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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Technical note
23 Jun 2016
Review status
A revision of this discussion paper was accepted for the journal Hydrology and Earth System Sciences (HESS) and is expected to appear here in due course.
Technical Note: Monitoring of unsteady open channel flows using continuous slope-area method
Kyutae Lee1, Ali R. Firoozfar2, and Marian Muste2 1Environmental Sciences Division, Oak Ridge National Laboratory, Tennessee, USA
2IIHR – Hydroscience & Engineering, Univ. of Iowa, Iowa, USA
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 slope, resulting in higher discharges than steady-based discharges, while the trends are reversed in the falling stage. The CSA method is demonstrated on Clear Creek near Oxford (Iowa, USA), where it shows that CSA-based discharges deviate from the United States Geological Survey (USGS) steady-based discharges by 10 % or less. The degree of hysteresis is also shown to vary depending on event scale (e.g., magnitude of unsteady forces) and vegetation condition. This evidence confirms that the CSA method has promising capabilities for dynamically tracking unsteady flows in natural streams. However, the use of a single channel bed slope (conceptually equal to the water surface slopes at every stage in uniform flow conditions) is not adequate in estimating the channel roughness, because non-uniformities of natural channels result in varying steady slopes at each stage. Therefore, "steady non-uniform slopes", which represent steady water surface slopes changing with stage, are considered. It is assumed that they can be estimated by simply averaging a pair of measured unsteady water surface slopes corresponding to the rising and falling limbs at the same stage, and the differences between steady non-uniform slopes and unsteady slopes can be considered as the effects of flow unsteadiness. The stage-dependent channel roughness values are approximated using these steady non-uniform slopes. While this approach is subject to validation in future research, the results are quite promising in this study.

Citation: Lee, K., Firoozfar, A. R., and Muste, M.: Technical Note: Monitoring of unsteady open channel flows using continuous slope-area method, Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-289, in review, 2016.
Kyutae Lee et al.
Kyutae Lee et al.


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Short summary
Accurate estimation of stream/river flows is important for the public safety during floods as well as for the efficient use of limited water resources for hydropower generation and irrigation. In this paper, we have investigated a feasibility of the continuous slope area (CSA) method which utilizes the measurements of instantaneous water surface elevation changes, and the results showed promising capabilities of the suggested method for the accurate estimation of flows in natural streams/rivers.
Accurate estimation of stream/river flows is important for the public safety during floods as...