Wednesday, October 5, 2022

B’luru’s flood woes: Overpopulation, concrete jungle, altered landscape

Many urban areas in India face flooding and water stagnation related issues during the monsoon season. Though different cities are in different topographical and meteorological settings, the causes behind flooding are often the same rooted in where we build, or expand, our cities and how we construct roads, buildings and other structures.
Coastal cities, like Mumbai and Chennai, are also vulnerable to storm surges and other sea-level related challenges such as sea water entering (during high tide) to impact the discharge capacity of the city’s storm water drains. If we must work towards solutions, it is necessary for us to have a deeper understanding of the growth patterns of our cities, their terrain, their infrastructure, and their ecosystems.
Let us take the case of Bengaluru, which has been severely affected by floods this year. Bengaluru is located at the top of a ridge which is the water divide between the watersheds of the Kaveri and the Ponnaiyar (Dakshina Pinakini) rivers. The city has numerous valleys which act as conduits carrying water to these two major rivers.
Original settlements were concentrated on the ridges while the valleys were used for agriculture. To irrigate these lands, bunds were erected to retain the water — creating lakes. Each lake had its own command area which it irrigated.
The older streams that once flowed were redesigned to create artificial canals (kaluve) which were used for irrigation of the command area of each lake and for carrying excess water downstream. Several minor drains that carried water in these command areas ended up under private ownership.
The city’s population, which stood at 1.6 lakh in 1901, is estimated to be more than one crore today. This rapid and extreme growth has triggered a massive demand for land and the city began sprawling out. Ignoring the topography of the land, construction began in the valleys and ridges, which in turn altered the original topography, with minor drains disappearing.
The new structures not only impacted water infiltration into the soil but also began obstructing the movement of water in the valleys. Most of the drains on private properties disappeared, while public ownership drains proved insufficient to carry water during heavy rainfall days. These existing canals, which were not created for inundation requirements, proved inadequate to the task of carrying excess rainwater. Extreme amounts of engineering, sewage flow and clogging further hampered flow in the canals.
The bulk of the flooding and stagnation in Bengaluru happened because of the obstructions in the valleys. There were very few instances of flooding outside the valley. The isolated cases of stagnation in the ridges were largely due to the quality of engineering of the roadside storm water drains.
Let us look at two cases of extreme flooding this year — The RMZ EcoSpace in the Outer Ring Road (ORR) and Rainbow Drive in Sarjapur Road. These spots have seen flooding in the past as well. The topography has been significantly altered by construction activity and drains and culverts are proving insufficient, resulting in water stagnating. The topography and the construction activity in these valleys and their flood plains are shown below.
With the city expanding at a rapid rate, new areas are becoming prone to flooding. This year, a few sections of the under construction MysuruBengaluru expressway were inundated. The visualization below shows how the landscape changed when the expressway and tollgate were constructed in the middle of the valley. This construction resulted in severely obstructing water flow that then resulted in floods.
The city master plan does not protect the valleys completely and is instead focussed on protecting a small and fixed buffer area around identified water bodies and streams. In cities like Bengaluru, master plans have not succeeded in regulating the land market efficiently. The legalization of ad-hoc development and the huge differences between the vision of the master plan and the market, means that the city’s built-up growth has huge deviations from the proposed land use plan.
Over concretization resulting in reduced infiltration into the soil, unplanned urbanization, changes to the topography, lack of infrastructure to carry rainwater during heavy precipitation events, and lack of buffers to hold surface water all lead to urban flooding. Bengaluru was affected this season, but it could be any other Indian city that faces high rainfall rates.
It is critical we begin understanding and respecting the topography of our cities. Since the investments we make in these critical areas are long-term, and would lead to further complexities in the future, it is necessary that flood zonation maps based on topography get integrated into city master plans.