Rivers

The river network is the basis of river model computations in MIKE+. The river network is defined as a combination of connected river segments and computational points. Module-specific features are added using the river network as a base.

 

Figure: Example of a river network in MIKE+

River networks may comprise of a single or several river segments or branches, and options for connecting river branches as well as defining specific chainage points are available.

River model calculation points comprise of h- and Q-points:

• h-points: Water level (h) calculation points. All cross sections as well as start- and end-points of river branches are h-points in the model. Moreover, h-points are automatically inserted to ensure that the maximum distance between h-points (i.e. grid spacing) is less than or equal to the defined ‘Max dx’ value.
• Q-points: Discharge (Q) calculation points. Q-points are automatically inserted midway between h-points in the entire model area. Additionally, hydraulic structures are included as Q-points in the numerical model.

Inflows to the River network are defined through Boundary Conditions River Network.

The Rivers editor contains data on defined river branches in MIKE+.

One may use the ‘Insert’ button to add river branch data in the editor.

It is however recommended to define river branches via the main Map or through import of geometric data (e.g. shapefiles) for quicker definition of river alignment coordinates. See Creation of a river Network.

Use the ‘Delete’ button to remove river branch data from the editor.

 

Figure: The Rivers editor

Identification

General properties for river branches are specified under the Identification group:

• Name: The descriptive name of the river. Multiple rivers can have the same name, if they are defined with a different range of chainages.
• Apply: This check box allows to toggle the Active status of the river on and off. The simulations will omit all rivers (and their structures) that are not active.
• Connections: Provides information on upstream and downstream linkages to other networks. Connections may be to other river networks or to Collection System Networks.
  • Define connections using the corresponding ellipsis button or arrow beside the Upstream or Downstream connection input box.

 

Figure: Connections groupbox

• The ellipsis button gives access to the ID Selector window from which one may select a CS node or river branch element to connect from the list of available nodes or river branches in the project.

 

Figure: Select an element to connect from the list of available Nodes or River branches in the project presented in the ID Selector window that is accessed via the ellipsis button

• Alternatively, define river branch connections via the main Map using the arrow button.
  • Define the location of the connection to another river branch via the ‘Chainage’ input box.

Geometry

Geometric information for a river branch are defined via the Geometry tab in the editor.

 

Figure: The Geometry tab page on the Rivers editor

Topo ID

Define a cross section data set for a river branch via the Topo ID input box. Use the ellipsis button to access the ID Selector window, which lists available topographical data sets for a river branch.

Topo ID is an abbreviation for `Topographical Identifier', which refers to the same variable defined as part of cross section definitions. Also see Cross Sections.

The cross section file accessed via the Cross Sections editor offers the possibility to store several measurement campaigns (e.g. from different years or before and after the construction of a structure), and these are identified through different Topo ID’s. This is to enable simulation of alternative scenarios once the river network has been configured. The topographical data used in a simulation is defined by the Topo ID.

Note

The River ID in the Rivers editor must correspond to the Branch ID in the cross section file for data presented in the Cross Sections editor to detect available cross sections data for a river branch in the Rivers editor.

 

Figure: Select a TopoID for a river branch from the list of available sets on the ID Selector window accessed via the ellipsis button

Max dx

The grid spacing or the maximum distance between two adjacent water level (h) calculation points in the numerical scheme used during river model simulations.

Calculation points in the River model are defined as follows:

• At locations where Cross sections are present in the cross section file, h-points are always created.
• At locations where rivers are connected, a ‘connection node’ is created internally in the simulation engine and h-points are automatically created at these locations as well (in case connections are defined where no cross sections are defined).
• If the defined value of Max dx is smaller than the distance between cross sections, a number of additional h-points are automatically inserted at engine-defined locations to ensure that the maximum distance between h-points is less than or equal to the defined Max dx. The additional h-points generated from the Max dx criteria in the model use linearly interpolated values for hydraulic parameters (i.e Area, Radius, Width, etc.) from the nearest physically defined up- and downstream cross sections.

Max dx may therefore be used to increase the spatial resolution of the river model by forcing an additional number of calculation points between physically-defined cross sections. (The lower the Max dx value, the higher the resolution).

Flow direction

The flow direction may be set as positive or negative, and indicates how the chainage definition is interpreted with respect to the flow direction.

If flow direction is defined as positive, the main flow direction will be assumed to go from low chainages to high chainages in the river branch and the corresponding discharge will get a positive sign in the results (whereas the reversed flow direction will get a negative sign). If flow direction is defined as negative, the main flow direction is defined in the opposite way: positive discharge results will describe water flowing from high chainages to low chainages, and negative results in the opposite direction. This main flow direction is per default reflected with an arrow along the river on the map.

The default assumption is to define flow direction positive with increasing chainages looking downstream of the river. Hence, if a river has been defined with chainage 0 (zero) at the river mouth and chainages increasing upstream, the flow direction must be defined as negative.

