Prelim/final check of existing V-ditch, new pipes

[kkoloj]

In doing some preliminary/final calculations to check the capacity of an existing concrete V-ditch I wanted to summarize, and ask about, the design steps I use. There are two residential sites, one 24000 sf and another 16000 sf. The larger site will be connected via a 6" or 8" pipe (85' long) to the smaller site and then emptied into an existing concrete V-ditch, which is about 40' away from the smaller site.

(1) Estimate Tc for each site (cons. using 5 min.)
(2) From IDF curves, select 100-yr storm, city's rec's. (I=6.2"/hr)
(3) Calculate demand from each site (Rational Method, C=1)
(4) Calculate capacity of new onsite and offsite 6" and 8" ABS/PVC lines (using Manning's, n=0.017)
(5) Calculate capacity of existing offsite concrete V-ditch (using Manning's, n=0.02)
(6) Compare demands and capacitys.

Finally, what qualities of the existing V-ditch could create problems (other than size), such as slope changes, roughness changes, etc. It is a relatively straight shot the a street. Any comments would be greatly appreciated. Thank you.

 

[brandonbw]

Have you seen the v-ditch physically? Is it clean? Are there cracks showing that somehow the swale has been destroyed over the years?

Another example I know is from a project I worked on that was recently built where the daylight line of the swale I designed did not match up to the survey and the contractor had to adjust in the field. This could lead to a swale not having enough slope to clean the drain enough, as the swale I designed was just barely enough to be 5% for self cleaning.

B+W Engineering and Design
Los Angeles Civil and Structural Engineering

http://bwengr.com

 

[gbam]

Wow Mannings of 0.02 for concrete? The ditch must be in poor shape or it was hydraulically placed (Gunite). If the ditch is had troweled or smooth Mannings would be about 30% lower. If by demand you mean discharge then the process seems ok. The only comment I have is the C=1. For a conservative approach you should be OK but mention that in your report. Will there be no landscaping or depressions within your site?

 

[francesca]

A half acre site, with Tc = 5 min and C = 1 is going to need a much bigger pipe than 8"!

With the pipes, you also have to consider entry/exit of the pipe. I used to use a nomograph that considered pipe diameter, headwall (or lack thereof) and flow and it produced the headwater depth. More robust drainage criteria require you to determine if you have entry or exit control and determine headwater depth with an equation/software.

 

[kkoloj]

(A) brandonbw: I have seen the V-ditch and there is a lot of dirt in it. There are very little cracks (that I can see), but the finish is pretty rough. My final plans will require soil erosion protection of the adjacent slope (the V-ditch is below our two lots by about 13 ft). The V-ditch is at about 20% slope.

(B) gbam: I just selected 0.02 for my preliminary calc's., just to get an idea. But, the finish is pretty rough (rougher than broom). Yes, for a first pass, I used C=1. I was just trying to see how close we were to the V-ditch's capacity. I can refine the analyses.

(C) francesca: Can we discuss more? For example, for a 24000-sf lot with I=6.2"/hr, I get Q=3.5 cfs (C=1). Then, using Manning's with Q=(.463/n)*D^8/3*S^1/2 with S=.12, D=8", n=0.017 I get Q=3.2 cfs, which means about an 8" pipe (3.2~3.5)?

(D) For the 13' drop in elevation from the lots to the V-ditch (which occurs in a horiz. distance of 13'), would an energy-dissipating device be advised, or simply an underground, oversized pipe?

Thanks again very much. I truly appreciate the input.

 

[gbam]

For (D) one can use a downdrain that outlets into a below grade grated concrete box to bubble the discharge out. Your flow rate is more of a nuisance flow (Minor). Our State DOT has a standard detail that we use for freeway downdrains, maybe check with your DOT for a similar detail. If not check this website for our details:

http://www.azdot.gov/highways/Roadw...rent/PDF/2007ConstructionStandardDrawings.pdf

 

[francesca]

kkoloj, I'm used to seeing a higher I for a 5-minute Tc. Your Rational method calc checks out. However, you can't size the pipe only on the flow in the pipe, you need to check inlet control. TxDOT has an equation for calculating your headwater depth under inlet control.

http://onlinemanuals.txdot.gov/txdotmanuals/hyd/hydraulic_operation_of_culverts.htm

I'm curious about the slope you've used (0.12 ft/ft) when you're saying a 13' drop over 13 feet = 1 ft/ft. If it truly is that steep, I'd look at alternatives, like a deep area drain-type inlet structure with a flatter pipe outletting in the v-ditch. You definitely don't want to be jetting water out of a small pipe with such high velocity and potentially perpendicular to the ditch. If you went for a deep area drain, that would nullify your inlet control and you'd be able to use your small pipe because the headwater would be contained within the inlet structure.

 

[kkoloj]

gbam: Thank you for all your input, again, I truly appreciate your time. I will surely check out the site.

francesca: Yeah, I didn't explain things very well, and it's hard to grasp the site without pictures/plans. Essentially, I have two rectangular lots side-by-side. Each lot has a single pipe that has collected all their respective discharge and exits the site (crosses prop. line). I need to connect one with the other, the pipe design with about 12% slope, then perp. to take the combined discharge down a 1:1 slope to the V-ditch, the V-ditch has about 20% slope. I would like to oversize the pipe down to the V-ditch, put in an energy dissipating device just before the V-ditch, then let the V-ditch do its thing? Does this make more sense? See attachment.

 

[beej67]

Steep pipes and channels can convey a lot of flow, but drainage engineers avoid them because flows of that high velocity tend to erode the pipe out over time, and also tend to cause erosion problems at their discharge points. Most design codes in my neck of the woods specify a maximum velocity of 15 feet per second to design for. Your 1:1 is going to be considerably higher than that, and your 12% slope is probably pushing it.

What drainage engineers typically do to avoid this, is put drop structures in their system. Let the water fall down a manhole, splash in the bottom of the manhole, and then exit down a flatter pipe.

Where does this existing 20% slope concrete V channel discharge? Is there an erosion problem there? Are you going to make that erosion problem worse?

Some other comments ..

Is your residential lot going to be paved from one property line to the other? Typically when designing storm drainage for residential lots, you don't see a C factor over about 0.5.

Do you really need to convey the 100 year storm in your pipes? Typically code requires you to design your pipes for a 25 year storm (sometimes less) and then check the 100 year conditions to make sure nothing crazy and litigious happens.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[dicksewerrat]

Look in your old textbook for some up to date 'n' values. PVC is about .010 as is HDPE.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[brandonbw]

I would put a splash wall at the end of the V-Ditch so where the pipe outlets, the water hits that wall and travels down the ditch.

I would always take into account is this pipe ever going to be cleaned? Doubtful, and that's why I wouldn't use something to bubble the water out of. That manhole or device will just be clogged with junk and basically fill up backing that pipe up over time. That's why I also asked what the slope of the channel is to determine if it is self cleaning.

For PVC I use capacity Q=K*SQRT(Slope)
for n=0.09
4" K=2.68
6" K=8.08
8" K=17.44
10" K=31.61

Maybe you want more than 1 pipe as an outlet?

B+W Engineering and Design
Los Angeles Civil and Structural Engineering

http://bwengr.com