Land Building Models: Uncertainty in and Sensitivity to Input Parameters

The purpose of this Coastal and Hydraulics Engineering Technical Note CHETN is to document a detailed analysis of land building spreadsheet models applied to estimate the net benefits from proposed river diversions. Many of the issues with the key input parameters for the spreadsheet models reviewed in this document will be faced by more complex two- and three dimensional models when they are applied. The CHETN starts with an overview of the various models that are being applied for land building in coastal Louisiana. The sensitivity of land building estimates to the specification of various key input parameters is analyzed and discussed, highlighting issues associated with those parameters. Coastal Louisiana has lost over 1.2 million acres of land since 1932, and land loss continues at a rate of over 15,000 acres per year. Water and sediment diversions have been proposed to mitigate land loss and rebuild land in the Mississippi delta. Numerous questions exist regarding the efficacy of diversions for building land and quantifying changes in both the receiving area and river. Freshwater flow diversions can offer significant mineral sediment and nutrient inputs to marshes that result in both inorganic and organic accumulation of soil. However, the capability to estimate land gain is limited. Several land building models have recently been developed and applied in south Louisiana. Boustany 2007 introduced a screening level model for assessing both the sediment and the nutrient benefit of flow diversions over longtime scales. This model has been applied to screen Coastal Wetland Planning, Protection, and Restoration Act CWPPRA project alternatives. The Boustany model calculates the annual land gain from sediments and nutrients separately and adds these values to the existing area, which is adjusted each year by a constant annual land loss value. Nutrient benefits are based on the potential of the diversion to introduce nutrients to support wetland vegetation.