Rambassun (Sandeep) Sewpal
Chartered Construction Manager and Chartered Architect,
Principal at Sandeep Sewpal Architect, RIBA Chartered Practice
Designers are currently faced with a new challenge which is designing for floods. The issue is becoming increasingly relevant for many reasons. The amount of natural soak-away available has been reduced by increasing urbanisation and higher densities of development and furthermore strained the existing drainage infrastructure. During the recent floods, we have noticed that people and properties that are not within currently recognised and defined floodplains are increasingly at risk of flooding, usually from surface water. The pressure for new housing and current policies that prioritise the use of green-field land (agricultural land under sugar cultivation), much of which is already at risk of flooding, may well mean more new development within the floodplain.
The Sources of Flooding
There are six recognised types of flooding which are namely, Tidal Flooding, Fluvial Flooding, Ground Water, Pluvial Flooding, Flooding from Sewers and Flooding from Man-made Infrastructure. In Mauritius, we are at risk from all six types of flooding. During the recent floods, many places across the country have experienced four (4) types of flooding simultaneously. During the heavy rain, rural and urban land could not absorb the surface water due to the large proportion of non-permeable surfaces, a problem often exacerbated by overloaded and out-dated drainage infrastructure. These circumstances, combined with intense rainfall, gave rise to localised flooding in many places of the country. At the same time, water level exceeded the river defences due to blockages of water courses and flood channels. In many places including coastal regions, ground water flooding happened as the level of water within the rock or soil that makes up the land surface (known as the water table) rose. To complicate the matter further, many urban areas experienced flooding from sewers. This phenomenon occurred where there are combined storm and foul sewers and their capacity is exceeded due to large amounts of surface water run-off in a short time. Poor cleaning and maintenance led to blockages that ultimately caused flooding resulting in water oozing out of manholes.
How can good design help?
It is essential to take an integrated approach to develop integrated control and mitigation measures. Civil and structural engineering expertise is required for the design of flood defences, barriers and underground drainage systems. Landscape architecture has also an immense role to play in the control of flood risk and the mitigation of its effects through the design of the public and private realm. The possibility exists to develop holistic design responses by identifying flood risk at the outset. First, it is important to understand the range of issues that gives rise to flooding.
The design process needs to be a step by step procedure that will firstly make an assessment of the risk (Flood Risk Assessment). Flood risk assessments (FRAs) aim to identify, quantify and communicate to decision makers and other stakeholders the risk of flooding to land, property and people. Secondly, it needs to prioritise spatial planning (Land Use Planning) to avoid placing new development in risk areas or at least substitute vulnerable uses wherever possible.
Following attempts to avoid placing vulnerable uses in hazardous areas the third step is to reduce the probability and severity of a flood to any parts of the scheme still at risk. Tidal and fluvial flooding can be controlled with coastal defences, river walls, barriers and barrages. These measures provide a primary defence against the risk. The risk of flooding from rainwater run-off and exceedance of drainage capacity can be reduced in a number of ways. Sustainable Drainage Systems (SuDS) integrated with the building and landscape design of a scheme will help control run-off generated by the development and also manage run-off from adjacent land.
Having put in place measures to control the flood risk, the next step is to consider how design proposals can minimise the potential consequences of a flood upon occupants, property, the public realm and the utilities infrastructure. Although we are making allowances to minimise flood risk, we are arguably entering a period of greater uncertainty because it is becoming harder to predict in advance a sudden burst of heavy rainfall over a short period of time over a particular area. Schemes should therefore as far as possible plan for the worst case in order to reduce the impact of a flood. Finally, proposals need to be re-assessed to check their impact upon future occupants’ safety, neighbouring areas, wildlife and ecology.
The design process is iterative which means that we need to calculate the result by repeating the cycle of operations. Therefore, it is imperative to understand the potential and type of flooding first and then develop proposals. After implementation of the proposal, the impact needs to be assessed and if necessary alterations and revisions considered.