1. Urban Storm Drain Design: Inverted Siphons and other Sewer considerations

2. General theory
“Water seeks it’s own level”, i.e. if there is a hydraulic connection, total energy will be uniform throughout the fluid
“Inverted siphon” is a term applied to a pressure flow reach within a system that operates in open channel flow elsewhere.
These appurtenances are used where some other fixed feature spatially conflicts with the open channel.
An example would be where a sanitary sewer line, which must maintain existing location and grade, would have to pass through a storm sewer conduit

3. Inverted Siphon
Siphon is pressurized in operation

4. General theory
Inverted siphons involve a junction at each end of the siphon that is deeper than would be in the absence of the siphon, and one or more conduits that carry pressure flow between the junctions.
Inverted siphons are submerged flow conduits
Always carry pressure flow
Do not drain -- they are traps for water, sediment, and debris
Inverted siphons should be avoided if possible.
If there is no alternative, consult DES-HYD for design.

5. Alternatives
As an alternative to inverted siphons for utility conflicts, consider installing a special purpose junction box and passing the utility through the box

6. Outfalls
The “outfall” is the downstream end of storm sewer system; where it empties into a stream or other receiving water
Location, configuration, size, and details of the outfall may have many impacts (environmental, public safety, system performance, etc)

7. Location
Outfalls should ideally be located in places that are accessible for inspection and maintenance, but do not draw public notice or attention
Locations may be subject to environmental regulation (MS4) and permitting
Consider the effects of introducing flow at a particular spot- quantity, momentum balance, sediment transport, and function of the receiving water

8. Elevation
The nature of the receiving water is critical to the selection of the outfall flow line elevation
If sufficient topographic relief is available, a short outfall run with a junction box incorporating a significant drop from trunk line to outfall run is desirable
The outfall should preferably not terminate in a significant drop unless there is well-designed scour protection underneath it (steep paved slopes are not well-designed scour protection!!!)

9. Elevation
The outfall should preferably not terminate low enough in a stream to promote blocking or clogging by sediment
Outfall design usually involves a compromise between scour potential and clogging potential.
Consider conditions other than the design conditions!

10. Submergence
In some cases, outfalls must be located such that they are subject to submergence, either by streamflow or by tidal flow
Every effort should be made to keep the line self-cleaning even when intermittently submerged (how?)
Consider stage/frequency and or tidal range/frequency

11. Velocity control
Exit velocity and momentum direction are critical elements of outfall design
Velocity should be low enough to prevent scour, but high enough to prevent clogging
In sanitary sewer design, engineers try to achieve 2-10 feet-per-second at the daily peak flow to keep entrained solids moving towards treatment plant.
Similar velocity range may be useful in storm sewers – 10 feet per second is quite fast!

12. Velocity control
Consider momentum effects on the receiving water if it is flowing water
A rigorous hydrologic examination of the receiving stream may be necessary to estimate an expected stage and velocity during storm sewer outflow
The characteristic times are likely to be very different

13. Intrusion control (public safety)
Outfalls, particularly large ones, should not facilitate easy entry and traverse by the public (particularly children)
As stated earlier, a junction box with a significant drop (>6’) close to the outfall discourages entry (a ladder is very difficult to get into a junction box from the outfall)
Gates, grates, etc may be necessary, but should be avoided if at all possible (they trap debris and present a danger should anyone enter the system upstream)

14. Intrusion control (public safety)
In conflict with earlier advise on trunk line configuration, a short outfall run may be designed with a wide, low box shape to discourage easy entry
Any measure that completely prevents intrusion also completely prevents escape in the event that a person enters the system higher up. Consider “discouraging” rather than “excluding” intrusion

15. Pollution
Storm sewers provide an immediate and efficient flow connection between the roadway and receiving waters.
A high potential for exacerbating the environmental effects of a chemical spill on the roadway or adjacent property any pollutants that end up on the roadway (oil, coolants, deicing chemicals, etc)

16. Pollution Consider sensitivity of the receiving waters.
Hazmat traps are available, but effectiveness is unknown
Questions to consider:
What will happen in the event of a spill?
Normal contaminants?
Do you need to do primary treatment on outflow?