Preissmann Slot and Baseflow Correction
InfoWorks ICM allows you to compensate for the effects of the Preissmann Slot and Baseflow on storage in an InfoWorks network.
- the Preissmann Slot increases storage in pipes by a small amount
- baseflow reduces storage in pipes by a small amount
In the past, these slight variations in storage were assumed to cancel out. This is a good approximation, and is still perfectly valid in many cases. However, you may want to compensate more accurately for these storage variations.
Numerical Correction
InfoWorks ICM compensates for these slight numeric variations in the following way:
- Preissmann Slot - InfoWorks ICM reduces the cross section area of the manhole to provide a reduction in volume equivalent to the volume of the slot.
- Baseflow - InfoWorks ICM increases the cross section area of the manhole node to provide an increase in volume equivalent to the volume occupied by baseflow in the pipe.
Because the Preissmann Slot correction involves reducing the plan area of the manhole, it is possible for the sum of all storage compensation parameters to be negative. The Minimum Plan Area at Nodes values in the Simulation Parameters ensure that the effective plan area of a node can never fall to zero.
Instructions
The process has two parts. You must first set up the parameters used for numerical correction, and then carry out the correction itself on the network.
Setting Up
To set up numerical correction parameters:
- With a network open, select Storage operations | Storage options from the Model menu. This displays the Storage Options dialog.
- On the Numerical Correction Page, check the Active checkbox for Preissmann Slot and/or Baseflow depending on which options you want to activate
- For any option you have activated, click on the Define button to adjust the default distribution settings. This displays the Storage Compensation Distribution dialog. You can choose to split the storage between upstream and downstream manholes, and to further split the storage between shaft and chamber of the manhole. See Choosing Where to Apply Additional Storage for more details on these options.
- If required, choose a user defined flag to mark all values changed when the correction is applied. You can display the User Defined Flags dialog and create a new flag if you want.
- Click OK on the Storage Options dialog. The parameter setup is now complete.
Once you have set up the numerical correction parameters, you still have to apply numerical correction to the network.
Applying Correction
To apply numerical correction:
- Set up the numerical correction parameters as described above
- Select the network objects that numerical correction is to be applied to
- Choose Storage operations | Numerical correction from the Model menu
- InfoWorks ICM calculates the correction and enters the calculated adjustments in the Chamber Additional Storage: Correction and Shaft Additional Storage: Correction fields for the manhole
