While both these disasters – floods and drought – can be blamed on global climate change, floods in urban areas are also largely as a result of the geography of the urban built environment, specifically the preponderance of impermeable surfaces like tarred roads, paved courtyards city squares and, of course, roofs.
Unlike in nature, where rainwater is absorbed by vegetation, and finds its circuitous way into sinuous rivers with permeable beds and vast amounts of riparian vegetation, in the cities, rainwater just pours over roofs, down drain pipes, across and along roads and into hard-surface storm water drains.
Ultimately, the rainwater ends up in urban rivers, many of which have been canalised or otherwise radically changed from their natural state. Thus the problem is the rain is not soaking into the ground, into rivers or wetlands, recharging aquifers and feeding plants. In short, there is too much water where we don’t want it, and not enough where we need it. This imbalance negatively impacts the whole urban hydrological cycle.
This, along with the “harvesting” of water from urban rivers and/or boreholes, results in a lowering of the water table, which further decreases the ability of the ground to absorb rainwater. It’s a vicious circle, resulting in unsustainable cycles of low water levels in urban streams interspersed with episodes of dramatic and destructive flooding – both because the rivers have lost their ability to absorb rain, and because it is delivered too rapidly through hard-surface run-off.
Urban flooding – the Servest approach
That’s the bad news. The good news is that the solutions are actually relatively simple, and are most effectively done by emulating nature, and trying to ameliorate the more destructive elements of urban design.
The Landscaping & Turf division of Servest does just that. Their integrated approach to addressing the issues surrounding urban flooding involves natural, and/or bio-engineered solutions that are implemented in the short, medium, and long term, and that are ultimately aimed at creating resilient and regenerative urban landscapes that can support the complex hydrological cycle associated with urban systems.
This is achieved by promoting and restoring resilience, biodiversity and ecological integrity, by re-establishing natural physical processes like flow and sediment size, natural physical features like gradient and river shape, and biodiversity and natural habitats.
At the onset of each project, it is important to identify and understand the connections of the river or strtream with its catchment, and with other rivers and floodplains, and then to find immediate, if imperfect, short term mitigation, medium term strategies and ultimately, long term sustainable solutions.
Short term solutions usually involve increasing riverbank stabilisation by maximising vegetation cover within the riparian corridor, and possibly some other short term, mechanical interventions.
Medium term solutions involve increasing the lateral river movements, to allow for the creation of a dynamically stable riparian zone that can accommodate flood events. There are various ways to achieve this, but a simple solution is through the creation of floodplains and wetlands. In other words, allowing – but containing – a certain level of flooding.
Long term solutions involve a completely integrated approach aimed at reducing the incidence and severity of flooding. This is usually done in conjunction with local stakeholders who assist with in-situ approaches to reducing the rate and/or volume of overland stormwater runoff from urban infrastructure, and increase groundwater replenishment in-line with sustainable drainage systems (SuDS) principles.
Depending on the project, solutions may be implemented at source, in small neighbourhoods, or even regionally. At-source solutions include rainwater harvesting and/or rain gardens. Neighbourhood-scale solutions may include bioswales and/or bioretention areas, while regional strategies include the construction of wetlands and retention ponds. The creation of rain gardens, bioswales and wetlands has the added advantage of biologically “cleaning” run-off before it is returned to rivers. And all of these strategies, when successfully applied, will ultimately result in the rejuvenation of urban rivers, the return of birds and animals and perhaps even fish, making urban waterways the pleasant little havens of peace and tranquillity that they should be.