Modeling Turbidity in a Water Supply Reservoir: Advancements and Issues

The development and testing of a “turbidity” model is documented for a water supply reservoir, Schoharie Reservoir, NY, where inorganic terrigenous particles received during runoff events in turbid density currents from the primary tributary cause distinct periodic degradation. The model state variables are fractions (two or three) of the beam attenuation coefficient at $660\mathrm{nm}$ $\left({\mathbf{c}}_{660}\right)$ , a surrogate optical metric of turbidity. The fractions of ${\mathbf{c}}_{660}$ correspond to slow and rapidly settling components; the latter implicitly accommodates particle aggregation. The transport framework is a two-dimensional (laterally averaged), independently tested, hydrodynamic model. Model testing is supported by detailed measurements of the dynamics of tributary and meteorological drivers and ${\mathbf{c}}_{660}$ within the reservoir, during and following twelve runoff events. The model is demonstrated to meet the demanding temporal and spatial predictive needs of water supply lakes and reservoirs, by performing well in simulating the timing and magnitude of ${\mathbf{c}}_{660}$ peaks, the vertical and longitudinal patterns of ${\mathbf{c}}_{660}$ , diminishment following runoff events, and the dependence of impact on magnitude of a runoff event. Further advancements in turbidity modeling, including multiple particle size classes as state variables and explicit representation of particle aggregation and resuspension inputs, are considered.