| Product(s): | StormCAD, SewerGEMS, CivilStorm |
| Version(s): | CONNECT Edition |
| Area: | Calculations |
Problem
Unexpected spread and depth results for a curb inlet in sag with local depression. Or, the results do not match GEOPAK.
Solution
In general, inlet capture, depth and spread calculations build upon the HEC-22 standards. (see HEC-22 manual) GEOPAK (or other applications) may be making more simplified (or different) assumptions or interpretations of HEC-22 for certain situations.
The first thing to note is that the gutter depth reported at the catchbasin may be different depending on the version you're using. Starting with the CONNECT Edition (10.00.00.40 and greater), the gutter depth is always measured from the bottom of the local depression. This can differ from other applications if they do not account for the local depression in the gutter depth figure. If you are using V8i or earlier, upgrade to the CONNECT Edition as this change should result in more logical results.
The second point to consider is that the datum point for weir flow calculations is based on the bottom of the gutter depression, as the bottom (datum point) of the weir is essentially the top of the local depression. Meaning, the beginning of the local depression slope as seen in the illustration below:
Gutter flow "spills" into this weir and the head in the weir equation is measured from that point.
As the local depression is increased, the bottom/low point is lowered, but the datum point above which the weir head is measured is still the top of the local depression (see red line above) So, the spread would stay the same (water surface elevation stays the same) but the depth would increase (bottom point is lower).
Here is the general flow of how the program (as of CONNECT Edition) is computing depth and spread for this type of situation, using a 4" depression as an example:
1) Two versions of the weir equation are compared and the one with the largest capacity is selected
- Checks HEC-22 equation 4-28
- Checks HEC-22 equation 4-30 (applicable to curb inlets without depression) and subtracts the local depression
2) Gutter depression height is added. The weir equations 4-28 and 4-30 solve for a HEC-22 "d", which does yet include gutter depression.
3) Orifice flow is considered, with local depression subtracted
4) The larger of the weir (step 2) and orifice (step 3) depths is selected
5) Local depression height is added
6) Compute spread based on cross slopes.
- See help topic "Composite Gutter Section"
- With a composite gutter section with multiple different slopes, the spread is computed in steps, and depending on whether it extends past the local and/or gutter depression, it determines which slope(s) need to be considered to compute the final spread.
Spread stays the same but depth increases, when using a depressed gutter
For a curb inlet in sag with a local depression, you may notice the spread does not change between using a continuously depressed gutter versus having no gutter depression ("Depressed Gutter?" = True/False) yet the depth increases.
As noted further above, as a local depression and/or gutter depression is added, the bottom (low point) is lowered, but the datum point above which the weir head is measured is still the top of the local depression (see red line above) This is why the spread stays the same (water surface elevation stays the same) but the depth increases (bottom point is lower) in this situation.
Consider FHWA HEC-22, pp. 4-59 HEC-22, 3rd edition Urban Drainage Design Manual (dot.gov):
"The weir location for a depressed curb-opening inlet is at the edge of the gutter, and the effective weir length is dependent on the width of the depressed gutter and the length of the curb opening..."
(The weir length referred to here is the red line in the illustration further above.)
"...The equation for the interception capacity of a depressed curb-opening inlet operating as a
weir is:
Qi = Cw (L + 1.8 W) d1.5 (4-28)
where:
Cw = 1.25 (2.3 in English Units)
L = Length of curb opening, m (ft)
W = Lateral width of depression, m (ft)
d = Depth at curb measured from the normal cross slope, m (ft), i.e., d = T Sx"
The HEC-22 manual goes on to say:
"Experiments have not been conducted for curb-opening inlets with a continuously depressed gutter, but it is reasonable to expect that the effective weir length would be as great as that for an inlet in a local depression. Use of Equation 4-28 will yield conservative estimates of the interception capacity."
So, the OpenFlows products use equation 4-28 for both the local depression case and the local depression plus depressed gutter case. And, since “d” in these equations is measured from the normal cross slope, “d”, and therefore the spread result, will be the same for both. However, the OpenFlows software will ultimately report the total depth of water at the curb with the depression included, rather than the projected “d” used by equation 4-28. (see also "Where is the Gutter Depth measured from?" below.)
See Also
Abnormal large spread on curb inlet in sag