Strom Sewer Anlysis - HGL Issues 2

[jwooten]

I am analyzing an existing system and have found a few instances where the Hydraulic Grade Line exceeds the elevation of grate inlets in th parking lot. What is the best way to account for this problem? I realize that runoff cannot enter the grate at the point in time that the HGL raises above the grate. I am not sure how to model and present backwater conditions for the inundated parking lot. Any comments or recommendations appreciated.

 

[RWF7437]

"I realize that runoff cannot enter the grate at the point in time that the HGL raises above the grate."

This is probably not true. Unless flow in the system is zero. water is entering the grate through orifice flow.
If you know the elevation of the HGL then you know the level to which water will pond in the parking lot. Whether or not this ponding is a problem or may be permitted is a question you must decide. But you can represent it by plotting the area ponded on a contour map of the lot.

 

[psmart]

The ponding in the parking lot may also attenuate your peak flow, but you would have to do a hydrograph routing analysis in order to determine the exact effects.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[jgailla]

What is the purpose of the analysis of the existing system?
Is someone buying the property?
Will you be designing any renovations to the existing system?
It seems to me that it is a fairly complicated problem to find out exactly how deep the water is in the parking lot during a given storm event, and the purpose of the analysis could lead to some simplifying assumptions.

 

[gbam]

jwooten;
RWF hits a point. Flow will still enter through the grate; however, it will be at a greater head than the HGL. I have ran across guidance regarding trash racks. I think the additional grate head loss was 1.5 times the difference in exit and approach velocity head.

 

[jwooten]

The analysis is for peer review of another's design that was built. Legal issues. I will say no more.

I would like to represent any flooding issues accurately within reason. I can map or topo the HGL elevation at each "problem inlet." That is easy, and I do it all the time for a sanity check on inlet spread.

What is the best way to calculate the actual head over the grate considering the HGL and contributing drainage area inflow combined?

 

[cvg]

you would need to use hydrographs and routing along with head loss factors for the grates, all of which could be done in SWMMM, Stormnet, Hydrocad or other software

 

[RWF7437]

"The analysis is for peer review of another's design that was built. Legal issues. I will say no more."


If this is a case that will go to Court you had better be sure that:
a. You are qualified to do a "peer review"
b. That the method of analysis you use follows the best available practice.
c That the original designer followed similar practices AND met local requirements for the design.
d If the original designer did not do those things be prepared to prove it beyond merely a difference of opinion.

good luck. I'll say no more.

 

[beej67]

RWF has the right of it. (again)

I'm in a legal case right now that's very similar, as an expert witness defending another engineer's design.

The short answer is that you can go into your modeling program, and jack the inlets up really high, and then see where the HGL comes to in those artificially jacked up inlets, and that will tell you how much head it takes to drive the stormwater through the system as it was designed.
So then once you've established how much head it takes to push the design storm through your system, go back and look at the grading plan and see if the water can reasonably pond that high without spilling downstream. Then follow the water and see where the flooding problem starts. Etc.

Pay very close attention to manning's roughness assumptions, and check them vs manufacturer's recommendations, especially when it comes to HDPE or helical CMP pipes, which sometimes don't follow the "general guidelines" we all use. Also check those assumptions vs your local regulations.

That's the easy way. The hard way is to do dynamic routing of a design storm with a specified duration. (e.g. SWMM) The hard way often shows less actual flooding than the easy way though. Keep in mind that the Modified Rational Method, which is what most of these StormCAD style programs use to design pipes, isn't really a great way of determining what a true HGL in a pipe is, but rather it's a way to determine where the worst case HGL might land for a given 'year' storm. Those programs usually re-figure the intensity at each node in your network based on longer and longer Tcs, which means the I is dropping, which means the design storm it's looking at changes from node to node in the network. Any 'hydrograph' approach to the problem is going to choose one design storm of a specific duration and route that storm through your system.

If it's a legal issue, then you're likely best approach is to choose what passes for accepted practice in your region, which in my experience varies widely even just in the South East.