Water is a precious resource and should be treated as such. In many urban areas, water is merely viewed as a problem, and any precipitation is rapidly expedited out of the city in a gray infrastructure network of pipes and sewers. We can do better! We can also do it cheaper.
Nature-based solutions are low-tech, cost-effective, and nature-inspired solutions that help build urban resilience by working with nature instead of against it. Examples of nature-based solutions are green roofs, rain gardens, bioswales, and living walls.
An essential task of many urban nature-based solutions is restoring the natural water cycle. This is crucial as for the green to happen, we need to deal with the blue. As my hero, marine biologist Prof. Sylvia Earle once said: "No water, No life. No blue, No Green".
If we don’t take care of the urban water cycle, our chance of highly biodiverse urban areas is also diminished. It is all interlinked and begins with access to water in the right amounts at the right time.
The Urban vs. Natural Water Cycle
The natural water cycle is disturbed in urban areas mainly due to the sealing of the ground. This sealing of the ground is a crucial problem as water can no longer percolate into the ground and replenish the groundwater. This non-percolated water accumulates on the surface and can cause flooding during high-intensity rain events.
How do we solve this?
There are many interesting solutions, ranging from parks to rain gardens and bioswales. However, due to the lack of free space, or costly land, in densely populated urban areas, we frequently run into issues with these at-grade solutions. Often stormwater is led into the mixed canalization systems and expedited out from the city to an expensive sewage treatment plant.
However, there is a simple and easy solution for such areas: we can use roof areas to manage stormwater. One of the purposes of green roofs is to re-establish the natural water cycle in urban areas.
The aims are to reduce the volumes of water that needs to be treated by sewage treatment plants, use the evaporative cooling of the plants to cool buildings and surrounding areas (reducing urban heat islands effects), reduce horrendous combined sewer overflows, and reduce flooding.
Green roofs can provide these ecosystem services, but not every green roof can provide all of them at every location. Some of these services can only be provided by a subgroup of green roofs.
The Natural Water Cycle
In a forest, rain falls onto the ground and percolates into it. Some of the water is directly taken up by plants, some is absorbed into the ground, and the part that cannot be absorbed continues flowing along its path. The part of the water absorbed by the ground and taken up by the plants will slowly be returned to the sky as water vapor through transpiration and evaporation.
Transpiration notwithstanding, evaporation is also substantially more effective in a forest compared with asphalt or paved surfaces. The forests’ evaporative surface area is vast compared with an asphalt road; think of all the leaves, crevices, sticks, and mosses – all increasing and adding to the evaporative surface area.
The water that is evapotranspired is the volume of water that the system can retain. This water will not become runoff.
The water that is not absorbed or taken up by roots continues its journey above and through the ground. Some of it will percolate down into the groundwater, and some will directly replenish streams and lakes. The flow of this water is slowed down by the friction provided by sticks and stones along its way. This is stormwater detention (or slowing down of water).
We can look at a system’s detention capacity by looking at peak inflow vs. the peak outflow rates, e.g., there is a rainfall coming down at 500L/s/ha, and the saturated system slows this to reach an outflow rate of 10L/s/ha. Also, the system provides a shift in the peak flow of, e.g., 30min. This means that the system provides detention.
It is worth noting that the filling up of the dry system is not detention – this is retention. This delay is not peak outflow delay and is the most common misunderstanding of this concept. If the water remains in the system and is evapotranspired, the volume represents retention.
Delayed Outflow and Peak Flow Reduction
We are trying to re-establish the natural water cycle on a green roof to recreate the path of water through a forest but on a roof. A large portion is soaked up by the green roof substrate and is evapotranspired by the plants. This water represents the retained water. The annual retention capacity of a green roof is somewhere between 40 to 70% depending on type and location. Once these profile types become saturated, they essentially become a pipeline, a bit like a soaked bath sponge. This water is the remaining 30 to 60% that becomes runoff annually.
The runoff is generated by extreme storm events or repeat storm events, i.e., it rains many days in a row, which is a common weather phenomenon. To prevent flooding, erosion, and devastation, each construction project needs to provide the municipality/stormwater agency with a stormwater plan that proves that the maximum allowable outflow rates are met during a specific set statistical storm, e.g., a 100-year storm event. This means that if this 100-year storm event happens, the project maximum allowable outflow rate will not be exceeded, e.g., 10L/s/ha. This is essential to provide security for health, ecology, and the economy as flooding events can be disastrous.
Though excellent at retaining water, a traditional green roof is not capable of detaining stormwater to a sufficient degree to be relevant for a stormwater engineer. However, some green roof types and concepts provide detention, e.g., Blue-Green roofs, Blue Roofs, and Purple-Roofs.
Learn more about the detention capacity of the Purple-Roof concept in this video:
The Green Solution for Urban Areas
This article is not saying that detention-type green roofs are superior to other types of roofs. What it is saying is the right roof in the right place.
Suppose you live in a rural area or a small town where land is inexpensive. In that case, your best green nature-based stormwater solution is most likely a rain garden and a traditional green roof, or even a rain garden and no green roof (but perhaps you want the green roof for other reasons than stormwater management – there are so many reasons to get a green roof!).
Nonetheless, if you are building in an urban area where land is expensive, and a rain garden would be a financial disaster, a detention-type green roof is your best friend. We have seen many projects where massive sums have been saved by managing stormwater on the roof instead of the ground.
Detention-type green roofs can provide a significant ROI in urban and industrial settings.