Hydrology in Los Angeles?

[brandoncdg]

I have seen this done a few different ways and was curious for those working in the Los Angeles area. What software do you use to figure out the Stormwater Flowrate/Q on site?

I was always told to use the LA County Hydrology manual and input the numbers from the 50-year 24 hour Isohyet maps into the TC calculator to find the Peak Flow rate.

Now my partner does it a different way and uses an older 1990 LA County Hydrology manual that has K values on a map for different areas that you plug into an equation using a Capitol Flood Q's by Rainfall Zone table.

My partner who has 20 years more experience than I do, says that the way everyone taught me to use the TC Calculator is wrong and gives wrong numbers.

So what does everyone else do for finding a site's Peak Flow Rate to size the storm drain pipes? Especially for those that have only seen the current LA County Hydrology Manual.

Civil Development Group, LLC
Los Angeles Civil Engineering specializing in Hillside Grading

http://www.civildevelopmentgroup.com

http://www.civildevelopmentgroup.com/blog

 

[Ryb01]

Try contacting your local govering agency (i.e City or municipality) and ask for the most current development/engineering standards.

For small sites (only a few acres/hectares)you could probably get away with using a rational method to calculate flows and further be able to size pipes. There is a number of ways to calculate time of concentration, you would need to select a method that is most appropirate for your site. The HEC-22 manual provides a few different ways of establishing a TC.

If you need to design any kind of attenuation facility or the siet is over a few acres, the rational method should not be used but rather using software like HydroCad or SWMM. These two pieces of software (as an exmaple) allow you to produce hydrographs for your site which therefore allows you to produce accurate attenuation calculations for required storage.

For the LA region, I would assume (haven't done work there)that you would use a 24-hour SCS Type I storm distribution and local rainfall data as a starting point for developing your flows. Again, you local engineering standards should provide this information.

Hope this helps.

 

[brandoncdg]

Thanks for responding. Though, I am looking for people who have done work in the County/City.

http://ladpw.org/wrd/publication/

Also, I am looking for info for sites less than an acre. Like 20,000-40,000 sq. ft.

I am curious if there is an easier software to use? Something better? Just more information from others.

Civil Development Group, LLC
Los Angeles Civil Engineering specializing in Hillside Grading

http://www.civildevelopmentgroup.com

http://www.civildevelopmentgroup.com/blog

 

[cvg]

I have the same version as your partner, originally published in 1971 and then updated in 1989. I would not dream of using that version instead of the 2006 version which supports your method. However, if he can get permits with the county approved using the old method than more power to him. What could be easier than the county supplied software? Unless you want to pay money for some commercial stuff. Does Bondamin or AES still produce hydrology software for LA County? See link below

thread162-201630

 

[brandoncdg]

He told me my version gives a wrong Q though. I was taught the current way using the TC Excel calculator. I don't mind buying software that gives us better answers. So are you saying we are both using the right stuff then?

Yes my partner is super old school, and can convince a lot of city engineers he is right basically in any situation. I love him for that, as that to me is a true engineer!

Civil Development Group, LLC
Los Angeles Civil Engineering specializing in Hillside Grading

http://www.civildevelopmentgroup.com

http://www.civildevelopmentgroup.com/blog

 

[freddyfred12]

I am working in the L.A. area (this is my favourite L.A. site:

http://www-scf.usc.edu/~yukimura/living-california.html)

and if you need more information about the most current developement/ engineering standards you can simply contact the govering agency of L.A. and ask.