This video will cover how to delineate and assign roughness values
to areas that represent land use types, for use in 2D models.
Like other model parameters, it is important to understand what the application of the model will be
so that appropriate land use detail and roughness value parameters can be specified.
2D models require roughness parameters assigned at each element or sell face, depending on the type of model.
To facilitate assigning the roughness values models use a dataset representing varying land use types with associated roughness values.
There are generally two methods used to develop a dataset that represents the land use of the project site.
The first, and most common, is to manually delineate areas of common roughness properties from aerial imagery.
The second, is to obtain an existing land use dataset online or from a local agency.
The USGS National Land Cover Database is one example that is available for the entire U.S.
This dataset is useful for some hydraulic analysis applications,
however, it is typically too coarse to sufficiently represent the land use for detailed hydraulic analysis of transportation infrastructure.
For either method it is important that the aerial image used to designate the land use areas be consistent with the terrain data.
For example, if a river channel has migrated, an older terrain data set may show the channel in a different location
than a recent aerial image. This could lead to erroneous land use areas for the terrain data used to develop the model.
It is also important to sufficiently detail land use designations for the purpose of the study,
but not more detailed than the element sizes in the mesh.
It is good practice to develop a simple land use designation to start with, similar to a coarse,
and add detail as necessary for the model application and mesh detail.
If the model domain includes buildings,
there are generally two methods for representing buildings within the mesh and land use designations.
The simplest method is to delineate locations with buildings as a single large area,
and assign a composite roughness value to account for the obstructions within that area.
The second method requires editing the mesh to represent building locations. This requires a more detailed mesh.
With the mesh elements aligning with the buildings, an unassigned land use type or a hole in the mesh
can be used so that hydraulic results are not calculated within building footprints.
For the last method, the roughness values for areas that are not buildings should be representative of the remaining area.
This value will be less than the value applied to the large generic building land use type.
Manning's n values are assigned to each land use area.
Manning's n values can be determined from several resources listed in the 2D reference document.
Existing n value resources were developed for application on 1D models.
Because 2D models account for several 1D model assumptions,
differences in n values of up to 10 to 15% have been observed based on calibration data using 2D models.
In some cases differences could be greater.
More research is needed, and is ongoing, to assess differences between the two types of models and develop improved guidance.
Using the existing Manning's n resources is best practice unless calibration data is available.
Some models allow the option to assign depth varied roughness values. There is little guidance on depth varied n values.
If a site condition warrants depth varied n values, it is up to the modeler to determine appropriate values.
Some examples of depth varied n values include; steep rocky channels,
orchards, and dense vegetation that lays down during a flood, such as willows and some crops.
For rocky channels and willows,
the n value decreases as flow depth increases.
For orchards, or vegetation with a dense upper canopy, the n value would increase as flow depth reaches the tree canopy.
Models should be calibrated to known water surface elevations if data is available.
Calibrating to multiple points at varying flows is ideal, however, any calibration data is helpful.
When calibrating to observed data,
remember that representing hydraulic controls in the terrain data has the largest impact on model results.
So, make sure the mesh adequately represents the terrain first.
With a sufficient mesh roughness values can be adjusted within reason so that model results better represent observed values.
If calibration data is not available, it is best practice to test land use types and roughness values
to determine confidence in the model results at the project area.
Initial adjustments to parameter should be made within the extreme range of resemble error.
This step helps the modeler better understand how the model reacts or is sensitive to specific changes.
Secondary fine tuning should include smaller adjustments until the modeler has confidence in the results.
Determining appropriate land use types and associated roughness values is an important step to develop 2D models.
Because this process requires user judgment, it is best practice to calibrate or assess model sensitivity to assigned parameters.
a one page summary sheet outlining key steps of delineating and assigning roughness values for 2D models,
and additional resources is available at this link.