Urban Storm Drain Design: Laterals

Sizes• Laterals carry water from inlets into junction boxes,

where it leaves by a trunk line.• The HGL of laterals can be independent of- but

above- that of the trunk line runs• It may be cheaper in the long run to have some

laterals oversize and consistent with others rather than specify a small quantity of smaller pipe

• Laterals often spill into junction boxes much higher than trunk lines enter and leave; they may be allowed to protrude a small amount to facilitate construction

Road crossing• Laterals often completely or partly cross the

roadway and may need to be constructed in phases• Should be located deep enough to clear pavement

construction!

Slope• If the trunk line is located reasonably deep, slope of

laterals is fairly free.• It is much preferred to have laterals enter high, with

relatively low velocity and plunging flow, than to bring them in low, with high velocity entering a larger stream (momentum distribution).

• Laterals can often be planned such that the length and slope of many laterals is the same, facilitating construction.

Box exit• The “bell” or “groove” end of pipe is oriented

upstream• With most inlets, the lateral emptying the box exits

from the “front”, or roadway, wall of the box (despite what is implied by the standard)

Junction with other appurtenances• Small storm sewer systems occasionally flow into

other drainage structures, such as box culverts• Consideration of lateral momentum is important if a

storm sewer system enters a culvert low in the culvert wall

• Beware of badly unbalanced flow in multiple boxes if a lateral flows directly into a multi-box culvert

Junction boxes/manholes• Junction boxes are connections between lines; they

usually serve also as manholes• Manholes are points of access and ventilation in a

system. They may coincide with junction boxes, but may also be located based solely on access and ventilation needs

Location• System-wise, junction boxes should be located

wherever laterals join a trunk line, or where there is a need to change conduit size or configuration (there are few reasons to change conduit size unless there is a change in discharge, i.e. a lateral enters)

• Geographically, junction boxes are typically located within the roadway

• Plan trunk lines and junction boxes to ease construction of the entire project- for instance, within the roadway in the first phase of a multi-phase roadway reconstruction process

Location• Avoid placing junction boxes in a wheelpath;

between wheelpaths or within an auxiliary lane is good

Entry/exit of trunk lines• Trunk lines will always exit junction boxes flush with

the bottom of the box (Duh!)• Trunk lines should enter junction boxes at an

elevation sufficiently high above the exit that the HGL of the entering line is at or above the HGL of the exit line. Some assumption about loss within the box should be made (there is a formula, use it if you want, but a general assumption is adequate)

• Remember that the HGL of the exit line will include discharge entering from laterals in addition to that entering from the trunk line

Entry of laterals• Laterals may enter a junction box at any elevation

such that the HGL of the lateral is at or above the HGL of the trunk line upstream

• They may be considerably higher- and flow plunge into the box, to minimize trenching and standardize lateral configuration from case to case

• If they will not physically conflict with through flow, they may protrude into the box slightly to ease construction

Losses• There will be some loss of energy through a junction

box. There is a formula, but such precision is rarely justified.

Size• Junction box planform size is implied by the

standard, which refers to conduit sizes for all entering and exiting conduits

• Junction box depth is determined by the roadway PGL and the exit conduit flow line elevation

• Flow line elevations can sometimes be manipulated to ease construction, i.e. to accommodate precast boxes.

Spacing• In areas of rapid change in topographic elevation,

junction boxes may need to be spaced relatively closely in order to “stairstep” and control energy (velocity) in the trunk line

• Normally, spacing is set by the need for entry of laterals due to inlets; it is closely associated with inlet spacing

• In cases of long trunk line runs with no junctions (such as long lines to a remote outfall), refer to the HDM for maximum manhole spacing for access and ventilation purposes. Use judgement.