Three‐Dimensional Effects of Artificial Mixing in a Shallow Drinking‐Water Reservoir

Abstract

Studies that examine the effects of artificial mixing for water‐quality mitigation in lakes and reservoirs often view a water column with a one‐dimensional (1‐D) perspective (e.g., homogenized epilimnetic and hypolimnetic layers). Artificial mixing in natural water bodies, however, is inherently three dimensional (3‐D). Using a 3‐D approach experimentally and numerically, the present study visualizes thermal structure and analyzes constituent transport under the influence of artificial mixing in a shallow drinking‐water reservoir. The purpose is to improve the understanding of artificial mixing, which may help to better design and operate mixing systems. In this reservoir, a side‐stream supersaturation (SSS) hypolimnetic oxygenation system and an epilimnetic bubble‐plume mixing (EM) system were concurrently deployed in the deep region. The present study found that, while the mixing induced by the SSS system does not have a distinct 3‐D effect on the thermal structure, epilimnetic mixing by the EM system causes 3‐D heterogeneity. In the experiments, epilimnetic mixing deepened the lower metalimnetic boundary near the diffuser by about 1 m, with 55% reduction of the deepening rate at 120 m upstream of the diffuser. In a tracer study using a 3‐D hydrodynamic model, the operational flow rate of the EM system is found to be an important short‐term driver of constituent transport in the reservoir, whereas the duration of the EM system operation is the dominant long‐term driver. The results suggest that artificial mixing substantially alters both 3‐D thermal structure and constituent transport, and thus needs to be taken into account for reservoir management.

A near-term iterative forecasting system successfully predicts reservoir hydrodynamics and partitions uncertainty in real time

“…We applied and evaluated FLARE at Falling Creek Reservoir (FCR), a dimictic, eutrophic reservoir located in Vinton, Virginia, USA (37.30°N,79.84°W). FCR is a shallow (maximum depth=9.3 m, mean depth=4 m), small (surface area=0.119 km 2 ) reservoir [ Chen et al 2018 ]. The lake exhibits summer thermal stratification from May to October and is ice-covered from January to February or March [ Carey et al 2019c ].…”

Whole-Ecosystem Experiments Reveal Varying Responses of Phytoplankton Functional Groups to Epilimnetic Mixing in a Eutrophic Reservoir

“…This possibility is supported by 3-D hydrodynamic modeling of the same reservoir by Chen et al (2018). Using field turbidity data, Chen et al (2018) found that the intensity of EM operation in FCR was an important driver of short-term particle transport, and that mixing facilitates the transport of particulate matter from the shallow upstream region to the deep lacustrine region in the reservoir [26]. The increased turbidity at the deepest site in FCR after the first mixing event coincided with an increase in volume-weighted TP throughout the treatment zone, as well as an increase in volume-weighted green algae, cyanobacteria, and cryptophyte biomass and total phytoplankton biomass in the treatment zone of FCR.…”

“…As shown in Figure 9, the increase in turbidity at the deepest site in FCR after EM1 likely originated in shallow, turbid upstream regions of the reservoir and was subsequently entrained downstream over the course of the week post-EM1. This possibility is supported by 3-D hydrodynamic modeling of the same reservoir by Chen et al (2018). Using field turbidity data, Chen et al (2018) found that the intensity of EM operation in FCR was an important driver of short-term particle transport, and that mixing facilitates the transport of particulate matter from the shallow upstream region to the deep lacustrine region in the reservoir [26].…”

“…In 2016, the EM in FCR was activated twice with different durations and mixing intensities during our monitoring period (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26). Because these mixing experiments were conducted using a newly-installed mixing system in a drinking water reservoir from which managers were actively withdrawing water at the time of our experiments, we were limited to operating the mixing system for relatively short trial periods due to management concerns about the effects of mixing on water quality.…”

“…The duration and flow rate of the two epilimnetic mixing events were chosen to preserve thermal stratification, as required by the reservoir managers. The two events were designed to be complementary, using different flow rates and durations to achieve the same change in metalimnetic boundary depth (a decrease of ~1 to 1.3 m) during both experiments (Text S1) [26]. In addition, mixing was conducted with the goal of homogenizing phytoplankton biomass concentrations throughout the treatment zone (0-5.5 m depth) at the deepest site of our experimental reservoir, which we determined by real-time monitoring of phytoplankton biomass depth distributions during our mixing experiments.…”