together we’ll keep the polder dry

Motivation

Climate change is making rainfall heavier and less predictable, increasing the risk and intensity of future flooding events. To reduce flooding in the future, greenhouses in Delfland, the Netherlands, are working on optimizing the use of the available space for rainwater storage and the enhancement of the ‘sponge’ effect in the area. Doing so, the water is retained for longer periods, infiltrated, and used in the greenhouses.

Water basins in the greenhouse sector

The greenhouse sector covers approximately 10% of Delfland’s area. The greenhouse roof areas are designed to discharge all their storm water to a reservoir, to be used for the growing of plants within the greenhouses themselves. When the water in the reservoirs reaches a certain level, the surplus water is discharged into the local canals through an overflow.. When heavy rainfall occurs, this sudden discharge of large amount of water into the surface water can lead to stress on the system or even flooding.

A shared vision and motivation

The water authority of Delfland, the municipality of Westland, Glastuinbouw Nederland (the organization of growers) and the local growers themselves have a shared vision of storing storm water more efficiently in the water basins. They believe that a more effective use of the water basin could prevent the area from flooding at a reasonable cost, maintaining profitability for the growers and with a minimal use of space. Rainlevelr is the result of this vision.

Within the project, the water authority of Delfland encourages horticulturalists to use a new system of water level management in their reservoirs. Delfland provides the funds for the installation of the new system, but horticulturalists are then expected to maintain it. Through the new system, managing the surface level water becomes easier for the water authority and damages are reduced for the horticulturalists.

The concept of Rainlevelr

Rainlevelr is a project founded in the collaboration between local horticulturalists, the water authority of Delfland and the municipality of Westland. The main goal is to reduce water stress with the help of a new system through which the water authority can request horticulturalists to change the waterlevel in their waterreservoirs depending on the expected rainfall.

All measurements made in the reservoir will be visible real time for the water authority in the LetsGrow.com portal, connected to the climate computers of the growers who see the data as well and can control the water levels themselves. When heavy rainfall is expected, the water level can be lowered beforehand to prevent overflow. The extra buffer space can then take in the water, and thereby reduce the stress of the surface water system. In the future it will be possible to fully automate the operation of the valves. The valves and measuring devices can be placed when a new water basin is built or an old one is renovated.

Co-operation with the project is on a voluntary basis. The water authority of Delfland has a warning system for heavy rainfall and sends out a request to create capacity in the water basins, when necessary. The grower themselves then decide whether to follow up on this request or not. They remain responsible for their own decision. Through this division of responsibilities, the execution of the project remains simple. Moreover, it justifies the fact that growers are prepared to actively participate to help keep all our feet dry.

Initially, Delfland will cover the installation cost. The growers will then use and maintain the basin themselves. Through this cooperation the risk of flooding and water damage is reduced.

Recruiting new participants

Informed in time

The most opportune moment for realising the concept of Rainlevelr is during new construction of renovation. It is therefore important to establish contact with horticulturalists in order for plans to be alligned. Through organising meetings and congresses, awareness on the system  and its possibilites is increased.

Within the project, Project ambassadors are responsible for creating awareness. The current ambassadors are:

• The current participating horticulturalists
• Colleagues from Delfland and Westland visiting growers
• Glastuinbouw Nederland (the organization of growers)
• Suppliers and builders of water reservoirs

The current participants are the best ambassadors. The reason being that they are the neighbours, family and friends of other growers. Next is the group of suppliers and contractors, because they talk to the growers at the right moment: When there are plans to renovate or build a basin. The contacts within this group are expanding, and some even mention Rainlevelr on their websites and in their offers (in Dutch).

Informative conversation

When a grower has plans for the replacement of the foil or more extensive renovation/ building projects, contact is sought with the grower for a non-binding conversation.

General conditions to participate

However, not every water basin is suitable:

1. Condensation water is not allowed into the water basin. Condensation water is polluted with pesticides and therefore is not allowed to discharge into the Delflands water system.
2. The online information must be available in LetsGrow. It is vital for Delfland to see the water level in the basin to ensure the correct management advice ahead of rainfall events.

The result

By the end of march 2020 there are 25 participating growers and together there are 139 ha of greenhousing connected to Rainlevelr. There are even several new connections being installed this year. It is estimated that by the end of 2020, a total of nearly 200 ha will be connected. This estimate can be seen in the graph below, as well as the current locations on a map underneath.