Understanding river models

When building rivers in InfoWorks ICM, it is useful to have reliable survey data available from an external source. This is typically river cross-section data, bridge cross-section data, and a center line for the length of river to be modelled. It is also advantageous to have good ground model data for the surrounding area, to capture items such as banks, ponds, and ditches. This is a necessity to model the flood plain in 2D.

A river reach in ICM is modelled as a link. The link is digitized between two nodes, typically break nodes, to define the center line of the reach. Once the river center line has been defined, reach geometry can be built from line object data. River reach cross section and bank parameters, including geometry, can be built from line data. Cross-section data is stored as cross section lines, containing X, Y, and Z data, as well as associated roughness and panel markers. The levels of the banks are stored as bank line data. It is uncommon to have this information available to import, so there are tools in ICM to automate their creation and populate the data as necessary.

Unlike sewer models, in the majority of cases, a break node is defined between river reaches and structures. These have no definable area. There are several structures associated with river modelling, namely, bridges and culverts. Finally, a river model will terminate with an outfall at the downstream extent.

It is possible to import network data from existing river models into ICM. Typically, ICM can import cross-section data, center line data, and bank information.

Note: Historically, river models are run as a steady state model, while ICM typically deals in dynamic state analysis. These performance requirements are very different, and a steady state model may need a lot of work for it to perform in a stable manner under dynamic analysis. One of the common pitfalls for river models is that the initialization process in ICM can take some time with long or large river models, but it is essential that this is completed successfully before undertaking a simulation.

Line data is imported in two parts through the Open Data Import Centre (ODIC).

First, each line object must be created with a unique ID. Secondly, import the X,Y, and Z data, which is stored as a subtable, usually in CSV format. These line objects are used to store the data, which are then used to build the information into the River Reach links. It is the River Reach object that is used during the simulation. Therefore, a disconnect can come between the line data and river reach data, but validation warnings can help with these cases.

Cross-section line data is stored in the Section Data grid, with an entry for each vertex in the cross section. This data can be edited on either the Cross Section Line Grid Window of the Lines Grid or the Cross Section Line Property Sheet.

The coefficient of roughness is defined in the cross sections as the Mannings n roughness. The table shows some typical values.

This river section has brush with trees on the left. It could be assigned a roughness between 0.05 and 0.15. The main channel looks relatively straight and clean, so it may get a value of between 0.03 and 0.035. The right-hand side is stoney with cobbles, lending itself towards a value of 0.035. The appropriate roughness always comes from the judgement of the user and any model calibration that takes place.

Bank lines allow the exchange of flow along the left and right boundaries of a river reach link, between either a 1D storage area, another river reach, or a 2D zone. Bank line data can be edited on either the Bank Line Grid Window of the Lines Grid or the Bank Line Property Sheet.

Bank line data may not be readily available for the entire model. The data can come from a range of sources, including from tools within ICM. There are three mechanisms to model offline storage for bank lines. The appropriate method, or combination of methods, depends on the scope of the project and the level of detail required.

• Create from section ends: Left and right bank lines can be automatically generated from the end vertices of river sections of selected river reaches. The bank lines can subsequently be used to build river reach bank profiles.
• Create from survey data: Import the banks directly from survey data. Use the previous tool, and then update or override the data, or simply import them as new objects from the ODIC.
• Create from ground model: Line section elevation data can be updated by sampling a ground model. Elevations at existing vertices can be updated and new vertices inserted between existing vertices.

There are several geometry tools provided in ICM to help speed up the model build process and to ensure that everything is aligned correctly. These tools can extend cross sections, rotate selected lines, and straighten selected lines. The straighten selected lines tool can be particularly useful to ensure that the river cross sections are the true width of the river and have not been artificially lengthened due to wavy or zigzag lines.