Maintain-for-less in the Lower Mississippi RiverPreliminary study of scenarios to reduce dredging activities and associated costs

Ocean Going Vessel at the mouth of the Mississippi River, Photo: Bos, 2010

>> Dredging activities: Problem statement: There is a federal funding shortfall for maintenance dredging of the Mississippi River, which causes a threat to the commercial navigation industry.

>> Objective: Evaluated scenarios that would reduce the dredging activities and costs between Venice and the Gulf of Mexico.

>> Methodology: A two-dimensional depth-averaged numerical model was used of the Lower Mississippi River to evaluate the scenarios.

>> Scenarios:

1. Donothing

2. ClosingWestBayDiversion

3. ConstructAdditionalRiverSedimentDiversion

4. ConstructRiverTrainingWorksbetweenWestBayDiversionandHeadofPasses

>> Optimal scenario: Model runs of the scenarios show that construction of River Training Works would reduce Mississippi River dredging by about 45% or $13.1 million annually. The estimated first of construction would be $5 million.

>> Next steps: This preliminary study provides a reliable initial conclusion that dredging costs can be significantly reduced. A wider range of scenarios needs to be modelled in greater detail to more accurately define the optimum benefit to cost ratios.

Key Takeaways

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The Study area ranges from Point a la Hache at river mile (RM) 49 Above Head of Passes (AHP) down to the mouth of the river at the Gulf of Mexico at RM 22 Below Head of Passes BHP).

Dredging challenge>> Downstream of Venice, the Mississippi River becomes wider and the flow is distributed. This causes

a reduction in velocities thereby increasing sediment deposition.

>> In 2010, 14.1 million cubic yards was dredged costing $74.1 million, which is higher than the long-term average. This year (2011), the USACE is facing a significant shortfall in Federal funding.

>> Lack of maintenance of the navigation channel depth results in restrictions for cargo, or even worse, like in June 2011 when an oil tanker ran aground in the Mississippi River 10 miles below Venice. Entire Blockage of the navigation channel would have a significant economic impact, as daily blockage costs are estimated at $ 295 million/day (source: Richmond1).

1 U.S. Rep. Cedric Richmond, D-New Orleans, 2011

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The figure shows the bed topography of the river in feet NAVD88, represented by the two-dimensional depth-averaged morphodynamic model. The river bed level upstream of the study area is 100 ft deep and turns shallower to 45ft deep towards the mouth of the river. This shallow part of the river below Venice is dredged every year.

Modelling A modelling tool was developed to simulate the scenarios.

1. A two-dimensional depth-averaged schematisation of the Lower Mississippi River has been developed with the open-source software package Delft3D from Deltares.

2. The model domain is 69 miles from Point a la Hache (RM 49 AHP) down to the mouth of the river (RM 22 BHP), the distributaries and spillways are included.

3. The bed topography has been derived from the hydrographic survey from 2004 (USACE).

4. Hydrodynamic model calibration were based on discharge rating curves obtained from the discharge data set (2001-2009) observed at Talbert Landing and the stage data from five gauges in the study area (Point a la Hache, Empire, Venice, Head of Passes and East Jetty).

5. The hydrodynamic part of the model was verified with a 2010 data set.

6. The simulated sediment transport consists of non-cohesive sediment (sand) and cohesive sediment (silt and clay or ‘mud’).

7. The morphological behaviour in the model was calibrated against yearly bed levels and suspended sediment transport rates and dredging volumes.

8. Bed level changes were calibrated with bed topography from December 2010 from RM 10 AHP down to the mouth of the river (RM 22 BHP). No updated bed topography was available from RM 49 AHP down to RM 10 AHP.

Modelingassumptions

1. The model focuses on the main stem of the river (navigation channel). A hydrodynamic HEC-RAS (1D) model was used to determine the outflow at distributaries (Davis, 2010).

2. The river is the dominant factor in the large-scale hydrodynamics of the Lower Mississippi River. Waves, wind and tide were excluded from this modelling parameters because they play a limited role.

