Jun 26, 2022 · The Pressure head formula may be expressed as, H= (p/w) Where P is Pressure head [4] To avoid severe hydraulic jump and/or backpressure, good engineering practice requires the vertical leader transition into a larger size horizontal storm drain per the GPM flow indicated in Table 2 for 1/8" and 1/4" sloped storm drains 7, 0 Compressor Type L