Abstract
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.
Highlights
A conventional slope area method has been used as a means of estimating peak discharges based on high watermarks after large flood events, as those events are commonly rare and pose a measurement challenge in obtaining reliable sample data
The water surface slopes are computed using the measured water surface elevations at each pressure transducer and the distance measured along the centerline of the channel between them
This study provides evidence of the continuous slope area (CSA) method’s capabilities for dynamically tracking unsteady water surface slopes in natural streams, while it still needs more studies for successful use of the method
Summary
A conventional slope area method has been used as a means of estimating peak discharges based on high watermarks after large flood events, as those events are commonly rare and pose a measurement challenge in obtaining reliable sample data. A continuous slope area (CSA) method is the approach that utilizes the same physical principle of the conventional slope area method; it uses at a minimum a pair of upstream and downstream water level sensors to continuously measure drops in water surface elevations instead of high watermarks. This enables hydrologists to continuously estimate stream/river discharges throughout a hydrologic event.
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