Measures to mitigate direct flood risks at riverbank filtration sites with a focus on India

The direct entry of flood water, resultant contamination and damage to groundwater and riverbank filtration (RBF) wells frequently cause the disruption of drinking water production in flood-prone areas worldwide, especially in India and other regions of South and South East Asia influenced by the monsoon. The aim of this paper is to identify critical flood-prone points in the present practice of well construction in India. The focus is on field investigations conducted on wells at the RBF sites in Uttarakhand and at some sites in Germany. Methods to determine whether the annular space of vertical wells and caissons of large-diameter wells are watertight were tested for their practical application in the field in India and Germany. The investigations showed that for a caisson well in poor sanitary condition, flood water can short circuit into the well within 20–45 min along preferential flow paths to a shallow groundwater table. In case of vertical well chambers in Germany, >300 L of water seeped into the well chamber during a 10-min observation period. The sealing of the annulus between the borehole wall and casing of vertical wells and watertightness measures to well chambers and their covers and well heads to prevent short circuiting are discussed in context to prevalent practice in India.