Local effects of longitudinal dams on flow and morphology: a physical scale study
Groynes increase the resistance in the river and cause a rise of upstream water levels during high discharges, increasing flood risk. Longitudinal dams can serve as an effective flood mitigation measure, replacing groynes as a river training structure. In contrast to the large-scale effects on the flow, the local morphological effects of a longitudinal dam are poorly understood. In this study we use a physical scale model to quantify the impact of the longitudinal dam on flow and turbulence structures, and the flow interaction with the river bed. We measured the flow field with an acoustic Doppler velocimeter (ADV). The river bed levels were measured with a laser scanner. The ADV-data was filtered with a phase-space thresholding method and a spectral filter. The phase-space method improves the estimates of the velocity covariance matrix. Between the two groynes in the scale model, the flow recirculates both for the high and the low flow conditions. Between the groyne closest to the dam and the inlet to the region behind the dam, the flow pattern is similar, but, under low flow conditions, the rate of recirculation is smaller. Compression of the recirculation cell may explain the higher turbulence intensity observed in the region adjacent to the inlet region. Under high flow conditions, the water flows over the dam, and the turbulence intensity is lower, relative to low flow conditions. Under low flow conditions, the flow around the head of the dam shows strong convergence and an increase of turbulence intensity. During high flow, the turbulence intensity is lower and the flow is no longer forced around the dam. The model performs well in the area between the groynes, and the longitudinal dam is not expected to cause strong scouring or excessive turbulence intensities. These results suggest that a strong morphological response near the inlet region, which could jeopardize navigation on the river, is not to be expected.