Loss coefficient for pits and junctions

This information is reproduced with permission from Road Design Guidelines Part 7, Drainage (Vic Roads, 1992).

Head loss factors for pits

Description​ Qu Ql Qg k
Inlet pit with one outlet pipe:​ ​ ​ ​ ​
(a) side entry​ -​ -​ =Qo​ 10​
(b) grated pit​ -​ -​ =Qo​ 5​
Inlet pit on through pipe​ ~0.9Qo​ -​ some​ 0.5​
~0.7Qo​ -​ ~0.3Qo​ 1.3​
~0.5Qo​ -​ ~0.5Qo​ 2.1​
Junction pit on through pipe​ = Qo​ -​ -​ -​
Inlet pit on through pipe with laterals​ ~0.9Qo​ some​ some​ 0.5​
~0.7Qo​ some​ some​ 1.1​
~0.5Qo​ some​ some​ 1.5​
~0.3Qo 0.7Qo​ some​ 2.0​
Junction pit on through pipe with laterals​ ~0.9Qo some -​ 0.5
~0.5Qo​​ ~0.5Qo​​ -​ 1.5​
~0.2Qo​ ~0.8Qo​ -​ 2.0​
Inlet pit on L bend​ -​ ~Qo​ some​ ​1.5
Junction pit on L bend​ -​ =Qo​ 1.3​
Inlet bend on T junction with laterals​ -​ ~Qo​ some​ ​1.8
Junction pit on T junction with laterals​ -​ =Qo​ 1.6​
Drop pit​ ​ ​ ​ ​
(a) direction change less than 45 degrees​ ~Qo​ -​ some​ ​2.0
(b) direction change more than 45 degrees​ ~Qo​ -​ some​ ​2.5


Figure C1 part - Full outflow from a junction pit

  1. Qu = flow from upstream pipe; Qo = flow out of pit; QL = flow from lateral pipes; Qg - flow from above the water level; k = pit head loss coefficient

  2. The pipes are assumed to operate below the water level in the pit. Flows entering from above the water surface should be added to Qg

  3. Where part full flow occurs in the outlet pipe, tests have shown the water surface is significantly higher. Assume the HGL to be at the pipe obvert and add

  4. Where the design flows are between the tabulated values, interpolate between the k values

  5. Where flow is deflected through a horizontal angle at a pit, add the coefficient from Figure C2 to the k value from the table, except at drop pits. For drop pits, use the values from the table

  6. If Du/Do is less than 0.9 or a better estimate of k is required, refer to design charts in Sangster et al


Figure C2 - Head loss factors for angles in pits

Losses at angles in pits are added to the other losses at the pit, except at drop pits. For losses at drop pits, don't use this figure. Refer to the Head loss factors for pits  table.


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