What does the volume-averaged water age distribution function reveal about flow fields in rectangular shallow reservoirs?

Shallow reservoirs are hydraulic structures widely used for water storage or as sedimentation tanks. Their design and sizing are intricate due to the complex flow fields developing in such structures, hence suitable indicators are needed to evaluate their hydraulic performance. Based on the outcomes of a depth-averaged computational model, we analysed the potential of the distribution function of the volume-averaged water age to unveil in a concise way valuable information on the flow field in rectangular shallow reservoirs. Ten different reservoir layouts were examined. In all cases, the shape of the computed distribution function reveals a remarkable amount of information on the flow field in the reservoirs. The distribution functions exhibit a sequence of steps, followed by an exponential decay, which may be related to fast and slow pathways travelled by water particles along their routes across the reservoirs. As such, the distribution function of the volume-averaged water age was found to provide valuable information for assessing the hydraulic performance of shallow reservoirs, while achieving an effective reduction in the problem dimensionality.