1D Manholes

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Introduction

Manholes are typically chambers used to provide one or all of the following; maintenance access, change in culvert direction, connections and change in culvert dimensions. By default, manholes are automatically created within TUFLOW at all culvert nodes, any manually created manhole will override the automatically created manhole.

Manholes are used at culvert junctions to dissipate energy due to:

  • Expansion/contraction of flow within the manhole chamber and outlet culverts.
  • Change in direction of the culverts (i.e. a bend/deflection).
  • Change in height, width or diameter and/or invert level of the adjoining culverts.


The presence of a manhole at a junction point will override the exit loss of any culvert discharging into the manhole and entrance loss of any culvert taking from our of the manhole.

There are 3 types of manholes:

  • "C" for circular chambers.
  • "R" for rectangular chambers.
  • "J" for junctions without a chamber


Manhole Losses

When modelling conduit pipe flows, the head losses that the flow in pipe are subject to are made up of major losses (or friction losses) and minor losses (or local losses). Major losses are caused by forces between the flow and wetted perimeter of the conduit. Minor losses are caused by disruption to the flow due to bends, cross-sectional changes, fittings such as manholes and steps in the bed profile. Major losses are represented through the specification of a friction coefficient. The representation of minor losses, particularly for gravity networks, is at manholes and requires separate treatment. The default TUFLOW/Estry manhole loss approach uses the Engelund method explained in section 5.12.5.4 of the TUFLOW user manual although it is also possible to use a more simplified fixed manhole headloss approach too. The Engelund approach provides an automatic method for determining the loss coefficients as presented below. Of note is that the coefficients are recalculated every timestep, and therefore vary depending on the flow distribution between inlet and outlet conduits and the depth of water within the manhole. The losses represented are as follows:

  • Kentry covers the expansion of flow within the manhole at the outlet of an inlet conduit. The coefficient is applied as the exit loss of the inlet conduit.
  • KƟ represents the losses due to a change in direction (i.e. a bend) between inlet and outlet conduits. KƟ is determined automatically by TUFLOW based on the angle of the digitised lines of the conduits. For the inlet conduit the last two vertices of the line are used and for the outlet conduit the first two vertices.
  • Kdrop is the loss coefficient due to a change in invert level and conduit height between inlet and outlet conduits.
  • KƟ and Kdrop are added and applied as an energy loss for each outlet conduit.
  • Kexit covers the contraction from the manhole and re-expansion of flow within the entrance of an outlet conduit. It is applied as an entrance loss of the outlet.
  • Km is the user-defined manhole exit coefficient. The resulting headloss value is then applied, when sub-critical flow is experienced, to the standard head loss equation, i.e. dh = K*V2/2g. Where K is the loss coefficient, V is the conduit velocity and g the gravitational constant. The equations used for the Engelund loss approach are provided below:
    Engelund Equations.PNG
    Where:
    Qp = Flow in Conduit
    Qim = Total flow into manhole
    Qom = Total flow out of manhole
    yi = Height of inlet conduit
    yo = Height of outlet conduit
    hi = Inlet conduit invert
    ho = Outlet Conduit invert
    θ = Angle in degrees of inlet conduit relative to outlet conduit(θ = 0° "when the culverts are in line," θ=90° when the outlet culvert is at right angles)
    Qp = Flow in conduit outlet
    Wm = Flow width in manhole (1d_mh width attribute)
    ym = Depth of water in manhole
    Am = Flow are in manhole
    A'm = Effective flow are in manhole
    Ap = Flow area of conduit
    Km = Manhole Loss Parameter (1d_mh Km attribute)
    Kb = Bend Loss Coefficient (1d_nwk Form_loss attribute)
    Kf = Fixed Loss (1d_mh K_Fixed attribute)
    KBendmax = Upper limit to sum of Kθ and Kdrop (1d_mh K_Bend_Max attribute)

    Below are three worked examples of the application of the Engelund method applied to a simple model for the following scenarios:
  • Single Incoming/Outgoing Pipe with no angle and no drop
  • Single Incoming/Outgoing Pipe with incoming bend and drop in levels
  • Multiple Incoming Pipes with incoming bend and drop in levels

