Mississippi Barrier Island Restoration

The Mississippi Coastal Improvements Program (MsCIP) was initiated after Hurricane Katrina in 2005. The aim was to return the sediment budget of the Mississippi Barrier Island system to its natural state as much as possible. A sound understanding of the physical processes driving the dynamics of the system is required for the successful design and implementation of such a plan. In this project, Deltares performed a numerical modelling study to establish the requisite knowledge and evaluate the consequences of implementing the plan.

hurricane Katrina

Objectives

Important to know was if the barrier island restoration would affect the sediment dynamics of the system. In this way we could answer practical questions for its implementation, e.g.:

  1. How will the restoration of Ship Island (for example, the closure of Camille cut and nearshore sand placement) affect sediment transportation?
  2. To what extent will the sand extracted from offshore borrow sites impact on the erosion and deposition of the barrier islands?
  3. To what extent will the restoration of Ship Island affect operations in the nearby shipping channel (Ship Island Pass)?

Approach

We set up a chain of numerical models, including Delft3D, XBeach and UNIBEST-CL+, which we used to derive a comprehensive sediment budget for the Mississippi Barrier Island system. The sediment budget distinguished between the contributions of year-averaged conditions and hurricanes. It was found that the contributions of year-averaged conditions and hurricanes to long-term, average, net longshore transport are of a similar order of magnitude (although the mean annual percentage of hurricanes is in the order of 3%).

Results

The numerical models and our understanding of the system were then used to answer the key questions of the study. The following conclusions were drawn:

  1. The restoration of Ship Island has a local effect on sediment transport along and around Ship Island in hurricane conditions. In year-averaged conditions, sediment transport along the tips of Ship Island and through the passes is expected to increase slightly (0-10%).
  2. Due to its small size and limited excavation depth the borrow area south of Ship Island will not negatively impact the overall morphological development of the restored Ship Island.
  3. The restoration is expected to result in a limited increase in sedimentation in Ship Island Pass, primarily during hurricane events. In year-averaged conditions, the effect is predicted to be minimal. The model results indicated that, during severe hurricanes, sedimentation in the channel increased by 10% to 30% while, with milder hurricanes, the increase is expected to be in the order of 0% to 10%.

Using these answers, the client was able to proceed with the implementation of the plan.

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