Addressing flooding in Geldrop-Mierlo with blue vein plan

It is no longer news by now that we are experiencing more frequent heavy precipitation. There seem to be patterns in the spatial distribution of the downpours. The southeast corner of Noord-Brabant is relatively often 'the victim'. So is the municipality of Geldrop-Mierlo.

In early 2020, the municipality of Geldrop-Mierlo asked Iv-Infra to consider a long-term vision for collection, transport and infiltration of rainwater for the entire municipality, or a blue artery plan. Parallel to this, work began on a new basic sewerage plan (BRP) which would allow for 1) an integrated approach and 2) a specific zoom-in on flooding locations.

In the summer of 2020, when Iv-Infra had just started working on the sewer model for the BRP, the municipality of Geldrop-Mierlo was again hit by a downpour resulting in major flooding. Streets turned into rivers, living rooms flooded and pits became swimming pools. This major nuisance demands a response, an approach and action from the municipality. But where do you start and can you solve everything? And how do you explain this to residents who keep getting water in their living rooms? 

Time for action

In addition to a vision for the (medium) long term, acute measures were also needed. The focus on the BRP and Blue Vein Plan was temporarily shifted to picking the "low-hanging fruit," or the relatively easy measures to take. Financial resources were made available by the College of B&W and the necessary capacity was made available on a broad front. This allowed administrators, policy staff, project leaders and field service employees to contribute to the approach. This was an important first step that allowed concrete follow-up actions to be taken quickly. 

In such calamities, the provision of information is very important, in addition to acute help, of course. When a few hundred reports come in, personal handling is impractical. Therefore, after a public information evening, discussions were held with smaller groups, delegations of residents from specific neighborhoods or districts. Because the residents were involved from the first moment in solving the problem, they had great input in developing ideas. 

Solutions that intervene on public space require careful balancing of considerations of all interests and creation of support. These discussions were also very valuable as preparation for the risk dialogue that will be held when the climate policy is drafted. The entire process of arriving at the realization of a measure has a long lead time. Maintaining a steady flow of information to residents and businesses is therefore important. Even when there is little news to report. The municipal website is an important channel where a lot of information can be shared. But an e-mail message to concerned residents, for example, also prevents them from feeling forgotten.

Water Ambassadors

During a public information evening, the issues and the intended approach were explained in outline form. Ten areas were then identified and two representatives (the water ambassadors) per area were invited for a discussion with an expert from the municipality. The functioning of a sewage system in general and the processing of rainwater were explained. Real expectations were also created for the residents about the possibilities and impossibilities. The residents experienced these discussions as positive. Parallel to the talks, surveys were conducted to further identify the problems. Together with the already known complaints and reports from the past, a complete overview of all water-related problems could thus be obtained.

All input and site analyses resulted in 84 measures. Some of these are the easily implementable measures at ground level, such as lowering curbs, installing thresholds or just removing them. But also the installation of vent covers, check valves, cleaning up waterways, reducing the size of culverts and creating emergency outlets will lead to a better functioning sewer system and less flooding. The somewhat more severe measures involve the construction of a number of storage cellars and the creation of open water storage areas (wadis). 

Iv-Infra worked out these measures using an integrated approach, from the initial design to supervision of execution. A number of measures have already been implemented or are in preparation, including a large storage cellar. Most of the measures are expected to be completed by mid-2022.

Long-term: providing structure

In parallel with the development of the measures, Iv-Infra prepared a calculation model. A 2D calculation was used to further analyze the systems and surface runoff. These considerations revealed various local improvement measures for the systems, as well as areas for future disconnection. The calculation results were tested against practical experience and these are broadly consistent. Differences do exist, but can often be explained by the fact that homes are located higher up, so that nuisance or damage does not occur. 

In order to provide direction and structure for future measures, a comprehensive analysis of area characteristics was conducted. Data such as altitude, groundwater levels, soil conditions, paved surface, greenery and surface water were collected and mapped. By combining these data, promising areas were identified for water storage or infiltration. For stormwater runoff, as a result of this analysis, a proposal was made for a blue vein structure that provides for optimal and municipality-wide distribution and collection of water in the (medium) long term.