Regional debris flow susceptibility analysis in mountainous peri-urban areas through morphometric and land cover indicators

Abstract

Abstract.A method for assessing regional debris flow susceptibility at the watershed scale, based on an index composed of a morphometric indicator and a land cover indicator, is proposed and applied in 106 peri-urban mountainous watersheds in Bogotá, Colombia. The indicator of debris flow susceptibility is obtained from readily available information common to most peri-urban mountainous areas and can be used to prioritise watersheds that can subsequently be subjected to detailed hazard analysis. Susceptibility is considered to increase with flashiness and the possibility of debris flows occurring. Morphological variables recognised in the literature to significantly influence flashiness and occurrence of debris flows are used to construct the morphometric indicator by applying principal component analysis. Subsequently, this indicator is compared with the results of debris flow propagation to assess its capacity in identifying the morphological conditions of a watershed that make it able to transport debris flows. Propagation of debris flows was carried out using the Modified Single Flow Direction algorithm, following identification of source areas by applying thresholds identified in the slope-area curve of the watersheds. Results show that the morphometric variables can be grouped into four indicators: size, shape, hypsometry and (potential) energy, with energy being the component that best explains the capability of a watershed to transport debris flows. However, the morphometric indicator was found to not sufficiently explain the records of past floods in the study area. Combining the morphometric indicator with land cover indicators improved the agreement and provided a more reliable assessment of debris flow susceptibility in the study area. The analysis shows that, even if morphometric parameters identify a high disposition to the occurrence of debris flow, improving land cover can reduce the susceptibility. However, if favourable morphometric conditions are present but deterioration of the land cover in the watershed takes place, then the susceptibility to debris flow events increases. The indicator of debris flow susceptibility is useful in the identification of flood type, which is a crucial step in flood risk assessment especially in mountainous environments, and it can be used as input for prioritisation of flood risk management strategies at regional level and for the prioritisation and identification of detailed flood hazard analysis. The indicator is regional in scope, and therefore it is not intended to constitute a detailed assessment but to highlight watersheds that could potentially be more susceptible to damaging floods than others in the same region.

A spatial multicriteria prioritizing approach for geohydrological risk mitigation planning in small and densely urbanized Mediterranean basins

“…o Ruggedness number: is related to flash flood potential and high erosion 232 rate (Patton and Baker, 1976). o Hypsometric integral: it is correlated to the stage of geomorphic 234 development of the catchment, is an indicator of the erosional stage and is related to several geometric and hydrological properties such as flood 236 plain area and potential surface storage (Rogelis and Werner, 2014 (Howard, 1990;Rakesh et al, 2000 Considering the percentage on the catchment surface for the floodable area (set 1) and for 262 the area exposed to risk level R1-R4 (set 3) is similar to weighting with the catchment extension. Surface area, then, is implicitly part of the process of computation.…”

Flash Flood warning in context: combining local knowledge and large-scale hydro-meteorological patterns

“…For each of the 12 communities, hydrological catchments are delineated, using the global SRTM 90m Digital Elevation Model (DEM) v4.1 (Jarvis et al, 2008). Geomorphological indicators related to surface and morphometric characteristics known to characterize flash flood risk are identified (Bajabaa, et al, 2013, Farhan, et al, 2016, Rogelis and Werner, 2014, with some linked to the local knowledge (such as indicators related to slope, soil type, land or vegetation cover). The identified indicators and references are described in Table 1.…”

“…Some studies use equal weighting (Zogg & Deitsch, 2013), or weighting based on the indicator ranking (Karmokar & De, 2020). Here we apply a weighted method based on Principal Component Analysis (PCA) to reduce the dimensions in each class (Chao & Wu, 2017, Rogelis & Werner, 2014. Based on the four Principal Component results, a ranking of flash flood susceptibility of the 12 catchments is calculated, following Eq.…”

Modelling of the hydrological connectivity changes in the Minjiang Upstream after the Wenchuan earthquake using satellite remote sensing and DEM data

“…Mass wasting processes in mountainous terrain are not only dependent on the overall terrain, but also on the spatial organization and the internal connectivity of various topographic units (Borselli et al, 2008;Blahut et al, 2010;Croke et al, 2005;Rogelis et al, 2014). For a given point, mass wasting processes depend on its size of upslope source zone and position in the catchment (Fig.…”