Slope‐area Discharge Gaging in Mountain Rivers

Tests of the slope‐area gaging technique at 16 sites in gravel and boulder‐bed rivers with slopes of 0.4–4% in upland Britain show that the field measurements must include detailed surveys of bed cross section and transverse water surface profile and that the major source of error in predicted discharge is the evaluation of the flow resistance coefficient. For the 44 data points errors in predicted discharge compared with measured discharge are $-19\%$ on average but range as high as $-60\%\text{.}$ Surveying errors are shown to account for errors of about $\pm 9\%$ but the major source is the over‐estimation of the flow resistance coefficient. Using an empirical resistance equation the range of error can be reduced to about $\pm 25\%$ to $\pm 30\%\text{,}$ but further research is needed in this area. It is found that inclusion of the velocity head gradient in the method may produce misleading results and tests show that representation of mean water surface elevation at a section by elevations measured at the bank only can cause errors comparable with those arising in the evaluation of flow resistance.