flat area hgl

[johnhan76]

I am working in a flat topographical area. Our pipes are allowed to surcharge provided that the HGL doesnt reach the street. What type of factor of safety should I use if any. I am already designing for the 25 year storm and I am using the SCS method (conservative( to size the pipes. Thanks.

 

[gbam]

First, what are your freeboard requirements? Then make sure that your system HGL is acceptable per your design constraints. For instance we use a 1 foot freeboard requirement for manholes and cbs.

 

[johnhan76]

The requirement is no flooding on the street (ie 0' freeboard) That is the design constraint. I dont know if that is good enough though - just wanted to touch base with fellow flat landers.

 

[gbam]

I would probably design the system using some sort of freeboard, 6" maybe. Then you would have a feel for the size of the system and the cost. If using the freeboard gives a result you can live with then go with it. All I can say is that a conservative approach can't hurt!

 

[dicksewerrat]

That will be an expensive system. Are you designing all the way to the outfall in a river or pond? If not you will have a large restriction where it connects to the old system.

 

[johnhan76]

we are designing to a proposed detention pond - yes the system will be expensive because of the 25 year storm and because of the tailwater the pond creates that requires even larger pipes - typical of flat areas

 

[treg]

You didn't say what the required storm is. If the requirement is for the 10 yr storm, I sould guess using the 25 yr storm would give you more than enough safety factor.

 

[johnhan76]

the requirement is for the 25 year storm

i guess i could design for the 50 as a safety but that seems excessive for a residential development

 

[PECPESC]

In Dallas, Texas they require that the HGL be at least 0.5' below the throat of any inlet using the 100 yr event.

 

[gjeppesen]

You might want to do a little digging. Designing storm sewers to the 25 yr storm for a subdivision seems very excessive. The backbone system I can understand (pond/outfall). For example, where I am in Florida, the backbone system must be designed to the 25 yr storm, but the storm sewers can be designed to the 10 yr or even the 3 yr in some cases.

 

[johnhan76]

yes it is excessive - but how will the pond work if if the storm pipes cant get the flow to it? in flat areas most of the inlets are in a sag - therefore you dont have much overland flow to the pipe in a flood. It just builds up until it builds up enough head to push it out. Youve got to make sure that your finish floors are up high enough.

 

[gjeppesen]

Correct me if I'm wrong everybody. Storm sewers are sized for the peak rate of a given storm frequency. Like I said, the storm sewers in my area can be designed for the 10 yr storm (Rational), but the backbone system needs to be designed to the 25 yr storm (routing with SCS). I think the reasoning is that the 10 yr designed pipes can handle a 25 yr storm except for a few hours at the peak (and by this point the roadway is flooded or almost flooded anyhow, so what's the difference).

Also, I don't know the exact reason behind it, but I've been told that the Rational Formula produces higher runoff values say SCS, etc.

 

[BigKing]

In western Washington (not flat but plenty of rain) most conveyance systems are now sized for the 25 year storm and for the most part the transistion is being made from SCS to HSPF or other forms of continuous modeling. In terms of the original question we do not use a factor of safety except in the fact that we always look at what will happen if surcharging occurs beyond what we expected (ie what if a trash bag is stuck in the pipe and it restricts flow and we have a surcharge above the rim of the catch basins). So essentially our factor of safety is an assessment of where the water will go in the event of surcharging, and what is risk of significant proprty damage. If a surcharge occurs and it stays in the street and receeds after 30 minutes that is obviously much different than surcharging that will end up in the cellars of nearby homes.

 

[zarar]

If the criterion is that the water should not come on the streets, then I would choose the pipe sizes based on the 10 year storm and then find the Hydraulic grade line based on the 100 or 50 yr storm flow in the pipes and see at what point the hydraulic grade intersects the road profile.Your pipes which are desgned for 10 year storm should be able to contain higher storm events flows below the road surface for a quite distance from the pond, with some variation in the pipe sizing.
But watch out if you are connecting the basement drains to storm, here in Canada we have three pipe system(Storm, Sanitary &FDC) , we use third pipe(FDC) to basement drains and it has no connection with the storm and outlets in creeks or natural water courses or sometimes in the Bypass line of Storm system.
zarar

 

[gbam]

Like I said before Start with Something then work out the bugs. If you choose the road elevations as your constraint your system will be initially sized. Then change a few pipes to see what happens. You might find that a small increase in pipe size and therefor cost can give some decent results. Once you are happy then run a different storm thru the system, say a 50-yr, to see what happens. Just make sure you are satisfied with the result.

 

[PECPESC]

Again, in Dallas, Texas, all storm drains are designed for the 100 year event. They formerly used a lesser event but that changed several years ago. 36" & 48" pipes in 100 lot subdivisions are not uncommon.

It all depends on your local requirements.