3. The Lower Mississippi was assumed to be well-mixed. The influence of density gradients was neglected.

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Location of Lower Mississippi River Reaches #1 to #6

Dredgingreaches

The dredging study area in the model was split into six reaches (see figure on left page). Moreover, the model results can be compared with the observed dredging volumes per reach.

Simulateddredgingvolumes

ReachNo. Reachname Lengthofreach[Miles]

Dredgingvolume[*106 cubic yards/year]

Dredgingvolume[*106 m3/year]

# 1 West Bay Diversion up (WBD up): 7.8 1.6 1.2

# 2 West Bay Diversion down (WBD down): 4.2 8.0 6.1

# 3 Head of Passes (HOP): 1.3 2.0 1.5

# 4 Southwest Pass up (SWP up): 6.6 0.15 0.11

# 5 Southwest Pass down (SWP down): 10.8 1.7 1.3

# 6 Entrance: 2.2 1.7 1.7

Total: 32.9 15.1 11.6

Modelverification

The observed average yearly dredging volume for the whole dredging reach is 17.5 million cubic yards/year (13.4 million m3/year) (1990-2010) and the simulated average yearly dredging volume under the Do Nothing scenario is 15.1 million cubic yards/year (11.6 million m3/year). The model dredging volumes are presented per reach and in total as displayed in the table. As expected, most dredging activities are in reach #2, because of the widening of the river and the distribution of flow into the distributaries.

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# 1 WBD up # 2 WBD down # 3 HOP # 4 SWP up # 5 SWP down # 6 Entrance # 7 Total volumes0

2

4

6

8

10

12

14

16 x 106 Simulated dredged volumes per section and total, 4 scenarios

Dre

dged

vol

ume

[cub

ic y

ards

]

Baseline scenarioModifying West Bay DiversionControlled upstream sediment diversionImplemented River training works

This figure presents the simulated dredging volumes of every scenario per reach and for the entire dredging reach. The Do Nothing scenario shows that reach #2 accounts for about 50% of the total dredging volume.

ScenariosThe results of the simulated scenarios are described below. The effects of each scenario on the dredging volumes are shown on the graph on page 10.

The following four scenarios have been modelled:

1. Do Nothing

2. Closing West Bay Diversion

3. Construct Additional River Sediment Diversions

4. Construct River Training Works between West Bay Diversion and Head of Passes

1. DoNothing

The Do Nothing scenario was used to define a baseline and to verify the observed dredging activities. The model results of the other three scenarios can be compared against the Do Nothing scenario to determine if expected benefits are achieved.

Results:The simulation results show that highest dredging volumes per river mile occur in Reaches 2, 3, and 6 with Reach 2 having the greatest volume. The high dredging volumes in Reaches 2 and 3 result from river flow passing into distributaries causing river velocity to decrease and accordingly depositing sediment in the channel. The high dredging volume in Reach 6 results from velocities slowing when the river discharges into the Gulf of Mexico.

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Lower Mississippi River, trifurcation at Head of Passes, source: USACE, 2010

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2. ModifyWestBayDiversion

In this scenario a closed-off West Bay Diversion was simulated to quantify the maximum effect that the scheduled closing of the West Bay Diversion will have on the sedimentation. It is expected that a closed diversion would decrease the required dredging because the diversion interferes with the flow field. This especially occurs downstream of the entrance of the diversion.

Results:The simulations with the closed-off West Bay Diversion show less dredging requirements (see graph page 12). Especially in Reach 2 (reduction of 40%) the impact of the closure is significant. The flow pattern with a closed diversion is more concentrated in the main channel, which enhances the sediment transport rate and a reduction of the sedimentation processes near West Bay. It shows that the location of the West Bay Diversion is not optimal. As a result of less sedimentation in Reach 2, the sediments are transported through the Southwest Pass to the mouth of the river and settle in this area with increasing dredging requirements in Reach 6. The overall reduction in dredging activities is about 25%.

Barataria Bay Waterway near Bayou St. Denis, source: USACE, 2011

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3. ConstructAdditionalRiverSedimentDiversion

A strategically placed uncontrolled sediment diversion at RM 33 AHP was modeled which would divert 5% of the flow and about 5% of the sediment into the delta during the wet period (January – June). A control structure would regulate the diversion and would be closed in the dry season (July – December). During the dry season it is not beneficial to use the sediment diversion because the river barely transports any sediment. The already scarce water in the river is necessary to provide the navigable depth.

