Calculating geomorphic indices in SE Asia using a SRTM derived DEM: a worked example from West Sulawesi, Indonesia

Abstract

Geomorphic indices like stream length gradient index (SL), mountain front sinuosity (Smf) and ratio of valley floor width to valley height (Vf) have previously been used to detect active tectonic features from observations of first order rivers and regional fault systems in areas where accurate topographic maps were available. However, areas in equatorial South East Asia present many problems, being remote, covered by dense forest and lacking full, detailed topographic surveys. Here a new approach is attempted, using results coming from the comparison of SRTM derived DEM's (SRTM-DEM) to traditional topographic 1:50,000 survey derived DEM's (TS-DEM). Neo-tectonic activity on the Palu-Koro fault in West Sulawesi was assessed using geomorphic indices calculated for major river systems (3rd up to 7th strahler order) derived from SRTM-DEM. Mountain front sinuosity indices are similar whether derived from SRTM-DEM or TS-DEM. Likewise, although the ranges differ, valley floor width to valley height ratios returns the same average value. The stream length index was influenced by systematic sensor errors in areas of extreme local relief on SRTM-DEM. A visual classification of river deflections and water gaps was performed in order to identify the best locations for wider application of geomorphic indices on the SRTM-DEM. While geomorphic calculations on SRTM-DEM's break down for flood plain valleys, yielding erroneous results, geomorphic indices may be successfully used to identify secondary fault systems in other parts of West Sulawesi.