The historical approach to water that fell onto cities has been to quickly get rid of it. Water was destructive and harbored illness. The prevalence of combined sewers in infrastructure across America shows that rainwater was as desirable to have around as human waste, and best discarded together. As stormwater volumes have increased due expanding development and the paved surfaces associated with growth, the issue has become more troublesome.
Even as volumes increase, we are beginning to see that water is not something to immediately discard. It is a valuable commodity and will become more so in the future. There is a shared opinion that future conflicts will be about water, not oil. Do we really want to discard something that costs nothing to receive and that could be reused, while at the same time using water treated to human consumption standards for flushing toilets and irrigating lawns?
There are three strategies for managing urban stormwater; evacuate it, slow it down or keep it. The evolving trend in stormwater management is to peel back the layers of concrete that swiftly evacuate rainwater and seek ways to achieve detention, absorption and retention. Infiltration is the gold standard, but in a true urban environment there is little remaining soil into which rainwater can infiltrate. There is no strata of native lying just below the surface, waiting to be exposed. Any soil intended for the purpose of infiltration must be placed onsite by design and edged in a structurally sound manner.
It is not feasible to envision a system that does not include buried stormwater conveyance. Structural engineers are understandably leery of allowing water to sit underground, weakening footings and fill. However, intervening between the water that falls and the systems we've built to shunt it away can have a huge impact. Connections to the storm system can be designed with the motivation that they will be utilized only as overflow in extreme events. U-shaped interventions can be the standard- grading and drainage systems that direct water into constructed wetlands at the surface. Water only enters the subterranean system of chamber and pipes during heavy storm events. Rainwater from the majority of events will stay into the surface areas onto which it was diverted where it can evaporate or be absorbed by plants.
One force against adoption of green infrastructure is budgetary. It is a difficult task to convince a city manager to build what appears to be a duplicate system of green infrastructure on top and gray infrastructure below. But the cost savings become apparent when green interventions across a city keep significant volumes of water out of catch basins and out of treatment facilities at the end of the pipe. Take a look at the Anacostia River Tunnel and the Blue Plains Treatment Facility in DC for an understanding of treatment cost.
If done well, constructed wetlands can be designed as an aesthetic feature. However, the soil medium needs protection from passersby (if pedestrians can walk on a surface, they will). This can be done through a variety of methods such as grade changes or grates. Maintenance crews should be trained to avoid compaction of the soil.
One key reality will inform the design of a constructed wetland- real estate values. Intensity of the structural design is directly proportionate to the value and availability of open space on the property in question. In urban environments, space at at a premium. Every square foot matters when trying to fit buildings, cars and people along the same block. Think of any standard pond constructed next to the parking lot of any typical big box store. It probably has a fountain in it to promote aeration. In the urban environment, that pond loses the luxury of space and becomes a constructed wetland filled with workhorse plants like sedges that can survive pollution and harsh conditions. While obviously not a natural feature, the functional aspects of natural systems are being replicated through biophilia.
The next big trend will be about storage of rainwater for reuse. The single biggest cost deterrent in a rainwater harvesting system is the storage, while treatment and monitoring add maintenance requirements. For now, harvesting mechanisms are infrequently included in construction projects. But necessity is the mother of invention; water-poor cities such as Las Vegas have embraced water-saving practices that the water-rich east coast feels no pressure to consider. As we move forward into the coming decades, demands and pressures will continue to shift on our cities and managers will look to new infrastructure practice to achieve health and resiliency for their citizens.