Urban Storm Drain Design: Conduit design

Sizing Conduit size is computed based on the discharge expected at the upstream node; Typically is done by the rational formula, applied to the sum of the areas contributing to that node; Subareas are pre-multiplied by their respective runoff coefficients (ΣC*A) Intensity is calculated for the travel time coinciding with the longest flow path leading to that node; TxDOT traditionally designs conduits to flow as open channels (free surface inside the conduit) at the design discharge.

Conduit shape Boxes are commonly used where headroom is constrained; Circular sections are easily the most economical, usually being ¼ to 1/5 the cost of box sections; Other section shapes are available, but should not be used unless there is a compelling reason (contact DES-HYD) Boxes are often used in an attempt to minimize trench protection cost by closely following ground contour. THIS IS USUALLY NOT EFFECTIVE. Trenching cost is less than the difference in material cost.

Cost comparison

Conduit materials Reinforced Concrete is the most common material for storm sewers. Corrugated metal is available, but is discouraged by TxDOT for storm sewer construction. Various plastic materials have been proposed in recent years; beware of “low Manning’s n” claims. RC pipe or precast boxes are the easiest to construct- backfilling lighter materials without “float” is much harder than it looks. Structural design/bedding condition is important.

Trunk lines Trunk lines should follow ground contour in only the most general way- trunk line profile should be dictated by velocity/energy/depth management needs. Trunk lines run from junction box to junction box, they should not run through other appurtenances (inlet boxes) nor should there be hidden pipe junctions (Ts or Ys) Trunk lines should enter and leave junction boxes such that there is no backwater in upstream conduits

Trunk lines Trunk lines should always stay the same size or increase in size in the downstream direction, never decrease. Velocity in trunk lines should stay the same or increase by small increments in the downstream direction, never decrease. Trunk lines should be designed to maximize the length of runs of the same diameter, rather than changing diameter frequently

Design discharge Procedure in HDM Used to size conduits

Velocity –travel time Storm sewers should be designed such that velocities are maintained at levels similar to those in natural overland flow. This minimizes the effects of changing timing of contribution in receiving streams.

Flowing as open channel – D/d TxDOT procedures assume conduit flow is as an open channel at design discharge Ratio of depth to diameter (D/d) is an important metric of open-channel flow Flow efficiency increases as D/d increases until D/d reaches.5 Flow efficiency diminishes as D/d increases past.5, but discharge still increases until D/d reaches.85 After D/d reaches.85, the computed discharge diminishes; in reality flow becomes unstable (oscillates and surges).

Surcharge flow If the discharge must be greater than the pipe will carry as an open channel at D/d of.85, flow will become pressure flow by building up head (surcharging) in junction boxes.

Hydraulic grade line The Hydraulic grade line should be relatively uniform within any conduit run, i.e. should not include an backwater effects The HGL of laterals should be matched or above the HGL of the trunk line at junctions Hydraulic drops of laterals into junctions in order to “disconnect” the laterals from trunk line influences are good practice and allow consistency of lateral construction

Profile grade Profile grade of storm sewer trunk lines may be “stairstepped” to control energy in cases of significant topographic relief Profile grade of storm sewer trunk lines should always include some sort of drop at junctions (where additional flow comes in) to at least match HGL upstream and downstream Profile grade should be driven by hydraulic considerations rather than topographic considerations