Flood Risk In Urban Areas Research Articles

Flood risk and context of land-uses: Chennai city case

India witnessed increased flooding incidences during recent past especially in urban areas reportedly since Mumbai (2005) as a mega disaster. Other South Asian cities like Dhaka, Islamabad, Rawalpindi, besides many other cities in India, are also reportedly been affected by frequent floods. Flood risk in urban areas are attributed to hazards accelerated by growth in terms of population, housing, paved-up areas, waste disposal, vehicles, water use, etc. all contributing to high intensity – high load of runoff. Reduced carrying capacity of drainage channels is also a key concern. Haphazard growth of low-income habitations and un-organised trade added to challenge. Spatial dimensions of all these flood factors are often characterised by land-use and changes. Chennai, a coastal mega-city is fourth largest metropolis in India, has a history of over 350 years of growth. Meteorologically there is no major upward or downward trend of rainfall during 200 years, and a decrease in last 20 years with a contrast record of increasing floods have been experienced. Analysis of land-use changes over the temporal and spatial scale has been undertaken for Chennai city in order to understand the patterns on green-cover, built-up area and consequences on hydrological settings. Land-use issues like decreased natural areas, loss of water bodies, encroachment of river/streams and other drainage channels, uncontrolled multiplication of built-up areas, have been identified as contributory factor to flood risk in Chennai. The paper discusses flood risk reduction and management strategies in urban context with example of the Chennai city and draws attention of land-use planners and disaster management experts to integrate their efforts for better and sustainable results. Key words: Chennai city, floods, land-use changes, national guidelines.

Attribution of flood risk in urban areas

Flooding in urban areas represents a particular challenge to modellers and flood risk managers because of the complex interactions of surface and sewer flows. Quantified flood risk estimates provide a common metric that can be used to compare risks from different sources. In situations where there are several organisations responsible for flood risk management we wish to be able to disaggregate the total risk and attribute it to different components in the system and/or agents with responsibility for risk reduction in order to target management actions. Two approaches to risk attribution are discussed: Standards-based attribution, which is a deterministic approach, based upon the performance of different engineering components in the system at their “design standard”. Sensitivity-based attribution, which apportions risk between the variables that influence the total flood risk. Whilst both these approaches are feasible for the small system considered here, in practice urban flooding systems involve tens of thousands of variables. The only feasible approach to tackling this problem for large urban systems is therefore by hierarchical simplification of the system, with the attribution analysis being applied in several tiers of detail. In this paper, the applicability of a hierarchical approach is demonstrated in the context of sewer pipe blockages. The results demonstrate the potential of attribution methods to support the development of integrated urban flood risk management strategies, as they can identify the forcing variables and infrastructure components that have the most influence upon flood risk.

Simulation of floods caused by overloaded sewer systems: extensions of shallow‐water equations

AbstractThe outflow of water from a manhole onto a street is a typical flow problem within the simulation of floods in urban areas that are caused by overloaded sewer systems in the event of heavy rains. The reliable assessment of the flood risk for the connected houses requires accurate simulations of the water flow processes in the sewer system and in the street.The Navier–Stokes equations (NSEs) describe the free surface flow of the fluid water accurately, but since their numerical solution requires high CPU times and much memory, their application is not practical. However, their solutions for selected flow problems are applied as reference states to assess the results of other model approaches.The classical shallow‐water equations (SWEs) require only fractions (factor 1/100) of the NSEs' computational effort. They assume hydrostatic pressure distribution, depth‐averaged horizontal velocities and neglect vertical velocities. These shallow‐water assumptions are not fulfilled for the outflow of water from a manhole onto the street. Accordingly, calculations show differences between NSEs and SWEs solutions.The SWEs are extended in order to assess the flood risks in urban areas reliably within applicable computational efforts. Separating vortex regions from the main flow and approximating vertical velocities to involve their contributions into a pressure correction yield suitable results. Copyright © 2005 John Wiley & Sons, Ltd.