Human development in urban areas is making urban flooding worse during rainy season. This is because most of the ground surface in urban areas is covered by impermeable buildings and roads, which rapidly divert rain water to drains and do not allow percolation of the water down through the soil. Increased runoff reduces groundwater recharge, thus lowering the water table.
A number of drainage systems in many urban areas of Odisha have collapsed resulting in flooding. This can be seen during rainy seasons in major cities like Bhubaneswar, Puri and Cuttack. lack of environmental considerations in city planning and unauthorized construction has led to blocking of natural drainage system.
In Bhubaneswar, Ekamara Kanan, Jayadev Vihar, Gajapati Nagar, Sainik School, Vani Vihar, west of Mancheshwar, Acharya Vihar, Iskcon Temple area, Aiginia, Jagamara and Pokhariput are the areas through which the natural drains pass. In Bhubaneswar most of the existing drains are in dilapidated condition and not working properly. A number of drainage channels are also encroached by unauthorized construction.
A proper storm water design means a proper knowledge of a collection of data like intensity of rainfall, runoff details and location of the catchment area etc. The peak rate, volume, and timing of runoff are important characteristics in the planning and design of storm water management practice. It appears most of the time that many of the data may not be correct and thus the design of the storm water drains with these data resulted urban flooding.
Urban drainage includes two types of fluids viz. wastewater and storm water. Both storm water as well as waste water needs to be considered for the drainage system planning and design. Proper disposal of surplus rainwater is essential to avoid water logging problems. It is imperative to plan and design surface water drains appropriately so as to dispose of surplus rain water within minimum time. Storm water drainage network should be designed with proper hierarchy. The drainage and slope map can also help in the planning and design of storm water drainage and urban flood management.
When planning new drainage projects, the conditions of the associated existing drainage system and the proposed measures should be checked for reference. A storm water drainage system should be designed to collect and convey run-off generated within a catchment area during and after rainfall events, for safe discharge into a receiving water source. The catchment is the area draining surface water to a particular location or outlet point. Catchment is the basis for planning and designing the storm water drainage systems in all cities.
The magnitude of peak flows that have to be accommodated will depend primarily on the intensity of rainfall and the size, topography, soil type, configuration and land use of the catchment.
The Government regularly publishes maps and town plans from which information on land use and topography of catchment areas can be extracted. Accurate contours are also necessary for determining the boundaries of a watershed/catchment and for computing directions of flow. For that, topographical sheets may be collected from Survey of India (SOI) department. These topographical maps are used to prepare slope maps. Slope maps are obtained from the toposheet maps by measuring the distances between the contour lines. Detailed contour maps should be prepared for proper delineation of drainage catchments.
Keeping in view the projected rate of urbanization, it is imperative to consider a 50-year planning horizon. Because the development is bound to take place during this period and it will be difficult to upgrade the underground drains once they are laid.
Currently, in Bhubaneswar about 10 drainage channels run west to east for discharge of waste water and storm water. But due to upcoming structures in these areas, the floodwater cannot drain properly and creates water logging. Another reason is that the entire city has not been covered with storm water drains. During the initial planning of neighbourhoods in Bhubaneswar, the area even for the smallest housing street has been fixed at a minimum of 30 feet. Only about 9 feet out of these 30 feet actually carry traffic. The remainder used for storm water drainage and avenue trees. These trees and the surrounding green areas help to reduce urban flooding in the city. The overall widths of the land earmarked for roads and streets is not determined by traffic alone, but by the requirements for storm water drainage and services(overhead electric lines, water, etc.) But in today’s planning, there are narrow roads. These roads do not allow to accommodate storm water drainage and other facilities. There is also no space for avenue trees. For storm water drainage purpose, 30 feet is commonly accepted as a minimum width of the road. There is a need to increase road width in different neighbourhoods.
With the rising population, new developments are bound to come up, but first priority should be given to water discharge system. Ideally, the natural drains should have been widened (similar to road widening for increased traffic) to accommodate the higher flows of storm water.
Besides, a master plan for drainage is highly needed for Bhubaneswar. The natural widths of all major drainage channels in the city should be protected, and there should not be any encroachment at least 4 to 5 metre on either side of the channel.
In Bhubaneswar the planning authorities should prepare storm water drainage manual for urban drainage design. The manual should provide policy and design requirements for storm water management throughout the city. Rainfall Intensity - Duration - Frequency (IDF) Relationships for rainfall record should be developed for Bhubaneswar city. Besides, integrated storm water drainage plan should be incorporated with the traffic and transportation planning of the city.
(Dr Praharaj is a faculty member, Department of Architecture, College of Engineering and Technology, Bhubaneswar)