Evaluating the feasibility and performance of Nature-based Solutions in Can Tho, Vietnam

Can Tho, the largest city in the Mekong River Delta, is experiencing rapid urbanisation that is causing many typical urbanisation-related issues, including the increasing flood risk. The flooding area has expanded from 30% to 50% of the total city area due to urbanisation and climate change. Due to the low topography and poor capacity of drainage systems, the city may sometimes remain inundated for up to three hours after the rain event has ended. It is essential to develop effective and also sustainable management strategies for the city to mitigate risk of flooding, especially surface water flooding caused by extreme heavy rainfall.

Nature-based Solutions (NbS) are proposed and widely promoted globally as a sustainable strategy for managing flood risk and creating other benefits. For flood risk management, NbS can help a city reduce surface runoff and subsequently release pressure on drainage systems through infiltration and interception, thus mitigating flood risk. Numerical modelling has been widely used to support the design and assessment of NbS. Conventionally, NbS modelling is achieved by integrating a hydrological model with NbS simulation modules though a one-way coupling method. Such models are incapable of fully describing the rainfall-runoff-flooding processes dynamically interacting with NbS measures, and therefore can only provide limited information such as temporal and spatial variation of runoff removal rate for NbS design and evaluation.

In this work, a 2D hydrodynamic flood model is adopted and further developed by coupling with compatible NbS simulation approaches to overcome the existing NbS restrictions. The new modelling framework is applied in Can Tho city to evaluate the feasibility and performance of different NbS against various evaluating metrics. The simulation results indicate that green roofs, rain gardens, and bio-retention cells can effectively reduce inundation area, flow rate, and runoff volume to protect localised infrastructure and key buildings under certain rainfall scenarios. However, dramatic change of flow velocities is observed near the key infrastructure and structures following the implementation of a rain garden, posing higher risk to pedestrians and vehicles. In-depth analysis of the hydrological performance of bio-retention cells further indicates that their designed capacity is not sufficiently exploited due to the inappropriate installation location, further demonstrating the advantage of the proposed model for better planning and design of NbS to achieve optimised performance.