Keywords

1 Introduction

Climate change and urbanisation pose remarkable threats to flooding and degradation of water quality in urban areas [42]. Urban flooding is the greatest challenge to human safety and sustained economic growth [25]. Climate change is a concerning factor in changing the precipitation patterns worldwide. Every year, rainstorms and related urban flooding issues are exaggerated in terms of frequency and severity. The main reason for urban flooding is the accumulation of local rainfall runoff due to inadequate drainage and buffer capacity, known as Pluvial flooding [20]. Pluvial flooding is a new approach that updates the current urban drainage models to urban flood models [9]. The externalities of urban flooding can be triggered by intensified rainfall, long-duration precipitation, close repetition of precipitations or a combination of externalities create a full-scale urban crisis by electrical failure, road congestion, interruption in business and influx of polluted water. These are the major problems which can impact heavily in the city dwellers. In order to adapt and mitigate climate change, the local urban body has critical roles. But, local urban bodies in low and middle-income countries cannot reduce climate change impacts [62]. Flood protection is one of the traditional functions of any drainage system and it remains a significant issue in many cities because of economic and health impacts [13]. The main challenges to implementing a sustainable urban drainage system and developing an urban flood resilience model [24]. There has been considerable research and tools developed to analyse fluvial and coastal flooding, but models for pluvial flooding are less advanced. Therefore, modelling and a better understanding of the risk of stormwater flooding is needed urgently for flood risk management [12].

Climate change has varied consequences in different parts of the world [36]. North-East India can be considered a distinct macro-region within the Indian landmass [49, 71]. The Northeast has a diversified climate, with extremely cold winters and semi-humid summers, particularly in the south. The region has a monsoon climate, which gets high or extremely heavy rainfall only during the summer months of June to September. The primary source of rain is the south-west monsoon, with June being the wettest month. There are three seasons in the area, i.e., winter, summer and rainy season. Between valleys and mountains, there is a climate variation. In January, the average temperature in the Assam Valley region is around 16 °C. In contrast, the maximum temperature in the hilly parts of Arunachal Pradesh and Nagaland is 14 °C and the minimum temperature is below freezing. Summer temperatures in the lowlands range from 30 to 33 °C, while in the hills, the average summer temperature is 20 °C and the average lowest is 15 °C [18]. The average annual rainfall is 2000 mm with local variations (1500–12,000 mm). North East India's climate has drastically changed over the last couple of years. Rainfall patterns over the region in the last century have considerably changed, resulting in its overall drying up [60].

This region had been treated as a backward region because of its rurality. However, but in the second half of the twentieth century, the urbanisation process blooms up in the northeastern region [35]. Agartala is the -growing and second-largest city in Northeast India, located in Tripura [16]. Due to the rapid unplanned urbanisation of Agartala. The city faces urban issues like road congestion, solid waste management, development of slums, sanitation and urban flooding [22,23,46]. Urban flooding is the most crucial issue in cities with significant impacts on the man-environment relationships [24, 26, 30]. The degree and intensity of urban flooding have increased in many Indian cities, as has been reported in the past two decades [1, 5–7, 15, 17, 23, 32, 37, 45, 51, 53, 72]. The spatiality of urban units plays an essential role in urban flooding along with meteorological, hydrological and anthropogenic factors [42, 67]. Due to those inevitable factors, the city's drainage system is facing delinquency. As a result, urban flooding has taken place, especially during the monsoon season [22, 28, 29]. Infrastructure planning is required to improve the city's drainage system, especially for sewage and stormwater [3, 11, 48, 73]. The cities are expanding and growing exponentially, simultaneously the natural drainage systems are traumatised due to development activities in the city [32, 33]. The capacity of water retention and surface runoff of the city has a negative relationship, increasing flood potentiality [38–40, 64, 69]. Unplanned urban infrastructure consequences urban flooding where environment and resource conservation are least prioritised in the development process of urbanisation, through such relationship between urbanisation and development is a vital policy concern [68]. But in micro-level planning, some important parameters such as city slope and its topography, watershed and its catchment areas are needed to be given prior attention which is eventually not being paid.

Agartala is frequently affected by floods during monsoon after heavy rainfall [4, 59]. The present scenario of the city drainage infrastructure system in terms of coping up with urban flooding is not up to the mark. Therefore, the study aims to identify the causal interference of urban flooding in the city of Agartala and explore technical strategies for managing urban flooding in Agartala and minimising the damages due to floods for mankind.

