Coastal Hydrodynamic and Morphodynamic Response to Tropical Cyclones II
Presiding: G W Stone, Louisiana State University; B A Murray, Duke University
OS34A-01 15:30h
Current Measurements From the SEED Field Experiment
To understand the mechanisms that transfer properties, such as heat, salt, energy, marine organisms, and materials, across the shelf slope, the Naval Research Laboratory has undertaken an intensive measurement program as part of its Slope to Shelf Energetics and Exchange Dynamics (SEED) project. With the primary focus on the currents, 14 acoustic Doppler current profilers (ADCPs) were deployed on the shelf and down the slope just west of the DeSoto Canyon in the northeastern Gulf of Mexico in May 2004. The ADCPs were serviced in November and all are scheduled for recovery in May 2005. The ADCPs measured nearly full water column current profiles on the shelf at depths of 60 and 90 m with 2 m resolution. On the slope current profiles in the upper 500 m were measured at water depths ranging to 1000 m with 10 m resolution. Additional current measurements were made 100 m above the bottom for the deep moorings. Preliminary results for the first six months show a mean two-layer flow pattern on the shelf and slope, with currents towards the northeast (onshore) in the upper layer and currents toward the southwest (offshore) in the lower level. There is a large monthly variability in the flow. Cross shelf exchange processes appear to be mainly driven by storm and eddy events. Fortuitously, Hurricane Ivan passed directly over the moorings and forced very intense flows.
OS34A-02 15:45h
Current Measurements Under Hurricane Ivan
The Naval Research Laboratory has undertaken an intensive measurement program in the northeast Gulf of Mexico as part of its Slope to Shelf Energetics and Exchange Dynamics (SEED) project. To understand cross-shelf exchange processes, with the primary focus on currents just west of the DeSoto Canyon, 6 acoustic Doppler current profilers (ADCPs) were deployed on the shelf bottom at depths ranging from 60 to 90m, and 8 Long Ranger ADCPs were deployed about 500 m below the surface on the slope at depths ranging from 500 to 1000 m. Additionally, the deep moorings contained a current meter located 100 m above the bottom. Fortuitously, the eye of Hurricane Ivan passed directly through the array without disrupting the instrumentation. For the first time, extreme storm induced currents spanning from just west of the eye to just east of the region of maximum wind stress were measured. Currents in excess of 2 m/s and 1.5 m/s were measured on the shelf and slope, respectively. The currents directly forced by the storm were highly barotropic, even in 1000 m of water. Strong Ekman downwelling occurred in advance of the eye and upwelling followed the passing of the eye, causing temperature to range nearly 11 degrees C at the bottom on the shelf. The maximum wind stress occurred in the northeast quadrant of the storm, but contrary to general expectations, maximum currents were recorded in the northwest quadrant of the hurricane.
OS34A-03 16:00h
Realtime Forecast of Hurricane Ivan's Storm Surge
The SEED observational program (see in this session) will serve as a validation data set for a realtime ocean prediction system developed at NRL in support of naval operations. Based on the Naval Coastal Ocean Model (NCOM), the portable system is easily and rapidly relocatable. Using the system, analysis and prediction can be produced for any part of the word, usually within six hours of the request. The system has the capability of multiple 1-way nesting domains. The open boundary conditions for the outer nest are usually extracted from a global version of NCOM with approximately 1/8° midlatitude resolution that is operational at the Naval Oceanographic Office. A preliminary configuration for the SEED data-model comparison is comprised of an outer nest for the GOM at about 5 km resolution and an inner domain for the Mississippi Bight at about 1.5km resolution. The system started running in a pseudo-operational mode (realtime forecast with forecasted winds) on August 15 2004. Therefore, we were able to model in realtime the effects of Hurricane Ivan on the GOM and coastal area. The model was able to predict the storm surge in the Mississippi Bight, but the representation of the location and time of the Ivan landfall was affected by errors in the forecasted winds.
