This option allows the user to adjust roughness coefficients with changes in flow. This feature is very useful for calibrating an unsteady flow model for flows that range from low to high. Roughness generally decreases with increases flow and depth. This is especially true on larger river systems. This feature allows the user to adjust the roughness coefficients up or down in order to get a better match of observed data. To use this option, select Flow Roughness Factors from the Tools menu of the Geometric Data editor. When this option is selected, a window will appear as shown in Figure 5-92.
As shown in Figure 5-92, the user first selects a river, reach, and a range of cross sections to apply the factors to. Next a starting flow, flow increment, and a number of increments is entered. Finally, a roughness factor is entered into the table for each of the flows. The user can create several sets of these factors to cover a range of locations within the model. However, one set of factors cannot overlap with another set of factors. Hence, you can only apply one set of roughness change factors to any given cross section.
Note: Flow Roughness factors is available as an option in both the Geometry Editor (saved with the geometry data) and the Plan Editor (saved in the plan file). If you put these factors in the geometry editor, they will be applied to every event that uses that geometry file. Normally you would do this when flow roughness factors are needed for a wide range of events, and you can come up with a set of factors that works well from low to high events. If you want to use flow roughness factors for real time calibration, or you believe they are much more event specific, then you would place them in the Plan File. When placed in the Plan file, they only get applied to that event being run with that plan.
Additionally, if you place these factors in both the geometry and the plan file, then you will be factoring the roughness twice. Factors placed in the geometry file get used during the Geometry Pre-Processing, and they will affect the hydraulic tables that get computed. Factors placed in the Plan file are used in real time during the unsteady flow computations, and they get applied each time step at each cross section location.
Figure 5 92. Flow versus Roughness Change Factors Editor
Seasonal Roughness Change Factors. This option allows the user to change roughness with time of year. This feature is most commonly used on larger river systems, in which temperature changes can cause changes in bed forms, which in turn causes changes in roughness. This factor can be applied in conjunction with the flow roughness change factors. When applying both, the seasonal roughness factor gets applied last.
To use this option, select Seasonal Roughness Factors from the Tools menu of the Geometric Data editor. When this option is selected a window will appear as shown in Figure 5-93.
As shown in Figure 5-93, the user first selects a river, reach, and range of river station to apply the factors to. Next the user enters the day and month in the Day column, for each time that a new roughness factor will be entered. By default the program will automatically list the first of each month in this column. However, the user can change the day to whatever they would like. The final step is to then enter the roughness change factors.
Figure 5 93. Seasonal Roughness Factors Editor