Final Report: Characteristics of Ship Waves and Wind Waves in Mobile Bay

EPA Grant Number: R827072C022
Subproject: this is subproject number 022 , established and managed by the Center Director under grant R827072
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: Alabama Center For Estuarine Studies (ACES)
Center Director: Shipp, Robert L.
Title: Characteristics of Ship Waves and Wind Waves in Mobile Bay
Investigators: Chen, Q. Jim , Douglass, Scott L.
Institution: University of South Alabama
EPA Project Officer: Packard, Benjamin H
Project Period: September 1, 2001 through December 30, 2003
RFA: Alabama Center For Estuarine Studies (ACES) (1999) RFA Text |  Recipients Lists
Research Category: Targeted Research

Objective:

Mobile Bay is a relatively wide and very shallow estuary. The natural forcing of high-speed winds associated with storms and hurricanes can generate surface waves that are responsible for the loss of wetlands and habitats in Mobile Bay. The wind waves generated by the frequent, moderate wind speed may be significant to the sedimentary dynamics of Mobile Bay. On the other hand, the large volume of ship traffic through the deep ship channel and in the Port of Mobile can also generate surface waves that could have the similar impact as the wind waves do. In comparison with the understanding of wind waves, however, little is known about the characteristics of the ship waves in Mobile Bay. Furthermore, the effects of the surface waves caused by either winds or ships on the sediment suspension or the turbidity in this shallow estuary are virtually unknown.

The long-term goal of the study is to quantify the natural and human-induced impacts on the physical processes in Mobile Bay and to understand the consequences of those impacts on the ecosystems and habitats. The specific objectives of the project are to develop a platform for the investigation of wave processes in the Mobile Bay estuary, and to characterize the surface waves generated by the natural forcing of episodic winds (storms and hurricanes) and sustained winds (with the most frequent occurrence of wind speed and direction) as well as by the human activity of cargo transport. This will allow us to compare the impacts of both types of surface wave on sediment suspension and alongshore sediment transport in Mobile Bay.

Summary/Accomplishments (Outputs/Outcomes):

Significant progress has been made in developing a platform for modeling the wind waves and ship waves in Mobile Bay. Work completed includes mining and analysis of the wind, wave, ship, and bathymetric data collected in the Mobile Bay estuary, development of curvilinear grids of the entire Mobile Bay estuary for numerical computations of surface waves and currents, and setup of a state-of-the-art wave model, SWAN (Simulating WAves Nearshore, Booij, 1999) for 1 the Mobile Bay estuary. Verification of the SWAN model against the wind wave data collected in Mobile Bay has been carried out. A numerical technique has been developed to simulate the propagation of vessel wakes in Mobile Bay within the framework of the SWAN wave model.

First, we obtained the surface wave data collected by the USGS (Pendygraft and Gelfenbaum, 1994). Because no electronic form of the data is available, we have digitized the plots of wind speed, wind direction, wave height, wave period, and water depth in the USGS report. An analysis of the data has led to the selection of a dozen representative events for the calibration and verification of the numerical model. These digitized data sets are valuable for the study of wind waves in Mobile Bay. We also obtained the bathymetric data of Mobile Bay and its adjacent area from the Northern Gulf of Mexico Littoral Initiative (NGLI) program and the National Ocean Service (NOS) Hydrographic Survey Data. Data reduction and interpolation were carried out to determine the water depth at each point on the curvilinear grids for Mobile Bay. The bathymetry on the curvilinear grid of Mobile Bay will serve as the basis not only for wave modeling, but also for simulations of wind-driven circulation in the Mobile Bay estuary. In addition to the surface wave and bathymetric data, we have obtained access to the vessel/barge data at the Port of Mobile.

In order to set up a numerical model for the prediction of surface waves and currents in Mobile Bay, computational grids need to be generated. Traditionally, rectangular grids with fixed grid spacing are used for numerical models based on the finite difference method. Though this type of grid is easy to generate, it is not suitable for modeling waves and currents in shallow estuaries, such as Mobile Bay, where the geometry of the shoreline is rather complex and there often exist deep ship channels. Moreover, the shallow water depth near the shoreline requires very fine resolution in the near-shore areas where detailed wave information is needed in order to mitigate wetland losses and shore erosion. Consequently, modeling waves and currents in estuaries needs computational grids that are able to fit the complex boundaries and provide fine enough resolution near the shoreline and ship channels. As part of the wave modeling effort of the project, we have developed methods to generate such curvilinear computational girds for the Mobile Bay estuary. Figures 1a and b respectively show the bathymetry and a computational grid for Mobile Bay. Notice that such a grid can be used not only for wave modeling, but also for the simulation of estuarine circulation that interacts with the surface waves.

