Characterizations and modeling of turbidity in a water supply reservoir following an extreme runoff event

Abstract

The findings from an integrated program of short- and long-term monitoring, individual particle analyses (IPA), and mechanistic modeling to characterize and simulate the turbidity (Tn) effects of an extreme runoff event (2011) on a water supply reservoir were documented. A robotic profiling platform and rapid profiling instrumentation resolved turbidity and temperature (T) patterns in time and space in the reservoir. Metalimnetic enrichment in Tn following the event was reported and attributed to the entry of turbid stream water as density currents, or plunging inflows. The diminishment of high Tn levels following the event was well represented by a first-order loss rate of about 0.023 d−1. The highest Tn levels were avoided in water withdrawn for the water supply following the event by selection of vertical intake alternatives, although Tn values in the withdrawal remained distinctly above typical baseline conditions for nearly 2 months. Based on IPA, the Tn-causing particles were mostly clay minerals in the 1–20 μm size range. The operation of sorting processes determining settling losses from the minerogenic particle population, according to their size and shape, following the runoff event was resolved. The set-up and testing of a mechanistic Tn model, composed of 2 submodels, a 2-dimensional hydrothermal/transport submodel, and a Tn kinetics submodel, is described. The hydrothermal/transport submodel was tested separately and performed well in simulating the dynamics of the reservoir’s stratification regime and the entry of the dense streams as plunging inflows during the extreme runoff event. The overall Tn model needed to represent the loss processes of both settling and coagulation to perform well in simulating the in-reservoir and withdrawal Tn patterns following the runoff event.