Turbidity Turbidity is the cloudiness or haziness of a fluid caused by individual particles and suspended solids in water that individually may are generally invisible to the naked eye but collectively form the haze. The measurement of turbidity is a key test of water quality and its measure can be used as an early warning that something has entered the drainage system. Increased turbidity is often associated with heavy rain where soil / clay can be washed into the water, however it can be used to detect substances such as milk, suspended powders, mixtures, paints, etc that could be spilt and enter the drainage system. Modern sensors are available to measure turbidity. Many of these look at the optical properties of the medium either looking for optical transmission or back scatter (reflection). By deploying turbidity sensors up stream in a drainage system it is possible to close of the exit from a site using a DrainstopperTM or FlapstopperTM at the exit from the site.