Authors Er. Vivek Abhyankar, Founder of SGAWings Civil Engineering Consultant and Advisor (OPC), Mumbai & Dr. N. Subramanian, an award-winning author, consultant, and researcher, based at Maryland, USA, present a brief review of the causes, effects, and remedies to prevent drowning of infrastructure in Indian cities during the monsoons.
On July 26th 2005, Mumbai experienced severe flooding (reported as once in a hundred year phenomenon by the officials) and about 900 mm rainfall was recorded within four hours duration. Most of the city infrastructure (roads, railways etc) was drowned, as water accumulated everywhere. The local municipal corporation decided to take action and declared many new plans like banning polythene carry bags of lesser than 20-micron thickness, cleaning of the local Mithi River, relocation of slump areas/huts outside city borders, use of pervious road tops, cleaning of drains before the monsoons, and so on.
However, due to the constant change of government, not much implementation was done, and whatever was implemented was not adequate. Since then, many other Indian cities have experienced severe flooding situations (Chennai in 2015, Kolhapur and Sangli in 2016, and others). But still, the governments take a ‘band-aid’ or ‘fire-fighting’ approach and no concrete proposal has emerged. The lives lost in these floods and the damages caused to the buildings and the infrastructure is beyond the imagination of anyone (especially the sympathetic value of a lost family member in such calamities). Hence, a brief review of the causes, effects and the remedies of the drowning of infrastructure are provided. In order to bring big changes in the society, it is not enough if the governments alone change the rules, but each and every citizen should follow the rules and put in their own efforts.
Key Reasons of Water Accumulation and Infra Damage
The garbage dumped by the people in the city nalas, drains, the debris dumped by illegal construction works, lack of maintenance and cleaning of drains before the start of rainy season, dug-up roads and open pits, etc., are key reasons of water accumulation and damage. Some of the low-lying areas in cities (lower than the sea level) experience inward flow of water i.e., from sea towards land during high tides. If the timing of such high tide coincides with heavy rain, then the densely populated cities situated on shorelines like Mumbai, Chennai, Bhubaneswar, Kochi etc, may experience severe floods. Figs.1 to 4 show the effect of severe flooding in a few major cities in the past couple of years. Urbanization of the landscape profoundly affects how water moves both above and below ground during and following storms, the quality of the storm water, and the condition of nearby rivers, lakes, and estuaries.
Causes of Flooding
As per engineering principles, if the inflow of water is equal to the outflow, flooding will not occur. The pipes and drains which carry clean and waste/tainted water, respectively are similar to the arteries and veins of the human body. If any of them get clogged, then the body shows symptoms of chest pain, tightness and shortness of breath, etc. In a similar fashion, the drains and pipes will leak, burst or overflow leading to or adding to the situation of inflow greater than the outflow and the resultant flooding.
Drinking water pipes usually do not get clogged in major cities (but the leakage due to illegal punctures and damages owing to the theft of clean water is a major worry of municipal corporations). However, in the case of drains and sewer lines, the choking occurs mainly due to the plastic bags and garbage thrown by the local people (Fig.5). Tree roots invading and blocking the sewer pipes (80% of blockages are due to this), deterioration of old pipes, and flushing foreign objects (such as disposable nappies, wet wipes, cotton buds, and sanitary napkins) down the toilets are common causes of blockage of sewer lines.
Moreover, the storm-water drains constructed in major cities were designed for the outflow based on the calculation of runoff at the time they were built. Nowadays, the situation has changed considerably. Many houses and apartments construct impermeable concrete platforms/patios, using interlocking paver blocks around the houses, preventing water infiltration. In addition, the roads and platforms are also paved with concrete or asphalt, which again do not permit the water to infiltrate.
Thus, the entire water generated during the rains tries to flow into the old storm-water drains, resulting in an overflow. This situation is compounded by nature also; cloud bursts are sudden and with very heavy rainfall, usually local in nature and of brief duration, resulting in rains higher than 100 mm per hour within an area of 10x10 km. Such heavy rainfall requires careful consideration as it may result in 10-12% of annual rainfall within just one hour.
