Evaluation of the Slope-area Method for Continuous Streamflow Monitoring

Low-cost pressure transducers equipped with capabilities to transmit the data in real time enable to conveniently measure the water surface elevations at remote locations. If a pair a sensors is set up at two close locations on a reach of quasi-uniform flow, the water surface slope on that reach can be derived at fast sampling rates. This arrangement opens up the possibility to dynamically monitor both steady and unsteady flows. Especial useful is the potential to capture the hysteresis, a phenomen inherent to flood wave propagation generated by any precipitation occurring in the stream drainage area. This phenomenon is typically ignored by the stage-area method but better captured by the index-velocity method. These two conventional methods for continuously monitoring streamflows require, however, development of rating curves a lengthy data collection program.

The alternative monitoring method discussed in this paper is the continuous slope area (CSA) method, a simplified version of the slope-area method typically used to estimate discharges for peak flows occurring during floods. The estimation is based on Manning’s equation with the geometry of the cross section known from a local survey, assumed Manning’s coefficient, and the free-surface slope estimated using surveys of the high-water marks left after the passage of large flows. The CSA method uses a pair of water level sensors to continuously measure the fall in water surface elevations instead of high-water marks. Successful implementation is highly dependent on the selection of a suitable site and reliable estimation of the Manning coefficient.

The CSA method is implemented on a small stream (Clear Creek, Iowa, USA). It is shown that for steady flow conditions the CSA method performs well compared to the estimates provided by a stage-discharge United States Geological Survey (USGS) gaging station collocated with the experimental site. The implementation of the CSA method in unsteady flows displays loops in the stage-discharge relationship even if the stream is small and the storm event producing the propagating wave is an ubiquitous rain occurring in the drainage area of the station. To the knowledge of these authors, it is for the first time when the phasing between the peaks of the flow variables (i.e., free-surface slope, bulk velocity, and stage) during unsteady flows is captured through field experiments. The experimental evidence provided by this paper confirms that the CSA method has promising capabilities for estimating steady flows as well as dynamically tracking unsteady flows in natural streams.