Prepared in cooperation with the U.S. Army Corps of Engineers

Effects of Proposed Navigation Channel 

Improvements on Sediment Transport in Mobile 

Harbor, Alabama

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2018–1123

Effects of Proposed Navigation Channel 

Improvements on Sediment Transport in Mobile 

Harbor, Alabama

By Davina L. Passeri, Joseph W. Long, Robert L. Jenkins, and David M. Thompson

Prepared in cooperation with the U.S. Army Corps of Engineers

Open-File Report 2018–1123

U.S. Department of the Interior

U.S. Geological Survey 
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

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Suggested citation:

Passeri, D.L., Long, J.W., Jenkins, R.L., and Thompson, D.M., 2018, Effects of proposed navigation channel 

improvements on sediment transport in Mobile Harbor, Alabama: U.S. Geological Survey 

Open-File Report 2018–1123, 22 p., https://doi.org/10.3133/ofr20181123.

ISSN 2331-1258 (online)
iii

Acknowledgments

This research was chartered by the U.S. Army Corps of Engineers Mobile District. The authors thank Soupy 

Dalyander and Elizabeth Godsey for reviewing this publication. Betsy Boynton and Rebekah Davis supported the review, editing, and publishing process. 

v

Contents

Abstract .......................................................................................................................................................... 1

Introduction..................................................................................................................................................... 1

Modeling Approach......................................................................................................................................... 3

Proposed Navigation Channel Modifications .............................................................................................. 3

Model Description ....................................................................................................................................... 3

Model Setup................................................................................................................................................ 4

Boundary Conditions .................................................................................................................................. 5

Morphological Tide.................................................................................................................................. 6

Simulations ................................................................................................................................................. 7

Modeling Results............................................................................................................................................ 7

Model Performance .................................................................................................................................... 7

Modeled Versus Observed Bed Level Changes ..................................................................................... 8

2010 Climatology ........................................................................................................................................ 9

10-Year Climatology.................................................................................................................................. 12

Discussion .................................................................................................................................................... 16

Summary and Conclusions........................................................................................................................... 19

References Cited.......................................................................................................................................... 21

Figures

1. The Mobile Harbor study area, Alabama, including Mobile Bay and the navigational channel,

which consists of the upper bay channel, the lower bay channel, and the entrance channel ............2

2. Delft3D model domains and initial elevations, Mobile Harbor, Alabama. ...........................................4

3. Observed bed level changes in Mobile Harbor, Alabama. A, from 2002 to 2014. B, from 2002 to

2015. C, from 2009 to 2014................................................................................................................9

4. Modeled bed level change in Mobile Harbor, Alabama. A, after 10 years in the 10-year existing

condition simulation. B, after 5 years in the 10-year existing condition simulation ...........................10

5. Changes in bed level for the 2010 simulations, Mobile Harbor, Alabama. A, existing conditions.

B, with-project conditions. C, difference in final bed level between existing and with-project

conditions .........................................................................................................................................11

6. Changes in bed level for the 2010 simulations, Mobile Harbor, Alabama. A, existing conditions

accounting for 0.5 meter of sea level rise. B, with-project conditions accounting for 0.5 meter

of sea level rise. C, difference in final bed level between existing conditions accounting for

0.5 meter of sea level rise and with-project conditions accounting for 0.5 meter of sea level rise ...12

7. Percent change in the volume of sediment eroded or deposited in the entrance channel, Mobile

Harbor, Alabama. A, between 2010 existing and 2010 with-project conditions. B, between 2010

existing with 0.50 meter of sea level rise and 2010 with-project with 0.50 meter of sea level rise. ......14
vi

8. Changes in bed level for the 10-year simulations, Mobile Harbor, Alabama. A, existing

conditions. B, with-project conditions. C, difference in final bed level between existing and

with-project conditions ......................................................................................................................15

9. Changes in bed level for the 10-year simulations, Mobile Harbor, Alabama. A, existing conditions

accounting for 0.5 meter of sea level rise. B, with-project conditions accounting for 0.5 meter

of sea level rise. C, difference in final bed level between existing conditions accounting for

0.5 meter of sea level rise and with-project conditions accounting for 0.5 meter of sea level rise ...16

10. Percent change in the volume of sediment eroded or deposited in the entrance channel, Mobile

Harbor, Alabama. A, between the 10-year existing and 10-year with-project conditions. B,

between the 10-year existing condition with 0.50 meter of sea level rise and 10-year with-project

condition with 0.50 meter of sea level rise. ......................................................................................18

Tables

1. Characteristics and percent occurrence of wave conditions for each wave bin for 10-year

climatology and 2010 climatology, Mobile Harbor, Alabama ..............................................................6

2. Coefficients of determination and root mean square error values for through-channel and crosschannel velocity components during flood and ebb tide at inlet, volume flux during flood and ebb

tide at inlet and water levels at the Dauphin Island tide gage, Mobile Harbor, Alabama ....................8

3. Volume of sediment eroded or deposited in the entrance channel at the end of the 2010

simulations, Mobile Harbor, Alabama ...............................................................................................13

4. Volume of sediment eroded or deposited in the entrance channel for the 10-year climatology

simulations, Mobile Harbor, Alabama ...............................................................................................17

5. Shoaling volume in the entrance channel at the end of each year for the 10-year simulations,

Mobile Harbor, Alabama ...................................................................................................................19
vii

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Length

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi)

meter (m) 1.094 yard (yd)

Volume

cubic meter (m3) 6.290 barrel (petroleum, 1 barrel = 42 gal)

cubic meter (m3) 264.2 gallon (gal)

cubic meter (m3) 0.0002642 million gallons (Mgal)

cubic meter (m3) 35.31 cubic foot (ft3)

cubic meter (m3) 1.308 cubic yard (yd3)

cubic meter (m3) 0.0008107 acre-foot (acre-ft)

Flow rate

cubic meter per second (m3/s) 70.07 acre-foot per day (acre-ft/d)

meter per second (m/s) 3.281 foot per second (ft/s)

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

Datum 

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). 

