orange B19b B20 B19a

10

LC2b LC2a Lake Charles LC6b LC6a LC5

27

165

LC4

LC3

LA3

10

LA2 LA8

90

Lafayette

167

14

LA7

Abbeville LC8b

LF3 LF5

Sa bin eL ak e

LC7 LC12

82

Ca lca La sieu ke

Port Arthur 29º50'

27

LC13

LC11

LC9

Cameron LC8a LA12 LA11

82

Grand Lake

LA9 LA10

White Lake

LA9b

N

0 0 10

10 20

20 Miles

Gulf of Mexico

Kilometers

29º30'

Storm-surge depth, in feet

Greater than 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Land-surface elevation, in feet above the the North America Vertical Datum of 1988 (NAVD 88) Greater than 30

0

Data-collection site and identifier, by sensor type

LC6b LC5 LC7

Barometric pressure Water level Barometric pressure and water level

Figure 9. Map showing locations of storm-surge sensors and computer-generated storm-surge depth, in feet, on September 24, 2005, at 3 a.m. in southwestern Louisiana and southeastern Texas.

Contact Information

Benton D. McGee, Supervisory Hydrologist ([email protected]); Roland W. Tollett, Hydrologist ([email protected]); and Burl B. Goree, Hydrologic Technician ([email protected]) U.S. Department of the Interior U.S. Geological Survey Louisiana Water Science Center 3535 S. Sherwood Forest Blvd., Suite 120 Baton Rouge, LA 70816

Ve rm La ilio ke n

Orange Intracoastal City

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