Strategies to mitigate urban flooding through planning interventions: a case of Bhopal city

Abstract

Flooding is a global phenomenon which occurs more frequently than any other water related disaster and affects hundreds of cities every year. In some cases it causes a serious alarm when water goes beyond the limits to cope up by the community and have a huge devastation in terms of physical infrastructure, social life, economic activities, ecological and environmental aspects of the city. Due to rapid urbanization and unplanned growth of the cities in peripheral areas, the impact of urban flooding increases to many folds resulting into huge economic losses and severe damage to buildings and infrastructure. The inundation of urban areas from hours to several days due to heavy rain or overflow from the water bodies makes it very difficult for the community to cope up. Urban flooding is more of a human induced disaster rather than natural and is very different from the floods in rural areas. Urban flooding is highly influenced by the anthropogenic factors besides metrological and hydrological effects. Urban areas are affected more severely due to: higher concentration of population; more impermeable surfaces which increase the rate of runoff; poorly planned or organic growth of cities; urban sprawl; poorly designed and maintained storm water drains; and, inadequate solid waste management resulting into choking and blockages in the drains. Unplanned urbanization and poor management of cities are therefore the main factors behind flooding in urban areas. As per the National Disaster Management Authorities (NDMA) guideline, the imperviousness and concretization in urban areas increases the flood peaks to 1.8 to 8 times and flood volumes up to 6 times. Mostly the slums and squatter dwellers and lower income groups are more vulnerable to urban floods as they tends to live in informal settlements with limited or no provision for housing due to regional disparities. Developing countries are higher at risk especially the Asian countries including India. The study highlights the problems due to vulnerability of rapidly growing cities to urban flooding through four major concerns. Firstly, the pace with which urban areas grows increases the imperviousness that results in decreasing infiltration and percolation rate of water into the ground. Secondly, the unplanned urbanisation experienced by the urban areas. Thirdly, the inadequate storm water drains within the city to carry run-off water and solid waste management. Lastly, lack of coordination and integration among institutions responsible for managing city. Urban flooding has now become a challenge to urban planners and policy makers for making cities more resilient to urban floods. This emerging issue of urban flooding must be addressed through various planning strategies to guide the urban development and make cities more resilience against urban flooding. Therefore there is need of the study to understand the factors and to quantify the vulnerability of urban areas due to unplanned and unrealised development. Aim of the study is to recommend mitigation measures for urban flooding through planning interventions. This aim is achieved through four objectives. The first objective is to identify the factors responsible for urban flooding through reviewing literature as well as various case examples from different cities across the world and India. The second objective is to assess the previous flooding events that have caused huge destructions in selected city. Third objective is to analyse and validate the identified factors with respect to the selected city which is done in two different levels- macro and micro level by delineation of the study area. In the last objective, planning strategies are recommended to mitigate the urban flooding and for better management of excess rain water during monsoon season. A detailed literature review was carried out to comprehend the critical issues regarding urban flooding and for identifying factors responsible for it. Literature review also guided in understanding the substantial findings through theoretical and methodological contributions to a particular topic. It was observed after reviewing different case examples from different context that there are metrological, hydrological and human induced factors which are responsible for the huge devastations in urban areas during rainy seasons. Strategies from various best practices were also reviewed to understand the mitigation measures which are adopted by different cases. Bhopal city has been selected to conduct the study on the basis of four selection criteria’s; undulating topography; well distributed rainfall pattern; rapid urbanisation in terms of built up and population in 2 decades; and the past two events of severe inundation of the city in last decade. For analysis, Macro area based on the most flood affected catchment area of Bhopal in two events of flooding has been delineated. In macro study area, factors are analysed at broader level i.e. the catchment level of Patra Nallha. Micro study area is delineated as one of the highest impervious microwatershed area delineated for runoff discharge calculation in the macro area. Detailed factors are analysed at micro study area. This research study