Section 6: Conduit SystemsAnchor: #i1017027
The storm drainage conduit system transports the runoff from the surface collection system (inlets) to the outfall. Although it is an integral component, analyze the conduit system independently of the inlet system.
An inlet location in a storm drain system basically controls the need for a conduit, its slope and horizontal orientation, and its minimum cover requirements.
The configuration of laterals and trunk lines is controlled by the locations of all inlet and roadway layouts and is also affected by utility and foundation locations.
The longitudinal slope of the conduit affects its capacity. The slope of the subject run is tentatively established during the system planning stage of design. Typically, the slope will be approximately parallel to the surface topography. However, you may have to adjust conduit slopes to adapt to critical elevations (such as outfall elevations). You can adjust individual run slopes as necessary to increase capacity, avoid conflicts with utilities, and afford adequate cover for the conduit.
Avoid circular pipe sizes less than 18 in. (450 mm) diameter for main trunk lines or laterals because of difficulties in their construction and maintenance. Some designers prefer to limit the minimum circular diameter to 24 in. (600 mm). Consider the following recommendations on conduit dimensions:
- Standard size pipe use in conduits -- Do not use non-standard sizes of pipe. It is rarely cost effective to specify pipe dimensions requiring special fabrication. Consult with local fabricators, become acquainted with stockpiled dimensions, and use those commonly manufactured sizes in the design.
- Larger versus smaller conduit dimensions -- Avoid discharging the flow of a larger conduit into a smaller one. The capacity of the smaller conduit may technically be greater due to a steeper slope. However, a reduction in size almost always results in operational problems and expenses for the system. Debris that may pass through a larger dimension may clog as it enters a smaller dimension.
- Soffit and flow line placement in conduits -- At changes in size of conduit, make an attempt to place the soffits (top inside surfaces) of the two conduits at the same level rather than placing the flow lines at the same level. Where flow lines are placed at the same level, the smaller pipe often must discharge against a head. It may not be feasible to follow this guideline in every instance, but it should be the rule whenever practicable.
- Conduit length -- You may approximate the length of the conduit for these calculations. Often, the length is indicated as from centerline-to-centerline of the upstream and downstream nodes of the subject conduit run. Use the length with the average flow velocity to estimate the travel time within the subject run. Establish the length of the run during the first phase of the storm drain system design in which the inlets are located.
NOTE: These are not pay lengths of conduit; the standard specifications provide that pay lengths include only the actual net length of pipe and not the distance across inlets or manholes where no conduit actually is placed.Anchor: #i1017082
Place manholes or combination manholes and inlets wherever necessary for clean-out and inspection purposes. It is good engineering practice to place manholes at changes in direction, junctions of pipe runs, and intervals in long pipe runs where the size or direction may not have changed. The table below provides recommended maximum spacing criteria for manholes.
12 – 24
300 - 600
27 – 36
675 – 900
39 – 54
1050 – 1350
Round the invert (bottom) of the manhole section to match the inverts of the pipes attached to the manhole to minimize eddying and resultant head losses. For manholes larger than the incoming or outgoing pipes, expansion losses can sometimes be significant.
Detail manholes that are intended as combinations with other functions to include facilities that will serve all the intended functions. In such cases, you may need to consider junction losses.
At junctions of pipelines, right angle intersections are simpler to construct. However, an acute angle junction reduces head losses, and you should consider it where practical. See Figure 10‑18 for the contrast. Where junction losses may be of particular concern, consider using acute angle junctions.
Figure 10-18. Head Losses at IntersectionsAnchor: #i1017117
Inverted siphons carry flow under obstructions such as sanitary sewers, water mains, or any other structure or utility that may be in the path of the storm drain line. Use them only where avoidance or adjustment of the utility is not practical. The storm drain flowline is lowered at an obstacle and is raised again after the crossing. In the design of inverted siphons, we recommend a minimum flow velocity of 3 fps (1 m/s).
In general, the conduit size through the inverted siphon used as a storm drain system should be the same size as either the approaching or exiting conduit. In no case should the size be smaller than the smallest of the approaching or exiting conduit.
Because an inverted siphon includes slopes of zero and adverse values, account for head losses through the structure using outlines in Chapter 6, Hydraulic Grade Line Analysis. The sources of these losses can be friction, bends, junctions, and transitions.
If the losses are unacceptable, you may need alternative means of avoiding the utility conflict. Provide maintenance access at either or both ends of the inverted siphon as indicated in Figure 10‑19.
Figure 10-19. Inverted SiphonAnchor: #i1017155
Conduit Capacity Equations
Refer to Chapter 6 for calculating channel (conduit) capacity and critical depth.Anchor: #i1017165
Conduit Design Procedure
In this procedure, nodes represent point definitions in the network such as junctions and inlets. Runs represent the conduit connections between nodes. A storm drainage system is characterized as a link-node system with runoff entering the system at nodes (inlets) that are linked together (by pipe or conduit runs), all leading to some outfall (outlet node). The procedure entails proceeding progressively downstream from the most remote upstream node to the outlet. The peak discharge at each node is re-computed based on cumulative drainage area, runoff coefficient, and longest time of concentration contributing to the particular node.