2 Study Area

Agartala, the capital city of Tripura, is located in between 23°45′ to 23°55′ N latitudes and 91°15′ to 91°20′ E longitudes (Fig. 1). The city is located on the flood plain of the Rivers Haora and Kata Khal. The physiographic structure of Agartala City is just like saucer-shaped and characterised by Tilla (relatively high land) and Lunga (low land) topography [63]. Maharaja Krishna Manikya built the old Agartala City in the year of 1748. That city was situated in between two uplands, Kunjaban tilla (23°51′ N and 91°16′ E) in the northern side and college tilla (23°49′ N and 91°17′ E) in the southern side. Due to the regular water logging situation, Maharaja Krishna Kishore Manikya had shifted his capital from old Agartala to present Agartala (our study area) in 1849. After 25 years of its establishment, in 1874, the Agartala Municipal Council was formed to provide better services to the city dwellers [61]. In 2014, it became upgraded into Agartala Municipal Corporation (AMC) which represents the local urban governance of the Agartala City [16]. Since its statehood in 1972, the city has become the nerve centre of all administrative, political, cultural and commercial activities of the state. The city has emerged as an essential border-trading centre with international linkage with Bangladesh. National Highway (NH-8 is passing through the city (Fig. 1. Maharaja Bir Bikram Airport [23°53′33.96″ N and 91°14′37.81″ E] is located about 11.75 km north-west from the Central Business District (CBD of Agartala City. Agartala railway station is [23°47′35.83″ N and 91°16′42.92″ E] located in the south-eastern part of the city. The city consists of 49 wards which are broadly divided into four planning zones (North, Central, East and South and shares. An international border with Bangladesh on the western side, and Jirania Rural Development (RD Block, Mohanpur R.D. Block and Dukli R.D. Block are situated in the east, north, and south of the city. The total area of AMC is almost 76.150 km2 with 5,26,292 population.

Fig. 1
An outline map of India highlights the Tripura region. On the right is a map of Tripura that highlights Agartala. Below is a zoomed-out satellite map of Agartala. It marks railway stations, airport, railway tracks, major roads, and international boundaries.

(Source Prepared by the authors, 2021; Data extracted from SRTM DEM, Google Earth and Hand-held GPS Receiver, 2021)

Location map of the study area

Full size image

The city holds on important international significance due to its strategic location with Bangladesh. Integrated Check Post (ICP) for trans-border trading is located only about 3 km west of the CBD of Agartala City. Agartala serves good regional connectivity with all the nearby cities of North East India as well as with other cities of India as the city has good connectivity through Roadways, Railways and Airways [27,28,55, 63]. The recent construction of a broad-gauge railway line in 2016 have also strengthened railway connectivity [57].

The city population varies from the inner to the outer part of the city. High population distribution can be observed in the central part of Agartala, whereas a sparse population is observed in the peripheral region of the city. So, most city people started to live in the central part of the city to avail the basic services and infrastructure facilities. From 1971 to 2011, there has been an upward trend of population density in the city. The highest rate of increase in population density has been observed from 1971 to 1981 due to political migration [16]. However, in 2016 when the city was expanded from 35 to 49 wards by accumulating more than 17.31 km2 area.

3 Methodology and Database

In order to identify the flooding areas, the general slope of the city has been analysed through the Digital Elevation Model (DEM). SRTM DEM has been collected from the USGS earth explorer platform and United States Geospatial Server (USGS). Drainage dynamics have been analysed through stream order and the watershed map has been depicted to identify the flow of the water's direction. In the next step, the land use map has been overlaid to analyse the relationship between catchment areas to see whether the urbanisations are swallowing those or not. Triangular irregular networks (TIN) map has been prepared to understand the overall physiographic structure of the city. Simultaneously, satellite-based Tropical Rainfall Measuring Mission (TRMM) data from 1989 to 2019 has also been collected and analysed. A drainage network map based on SRTM DEM has prepared to identify urban flooding and respective flood vulnerable areas. According to the duration of waterlogging, the city has been classified into four different vulnerable zones, very high, high, low and very low (Fig. 2). In the very high zone the duration of water logging is more than 72 h. Between 48 and 72 h water logging situated areas are demarketed as high zone. Wherever, 24–48 h and less than 24 h water logging areas of the city are classified as low and very low vulnerable zone, respectively. The Spatio-temporal changes of the urban areas have been monitored through Landsat Satellite Imageries of different years and respective data have been collected from USGS earth explorer. The detailed data summary between (2000–2021) is reported in Table 1.

Fig. 2
A satellite map of Agartala marks the flood zonation of the city. An inset bar graph represents the percentage of regions with very high, high, low, and very low flood vulnerabilities.