Elevation, as used in this report, refers to the distance above the vertical datum. 

Abbreviations 

ADCP

CoNED

DEM

ECMWF

HYCOM

lidar

NGDC

NOAA

R2

RMSE

SLR

U

V

acoustic Doppler current profiler 

Coastal National Elevation Database 

digital elevation model 

European Centre for Medium-Range Weather Forecast 

Hybrid Coordinate Ocean Model 

light detection and ranging 

National Geophysical Data Center 

National Oceanic and Atmospheric Administration 

coefficient of determination 

root mean square error 

sea level rise 

through-channel velocity

cross-channel velocity

Effects of Proposed Navigation Channel 

Improvements on Sediment Transport in Mobile 

Harbor, Alabama

By Davina L. Passeri, Joseph W. Long, Robert L. Jenkins and David M. Thompson

Abstract

A Delft3D model was developed to evaluate the potential effects of proposed navigation channel deepening and widening in Mobile Harbor, Alabama. The model performance was 

assessed through comparisons of modeled and observed data of water levels, velocities, and bed 

level changes; the model captured hydrodynamic and sediment transport patterns in the study 

area with skill. The validated model was used to simulate changes in sediment transport for existing conditions and with the proposed modifications to the navigational channel (with-project), 

with and without accounting for 0.5 meter (m) of sea level rise (SLR). Each scenario was simulated for 1 year with a wave climatology representative of the year 2010 as well as for 10 years 

with a longer-term wave climatology spanning from 1988 to 2016. Bed level differences for the 

existing and with-project 2010 simulations were minimal, ranging from −0.11 to 0.11 m offshore 

of Pelican Island and −0.81 to 0.22 m offshore of the Fort Morgan Peninsula. For the simulations accounting for 0.5 m of SLR, differences in bed levels from −0.20 to 0.32 m near Pelican 

Island and −0.38 to 0.34 m offshore of the Fort Morgan Peninsula. The proposed modifications 

reduced the channel shoaling volume by 4.77 and 8.09 percent for the 2010 simulations without 

and with 0.5 m of SLR, respectively. For the 10-year simulations, bed level differences for the 

existing and with-project simulations ranged from −3.17 to 3.94 m for the simulation without 

SLR and −1.92 to 1.47 m for the simulation with 0.5 m of SLR. The with-project condition reduced the entrance channel shoaling volume by 5.54 percent for the simulation without SLR and 

14.98 percent for the simulation with 0.5 m of SLR. 

Introduction

Mobile Harbor is in southwest Alabama in the northern Gulf of Mexico (fig. 1). The 

U.S. Army Corps of Engineers (USACE) proposed to deepen and widen the existing navigation 

channel in Mobile Harbor as part of an economic analysis to determine the feasibility of channel improvements. To evaluate the potential effects of channel deepening and widening on the 

morphology of the ebb tidal shoal and adjacent areas, the USACE Mobile District requested the 

support of the U.S. Geological Survey in numerical modeling of waves, currents, and sediment 

transport for the Mobile Harbor General Reevaluation Report. A numerical modeling approach 

was implemented to quantify relative changes in sediment pathways and the morphological 

response on the ebb tidal shoal because of the increased channel dimensions. A Delft3D model 

was developed to simulate changes in sediment transport under existing conditions and accounting for 0.5 m of sea level rise, with and without modifications to the navigation channel. Each 

scenario was simulated for a 1- and 10-year period; the 1-year simulation used a climatology 
representative of the year 2010, and the 10-year simulation used a long-term wave climatology 

for the region. Model output was used to infer potential effects to sediment delivery at the inlet 

ebb tidal shoal and towards Dauphin Island, Alabama.

Modeling Approach

A Delft3D model was developed and used to quantify relative changes in sediment 

transport and the morphologic response on the ebb tidal shoal under existing conditions and with 

Figure 1. The Mobile Harbor study area, Alabama, including Mobile Bay and the navigational channel, 

which consists of the upper bay channel, the lower bay channel, and the entrance channel.

2
3

the proposed channel modification. Details on the development of the model grid, initial model 

elevations, and boundary conditions are provided herein. The model grid and initial elevations 

are provided in Passeri and others (2018).

Proposed Navigation Channel Modifications

Mobile Harbor includes Mobile Bay, which connects to the Gulf of Mexico through the 

Mobile Bay inlet bounded by the Fort Morgan Peninsula and Dauphin Island (fig. 1). North of 

Dauphin Island, Mobile Bay connects to the Mississippi Sound through Pass aux Herons (fig. 1.). 

The Mobile Harbor navigation channel spans the length of Mobile Bay and includes the entrance 

channel, which extends from the mouth of Mobile Bay southward into the Gulf of Mexico, and 

the lower and upper bay channels, which extend from the mouth of the bay northward (fig. 1). 

