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| Note: The stability criteria must also be satisfied for a model to run correctly. If this is not the case, the following error appears during runtime: Routing parameters have caused instability in storage routing. Refer to Stability criteria for more information. |
This is a simplification of the full momentum equation and assumes that diffusion and dynamic effects are negligible. The method uses index flow in flux, storage and mass balance equations. A weighting factor is used to adjust the bias between inflow and outflow rate, hence allowing for attenuation of flow. The storage routing equation is shown below and some of its terms are represented diagrammatically in Figure 1.
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Piecewise storage function
Link Flow travel time can also be set using a piecewise linear editorfunction. This describes a series of relationships between reach index flow rate q¯ versus travel time. The slope of the curve is the same as that for index flow rate versus storage so the rating curve can be derived using dead storage (zero flow) as the starting pointrate versus travel time. The data points can be entered manually or imported from a .CSV file, the format of which is shown in Table 2. Quadratic interpolation is used to find points in each defined segment on the curve (as in BigMod where x = 1).Piecewise routing allows you to specify how k varies with flow. If x = 1, then k must always be less than or equal to the time-step. In BigMod routing, the highest value of k is found in the travel time relationship, and the reach should be sub-divided into sufficient divisions such that the highest value of k for each division is less than the half the time-step.. See the SRG for details: Link storage routing - SRG.
To get hydraulically valid results, the maximum travel time can not be more than the timestep divided by the inflow bias ( x ) per division. For example in a daily model, if you have three divisions and x = 1, the maximum travel time can be no more than 3 days. If x = 0.5 the maximum travel time could not be any more than 6 days
Figure 3. Storage routing link, Piecewise
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Rating curves (Figure 4) are used to describe the physical characteristics of the reach and convert a flow into a level, ie. they produce an output of level. They can be specified in one of two ways:
- Using the Cross Section Editor to specify a physical cross section, and then generating the rating curve; or
- Directly entering the rating curve - the piecewise linear editor allows you to define relationships with respect to water level, discharge rate, reach width and dead storage. You can define multiple rating curves for a reach, each scheduled to commence on a particular date.
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- Right click Rating Curve and choose Add Rating Curve;
- Today’s date will automatically be entered for Start Date. To change this, click the calendar on the right side (see Working with date-pickers);
- Enter the water level, discharge rate, reach width and dead storage; and
- Enter an appropriate value for Overbank Flow Level.
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Figure 4. Storage routing link, Rating curve
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Ownership must be enabled at the scenario-level (using Edit » Ownership) prior to configuring ownership at storage routing links. Refer to Ownership for details.
References
Davis, C.V., and K.E. Sorensen (1969) Handbook of Applied Hydraulics, 3rd Edition, C.V. Davis and K.E. Sorensen (eds). McGraw-Hill.