Under the General tab the following types of information may be specified.
Location
ID. String identification of the culvert. ID will be used in structure results and hence makes it easier to identify a specific structure if setup contains many structures, or if multiple structures are defined at the same location.
Branch name. The Branch name is automatically registered where the culvert is located.
Chainage. Chainage at which the culvert is located.
Type. The location type may be defined as Regular, Side Structure or Side Structure with Reservoir. See Structure types definition (p. 114) for details.
Graphics
Horizontal offset from marker 2. The horizontal position of the culvert within the cross section can be defined. By default the centre of the culvert is positioned at marker 2. Note that the position is used only to plot/display the culvert and has no impact on the simulation.
Attributes
Valve. Valves may be included to control the direction of the flow.
· None: No valve regulation applies.
· Only Positive Flow: Only positive flow is allowed, i.e. whenever the water level downstream is higher than upstream the flow through the structure will be zero.
· Only Negative Flow: Only negative flow is allowed, i.e. whenever the water level upstream is higher than downstream the flow through the structure will be zero.
· No Flow: The flow through structures will always be zero, regardless of upstream and downstream water levels.
Head loss
Head loss variables are included in the calculation of the flow direction dependent energy loss occurring for flow through hydraulic structures. They include:
Manning's n. Manning's bed resistance number along the culvert (for calculation of the friction loss present in a structure with a certain length).
Positive/Negative flow. Headloss factors are defined for Inflow, Outflow, Free overflow and Bends. Different loss factors can be applied depending on the flow direction across the structure.
Flow blockage
Apply flow factor
When this option is active, the discharge computed through the culvert is multiplied by a flow factor. This factor's value is specified in the Flow factor field. The factor is a dimensionless factor, and a value of 1 means that no change is applied to the computed discharge. A value lower than 1 can typically be used to describe the reduction of the flow through the structure due to obstacles, like debris, restricting the flow area in the structure.
Geometry
Here, the geometry of the weir is defined.
Type. The type of cross sectional geometry of a culvert can be specified as:
· Rectangular: The geometry is specified by width and height.
· Circular: The geometry is specified by the diameter.
· Irregular: The geometry is specified using a level/width table or a depth/width table. Select the relevant table and click on the Append button to include rows. Note that the values in the first column (level or depth) must be increasing.
· Cross Section DB: The culvert geometry is specified in the cross section editor. Select the branch name, TopoID and chainage of the cross section, which describes the structure’s geometry. The selected cross section must be a closed section.
Upstream invert. Invert level of the upstream end of the culvert.
Downstream invert. Invert level of the downstream end of the culvert.
Length. Length of the culvert.
No. of culverts. Number of parallel culverts.
Section type. For rectangular and irregular culverts the culvert type must be specified as either open or closed. For circular culverts it is closed and for culverts defined with a cross section the cross section definition determines if the culvert is open or closed.
Note: Since a culvert is defined as a structure causing a contraction loss, a friction loss (bend loss) and subsequently an expansion loss, the geometry of the culvert must be such that the cross sectional area at the inflow is less than the cross sectional area upstream of the culvert for all water levels. Similarly, the cross sectional area at the outflow end must be less than the cross sectional area immediately downstream of the culvert.
For culverts an additional tab is shown for flow conditions. Different hydraulic relations are calculated automatically by clicking ‘Calculate Q/h relations’ once the no. of Q/h relations have been specified.
Q/h relations
The Q/h relations are given as Q/y relations (where y is depth above the lowest of the Upstream and Downstream invert levels). The Q/y relations table also shows the type of flow occurring for the actual levels. Please note that the flow-type definitions in the tables are only flow indicators derived from the Q/h table calculation, and these indicators are not directly impacting the calculation results. Hence, there will be no effect of changing these flow-type definitions on the calculation results.
The possible types are:
· No Flow: No flow occurs at the first level (y = 0) and when the valve regulation flag prohibits flow in one direction.
· Inlet C: The flow at the inlet is critical.
· Outlet C: The flow at the outlet is critical. A backwater curve using a fine resolution is calculated to relate the discharge to the upstream water level in the river.
· Orifice: The flow at the culvert inlet has an orifice type formation.
· Full: The culvert is fully wet with a free discharge at the outlet.
Orifice flow coefficients
Orifice type discharge is calculated based on the orifice coefficient tables shown in the this menu. These coefficients can be edited, added or deleted, if required. After editing the Orifice flow coefficients it is required to recalculate the Q/h relations using the ‘Calculate Q/h relations’ button.
Hydraulic parameters
The hydraulic parameters is a table of level dependent parameters used in the calculation of Q/h relations. The Hydraulic parameters are also used internally in the hydrodynamic engine when calculating the structure flow for other flow conditions than the free outflow. Hydraulic parameters includes Flow Area, Hydraulic Radius and Conveyance calculated for the level ‘Y’ (depth above culvert invert).
To compute the Q/h relations, the nearest upstream and downstream cross sections are used. The cross sections must be located within a distance which is smaller than the user-defined maximum grid-spacing (‘Maximum dx’) for the branch in question. The Q/h relations cannot be calculated unless cross sections are defined.
Note that Q/h relations must be re-calculated if any changes are made to the culvert and/or up/down-stream cross sections. This is done by either pressing the button ‘Calculate Q/h relations’ for an individual structure or by selecting ‘Recalculate flow conditions...’ in the ‘Run’ drop- down menu in the MIKE Zero menu bar for recalculating all structures.
Allow for recalculation. Q/h relations can either be calculated automatically or entered/edited manually. If the Q/h relations have been modified manually it is possible to lock the relations by unchecking ‘Allow for recalculation’. This will ensure that Q/h relations for this structure are not recalculated through the menu ‘Recalculate flow conditions...’.