The existing depth at the entrance channel is 14.33 meters (m, North American Vertical Datum 

of 1988) with an additional 0.61 m for advanced maintenance (that is, additional dredging depth 

to avoid re-dredging) and 0.61 m for allowable overdepth dredging (total of 15.54 m). The existing depth in the lower and upper channels is 13.72 m with an additional 0.61 m for advanced 

maintenance and 0.61 m for allowable overdepth dredging (total of 14.94 m). The proposed project depths would deepen the entrance channel to 15.85 m with an additional 0.61 m for advanced 

maintenance and 0.61 m for allowable overdepth dredging (total of 17.07 m), and the lower and 

upper channels to 15.24 m with an additional 0.61 m for advanced maintenance and 0.61 m for 

allowable overdepth dredging (total of 16.46 m). The turning basin (fig. 1), at the northernmost 

part of the upper bay channel would be widened 76.2 m southward. The channel from the mouth 

of the bay northward for 8.04 kilometers would be widened from 121.92 to 152.4 m to include a 

passing lane.

Model Description

Delft3D (developed by Deltares; see Lesser and others, 2004) is an integrated process-based model consisting of multiple modules used to simulate wave propagation, wave and 

tidal currents, sediment transport, and morphologic change. The FLOW module (Deltares, 2018a) 

solves the nonlinear shallow water equations for incompressible free surface flows in two 

(depth-integrated) or three dimensions. The WAVE module (Deltares, 2018b) solves the spectral 

action density equation and computes wave radiation stresses and gradients that drive nearshore 

circulation. When coupled with the FLOW module, the WAVE module accounts for the effects 

of water level variations and wave-current interaction processes such as frequency shifting. The 

sediment transport module solves for suspended and bed load sediment transport. To calculate 

suspended load, the three-dimensional advection-diffusion equation is solved, accounting for sediment concentration, flow velocities, eddy diffusivity, and sediment settling velocity. For bed load 

transport of non-cohesive sediments, the transport equation is solved accounting for bed slope, 

bed composition, spatially variable bed friction coefficients, and concentration of available sediment. Breaking-induced shear stresses, mass flux, and bed shear stress are included in the transport of suspended sediments and fluxes from bed load sediments. The transport module evolves 

bed morphology on the basis of mass fluxes between suspended and bed load sediments. More 

detailed information on the Delft3D model is provided in Lesser and others (2004).

Delft3D was operated using the mormerge approach (Roelvink, 2006), which is a configuration of the model in which multiple simulations run simultaneously with identical initial 
4

bed conditions but with unique wave forcing. Each simulation is assigned a weight according to 

the percent occurrence of the wave conditions from a wave climatological assessment. At each 

half model time step, the current bathymetry from each of the simulation bins is combined using 

a weighted-average to form a new shared bathymetry that is passed back to each simulation and 

applied as the bathymetry for all the concurrent simulations for the next time step. The cumulative effect is a computationally efficient way to perform long-term morphological predictions.

Model Setup

For this study, three computational grids were used (see grid extents in fig. 2). The FLOW 

module uses a curvilinear grid consisting of 1,368 x 657 grid points. Cross-shore grid resolution 

ranges from less than 5 m over Dauphin Island and in the surf zone to greater than 300 m in the 

northernmost reaches of Mobile Bay. The alongshore grid resolution ranges from 40 m at Dauphin Island and across the Mobile Bay inlet to 100 m grid spacing at points in the southeastern 

quarter of the grid. The WAVE module uses two grids: a coarse outer grid and a nested fine grid. 

The coarse outer grid covers the study area with 245 x 449 grid points. It has variable alongshore 

resolution ranging from 250 to 325 m and variable cross-shore resolution ranging from 15 to 

300 m. The spatial extent of the nested fine grid is limited in latitude to the mouth of Mobile Bay 

where substantial wave-current interactions are expected and higher resolution is required. The 

fine grid consists of 1,367 x 458 grid points with a variable cross-shore resolution less than 5 m 

Figure 2. Delft3D model domains (computational grid extents) and initial elevations (existing condition), 

Mobile Harbor, Alabama.
5

at the mouth of the bay to more than 250 m offshore and to the north. The alongshore resolution 

of the fine grid is 100 m along Dauphin Island and becomes coarser east of the Mobile Bay inlet.

Two digital elevation models (DEMs) were created for this study and used to initialize the 

model to represent (1) the existing bathymetry of Mobile Harbor and the navigation channel and 

(2) the proposed channel modifications. The base DEM was derived by combining the Coastal

National Elevation Database (CoNED) topobathymetric DEM for Mobile Bay (Danielson and

others, 2013) and the National Geophysical Data Center (NGDC) coastal DEM (National Geophysical Data Center and others, 2009). The NGDC coastal DEM covers the full extent of the

modeling domain and was used primarily for offshore regions that were not included in the

CoNED DEM, which contains more recent elevations in the coastal areas. A 2015 bathymetric

survey at Dauphin Island (DeWitt and others, 2017) and a 2015 airborne light detection and ranging (lidar) survey of Dauphin Island (U.S. Geological Survey, 2016), also were merged into the

DEM using the controlled interpolation methods of Plant and others (2002). For updated coverage, the USACE Mobile District provided elevations within Mobile Bay, including the navigation channel, based on a composite of recent bathymetric surveys (taken by the district) for the

existing condition in addition to the altered bathymetry for the proposed with-project condition.

These data were incorporated within Mobile Bay east of Pass aux Herons and within the entrance

channel limited to the south by the 16-m contour. For depths greater than 5 m, a region was defined using a contour of the minimum difference between the USACE depth and the underlying

merged product of NGDC, CoNED, and the 2015 bathymetric and lidar surveys to ensure a continuous bathymetry. The USACE depth was then interpolated onto the FLOW grid and applied

only to this defined region.

