Deadly flash flood event in Mauritius on 30th March 2013 – How SUDS can help reduce flooding risks

Deadly flash flooding event in Mauritius on 30th March 2013

Deadly flash flooding event in Mauritius on 30th March 2013

It is very saddening and shocking to see that 11 people have been victims (so far) of the tragic flash flood event on the 30th March 2013 in Mauritius. My thoughts are with the families who lost their dear ones and those who have lost their property and belongings during this event.

I am not an expert in drainage system design or flood risk management, however I would like to share some information that I have gathered during my career, regarding reduction of flooding risk through the implementation of SUDS (Sustainable Urban Drainage Systems). I am providing the information below, in the hope that the engineers/designers who will review the drainage system in Mauritius after this terrible event, might take SUDS into consideration. 

Flash floods are terrifying events, that can be very destructive and deadly, as we have witnessed. Localised heavy rains, together with inadequate drainage and increasing urbanisation can lead to severe problems due to flash flooding after sudden heavy rainfall. Cutting down vegetation to replace them by artificial surfaces such as impermeable concrete and tar means the area loses its ability to absorb rainwater. The rain is therefore directed into surface water drains instead, causing them to be overloaded and resulting in floods. Whatever we do on land will affect the hydrology of an area.

All it takes for flooding to become a problem is for 40% of vegetated area to be cleared and replaced by impervious surfaces such as impermeable concrete. The amount of surface water runoff will then double and flow twice as fast. Extreme rainfall events can cause hundreds of mm of rainwater to fall within a few hours – this amount of water flowing at high speed on impervious concrete areas will certainly overload drainage systems, which cannot dispose of the water as quickly as they are receiving it.

There are several ways of managing stormwater. The more traditional way is to divert water runoff from drains into rivers (“rapid conveyance” approach). However, the water runoff is sometimes so rapid that the rivers just cannot cope with the sudden deluge, leading to flooding downstream. When drains and rivers are blocked by debris, the water flow is impeded, worsening the flooding event.

Retention ponds hold water run-off back and slowly release it to mitigate flooding

Retention ponds hold water run-off back and slowly release it to mitigate flooding

Another way of managing rainwater runoff is to use the “control at source” concept. This means increasing the time that the water takes to reach streams by controlling it at source. This is a totally different approach from the rapid conveyance method. Water runoff is first captured and then gradually released or allowed to infiltrate the soil. Therefore, instead of a sudden, excessive water flow to areas downstream, the water flow is mitigated and reduced, making it more likely for drains and streams to be able to cope with it.

A swale

A swale

However this requires that the water runoff from the site must be similar to or less than levels before land-clearing took place. This means that developers should be responsible for limiting the rainwater discharges due to their construction when a vegetated site is changed into an artificial impervious one. Instead of merely building drains to cater channeling of rainwater away from their development site, they should contain the water instead and release it gradually or allow it to seep into the soil. We should not work against nature, but with it. We should try to mimick nature’s natural hydrological cycle through a combination of infiltration, storage and delayed stormwater flow. The approach to replicate natural drainage systems is termed the SUDS (sustainable urban drainage systems) approach, sometimes termed “sustainable drainage systems” as they are not restricted to urban areas only. SuDS have a low environmental impact by managing water runoff through collection, storage, and natural cleaning before it is allowed to be released slowly back into water courses (drains and rivers). Usually SuDS constitute a combination of water retention ponds, wetlands, underground storage tanks, infiltration, permeable surfaces, swales, and filter drains with the best combination of structures and techniques being specific to the site in question.

Permeable paving especially useful in car parking spaces to allow for subsurface infiltration

Permeable paving especially useful in car parking spaces to allow for subsurface infiltration (image courtesy of RIBA Sustsainability Hub)

The way we manage rainwater runoff should be reviewed and alternative ways of managing stormwater could be explored and evaluated, as well as the establishment of by-laws to guide the implementation of more sustainable and natural stormwater management practices, replicating natural attenuation, infiltration and drainage, rather than relying solely on concrete-lined drains. The benefits of both water storage and infiltration strategies extend beyond that of controlling water runoff. They also improve water quality, encourage bio-diversity, add amenity value and help us adapt to and mitigate the negative impacts of climate change.

Emma Ozsen
Sustainability Consultant

GE Says Better Incentives Needed To Stimulate Widespread Water Reuse and Recycling

On World Water Day GE Announces New White Paper Highlighting Successful and Practical Incentive Policies for Water Reuse

London, United Kingdom—March 23, 2011—The world’s urban and industrial water use is projected to double by 2050, yet one fifth of the world’s population, or some 1.2 billion people, already lives in areas of water scarcity. One of the best ways to stretch our planet’s dwindling supply of available water is through increased reuse and recycling, yet progress in these areas has been limited for a host of economic, political and social reasons.
One major stumbling block is a lack of effective incentives, according to a new white paper to be issued by GE (NYSE: GE). The paper describes the multifaceted nature of the problem and highlights various incentive policies and structures from around the world to illustrate those which have been effective in encouraging water reuse and recycling. GE will present the white paper at its Water Summit, From Used to Useful — Middle East, taking place on April 5-6 in Saudi Arabia.
“Our goal is to stimulate action to preserve fresh water supplies,” said Heiner Markhoff, president and CEO—water and process technologies for GE Power & Water. “Cost-effective technologies already exist to solve virtually all water challenges, thus the focus needs to be placed on the human side of the equation. In that regard we see four main approaches: increased education and outreach so that people can see the need and the benefits;  removal of bureaucratic and other barriers; effective use of mandates and regulations; and establishment of effective incentives, which is the focus of our latest white paper.” Continue reading