Spatial heterogeneity of debris-flow watershed

<p>The intermittent surge is the basic manifestation of viscous debris flow, which emerges universally over the world, especially exemplified by those in Jiangjia Gully (JJG), a valley famous for its high frequency and variety of debris flow surges. It has been found that the surges originate from various sources in the watershed, thus identifying the source areas plays a fundamental role in studying the mechanism and process of surge developing. Advancement of GIS provides an apparent convenience in geospatial analysis of the watershed, which is used as a dominate tool in this paper.</p><p>In this study the JJG is divided into 97 tributaries (sub-watershed) and the hypsometric analysis is performed for each, from which derive the height of inflection points and the gravitational potential energy, coupled with the fitted parameters of specific power function. Then the morphology parameters, including slope, roundness, vegetation and soil, are revealed in tributaries. Besides, spatial autocorrelation among tributaries is quantified both globally and locally through Moran’s I and Getis-Ord G<sub>i</sub>*, so that the HI spatial distributions are quantified and visualized. In particular, hot spots are conspicuously visible and highlight the geologic meaning of the HI when exploratory spatial data analysis is applied to the data distributions through local indices of spatial autocorrelation.</p><p>The results show that H-curves approximately present as S-shaped, and the integral values (HI) range from 0.18 to 0.69 and show positive relationship with both gravitational potential energy and the height of the inflection points. By the HI value, the tributaries are identified as in 5 phases of evolution. The younger tributaries (HI>0.49) make up the majority, which are expected to be the main possible sources for debris flows. Additionally, the slope distribution of tributaries all conform to the extreme distribution while the curves for the upstream, where the HI of tributaries generally manifest higher coupled with larger roundness, tends to skew to the right.</p><p>Finally the correlation between possible sources are explored through geospatial analysis. The spatial association in JJG provides an explanation of the debris flow source areas. Global spatial autocorrelation manifests significantly clustered (Moran’s I shows 0.449, passing the significance test) while tributaries with high HI value concentrate mainly in the Menqian Valley. Moreover, the drainage form of Menqian Valley represents a large possibility of debris flow source area with the respect of that being in Duozhao Valley.</p><p><strong>Keywords: </strong>debris flow source area; hypsometric analysis; topographical characteristics; spatial autocorrelation; evolutionary phases</p>

χ-maps and Hypsometric Analysis for River Basin Management and Prioritization: The Case of Bohol River Basins, Central Philippines

Topographic indices represent and simplify complex surficial processes fundamental to characterizing landform dynamics and to prioritizing river basin conservation and management goals. A method of characterizing landform dynamics integrating the steady-state river channel elevation and the hypsometric analysis is presented. The chi (χ) metric, a proxy for the steady-state river channel elevation, gages the stability of drainage divides while hypsometric analysis quantifies the stages of basin geological development. Using a 30m SRTM DEM and the TopoToolbox tool in MATLAB, χ - values along stream networks are computed. At the channel heads of opposing stream networks of a divide section, equal χ – values indicate a stable divide while across difference in χ - values suggest unstable divide with a potential to migrate from low χ – values towards the high χ – values side of the divide. To classify the degree of potential divide mobility, the quantity called mean chi difference (χmd) is proposed. The features of the aggressive and victim river streams are visually differentiated using their elevation profiles, map-view arrangement and pathways to discharge points. Hypsometric analysis examines the erosional stages of basins indicated by the hypsometric integrals (HI) and hypsometric curves. A basin and its subbasins show different levels of geologic development that the disaggregation of large basin into small hydrologic units enables the identification of areas of different erosional stages. The prioritization of subbasins considers the intersection of highly mobile divides and highly erosional areas. Over the study area, nine subbasins are identified which are all located at the headwaters of major basins in the island. A considerable earthquake-triggered landslide has been found in one of the identified subbasins. The study presents a new approach in the initial characterization of landforms in order to facilitate the identification and prioritization of highly erodible areas for high consideration especially at the local or village level.

Assessment of Soil Erosion in Siruvani Watersheds Based on USLE and Hypsometric Curve Methods

Soil erosion poses a serious threat over the maintenance activities of a reservoir and its watershed. This study has been taken us to assess the extent of soil erosion in the watersheds of the Siruvani Reservoir located in the district of Kerala. Two methods namely, Universal Soil Loss equation (USLE) and Hypsometric curve methods are adapted in this study. Hypsometry of watersheds (area-elevation analysis) has generally been used to reveal the stages of geomorphic development (stabilized, mature and young). The watersheds of Siruvani Reservoir were delineated from the generated Digital Elevation Model (DEM) using Geographic Information System (GIS). Various set of data like land use map, rainfall and soil map have been used for this analysis to generate five factors namely finally, the soil erosion risk map was created to identify the regions which are susceptible to erosion. Hypsometric analysis deals with measurement of the interrelationships between basin area and altitude of basin which has been used to understand the influence of various factors such as climate, geology and tectonic changes. GIS provides advanced tools to obtain hypsometric information and also helps to estimate the associated parameters of landforms. The entire study area has been sub-divided into 5 watersheds for hypsometric analysis and their area ranges from 1.77 to 6.94 km2. The hypsometric curve of the whole the watersheds reflects the mature geomorphic terrain whereas hypsometric integral indicates that the drainage basin has already eroded per cent of land masses. These findings would emphasize the importance of soil and water conservation measures to be taken up in the Siruvani watersheds for controlling further erosion, reducing the sediment outflows and conserve water.

Assessment of Soil Erosion in Siruvani Watersheds based on USLE and Hypsometric Curve Methods

Soil erosion poses a serious threat over the maintenance activities of a reservoir and its watershed. This study has been taken us to assess the extent of soil erosion in the watersheds of the Siruvani Reservoir located in the district of Kerala. Two methods namely, Universal Soil Loss equation (USLE) and Hypsometric curve methods are adapted in this study. Hypsometry of watersheds (area-elevation analysis) has generally been used to reveal the stages of geomorphic development (stabilized, mature and young). The watersheds of Siruvani Reservoir were delineated from the generated Digital Elevation Model (DEM) using Geographic Information System (GIS). Various set of data like land use map, rainfall and soil map have been used for this analysis to generate five factors namely finally, the soil erosion risk map was created to identify the regions which are susceptible to erosion. Hypsometric analysis deals with measurement of the interrelationships between basin area and altitude of basin which has been used to understand the influence of various factors such as climate, geology and tectonic changes. GIS provides advanced tools to obtain hypsometric information and also helps to estimate the associated parameters of landforms. The entire study area has been sub-divided into 5 watersheds for hypsometric analysis and their area ranges from 1.77 to 6.94 km2. The hypsometric curve of the whole the watersheds reflects the mature geomorphic terrain whereas hypsometric integral indicates that the drainage basin has already eroded per cent of land masses. These findings would emphasize the importance of soil and water conservation measures to be taken up in the Siruvani watersheds for controlling further erosion, reducing the sediment outflows and conserve water