urban drainage and sewer asset

Establishment of rainwater plans

Reference case FARYS' working area

Introduction to the challenge

Rainwater management is becoming increasingly challenging as a result of intensive urbanisation, the continuing increase in paved surfaces and the expected impact of climate change on precipitation. Rainwater plans with a region-wide vision on rainwater management provide the answers.

Our approach

Stapsgewijze aanpak met basishemelwaterplan en detailhemelwaterplannen

Establishment of rainwater plans
For the establishment of rainwater plans, HydroScan developed its own methodology for both basic and detailed rainwater plans. This methodology is based on, and complies with, the relevant recommendations of the Coordination Commission on Integrated Water Policy (CIW) but can be seen as a more concrete implementation of those recommendations.

Methodology for basic rainwater plans
In the basic rainwater plans, the available information is used as basis to develop a vision on rainwater management within the municipality, with a strong focus on the use of at-source measures and collective buffer and infiltration facilities.
This process involves the following steps:
  • Making policy choices (e.g. relating to collective buffering, construction of permeable paving, canalisation of the public space, etc.) in collaboration with the client and any other stakeholders;
  • Development of a "drainage database" in GIS that groups all relevant data in a structured manner (e.g. sewer database, DTM, identified bottlenecks, etc.);
  • Geographical analysis of most suitable locations for additional at-source measures;
  • Delineation of sub-basins and main rainwater drainage lines;
  • Prioritisation of sub-basins based on identified bottlenecks and the existing rainwater drainage system;
  • Estimation of the required buffering per sub-basin based on existing and future paving;
  • Indicative estimation of the drainage capacity of existing rainwater drainage lines;
  • Development of a buffer strategy for each sub-basin: identification of generally applicable at-source measures, development of different scenarios for collective buffering and infiltration;
  • Creation of overview maps with vision and detailed sheets with measures per sub-basin.
Depending on the available budgets, the choice can be made to implement certain steps only for top priority sub-regions.

Methodology for detailed rainwater plans
In the detailed rainwater plan, the vision from the basic rainwater plan is elaborated in greater detail (possibly for a specific area within the municipality), including hydraulic optimisation of the main rainwater drainage lines based on hydraulic modelling.

This comprises the following activities:
  • Detailed analysis of the identified bottlenecks within the municipality based, among other things, on intensive workshops with the municipal departments involved;
  • Development of an integrated water model that simulates both the run-off from fields, drainage via the sewerage systems (including water-on-street modelling) and the existing watercourses. The model's level of detail can be differentiated for the different areas in relation to existing bottlenecks;
  • Extrapolation of the model for different (climate) scenarios and different precipitation extremities and further analysis of the causes of the bottlenecks;
  • Development of scenarios to resolve bottlenecks and further optimisation of the main rainwater drainage lines based on the model;
  • Risk assessment of the different bottlenecks, based on the likelihood of occurrence of flooding events under different (climate) scenarios and the vulnerability of the existing infrastructure;
  • Cost estimate of proposed measures and assessment of avoided damage and costs after implementation of the measures;
  • Prioritisation of different measures based on a cost-benefit analysis;

Our solution

Rainwater plans with focus on the use of at-source measures and collective buffer and infiltration facilities

The above methodology has already been applied successfully in various cities and municipalities within Farys' working area:
  • City of Ghent: Establishment of detailed rainwater plan for various municipal districts
  • Municipality of Beersel: Establishment of detailed rainwater plan
  • Municipality of Melle: Establishment of basic rainwater plan
  • Municipality of Buggenhout: Establishment of basic rainwater plan (under development); Establishment of detailed rainwater plan for eastern part (centre of Buggenhout, Opdorp, Hoge Briel) planned

In the municipality of Beersel, the detailed rainwater plan is used as an operational tool that helps with the planning of preventive projects and with setting the right priorities. 

Our solution

Proactive and preventive approach to prevent or mitigate the impact of exceptional rainfall

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