OS34A-04 16:15h
The Cause of the I-10 Bridge Collapse and the Vulnerability of Coastal Bridges in Hurricane-Prone Areas
Four hurricanes devastated the Florida and Alabama coasts in 2004. Among them, Hurricane Ivan (2004) caused the collapse of the bridge decks on Interstate Highway 10 (I-10) crossing Escambia Bay in Pensacola, Florida. The goal of the study is to identify and understand the cause of the bridge collapse in order to prevent similar bridge failures in hurricane-prone areas. Two state-of-the-art numerical models have been evaluated, tested and applied to the U.S. Central Gulf Coast for the predictions of storm surges and surface waves on coastal roadways. A methodology of coupling the advanced storm surge model, ADCIRC, with the third-generation spectral wave model, SWAN, have been developed and validated. To reach our specific goal, first, a regional-scale storm surge model for Pensacola Bay is tested against field measurements of surge hydrograph and high watermarks collected by the U.S. Army Corps of Engineers Mobile District. Next, the validated surge model is used to predict the non-stationary water surge and depth-averaged currents in Pensacola Bay generated by Hurricane Ivan. After that, the surge model is coupled with the unsteady wind wave model to predict the storm waves and the elevated mean sea levels (wave setup) along the I-10 Bridge. We then compare the predicted maximum surge heights with the measured high watermarks from the post-Ivan survey and excellent agreement is found. We also correlate the spatial variation of the predicted wave energy flux with the exact locations of the failed bridge decks. The predicted surge and wave conditions along the bridge allow us to estimate the wave slamming and uplifting forces on the bridge decks. The detailed hydrodynamic analysis reveals the cause of and solution to the bridge collapse during Hurricane Ivan. The methodologies we have developed can be utilized to evaluate the vulnerability of coastal bridges in hurricane-prone areas and improve the design of coastal transportation infrastructure. Detailed results will be presented at the conference. The study has been supported by the Federal Highway Administration.
OS34A-05 16:30h
Hurricane waves on Storm Surges in Coastal Zone
During a tropical storm, high winds and low pressure generate abnormal sea water levels and currents. We present a time series of 48 hours water levels combining wave effects on surges during hurricane Opal (1995). The offshore hurricane ocean wave is modeled by a recent parametric ocean wave model using the best track data. The post-storm meteorological input data are applied to the calculation of surges and storm waves. A scheme is used to simulate the effects of wave-surges near the coast: the time varying surges is obtained first and then the modified mean water level is used for coastal wave calculations. The high water levels at the gauge station and on-site debris line are collected for model comparison. The hurricane waves and coastal waves are calculated near the eye center and on the beach slope, where the waves break and produce a mean free surface wave set-up. We examined the maximum wave set-ups at the shoreline, the wave run-up on the beach, adding up the storm surges and predicted tides at the station. It was found that the inland flooding is the combined results of storm surges elevating the water level, causing the wave breaking further landward and a higher wave run-up. The accuracy of hurricane waves is significant to the determination of inundation zone.
http://www.oceanweather.com/8thwave/Papers/O4.pdf
OS34A-06 16:45h
Observations of multiple-barred beach response to tropical storms
Multiple sand bars occur along the coastline of the Mississippi Sound, on the USA's Gulf Coast. Observations of the response of these bars have been made using an automated video camera system in Bay St. Louis, MS (http://visser.nrlssc.navy.mil/LOUIE). At this site, up to 10, approximately shore-parallel sandbars with a cross-shore spacing of about 20 m have been monitored over a 3 year period. During this time numerous tropical storms have passed the area, causing large fluctuations in water levels, wave heights, and nearshore currents. We present an analysis of the relationship between these strong changes in the local forcing and the corresponding bar response. Results so far indicate that the multiple bars are remarkably unaffected by the impulsive forcing due to large storms, and evolve at a much slower time scale.
http://visser.nrlssc.navy.mil/LOUIE