The water drainage system in North Bangalore – roughly the area between IISc and Kodigehalli – may become “unreliable”, with rainwater runoffs exceeding the capacity of the water drainage network, finds a recent study from IISc. This can obviously cause urban flooding, among other problems.
The study further shows that there is a chance of flooding if the probability of storm event exceeds the estimated limit of 50% (or lower) in any year.
Most of us have seen pictures of India in the early twentieth century: lesser houses, scattered roads and a few commercial hotspots here and there. In less than a century, things have changed at an unprecedented rate -- and are still changing. To accommodate the increase in anthropogenic requirements, a lot of rural landscapes are becoming cropland, or a mosaic of buildings.
Development thus leads to changes in the land cover and land use patterns of a given area. Land use refers to the purpose or the utilization of the land by the people and land cover refers to the habitat type at a given area: forests, wetlands or urban area, for example.
This urban revolution is leading to many consequences on the landscape itself. One such change is the manipulation of the network of storm water drainage system. The rainwater that pours down follows a pathway in the form of streams, reaches the rivers and eventually the sea. This path is, however, naturally determined by the elevation of the land, sloping pattern, substrate type, dimensions of the conduit and other factors. Alteration of land usage and land cover results in manipulations of these natural drainage systems that aid in water reaching the sea.
R. L. Gouri from Prof. V.V. Srinivas’ group from the Department of Civil Engineering, Indian Institute of Science, set out to understand the reliability of an 18km stretch of storm water drainage network in Northern Bangalore. This drainage network, covering an area of about 73 km2, was divided into 34 sub-watersheds for ease of analysis.
For examining this network, the duo gathered data on daily and sub-daily rainfall of different seasons for more than 20 years, and collected information on the substrate type and land use and land cover for the year 2006. The collated information was modelled using Storm Water Management Model (SWMM) to estimate quantity and velocity of flow in drains. These estimates together with drain width, depth, slope and roughness related parameters were analysed to assess the reliability of the current drainage network.
The team has opted a method called AFOSM (Advanced First Order Second Moment Method) for reliability assessment, considering three modes of failure that could occur to a drainage network. If velocity of flow exceeds a particular threshold, it could lead to erosion in drain; on the other hand, if the velocity of flow is lower than the minimum requirement it could cause silt deposition in drain, which would alter capacity of drains. Another mode of failure occurs if the runoff exceeds the capacity of drains.
The reliability analysis carried out by the team in all the 34 sub-watersheds has revealed that most of the drains are prone to all the three modes of failure. They have found out that flow in most of the drains could exceed their design capacity even when the drains receive flow from rare (extreme) storm events with a return period of two-years.
The authors thus conclude that the current drainage network is not reliable, as extreme storm events with return periods (rarity) greater than two-years are inevitable. Redesigning the storm water drainage system by considering the failure modes could help in better management of the storm water, thus reducing the frequency of flooding in the study area. In order to control erosion and depositions in drains due to runoff and to enhance design capacity of drains, these analysts also recommend considering changes to width, slope and substrate of drains for further analysis in designing the water drainage system.
On any solutions to enhance the reliability of a drain network apart from redesigning them, Prof V.V. Srinivas said “Capacity of a drain may get reduced due to factors such as clogging and silting, whereas changes in runoff into the drain could be expected from climate and land-use/land-cover changes. Presence of lakes in a drain network, however, enhances reliability of the network. Hence citizens can work towards prevention of encroachment of lakes, and maintaining capacity of drains by ensuring that they are free from waste material that causes clogging”.
About the authors
Prof V V Srinivas is an Associate Professor at the Department of Civil Engineering, IISc. R L Gouri is a research scholar in his research group.
About the paper
The paper appeared in the journal Aquatic Procedia last month.