Associate Public Health Engineer at BuroHappold Ashveen Jeetun discusses flash floods in the UAE

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Ashveen Jeetun, BuroHappold Engineering, Flash flooding, Drainage, Cloud seeding

Flash flooding is becoming a yearly recurring event in the UAE. This can lead to a direct impact on costs and disruption to businesses and individuals.

Losses of life, though rare in the Middle East, can also happen. Cloud seeding in the UAE is also bringing an increase of about 30%-35% in annual rainfall.

Related

According to the National Centre for Meteorology, cloud seeding in the UAE started in the 1990s and has since placed an increasing focus on missions. In 2018, the UAE conducted 242 missions.

There are many reasons why buildings flood and, very often, it is often a combination of factors that lead to a particular building flooding.

Can the design of a storm water system be enhanced to reduce the risks? In this article, we are going to investigate some of the factors that contribute to flooding.

Using a hospital building as an illustration, we’ll outline some principles that can be adopted to help mitigate the risks of flooding.
Selection of the correct storm design criteria is an important component of design against flooding.

The Municipality prescribes the minimum design criteria, which the engineer can use for the design of the rainwater system. But some developments are more at risk than others.

Sites located within a depression are at higher risk as water from the surrounding areas will flow towards the development. 

A high water table normally allows less water to naturally infiltrate into the ground as the soil gets saturated quicker than a development with a low water table.

Large portions of hardscapes mean a higher volume of storm water run-off in a shorter time, which results in higher storm water level.

Therefore, higher storm intensities than required by Municipalities are often required. It is not unusual for the engineer to be challenged by the client about the selection of more intense criteria.

It is the responsibility of the engineer to select the right criteria and to convince the client about the selection.

Some of the factors to be considered while selecting the criteria are grading, the use of the proposed building, content of the building, water table level, and the development proposal in terms of hardscape, softscape, location of the entrances.

It is also encouraged to use more than one design criteria on certain developments.

For example, the minimum municipality criteria can be used at the external car park of a hospital and a more intense storm criteria is used at the main building, main entrance, utilities for uninterrupted power and water supply, and the emergency entrance of the hospital.

This will allow the major areas to operate normally in the event of an intense storm.

Similar considerations should be given to other types of buildings where critical areas are designed at higher storm events.

Grading is an important factor that requires careful consideration. MEP engineers can often design storm water with information within the plot.

The surrounding plots or roads information like grading and slopes are rarely taken into consideration.

No matter what criteria an engineer specifies, the building will be at risk if the surface water from adjacent plots enters the site.

It is therefore important to check information from the surrounding plots. If the plot to be developed is at a low point, then mitigating measures like raising building and basement entrance levels must be considered to reduce the risk of flooding.

The development grading is normally done by the landscape architect. What may be aesthetically nice to look at may not be the best solution in terms of surface water runoff.

The engineer, working closely with the landscape architect at the beginning of the design phase, can help reducing flood risks.

The site grading could be designed to convey storm water away from entrances. The number of gullies required could be reduced, thus reducing maintenance of gullies, which often get blocked with sand.

Figure 1 shows that an example of a hospital with existing road levels in red and the proposed level in black.

The building was constructed at a level which is at least 300mm above the gate level.

Extra measures were taken to set the car park at a lower level than the emergency and main entrance.

In case of an intense storm, the car park will be used as ‘sacrificial area’ to allow water to build up in this area instead of flooding the building and its access, thus allowing uninterrupted operation of the hospital.

While the engineer is responsible for the selection of criteria, it is important to check how the site will behave in more intense storms, especially if the site is designed to the minimum authority requirements. This will help in identifying the areas at risk during an intense storm.

When identified, either the design could be modified to manage the risks, or the flood could be shifted to another area like an external car park or a landscaped area.

This has been found to not be regularly practised in the Middle East; or a requirement from the authority. But by doing so, the building management will be aware of the areas that are susceptible to flooding and adequate flood management plans could be made.

Sustainable drainage systems (SuDS) is a concept that reduces flood risk and offers additional advantages. Construction projects disturb the natural and slow drainage system of rainwater.

SuDS are designed to mimic the natural drainage by allowing slow run-off, filtration on the surface, thus reducing pollution, recharging ground water, and even promote wildlife.

Incorporating SuDS in a project will reduce the rate of run-off and therefore the risk of flooding. A useful concept used in SuDS incorporation is the SuDS management train.

The management train starts with source control (prevent or reduce water run-off by reducing hardscape or by controlling the run-off).

Source control reduces the volume of rainwater and increases the time required of rainwater entering the drainage network.

Green roof is a roof that is partially or completely covered with vegetation and a growing medium. Green roofs are not only an important and effective source control, but also help in reducing the cooling load of HVAC systems.

A blue roof also helps in reducing the flow rate of rainwater by allowing water to build up on the roof with a slower discharge rate in the rainwater network. Source control also helps in having smaller pipe diameters inside the building.

The next step of the SuDS management train is the site control. At this stage, rainwater from the building and from the site is managed by using several techniques like a rain garden, swales, infiltration tanks, detention basins, porous surfaces.

All these measures allow the storm water to be managed on the site with only a small portion discharging into the Municipality network.

With the occurrence of drought and inundation in the UAE, a lot of the plants are well adapted for the incorporation of SuDS. Most of these are grasses and perennials.

Incorporation of SuDS can help a project in many other ways.

Benefits include lower cost of drainage when planned landscapes are redesigned as a SuDS feature; better aesthetic; reduced risk of flooding on site; less run-off in Municipality network thus reducing the load on the public network; reduced urban heat island effect; and less maintenance and risk of blockages.

The use of software by civil engineers is common in the Middle East and European countries such as the UK. This is not typically the same for MEP engineers.

Software design packages such as microdrainage and StormCad help engineers to simulate storm water systems at different storm intensities and identify areas of the site where drainage may occur. 

Early flood warning systems can be used to give building operators time to react to a flood alert.

Sensors installed across several critical areas around the site could monitor water level and send an alarm when there is a flood risk.

Various forms of alert systems can be used for notification of potential danger.

Alerts that can be generated include a phone call or SMS – this is the most common mode being used to generate automated real-time alerts in case of flood monitoring systems; a siren – another common form of an alert, a loud siren would be generated when the water level exceeds the user-defined threshold; and email – which could also be sent in addition to the above two methods, notifying the user of a potential threat.

The system could be integrated with some automated actions, for example starting an emergency pump to flood a sacrificial area of the site, automatic closure of penstock to prevent water from the Municipality network to enter the site drainage system, activation of flood barriers, closing of car parks and the like.

Design against flooding comes with the difficulty of accurately predicting a storm pattern, however the design could be enhanced to provide better building and life protection.

The techniques to be used on a development varies and incorporating the above measures will help in reducing flood risks and serve to protect occupants, as well as properties, in the Middle East.

It is important for engineers to discuss the techniques early in the design stages so that the architectural and landscape planning develops with these techniques.

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