Sediment management for sustainable ecosystem restoration of coastal Louisiana

摘要

The Mississippi River Delta Plain (MRDP) in southeast coastal Louisiana exemplifies an ecocatastrophe which was triggered by geological and geophysical processes resulting in the present degradation of the coastal landscape. Between 1932 and 2016, Louisiana has lost approximately 4,800 km~2 of coastal land, much of which was lost between 1932 and 1985 (～4,000 km~2; ～76 km~2/yr). Since 1985, cumulative wetland loss has continued at a rate of ～25 km~2/yr (Couvillion et al. 2017). This rate of land loss threatens a range of key national assets and important communities. A Coastal Master Plan for coastal protection and restoration was developed by the Louisiana Coastal Protection and Restoration Authority (CPRA) to mitigate land loss and degradation of the ecosystem and reduce flood risk (CPRA 2017). This multi-decadal, multi-billion-dollar plan prescribes a portfolio of protection and restoration projects to reduce land loss, maintain and restore coastal environments, and sustain communities. The large-scale effort to restore coastal Louisiana is made more challenging by reduced sediment supply from the Mississippi River mostly due to separation of the delta plain from the river by levees, rising sea levels, subsidence, oil and gas activities, local flood-control levees, and navigation infrastructure. Sedimentological restoration to restore geomorphic form is critically important to offset land loss. Many of the environmen-tal conditions that impact land loss are influenced by climate change. With climate change impacts (e.g. sea level rise and increased storm intensity), the quantity of sediment needed for sustainable restoration will increase, while at the same time the accessibility to the sediment resources will become more challenging. Thus, restoration of coastal Louisiana under changing environmental conditions is a two-pronged challenge: 1) restoration and protection projects must be robust enough to counter varying future environmental scenarios; and 2) the quantity of sediment resources available for critical restoration projects is likely to decrease, rendering the dredging and utilization of the sediment resources more difficult and costly. To address these challenges, the Louisiana Sediment Management Plan (LASMP) identifies and delineates potential sediment sources for restoration and provides a framework for managing sediment resources wisely, cost effectively, and in a systematic manner. To this end, there is consensus that replicating natural processes by diverting sediment into wetlands via sediment diversions will be the most effective sediment management practice. Evaluating project design, construction goals and objectives, project monitoring, adaptive management, and future project and policy innovations in all aspects of a coastal restoration program (planning, policy, and implementation) provides an efficient and effective basis for managing sediment resources.