Initiation of stagnation in drinking water storage tanks

Using temperature as a conservative tracer, the initiation of stagnation by buoyant jets was investigated in two drinking water storage tanks operated in fill-and-draw mode. The problem concerns the risk of water quality degradation caused by excessive ageing in stagnant zones. By measuring flow rates into the tanks and temperatures at several points, the initiation of stagnation could be related to the source parameters: the momentum flux M-0 and the buoyancy flux B-0. In agreement with previous studies, the results indicated that the jet/plume transition length scale, L-m = M-0(3/4)/\B-0\(1/2), had to exceed a certain fraction of the depth in order to avoid stagnation resulting from stratification of the water mass. The study provides insights into the parameters governing initiation of stagnation and illustrates how stratifications generated by positive and negative inflow buoyancy can affect the water exchange.