Overview: zero-order basins and problems of drainage density, sediment transport and hillslope morphology

Abstract

Overview: basins and problems of drainage density, sediment transport and hillslope morphology W.E. DIETRICH, S.L. RENEAU AND CJ. WILSON Department of Geology and Geophysics University of California, Berkeley, CA 94720, USA ABSTRACT Soil-mantled hillslopes are divided into a network of hollows. Analogous to channel networks, hollows can be assigned a magnitude based on the number of unbranched tributary hollows they have. The spacing between the smallest, magnitude one, hollows may be determined in part by the minimum size required for a thickened colluvial deposit to overcome lateral root strength resis­tance. The source area size above channel heads varies inversely with hollow gra­dient, except where groundwater flow is strongly forced to the surface by bedrock inhomogenekies. Deposits in hollows record long periods of erosion from side slopes and noses, and dating of these deposits provides a means to estimate local sediment transport rates. Surveys of nose to hollow slope profiles may allow esti­mate of the diffusion coefficient in a slope dependent transport equation. Hollows appear to possess a characteristic three-dimensional geometry, the properties of which are not well-established. INTRODUCTION Most soil-mantled hillslopes in humid to semiarid landscapes are finely divided into small unchanneled valleys which because they focus runoff and sediment transport are the dominant sites of saturation overland flow, periodic gullying and debris flow generation. To some, the current interest in these valleys, most often referred to as hollows and zero-order basins, is a fad that will soon pass. They would argue that much of the work is very descriptive, is overly concerned with details of small areas of hillslope or, in fact, is a reanalysis of earlier, nearly forgotten ideas. The abundant terminology proposed to describe these features and the collu­vial deposits that collect in them, including swale, hollow, hopper, dale, dell, zero-order basin, headwall, cove, rampa, wedge and colluvial-filled ravine, adds to this perception; it certainly adds to the confusion about what we are in fact studying. We argue here that research on unchanneled valleys can provide the key to many outstand­ing problems in hillslope geomorphology. Not only does analysis of these features provide a way of looking at hillslopes and conceptualizing the relationships between diverse hydrologie and geomorphic processes, quantitative observations can provide unique data on rates of processes and the relationship of process to form. The papers in this basin ses­sion represent the new directions that research in the future will head. Previous literature on this topic is reviewed in each of the contributed papers of this session. In this overview we relate the research reported in this session to the basic problems of controls on drainage den­sity, rates of sediment transport on hillslopes and to three-dimensional hillslope morphology. DRAINAGE DENSITY Hollow network Drainage density is widely argued to be the prime determinant of the overall morphology of the landscape (e.g., Chorley et al, 1984). Although many correlations have been attempted, studies of controls on drainage density in semi-arid to humid landscapes have been limited by reliance on interpretation of topographic maps or aerial photographs in which the channel head canoot be directly determined, and by a lack of field observations about processes at and 27