Determination of erosion risk level at the downstream of Kelantan river

Abstract

Riverbank erosion is an undesired phenomenon and was always given a special attention by engineers and authorities in the past. Human activities are believed accelerated the process of erosion at either terrestrial area or in the river system. The hydraulic force will prompt toe erosion at the water edge while percentage of soil composition always induced potential mass wasting at the upper riverbank section. The second category of erosion was given special attention in this study. The aftermath of 2014 catastrophe flood in Kelantan initiated gradual erosion along the Kelantan River network. It is believed that the vegetative cover at the riverbank section was washed away by the flooding event. The exposure of bare soil is susceptible to have erosion should the rainfall erosivity exceeds the capacity of soil to withstand the interlocking texture. Therefore, this study was performed to assess the potential risk of riverbank erosion at the area where no vegetative cover was detected. The study area was focused on the downstream part of Sg Kelantan River basin due to dense of population and active socio-economic activities. The hand Auger was deployed to extract the soil composition along the riverbank section. The device was hammered at an approximate 50 mm- 1 meter beneath the ground surface to sample the undisturbed soil. The soils were brought to the lab for sieving and hydrometer analysis. The main purpose laboratory work is to obtain a complete grain size distribution. The final grain size distribution was mapped against the RON index to obtain the potential risk of either "low", "moderate", "high", "very high"or "critical"erosion. The output of this study revealed that point number 3 (N 6°11'50", E 102°14'5") and point number 4 (N 6°12'22", E 102°14'5") are susceptible to have critical erosion risk level based on soil composition. These two points were located downstream from Sungai Kelantan and near with the residential area. It is expected that high rainfall erosivity will induce severe riverbank erosion at these two points.