THE IMPACT OF MEASUREMENT TECHNIQUE ON QUANTIFYING SEDIMENT SUPPLY FROM THE MISSISSIPPI-ATCHAFALAYA RIVER IN SUPPORT OF COASTAL RESTORATION

Sediment supply from the Mississippi-Atchafalaya River (MAR) is critical to the future sustainability of the deltaic coast of southeastern Louisiana in the face of threats from eustatic and relative sea level rise, tropical storm erosion, and ecosystem shifts associated with changing climate. MAR levees constructed for flood protection across this landscape have altered the sediment supply to the coastal zone in multiple ways including (1) limiting overbank flooding in certain reaches, (2) creating, modifying or sealing off channel exits, and (3) limiting lateral channel migration. All these processes have modified the total supply and seasonal timing of sediment release to the coastal zone. Quantifying the impact these modifications have had is strongly influenced by how riverine sediment monitoring is conducted, particularly that associated with fixed, long-term stations that are the best method for tracking the progressive impact of river-training works. Station location will impact total sediment loads measured depending on whether overbanking is occurring at the site, batture extent upstream of the station, and, further downstream, the impact of tidal and estuarine processes. Calculated loads are also influenced by frequency and method (boat transect versus fixed sensor) of sampling. This is particularly critical for tracking the mass of washload (fines), whose supply to the lowland MAR reach is impacted by hysteresis in the catchment. Bed material load sand mass is more directly impacted by station location relative to underlying bar morphology. Future standardization of MAR station locations and field and laboratory methodologies is critical for providing consistent and accurate sediment supply numbers for planned coastal restoration activities such as river diversions, and sand mined for barrier island restoration and wetland creation.