Boundary Conditions

Wave Climatology

The wave climatology was developed using output from the European Centre for Medium-Range Weather Forecast (ECMWF) ERA-Interim reanalysis model (Dee and others, 2011). 

For the 10-year simulations, significant wave height (Hs), peak wave period (Tp) and mean 

wave direction (Dm) from January 1, 1998, to January 1, 2016, at a model grid point at longitude −88.125 W and latitude 30.000 N were used to define the regional wave climatology. Periods with waves not directed towards shore between 110° and 250° (nautical convention) were 

assumed to minimally affect the study site and therefore were removed from the time series. To 

validate the model wave height, data from National Data Buoy Center buoy 42040 (National Data 

Buoy Center, 2018) and from an ECMWF model (Dee and others, 2011) grid point about 6 kilometers away from the buoy were compared for times of overlapping data. A linear regression 

analysis revealed that using a correction factor of 1.22 improved the modeled wave height; the coefficient of determination (R2

) was 0.86 and the root mean square error (RMSE) was 0.26 m. The 

Energy Flux Method of Benedet and others (2016) was then used to derive a binned (grouped) 

wave climatology where wave direction and height bin boundaries were defined such that all bins 

contained an equal amount of wave energy flux. The wave climate was divided into nine wave 

classes (three directions and three heights). For each defined bin, wave period is the mean period 

of the bin, wave direction is mean direction of the bin, and wave height is calculated from the 

mean wave energy flux in the bin assuming linear wave theory (table 1). For the 2010 simulations, 

the wave climate was derived similarly using ECMWF ERA-Interim data for 2010 (table 1).
6

Table 1. Characteristics and percent occurrence of wave conditions for each wave bin 

for 10-year climatology and 2010 climatology, Mobile Harbor, Alabama.

Row Significant Wave 

height (Hs), in meters

Peak wave period (Tp), 

in seconds

Mean wave direction 

(Dm), in degrees

Occurrence, in 

percent

10-year climatology

Bin 1 0.59 6.24 129.2 26.2

Bin 2 0.59 6.43 154.01 25.4

Bin 3 0.58 5.75 199.77 28.9

Bin 4 1.21 7.3 128.1 5.3

Bin 5 1.18 7.49 154.48 5.4

Bin 6 1.23 7.22 195.49 5.2

Bin 7 2.65 9.09 126.94 0.9

Bin 8 2.17 8.6 155.06 1.4

Bin 9 2.26 8.68 198.13 1.3

2010 climatology

Bin 1 0.61 6.36 130.13 24.55

Bin 2 0.61 6.52 155.87 23.43

Bin 3 0.61 5.55 201.33 27.69

Bin 4 1.03 7.02 129.71 7.85

Bin 5 1.17 7.75 157.27 5.16

Bin 6 1.39 7.41 197.34 3.92

Bin 7 1.63 8.02 133.33 2.69

Bin 8 1.67 8.13 158.77 2.8

Bin 9 2.01 8.37 201.29 1.91

Morphological Tide

In addition to the wave forcing, a tidal time series or “morphological tide” was applied at 

the model boundaries to capture current velocities and morphological change associated with the 

neap-spring tide cycle. The morphological tide was calculated following the method of Lesser 

(2009), which is applicable in locations where the lunar diurnal K1 and O1 tidal constituents substantially contribute to the tidal signal, as is the case in the study domain. Tidal constituent amplitudes and phases were obtained from the National Oceanic and Atmospheric Administration 

(NOAA) tide gage (8735180) at the eastern end of Dauphin Island (fig. 1) and used to generate 

the amplitude and phases of the morphological tide. These were applied at the boundaries of each 

Delft3D simulation. For model stability, a consistent and progressive phase shift also was added 

to the morphological tide constituents of each successive wave bin.

Simulations

To assess the model performance, two deterministic simulations were conducted to 

compare modeled current velocities and water levels with collected data. Acoustic Doppler 

current profiler (ADCP) measurements were collected at the Mobile Bay inlet from August 27 
7

through 29, 2015 (representing the flood tide), and December 19 through 11, 2015 (representing the ebb tide). For each deterministic simulation, the existing Mobile Harbor DEM was used 

as the initial depth input with boundary conditions of modeled wind, wave, and water levels 

from the NOAA Hybrid Coordinate Ocean Model (HYCOM) (Bleck 2002) and the NOAA 

Wavewatch3 model (Tolman 1989). In comparing the modeled HYCOM water levels to the observed water levels at the Dauphin Island tide gage (station 8735180), the HYCOM water levels 

on average were 0.21 m lower than the observed; therefore, an offset of 0.21 m was added to the 

HYCOM water levels. Each simulation was spun-up for 12 hours before the first observation. 

In addition, a 6-month deterministic simulation from June 19 through November 20, 2005, was 

done to compare modeled water levels with observations at the Dauphin Island tide gage (Center 

for Operational Oceanographic Products and Services, 2018). 

For the 2010 and 10-year simulations, four scenarios were examined: existing conditions 

(that is, existing bathymetric conditions of the coastal nearshore areas with no channel modifications), with-project conditions (that is, with the proposed channel modifications), existing 

conditions with a moderate sea level rise (SLR) of 0.50 m, and with-project conditions with a 

moderate SLR of 0.50 m. For the 10-year simulations, the channel depths were reset to the initial 

depths at the start of each year, assuming annual dredging would take place. Additionally, a 

volume of 503,606.21 cubic meters (m3) of sand was added to the DEM in the southern section 

of the Sand Island Beneficial Use Area, on the 10-m contour southeast of Pelican Island (fig. 1), 

at the end of each year to account for the average annual volume of maintenance dredge material 

placement during 1999–2015.

