Sediment reduction effect of level terrace in the hilly-gully region in the Loess Plateau

The sediment reduction effect of level terrace in the Loess Plateau is likely to have been underestimated before. By analyzing the water and sediment regulation mechanism of the terraces in the basin,it is realized that the level terrace can not only significantly reduce the sediment yield from itself,but also intercept the sediment yield from the upper area,and can achieve the valley sediment reduction by reducing runoff flowing from slope to valley. The sediment reduction potential of level terrace with ridges in watershed is up to 65 %. Based on the measured data of the third and the fifth sub-district of the loess hilly region in different periods,the sediment reduction effect of different scale level terraces is analyzed under average annual rainfall condition. The concept of terrace ratio is introduced to build the relationship between terrace ratio and sediment reduction magnitude,which can be used to quantitatively evaluate sediment reduction effect of level terrace on a large spatial scale,When the terrace ratio is less than 30 %,the magnitude of sediment reduction is basically proportional to the terrace ratio;and when the terrace ratio is larger than 35 %,the sediment reduction effect is basically stable at about 90 %. The flood sediment concentration reduction effect of level terrace is not very obvious;this characteristic can be used to measure the water-reducing effect of level terrace in the river basin.