Channel Friction Coefficient Method

This indirect method, also referred to as the slope-area method, consists of measuring flow depth at a suitable cross-section and substituting the measured depth into an equation for uniform flow (such as the Manning equation) or critical flow. To complete the calculation, one must estimate the friction coefficient of the channel where the flow is to be measured, and know the channel slope and geometry.

The inference of flow rates from measured depths of flow is a rather inaccurate procedure. The main sources of error arise from the lack of uniformity and steadiness of flow, and the lack of certainty in estimating the friction coefficient.

Improved accuracy can be achieved by performing calibration in place, and developing an empirical rating curve for each measuring cross-section. In this case, the channel discharge (Q) is measured, generally by current meters, for various depths of flow, and the cross-section rating curve (Q vs depth of flow) is developed. This curve is then used to convert the observed stage to discharge.

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