Modeling Results

The results of the Delft3D simulations are presented herein. To evaluate the model performance, output in the form of water levels, velocities, and bed level from the deterministic simulations were compared with observations. To assess the effects of the proposed channel modifications, the final bed levels were extracted as output from the model at the end of the 2010 and 

10-year simulations, with and without 0.50 m of sea level rise (SLR). Model output from each

simulation is provided in Passeri and others (2018).

Model Performance

Modeled Versus Observed Water Levels and Velocities

Modeled water velocities were interpolated to the ADCP transect at the Mobile Bay 

inlet. Modeled and observed water levels were rotated from their respective native coordinates 

to stream-wise coordinates so that the resulting velocity constituents were a stream-wise U

(through-channel) velocity and a V (cross-channel) velocity. The R2 and RMSE values between 

the modeled and observed U and V velocities in the Mobile Bay inlet are summarized in table 2. 

The R2 values for the modeled and observed U velocities during ebb and flood tide are 0.93 and 

0.66, respectively. The R2 values for the modeled and observed V velocities during ebb and flood 

tide were 0.79 and 0.30, respectively. An additional comparison of modeled and observed volumetric fluxes calculated across the transect was done for the two ADCP observational periods 

(table 2). Fluxes were defined as stream-wise, depth-averaged velocities multiplied by water 

depth and integrated over the observation transect. A linear fit and R2 value was calculated for the 

ebb and flood tide fluxes, resulting in values of 0.98 and 0.79, respectively. The high skill during 
8

ebb tides shows the model’s ability to accurately capture the ebb-dominant behavior of the inlet, 

which affects sediment transport out of the bay.

The observed water levels at the Dauphin Island tide gage were compared with modeled 

water levels extracted from the closest grid point to the tide gage location (table 2). The R

2

 value 

between the observed and modeled water levels was 0.68. Model error is likely due in part to the 

absence of lower frequency harmonic constituents in the boundary forcing.

Modeled Versus Observed Bed Level Changes

Modeled and observed changes in bed levels were compared to evaluate the model’s capability to accurately simulate sediment transport. The USACE Mobile District provided changes 

in bed levels at various locations in the study area from the periods of 2009–14, 2002–14, and 

2002–15 (the range of uncertainty is plus or minus [±] 0.61 m). The changes in bed level were 

calculated from bathymetric surveys by Byrnes and others (2012), Flocks and others (2017), and 

the NGDC (National Ocean Service, 2014). Changes in bed levels from 2002 to 2014 and 2002 

to 2015 indicate erosion and deposition along the 5-m contour extending from Pelican Island, 

and deposition along the eastern edge of the navigation channel offshore of the Fort Morgan 

Peninsula (figs. 3A, 3B). These changes were compared with the modeled change in bed level at 

the end of the 10-year existing simulation (that is, the year 10 final bed level minus the year 1 

initial bed level). The simulation shows similar patterns of erosion and deposition along the 5-m 

contour and along the navigation channel (fig. 4A). It is important to note that the simulation was 

not initialized with 2002 bathymetry, so the magnitude of differences is not expected to match 

exactly. Additionally, the magnitude of the sediment placed in the Sand Island beneficial use area 

is not expected to match exactly because an annual average was applied in the simulation.

Observed bed level changes on the ebb tidal shoal between 2009 and 2014 indicate plus 

or minus (±) 1 m erosion and deposition in between the 5- and 10-m contours (fig. 3C). For 

comparison, bed levels were extracted after year 5 in the 10-year simulation and used to calculate 

the change in bed level. Similar to the observation, there are patterns of erosion and deposition 

between the 5- and 10-m contours, as well as the dredge placement in the Sand Island Beneficial 

Use Area (fig. 4B). The magnitude of the difference is less than the observed data, but again, this 

Table 2. Coefficients of determination and root mean square error values for through-channel and crosschannel velocity components during flood and ebb tide at inlet, volume flux during flood and ebb tide at inlet 

and water levels at the Dauphin Island tide gage, Mobile Harbor, Alabama.

[R

2

, coefficient of determination; RMSE, root mean square error; U, through channel; m/s, meter per second; 

V, cross channel; m3/s, cubic meter per second; m, meter]

Constituent R 

2

RMSE

Ebb U velocity 0.93 0.11 m/s

Ebb V velocity 0.79 0.06 m/s

Flood U velocity 0.66 0.12 m/s

Flood V velocity 0.30 0.07 m/s

Ebb tide flux 0.98 1.53×106 m3

/s

Flood tide flux 0.79 1.85×106

 m3/s

Water level 0.68 0.09 m
9

Figure 3. Observed bed level changes in Mobile Harbor, Alabama. A, from 2002 to 2014. B, from 2002 

to 2015. C, from 2009 to 2014. Differences greater than 0 indicate deposition, and differences less than 0 

indicate erosion.

simulation was not initialized with 2009 bathymetry and does not include tropical storms that 

would have occurred during this period.