Results: The simulation with the upstream sediment diversion at RM 33 AHP also simulates less dredging activity (see graph page 12). The results show that the dredging volumes decrease for every reach. The assumption that less sedimentation would occur over the entire reach seems realistic. The resulting reduction in dredging activities is about 30%. Moreover, the diversion contributes to land building and has, therefore two significant benefits: on the one hand a reduction in dredging activities and on the other a contribution to wetland building.

Cost Benefit Analysis: An upstream controlled sediment diversion contributes to lowering the dredging activities. This scenario reduces the dredging requirements by 30%, with the about $8.5 million of yearly savings. The estimated first costs of construction for the controlled sediment diversion are in the range of $40 to $150 million. The costs are comparable with the built West Bay Diversion, a diversion without control structure (costs $33.3 million2) or the planned Myrtle Grove, with control structure (costs $144.3 million3). An $8.5 million annual savings in dredging could support a diversion costing up to about $150 million.

2 Fact Sheets: West Bay Diversion (September 2010) and Myrtle Grove (Januari 2011), USACE

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4. ConstructRiverTrainingWorks

The flow field of the Do Nothing scenario (Baseline) is presented on the left page. The hydrodynamic behavior of the area between West Bay and Head of Passes shows that the flow velocities significantly decrease (red turns into orange/yellow) due to flow through the distributaries and the widening of the river. The intent of the River Training scenario is to narrow the river channel by using groynes, which will enhance a concentrated flow field in the main channel. The right figure (on the left page) shows the flow field of this scenario with the implemented river training works. It shows the flow velocity remains constant when passing the distributaries and the widened river. The increased flow velocities in the problem area amplify the sediment transport and reduce sedimentation processes and, therefore reduce dredging activities.

Results:This scenario shows promising results (see graph page 10). The reduction of dredging required is as high as 45%. Even after the implementation of training works significant sedimentation occurs in Reaches 3, 4 and 6. The simulated aggradation of the bed in Reach 3 still is more than 3-4.5 ft/year, with a 750 ft navigation channel width. The river channel should be even narrowed more at the trifurcation. The increasing simulated dredging activity in Reaches 4 and 6 are the negative effect of the implemented training works. The sedimentation problem is moved into these reaches. This effect can be reduced by strategically placed training works in the Southwest Pass and at the entrance. In this way the sediments can be transported into the Gulf of Mexico. Additional modeling needs to be done to determine the optimal design of the implementation of the river training work in the entire dredging reaches.

CostBenefitAnalysis: Building river training groynes costs approximately $425 /ft. The simulated scenario uses 12,000 ft of training works at an estimated first cost of $5.1 million. Dredging costs are approximately $1.7 /cubic yard. The investment in river training works is cost effective. The model results of this scenario indicate that construction of training works would reduce the yearly dredging volumes up to 45% (7.8 million cubic yards), which indicates that within one year the investment cost can be recovered. A disadvantage of river training works is that the sediments would not be used for wetland building.

Lower Mississippi River, construction of West Bay Diversion, source: USACE, 2003

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Conclusions This preliminary study confirms that there is great potential to lower the yearly dredging activities and associated costs. The following specific conclusions are drawn:

>> Complete closing of the West Bay diversion results in a reduction of sedimentation in the Mississippi River. In stead of complete closing, modification of the West Bay Diversion could be considered and further investigated.

>> Building river training works and/or an introduction of a controlled sediment diversion upstream reduce the dredging volumes significantly. The model simulations indicate that the implementation of training works results in a reduction of about 45% in dredging volumes, whereas an additional controlled river diversion results in about 30% reduction.

>> A preliminary cost benefit analysis concludes that the investment of building river training works can potentially be recovered within one year and that a sediment diversion could be cost neutral.

>> A wider range of scenarios needs to be modelled in greater detail to more fully define the benefit to cost ratios and optimum plan.

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Ocean Going Vessels at the mouth of the Mississippi River, Photo: Bos, 2010