is an attempt to highlight the responsible factors for flooding in Bhopal especially the failure of urban planning and to suggest some mitigation measures through planning interventions. Analysis has been done in two stages, first is to assess the impacts of previous flooding events of Bhopal that had happened in 2006 and 2016. Second stage is to analyse the factors responsible for flooding at macro level and micro level. Data collection was done through both primary and secondary sources. Most of the data was collected through secondary sources from various departments in Bhopal whereas primary survey includes observatory surveys, photographic documentations and focussed group discussion. First stage of the assessment shows that the 2006 event was the extreme event of heavy rainfall in which city had received 54 percent of excess rainfall. The physical infrastructure of the city fails to cope up the additional rate of runoff from paved surfaces and results in the affectation of 17 percent of the total Bhopal’s population and 26 people were reported as died and 8728 houses were partially or completely damaged during the event. Govindpura area was identified as the most affected area within the city. Next event was recently happened in July – August 2016 in which city had received 17 percent of excess rainfall and the destructions were as similar as 2006. Total 40 thousand people got affected and 5 – 6 people lost their lives as per the news reports and around 8519 houses are affected during the event. Old Bhopal and Govindpura area was identified as the most affected areas in the 2016 event. Now, to understand the reasons behind flooding, further analysis has been done at two different levels referred here as macro and micro area. Macro area was delineated as the most flood affected catchment i.e. Patra Nallha catchment area which was severely got affected during both the events. Various factors like, rainfall pattern analysis, topography and elevation analysis, urban morphological changes, violation and non-considerations of DCRs has been analysed in the context of Bhopal city and highlight the issues at the catchment level. Due to high imperviousness in the area, runoff discharge is also calculated by using rational method to quantify and compare the capacity of Patra Nallha and the runoff generated from the paved urban areas. Patra Nallha has 38.13 cubic meters/second of capacity to receive the runoff but the area generates 44 cubic metres/ seconds of runoff discharge. Through analysis, the study find out the main issues lies in the lack of enforcement and implementation of development control regulations and the destruction of natural drainage system by the human interventions. A micro watershed area has been selected where 85 percent of the area are under paved surfaces. Assessment of some detailed factors are highlighted at micro level due to time constraint and their responsiveness at smaller level. After analysing the situation it has been found that there is a lack of sewerage system and storm water drains in terms of capacity and management in the study area. Low frequency of waste collection by the municipality and lack of waste collection infrastructure in the area. Violation of building byelaws by extending their building structures either on the Nallha side or on the road area which creates high congestion with no breathing space. Encroachments by slum dwellers and illegal construction over the Nallha area has been identified the main concern for the reduction in the width and capacity of the Nallha. Other factors are including narrowing of culvert’s design results in hinders the flow of water and responsible for back water effect. Roads are at higher level than the plinth of houses which creates submersion of their ground floors. An attempt has been made to recommend strategies by considering the issues which are identified in the study to mitigate flooding in urban areas. Impacts can be reduced by proper land use planning and management of water system. The study recommends the riparian buffer widths considering the orders and width of the streams to control flood attenuation, stream bank stability and to improve the quality of water. For low lying area, study has recommended some measures to reduce the impacts of flooding especially in water-logged areas. In Bhopal, the industrial area is situated at low lying region. To protect the industries by the spillage of water, the study recommend to have a green multipurpose levees along the water streams. Strategies are made to rejuvenate the Patra Nallha through widening its width and capacity by removing encroachment from the Nallha’s area and its buffer region. To manage the additional runoff from the paved surfaces, sustainable drainage systems are proposed in the urban roads. Rain water harvesting to store additional runoff during heavy rains could act as water storage system. Proper Land use planning must be encouraged in the flood risky areas, with minimum criteria for the provision of urban development and built up. Working in collaboration with various departments, agencies, NGOs with public participation can improve the situation in way better than what we experienced presently. There must be a cohesive participation and coordination in every planning process.