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Levee failure during the 1993 Missouri River flood caused discharges with large hydraulic heads to flow through constricted openings. These discharges produced deep, extensive scours and deposited large quantities of sediment on the Missouri River floodplain. Six representative sites were selected to study the effects of levee breaks on floodplain scour and sedimentation. Emphasis was placed on determining whether these sites were net sinks or sources for flood sediment and on documenting particle-size and soil-chemistry characteristics of the sediment. Four of the sites have scours that remain connected to the Missouri River during low flow, whereas two sites have unconnected scours. Scour volumes ranged from 150 to 720 acre-feet at the connected-scour sites and were 94 acre-feet or less at the unconnected-scour sites. Scour volumes at depths below the pre-flood elevation of the floodplain ranged from 89 to 95 percent of the total scour volumes at the connectedscour sites and were 65 and 89 percent of the total scour volume at the unconnected-scour sites. The maximum scour depths ranged from 20 to 51 feet below the average pre-flood elevation of the floodplain. The net sediment volumes (total sediment deposited during the 1993 flood minus the scour volume) ranged from -340 to +4,200 acre-feet at the connected-scour sites and were less than 20 acre-feet at the unconnected-scour sites. Deposits thicker than 1 foot consisted mostly of sand. The areas covered with 2 feet or more of sand ranged from 2.3 to 840 acres at the connected-scour sites and were less than 35 acres at the unconnectedscour sites. Sediment volume ranged from 26 to 680 percent of scour volume at the connectedscour sites and from 117 to 162 percent of scour volume at the unconnected-scour sites. Ratios of deposition to erosion at connected-scour sites indicate that some of the sites were net sources for sediment in transport by the flood, whereas others were net sinks. The ratios at the unconnectedscour sites indicate that the volume of sediment deposited downstream from the scours is nearly equal to the volume of the scours. However, flood-sediment deposits are coarser than much of the scoured sediment, so these estimates represent minimum total fluxes of sediment onto the floodplain. The potential significance of connectedscour levee-break sites as sinks for transported flood sediment is exemplified by a site 5.5 river miles downstream from Hermann, Missouri. The net mass of flood sediments (7.7-12.6 million tons) deposited on the floodplain was estimated to be 10 to 16 percent of the total sediment load transported by the Missouri River past Hermann during the 1993 flood. In contrast, a connectedscour levee-break site near Arrow Rock, Missouri, had a sediment volume that was only 51 percent of the scour. The net loss of sediment from this site may be related to local flow hydraulics or increased sediment transport capacity of

Scour, sedimentation, and sediment characteristics at six levee-break sites in Missouri from the 1993 Missouri River Flood