Gully Development in Loess Hilly Area of Gorgan, Northeast of Iran

摘要

Gully erosion is one of the most important kinds of soil erosion that causes soil degradation. Also sediment yield of eroded gully causes many problems in downstream areas. Loess is" one of the erodible soils that have good distribution in Golestan province. Because of the erodibility of these soils all of kinds of erosion can be seen in loess area. Also because of enough depth of soil gully erosion is common over these soils. For studying of gully development trend in loess hilly area of Gorgan, Golestan province three typical gully selected with 40 cm-50 cm depth, 90 cm-110 cm width and 300 cm-500 cm length. The slop of area was 12%. Average of annual rainfall is 700 mm. To measure the surface area and changes from cutting back or bank collapse a rectangular grid of erosion pin was set out at an appropriate interval. After each rainfall the dimensions of gully measured again. From 43 rainfall event in one-year study only 6 of them had measurable changes in gullies. In each rainfall event intensity, depth and time of rainfall data was collected from Hashem abad station in 3 km far from study area. The results of this study showed that after each rainfall the amount of degraded soil was between .0.1 m~3 - 0.5 m~3. The total of soil eroded in one-year study was about 0.6 m~3 for each gully. The amount of gully development from head and two sides in one year was 10 and 7 cm respectively. Statistical analysis showed that volume of eroded soil has significant relationship with depth of rainfall. This survey also showed that depth of gully after each rainfall doesn't increase necessarily and usually because of head cutting and side bank collapse, depth of gully decrease. But after washing and transporting of eroded soil depth of gully increase again. It must be mentioned that depth of rainfall and volume of runoff and also duration of rainfall has important role in cycle of increasing and decreasing of gully depth.