Another problem is the illegal construction of buildings in areas which were originally designated as water bodies. This will not only result in the constructed buildings being inundated during heavy rains, but also prevent storage of water in lakes and flooding of cities. For example, there are many roads in Chennai called Lake-view Road, Tank Bund Road, New Tank Street, etc., but there are no lakes!
Scientists involved in the Intergovernmental Project on Climate Change (IPCC) had also warned that a 1.5oC increase in global temperature will generate a global sea-level increase of about 0.3 to 1 m by 2100. According to them, even if global warming is stopped immediately, sea levels may continue to rise, and affect several cities along the seacoast.
As mentioned earlier, majority of the land in metro cities is covered with concrete/bitumen pavements or tiles. In Mumbai, after the historic 2005 floods, the municipal corporation decided to lay paver blocks on walkways and shoulders. Technically, this decision was good, but its implementation was erroneous. Many local contractors laid the paver blocks on a hard concrete bed (instead of a sandy bed), while a few used inferior quality paver blocks, which got worn out under pedestrian movement. A few contractors used the paver blocks even on surfaces where vehicles ply and on a few bridges as a wearing coat; this increased the operating cost of vehicles and the fuel consumption. Thus, the whole purpose of using paver blocks failed.
The most simple and effective solution is to keep the city clean such that there is a balance of inflow and outflow. The Municipalities, Corporations, Metropolitan Development Authorities etc. should prevent unauthorized constructions in areas earmarked for lakes. Such encroachments along waterways should be removed by providing alternate sites for them to construct houses. Some other possible flood control measures are listed in Fig. 9.
Installing flood barriers (Fig.10) along the sides of local rivers, channels, canals, and streams flowing through the city are found to be effective in containing the flood water within the river. Flood barriers can be vertical, inclined, fixed, floating, or automatic.
The world’s largest storm surge barrier is the Maeslantkering in the Netherlands; it automatically closes when needed (Fig.11). Constructed during 1991-1997, it is one of the largest moving structures on earth and consists of two 22 m high and 210 m long steel gates, to which 237m long steel trusses were welded. Standing upright, the arms are as high as the Eiffel Tower, and each one weighs about 6,800 t. It operates with ball-and-socket joints, each of which is 10 m in diameter and weighs 680 t. The cost of construction was 450 million euro.
It may be interesting to note that the levee system of the New Orleans area in Louisiana, USA, withstood the Category 4 Hurricane Ida that struck the area on 29th August 2021, and had winds up to 240 kmph.
Levees (earthen embankments) and floodwalls are typically built parallel to a river, to reduce the risk of flooding of a city. The levee system failed during Hurricane Katrina on the same day in 2005 resulting in catastrophic damages to infrastructure. Authorities felt that the present situation may be due to the $14 billion hurricane risk-reduction system consisting of levees, pumps, seawalls, floodgates, and drainage built by the US Army Corps of Engineers that provides enhanced protection from storm surge and flooding in the greater New Orleans area following Katrina in 2005. Although the flood waters overtopped some levees and others did fail, more extensive flood damage was averted.
Pervious Concrete Pavements
Use of pervious concrete pavements is found to be effective in better drainage/retention of large volumes of rainwater. Pervious concrete is a concrete with no fine aggregates. It has a 15-25% void structure, allowing for 120-320 liters of water per minute to pass through each square metre, with typical average flow rate of 3.4 mm/s (200 L /m2/min) or more (Fig. 13). This flow rate is greater than that generated during any rain event, allowing water to flow through it (Subramanian, 2008).
