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.

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Emma Ozsen
Sustainability Consultant
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Beautiful earth and stone structures at the School of Visual Arts of Oaxaca (Mexico)


Image copyright, Luis Gordoa

The School of Visual Arts of Oaxaca was developed at the request of the artistFrancisco Toledo, in collaboration with the Universidad Autonoma Benito Juarez. The lack of a master plan integrating the pre-existing buildings led to the project being designed as a large garden rather than just another building. At the same time, the building under construction on the campus were producing huge amounts of earth. These contingent factors suggested raising a bank of earth to provide the isolation necessary for an art school. Due to calendar and budgetary issues, the school was planned in three stages. The first two have already been built, giving form to what’s become known as “the crater”, which defines its perimeter with regard to the rest of the campus. Continue reading

What Is Sustainable Forestry?


Reducing paper usage has been one of the most iconic and understandable of the green movement’s mandates. The intricacies of forest management have remained obscure to most of us, however,  although forest management is the real key to preserving woodland habitats as well as our wood supply. Most consumers now recognize that it is an eco-friendly choice to look for the Forest Stewardship Council (FSC) label on wood products, to guarantee that they are sustainably produced … but what does this label mean, exactly, and is there more to the story of sustainable forestry?
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The power of building codes: why fewer died in Chile’s earthquake than in Haiti’s


According to an article on Bloomberg.com: “Earthquakes, heat waves, floods, volcanoes, super typhoons, blizzards, landslides and droughts killed at least a quarter million people in 2010 — the deadliest year in more than a generation. More people were killed worldwide by natural disasters this year [2010] than have been killed in terrorism attacks in the past 40 years combined”.
This is an alarming statement!


Image copyright: BBC

Even with the most sophisticated weather tracking tools, it is not always possible to predict when a natural disaster will occur.  Although we can track how a cyclone develops, it is very difficult to tract the exact path it will take and the severity with which it might hit when it reaches land.  However, there is one thing we can do: prepare and adapt.  The way we design and build our buildings can save lives. This is the reason why we build reinforced concrete structures in Mauritius, and rightly so, because we are prone to cyclonic conditions.

When it comes to earthquakes, it is estimated that 80% of deaths that occur during an earthquake is attributed to the collapse of buildings. As an example, in Haiti, where most of the population lived in poorly built structures, about 220,000 people were killed.  In contrast, the earthquake in Chile, which was about 500 times stronger than that of Haiti, caused less than 1,000 deaths.  One of the reasons for this is better building codes and better construction.

An interesting article below from BBC, talks about a system that helps buildings stay up, called the “strong columns weak beams” system, and helped prevent Chile’s buildings from collapsing.

Link to article: http://news.bbc.co.uk/2/hi/americas/8543324.stm

Rebuilding a Sustainable Haiti: Best Green Building Practices in Developing Countries


The January 12, 2010 earthquake in Haiti destroyed or severely damaged an estimated 200,000 homes, 30,000 commercial buildings, and 180 government buildings.Since the earthquake, building practices and guidelines in Haiti have been a topic of global discussion with many organizations inside and outside the country hoping to rebuild a more sustainable Haiti.

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What is sustainable construction?


A few tips to help you incorporate sustainable practices into your next project:

Sustainable construction brings together the principles of green building methods and eco-friendly values in a bid to lighten the environmental impact of residential and commercial structures. Continue reading

French Green Building Gets Highest BREEAM Award


A multi purpose green building in France has scored a first – “Spring” in Nanterre is the first building to achieve an interim ‘Outstanding’ rating under BREEAM Europe Commercial 2009. The development by Bouygues Immobilier and Archon Group has achieved 90% – the highest ever score under BREEAM for a building at the design stage.
Read more below:
http://www.greenbuildingpress.co.uk/article.php?category_id=1&article_id=655

Travelodge modular construction by Verbus Systems


Travelodge 120-bedroom container hotel opens in Uxbridge, UK! Developed by Verbus Systems, the completed design uses 86 containers of different sizes, retrofitted into bedrooms and assembled onsite. Travelodge plans to follow up with a 307-bedroom version at Heathrow.
Read more below:

http://www.verbussystems.com/projects/project-b.html