    Example 1: Single Incoming/Outgoing Pipe with No Incoming Angle or Drop


    A simple TUFLOW/ESTRY model was set up as shown in Figure 1 with the parameters for the manhole and the links upstream/downstream of the manhole shown in Table 1.
    Schematic 1.png
    Figure 1: Example Model 1 Network Schematic
    Long Profile 1.png
    Figure 2: Example Model 1 Long Section

    Parameter Upstream Conduit Downstream Conduit Manhole
    ID UC-2 DC-1 Manhole
    Type R R R
    Invert Level (m AD) N/A N/A 99.6
    Upstream Invert Level (m AD) 99.8 99.7 N/A
    Downstream Invert Level (m AD) 99.7 99.6 N/A
    Width(m) 1 N/A 1
    Form Loss 0 1 N/A
    Flow Width(m) N/A N/A 1.8
    K_Fixed N/A N/A 2
    Km N/A N/A 1

    Table 1: Relevant Model Parameters for Hand Calculations

    The model was run with a steady inflow of 2m3s-1. The resulting losses from the 1 hour steady inflow simulation are presented in the following figure.
    Manhole Results 1.png
    Figure 3: Example Model 1 Results-Loss Coefficients

    Example 1 - Entry Losses

    Entry losses are applied as an exit loss on the incoming conduit and are calculated as follows. K entry.PNG
    Where: Vp.PNG
    With other parameters defined in the section above. The calculated flow area in the manhole is 3.6m2 and 1m2 in the adjacent culvert whilst the flow is 2m3/s for both Qp and Qom. Therefore, Vm equals: Vm1.PNG
    And Vp: VP1.PNG
    Using these in the Kentry equation provides a loss coefficient of: K entry1.PNG
    The value matches the downstream loss coefficient for the upstream conduit shown in Figure 3. The loss values are also shown in the *_TSL_P.shp file with the third value for the upstream conduit providing the entrance loss value.

    Example 1 - Exit Losses

    Exit losses are applied to the upstream end of the outgoing conduit and are calculated as follows: K exit.PNG
    From table 1, we can see that Km is set to 1. Ap and A’m are 1m2 and 3.636m2 respectively. Therefore, Kexit is: K exit1.PNG

    Figure 3 shows that the calculated value matches those provided in the TUFLOW results. The value is also reported as the first value for the downstream conduit in the *_TSL_P.shp layer.

    Example 1 - Outgoing Conduit Losses

    The loss coefficient for the outgoing pipe represents the losses due to the incoming angle of the upstream conduit, any drops in inverts levels between the incoming and outgoing conduits, bend losses and any additional form losses. It is calculated as follows:

    K outletpipe.PNG

    The default value of KBend_max is set to 4 but can be changed via the 1d_mh K_Bend_Max attribute. As shown above we have no angle for the incoming pipe and no drop in invert levels. Table 1 shows the K_Fixed is equal to 2 and the outgoing pipe has a form loss coefficient, to represent bend losses of 1. Therefore Koutletpipe is: K outletpipe1.PNG

    This matches the value within Figure 3 and the middle value for the downstream conduit in the *_TSL_P.shp, representing the outlet conduit losses, of 3.

    Example 2: Single Incoming/Outgoing Conduit with Incoming Bend and Drop in Invert Levels


    In this example, we will use the same simple setup but this time there is a 90 degree angle between the incoming and outgoing pipe as shown in the plan view in the figure below.
    Schematic 2.png
    Figure 4: Example Model 2 Network Schematic

    There is also a drop between the invert elevation of the incoming and outgoing conduits as shown in the difference in invert levels in table 2 below and the long profile in figure 5 . All other model parameters were kept the same including boundary inflows.

    Parameter Upstream Conduit Downstream Conduit Manhole
    ID UC-2 DC-1 Manhole
    Invert Level (m AD) N/A N/A 99.4
    Upstream Invert Level (m AD) 99.8 99.5 N/A
    Downstream Invert Level (m AD) 99.7 99.4 N/A

    Table 2: Updated Model 2 Parameters for Hand Calculations

    Long Profile 2.png
    Figure 5: Example Model 2 Long Profile

    The resulting simulated losses are shown in the below figure 6.
    Manhole Results 2.png
    Figure 6: Example Model 2 Results-Loss Coefficients

    Example 2 - Entry Losses

    In this example, the flow area in the manhole is 4.12m2 and 1m2 in the adjacent culvert whilst the flow is 2m3/s for both Qp and Qom. Therefore, Vm equals: Vm2.PNG
    And Vp: Vp2.PNG
    From this: K entry2.PNG

    Which matches the value in figure 6.

