Flood safety in delta cities

Abstract

Flood safety in low-lying delta regions has been a challenge of all ages. Societies worldwide have been trying to protect themselves against flood hazards for centuries. Floods in vulnerable urban regions are often caused by heavy rainfall, high river flow, extreme sea levels, or combinations of those three. Currently, there is a lack of tools and experience in assessing the effects of combined pluvial, riverine and coastal flood risk reduction measures for delta cities. The objective of this study is to develop a comprehensive risk-based model, which supports decision making in delta cities on measures against pluvial, riverine and coastal floods. An idealised hydrologic model has been developed to evaluate flood extent and frequency due to combination of these three factors in urban environments. The model quantifies pluvial, riverine and coastal flooding and identifies the contribution of different weather events to floods in a delta city. The city is schematised by a typical lay-out, in which different elements of a city’s flood protection system are connected. The city was drafted as a storage basin with varying areal sections as a function of the city’s elevation levels; incoming and outgoing water fluxes; and different strengths of flood protection elements, including waterfront resistance, discharge capacity and storage capacity. By calculating the hydrological budget of water, flood levels in the city can be computed as a function of precipitation and water levels in time. The computed flood levels are coupled with stage-damage relationships to define the direct economic damages. By comparing the economic damage and the investment costs, an optimal mix of measures (river flood defences, water pumps, storage areas) can be defined. The model serves as a practical tool that can be used in the search for an optimal combination of flood risk reduction measures. The study has adopted the City of Hoboken in New Jersey (United States) as a case study to apply the idealised risk model. Because of Hoboken’s history as an island, the low-lying area inundates when water levels in the Hudson River increase due to storm surges. In combination with severe rainfall, the impermeable surface and a lack of drainage, floods also occur during high rainfall events, especially when the excess water cannot be drained into the river. The consequences of a flood can be very large because of Hoboken’s economic valuable character and central location. Different combinations of flood risk reduction measures were included by the model consisting of waterfront protection; increasing storage capacity within the city; and increasing discharge capacity. For every combination the reduced flood risk, compared to the current situation of the city, has been calculated. The sum of the investment costs and the expected damage or risk, are the total costs for the combination of the system. The investment costs of each combination have been balanced with its corresponding benefit (i.e. risk reduction). In this study the economic optimum is defined as the combination with the lowest total costs. For Hoboken the elements for computing the present value of the risk and investment costs in this study were a preliminary estimation. Recommended areas for further improvement of the model are: uncertainty and sensitivity analyses for improving robustness of the model; a study to the dependency of input variables and system elements; and investigation to the effects of correlated surges and precipitation on the model results. Despite its assumptions, the model serves as a practical and convenient tool, which can be used in the search for an optimal combination of flood risk reduction measures in delta cities. It can quickly identify the different flood events in a delta city and it can quantify their frequencies and scales. For this matter, it offers realistic insight in the rates between pluvial, riverine and coastal flooding and the required flood risk reduction measures. As a result, the presented comprehensive flood risk model in this study is an effective and efficient tool for urban planners, engineering companies and decision makers of delta cities.