With the bathymetric data in place and the development of the curvilinear grids, we set up the state-of-the-art, third generation wave model, SWAN for Mobile Bay. The model successfully simulates the significant wave heights and peak wave periods measured at the mid bay location indicated by the star in Figure 1b under non-stationary wind conditions. Good agreement between the model results and measurements has been found. Figures 2a and b illustrate a snapshot of the modeled wave field in the entire Mobile Bay estuary during a wind event. The colors and contours indicate the spatial distributions of the predicted significant wave heights and peak wave periods. The arrows depict the mean wave direction at each location. A northerly wind drives this wave field and the measured wind speed at 10 m above the water surface is 15 m/s. Figures 3 illustrate the comparisons of the SWAN predictions and measurements.

Figure 1. Bathymetry in Mobile Bay

Figure 1. A curvilinear grid with varying spacing.

Figure 1. (a) Bathymetry in Mobile Bay, (b) a curvilinear grid with varying spacing.

Figure 2. (a) Distribution of modeled significant wave heights (contours) and mean wave directions (arrows), (b) distribution of average wave periods. The blue lines represent the ship channel.

Figure 2. (a) Distribution of modeled significant wave heights (contours) and mean wave directions (arrows), (b) distribution of average wave periods. The blue lines represent the ship channel.

Figure 3. Measured (blue lines) wind speed and direction, measured and computed (green lines) significant wave heights and wave periods, and computed wave directions.

Figure 3. Measured (blue lines) wind speed and direction, measured and computed (green lines) significant wave heights and wave periods, and computed wave directions.

In addition to the comparison with the field data, the SWAN model was also tested against laboratory experiment. Figure 4 shows a computational grid over a laboratory wave basin with a circular shoal on an otherwise flat bottom. The physical experiment on wave propagation over the shoal was conducted Chawlar and Kirby (1996) at the University of Delaware. We tested SWAN against the laboratory data collected at seven instrument arrays A to G, where the small circles indicate the wave gage locations. The large circle in Figure 4 represented the footprint of the circular shoal. Directional, random waves were generated at the south boundary and propagated northward in the absence of wind forcing. The computational grid for the wave prediction model features finer resolution on top of the shoal in order to resolve the large gradients of the water depth and wave field.

Figure 5 depicts model/data comparisons at those seven arrays. Circles are the measurements and solid lines are the model results given by SWAN. Excellent agreement is found. In comparison to the model prediction given by a rectangular computational grid, it was found that the curvilinear grid agrees better with the data in the area near the top of the shoal. This demonstrates that curvilinear grids with variable grid spacing are superior to rectangular grids with fixed grid spacing wherever there are large gradients of water depth or wave field. The test supports the PI’s choice of curvilinear computational grid in the present study to develop a wave model for the entire Mobile Bay where abrupt changes in water depth do exist.

An innovative technique has been developed to model the propagation of ship waves generated by vessels in the ship channel in Mobile Bay. No such applications of the SWAN model for ship waves have been found in the literature. We split Mobile Bay into the eastern and western bays about the ship channel and specifying the wakes as a boundary condition along the ship channel.

Figure 4. Computational girds for wave propagation over a circular shoal in the laboratory. Small circles indicate the measurement locations. The large circle is the footprint of the circular shoal on an otherwise flat bottom.

Figure 4. Computational girds for wave propagation over a circular shoal in the laboratory. Small circles indicate the measurement locations. The large circle is the footprint of the circular shoal on an otherwise flat bottom.

Figure 5. Model (solid lines) and data (circles) comparisons at seven transects.

Figure 5. Model (solid lines) and data (circles) comparisons at seven transects.

A methodology to estimate the ship waves in the ship channel has been developed on the basis of Sorensen’s (1997) formulations. This enables the non-stationary, curvilinear wave model to predict the transformation of boat wake energy in Mobile Bay.