The 'Flow direction' also controls the definition of the left and right sides of the river, which are important for the correct definition of cross sections: the sides are observed looking in the direction of the main flow:

• For a positive flow direction, the left and right sides are observed looking in the direction of increasing chainages
• For a negative flow direction, the left and right sides are observed looking in the direction of decreasing chainages.

Start chainage

The chainage of the first point in the river branch. To change this chainage, it is required to insert a User defined chainage point in the first point of a branch.

End chainage

The chainage of the last point in the river branch. To modify this chainage, a User defined chainage point must be inserted in this last point of the branch.

River type

There are three different river types available:

• Regular: A regular river is a 'normal' river consisting of a number of computational points defined from cross sections. A minimum of one cross section is required. The hydrodynamic simulation in a regular river is based on a solution of the de Saint-Venant equations.
• Structure link. A structure link is typically a short river used to connect a flood plain to the main river. Structure links do not require cross sections to be specified but a river structure must be specified on the river and the hydrodynamic simulation is therefore based on an energy loss calculation.
• Simple routing. Routing is a simplified hydraulic calculation. The concept of routing is basically the transformation of a hydrograph entering at the upstream end of a branch into a downstream outflow, using a simple equation. Cross sections are not necessarily required on routing rivers. Routing options are defined in the 'Routing' tab of the Rivers editor.

A structure link generally represents the embankment geometry between parallel rivers (e.g. main river and flood plain river) or between a main river and a storage area. Structure links are modelled as a single structure river of only three computational points (h-Q-h). A structure link doesn't require cross sections to be defined: the h-points are automatically assumed to be made of very wide geometries. The Q-point in the middle of a structure link is defined through a structure, and any type of river structure is accepted. This structure must be specified in the corresponding structure editor (e.g. Culverts editor), but when changing the river type to a Structure link, MIKE+ offers the option to automatically insert an 'open culvert' type of structure, which can represent an overflow weir with a length to include the friction loss component and which is well suited for describing overbank spilling. The friction loss is computed using the length defined by the upstream and downstream chainages of the river.

Typical structure links describing overbank spilling is illustrated in the figure below.

 

Figure: Longitudinal representation of a structure link

Note

Following the internal formulation of the structure link where no true cross sections are defined in a structure link, the usage is restricted to Hydrodynamic simulations ONLY. Thus, for e.g. Water Quality calculations the setup should be void of structure links. They should in that case be replaced by 'regular' rivers with actual cross sections defined.

Routing can be used to model the hydraulics of upstream tributaries and secondary rivers, where the main concern is to route water to the main rivers.

Routing is a simplified hydraulic calculation. Normally, simulation of hydrograph mitigation along a river is based on the shallow water equations, which require cross section information. However, routing doesn't require such cross sections. The concept of routing is basically transformation of a hydrograph, i.e. calculating the modified hydrograph (different peak value and adjusted shape) at a given location based on the hydrograph at the upstream point and simple parameters. Typically, routing may be used to represent a reach of a river, a reservoir or a structure.

Note

The following applies to routing rivers:

• Routing rivers cannot be used to model a looped part of a river network.
• Use of routing rivers requires that all rivers located upstream of a routing river are also routing rivers. Routing can however be combined with regular rivers, if these are located downstream the routing rivers.
• Runoff from catchments can be included in a routing river.
• The routing method does not support the use of structures on routing rivers.
• The routing method does not account for backwater effects.
• If no routing point is inserted in a routing river (in the 'Routing' tab), the outflow will equal the inflow.

Edit coordinates

In some cases it might be necessary to update the coordinates of the vertices of the river branch on the Map.

When a specific river branch is selected in the overview table, click on the Edit coordinates button to access the Edit Coordinates of Vertices dialog (figure below). This table contains the X and Y coordinates of the vertices of the selected branch, which can be edited (copy and paste is also supported).

New rows may be added or existing rows deleted. ‘Insert’ adds a new row before (above) the active row and ‘Append’ adds a new row after (below) the active row.

Click on OK to update the river branch alignment on the Map after changes.

 

Figure: The Edit Coordinates of Vertices dialog

Chainage Points

A user defined chainage point is a specific location with a controlled chainage. Data on user defined change points are listed in the Chainage Points tab page of the Rivers editor.

 

Figure: The Chainage points tab page in the River editor

If a river branch does not contain any user defined chainage points the start chainage is per default set at 0 chainage and the end chainage equivalent to the length of the branch calculated based on the geographical length of the branch shape. Both the start and end chainages may be controlled by inserting user defined chainage points.

The chainage along a river branch between two user defined chainage points are linearly calculated between the two defined chainage values. The chainage along a branch between a user defined chainage point and a branch end is calculated s the geographical distance from the defined chainage value.

Add user defined chainage points to the River network via the main Map.

Use the Edit Features toolbox on the River Network menu ribbon (see Digitise a River Network on the Map). Set the target layer to ‘User defined change’ and use the ‘Create’ tool to define user defined chainage points on the Map.