2010 Climatology

The change in bed level at the end of the 2010 simulation for the existing and with-project conditions is illustrated in figures 5A and 5B. Both simulations indicate erosion and deposition along the 5-m contour extending out from Pelican Island, as well as offshore of the Fort 

Morgan Peninsula. The difference in the final bed levels between the existing and with-project 

conditions is shown in figure 5C. Results indicate that there are minor changes in bed levels near 

Pelican Island (ranging from −0.11 to 0.11 m) and offshore of the Fort Morgan Peninsula (ranging from −0.81 to 0.22 m) with the proposed channel modification; these changes were confined 

within the 5-m contour. Similarly, figures 6A and 6B illustrate the change in bed level at the 
10

Figure 4. Modeled bed level change in Mobile Harbor, Alabama. A, after 10 years in the 10-year existing 

condition simulation. B, after 5 years in the 10-year existing condition simulation. Differences greater than 0 

indication deposition, and differences less than 0 indicate erosion. 

end of the 2010 simulation for the existing condition with 0.50 m of SLR and the with-project 

conditions with 0.50 m of SLR. Similar patterns of erosion and deposition can be seen along the 

5-m contour offshore of Pelican Island and the Fort Morgan Peninsula. Again, there are minor

changes in bed levels for the with-project conditions ranging from −0.20 to 0.32 m near Pelican Island and −0.38 to 0.34 m offshore of the Fort Morgan Peninsula within the 5-m contour

(fig. 6C).

The volume of sediment eroded and deposited in the entrance channel at the end of

the 2010 simulations was calculated by dividing the entrance channel into 15 sections of

equal length. The volumes in each section and across the entrance channel are summarized

in table 3; the percent change in each section is illustrated in figure 7. The change in volume

across the channel for the existing and with-project scenarios is 45,860 and 43,670 m3respectively, indicating that the channel is shoaling (sand is being deposited in the channel) for both

scenarios. The deeper channel (with-project condition) reduced the overall shoaling volume
11

Figure 5. Changes in bed level for the 2010 simulations, Mobile Harbor, Alabama. A, existing conditions. 

B, with-project conditions. C, difference in final bed level between existing and with-project conditions. For 

A and B, differences greater than 0 indicate deposition, differences less than 0 indicate erosion. For C,

differences greater than 0 indicate the with-project bed level is shallower, and differences less than 0 

indicate the with-project bed level is deeper. 

by 2,190 m3

(4.77 percent). Under 0.50 m of SLR, there is less shoaling with channel volumes

of 20,662 and 18,991 m3for the existing and with-project conditions, respectively. Similarly,

the with-project condition reduces the channel shoaling volume by 1,671 m3

 (8.09 percent)

from the existing condition. Changes in shoaling volume are negative at most sections of the

entrance channel, meaning that less sand is deposited for the with-project condition, as shown

in figure 7. However, a few sections in the middle of the entrance channel (6 through 9) and

sections 13 and 15 have positive changes, indicating that more sand is deposited in these sections with the deeper (with-project) channel.
12

Figure 6. Changes in bed level for the 2010 simulations, Mobile Harbor, Alabama. A, existing conditions 

accounting for 0.5 meter of sea level rise. B, with-project conditions accounting for 0.5 meter of sea level 

rise. C, difference in final bed level between existing conditions accounting for 0.5 meter of sea level rise 

and with-project conditions accounting for 0.5 meter of sea level rise. For A and B, differences greater than 

0 indicate deposition, differences less than 0 indicate erosion. For C, differences greater than 0 indicate 

the bed level for the with-project condition with 0.5 meter of sea level rise is shallower than the bed level 

for the existing condition with 0.5 meter of sea level rise, and differences less than 0 indicate the bed level 

for the with-project condition with 0.5 meter of sea level rise is deeper than the bed level for the existing 

condition with 0.5 meter of sea level rise. 

10-Year Climatology

The change in bed level at the end of the 10-year simulation (that is, the difference 

between the final bed level at the end of year 10 and the initial bed level at the start of the simulation) for the existing and with-project conditions is shown in figures 8A and 8B. Similar to 

the 2010 simulations, there is erosion and deposition in both simulations along the 5-m contour 

extending out from Pelican Island, as well as from the Fort Morgan Peninsula. The difference 
13

Table 3. Volume of sediment eroded or deposited in the entrance channel at the end of the 2010 simulations, Mobile Harbor, Alabama.

[Positive numbers indicate sand was deposited in the channel (shoaling); negative numbers indicate sand was eroded from the channel]

Section

(figs. 7, 10)

Sediment volume change, in cubic meters

Existing 

conditions

With-project 

conditions 

Existing conditions with 

0.50 meter of sea level rise

With-project conditions with 

0.50 meter of sea level rise

1 −171 −190 −85 −115

2 −1,144 −1,370 −563 −642

3 −13,012 −15,434 −6,668 −7,878

4 −12,306 −12,704 −6,458 −6,608

5 −21,733 −22,506 −10,157 −10,621

6 −21,858 −20,215 −12,144 −11,446

7 15,200 18,455 5,488 7,621

8 2,433 3,746 −1,569 −668

9 −3,903 −1,735 −6,546 −5,283

10 3,869 3,215 −1,891 −2,117

11 44,910 41,969 20,786 19,041

12 53,606 47,403 34,337 30,728

13 −4,859 −1,833 2,754 3,624

14 3,555 3,358 2,527 2,398

15 1,273 1,511 850 955

All sections 45,860 43,670 20,662 18,991

in the final bed levels between the existing and with-project conditions is shown in figure 8C. 

Results indicate that, with the proposed channel deepening, there are some changes in bed levels 

along the 5-m contour offshore of Pelican Island, ranging from −2.62 to 2.03 m. Offshore of the 

Fort Morgan Peninsula, there are larger changes in bed levels ranging from −3.17 to 3.94 m. The 

change in bed level at the end of the 10-year simulation for the existing and with-project conditions with 0.50 m of SLR is illustrated in figures 9A and 9B. There are similar patterns of erosion 

and deposition along the 5-m contour and near the Fort Morgan Peninsula for both simulations. 

