Technical Note: Monitoring of unsteady open channel flows using continuous slope-area method
Kyutae Lee1, Ali R. Firoozfar2, and Marian Muste21Environmental Sciences Division, Oak Ridge National Laboratory, Tennessee, USA 2IIHR – Hydroscience & Engineering, Univ. of Iowa, Iowa, USA
Received: 09 Jun 2016 – Accepted for review: 22 Jun 2016 – Discussion started: 23 Jun 2016
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.
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.