MODELLING STUDY OF BUOYANCY-DRIVEN EXCHANGE FLOWS OVER A DESCENDING BARRIER

摘要

Results are presented from a series of model studies investigating transient exchange flows resulting from the steady descent dh_b/dt of an impermeable barrier separating initially-quiescent fresh and saline water bodies of depth H_b, and respective densities ρ_0 and ρ_0+ (Δρ)_0. Parametric laboratory experiments have been conducted (ⅰ) to determine the characteristics of the time-dependent exchange flow over the barrier crest and (ⅱ) to quantify the temporal development of the near-bed brackish water reservoir formed behind the barrier. A model adapted from the steady-state maximal exchange flow case provided good qualitative agreement with laboratory measurements. Comparisons indicated that mixing-related head losses of between 7% and 3% are applicable for the exchange flow conditions observed in the early and late stages respectively of the barrier descent phase. The experimental study revealed (and the model predictions confirmed) that (ⅰ) the dimensionless time t_(det)/(g'H_b)~(1/2) of detection of the spreading, near-bed brackish reservoir at a given location increases with increasing values of the parameter g'H_b/(dh_b/dt)~2 and (ⅱ) the normalised thickness δ(x,t)/H_b of the reservoir at this location increases monotonically with increasing values of the modified time (t-t_(det)_/(g'H_b)~(12/).