In summary, we have developed a state-of-the-art computer model for the prediction of surface waves in Mobile Bay. With this advanced numerical model, we are not only capable of predicting surface waves generated by wind storms in Mobile Bay with remarkable accuracy, but also able to simulate the propagation of vessel wakes from the ship channel to the shoreline. Model results of wind waves and ship waves have been presented by the PI at two meetings at the Dauphin Island Sea Lab as well as at two national conferences. A journal article based on the findings of the project has been accepted for publication (Chen et al., 2004). The validated numerical model is serving as a platform for the study of wave dynamics in Mobile Bay. A direct, practical application of this model is the development of a wave atlas for Mobile Bay on the basis of the wind statistics at Dauphin Island and ship statistics at the Alabama State Dock. This is being pursued by the PIs.

Acknowledgments and Disclaimers:

This research has been supported by a grant from the U.S. Environmental Protection Agency’s Science to Achieve Results (STAR) program.

Although the research described in the article has been funded wholly or in part by the U.S. Environmental Protection Agency’s STAR program through grant (F-827072-01-1), it has not been subjected to any EPA review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.

References:

Booij N, Ris RC, Holthuijsen LH. A third-generation wave model for coastal regions. Part 1, model description and validation. Journal of Geophysical Research 1999;C4,104,7649-7666.

Chawla A, Kirby JT. Wave transformation over a submerged shoal. CACR Report No 96-03, Dept. of Civil Engineering, University of Delaware, Newark, DE, 1996.

Chen Q, Hu K, Zhao H, Douglass SL. Prediction of wind waves in shallow estuaries. Journal of Waterway, Port, Coastal and Ocean Engineering (accepted, 2004).

Pendygraft SL, Gelfenbaum GR. Wave data in Mobile Bay, Alabama from March, 1991–May 1992. U.S. Geological Survey, St. Petersburg, FL, OF 94-0017, 1994.

Sorensen RM. Prediction of vessel-generated waves with reference to vessels common to the upper Mississippi River System. US Army Corps of Engineers, ENV Report 4, 1997, p. 50.


Journal Articles on this Report : 1 Displayed | Download in RIS Format

Other subproject views: All 7 publications 1 publications in selected types All 1 journal articles
Other center views: All 86 publications 5 publications in selected types All 5 journal articles
Type Citation Sub Project Document Sources
Journal Article Chen Q, Zhao H, Hu K, Douglass SL. Prediction of wind waves in a shallow estuary. Journal of Waterway, Port, Coastal, and Ocean Engineering 2005;131(4):137-148. R827072C022 (2002)
R827072C022 (Final)
  • Abstract: ASCE Abstract
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  • Supplemental Keywords:

    RFA, Scientific Discipline, ECOSYSTEMS, Water, Geographic Area, Ecosystem Protection/Environmental Exposure & Risk, Aquatic Ecosystems & Estuarine Research, estuarine research, Ecology, Ecosystem/Assessment/Indicators, Ecosystem Protection, Environmental Chemistry, Soils, State, Chemistry, Restoration, Aquatic Ecosystem, Ecological Effects - Environmental Exposure & Risk, Aquatic Ecosystems, Terrestrial Ecosystems, Ecological Monitoring, Ecology and Ecosystems, Aquatic Ecosystem Restoration, wetlands, coastal ecosystem, watersheds, erosion, shorelines, estuaries, waves, Alabama (AL), anthropogenic impact, wetland stabilization, wind-wave models, coastal environments, ecosystem, environmental indicators, estuarine waters, water quality, human modifications, sediment dynamics, breakwaters

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    Progress and Final Reports:

    Original Abstract
  • 2002 Progress Report
  • 2003

  • Main Center Abstract and Reports:

    R827072    Alabama Center For Estuarine Studies (ACES)