(Source Prepared by the authors, 2021 using ArcGIS v.10.8, 2021; Data extracted from TRMM and SRTM DEM)

Flood zonation map of Agartala city

Full size image
Table 1 Sources of the data seta
Full size table

To represent the LULC for change detection and analysis of urban land use with respect to urban flooding, a series of processes have been done like data acquisition, geometric correction, image extraction, training site collection and supervised classification using the Maximum Likelihood Estimation (MLE) method. The Images are processed by the GIS and RS software like ArcGIS v.10.8 and Map Info v.17 to analyze the Agartala urban area change detection. The Maximum Likelihood (ML) classification algorithm is a well-known parametric classification used for supervised classification [47]. Handheld GPS receiver used for ground verification. A rainfall zonation map has been prepared based on TRMM data (1989–2019) using Inverse Distance Weightage (IDW) method in ArcGIS v.10.8 and Global Mapper v.22 software. Geospatial techniques like GIS and RS have represented the correlation between flooded areas, population density and road density of Agartala City.

4 Results and Discussion

4.1 Characteristics of Urban Flooding in Agartala City

Agartala receives a huge amount of rainfall during the monsoon from May to September [14] in the effect of south-west monsoon wind flowing over the Bay of Bengal. The city receives an average annual rainfall of 211.7 cm with 95 rainy days. The city receives maximum rainfall during the monsoon period [41]. During post-monsoon to pre-monsoon seasons (dry period), the city's groundwater level dries up because of the massive extraction of groundwater by the city to meet the water supply demand for the city dwellers [31]. The main reason, the fluctuation of ground water levels in the city is that there are no recharge techniques in the city and individual household levels and river water lifting from Haora River [10]. Therefore, the city needs to take long term measures to tap the excess rainfall during the monsoon in order to reduce the dependency on groundwater during the dry period. In previous flooding events of Agartala City, critical flood zone is located in the low-lying areas. Where around 4–5 days are required to drain out the water because the city does not have any high-tech water lifting stations to drain out the floodwater from those low-lying areas [8, 29, 65, 66]. As a result, urban flooding has become a regular event in the low-lying areas of Agartala City in every monsoon (Fig. 2).

In Agartala, about 37.38% area has been identified where water logging extends more than three days especially ward numbers 4, 15, 16, 17, 18, 22, 23, 24, 32, 33, 34, 35, 36 and 42. Maximum wards (16.48%) of AMC characterised with very high flood vulnerability are located in the central-west part of the city. About 34.32% of Agartala City falls under a high flood vulnerability zone where water has been logged for 2 to 3 days (Fig. 2). It has been observed that 19.90% and 8.45% of the area have low and very low flood vulnerability, respectively. Those areas are located in northern, south-eastern and southern parts of the city and the relative height of those areas is comparatively higher than the central and central-western parts of the city (Fig. 1). Vegetation cover is a commonly used indicator to evaluate terrestrial environmental conditions [34, 70]. The cumulative consequences of climate change and urbanisation on surface runoff get measure of the impact of climate change on urban flooding [2]. Enhanced green infrastructure (GI) in urban areas, such as green roofs, parks and green spaces, can significantly enhance the provision of fundamental ecosystem services through nature-based solutions, prevention of urban flooding one of that [75]. Due to climate change and anthropogenic activities, vegetation degradation has taken place in Agartala city significantly (Fig. 3). This change impacts urban flooding. Because GI always acts as a sponge that absorbs a huge amount of Strome water during monsoon. The drainage system is the most crucial factor for urban flooding [19]. A natural drainage system has been identified through the process of stream order. Strahler suggested an ordering technique, still is considered as most reliable method to categories the stream segments into different classes based on their significance and contribution to the drainage pattern [21, 27, 50, 52, 74].

Fig. 3
4 maps of Agartala city exhibit the green infrastructure in 2000, 2007, 2014, and 2021. There is a marked reduction of green infrastructure in the city in recent years.

(Source Prepared by the authors, 2021 using ArcGIS v.10.8, 2021; Data extracted from LANDSAT Series)

Green infrastructure of Agartala city [A. 2000, B. 2007, C. 2014 and D. 2021]

Full size image

Agartala City had well natural drain network to move out the flooded water from the city (Fig. 4). However, due to rapid urbanisation, the natural stream network has been changed. By using the Strahler method, about 56.20% were a first-order stream, 24.05% second-order stream, 11.46% third-order stream, 5.44% fourth-order stream and 2.69% fifth order stream are found in Agartala City (Fig. 4). The lower order stream takes a longer time (more than 72 h) to subside the flood situation wherever the higher-order stream takes a longer time to develop the flood situation by reducing flood with quicker time (less than 24 h) flood level. In Agartala City, only 0.16% of higher-order streams are observed. As a result, having immense pressure on higher-order streams required more time to subside the flood situation.

Fig. 4
A hill shed map of Agartala highlights the streams of different orders. An inset bar graph represents the percentage of steams of first, second, third, fourth, and fifth orders. The graph follows a decreasing trend.

