...
| Parameter | Type | Definition |
|---|---|---|
| Lag time | Time | This represents the time it takes for water to travel along the link and is a positive real number. If the specified lag time is not an integer multiple of the model time-step, it will be rounded to the nearest time-step. |
| Initial Storage | Volume | The amount of water deemed to be in the link on the first time-step. For example, if there is a lag of two days, and there is 10ML in the link at the start of the run, then 5ML is deemed to be flowing out each day (total initial storage divided by lag). |
...
| Tip |
|---|
...
| ||||||
A link configured for lagged flow routing is treated as a series of sub-reaches or 'divisions' of equal length, with the travel time in each division equal to one time-step. Water moves through the link progressively, without attenuation. You cannot configure fluxes, constituents or ownership on a lagged flow routing link. If lateral flows are significant and/or there is dead storage in the reach, you can model lagged flow routing using storage routing and the generalised non-linear storage option, as follows:
|
...
|
...
|
...
Lagged Flow when Fluxes are Significant or there is Dead Storage
If lateral fluxes (reach processes) are important, or there is dead storage in the reach, or both, and it is desired to lag flows without attenuation then it is necessary to use a Storage Routing link to do this. The approach that should be adopted uses generalised non-linear routing and is as follows:
- The lag is converted to the number of model time-steps, n, by dividing average travel time (ie. wave passage time) by the model time-step, and rounding off the result. However, the minimum value of n is one (i.e. n ≥ 1);
- Set up a routing reach where the number of divisions, ndiv = n, x = 1, m = 1 and K = dt (recalling dt is the model time-step). However, if ndiv = 1 and the average travel time is much less than the model time-step, and it is required to account for lateral fluxes but not otherwise change the hydrograph shape, then set K = travel time (recall that for stability K ≤ dt/x). For example, this would provide a means of adjusting for fluxes in a reach in a monthly model, where travel times are very small relative to the model time-step.
...
|