Drainage master plan brooks no delay
A number of major cities and towns in India reported a series of devastating urban floods in the recent decade. Major cities like Delhi, Kolkata, Mumbai and Chennai suffered flooding. Urban floods happen when a large volume of rain falls within a short time. Uneven distribution of rainfall, encroachment or blockage of natural drainage channels and lakes for converting them to high-value urban land are the main causes of flooding.
Urban flooding is significantly different from flooding in rural areas as urbanisation results in impermeable catchments causing flood peaks by up to three to four times. Many water bodies in urban areas have disappeared due to human-induced succession filled with wastes and development or slum encroachments. Unplanned urbanisation is the key cause of flooding. Urbanisation leads to paving of surfaces which decreases ground absorption and increases the speed and amount of surface flow.
The water rushes down suddenly into the streams from their catchment areas leading to floods. Various kinds of depression and low-lying areas near or around cities which act as cushions and flood absorbers are gradually filled up. This results in inadequate channel capacity. During heavy rains, the silted drains can’t carry full discharge.
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 Puri, Bhubaneswar and Cuttack. In Bhubaneswar, dwellings in and around Ekamara Kanan, Jayadev Vihar, Gajapati Nagar, Sainik School, Vani Vihar, west of Mancheshwar, Acharya Vihar, Iskcon Temple, Aiginia, Jagamara and Pokhariput are the areas through which the natural drains pass.
But due to upcoming structures in these areas, the floodwater cannot drain properly and creates waterlogging. The general topography of the Bhubaneswar municipal area has a gentle slope from west to east. The level difference is about 80 metres between the west end (Kalinga Nagar near Chandaka Road) and the east end (near Jharpada and Badagada).
Thus, the overall topography slopes from western highlands to eastern plains, rendering a natural advantage of drainage. There are 10 major natural drains running west to east. Some of these finally join the Gangua Nallah. Bhubaneswar’s storm water flows into these 10 drain channels. But the entire city has not been covered with storm water drains.
This apart, encroachment of natural drains by construction and dumping of waste materials make the storm water drains inefficient, resulting in blockage and chocking. The city’s drain No1 opens to river Kuakhai and the other nine drain channels (No2 toNo10) flow into the Gangua Nallah.
The starting point and outfall of the drains are as follows: Drain No1 (Patia-Chandrasekharpur to Daya West Canal Crossing to river Kuakhai); Drain No2 Sainik School Road Culvert to Railway Bridges (confluence with Drain No3) to Gangua Nallah; Drain No3 (OAP area)- field near Sainik School to Railway Bridges (confluence with Drain No2) to Gangua Nallah; Drain No4 (Vani Vihar)-Reserve Forest, Bharatpur to Daya West Canal Crossing to Gangua Nallah; Drain No5 (laxmisagar)-Janpath Road to Gangua Nallah; Drain No6 (Baragada)-Railway Bridges to Gangua Nallah; Drain No7 (Kedargauri)-Airport Road to Gangua Nallah; Drain No8 (Airport)-Joklandi Road to confluence with Drain No8 to Gangua Nallah; Drain No9 (Ghatikia)-Culvert on NH-5 to Pokhariput Railway Bridge to Gangua Nallah; and Drain No10 (Nicco Park)-lake near CRP Colony to Gangua Nallah.
The BMC maintains the ‘minor drains’, which comprise secondary and tertiary drains. These drains essentially convey storm water runoff from the road surface, household roof drains and form the catchment area in undeveloped or partially developed portions of the city to the major drains. Although 10 natural drains are flowing through the city, it is fou nd that because of encroachment many drains are not getting adequate space to discharge storm water. The drains are also choked owing to siltation. Buildings are often constructed without flood inundation in mind.
A large number of multistoried apartments have been constructed without considering the drainage pattern of the area. Also, in many areas, the widths the nullahs have been reduced. Another reason is that the entire city has not been covered with surface water drain. With the rising population, new residential areas are bound to come up, but first priority should be given to water discharge system. An integrated approach needs to combine watershed and land-use management with development planning, engineering measures and preparedness for flooding.
Apart from this, preventive measures like improvement of drainage efficiency and adopting rainwater harvesting system are some of the preventive measures to curb flooding. Natural streams and watercourses have formed over thousands of years due to the forces of flowing water in the respective watersheds.
Ideally, the natural drains should have been widened (similar to road widening for increased traffic) to accommodate the higher flows of storm water. But on the contrary, there have been large-scale encroachments on the natural drains. Management of urban flooding is an emerging subject, and as such, it has to be treated holistically in a multidisciplinary manner. In many cities, rainwater causes flooding on the road despite the existence of the underground drainage system.
It is seen that the inlets to drain water from the roads into the roadside drains are either not properly aligned or nonexistent leading to severe waterlogging on roads. Provision of a simple connecting element, namely, drainage inlet through which the water can flow from roadside drain into the underground drain can significantly reduce waterlogging on the roads.
Rainwater harvesting is a form of source control in which water can be converted into a resource. Due to urbanisation, groundwater recharge has decreased and the peak runoffs from rainfall and consequent flooding have increased. So, rainwater harvesting should be carried out extensively. This would serve the twin purposes of lowering the peak runoff and raising the groundwater table.
Urban water bodies also play a very important role in the management of urban flooding by reducing the storm water run-off by capturing it. In the past there have been many water bodies in Bhubaneswar, but over the years their number has been dwindling. All water bodies in the city should be protected.
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. The exact bed width, land width and the land requirement for drainage channel can be calculated after preparation of a drainage master plan of the various zones of the city. This would lead to a sustainable urban drainage system and reduce flooding in the city.
(Dr Mayarani Ptaharaj teaches in the Department of Architecture, College of Engineering and Technology, Bhubaneswar)