(Source Prepared by the authors, 2021; Data extracted from SRTM DEM through Hydrological Modelling)

Hill shed map with stream ordering

Full size image

This morphometric pattern is one among the response to hydrologic and anthropogenic conditions. However, due to rapid urbanisation since 1991, the maximum first, the second and third-order stream has been either wiped out or the stream has been converted into the respective lower-order stream. Surface runoff in those drainage networks has been obstructed and causes the water to be logged in the city for a couple of hours to a few days and creates a situation of urban flooding. Due to lack of water retention points, in few wards of Agartala City were severely flooded during every monsoon (see Table 2).

Table 2 Externalities of urban flooding
Full size table

The average rainfall intensity of Agartala city is more than 204.5 mm, which is considered as extremely heavy rain [41]. The average annual rainfall of Agartala is about 2146 mm. The seasonal rainfall varied from 68% (monsoon) to 1.4% (post-monsoon). It has been recorded that floods in Agartala city due to intense rainfall over just two days point to immediate attention required towards building urban climate resilience [66].

Due to flooding in Agartala, almost more than half of the city, that is 65% of people, have to struggle with their days without having any alternative occupation and cannot meet the demand of their daily basic needs. During flood time people have to take shelters in schools, colleges, or even on the rooftop as there are no proper flood shelters in Agartala city. Therefore, the socio-economic analysis of Agartala city have been categorised into four parts. The flood can be categorised in the following physical characteristics such as floodwater height, frequency and duration of the flood. The longer period of floodwater stagnation in the city rises the level of vulnerability impact upon the people. Surveys have been done for analysing the communities affected during a flood. Through which it is being observed most vulnerable communities are farmers, daily labours, rickshaw pullers and small business holders in the city. From the above chart, it is seen that the most vulnerable community during flood time is the farmer community because most of the farmers agricultural land are in a low lying region which gets waterlogged very frequently resulting in damage to agricultural crops. Whereas during flood causes death and leads to miseries to the communities like labours and rickshaw pullers as they become homeless, jobless and suffer from starvation. Poor household communities have to face great difficulty in balancing the loss of income than the non-poor community. Similarly, during flood time, the small business holder community faces the problem in storing and carrying their goods in godowns as half of the city goes under the water and most of their goods get damaged, and they have face loss in their income.

Agricultural crops and the damage of shelters are severely damaged due to floods as livelihood assets and property are closely related to each other. When property assets are affected, it directly impacts change of livelihoods. The survey shows that 60 and 44% properties are being damaged in terms of Houses and Agricultural lands. Due to intense flooding in Agartala, the agricultural lands and houses are being submerged severely, and most of the houses are made through local materials like bamboo, muds so, during flood time, the entire houses get washed away along with households furniture. The agricultural lands located near the river yearly lose a huge amount of agricultural crops due to heavy river bank silt erosion due to heavy siltation during flood have made the agricultural land more infertile and have decreased the productivity of the land. Financial losses depend on the city's intensity, duration and frequency of flooding; the average financial loss is 20,000–25,000 rupees per flood per family belonging to a poor, vulnerable community. Apart from the direct income loss, there are loss of retail goods and loss of working days, there is a direct economic loss of 5% of the monthly income due to flooding, with major loss of working days is about 3–5 days due to intense water logging in the city. Most of the community affected by the flood come from middle to lower-income groups. As a result, most people suffer from water-born and vector born diseases such as diarrhea, dysentry, cholera, maleria and respiratory problems. As the people belong to lower-middle-class communities, a result, they are more vulnerable to flood than that person living who live in high-class communities. Therefore it can be said that there is a significant correlation between the people’s income and the impact of flood, where the poor people fails to maintain the balance in terms of living standards during the time of the flood. Hence these people can afford neither medicine nor medical check-up during the time of the disaster, which results in death and chronic suffering from the water and vector born diseases.

5 Findings

From the above analysis following are the significant findings-

  • Urban flooding intensity in the city is very high due to high population density and poor drainage infrastructure.

  • The city's natural drainage system has been chocked due to the unscientific construction.

  • The city has a mayhem flood due to the degradation of the drainage network and pluvial activities.

  • The rapid change in the change in green infrastructure of Agartala results in the increase of surface runoff in the city.

  • The pluvial flooding of Agartala city mainly affected the poor and people living in low-lying areas.

  • Financial and property losses are common phenomena of urban flooding, mostly depending on the intensity, duration and frequency of flooding in the city.

6 Conclusion

Pluvial events change nature due to climate change. Spatial prioritisation based on urban flood analysis of Agartala City has been spotted. To resolve the issue of urban flooding, sustainable strategic city drainage network planning is needed for making the plan. As Agartala City has grown up in an unexpected way, like unfollowing the city development control rules, regulations and guidelines, the city is frequently facing urban flooding. Along with pluvial events, anthopogeomorphic factors are also responsible for hazards impacting human lives and livelihood.