Use the following steps for the design of conduit systems:
- Determine the design discharge
at each extreme node (inlet). The design discharge for a particular
run is based on the watershed area to the upstream node of the run,
the associated weighted runoff coefficient, and the rainfall intensity
based on the time of concentration (tc) in the
watershed. This time of concentration often is referred to as “inlet
time,” indicating it is the surface time of concentration in the
watershed to the inlet. If the tc is less
than 10 minutes, base the intensity on a tc of
10 minutes; otherwise, use the actual tc value.
Use this value of tc in Equation 10-35 for
rainfall intensity and compute the discharge using Equation 10-34.
Account for the actual time of concentration as this value eventually
may become significant even if it is less than 10 minutes.
- Q = peak discharge (cfs or m3/s)
- C = runoff coefficient
- I = rainfall intensity associated with a specific frequency (in./hr or mm/hr)
- A = area of the watershed (ac. or ha)
- z = 1.0 for English measurement and 360 for metric.
- If = rainfall intensity for frequency (mm/hr)
- tc = time of concentration (min)
- e, b, d = empirical factors that are tabulated for each county in Texas for frequencies of 2, 5, 10, 25, 50, and 100 years in Hydrology. (See Rainfall Intensity-Duration-Frequency Coefficients.)
If the inlet has been designed with carryover, either from or to the inlet, ignore the carryover rate(s) when considering the discharge into the conduit.
Base the intensity on the longest time of concentration leading to the upstream end of the run. This means that a recalculation of total discharge is necessary at each upstream end of a conduit run. It also means that you do not simply add discharge rates from approaching watersheds and/or pipe runs; rather, multiply the sum of contributing CA values by an intensity based on the longest time of concentration leading to the point in question.
- Size the conduit for pressure flow or for non-pressure
flow based on Manning’s Equation and the design discharge. The recommended
method is to design for non-pressure flow: conduit size will likely
be slightly larger than necessary to accommodate the design flow
under the terms of Manning’s Equation. For TxDOT, pressure flow
design means that the conduit has dimensions smaller than necessary
to accommodate the design flow under the terms of Manning’s Equation.
If it is necessary or useful to design conduits for pressure flow,
coordinate such design with the Design Division, Hydraulic Branch.
To size circular pipe, use Equation 10-36 (depending on material
type and associated roughness):
- D = required diameter (ft. or m)
- z = 1.3333 for English measurement or 1.5485 for metric
- Q = discharge (cfs or m3/s)
- n = Manning’s roughness coefficient
- S = slope of conduit run (ft./ft. or m/m).
For sizing other shapes, use trial and error: select a trial size and compute the capacity. Adjust the size until the computed capacity is slightly higher than the design discharge.
- Estimate the velocity of flow through the designed conduit.
Assume uniform flow as an average depth of flow in the conduit as
discussed in Section 2 of Chapter 6. Determine the cross-section
area, Au, at this depth. This is a straightforward
procedure for rectangular sections but much more complicated for
circular and other shapes. Manufacturers’ product information may
include tables of depth, area, and wetted perimeter. If not, calculate
area and wetted perimeter based on the geometry of the conduit.
Then calculate the average velocity of flow (Va)
using the continuity relation shown in Equation 10-37.
- Calculate the travel time for flow in the conduit from
the upstream inlet/node to the downstream node by dividing the length
of the conduit by the average velocity of flow. Add this travel
time to the time of concentration at the upstream end of the subject
run to represent the time of concentration at the downstream end
of the run.
NOTE: When accumulating times, base the time of concentration on the actual calculated times, even if it is less than the minimum of 10 minutes.
- Determine the total drainage area, cumulative runoff coefficient times area, and respective time of concentration. As you complete the design of the most remote runs and the design proceeds downstream through the system, determine the total drainage area, cumulative runoff coefficient times area, and respective time of concentration for all conduits incoming at a particular node before sizing the conduit run out of that node.
- Compute the peak discharge for the next run downstream
based on the total drainage area upstream contributing to each incoming
conduit/run at the node, the cumulative product of the runoff coefficient
and contributing area to each incoming conduit/run at the node,
the longest time of concentration of all incoming conduits, and,
if applicable, inlet time for the node. (This time is used to re-compute
intensity in the rational equation for sizing the next downstream conduit
NOTE: You can easily determine the area and runoff coefficient if you record the CA values for each watershed as you proceed with design down the system and sum them at each node.
- Continue this process until you have sized all conduits in the network. In each case, as runs and entering watersheds converge to a node, recalculate the peak discharge for which the exiting conduit is to be designed as the product of an intensity based on the longest time of concentration leading into the node and a summation of all CA values that contribute flow to the node. The discharge, so determined, is not the same as if you have added all approaching discharges because the procedure is fashioned to conform to the general application requirements for the Rational Method. In some instances, calculated discharges can decrease as you carry the analysis downstream (because of a small increase in the accumulated CA as compared to rainfall intensity). In such cases, use the previous intensity to avoid designing for a reduced discharge or consider using a hydrograph routing method.
- Develop the hydraulic grade line (HGL) in the system as outlined in Chapter 6. Calculate minor losses according to Chapter 10.
The analysis of a conduit requires the same consideration of hydrology as does design. The difference is that geometry, roughness characteristics, and conduit slopes are already established.
The analysis and accumulation of discharge must proceed from upstream toward downstream in the system. Develop the discharges in this way so that appropriate discharge values are available for the development of the hydraulic grade line analysis.