Supplemental water retention by flexible water level management and seasonal storage
As a rule, more precipitation falls in the winter than in the summer. More water also evaporates during the summer. As a consequence, water shortages occur in the summer, while winters have water surpluses. Water needs to be fed into urban water systems to compensate for summer shortages. In most cases, that water is of a lesser quality. Storing the winter’s water surplus can serve to create a clean water buffer for the drier summer period.
Climate changes will only cause the need for clean water to compensate for water shortages in summer months to increase. The various climate models present a picture of wetter winters and drier summers. All models indicate that summers will become drier. In most locations, the drier conditions will have an adverse impact on water quality: on the one hand because of internal salinisation caused by seepage and on the other as a result of external pollution, including pollution caused by salinisation when water from other areas is fed into the system. The deterioration of the quality of surface water will have a great impact on flora and fauna.
Forms of seasonal storage
Seasonal storage is an additional additional facility: the water volume to be stored comes in addition to the volume realised for storing precipitation peaks in the water system. The extra volume can be realised by reserving more space for surface water or by permitting and designing for greater fluctuations in water levels. Greater fluctuations in water levels demand special attention when banks are designed.
Another variant is to reserve a specific space in addition to the peak storage system to realise seasonal storage. That reservoir is kept empty during summer months and filled in the winter.
The dimensions need to be calculated to match summer water shortages. In practice, summer shortages will prove impossible to compensate using precipitation volumes buffered in the winter.
Riverbanks and quays near seasonal storage must be designed for significantly fluctuating water levels. Options include constructing a stepped quay or creating green embankment zones which are better suited to the changing water levels.
Seasonal storage by realising extra surface area
Realising additional surface area for storage can serve to create additional storage volume if the fluctuations in water levels remain unchanged. Part of the standard fluctuation of 30 cm, for example, is then earmarked for peak storage, while another part serves for seasonal storage.
This means that in the winter season water levels will fluctuate between the mid-level and the maximum level over the standard and the extra surface area, while in the dry summer season they fluctuate between the minimum level and the mid-level over just the standard area, since the peak storage volume required must be guaranteed at all times. The advantage to this method of seasonal storage is that the fluctuations are limited, which is good for flora along the banks.
Seasonal storage by realising extra storage height
Designing the surface area for storage to handle greater fluctuations in water levels is a way of realising seasonal storage capacity. The advantage to this form of seasonal storage as compared with the previous method is that it requires no additional surface area. In many locations, however, the height required cannot simply be created by limited drainage (the difference between the groundwater level and the level of the streets and buildings). Greater fluctuations in water levels will place demands on how the banks are designed and planted, since those banks will be exposed to more extreme conditions: during the winter season, large parts of the banks will be under water almost permanently, while those same parts of the banks will be almost entirely dry in the summer. Not many plants are capable of surviving those changes.
Oude Lek in EVA-Lanxmeer is an example of this form of seasonal storage. The banks slope gently, are covered in reeds and partly covered by a play beach. The maximum fluctuation in water levels is 0.3 m in this project [Pötz et al., 2009]. The current water level is shown on an information sign.
Flexible water level management
Flexible water level management involves anticipating precipitation that has been forecast. The water level is lowered before a shower is expected, to create additional water storage capacity. If this link between meteorological data and water level management is optimised, less surface water from other areas needs to be fed in. Since the water is almost always at the maximum level, salinisation is reduced. Less seepage occurs.
Specific seasonal storage facilities
Seasonal storage capacity can be given shape as a specific temporary facility: storage facility that is filled during the winter months or during extreme precipitation to compensate for shortages in the summer months. These facilities are generally kept empty during the summer, and as such can serve other functions during dry periods. Utilising this possibility for multiple uses of the spaces, places demands on the design of these facilities and limits the possible uses. Possible double functions include periodical events (festivals, concerts) or specific summer activities (camping). These facilities can during dry periods also be used as a playground.