Geohydrology is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth’s crust (commonly in aquifers). The terms groundwater hydrology, geohydrology, and hydrogeology are often used interchangeably. Groundwater engineering, another name for hydrogeology, is a branch of engineering which is concerned with groundwater movement and design of wells, pumps, and drains. The main concerns in groundwater engineering include groundwater contamination, conservation of supplies, and water quality.

Urban flooding is the inundation of land or property in a built environment, particularly in more densely populated areas, caused by rainfall overwhelming the capacity of drainage systems, such as storm sewers. Due to rapid, unscientific, unplanned urbanization across the globe, the carrying capacity of urban areas is often breached leading to impending disasters. The cases of floods in Chennai in 2016, Bangalore, Gurgaon in 2017 are instances of urban flooding. Floods and water-logging show that urban planners have paid scant respect to hydrology.

Unscientific urbanization affects Hydrology:

• Concretization:
  • Most cities had water bodies — lakes, ponds, streams, rivulets — which served three purposes: They replenished groundwater, catered to the city’s water needs and channelised excess rainfall to larger water bodies. Most such aquifers have fallen prey to concrete.
  • In Delhi, for example, a stream used to feed the Yamuna at about the place where the busy ITO today stands. It’s not without reason that the area is one of the most water-logged when it rains heavily in Delhi.
• Wiping out of the wetlands:
  • The encroaching of lakes in the cities by filling with mud and construction waste to recover the land leaves no place for water to get collected.g. Bengaluru had more than 250 lakes in the 1960s. There are scarcely 10 such water-bodies in a healthy state in the city today.
  • Chennai’s flood sink on its southern outskirts — the Pallikarnai marsh — was around 5,000 hectares in the 1950s. After the horrific floods in November last year, it was found that it had shrunk to 600 hectares.
  • The rivers, Cooum and Adyar, and the Buckingham Canal in Chennai have become dumping grounds. So have Mumbai’s wetlands near Sewri and other areas in New Mumbai. In fact, the city became alive to the river it has lost to real estate — the Mithi — after the terrible floods of 2005.
• Lack of proper drainage system:
  • With most cities lacking proper sewerage facilities, people dispose sewage in stormwater drains. The problem becomes worse when industries discharge their polluted water into such drains.
  • All this compromises the capacities of stormwater drains and also results in polluted water flowing into the larger water bodies.
• Illegal constructions:
  • Real estate activity blocks the path of water, the city roads get waterlogged.
  • Stormwater drains cannot take the burden of the water that once used to seep into the ground. Moreover, cities do not make the distinction between stormwater drains and sewage disposal outlets.

Significance of Hydrology:

• It helps to channelise excess amount of water to larger bodies.
• Maintain the lakes, ponds in the urban areas which helps to absorb the surface water.
• For upgrading the existing drainage system if it is not adequate to meet the challenges
• To identify the effect of blocking natural drainage system and to manage the issues

Measures needed:

Structural Measures:

• Conservation of wetlands in urban areas like lakes, ponds, streams.
• Construction of differential slope along sidewalks, roads to drain excess water into reservoirs.
• Strengthening of Storm water drainage system.
• Pre-monsoon desilting of all major drains to be completed by March 31 each year.
• Every building in an urban area must have rainwater harvesting as an integral component of the building utility.
• Concept of Rain Gardens to be incorporated in planning for public parks and on-site storm water management for larger colonies and sites those are to be developed.
• Suitable interventions in the drainage system like traps, trash racks can be provided to reduce the amount of solid waste going into the storm sewers.

Non structural Measures:

• National Hydro-meteorological Network as per NDMA is needed for all urban cities in India.
• Flood hazard assessments should be done on the basis of projected future scenarios of intensities and duration of rainfall and land use changes.
• Better forecasting of rainfall events; timely dissemination of information to the mass- ‘Nowcasting’ alerts or real-time weather updates.
• Restrict encroachments in natural drainage areas; clearance of river beds, proper implementation of Coastal Regulation Zone rules.
• Provisions for flood-proofing of buildings
• Storm water pollution control, i.e. source is controlled by imposing quality standards for wastewater and solid waste disposals in urban environments

Conclusion:

The understanding of the geohydrology goes a long way in preventing the urban floods. With climate change accelerating at unmitigated pace, it becomes all the more critical for planned urbanization.