Flood Risk and Urban Water Management

Today, more than half of the world’s population lives in cities and this urbanisation process is expected to continue. Most large urbanisations are located along rivers and along the shorelines of oceans. Flood risk may originate from high river discharges and peak flows, sea level rise, as well as from extreme storms. As the river basins are often densely populated or used for agricultural purposes, nowadays full attention is given to flood risk assessment and flood risk reduction.

In particular in developing countries cities grow fast, but the development of urban infrastructure lags behind. Drainage systems are often under-dimensioned to drain the expanded areas and not properly maintained and cleared, thus causing hydraulic congestion and flooding. At the same time the climate is changing and weather is getting more extreme, with locally extreme storms and higher rainfall intensities. Urbanisation in combination with climate change accelerates the problems; flooding is becoming a reoccurring event in many cities, damages increase, and the number of casualties grows.

An integrated approach is recommended to achieve sustainable development of the basin, while maximizing the net benefit from the floodplains or the river basin. Important part of this approach is effective operating of the flood control structures in the system, reliable forecasting of floods and early warning measures.

This calls for the development of sustainable urban drainage systems (SUDS) which can deal with large shares of paved or roofed areas. These urban areas have lost their ability to absorb precipitation (less infiltration, groundwater recharge and evapotranspiration) which is often the problem source.

Our urban drainage engineering focusses on an integrated and step-wise approach to design cost effective systems with low environmental impact to drain away and manage surface water run-off through collection, conveyance, retention and storage. These systems can be developed both on small (plot size) and large (cities) scales. It is essential that all stakeholders will be closely involved during the design, implementation and maintenance management process. The approach we adopt implies carrying out the next steps:

  • Mapping the catchment areas, identifying the sub-catchments, making a terrain model detailing the terrain topography, morphology and relevant features;
  • Mapping the present drainage system with all its components and dimensions;
  • Collecting hydrologic, land-use and soil data;
  • Setting up a simulation model for the present drainage system to identify the bottlenecks;
  • Evaluating various design alternatives to take out the bottlenecks (in an iterative way), in close collaboration with the stakeholders and the Client;
  • Preparing designs for the preferred option.

Solution options should not only focus at re-dimensioning the drainage network, but equally consider source handling (reducing run-off, upstream rerouting, temporary storage), impact reduction (measures to reduce casualties and damages), and maintenance. We are seeking for the right balance between conveyance and retention.

CDR has the engineers and specialists to find solutions for (urban) drainage problems.