Urban flood and effective management strategies
- Part 1 -

Bhavananda Mayengbam *

Heavy flooding in and around Imphal on March 31 2017 :: Pix - Shankar Khangembam

Water is essential for our life, however, too much, too little and too dirty water can be a problem for our life. One of the greatest problems related to too much water is flood. Global disaster data (Chart-1) of Centre for Research on the Epidemiology of Disasters (CRED) illustrated in DTE (2015) clearly shows number of natural disaster from 1970 to 2014 and it also further illustrates that flood is the most frequent global disaster.

The climate change will adversely affect our environment. The Intergovernmental Panel on Climate Change (IPCC) projects several extreme events such as temperature extremes, heat waves, increase in frequency of heavy precipitation, increase of maximum wind speed in tropical cyclone/ hurricane, rise in sea level and increase in drought/ floods etc.

Flood is one of the most devastating events in Imphal, [the capital city of Manipur, India] too. The census data of Imphal (2011) indicates a metropolitan population of 418,739. The average elevation of the city is 786 metres above MSL and receives an average annual rainfall of 1320 mm, with June as the wettest month.

The two major rivers namely Imphal River and Nambul River pass through the heart of the city and Imphal River has flood warning level at 785.9 M whereas Nambul River has at 771.2 M. During the monsoon season, apart from rising water level of the main rivers, incessant heavy rainfalls also frequently inundates many low lying areas of the city. As most of the secondary and primary drains such as Waishael drain etc. are encroached or filled with artificial deposits and solid wastes at many segments, the urban runoffs inundates in several adjoining areas during the rainy season.

Frequent floods contribute numerous casualties to both urban and rural areas thereby causing socio-economic and environmental hardship to the inhabitants. The absence of counter measures affects our food, water, health, security, business and settlements. The population growth and rapid urbanisation of Imphal has negative impact on our environment. In the process of urbanisation, wetlands, primary, secondary and tertiary drainage are encroached upon causing major water clogging and flooding in low lying areas of Imphal. Moreover, the present drainage systems are unable match the rapid urbanisation.

Rossi et al., (1994) define flood as overflow of water from rivers and drains in enormous volume. Flood can be transpired in several areas including coastal area, urban area, rural area, or river basin. Few and Matthies (2006) define heavy rainfall, extreme tidal waves, and structural failure such as flood breaching of river/ sea embankments, dams etc. as major causes of flood. Aside from physical injury and hardship, flood also damages infrastructure, property, business, and environment.

As flood is an excess of water on a normally dry land, a situation is created where the inundation is caused by high flow, or overflow of water in an established watercourse such as river, stream, drainage, ditch , pond or near the location where the rain fell. Subsequently, it can also strike anytime, anywhere without warning when a large volume of rain falls within a short time (NIDM, 2009).

Parker (2000) describes flood as a broad result of various events and processes that inundates an area by water. The water inundation characteristics can be different from one another. It can be shallow or deep, and it can also be slow flow or high velocity flow. Proverbs and Soetanto (2004) explain that during a flood, there are several events such as intense rainfall or coastal storms which ultimately clog or overflow drainage systems, and sewers and create excessive run-off and high groundwater level.

Types of flood :

ADPC (2006) defines flood into two different types as mentioned below, however, another category can also be added to this classification.

1 River floods occur because of water overflow from the river sides/ embankments. Water in the river slowly drains and rises to create overflows and this type of flood causes extensive inundation in lowland areas. River flood also can be separated into slow-onset flood and rapid-onset/flash flood and they are identified based on their characteristics.

Slow-onset flood last for longer period ranging from week to even month and consequently it can lead to heavy damage to infrastructure, agriculture and environment. Rapid-Onset flood usually last for one or two days, nevertheless, it can cause more damages to life and property as people usually have very less time to react, prepare and plan.

2 Localized and urban floods occur because of excessive rainfall in an area with insufficient drainage. Subsequently, the urban run-off cannot be drained into river due to lack/clog of primary, secondary and tertiary drainages. The cause can be rooted to several factors such as poor maintenance, to lack of urban planning and regulations, population growth and land encroachment on urban wetlands and drainage systems.

3 Marfai et al. (2008) argues that there is also another type of flood that must be included. It is called tidal flood or coastal flood. This flood is caused by high tides, waves, and rise in sea level. This type of flood generally occurs in coastal cities; however, there can be also mixed type of flood which employs overflow from rivers, and influence from high tide. In such condition, it is assumed that height of water, the level of inundation, the duration of inundation and the impact of flood not only depends on rainfall, but it also depends on tidal conditions.

The most frequent flood in Imphal comes under the 2nd classification and caused mainly by urban overflows and insufficient drainages to contain heavy continues rainfall. Subsequently, the urban runoff inundates several areas within a long or short time. The inundation type is shallow with slow flow or high flow characteristics depending on source and locations. The physical topography, land encroachment, poor urban drainage, lack of infrastructure planning and management aggravate the problem further. The city has been frequented with flood almost every year and requires a prioritized flood management policy.

Certain floods cannot be prevented; however the impact can be controlled and reduced by proper management. Floods occur quickly without warning, and to reduce the impact, effective strategies are essential and should be prepared through proper policy. The household adaptation strategies to cope the flood are mostly developed by the initiative of affected communities.

For instance, people have increased the plinth height or added few floors to their houses in order to avoid the inundation and save their valuables. These adaptation activities that have been prepared by local community reveal two important facets: – incapacity of institutions/government to improve primary, secondary and tertiary drainages and inability of the affected community to move to safer areas.

To be continued...