| Product(s): | PondPack |
| Version(s): | 08.11.01.56 |
| Area: | Calculations |
When computing an outlet structure with the Tailwater type set to Downstream Channel, one of the following errors occurs:
"The <structure type> structure with the id __ is out of range. If using a Downstream Channel tailwater type, make sure the channel has enough capacity."
"The <weir name> is setup incorrectly. Check that the Top of Weir = Maximum Headwater and the Maximum Tailwater <= Maximum Headwater. If using a Downstream Channel tailwater type, make sure the channel has enough capacity."
Problem ID#: 43780
When using the "downstream channel" option for the tailwater type of a composite outlet structure, the tailwater is determined by solving the mannings equation with the given channel physical parameters at each flow in the rating table. The "channel invert elevation" parameter represents the bottom of the channel at the point of basin discharge (where the outlet structure empties into it.)
Here are some possible causes and solutions:
1) Make sure that for the range of flows through the outlet (you can check using free outfall for tailwater) the corresponding tailwater is less than the corresponding headwater. Basically make sure the capacity of the downstream channel is greater than the outlet structure. This can be checked by configuring a conduit with the same entry from the channel catalog, then right click on it and choose "Channel Rating table".
If this is a channel with existing upstream flow, or if it discharges into another pond that can influence the tailwater, or if the channel could potentially cause reverse flow back into the pond, you will need to use a different approach involving ICPM (interconnected pond modeling). Instead of using a channel, use a pond route connected to a "fake" no volume pond used to communicate the channel HGL to the upstream pond. See this article, and the "ICPM Channel.ppc" example model in the "samples" folder within the PondPack installation folder.
If you happen to have Bentley CivilStorm or SewerGEMS, you could model this situation a bit easier (the pond will automatically see the tailwater from the downstream channel or conduit link element).
2) If using an irregular channel cross section for the downstream channel, make sure the bottom elevation of the irregular cross section is higher than the lowest elevation in the outlet structure (for example culvert downstream invert)
3) If using an irregular weir as the outlet structure, ensure that the elevation entries in the weir cross section table align with top of the pond. Check the root composite outlet structure entry to make sure that the pond headwater range is correct for the the respective pond, then select the invert of the weir for the "elevation" field in the weir entry. Lastly, ensure that the elevation entries in the weir cross section table match the top of the pond - these values are added to the "elevation" field. For example if the pond bottom is at 100 ft and the top at 110 ft, with the weir "elevation" set to 108, ensure that the bottom of the weir cross section is at an elevation value of zero in the table, and the top at an elevation value of 2.0 (108+2=110).
4) If using a rectangular weir, a possible reason could be due to the weir length being too small when used in the equation to calculate the flow. In the help document titled "Rectangular Sharp Crested Weirs" you will see equation 10.17, which is used for calculating the flow for contracted weirs. From a hand calculation of the part of the equation in parentheses, if you get a negative number, this can cause the notification in question. In order to fix this issue you could either fix the weir length if incorrect, switch the weir to suppressed and adjust the C coefficient to account of the contracted weir headlosses, or try combining two weirs into one new weir and use an irregular weir type to enter station vs. elevation to define it.
5) If the channel has enough capacity and there is no problems with the elevations or lengths, try using the elevation-flow-area option for the Tailwater Type:
a. Temporarily lay out a conduit element somewhere in the model and configure it with the same channel entry and inverts/length
b. Right click on the conduit and choose "Channel rating Table", then click the "Data Table" tab. Note the units for elevation, flow and area and ensure they display at least three decimal places (right click on the column header, choose "units and formatting" and adjust the display precision)
c. click the column header for "elevation", then press CTRL+C to copy the elevations to memory
d. Open the composite outlet structure in question and select "elevation-flow-area" as the Tailwater Type.
e. Open the elevation-flow-area curve, confirm the units are correct based on step b
f. Click the elevation column header and press CTRL+V to paste in the elevations from step c.
g. Click OK, close the composite outlet screen, generate the Channel Rating table again, and this time copy the column header for Flow
h. Go back to the composite outlet structure, open the elevation-flow-area curve, click the column header for Flow, the press CTRL+V to paste in the flow values
i. Repeat this process for Area.
j. In the elevation-flow-area table, delete the first row, which usually contain an extra set of zeros. You may also need to delete the third row if it contains a duplicate elevation entry.
k. Compute the composite outlet structure.
6) Another, more simplified option would be to use a constant tailwater equal to the normal channel elevation. Solve the Mannings equation to estimate depth in your channel based on an estimated average flow, then use that as a constant tailwater as the outlet structure tailwater type.
7) Upgrade to our CivilStorm product, which has two different fully dynamic solvers available to handle more complex situations like this without any special setup. You would simply connect the pond outlet to a channel element and the roadway runoff could be directed to that channel as well, and the solver would properly handle the backwater effects.
How to model a channel between two ponds
User Notification: "Number out of range. Looking for value that is less than lowest table value."