Slope Map

‡

‡

Figure 5.6.4. Steps in the preparation of a tsunami vulnerability map (adapted from Arulraj et al., 2006)

• The siting, drilling and testing of emergency tube wells with sealed collars

• Reticulation (network) water supply distribution systems should be adequately strengthened/

protected to resist tsunami waves.

• A national and local water resource risk management plan should be developed to ensure safe water in emergency situations in areas prone to tsunami damage. The involvement of local governments and communities, water managers, planners and non-governmental organizations is vital in creating awareness, reducing vulnerability of the population and participating in the implementation of aquifer risk management.

Relief and rehabilitation phases

Relief and mitigation of the impact of tsunami are required to rehabilitate the community especially with respect to their immediate water needs. Three aspects are very important: first and

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Figure 5.6.5. Influence of a tsunami on a costal aquifer, showing infiltration of saltwater from the land surface and water bodies (IWMI, 2005)

Figure 5.6.6. Influence of the tsunami on fresh water coastal lagoons and associated groundwater prior to and after tsunami (IWMI, 2005)

foremost is the evaluation of water requirements for the tsunami affected areas; second is the identification and evaluation of accessible good quality freshwater resources; third is the assessment of physical damage to and pollution of domestic and public water supplies including vulnerable aquifers as mentioned previously (Figs. 5.6.5 and 5.6.6). The immediate assessment of tsunami impact is carried out using space imageries, GPS and GIS and field investigations using mainly hydrogeological, geophysical and hydrochemical methods. The most direct and notable impact is the salinisation and pollution of shallow groundwater by sea water and various wastes and chemicals. Remediation measures may differ depending upon the salinisation/pollution process involved.

The rehabilitation of groundwater supplies affected by the tsunami in Sri Lanka is described in chap -ter 11.9. The important steps required for mitigating a drinking wa-ter crisis are discussed under immediate relief actions and long-term rehabilitation phase.

Relief phase

• Inspect water supply wells for both groundwater level and quality. Data is essential for the designation of methods and techniques suitable for well recovery and aquifer rehabilitation.

• Clean water supply wells by purging salt water, physical removal of debris, disinfect wells and purify water by in-situ chlorination.

• In purging wells care must be taken as excessive pumping may cause seawater intrusion by disrupting the saltwater-fresh water interface resulting in up-coning. Septic tanks should be inspected for possible damage and leakage of waste water and repaired if necessary to prevent groundwater pollution.

.

Rehabilitation phase

• The rehabilitation phase involves natural processes, through which groundwater quality is allowed to recover by infiltration of rainwater and surface water. This process may take months and up to a few years where groundwater is polluted by chemicals.

• Observing post-tsunami temporal changes in chemical quality of shallow aquifers helps to prevent up-coning of saltwater due to over pumping.

• Numerical models need to be developed for the tsunami impacted aquifers to estimate the time required for flushing of salinity in the inundated aquifers by simulating groundwater flow and transport of salts using aquifer hydraulic characteristics and boundary conditions.

IGRAC (International Groundwater Assessment Centre) has produced simulation models (http://igrac.nitg.tno.nl/tsunami1.html) that estimated that the process of aquifer rehabilitation takes in the order of several years for groundwater to regain its pre-tsunami groundwater chemical composition. However, it should be pointed out that this simulation was carried out following the 2004 Asian tsunami under several assumptions and uncertainties, such as insufficient knowledge of hydrogeologic parameters, amount and pattern of pumping, rate of leaching of salts through zones of low permeability, disruption of the fresh water lens and contribution from polluted surface water bodies. Nevertheless, the outcomes from simulation models are valuable for future guidance.

• Post - tsunami information gained through maps of inundated areas, groundwater vulnerability and groundwater quality maps, resistivity and other relevant surveys are useful in designing aquifer risk management strategy in emergency situations.

The following flow chart summarises tsunami risk management of groundwater resources.

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Study of time series on groundwater lev-els and hydrochemistry & assess inter-face, identify deep & safe aquifers using hydrogeological,hydrochemical,

geo-physical, and isotope studies Drill wells in the identified aquifers to be used, and arrange alternate supplies from inland sources and establish

struc-tures of rain water harvesting Reticulation system of water supply,

sanitation and wastewater collection and treatment system to be made resistant to tsunami impact. Register deep wells and furnish open wells with protective covers to be used in emergency

Planning for interaction with local up-coning due to excessive pumping

Simulation studies for estimating

5.7 Groundwater risk assessment and management in regions affected by storms Balbir Sukhija

Introduction

Storms may be defined as high velocity wind systems generated by large scale atmospheric pressure patterns across the surface of the earth. The hazards associated with storms include ocean storm surges, high winds and torrential rain. Depending on geographical and meteorological characteristics and intensity in different regions, large and intense storms are called cyclones, typhoons, hurricanes or tornadoes. Tropical cyclones are defined as intense cyclonic storms that originate over warm tropical seas. In North America, they are called hurricanes and are characterised by strong winds which are likely to damage in particular taller structures and those with large exposed surfaces. In Japan and South East Asia, tropical cyclones are called typhoons. Fig. 5.7.1 shows the areas of occurrence and number of tropical cyclones per year for the period 1952–1977. Strong tidal surges (several metres high) , strong winds (exceeding 300 km/hr) and high rainfall, often exceeding 1000 mm per day) in coastal areas, are common and have enormous impact on populations with loss of lives, inundation of low lying areas damage to houses, buildings and infrastructure, crop destruction, and pollution of water resources.

Heavy rainfall may be responsible for floods, land slides, and mud flows that may render millions of people homeless.

A new study by U.S National Centre for Atmospheric Research shows the increasing trend in frequency of Atlantic storms, linking them to recent global warming (Fig. 5.7.2 ).

Figure 5.7.1. Global annual distribution of tropical cyclones indicated by their numbers in different regions during 1952–1977 (after Gray, 1975)