The coordinates and chainages for user defined chainage points defined on the Map are shown on the table in the Chainage Points tab page on the Rivers editor.

Use the Delete button on the Chainage Points tab page to remove user defined chainage points. The Delete tool from he Edit Features toolbox may also be used to remove user defined chainage points via the Map.

Note

If a user defined chainage point is moved, the defined chainage is fixed and hence moving the point will update all chainages along the river branch.

Example

A river branch without user defined chainage values is created with a geographical length of 1000 m. The start chainage will be 0 and the end chainage will be 1000.

A user defined chainage point is added at 500 m (geographically) along the branch and given the chainage 750. The start chainage is now automatically updated to 250 and the end chainage to 1250.

Another user defined chainage point is added at 250 m (geographically) along the branch and given the chainage 100. The start chainage is now automatically updated to -150 and the end chainage is unchanged at 1250.

Wave Approximation

Wave approximation refers to the numerical solution and number of physical terms included in the Momentum equation applied in the Hydrodynamic simulation. For further details, please see the MIKE 1D reference manual.

By default, the wave approximation is selected for the entire river network in the MIKE 1D engine configuration dialog in Model Manager (see Configuration (‘Rivers, collection system and overland flows’ models)). But it is possible to apply a different wave approximation for some rivers. When the option 'Use specified local wave approximation' is selected, then it is possible to select the wave approximation for the current river, which will take priority on the global wave approximation.

Routing

The 'Routing' tab contains definitions of locations and properties for routing points along the river. A routing point is a location where a routing procedure is active, that is where change in hydrograph appearance occurs.

If no routing point is inserted in a routing river, the outflow will equal the inflow.

To allow for the insertion of routing point into a river, the river type must be defined as 'Simple routing', in the 'Geometry' tab. Routing points can be added or deleted using the 'Insert' or 'Delete' buttons above the table in the 'Routing' tab.

Multiple routing points can be defined on the same river if required.

Simple routing location

Specify the name and location of a routing point:

ID. String identification of the routing point.

Chainage. Chainage at which the routing method is applied. The chainage of a routing point will have effect on the downstream node in the river where the routing point is defined.

Flow routing method

Three routing methods are available. These methods are:

• Muskingum: a routing method that requires two parameters to be specified to describe the delay and the shape of the modified hydrograph. These parameters are fixed in time and space.
• Muskingum-Cunge: a modified version of the Muskingum routing method, for which the two parameters are automatically computed based on the diffusive wave model. The parameters are recomputed at each time step of the simulation. Hence, this method doesn't require any additional parameter to be specified by the user.
• No flow routing: the hydrograph is not transformed while passing the current routing point.

When the Muskingum method is selected, the two additional parameters below must also be specified.

Delay parameter. Time for the hydrograph to travel between the selected river node and the next downstream node. Its value may also be estimated as the observed travel time of the flood peak between the nodes.

Shape parameter. A dimensionless factor controlling the shape of the modified hydrograph. This factor depends on the shape of the modelled wedge storage and must be between 0 and 0.5. In natural rivers, it usually has a value between 0 (reservoir type storage) and 0.3, with a mean value of 0.2.

Note

the Muskingum-Cunge method computes the delay parameter and the shape parameter using the flow width and the flow area, and it therefore requires that a cross section exists at the routing location.

Water level method

Routing rivers do not per default consider any water level, as the primary purpose is to route a hydrograph through the river system and hence, only the discharge is a key state variable. It is, however, possible to determine water levels using one of the methods below:

• Rating curve
• Manning formula.

Both methods apply for steady-state flows and are thus approximations, but they are usually reasonable estimates as long as the water level does not change rapidly (e.g. during a flood).

When the 'Rating curve' method is used, it looks up the water level in a user specified Q-h relation table, selected from the list of tables defined in the 'Curves and relations' editor. The table must cover the range of discharges encountered during the simulation, and must be defined with the type 'QH relation'.

When the 'Manning' method is used, the Manning formula is applied during the simulation:

(3.1)   

Where n is the Manning (n) number, A is the cross-sectional area, S is the water surface slope, R is the hydraulic radius or resistance radius. For these parameters to be computed, it is required to specify a cross section at the same chainage along the river in order to get hydraulic parameters from its processed data. If no cross section exists at this location, it is not possible to use the Manning formula option.

Description

The Description tab is where free text descriptions for river branch data may be added. It offers options for providing asset and model management information for river model data, as well as attributes for quick model data query.

• Description: Free text description for the river branch data.
• Data source: Free text describing data source for the river data.
• Asset ID: Associated asset management system ID for the river data, if any.
• Status: Project-defined status information that may be used for model build management or e.g. model data query. Pre-defined codes are contained in the Status Code editor (second figure below) which may be accessed via the ellipsis button from the Status dropdown menu.
• Network type: Project-defined model network type that may be used for e.g. model data query.
• Add picture: Option for adding images associated with the river data.

 

Figure: The Description tab on the Rivers editor

 

Figure: The Status Code editor