What Causes Gravel Migration?
The cause of filter media migration and mounding is almost always associated with maldistribution of filter backwash water, frequently made worse by excessive backwash rates. Maldistribution of filter backwash water can be tied to three interdependent filter underdrain hydraulic characteristics: • Filter plenum hydraulics • Filter underdrain lateral hydraulics • Filter bottom block hydraulics A problem frequently found was a notable maldistribution of water from the central filter backwash plenum to the underdrain laterals. There was a nozzle-like tendency for water entering the central plenum to shoot to the far end of the plenum and impact against the end wall where velocity head would be converted to pressure head. This increase in pressure along the length of a lateral or plenum described above is the same hydraulic problem which occurs in most single lateral systems or false floor underdrains with low headloss. Teepee (Single Pass) Underdrain Teepee underdrains consist of a series
