HGL FOR A FAILING STORM WATER SYSTEM

[2858]

A client wants us (consultant) to determine the HGL through a storm water system that is drastically undersized (10Yr Storm).

The traditional way of adding up head losses results in HGL shooting out of the catch basins (in excess of 100 ft) in the street because of the friction head loss calculation. We explained to the client that the system is drastically undersized and old (clay pipe)and is in need of a new system. The client wants to pursue the option of floodproofing of the buildings instead of system replacement.

I know this is not the traditional way of solving the storm water problem to allow the street to flood and protect the buildings; however, the client wants us to develop a HGL taking in account street flooding. Obviously, the storm water spreads out over the road and is not trully 100 plus feet. We thought of two options:

1. Orifice flow up to the grate of the drainage structure (and whatever other minor losses) and broad-crested weir flow over the grate onto the road.

2. Given the height of drainage structure (grate elev. minus invert elev.), solve for the flow. This would be the flow it would take to reach the top of the road. Subtract this flow from the actual flow getting to this point. The net flow would be the excess flow in the street.

Then use this resulting excess flow in HEC-RAS (We have a full survey). Of course, you would cut cross sections which would include the roads and buildings.

It seems option 1 would not be accurate because you would not be using the existing topography. Option 2 sounds pretty cool if we can get it to work. But I don't know if it will because of the sag points in the roads.

Has anybody had to peform a HGL through a system taking into account the spreading of the water out of the catch basins and determining the W.S.E. next to the buildings in the inner city? What do you think of the options above?

Any tips would be helpful. Thanks

 

[RWF7437]

You might try:

Hydrocad, advertised on this web page.
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Also, SMADA written by Eaglin, Eaglin and Wanielista. About $95 I believe. Do a quick Google search for the University of Central Florida website.

SWMM, EPA's stormwater modeling program is "free" but might take a long while to learn.

Finally, if money is no object, check Haestad Methods programs.

If it is still available, my Alma Mater, the University of Illinois developed the Urban Drainage Area Simulator ( ILLUDAS) many years ago which is designed to simulate the condition you have.

After all this, I remain as skeptical as you that your client can save money by floodproofing buildings even if the regulatory agency in your neighborhood will allow him to. Before you spend a lot of time and money it would be good to know how your calculations are likely to be received at City Hall, the Courthouse or whoever will review and approve your plans. Are you in a regualtory floodplain which in the US is overseen by FEMA ?

Good luck,

Russ

 

[KRSServices]

I may be wrong, but by chance your client wouldn't happen to be the municipality itself, would it? It sounds as though some planner has the capacity to make technical decisions or is way too far into the clouds, resulting in the consultant chasing turkeys!

The existing system is a 1 in 10 design. What happens currently with a 1 in 15 or 1 in 20 event? Does the street flood at lowpoints due to the surcharging? Do the buildings in question flood if below the HGL?

Floodproofing the buildings will not make the undersizing problem disappear, but it will create an unmistakable headache if the consultant "cooks" the calculations to imply that floodproofing the buildings has merit in resolving an undersized storm system.

I don't mean to sound harsh, but you should stick to your guns and provide recommendations as to the upgrading of the storm system, because it sounds to me as the responsible solution. Secondly, if your calculations indicate that water will be shooting out of the CB's a 100 feet into the air, and that has not been witnessed or simply does not occur, then you may want to re-calibrate your model or modify some of your assumptions because something is not right.

KRS Services

http://www.krs-services.com

 

[2858]

When you calculate the friction loss on a substantially undersized pipe - For example, a 18"CMP with 100cfs, with the slope (assumed friction slope) x length of pipe (120 ft)can be calculated in excess of 100ft. This is almost impossible because the flow spreads out after surcharging the drainage structure (which this method does not take into account flow spreading out of the drainage structure). However, this method of friction head loss calculation (for storm water pipe)is widely used and still proves to be flooding(it just does not give an accurate W.S.E. from a substantially undersized pipe).

However, the client wants to see a more detailed determination of the W.S.E. as explained in my first message to see the W.S.E. next to the buildings. Normally our other clients would see that the pipes are drastically undersized and proceed with a new system. This is not the goal of this client as explained above.

Because of major expensive utilities and obstructions in the inner city, the client wants to persue floodproofing if possible. We have pushed the client to pursue a new system even trenchless technology.

We were planning and are planning to give him a more detailed determination (was not part of our scope but the client is increasing our scope) of the W.S.E., but recommending that floodproofing is not the route to go and replacement of the system is mandatory of regulations and the storm water system will behave much more efficently besides dangerous conditions. Also we will add the fact that blowouts will probably appear in the old system in the near future and will have to repair or replace the system in the near future anyway. So they will end up with a new system and floodproofed buildings.

However, thanks RWF7437 for the tips. I plan to pursue some of these methods.

I hope this clears up some of the questions about the client. I probably should have explained more about the situation. I just wanted to get to the point. Thanks for the responses; we had the same responses to our client.

 

[PECPESC]

I think what everyone is overlooking is the fact that what the client wants to do may violate the Phase II requirements of the Clean Water Act. Check the NPDES permit requiremnts for the municipality.

 

[mikeb8e]

If you are looking at flood proofing the buildings, and the pipe is as undersized as it sounds, I would ignore the flow through the undersized pipe and just calculate surface flow between the buildings. Use HEC-RAS or some other open channel flow software. You can get HEC-RAS off the net for free or a nicer version with widows editing for a little cost from someone like Heasted.

If the existing pipe is a vitrified clay pipe, it can not be too large and its affect on larger storms is going to be inconsequential. Hydrology for storm water pipe design is all based on statistical data anyway. If you are within 30% of an actual peak flood you're a genius! Why worry about the amount carried through the smaller pipe when it is obviously ridiculously undersized. If you are looking at floodproofing the buildings I assume you are considering 100 yr or greater events. In that case, I would definately assume the smaller pipe is inconsequential because of its size. In a 100 yr event, what happens if the smaller pipe is blocked by debris?

If you absolutely must calculate the hieght of the spout from pressurized flow you would have to consider the effects of impact and momentum between the water and the pipes or junction box structures. Normally we ignore these conditions because they are minor compared to friction losses. However, the loss of momentum would describe the spreading effect at the surface. You would use the momentum equation and the Energy equation similar to how you would determine the depth of a hydraulic jump in an open channel.

The momentum equation form is M1=M2+F/(Unit Weight of water). Good Luck.