Assessing Nature-Based and Classical Engineering Solutions for Flood-Risk Reduction in Urban Streams

Urbanization of stream ecosystems with the purpose of managing the flash-flood events is nowadays considered responsible for habitat loss and alteration of the natural flow regime with severe implications for the ecosystem functioning. Unsurprisingly, the river scientists have started seeking alternative options inspired from nature for mitigating the flood-risk and maintaining the stream at its natural state. With this article the authors demonstrate the effects of a nature-based solution (NBS) for managing an urban stream based on the use of bioengineering materials (e.g. plants) and the implementation of the actions that restore the stream to its natural form (e.g channel widening). The HEC-RAS software was employed to simulate the flow and hydraulic components of an approximately 800m long reach of an urban stream under three different scenarios of flood risk management with a design flow set to 400 m(3)/s. The first scenario was based on the current situation of the stream, the second scenario concerned the stream restoration by following the nature-based solutions, while the third scenario was based on the classical "grey" engineering approach of concrete channelization. Unmanned Aerial Vehicle (UAV) photogrammetry methods and the Pix4Dmapper software were used in order to develop a detailed 3D model of the studied reach that accurately captured the current geomorphology. The obtained results showed that with concrete channelization, the average and maximum flow of the stream increases significantly in relation to the current situation, from 2.48 and 4.88m/s to 9.82 and 11.22 m/s, respectively, while the average Froude number raises from 0.36 to 1.69 implying super-critical flows. In contrast, the NBS scenario retained lower flow velocities and average Froude number similar to those under the current conditions. In addition, a cost estimation analysis for both stream management techniques revealed that the NBS is much cheaper than the traditional channelization (1.1 mil (sic) vs 5.6 mil (sic)). In conclusion, our findings suggest that the future restoration of urban streams should consider the nature-based solutions since i) they can be effective with regard to the reduction of flood-risk, ii) are cheaper than the traditional "grey" techniques and, most importantly, iii) maintain the natural state of the ecosystem which improves not only the ecosystem functioning but also the aesthetic value within the urban context.