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Title: Investigation of discharge measurements of the Lower Mississippi River below Natchez, MS
Authors: Lewis, James W.
Brown, Gary L.
Ayres, Steven K.
Keywords: Mississippi River
Stream measurements
Publisher: United States. Army. Corps of Engineers. Mississippi Valley Division.
United States. Mississippi River Commission.
Coastal and Hydraulics Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: MRG&P;Tech Note No. 3
Abstract: Introduction: A significant challenge during the 2011 Mississippi River flood was estimating the actual volumetric flow of water within the river. There were large disagreements among the various gages and measurements in May 2011. To address this issue (among others) (USACE 2012), the U.S. Army Corps of Engineers (USACE), Mississippi Valley Division (MVD), initiated the Mississippi River Geomorphology and Potamology Program (MRG&P) in 2013, a revitalization of historic Mississippi River Potamology studies that were performed in the 1960s to 1980s (Biedenharn et al. 2014). The data collection and analysis within this study aim to improve the understanding of the water budget of the Lower Mississippi River through three primary objectives: to determine if there are systematic errors between gages in the Lower Mississippi River, to examine claims of a water volume loss between Baton Rouge and Belle Chasse, and to document extreme loss of flow from Natchez, MS, to the Head of Passes (HOP). For most rivers, the flow increases in the downstream direction because of an increasing drainage area reaching each downstream location, although there are plenty of exceptions. In general, the Lower Mississippi River is one of those exceptions, where the flow within the river does not always increase in the downstream direction. In addition to the man-made diversion through the Old River Control Complex (ORCC), there is also a significant decline of the Mississippi River discharge between ORCC and HOP. The majority of this water loss occurs through the delta distributaries and Fort St. Philip leakages (Sharp et al. 2013). It is also known that the Bonnet Carre Spillway (BCS) continuously leaks several thousand cubic feet per second through the needle gates of the structure, when the river stage is high enough to inundate the gates. However, even when all of the known diversions of water are accounted for, the water budget for the Mississippi River still does not close. This has led to speculation about possible additional discharges from the river, potentially associated with groundwater losses. Evidence of freshwater signals within coastal estuaries near the Mississippi River led Kolker et al. (2013) to hypothesize that a significant amount of water may be exiting the river through sandy underground paleochannels, connecting the river to adjacent bays. They demonstrated that the chemical signature of freshwater entering Barataria Bay is clearly evident. One of the primary means they used for estimating the volume of water associated with this signal was to associate the quantity of the groundwater flux with the residual of the Mississippi River water budget, computed by integrated existing observations of discharge in the river. This study is designed to determine, by direct observation, if there is indeed an unexplained residual in the water budget that could potentially indicate some other significant source/sink to the river (such as groundwater flux). The methods and results are given below.
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