Vertical Water Sponge - Water Storage Systems That Protect Against Flooding and Extreme Heat

Why do we need a new solution?

When houses, streets and squares cover a large portion of the available area, heavy rain quickly becomes a hazard. Insufficient infiltration capacity poses a risk of overloading sewer systems, flooding and damage to buildings and infrastructure. Urban planners are taking proactive sponge city measures to prevent such disasters. These include constructing aboveground and underground infiltration systems or retention basins, unsealing surfaces, creating green spaces, planting trees and installing green roofs. However, the extent to which such structural interventions are possible is often limited, primarily because the available space restricts their scope. Holger Wack’s team at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT has developed a solution in the vertical water sponge. 

What makes the project unique?

The vertical water sponge expands the toolkit for urban water management with an innovative technological approach that exploits the largely unused potential of facades, walls and other vertical structures on a large scale for water retention. In comparison with other sponge city measures, these retention systems only need a small footprint to absorb large quantities of water, with a corresponding reduction in the load on drainage and sewer systems. The core element of the new solution comprises storage modules with a moisture-permeable outer shell around a filling of mineral granules that can absorb and store water. This absorbs rainwater routed from the roof surfaces like a sponge and later releases it back to the surroundings by evaporation.

Not only do vertical water sponge systems have a small footprint, they can also be implemented relatively easily without intervening in the existing infrastructure. This means no costly civil engineering work such as creating underground retention basins and no elaborate maintenance or irrigation during dry periods such as is required for conventional green walls or green roofs. This saves money, time and resources.

However, the vertical water sponge also has other benefits. For example, the cooling effect due to evaporation could create cool spaces during the summer heat or positively affect the microclimate of city centers, or it could be integrated in building services to augment the air conditioning of individual buildings. Module surfaces with enhanced functionality are also conceivable. For example, vertical water sponge elements could be extensively planted with sedum and mosses, or they could be equipped with photovoltaic panels capable of increased power generation in the summer due to the cooling effect of evaporation on their rear surfaces. 

Who will benefit from the new technology?

The vertical water sponge concept is ideally suited for large-scale use in urban areas, in both existing and new buildings, in public spaces and in residential or industrial buildings. This provides urban planners, construction companies, architectural companies and licensed private and public building owners with a new sustainable tool for efficient rainwater management, flood prevention and modern, aesthetically pleasing design of facades and the urban landscape. As part of a comprehensive overall concept for public spaces, vertical water sponge systems could help prevent flood damage even during exceptionally heavy half-hour rainfall events of up to 44 l/m² that occur once every 100 years. The evaporative cooling effect of the vertical water reservoirs would also counteract the urban heat island effect, which is the overheating of urban areas caused by dense development and a lack of green spaces. This could improve the quality of life for the city population and reduce the load on the healthcare system during periods of extreme heat.

How does the new solution work?

The water storage modules (retention elements) are among the key system components. Their permeable outer shells should be as stable as possible while allowing the evaporated moisture to permeate out unhindered. A specially fabricated perforated sheet of anodized aluminum has proven itself in the Fraunhofer institute's demonstrator. For the mineral core storing the water, the focus is especially on water storage capacity and permeability. Over the course of the project, perlite and vermiculite have proven to be ideal filler granulate and have outperformed all alternatives tested thus far.

The retention modules are currently configured above each other. The top of each cascade ends with a control unit. The rainwater flows through a drainage system into this upstream intermediate storage element. Once a specific water level is reached, a type of sluice automatically opens downward and the water trickles through a water distributor and a sieve onto the water-retaining substrate. Another key question for the researchers is the optimal rate for the control unit to drain. During heavy rain, the process should proceed quickly enough to allow the mineral filler to completely absorb all of the water without losing valuable time.

If the control unit is at risk of overflowing, the excess rainwater is directed to the downspout and into the sewer system. This system functions with no digital controls or external power. It therefore remains fully operational even if the power supply fails in the event of a disaster. The architectural or planning company decides how to integrate the modules in the supporting structures. Possible implementations include elements integrated in the wall or offset facades, ideally with ventilation in the rear to enable the wall behind them to breathe.

Why is the Fraunhofer Future Foundation supporting this project?

Vertical retention systems could help prevent flooding and damage to buildings and infrastructure during heavy rainfall events in city centers with large paved areas. They would also support the natural local water cycle, since the moisture they store evaporates and thus returns to the environment. Evaporative cooling could help improve the urban microclimate on hot days and cool the immediate surroundings. This solution could be implemented for sustainable and resilient urban development (SDG 11), climate adaptation (SDG 13), sustainable water management (SDG 6) and heat protection.