Appendix C - Loss Coefficient for Pits and Junctions
The information in this appendix 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 | - | - | 0.2 |
| 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 ~0.5Qo | some ~0.5Qo | - - | 0.5 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
- 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
- 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
- Where part full flow occurs in the outlet pipe, tests have shown that the water surface is significantly higher. Assume the HGL to be at the pipe obvert and add
- Where the design flows are between the tabulated values, interpolate between the k values
- 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
- 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, do not use this figure. Refer to the previous table.
