Protected wells can potentially provide a water supply that is highly resilient to flooding. However, improper design and construction can make them vulnerable during flooding. The key vulnerabilities of wells during flooding are: (1) ingress or infiltration of contaminated waters; (2) lack of wellhead access due to flood waters; and (3) collapse of unlined hand dug wells when soil becomes saturated (WaterAid, 2006). This article covers the design, construction and retro-fitting of dug wells, tubewells and boreholes to achieve high resilience to flooding.
Protected wells can include tubewells, boreholes and (hand) dug wells. Tubewells and boreholes are small diameter bores into a water-bearing zone of the subsurface. The bore is encased with a tube for some or all of the depth. They are described in more detail in the article on Domestic water supply during drought. Dug wells are generally more susceptible to contamination than tubewells/boreholes, but protected dug wells can also provide “improved” drinking water. The advantages of dug wells include inexpensive construction and, generally, greater volume yield per depth (due to their usually greater diameter) (WaterAid, 2006; WaterAid Bangladesh, 2006; WHO, 1996).
The salient features of all protected wells include the following: (1) a concrete apron to direct surface water away from the well; (2) a sanitary seal (normally clay, grout, and concrete) that extends at least 1-3 m below ground to prevent infiltration of contaminants; and (3) a method to access water that enables it to be sealed following use. Handpumps can be fitted to most wells (including hand dug wells) to improve convenience and decrease the likelihood of contamination (WaterAid, 2006; Howard et al., 2006; Misstear et al., 2006).
Location is another key parameter in assessing the vulnerability of wells to flooding. Constructing drinking water wells in the vicinity of sanitation facilities can lead to contamination through subsurface transport of fecal pathogens, particularly during flooding. Wells should be constructed up the hydraulic gradient (usually uphill) from latrines and animal waste. The minimum recommended distance between a well and a single latrine is 30 m. However, in settlements where latrine density is high, greater distances are often needed (Misstear et al., 2006).
Many of the key vulnerabilities related to flooding can be identified by undertaking a “sanitary survey” of all drinking water wells. Sanitary survey forms with illustration to guide inspection for many varieties of wells can be found in Annex 2 of the WHO Guidelines for Drinking Water Quality (GDWQ) 2nd Edition (WHO, 1996). Additional well design issues that are relevant to flooding are covered in Chapter 6 of the GDWQ 2nd Edition. These include recommendations for the depth of the sanitary seal (3 m) and casing (to the water table) for tubewells. The minimum height of the casing above ground is recommended to be 30 cm. However, in flood-prone areas it should be higher (WHO, 1996).
In addition to protection of wells currently used for drinking water, sealing abandoned wells is also essential to protecting groundwater quality in flood zones (Howard et al., 1996). If an abandoned well in not properly sealed, floodwaters that inundate the abandoned well are likely to contaminate both shallow and deep groundwater.
Retro-fitting of drinking water wells by elevating handpumps has been undertaken systematically in floodprone areas of Uttar Pradesh, India. An example of a flood-proofed handpump is shown in Figure 1. Further detail of the costs and success of the program can be found below (District Bahraich, 2010).
This chapter addresses preemptive flood-proofing interventions that are generalizable to most types of flooding and in most settings. However, the magnitude, onset time, and setting can differ widely. Descriptions of the different classes of floods and guidance on preparation and response in urban (Smith, 2009) and rural (Mwaniki, 2009) settings have been made published by the Global WASH Cluster.
A warmer climate is highly likely to result in more frequent and intense rainfall and more flooding (IPCC< 2007). Flooding can lead to contamination of drinking water wells and can also prevent physical access when floodwaters are high enough.
Community health and economic activity require continuity of safe water supply. Sealing and elevating wells can prevent both contamination of drinking water and loss of physical access to the wellhead. Ensuring continuous access to drinking water decreases the likelihood that populations will be displaced during moderate flooding.
