PIT AVULSIONS AND PLANFORM CHANGE ON A MINED RIVER FLOODPLAIN: TANGIPAHOA RIVER, LOUISIANA

Floodplain mining creates a vulnerable landscape comprised of pits, bare land and stock piles with minimal vegetation. During floods, when stages can exceed bank-full by several meters, all or part of the channel may divert into a pit, re-routing the flow and transforming the planform and profile of alluvial landscapes. Using geospatial data from the mined Tangipahoa River floodplain for 1980 to 2004, the objectives of this study are to (1) assess if pit characteristics such as proximity to channel and size affect the potential for diversion; and (2) interpret relationships between pit diversions, channel changes, and floods. Of the 56 pits mapped in the floodplain in 1980, six had been captured by 2004. Captured pits were closer to the main channel than those which were not captured, but the areas of captured and noncaptured pits were not significantly different. However, avulsions into large pits caused more channel planform change than those into smaller pits. Other findings are that point bar areas increased 72% overall, more so in the mined reach, and that degradation exceeded 6 m in the mined reach, in contrast with aggradation of 1 m roughly 20 valley km downstream. Results will be useful in understanding landscape vulnerability and can be applied to river management and restoration.