Investigation of Pangkalan Floods: Possible Reasons and Future Directions

Abstract

Two flood events in Pangkalan, Lima Puluh Kota Regency on 3 March and 29 December 2017 have triggered some questions whether a dam downstream the catchment is solely responsible, or they were caused by other factors. In fact, several other major floods have been recorded after dam commission in 1998. Yet, the catchment have undergone rapid land use conversion into commercial plantation such as palm and Uncaria gambir Roxb (gambir) since mid-1990’s. To understand the flood, three contributing factors i.e. flood inundation by the dam and river tributaries, land use changes, and rainfall data were discussed and analysed, and some field works were conducted along the main river and flooded areas. Flood inundation was estimated using Digital Elevation Models (DEM) of 30 m in resolution and modeled using QGIS. Land use changes from year 1994 were analysed from Landsat and EVI (Enhanced Vegetation Index) time series. Rainfall data were collected and analysed using nearby station from year 1980. Flood inundation analysis showed that dam spillway at an elevation of 80 m is unlikely the main reason, since maximum flood elevation occurred at 92 m. Rather, the flood was likely induced by a constriction due to rock in the river at midstream catchment, since flood inundation model using this scenario exhibited similarities to the impacted areas. Furthermore, land conversion from forest to palm plantation and gambir was thought to increase runoff and also contributed to the floods. Yet, this manuscript needs to be followed by a model with reliable data such as river topography and high resolution DEM and automatic water level recorder (AWLR).