    Example 2 - Exit Losses

    In this example, A'm is 4.12m2, therefore Kexit:K exit2.PNG

    Which matches the result in figure 6.

    Example 2 - Outgoing Conduit Losses

    On this occasion, we have an 90 degree angle between the incoming and outgoing conduit. Therefore, we need to calculate Kθ using the following equation: Ktheta.PNG
    Qf is calculated as: Qf.PNG
    Qp and Qom are both 2m3s-1 and therefore Qf is equal to 1. The incoming conduit angle is 90 degrees which is cancelled out by the denominator. Kθ is therefore:Ktheta1.PNG

    We also have a drop between the incoming and outgoing pipes. Kdrop which represents losses due to this drop is calculated as follows:Kdrop.PNG

    Which is:Kdrop 1.PNG

    Therefore Koutletpipe is:K outletpipe2.PNG

    This matches the simulated value in figure 6.

    Example 3: Multiple Incoming Conduits with Incoming Bend and Drop in Invert Levels

    In this example we have multiple incoming conduit. Each upstream conduit has a flow of 1m3s-1 applied to it.

    Schematic 3.png
    Figure 7: Example Model 3 Network Schematic
    The conduits are the same shape and the same dimensions but have different invert levels as specified in the table below. All parameters for the manhole and other parts of the network are identical.

    Parameter Upstream Conduit 1 Upstream Conduit 2
    ID UC-2 UC-31
    Upstream Invert Level (m AD) 99.8 100
    Downstream Invert Level (m AD) 99.7 99.8

    The resulting simulated loss values are presented in Figure 8.
    Manhole Results 3.png
    Figure 8: Example Model 3 Results-loss Coefficients

    Example 3 - Entry Losses

    This time the flow area in the manhole is 3.57m2 and 1m2 in the adjacent culvert whilst the flow is 1m3/s for Qp and 2m3s-1 for Qom. Therefore, Vm equals:Vm3.PNG
    And Vp:Vp3.PNG
    From this: K entry3.PNG

    This matches the losses for both incoming pipes as shown in figure 8.

    Example 3 - Exit Losses

    A'm is 3.57m2 respectively. Therefore, Kexit:K exit3.PNG

    The calculated losses match the losses calculated by the TUFLOW presented in Figure 8.

    Example 3 - Outgoing Conduit Losses

    In this model we have two incoming pipes, one with an 90 degree angle between the incoming and outgoing conduit and one with zero. Recall that: Ktheta.PNG

    With Qf calculated as: Qf.PNG
    Qp is set to 1m3s-1 and Qom to 2m3s-1. Qf is equal to 0.5m3s-1. For the incoming conduit with an angle of 90 degrees, Kθ is therefore: Ktheta2a.PNG

    For the incoming conduit with an angle of 0 degrees, Kθ is therefore: Ktheta2b.PNG

    We also have a drop between the incoming and outgoing pipes. Kdrop which represents losses due to this drop is calculated as follows: Kdrop 2a.PNG
    and:Kdrop 2b.PNG

    Therefore, Koutletpipe is: K outletpipe3.PNG

    The calculated value matches that presented in figure 8 for the downstream conduit loss.

    Storage chambers

    Storage, including storage chambers or floodplain storage, can be manually defined using a 1d_na (1d_tab_empty) node that has an assigned elevation versus surface area table. For the purpose of this page, manmade storage chambers have been discussed although the method is the same for all applications.

    An example of a chamber:

    www.humes.com.au


    Methodology

    • Import 1d_na node.
    • Assign the name of the elevation vs area csv.
    • Specify the names for the elevation and surface area columns


    Storage chamber MI.JPG
    storage_chamber.csv
    Storage chamber csv.JPG

    For more information on storage nodes see Section 5.10.3 Storage Nodes (User Defined NA Tables) within the TUFLOW manual.


    Any further questions please email TUFLOW support: support@tuflow.com

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