A large reservoir can provide reliable access to water, control flooding, and be used to generate hydroelectricity.
But environmentalists who once advocated dams have now turned on them, claiming they upset local ecosystems by changing river flow patterns or by affecting nutrient and oxygen concentrations in downstream flows.
Damming a river and constructing a reservoir in a litigious culture armed with well-paid environmental lawyers requires balancing benefits and risks to avoid lawsuits.
Outside America, there is a little more common sense but the ecosystem is still a concern. In tropical reservoirs, water often becomes highly stratified, with nutrient-depleted but well-oxygenated upper layers and nutrient-rich but oxygen-depleted waters at depth. When water is let out of the reservoir to power turbines or stave off drought, its quality has consequences for life downstream.
Engineers plan to use the dam at the Itezhi-Tezhi Reservoir that blocks the Kafue River in Zambia for power generation. Compared with more-industrialized regions, the Kafue River area is poor in nutrients, as farmers do not use fertilizers. Therefore, the presence of the Itezhi-Tezhi Reservoir is currently removing nutrients needed for productivity in the downstream regions.
Using the Itezhi-Tezhi Reservoir as a case study, Kunz et al. devise an approach that should allow for optimizing the quality of the water being loosed from a reservoir without affecting the dam's capacity for power generation.
The authors' approach revolves around drawing from the two layers of the stratified reservoir water, balancing the mix of oxygenated shallower water and nutrient-rich deeper water. The authors note that though technology exists to re-oxygenate depleted reservoir water, the necessary equipment is often expensive to operate and maintain.
Using a biogeochemical model of nutrient cycling, the authors calculate that by using a mix of water from around 13 meters (43 feet) depth and from more than 30 meters (about 100 feet) depth from the Itezhi-Tezhi Reservoir, they could maximize the nutrient load while avoiding releasing hypoxic water.
Article: Manuel J. Kunz, David B. Senn, Bernhard Wehrli, Alfred Wüest, Elenestina M. Mwelwa, 'Optimizing turbine withdrawal from a tropical reservoir for improved water quality in downstream wetlands', Water Resources Research, doi:10.1002/wrcr.20358