Construction of new wells is very expensive and often requires drill rigs or other specialized equipment. Retro-fitting for flooding can generally be accomplished with basic construction supplies at or close to the ground surface. The costs of retro-fitting wells for drought by elevating the apron and handpump (figure 1) were estimated to be $315 per well in India (Districht Bahraich, 2010). By comparison, the costs of installing a new borehole are highly dependent on soil type, depth to the water table, and other factors; they have been reported to be between $1000-1500 in India and $10,000-15,000 in parts of Africa (Carter, 2006).
Some basic knowledge of water supply technology and public health principles is necessary to perform sanitary surveys (Smith and Shaw, no date). Experience drilling a given type of well and basic concrete construction skills are also necessary.
Undertaking a survey of population distribution, and water point location, elevation and condition can greatly improve the efficiency of flood-proofing programs. This survey should then be compared against floodplain maps to determine priority areas for well flood-proofing. This procedure can be used to ensure that the WHO emergency guidelines are already met when flooding does occur: (1) at least one functioning water point per 250 people and (2) the maximum distance from any shelter to a water point is less than 500 meters (Reed, 2005).
A training or certification program may be necessary for those carrying out sanitary surveys of wells in flood-prone areas. Some institutional capacity is necessary to determine if, where and how public funds should be allocated for constructing or retro-fitting wells.
Frequent flooding causing temporary lack of access to handpumps has increased demand from local citizens for flood-proofing (Districht Bahraich, 2010). Communities with alternative (e.g. piped) water supplies may be less likely to demand/ less willing to invest in flood-proofing wells.
Carter, R. (2006) Ten-step Guide Towards Cost-effective Boreholes: Case study of drilling costs in Ethiopia. World Bank Water and Sanitation Program. http://www.rwsn.ch/documentation/skatdocumentation.2007-06-04.3136351385...
District Administration, District Bahraich, Uttar Pradesh, India. (2010) Wat-San: Bahraich Model. http://www.recoveryplatform.org/assets/document/Bahraich%20WASH%20case%2...
Howard, G., Godfrey, S., and Boonyakarnkul, T. (2006) Sanitary completion of protection works around groundwater sources. In “Protecting Groundwater for Health” Eds. Schmool, O., Howard, G., Chilton, J. & Chorus, I. International Water Association. London. http://www.bvsde.paho.org/bvsacd/cd59/protecting/sect4-18.pdf
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Jaiswal, P. (2010) Hope floats in great flood. Hindustan Times. September 23, 2010 http://www.hindustantimes.com/Hope-floats-in-great-flood/Article1-603891...
Misstear, B., Banks, D. and Clark, L. (2006) Water wells and boreholes. Wiley & Sons, Ltd. West Sussex, England.
Mwaniki, P. (2009) Lessons learned in WASH Response during Rural Flood Emergencies. Global WASH Cluster. New York. http://www.humanitarianreform.org/humanitarianreform/Portals/1/cluster%2...
Reed, B. (2005) Minimum water quantity needed for domestic use in emergencies. WHO—Technical Notes for Emergencies Technical Note No. 9. World Health Organization, Geneva. http://www.who.int/water_sanitation_health/hygiene/envsan/minimumquantit...
Smith, M. (2009) Lessons learned in WASH Response during Urban Flood Emergencies. Global WASH Cluster. New York. http://www.humanitarianreform.org/humanitarianreform/Portals/1/cluster%2...
Smith, M. and Shaw, R. (no date) Technical Note 50: Sanitary Surveying. WEDC. Loughborogh University. Leicestershire, UK.http://www.lboro.ac.uk/well/resources/technical-briefs/50-sanitary-surveying.pdf
WaterAid (2006) “Technology notes.“
WaterAid—Bangladesh (2006) “Step by step implementation guide for tubewells.” Dhaka. http://www.wateraid.org/documents/plugin_documents/060721_tubewell_guide...
WHO (1996) Guidelines for Drinking Water Quality—2nd Edition. World Health Organization. Geneva. Chapter 6. http://www.who.int/water_sanitation_health/dwq/2edvol3f.pdf