Manhole Unit
    • 23 Oct 2022
    • 3 Minutes to read
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    Manhole Unit

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    Article Summary

    The manhole unit simulates discharge through a manhole from a surcharged culvert modelled as a conduit unit. The manhole outlet can be connected to a dummy boundary unit for linking to 2D, or to a reservoir to represent overland storage.


    Field in Data Entry Form


    Name in Datafile



    Upstream Label

    upstream conduit label




    Downstream Label

    downstream conduit label




    Manhole Label

    Manhole outlet label





    Manhole elevation (mAD)





    Manhole diameter (m)




    Calibration Coeff.

    Lumped calibration coefficient (includes discharge coefficient, constants [2/3,g],etc)




    Modular Limit

    Modular limit




    Loss Coefficient

    Loss coefficient


    > 0 or equal to 0


    Theory and Guidance


    With the increasing requirements for integrated modelling, particularly 1-d to 2-d linking for surface water flooding, a need has arisen to provide a unit to simulate discharge through a manhole from a surcharged culvert, modelled as a CONDUIT unit in Flood Modeller. The manhole outlet can be connected to a dummy boundary unit for linking to 2D, or to a reservoir to represent overland storage.

    The manhole unit can also incorporate an energy loss term, governed by Bernoulli's equation.

    Although it has previously been possible to model vertical discharge from a conduit within Flood Modeller by attaching a spill unit to a CONDUIT unit, the new manhole unit has a number of advantages, namely:

    • The discharge from a manhole unit, is modelled as a point source, whereas a spill unit removes discharge laterally along the length of the conduit unit.
    • The manhole unit enables the modeller to specify a loss coefficient to simulate energy losses.
    • The manhole unit is specifically designed for this purpose, whereas using a spill unit is somewhat unintuitive.


    The manhole unit may be considered as a hybrid junction unit, consisting of exactly three nodes, the first two of which must be the upstream and downstream conduit nodes. The third node represents the manhole outlet and is typically connected to a downstream boundary, or reservoir, although any other Flood Modeller unit may be connected (NB restrictions apply to the Direct Method, see below).

    The discharge through the manhole is determined by applying a standard weir equation to the pressurised head in a conduit over the circumference of the outlet. The weir flow may also be influenced by the water level of the outlet node, although the user may ensure that any discharge out of the outlet will be modular by applying an HT boundary unit with a level below that of the manhole invert.

    The outlet is assumed to be circular in shape, therefore the modeller needs to specify its diameter, as well as the invert level (typically ground level).


    Weir Equation

    Qm = cdbh13/2fd


    Loss Equation

    H1 = H2 + kv22/(2g)


    Continuity Equation

    Q1 = Q2 + Qm



    Qm = discharge through manhole

    Q1 = discharge through u/s conduit node

    Q2 = discharge through d/s conduit node

    cd = lumped discharge coefficient. For example, for a round-nosed weir equation, this value would be approximately 1.7.

    b= manhole circumference

    h1= u/s stage above manhole invert level

    hm= manhole label stage above manhole invert level

    fd= drowned flow reduction factor (=1 if modular; if drowned)

    H1 = total head at u/s conduit node

            H2 = total head at d/s conduit node

            k= loss coefficient

           v2= velocity in d/s conduit


    1. Although normal junctions are not permitted to be connected to reservoirs in Flood Modeller, this does not apply to the manhole unit since the natural flow direction of the manhole unit is well-defined (positive flow through the manhole node is defined as flow out of the conduit system).
    2. Reverse flow is permitted through the culvert - both the energy loss equation and the continuity equation will take account of this, with the nodes reversed as appropriate. However, as with all units, the Direct (steady) Method will fail if backflow is encountered.
    3. When using the Direct Method, the manhole outlet must be connected to an HTBDY-type unit.
    4. The manhole is assumed to be open - no account is taken of any lid or its weight; also, no account is taken of any water volume in the manhole shaft.

    Datafile Format

    Line 1 - keyword 'MANHOLE #REVISION#1'

    Line 2 - label1, label2, label3

    Line 3 - z, dia, cd, r, k

    Data file example


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