Integrating delta building physics and economics: Optimizing the scale of engineered avulsions in the Mississippi River delta

Background/Question/Methods

Land loss in the Mississippi River Delta caused by subsidence and erosion has resulted in habitat loss, interference with human activities, and increased exposure of populations to storm surge risks. A few projects have been built in order to divert sediment from the river to areas where land can be built and many more are under consideration as part of the state and federal planning processes.   Most diversions are small scale, although there have been some proposals for large engineered avulsions that would divert a significant fraction of the remaining available sediment.   However, there is debate over whether small or large diversions are optimal economically and socially.   From an economic point of view, the optimal size involves tradeoffs between scale economies in civil work construction, the relationship between depth of diversion and sediment concentration in river water, effects on navigation, and possible diminishing returns to land building at a single location as the edge of built land progresses into deeper waters.  

Results/Conclusions

In this talk, we present the results of a general analysis of scale economies and diseconomies in land building from engineered avulsions.  The analysis addresses the question: how many projects of what size should be built at what time in order to maximize the amount of land built by a particular time (50 years in the future)?    The analysis integrates three models:

1.     coarse sediment diversion as a function of the width, depth, and timing of water diversions (using our field measurements of sediment concentration as a function of depth),

2.     land building as a function of the location and amount of sediment diverted, accounting for bathymetry, subsidence, climate change, and other factors, and

3.     cost of building and operating the necessary civil works. 

The analysis explores general relationships between size, cost, and land building, and does not consider specific actual project proposals.  Sensitivity to assumptions about fine sediment capture, accumulation rates for organic material, sea level rise, and other inputs will be discussed. Our statistical analysis of past diversions indicates existence of strong scale economies in width, but not in depth.  Thus, given our preliminary work, if the goal is to build land, larger scale diversions deliver more land-building sediment at a lower cost than several smaller diversions.