 A minefield was installed in the river between Shornemead and Coalhouse forts, with mechanically operated mines in the shallower parts of the river and remotely detonated mines in the navigable channel.Friendly ships could thus pass freely but the mines could be detonated by a shore-based observer if an enemy ship tried to use the main channel.Coalhouse Fort took on the role of an Examination Battery controlling the river traffic in conjunction with the River Examination Service.