Articles | Volume 21, issue 3
Technical note
30 Mar 2017
Technical note |  | 30 Mar 2017

Technical Note: Monitoring of unsteady open channel flows using the continuous slope-area method

Kyutae Lee, Ali R. Firoozfar, and Marian Muste

Abstract. The advent of low-cost pressure transducers capable of directly measuring water surface elevation enables continuous measurements of dynamic water surface slopes. This opens up a new possibility of dynamically monitoring unsteady flows (i.e., hysteresis) during the course of flood wave propagation. Hysteresis in this context refers to a looped stage–discharge rating caused by unsteadiness of flows. Hysteresis is monitored in this study using a continuous slope area (CSA) method, which uses Manning's equation to calculate unsteady discharges based on continuously measured water surface slopes. In the rising stage, water surface slopes become steeper than a steady water surface slope, resulting in higher discharges than steady-based discharges, while the trends are reversed in the falling stage. The CSA method applied to Clear Creek near Oxford (Iowa, USA) estimates the maximum differences of peak discharges by 30–40 %, while it shows sound agreements for a low to medium range of discharges against USGS steady-based records. The primary cause of these differences is the use of a single channel bed slope in deriving Manning's roughness coefficients. The use of a single channel bed slope (conceptually equal to the water surface slopes at every stage in uniform flow conditions) causes substantial errors in estimating the channel roughness, specifically at high stages, because non-uniformities of natural channels result in varying (non-uniform) steady water surface slopes at each stage. While the CSA method is promising for dynamically tracking unsteady water surface slopes and flows in natural streams, more studies are still needed to increase the accuracy of the CSA method in future research.

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Short summary
Accurate estimation of stream/river flows is important in many aspects, including public safety during floods, effective uses of water resources for hydropower generation and irrigation, and environments. In this paper, we investigated a feasibility of the continuous slope area (CSA) method which measures dynamic changes in instantaneous water surface elevations, and the results showed promising capabilities of the suggested method for the accurate estimation of flows in natural streams/rivers.