The objective of the proposed Rapid Response Research grant is to undertake field campaign, in-situ sampling, and Lidar survey of selected regions of the Lower Mississippi River and surrounding landscape impacted by the 2011 extreme flood. PIs interest is to understand how deliberate but uncontrolled levee breach at Birds Point, and engineered diversions at Morganza and Bonnet Carré spillways impact both the streams and the surrounding landscapes. In the proposed effort they plan to (1) characterize flood deposits by sampling sediment depth, depositional area, and sediment grain size distribution and stratigraphy; (2) investigate soil morphology and their properties such as soil organic carbon, pH, cation exchange capacity, texture, and bulk density; (3) characterize response of landscape form and structure at all three offtake sites by characterizing the bathymetry, flow, and sediment transport; and (4) evaluate landscape impact through Lidar survey near and downstream of the Mississippi and Ohio River Confluence including large sections of the Birds Point New Madrid (BPNM) floodway. The data collected will allow them to assess if large episodic events, such as the present Mississippi flood, shape the long-term form and function of rivers and the surrounding floodplains, and the timescale of the impact on the flow and transport dynamics. Studies using this data will assist in future planning of water and sediment diversions in the lower Mississippi River basin.
The aim of the project was to collect a variety of data arising as a result of the 2011 flood in the Lower Mississippi River that could be lost through subsequent human activities and natural processes. Two areas were of interest: the New Madrid floodway impacted by the Birds Point levee breach, and the region of the Bonnet Carré and Morganza spillways. Findings from New Madrid Floodway Study: Large floods sustain natural flora and fauna, contribute sediment to floodplains, and can also produce substantial floodplain erosion. In human-dominated landscapes, flooding can result in extensive damage to property and land. A heterogeneous distribution of landscape susceptibility to erosion and deposition creates highly variable spatial patterns of flood impacts. In May 2011, the activation of the Birds Point-New Madrid (BPNM) Floodway, as a Mississippi River flood control measure, inundated a 540 km2 agricultural floodplain in southeastern Missouri, USA. This historic event, when seen as a scientifically unique large-scale field stress test, provides an unprecedented opportunity to analyze factors that are indicators of hotspots of floodplain erosion and deposition. Using pre- and post-flood high-resolution LiDAR datasets we find that erosion and deposition occurred over 12% and 8% of the Floodway, respectively, with corresponding average depths of 0.27 m and 0.3 m. Depths of erosion and deposition exceeding 1 m were generally localized near levee breaches. Eroded gullies up to 1 km long and 3 m deep were observed on agricultural land where floodwater flowed over a low ridge forming the outer bank of a relict meander scar of the Mississippi River. This vulnerability arises due to a combination of erodible soils, acceleration of flow due to locally steep topographic gradients associated with legacy fluvial landforms, and a lack of woody vegetation. Hotspots of erosional vulnerability can be mapped a priori using simulated flow velocities in the floodplain along with elevation, soil, and vegetation data. This analysis also indicates that establishment of woody vegetation in strategic locations may be an effective mitigation measure against future floodway erosion due to levee breaks. On the other hand, a wide corridor of riparian forest between the river and the main upstream levee breach reduced river-floodway connectivity, preventing major deposition of sediment within the floodway. This paper demonstrates that landscape vulnerability to potentially damaging erosion and deposition emerges from a combination of contributing factors that can be analyzed prior to future flood events, and could support mitigation measures. Findings from Spillway Study: After the 1927 flood of record on the Mississippi River, the Bonnet Carré Spillway in Louisiana was constructed as a flood control operation. When it is opened, the spillway diverts floodwaters from the Mississippi River to Lake Pontchartrain, to reduce the water discharge flowing past New Orleans. During the 2011 Mississippi River flood, which had the highest peak discharge since 1927, the Bonnet Carré Spillway was opened for 42 days, from 9May to 20 June. During this period, the average spillway discharge of 6,010cubic meters per second amounted to 10–20% of the total river flood discharge. We measured and analyzed the areal extent and thickness of new sediments in the floodway, following the 2011 Mississippi flood. We conservatively estimate that 31–46% of the total sand load carried by the Mississippi River during the period of spillway opening was diverted into the floodway. Only the upper 10–15% of the river water column was skimmed into the floodway. We find that local river conditions led to increased concentrations of suspended sand in the upper water column and thus led to diversion of sand from the river into the spillway. We conclude that an appropriate design of engineered river diversions in Louisiana can help mitigate coastal wetland loss. This study has profound implications for evaluating the sustainability of the Louisiana coastline, a subject not only of current national debate, but also a subject of global concern since many of the World’s great rivers deltas, and their populations, may face similar environmental dilemmas over the coming decades. In our paper, we demonstrate how the local channel planform properties of the Mississippi River maximize suspended sediment concentrations in the water column, and therefore boost sediment delivery through the Bonnet Carré engineered diversion. This work should strongly and positively influence, and optimize, the direction of future projects that seek to locate diversions along the Mississippi River for the purpose of promoting land growth in the delta. Publications: Jeffrey A. Nittrouer, James L. Best, Christopher Brantley, Ronald W. Cash, Matthew Czapiga, Praveen Kumar, and Gary Parker, Mitigating land loss in coastal Louisiana by controlled diversion of Mississippi River sand, Nature Geoscience, 5, 534–537, doi:10.1038/ngeo1525, 2012. Goodwell, A., Z. Zhu, D. Dutta, J. Greenberg, P. Kumar, M Garcia, B. Rhoads, G. Parker, R. Holmes, D. Beretta, Assessment of Floodplain Vulnerability during Extreme Mississippi River Flood 2011, submitted for publication, 2013.
