Sediment deficit and morphological changes of alluvial floodplain in wandering reach of the lower Yellow River after Xiaolangdi Reservoir operation

Alluvial floodplains are a prominent part of rivers and play a vital role in flood mitigation and sediment detention. In the sediment-laden lower Yellow River (LYR), the floodplains have lost the sediment supply and are experiencing competition between land development and protection since the start of the operation of the Xiaolangdi Reservoir (XLD) in 2000. The objective of this paper is to investigate the sediment distribution and morphological changes of the floodplains in a typical wandering reach of the LYR based on the evolution in measured elevations of cross-sectional topography. The results show that the floodplains have gradually shifted from net sediment deposition to deficit. The cumulative eroded volume since the full operation of the XLD in 2002 has been approximately 5.74 × 108 m3. The erosion rate was approximately 3.25 cm/year, and the most eroded sediment originated from the right floodplain. The sediment erosion results in the adjustment of the cross-sectional topography with abrupt vertical changes in the floodplains. Through discussion, the sediment deficit and morphological changes have mainly been as a result of no floods due to the operation of the XLD and the human use of floodplain sediment. The water–sediment regulation scheme (WSRS) of the XLD has resulted in a fully scoured river channel to enhance the flood discharge capacity and has effectively controlled the flood peak to reduce the inundated risk of floodplains. The floodplain has, therefore, lost its sediment supply. In addition, the intensity of land development and construction in the floodplain has increased for the hardly inundated risks, causing a larger sediment requirement to form abrupt vertical changes in the floodplain topography with artificial land features.