With the proposed channel modifications under 0.50 m of SLR, changes in bed levels were 

smaller than for the 10-year simulations without SLR and range from −0.86 to 1.07 m offshore of 

Pelican Island and −1.92 to 1.47 m offshore of the Fort Morgan Peninsula (fig. 9C).

The volume of sediment in the entrance channel at the end of the year 10 was calculated 

at each of the 15 sections and across all sections of the channel (table 4); the percent change in 

each section is illustrated in figure 10. At the end of 10 years, the changes in volume across the 

entire channel for the existing and with-project scenarios are 40,035 and 37,816 m3, respectively, 

indicating that the channel is shoaling (sand was deposited in the channel). The with-project 

condition reduced the overall channel shoaling volume by 2,219 m3 (5.54 percent). The change in 

volume across the entire channel for the existing and with-project scenarios under 0.50 m of SLR 

is 17,849 and 15,175 m3, respectively. The with-project condition reduced the overall channel 

shoaling volume by 2,674 m3 (14.98 percent). Like the 2010 simulations, the negative changes 

shown in figure 10 illustrate that less sand is being deposited at most sections of the entrance 
14

Figure 7. Percent change in the volume of sediment eroded or deposited in the entrance channel, 

Mobile Harbor, Alabama. A, between 2010 existing and 2010 with-project conditions. B, between 2010 

existing with 0.50 meter of sea level rise and 2010 with-project with 0.50 meter of sea level rise.
15

Figure 8. Changes in bed level for the 10-year simulations, Mobile Harbor, Alabama. A, existing conditions. B, with-project conditions. C, difference in final bed level between existing and with-project 

conditions. For A and B, differences greater than 0 indicate deposition, differences less than 0 indicate 

erosion. For C, differences greater than 0 indicate the with-project bed level is shallower, and differences 

less than 0 indicate the with-project bed level is deeper. 

channel, especially at the southern end. Again, a few sections in the middle of the entrance 

channel (6 through 9) and sections 13 and 15 have positive changes, indicating that more sand is 

deposited in these sections with the deeper (with-project) channel.

The shoaling volume across the entire entrance channel also was calculated at the end of 

each year in the 10-year simulation (table 5). Although elevations in the channel were reset to the 

initial depth at the beginning of each year, the shoaling volume at the end of each year was not 

equal for all simulations; the percent change in the volume varied from a 1.47-percent decrease 

to a 9.99-percent increase from the previous year. These fluctuations indicate that as sand shifts 

in offshore areas (especially near the Fort Morgan Peninsula), the resulting sediment transport 

into the entrance channel changes.
16

Figure 9. Changes in bed level for the 10-year simulations, Mobile Harbor, Alabama. A, existing conditions

accounting for 0.5 meter of sea level rise. B, with-project conditions accounting for 0.5 meter of sea level

rise. C, difference in final bed level between existing conditions accounting for 0.5 meter of sea level rise

and with-project conditions accounting for 0.5 meter of sea level rise. For A and B, differences greater than 0 

indicate deposition, differences less than 0 indicate erosion. For C, differences greater than 0 indicate the 

bed level for the with-project condition with 0.5 meter of sea level rise is shallower than the existing condition 

with 0.5 meter of sea level rise, and differences less than 0 indicate the bed level for the with-project 

condition with 0.5 meter of sea level rise is deeper than the existing condition with 0.5 meter of sea level rise. 

Discussion

The results and patterns from the existing and future with-project conditions indicated 

some changes in the overall dynamics of the system, especially for the 10-year simulations. 

There were minimal differences in morphologic change in the nearshore areas of Dauphin Island 

and Pelican Island because of the channel modifications (figs. 8, 9). This suggests that sediment 

delivery away from the ebb tidal shoal to these areas is similar under these two scenarios and that 
17

shoreline positions are unlikely to be affected because of the modified channel. Although comparison of the two simulations shows some spatial shifting of sand offshore of the Fort Morgan 

Peninsula, the patterns of erosion and deposition in the two simulations are quite similar. Based 

on these results, it also seems unlikely that these changes would alter sediment delivery to the 

peninsula, and only minor effects to the terminal end of the peninsula closest to the channel 

could occur.

A limitation in the modeling framework is the exclusion of peak wave and storm surge 

characteristics associated with tropical storms. Although larger wave heights from storms are 

included in the full time series of the waves used to define the climatology, the nine bins were 

defined using mean characteristics of all waves within each bin. Therefore, the model was not 

forced with wave heights larger than 2.26 m, which is smaller than peak wave heights observed 

during tropical storms in the Gulf of Mexico (for example, see Bilskie and others, 2016). Additionally, the simulation of each bin contains a tidal time series but does not include storm surge, 

which is associated with individual storms rather than the wave conditions represented by each 

bin. River inflow from the Mobile and Tensaw Rivers (fig. 1) also was not considered for this 

study because it was assumed that riverine effects on hydrodynamics and marine sediment transport would be minor around the ebb tidal shoal and Dauphin Island.

To simulate morphological change over decadal time scales, two-dimensional depth 

averaged velocities were used in the Delft3D simulations. This neglects the effects of vertically 

varying velocity profiles and boundary layer processes on morphological change. Studies have 

Table 4. Volume of sediment eroded or deposited in the entrance channel for the 10-year climatology 

simulations, Mobile Harbor, Alabama.

