They'd have to feed into an attenuation tank/pit/lake within Stanley's control. Can't simply discharge surface water onto your neighbours' land anymore, and move the flooding further downstream. Would have thought somewhere under the Coppice end car park would be an option. Could install a giant SUDS system under there, but it's unlikely to be cheap, unless it as part of a more comprehensive development at that end of the ground
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But the current groundwater levels are exceptional, and across the country, and the cost of further drainage infrastructure has to be balanced against both how often this level of incessant rain occurs and what else the money could be spent on.