Installation of pervious pavement differs from conventional concrete pavement. The concrete is usually placed over a base of clean, gap graded gravel or crushed rock (25 mm maximum size) that acts like a reservoir to hold water until it can infiltrate the underlying soil. A geosynthetic liner may be placed below the stone reservoir to prevent preferential flow paths and to maintain a flat bottom (see Fig. 14). Designs also may incorporate some method to convey larger volumes of storm-water runoff to the storm drain system, such as the inclusion of drainpipes below the pavement, diverting storm-water flow to supplementary catchment areas for potential reuse. It is important not to over compact the subgrade soils, as the key design feature of a pervious concrete pavement system is its permeability (Subramanian, 2008).
Green roofs capture and store rainfall that would otherwise fall on an impervious roof material. This is an engineered growing media designed to support plant growth. A portion of the captured rainfall evaporates or is taken up by the plants, which helps reduce runoff volumes, peak runoff rates, and pollutant loads.
Green roofs typically contain a layered system of roofing (Fig.15), and are designed to support plant growth by retaining water while preventing ponding on the roof surface. The roofs are so designed that the stormwater drains vertically through the different layers and then horizontally along a waterproofing layer towards the outlet. Green roofs usually have minimal maintenance requirements. Plant species are selected in such a way that the roof does not need supplemental irrigation or fertilization after vegetation is planted.
Green roofs are typically not designed to provide storm-water detention of larger storms, although it is possible to design them to meet these criteria. Green roof designs can also be combined with a separate facility to provide large storm control.
Use of Drones for Flood Management
Drones are being used effectively in flood management (Fig.16). Before floods, they can be used to take riverbed surveys as a kind of preventive application. During floods, they are fitted with high resolution mapping devices that give an eagle’s eye view of the flooded area. Experts can use drones for mapping or remapping the affected area for making quick assessment of damages caused by floods.
Another system that can help alleviate flooding and also reduce the water runoff of cities paved with concrete platforms and asphalt roads, is the rainwater harvesting system. In the conventional rainwater harvesting system, the water that is collected from the building terrace or pavement within the compound wall of a building, is channeled into an underground storage tank through a soak pit (filled with brick jelly) that is connected to the well (thus recharging ground water Fig. 17).
The problem with this type of system is that if the downpour is heavy, the tank or water collected in the soak pit will overflow since the surface area for percolation is not considerable. Moreover, for the system to function effectively, some filters should be provided to eliminate the pollutants during the first flush of rain. In addition, the contents of the soak pit have to be replaced periodically for effective filtration, or a mechanism should be available for removing any contaminants that accumulate at the bottom of the tank.
In 2002-2003, the Tamil Nadu government implemented a strict law mandating rainwater harvesting for every building in Chennai, however, it could not prevent the large scale flooding of the city in 2015!
Trash Capture Technologies
Use of garbage collecting nets is proving to be effective in recent times. But it requires periodic cleaning and replacement of the bags. Figure 18 shows such bags installed on the downstream face of a pipe culvert. The selection of any one of these trash capture technologies is based on the budget, capacity for operation and maintenance, local hydrology, and other site-specific considerations.
Other Storm-Water Best Management Practices (BMP)
- Bio-retention facilities that capture and store storm-water runoff and pass it through a filter bed of engineered soil media consisting of sand, soil and organic matter. Using such a facility, the filtered runoff may be collected and returned to the conveyance system, or allowed to infiltrate into the soil.
- Filtering systems capture and temporarily store the design volume and pass it through a filter bed of sand, organic matter, soil or other filtering media. Filtered runoff may be collected and returned to the conveyance system.
- Infiltration practices that capture and store the design volume before allowing it to infiltrate into the soil over a two day period.
- Vegetated open channels designed to capture and treat or convey the design storm volume.
- Storm-water ponds that are storm-water storage practices consisting of a combination of a permanent pool or shallow marsh that promotes gravitational settling, biological uptake, and microbial activity.
- Storm-water wetlands (explicitly designed to provide storm-water detention for larger storms) that create shallow marsh areas to treat urban storm-water which often incorporate small permanent pools and/or extended detention storage.
- Storage practices designed to provide storm-water detention (2-year, 15-year, and/or flood control).
The authors wish to acknowledge that the images used were taken from various websites on the Internet.
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