Information
The wetlands in the delta of the Mississippi (USA) are drastically subsiding and eroding. Many projects and researchers are going on to determine how this “drowning effect” of the delta can be stopped. The researchers say the delta is not durable anymore; the approach has enable to restore the delta and create new land.

One of the solutions that could be feasible is implementing river diversions in the levees of the Lower Mississippi. This means that cuts and gabs will be made in the levees along the Lower Mississippi, in order to divert water and most important sediment into the delta. Today the water and sediments in the river are mainly flowing straight into the Gulf of Mexico and not into the needy delta.

The Lower Mississippi has been canalized since 1800 for navigability. The shipping industry is a very strong player that is against large engineering changes in the river navigation channel. They demand 100% navigability through the Mississippi outlet/ inlet river channel because the entire innerland has to be accessible at any time. Therefore engineers have to maintain a certain depth of the river channel for navigation by dredging, it will be difficult to guarantee if changes have been made in the river system.

The objective
This project is a MSc-study of Delft Technical University, commissioned by Haskoning Inc. and Deltares. Matthijs Bos will carry out this study.

The main objective is to analyze and optimize trade-offs between delta (coast) building and river navigability.

Therefore a 2D numerical model in Delf3D software will be created; this model has to represent the behavior of the river and morphological effects. The model will be calibrated by flow and sediment data from surveys of the river. In addition the river diversion will be implemented in the model and evaluation will be made on the simulations.

Besides that, there will be cooperation with researchers from four different universities in the USA, data will be shared, discussions will be held and different numerical models will be compared and analyzed. This will help to make the research more reliable and to give it a good foundation.

Description of project

The FEMA guidance for levee certification for coastal levees says that “the freeboard must be established at one foot above the height of the 1% wave or the maximum wave runup (whichever is greater) associated with the 100-year still water surge elevation at the site” (FEMA, 2007). The extra 1ft is added to account for uncertainties in the design variables. A disadvantage of this deterministic approach is that the uncertainty in the various hydraulic variables is not taken into account explicitly. The Corps guidance on certification is for a probabilistic approach to be followed similar to the approach used in Corps planning studies; yet, the procedure for coastal levees and floodwalls has not been well defined.

Haskoning Inc. has developed and implemented an innovative probabilistic method to determine the levee and floodwall elevations. This method takes into account the uncertainties in the various hydraulic input parameters. The main advantage of this method is that this method does not include a (arbitrary) freeboard as it is the case in the current FEMA certification guideline. Another advantage is that the uncertainty in the water levels and the waves may differ from one area to another. This procedure enables the end-user to take this spatial variability into account in the hydraulic design process of flood protection systems.

The probabilistic approach essentially differs from a deterministic approach that has been applied before. A deterministic approach uses one (pre-selected) input combination and results in one output. No formal insight is obtained in the probability of these output results in a sensitivity analysis. Using the MCS approach, hundreds (or thousands) of outputs are obtained and this gives insight how the input uncertainty is reflected in the output uncertainty. The development and implementation of this probabilistic method for the HSDRRS around New Orleans is considered to be a necessary step forward toward a more rational way of including uncertainties into the design procedures of flood protection systems.