Conduit Design Procedure

In this procedure, points in the network such as junctions and inlets are referred to as “nodes”. Conduit connections between nodes are referred to as “runs”. 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 recomputed based on cumulative drainage area, runoff coefficient, and longest time of concentration contributing to the particular node.The following steps are used for the design of conduit systems (a more detailed explanation and example are contained in
HEC-22
):
  1. Determine the design discharge at each extreme node (inlet). Any bypass flow, either from or to the inlet, is ignored when considering the discharge into the conduit. Keep track of the cumulative runoff coefficient multiplied by the area (ΣCA) and the time of concentration. 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.
  2. Determine the design discharge for the first run (or any inlet lateral) based on the watershed area to the upstream node of the run (A), the associated weighted runoff coefficient (C), and the rainfall intensity based on the time of concentration (t
    c
    ) in the watershed. The rainfall intensity is calculated with Equation 10-33 using the larger of the actual t
    c
     value or a t
    c
     of 10 minutes. The discharge is computed using Equation 10-32. It is very important to record the actual time of concentration as this value will eventually become significant.
    EquationObject309369
    Equation 10-32.
    where:
    • Q = peak discharge (cfs or m
      3
      /s)
    • C = runoff coefficient
    • I = rainfall intensity associated with a specific AEP (in./hr or mm/hr)
    • A = area of the watershed (ac. or ha)
    • z = 1.0 for English measurement and 360 for metric.
    EquationObject310370
    Equation 10-33.
    where:
    • I
      f
       = rainfall intensity for design AEP (in./hr or mm/hr)
    • t
      c
       = 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 (50%, 20%, 10%, 4%, 2%, and 1% AEPs) in
      Hydrology
      . (See
      Rainfall Intensity-Duration-Frequency Coefficients
      .)
    NOTE: Chapter 4 references the new rainfall atlas,
    Atlas of Depth-Duration Frequency (DDF) of Precipitation Annual Maxima for Texas
    (TxDOT 5-1301-01-1). A table of factors correlating to this atlas, similar to
    Rainfall Intensity-Duration-Frequency Coefficients
    , for use with Equation 10-33, will be developed at a later date. The new table will replace this reference to
    Rainfall Intensity-Duration-Frequency Coefficients
    at that time.
    The intensity is based 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 conduit run. It also means that the discharge rates from approaching pipe runs are not simply summed; instead, the sum of contributing CA values (ΣCA) are multiplied by an intensity based on the longest t
    c
     leading to the point in question.
  3. Size the conduit based on Manning's Equation and the design discharge. The Department 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. To size circular pipe, use Equation 10-34:
    EquationObject311372
    Equation 10-34.
    where:
    • D = required diameter (ft. or m)
    • z = 1.3333 for English measurement or 1.5485 for metric
    • Q = discharge (cfs or m
      3
      /s)
    • n = Manning’s roughness coefficient
    • S = slope of conduit run (ft./ft. or m/m).
    For sizing other shapes, use trial and error by selecting a trial size and then computing the capacity. Adjust the size until the computed capacity is slightly higher than the design discharge.
  4. Estimate the velocity of flow through the designed conduit. Determine the cross-section area, A
    u
    , assuming uniform flow as an average depth of flow in the conduit as discussed in
    Section 2
    of Chapter 6. This is a straightforward procedure for rectangular sections but much more complicated for circular and other shapes. Then calculate the average velocity of flow (V
    a
    ) using the continuity relation shown in Equation 10-35.
    EquationObject312373
    Equation 10-35.
  5. Calculate the travel time, t
    t
    , for flow in the conduit from the upstream node to the downstream node by dividing the length of the conduit by the average velocity of flow. Add this travel time to the t
    t
     at the upstream end of the subject run to represent the t
    t
     at the downstream end of the run.
    NOTE: For this purpose, base the t
    t
     on the actual calculated times, not the minimum of 10 minutes used to compute intensity.
  6. Proceeding downstream through the system, determine the cumulative runoff coefficient multiplied by the area (ΣCA) and respective time of concentration at each node. Make sure to include all conduits and inlets coming to a particular node before sizing the conduit run out of that node. It may help to draw a stick diagram showing the cumulative CA and t
    c
    /t
    t
     values.
  7. Compute the peak discharge for the next run downstream based on the ΣCA to the node and the intensity based on the longest value of t
    c
     of all incoming conduits, and, if applicable, t
    c
     of any inlet directly at the node. The discharge, so determined, is not the same as if all approaching discharges have been added.
    In some instances, an increase in t
    t
     (which decreases I) with little or no additional CA can cause the calculated discharge to decrease as the analysis is carried downstream. In such cases, use the previous intensity to avoid designing for a reduced discharge, or consider using a hydrograph routing method.
  8. Develop the hydraulic grade line (HGL) in the system as outlined in
    Chapter 6
    . Calculate minor losses according to
    Conduit Systems Energy Losses
    . If the system was designed for full flow, calculate other losses such as junction, manhole and exit losses according to
    Conduit Systems Energy Losses
    .