Urban Storm Drain Design: Inverted Siphons and other Sewer considerations

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

Inverted Siphon Siphon is pressurized in operation

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.

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

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)

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

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!!!)

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!

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

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!

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

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)

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

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)

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?