Breathing cities: improving water quality in urban channels by controlling tidal flows, an idea for Ho Chi Minh City

Coastal cities are constantly facing new challenges in their relationship with the accelerated sea level rise associated with global warming, and with the quality of the water that flows in their urban canals. Indeed, some of the largest conurbations are innervated by a network of channels, which serve many purposes and provide preferential pathways for water exchange, but also make them prone to flooding. Flows periodically change their direction due to the tidal cycle, letting the fluxes of water, heat, and contaminants, enter and leave the city, like the breath of a living body. However, the presence of these channels is also a major threat of flood risk propagation, and in some cases motivated the construction of tidal gates to protect the city from high water levels. Ho Chi Minh City (HCMC) is an example where this strategy is progressively being applied.

In this work, we discuss how the existence of tidal gates for flood protection may be exploited to control the tidal flows in the urban canals with the aim of improving water quality. Interestingly, the channel network in HCMC is characterized by several closed loops, where the periodic “breathing” is not efficient to renew the water in the central branches of the network because the water tends to enter and leave the system synchronously at the two ends of the loop, dictated by the water levels at the two connecting sections in the estuary, where tide propagation is typically fast so that the levels are similar. Therefore, we propose to exploit the gates not only to protect the city from high tides, but also to induce a prevalent unidirectional flow in the closed loops by alternately opening and closing the gates according to the difference in water levels between the channels and the estuary. Careful management can promote the removal of pollutants with beneficial effects on water quality, and potentially contribute to the transport of sediments, thus reducing the need for dredging. We demonstrate how the system can work by means of hydrodynamic modelling, first considering an idealized closed loop and then extending the simulation to a realistic model of the HCMC channel network.