How do YOU design storm sewer?
How do YOU design storm sewer?
(OP)
I am wondering how some of the users here design storm sewer. I have always used a spreadsheet I put together that uses the rational method along with Manning's formula to design storm sewer. I have heard of some folks using HydroCAD to design storm sewer, and I have used it on some smaller sites, but never on a site with more than a half dozen catch basins. If you do use HydroCAD, how do you compare it to using the rational method in regards to design time and pipe sizing? As always, I try to walk the line between undersized and oversized, trying to keep the costs to a minimum. I am just about to design a site with about 50 catch basins, and am thinking of trying HydroCAD, but am a bit worried about design time and pipe sizing. Thoughts?





RE: How do YOU design storm sewer?
RE: How do YOU design storm sewer?
Speak with your reviewing agency to see if HYDROCAD is an acceptable modelling software.
Generally speaking there's no perfect/ideal or preferred method to design storm sewer other than to design to your specicific site criteria. The rational method works well for storm sewer design as it prodces conservative results, which is never a bad thing when designing an underground piping system.
I typically use a spreadsheet for small projects and/or minor system design, major system conveyance I'd use something a little more poweful than a spreadsheet.
Hope this helps.
RE: How do YOU design storm sewer?
However, formatting the output from StormCAD can be cumbersome. I still tabulate a spreadsheet so that I can track my HGL elevations (my local municipality requires a spreadsheet in a particular format anyway).
Just my two cents.
Thanks.
RE: How do YOU design storm sewer?
RE: How do YOU design storm sewer?
The plan reviewer guys got me hooked on Hydrocadd. Whenever they had a plan come in with a complicated HEC-1 model, they would put together a simple Hydrocadd model to check - if the results were in the ball park they moved on.
Robert
www.newrivereng.com
RE: How do YOU design storm sewer?
The spreadsheets do come in handy for the reviewing party though, since most don't have access to some of the larger modeling software.
RE: How do YOU design storm sewer?
1. What hydrology method is used to compute design flows (Rational, SCS, SWMM Runoff, etc.)?
2. What hydraulic method is used to calculate depths/water elevations? (uniform flow computation for each pipe segment, steady-state backwater analysis using energy equation, dynamic routing using SWMM or similar)
3. What computer software or manual method is used to do the above?
In my opinion, the most uncertainty surrounds question 1, the design flows. I agree with cvg that the Rational Method is not necessarily conservative - most side-by-side comparisons of flows calculated using the Rational and SCS methods that I've seen have higher flows from the SCS method. Rational Method seems to more closely match gage flows / actual catchment data without much conservatism, which is a good or bad thing depending on your situation and how you account for variability/factors of safety. Granted it all depends on your procedures for calculating input parameters.
Item 2, hydraulic method, sometimes is quite important and sometimes makes little difference. When you're designing a new system without a significant downstream backwater and can design in adequate freeboard, probably all methods will work well if used properly. But when you've got backwater effects, hydraulic restrictions, intentional or unintentional storage in the system, you need a more detailed analysis.
With regards to item 3 Computer Software, something that has puzzled me is that in a field where so many of the industry-standard water resources software is government-developed freeware (TR-20, TR-55, HEC-HMS for hydrology; HEC-2 and HEC-RAS for river hyraulics, HY8 for culverts, SWMM for dynamic hydraulics), steady state sewer modeling is the one big application where no government agency has stepped in with a widely-used, Windows-based software, something along the lines of StormCAD (I consider SWMM to be in a separate category). I guess the government has never felt the need for a public-domain application of this nature (though let me know if you know about one). Good thing for Haestad and the other software vendors. Maybe it's because it's possible to do the core analysis in a spreadsheet?
RE: How do YOU design storm sewer?
http://www.ladpw.org/wrd/Publication/index.cfm
RE: How do YOU design storm sewer?
RE: How do YOU design storm sewer?
RE: How do YOU design storm sewer?
http://www.udfcd.org/downloads/down_software.htm
RE: How do YOU design storm sewer?
Spreadsheet is a problem for me because of composite slopes. In order to get the spread and efficiency of an inlet on a composite slope, a trial-and-error procedure is needed - or Excel's goal seek. This is a manual calculation that must be done one by one whenever anything changes upstream. This is a pain to keep track of. There is no simple equation on composite slopes. Without using goal seek or the nomograph, you would have to use a conservative revision to the cross-slope.
I do not like this because we water-people are already using conservative estimates for all the parameters that we are not sure about. For this reason, many of our designs are little more than "guesses" that any intelligent person could come up with and defend. We need to do better than we have been and "pin down" some more of our numbers and stop using "engineering judgement" when we don't know the answers.
Use the software so we can at least get the numbers right for the stuff we DO know.
RE: How do YOU design storm sewer?
CDG, Civil Engineering specializing in Hillside Grading in the Los Angeles area
http://www.CivilDevelopmentGroup.com
RE: How do YOU design storm sewer?
RE: How do YOU design storm sewer?
CDG, Los Angeles Civil Engineering specializing in Hillside Grading
http://www.CivilDevelopmentGroup.com
RE: How do YOU design storm sewer?