[Positive numbers indicate sand was deposited in the channel (shoaling); negative numbers indicate sand was eroded from the channel]

Section

(figs. 7, 10)

Sediment volume change, in cubic meters

Existing conditions With-project 

conditions

Existing conditions 

with 0.50 meter of 

sea level rise

With-project conditions 

with 0.50 meter of 

sea level rise

1 −1,328 −1,581 −596 −742

2 −534 −1,108 −63 −230

3 −15,532 −18,680 −9,042 −10,819

4 −11,984 −12,367 −5,618 −5,687

5 −24,782 −26,482 −10,693 −11,355

6 −29,023 −28,022 −14,088 −13,393

7 10,243 13,260 5,626 7,250

8 −2,156 1,450 −6,203 −4,547

9 −11,460 −8,587 −16,910 −15,473

10 24,661 21,423 21,054 18,829

11 54,818 54,185 23,400 22,004

12 52,207 44,659 24,076 21,709

13 1,052 4,897 3,727 4,458

14 4,562 4,446 2,402 2,297

15 1,619 1,969 777 876

All sections 52,364 49,462 17,849 15,175
18

Figure 10. Percent change in the volume of sediment eroded or deposited in the entrance channel, 

Mobile Harbor, Alabama. A, between the 10-year existing and 10-year with-project conditions. B, between 

the 10-year existing condition with 0.50 meter of sea level rise and 10-year with-project condition with 

0.50 meter of sea level rise.
19

Table 5. Shoaling volume in the entrance channel at the end of each year for the 10-year simulations, 

Mobile Harbor, Alabama.

Period

Shoaling volume, in cubic meters (percent change in volume from the previous year)

Existing conditions With-project 

conditions

Existing conditions 

with 0.50 meter of 

sea level rise

With-project conditions 

with 0.50 meter of 

sea level rise

After year 1 38,442 37,482 15,459 12,808

After year 2 42,284 (9.99) 38,474 (2.65) 15,726 (1.73) 13,283 (3.71)

After year 3 41,705 (−1.37) 40,078 (4.17) 15,633 (−0.59) 13,268 (−0.11)

After year 4 41,583 (−0.29) 39,681 (−0.99) 15,879 (1.57) 13,509 (1.82)

After year 5 41,520 (−0.15) 39,677 (−0.01) 16,322 (2.79) 13,836 (2.42)

After year 6 41,470 (−0.12) 39,404 (−0.69) 16,687 (2.24) 14,234 (2.88)

After year 7 41,217 (−0.61) 39,035 (−0.94) 17,041 (2.12) 14,545 (2.19)

After year 8 40,798 (−1.02) 38,473 (−1.44) 17,218 (1.04) 14,651 (0.72)

After year 9 40,305 (−1.21) 37,907 (−1.47) 17,218 (0.00) 14,607 (−0.30)

After year 10 40,035 (−0.67) 37,816 (−0.24) 17,849 (3.66) 15,175 (3.89)

determined that overall sediment transport patterns and morphology change can be accurately 

simulated using depth-averaged velocities, but the inclusion of three-dimensional processes 

could change the patterns or magnitudes shown here (Hu and others, 2009; Lapetina and 

Sheng, 2015). However, the relative difference between simulations with and without project 

conditions would likely be comparable.

Summary and Conclusions

A Delft3D model was developed to evaluate the potential effects of proposed navigation 

channel deepening and widening in Mobile Harbor, Alabama. Comparisons of model output from 

deterministic simulations with observed data of water levels, velocities, and bed level changes 

indicated that the model was able to capture hydrodynamic and sediment transport patterns in 

the study area with skill (coefficient of determination [R2

] values were 0.93 and 0.66 for modeled 

versus observed through-channel (U) velocities during ebb and flood tide, respectively, 0.79 and 

0.30 for modeled versus observed cross-channel (V) velocities during ebb and flood tide, respectively, 0.98 and 0.79 for ebb tide flux and flood tide flux, respectively, and 0.68 for modeled 

versus observed water levels). The model was then used to simulate changes in sediment transport with and without modifications to the navigational channel and accounting for 0.5 meter (m) 

of sea level rise (SLR). Each scenario was simulated for 1 year with a wave climatology representative of the year 2010 as well as for 10 years with a longer-term wave climatology spanning 

from 1988 to 2016. Comparisons of model output for the with-project and existing conditions for 

the 2010 simulations indicated differences in bed levels ranging from −0.11 to 0.11 m offshore 

of Pelican Island and −0.81 to 0.22 m offshore of the Fort Morgan Peninsula. For the simulations 

with 0.5 m of SLR, differences in bed levels ranged from −0.20 to 0.32 m near Pelican Island 

and −0.38 to 0.34 m offshore of the Fort Morgan Peninsula. The with-project condition reduced 

shoaling in the entrance channel by 4.77 and 8.09 percent for the 2010 simulations without and 

with 0.5 m of SLR, respectively. For the 10-year simulations, there were larger changes in bed 

levels with the proposed channel deepening; at the end of 10 years, the largest changes were 
20

offshore of the Fort Morgan Peninsula and ranged from −3.17 to 3.94 m for the simulation without SLR and −1.92 to 1.47 m for the simulation with 0.5 m of SLR. The with-project condition 

reduced the entrance channel shoaling volume by 5.54 percent for the simulation without SLR and 

14.98 percent for the simulation with 0.5 m of SLR. Spatially, most of the entrance channel had 

less deposition except for the middle of the entrance channel, which had more deposition with the 

proposed channel modifications. Lastly, the shoaling volume at the end of each year in the 10-year 

simulations was not equal, indicating that offshore changes in bed levels especially around the 

Fort Morgan Peninsula affect the quantity of sediment that is transported into the channel.
21

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Passeri and others—Effects of Proposed Navigation Channel Improvements on Sediment Transport in Mobile Harbor, Alabama—Open-File Report 2018–1123

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