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
    R827072C001 Fluorescent Whitening Agents As Facile Pollution Markers In Shellfishing Waters
    R827072C002 Red Snapper Demographics on Artificial Reefs: The Effect of Nearest-Neighbor Dynamics
    R827072C003 Stabilization of Eroding Shorelines in Estuarine Wave Eliminates with Constructed Fringe Wetlands Incorporating Offshore Breakwaters
    R827072C004 Interaction Between Water Column Structure and Reproduction in Jellyfish Populations Of Mobile Bay (SGER)
    R827072C005 Effects of Variation in River Discharge and Wind-Driven Resuspension on Higher Trophic Levels in the Mobile Bay Ecosystem
    R827072C006 Results of Zooplankton Component
    R827072C007 Benthic Study Component
    R827072C008 A Preliminary Survey of Macroalgal and Aquatic Plant Distribution in the Mobile Tensaw Delta
    R827072C009 Fisheries-induced changes in the structure and function of shallow water "nursery habitats": an experimental assessment
    R827072C010 Effects Of Variation in River Discharge and Wind-Driven Resuspension on Lower Trophic Levels of the Mobile Bay Ecosystem
    R827072C011 Evaluation of Alabama Estuaries as Developmental Habitat for Juvenile Sea Turtles
    R827072C012 Effects of Salinity Stress on Natural and Anthropogenically-Derived Bacteria in Estuarine Environments
    R827072C013 The Role of Land-Use/Land-Cover and Sub-estuarine Ecosystem Nitrogen Cycling in the Regulation of Nitrogen Delivery to a River Dominated Estuary; Mobile Bay, Alabama
    R827072C014 Environmental Attitudes of Alabama Coastal Residents: Public Opinion Polls and Environmental Policy
    R827072C015 Synthesis and Characterization of an Electrochemical Peptide Nucleic Acid Probe
    R827072C016 Determinants of Small-Scale Variation in the Abundance of the Blue Crab Callinectes Sapidus
    R827072C017 Effects of Estrogen Pollution on the Reproductive Fitness of the Gulf Pipefish, Syngnathus scovelli
    R827072C019 A Model for Genetic Diversity Aquatic Insects of the Mobile/Tensaw River Delta
    R827072C020 Evaluating Trophic Processes as Indicators of Anthropogenic Eutrophication in Coastal Ecosystems: An Exploratory Analysis
    R827072C021 Effects of Anthropogenic Eutrophication on the Magnitude and Trophic Fate of Microphytobenthic Production in Estuaries
    R827072C022 Characteristics of Ship Waves and Wind Waves in Mobile Bay
    R827072C023 Methods Comparison Between Stripping Voltammetry and Plasma Emission Spectroscopy for Metals in Mobile Bay
    R827072C024 Changes in Water Conditions and Sedimentation Rates Associated With Construction of the Mobile Bay Causeway
    R827072C025 Cold-Induced Hibernation of Marine Vibrios in the Gulf of Mexico: A Study of Cell-Cell Communication and Dormancy in Vibrio vulnificus
    R827072C026 Holocene Sedimentary History of Weeks Bay, AL: Human and Natural Impacts on Deposition in a Gulf Coast Estuary
    R827072C027 Shelter Bottlenecks and Self-Regulation in Blue Crab Populations: Assessing the Roles of Nursery Habitats and Juvenile Interactions for Shelter Dependent Organisms
    R827072C028 Predicting Seagrass Survival in Nutrient Enriched Waters: Toward a New View of an Existing Paradigm
    R827072C029 DMSP and its Role as an Antioxidant in the Salt Marsh Macrophyte Spartina alterniflora
    R827072C030 A Preliminary Survey of Aerial and Ground-Dwelling Insects of the Mobile/Tensaw Delta
    R827072C031 Natural Biogeochemical Tags of Striped Mullet, Mugil cephalus, Estuarine Nursery Areas in the North Central Gulf of Mexico
    R827072C032 Resolution of Sedimentation Rates in Impacted Coastal Environments Using 137Cs and 210Pb Markers: Dog River and Fowl River Embayments
    R827072C033 Investigation of the Use of Pulse Amplitude Modulated (PAM) Fluorometry as an Indicator of Submerged Aquatic Vegetation Health in Mobile Bay
    R827072C034 Influence of Invasive Plant Species in Determining Diversity of Aquatic Vegetation in the Mobile-Tensaw Delta
    R827072C035 The Influence of Shallow Water Hydrodynamics on the Importance of Seagrass Detritus in Estuarine Food Webs
    R827072C036 Food Web Interactions, Spatial Subsidies and the Flow of Energy Between the Mobile Bay Delta and Offshore Waters: A SGER Proposal to the Alabama Center for Estuarine Studies
    R830651C001 Meteorological Modeling of Hurricanes and Coastal Interactions: A Stability Study For Vertical Pressure Levels
    R830651C002 Characterization of Glycoprotein Cues Used by the Parasitic Rhizocephalan Barnacle Loxothylacus texanus To Identify Its Blue Crab Host, Callinectes sapidus
    R830651C003 Survey of Diamondback Terrapin Populations in Alabama Estuaries
    R830651C004 An Assessment of Environmental Contaminant Levels in Water and Dragonfly Larvae Tissues from the Mobile/Tensaw Delta