Floodplain restoration to enhance urban climate change resilience

Abstract

Urbanisation and climate change are major threats to the urban hydrologic cycle due to increased surface sealing and consequently reduced evapotranspiration and infiltration. The increased amount of impervious surfaces inhibit the naturally-induced losses and result in accumulation of stormwater and urban inundation, which accounts for considerable amount of casualties and asset damage. Nature-based solutions (NBS) are the rapidly emerging concept for alleviating the consequences of urbanisation and population growth on natural systems. NBS deliver benefits not only for reduction of urban inundation but also for biodiversity and water quality, and other social, economic and environmental objectives.

The present study focused on building the 1D steady-state inundation model of Vuores Central Park (Tampere, Finland) in HEC-RAS that features a restored stream and its floodplain for urban flood reduction. Field measurements of channel geomorphology and open DEM and other datasets were used to delineate channel geometry. Gauged online measurements of stream flow and water level, and the SWMM (Storm Water Management Model) model of Vuores catchment were applied for approximation of the total inflows to the stream from upstream of the catchment based on land-use and computation of losses. These were the basis for simulations of model output comparison and inundation extents for three heavy rainfall events: one having a 50-year (Päijänne, 30.08 mm of precipitation) and two a 100-year statistical return period (Turku, 70.5 mm and Lempäälä, 43.19 mm).

HEC-RAS and SWMM models yielded comparable results with the gauged measurements during small rain events. The HEC-RAS model was consistent with the observed velocity and discharge for two out of four events. During the simulations of severe storm events, the Central Park NBS was projected to be heavily flooded. The total simulated inundation area for three events was at least 1.33 ha, 1.63 ha and 1.93 ha for 50-year Päijänne, 100-year Turku and 100-year Lempäälä events, respectively. The major area west of the Park was modelled to be exposed to inundation, including buildings. Model results were subject to large uncertainties related to setting boundary conditions and estimating inflows when extrapolated to rarely occurring storm events. Possible solutions to the flooding would be to consider channel vegetation maintenance to increase its conveyance, or to install more NBS upstream of Vuores Central Park to retain more water at the source. The developed HEC-RAS model can be used to perform simulations of dynamic inflows to design the best possible solution for preserving the infrastructure assets and their co-existence along natural ecosystem.