HOMS COMPONENT

A GUIDE TO SLOPE-AREA DISCHARGE GAUGING IN MOUNTAIN RIVERS

1.   Purpose and objectives

     The slope-area technique is a method of indirectly estimating the peak discharge of a flood wave after the flood has passed. The technique is here adapted for use in high-gradient mountain rivers (with slopes of typically 0.1-5%) and in countries where equipment and trained manpower are scarce.

2.   Description

     The slope-area technique assumes that flow depth and velocity for a given discharge are determined by channel cross-sectional shape, channel slope and boundary roughness, and that flood peak conditions are indicated by the high-water elevation, recorded either by trash lines or by suitable instrumentation. Peak discharge is then calculated from an equation involving channel characteristics, peak flow parameters and a flow resistance coefficient calculated, by an empirical equation developed for mountain rivers, from the ratio of depth to bed material size.

     Criteria governing selection of site for the field measurements are listed in the guide. Field data are collected after the passage of the flood. Channel shape and slope are obtained by surveying and the bed material size distribution is obtained by a grid sampling technique.  A step-by-step description of the office procedure for calculating the discharge is given, with a worked example.

     The procedure can be to some extent automated and simplified, using maximum stage posts to record high-water elevations, charts to give cross-sectional data and a pocket calculator to carry out the calculations.

3.   Input

     Field data: surveyed cross sections, including high-water elevations; distances between cross sections; sampled bed material sizes; and sketch map of fieldsite.

4.   Output

     Flow discharge for a specified water surface profile. Additional results can include flow parameters (mean depth, cross-sectional area and velocity) and bed material size distribution.

5.   Operational requirements and restrictions

     The fieldwork requires the services of a trained surveyor.  The office calculations are relatively simple and can be carried out using a hand calculator.  The automated procedure is designed for use by staff with limited mathematical ability and understanding of flow processes but its initial preparation requires the services of trained personnel.

     The method is limited to in-bank flows for channels with predominantly coarse bed materials (i.e. gravel and boulder-bed channels, not sand-bed channels). It does not allow for flow through vegetation and care should be taken in application to compound shape channels. The method is approximate and may be in error by up to 30% or more in the conditions of mountain rivers.

6.   Form of presentation

     A paper in English, with French and German abstracts, describing the equations, choice of site, field procedure, office calculations and automated technique, with a worked example.

7.   Operational experience

     The basic slope-area technique is well described in the engineering literature; the modifications for mountain rivers described here are based on development work in Scotland and test applications in the Yemen Arab Republic and in Indonesia.

8.   Originator and technical support

     Originator of modified technique: Dr. J.C. Bathurst, Institute of Hydrology. Technical support: Centre for Ecology and Hydrology, Wallingford, formerly the Institute of Hydrology, UK.

     Development work funded by Overseas Development Administration, UK.

9.   Availability

     From the HOMS National Reference Centre for UK.

10.  Conditions on use

     None

Last updated: 23 DEC 88)
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