In high-density urban areas in the Netherlands
Our climate is changing. Increasingly, we are facing intensive rain showers, with large amounts of water falling in short periods of time. Pumps alone cannot handle these large volumes of water. More space is needed to store water (temporarily), but space is not abundant in the densely populated Netherlands. This situation stresses the need for innovative solutions for storing water, combined with spatial functions such as housing, infrastructure, economic development and nature. For these urban areas, DHV identified cost effective innovative solutions to create extra water storage capacity.

Introduction
The Netherlands have been hit by serious local flooding due to heavy rain events several times in the recent past. Flooding in 1998, caused by 100 – 125 mm rainfall in 24 hours made everyone aware that more water storage capacity is needed. Also last year several cities had serious problems due to heavy rainfall. Flooding will occur far more frequently in the years ahead unless the capacity problem is solved. The last ten years the water boards have tried to create new surface water bodies for extra storage capacity. Now policymakers are aware that creating extra storage capacity by expanding the area of surface water is not feasible, especially in high density urban areas. The density of urban housing is too high and the open public green areas which are potentially available have a significant cultural heritage value and therefore cannot be used. Therefore, solutions must be sought by applying innovative water storage techniques.

Innovative water storage
A number of techniques and methods for innovative storing water can be distinguished, based on examples found both in the Netherlands as well as in other countries. They can be subdivided as shown in table 1.

Table 1. Techniques and methods for innovative water storage.

Feasibility of water storage techniques

Not all water storage techniques are feasible on every location in the urban area. Also each technique has different advantages and disadvantages. On the basis of the following criteria it is possible to evaluate different innovative techniques:

• Adaptation: the effort needed to fit a system into the urban environment under local conditions
• Emotional value: the contribution of water in the living environment.
• Accessibility: the accessibility for inspection and maintenance
• Water quality: the influence on water quality
• Added value: the possibilities for combination with other functions

Applying the criteria mentioned above to the innovative techniques results in the matrix shown in table 2.

Table 2. Score of each innovative technique on the criteria.

Promising innovations for use in high-density urban areas are:

• Water roof. Flat roofs can be used to store water. There is obviously a difference between traditional flat roofs and true water roofs. The latter are capable of storing far more water (see figure 1). The captured water can be used for several purposes, like aesthetics (water art) or practical usages (fire-fighting water). Roofs must be reinforced in order to store this volume of water and stored water will be drained slowly.

Figure 1. Water storage roof (photo: © Bert van Rootselaar, DHV)

• Vegetation roof. This is a form of a water roof with a high aesthetical value. While a vegetative roof has less capacity, the quality of the captured water will be better because of the plants present, which form a natural filter. Furthermore, the roof absorbs noise, insulates and reduces particulate matter. Vegetative roofs can be designed to allow their use as roof gardens.
• Enlarged drainpipes. Enlarging drainpipes along buildings have a limited influence on the architecture. Extra water storage is created relatively simply.
• Wadi. A ditch or sunken grass field can store water. A so called ‘wadi’ is created by making a storage trench below the ditch or the field. The trench consists of gravel that contains a drainage pipe. Both in the ditch and in the trench it is possible to store water for slow discharge into the surface water.
• Water square. Making a square suitable for water storage provides an opportunity to make 'living with water' visible to the public.
• Floating dam. This makes it possible to build a temporary quay. If the dam is constructed as a circle, a storage basin can be created. Normally, the dam is located in a trench below the ground, with the top side level with the ground. If water is supplied in the trench, the dam will float up.
• Water bag. This solution may be used under houses and / or basements. The bag will be filled under free flow conditions and must then be pumped out.
• Storage in crates. An underground reservoir can be made using hollow stackable plastic crates or blocks enveloped in a watertight film or enclosed by concrete. Such a reservoir can be made below a road or sports field. The low weight of the crates makes them ideal for areas with limited loading capacity.
• Storage basement. Peak rainwater storage is possible by building a basement structure below a surfaced or unpaved site.
• Airbag Water Storage. This innovative solution – which DHV has patented – consists of an inflated airbag below water. The airbag must be fastened to a structure to keep it under water. Water storage is created by letting air escape from the bag. If the level of the surface water rises, the airbag is emptied to an extent that eliminates the rise in the level. If storage is no longer needed, the airbag can be re-inflated. The bag may be fastened easily to a building located alongside the water (see figure 2). The major advantage of the airbag is that water storage is created without any increase in the water level.

Figure 2. Airbag Water Storage.

Comparison of costs

Every way of storing water has its own specific costs. In the traditional way of creating water storage, the government purchases land and turns that into open water. The allowed fluctuations in water level define how water can be stored on a square meter of land. For land acquisition as well as the realization of the storage the costs can be calculated and then converted to a price per m³ of water storage.

Similarly the costs of innovative techniques can be calculated, although in many cases these solutions do not require the acquisition of land as they co-exist on land with another primary function that accounts for the costs of the land.

The quantity of water storage that can be created by every technique is different for every specific urban area. Water roofs are only viable on large flat roofs. Water squares can only be applied beside buildings. For a specific business area in the Netherlands, the water storage capacity of every technique was calculated. Figure 3 presents both the costs of all techniques of storing water as well as the potential m³ of storage they can create. The illustration shows that land acquisition costs account for a large part of the total construction costs for traditional water storage. Innovative solutions that avoid the need to purchase land will be cheaper_[1]

Figure 3. Unit prizes and water storage potential for several techniques

The opportunities of innovative water storage are not negligible; for this specific business area potentially € 14.7 million could be saved by applying innovative techniques over creating traditional open water storage.

The costs of creating traditional water storage are often lower than those incurred for innovative ways of storing water, as long as the costs of acquisition are not accounted for. Digging new bodies of water costs about € 150 per m³. The cost of acquiring land for an extra square meter of water storage is approximately € 350 - € 600 per m³ in Dutch urban areas. The estimated cost of creating innovative ways of storing water varies from € 120 (Airbag Water Storage) to € 360 per m³ (water storage roofs). Vegetation roofs, used to store water as well as to enhance visual quality, water quality and ecology, cost approximately € 2,000 per m³ of water storage as they hold a very small quantity of water per m².

The main conclusion is that in densely populated areas with high land acquisition costs, innovative techniques for water storage will be financially more attractive then the traditional way of purchasing land for open water storage.

There are several methods which are technically sound, have lower costs and have adequate potential to be further developed. Making room for water in spatial planning may even create value. So the hurdles for the implementation of innovative water storage facilities lie not in technology or finance but in the way public and private parties jointly address the problem. Innovative techniques need to be implemented in the private sector domain, but serve a public interest. For all stakeholders (municipalities, district water control boards, housing corporations and companies) the way forward should be clear: start talking to each other, recognize each other's interests but also discover the shared interest. They must be bold enough to venture outside established frameworks.

[1] If allowance is made for multiple use of space, with some of the land acquisition costs being allocated to the innovative solution, this results in an even cheaper innovative solution.

About the Author
Emil C. Hartman, M. Sc and Steven B. de Boer, DHV BV
P.O. Box 1132, 3800 BC Amersfoort The Netherlands emil.hartman@dhv.com