Effect of polymer additives on dynamics of water level in an open channel

TL;DR

This study investigates how polymer additives affect water levels in open channels, revealing a counterintuitive rise downstream despite initial height decrease, and proposes mitigation strategies based on numerical simulations.

Contribution

It introduces a numerical model incorporating polymer dynamics and turbulence to explain water height variations caused by polymer additives in open channels.

Findings

Polymer addition initially reduces water height due to decreased friction.

Downstream water height can increase unexpectedly, matching experimental results.

A mitigation technique is suggested to prevent water rise in practical applications.

Abstract

The presence of a tiny amount of polymers (a few parts per million) in a fluid dramatically reduces turbulent drag. For this reason, polymer additives have been proposed to be used in flood remediation: in an open channel at a fixed flow rate, the decrease in friction due to polymer addition is expected to lead to a decrease in water height in the channel. However, in a recent field experiment, a counterintuitive transient increase in water height has been observed far downstream of polymer injection. We numerically investigate the effect of polymer additives on the water height in a long canal using the shallow water equations augmented with an evolution equation for polymer concentration that incorporates turbulent dispersion and polymer degradation. Just downstream of polymer injection, the water height decreases due to the decreased friction at a fixed volumetric flow rate. Further downstream, however, the height can increase, consistent with the experimental observation. We elucidate the mechanism of this unexpected rise in water height in the canal. We also suggest a technique to mitigate the water rise, as this is detrimental for practical applications.