Sediment and Nutrient Deposition Over A Reconnected Floodplain During A Large-Scale River Diversion, The Bonnet Carré Spillway 2011, 2016, and 2019.

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The Bonnet Carré Spillway is the most frequently used spillway of the Mississippi River and Tributaries (MR&T) System located 51 km upstream of New Orleans. The structure was constructed in 1931 to redirect any flow that exceeds the design capacity of the downstream levee system. This design capacity of the Mississippi River (MR) levee system below the Bonnet Carré Spillway is of 35,000 m3/sec, which includes the levees and floodwalls through the City of New Orleans. The area of the Spillway is approximately 28 km2 and was able to pass 8,900 m3/sec during the 2011 flood (open 42 days). During the 2016 (23 d) and 2019 (78 d) flood operations the Spillway passed a maximum discharge of 6,000 m3/sec. Sediment deposition during the 2011 flood was partially calculated by Nitrouer et al. (2012). In 2016 and 2019, before Spillway opening, we established several artificial marker horizons to measure sediment accretion and performed bathymetric surveys on several of the lakes across the Spillway. Discharge into the Spillway during each flood was calculated using a weir calculation by USACE for all studied floods. During the 2011, 2016, and 2019 openings, nutrient concentration samples were collected along a highway crossing midway down the Spillway. Suspended Sediment load was measured in 2011 and calculated in 2016 and 2019. After the Spillway drained in 2016 and 2019, accretion was measured above the artificial horizons and a greater number of natural horizons (grass roots, leaf pack, willow roots). Samples of new deposition were collected and analyzed for mass, particle size, carbon, nitrogen, and phosphorous concentrations. Bathymetry of the lakes surveyed before opening were resurveyed after closure in 2016. Depositional volume was calculated for those lakes and a ratio was developed for lake deposition relative to the surrounding spillway deposition. Sediment and nutrient concentrations in the MR were determined from samples taken by USGS during the floods in the MR at Belle Chase, LA and compared with the sediment load during the studied openings. During the Spillway operation, large volumes of river water (21.5 km3, 2.91 km3, 38.0 km3) and masses of sediment flowed through the spillway from suspended river material [(3.48 teragrams (Tg), 1.95 Tg, 5.35 Tg, respectively)]. However, sediment deposition exceeded the suspended load in 2011 (6.41 Tg using 2019 bulk density) and 2019 (5.95 Tg), while 2016 deposition was 1.17 Tg. Mean depth of accretion on the Spillway ranged from 28.5 mm in 2016 to 286 mm in 2011. These high trapping rates in combination with the particle size distribution indicate large contributions of bed material to Spillway sediment load and deposition during the 2011 and 19 openings. Deposition during the 2016 opening was 40% lower than the difference between river load input and the mid-spillway measurement location load. Measurements of deposition from calculated load at the mid-spillway surface water monitoring location to Lake Pontchartrain indicated a sediment export of 0.8 Tg in 2016 and 3.13 Tg in 2019. Nutrient deposition on the Spillway was substantial with 0.986 gigagrams (Gg) N, 23.1 Gg C, 0.662 Gg P in 2016 and 4.90 Gg N, 75.6 Gg C, 3.42 Gg P in 2019. The 2011 and 2019 deposition + load equated to 248% and 170% of river load input respectively, while the total 2016 measured deposition + exported load was 104% of the river load. The mass balance for deposition in relation to input and output loads indicate a bedload input of 5.14 Tg, 0.08 Tg, and 3.73 Tg respectively. These data suggest that both greater discharges through and longer openings of the Spillway are more effective at entraining riverbed sediment through the Spillway structure that leads to greater floodplain deposition. These data provide depositional properties and curves for the operation of diversions designed to promote land area growth in subsiding deltas.