Influence of the Urban Block Structure on the Flow Pattern Along a Flooded Street

Urban flooding is among the most prevalent disaster worldwide lately, and to minimize its risks it is necessary to improve flood hazard predictions. Nowadays, there are several studies and research on urban flood processes, but most of them considerer that the flow is concentrated only on the streets. However, the built-up areas (blocks) might modify the flow patterns at local scale due to their irregularity and the lateral inflows or/and outflows.

This paper focuses on the study of the variations in flow depth and flow velocity on a flooded street, depending on the structure of the adjacent built-up areas. The experiments were carried out on a physical model, using the following configurations under steady flow conditions: (i) a unique straight street, (ii) a straight street flanked by two lateral cavities, and (iii) a straight street with a lateral opening (building opening). An inlet flow-rate of 3 l/s at the entrance of the straight street and a slope of 0.12% were used. For each configuration, a downstream weir was adjusted to get a quasi-uniform subcritical flow along most of the reach. Water surface elevation was measured using an Ultrasonic Distance Sensor, velocity within the water column was measured using an Acoustic Doppler Velocimeter with a side-looking probe, and the surface flow velocity using LSPIV.

Due to the dimensions of the street and the flow depth (b/d=2.7), the results were analysed as those corresponding to a narrow channel, secondary currents with a longitudinal axis and the maximum flow velocity below the surface have been observed. In the case of the lateral cavities, it was observed that the mixing layers between the channel (street) and the cavities present a slight shift to the left bank, i.e., the left mixing layer is inside the cavity, and the right mixing layer inside the channel; compared to the first configuration, the flow depth also showed a slight general increase in the channel, upstream of the cavities, and a pronounced decrement immediately downstream of them. In the case of the side opening, the main feature is the strong asymmetry of the flow.