The invention relates to a method and apparatus for flood prevention, particularly floods caused by back flow of water through a water closet. The invention provides for provision of a sealant (1) which is flowable and expansible in wet or humid conditions, ie in water (2 ) in a wc bowl (3), the sealant being delivered to an outlet passageway (4) where the sealant (1) expands for obturating fluid flow, in this case water, back through the passageway (4) into and through the wc bowl to form a flood. The sealant (1) comprises a mixture of a clay-like sodium bentonite and sand. The sealant (1) is in a flexible container such as a nylon bag (5) which has through passageways to provide for contact of the bentonite with water in the wc bowl. Sodium bentonite swells to many times its dry size, for example 15-18 times its dry size on contact with water, forming a strong water-proof seal. The bag (5) is housed in a bag of dissolvable material such as polyvinyl alcohol (=PVA).
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