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Isopluvials vs. m&n values

Isopluvials vs. m&n values

Isopluvials vs. m&n values

I am working on a small project using hydroCAD in the Maple Valley, WA area.  The city apprears to have adopted the KCC stormwater design manual.  

According to the KCC manual, the design method to be used for an area under 10 acres where the runoff is not detained is the rational method.

Isopluvials of the area indicate a 2 year 24 hour value of 2.2 inches and a 10 year 24 hour value of 3.0 inches.

The m+n values provided within the washington state hydraulics manual do not include data on this area.  I averaged a couple *somewhat* near by cities to get reasonable n values.

Looking at the isopluvials are currently a bit higher than the number produced by the m+n values.  Is there a way to get more accurate m+n values for the area, or get hydroCAD to develop a IDF curve around the isopluvial values?  Should I just monkey with the m+n values until I get something closer?


RE: Isopluvials vs. m&n values

I assume that m+n are IDF coefficients?

That data really should be provided by the applicable ordiance (Maple Valley), if only to assure consistency between different studies.

RE: Isopluvials vs. m&n values

Hi Peter.

Yes, the m+n are the IDF coefficients.

I just put a call into the city engineer there, so we'll see if he has a preference on what to do or better values that I can't seem to find.  Beyond that hydraulics manual and the isopluvials provided by the KCC, I'm not sure where else to find data.

Sooo, I guess we will go from there.  It seems like the values should be provided, if the code they reference wants us to use the rational method for the site, but the smaller cities...you know it really wouldn't suprise me if they don't have these numbers.

RE: Isopluvials vs. m&n values

If I read my Google Earth map correctly, I see that Maple Valley is East of Kent, north of Tacoma and south of Tiger Mountain. I'm also guessing that "KCC" has something to do with King County, WA. It is also apparent that there are many subdivisions in the area, probably of fairly recent vintage.

This suggests at least two approaches, in addition to asking the City what they want.

You could also ask the City what was used on all those recent subdivisions ?  Surely they know or can easily find out from their records. If they don't know, ask the Engineers who designed those subdivisions.

Alternatively, you can look at the isopluvial maps and see which ones also pass through Tacoma, Kent or other Cities in the nearby area.  Because you are a little closer to the mountains you may find those isopluvials to be more closely spaced than they are to the west of Maple Valley.  This should allow you to use one of the 37 IDF Charts published in the Washington DOT Hydraulics Manual, Chapter 2, Appendix 2.1. If you don't have this book....get it.

Sorry to hear you have to use the Rational Method.

good luck

RE: Isopluvials vs. m&n values



This should allow you to use one of the 37 IDF Charts published in the Washington DOT Hydraulics Manual, Chapter 2, Appendix 2.1. If you don't have this book....get it.

You can download the manual now as a .pdf, I looked at Appendix 2-1 – USGS Streamflow Gage Peak Flow Records, it has some Maple Valley data...it shows the station name, latitude, longitude, hydrologic unit and drainage area.  

How does one get an IDF chart or the co-efficients out of this?


Oh, the city engineer hasn't returned the call.

RE: Isopluvials vs. m&n values


It appears we are looking at different versions or editions of the WSDOT Hydraulics Manual. In the edition I have, dated May 1989, Chapter 2 contains 37 IDF Curves for various areas of the State. Look for the same curves in whatever Edition you are working with. The Chapter and Appendix numbers may be different in your newer edition.

The stream gage records will not help you derive IDF curves but they would allow you to develop Regression Equations. Such equations would, in my opinion, be better than the Rational Method anyway but may not be accepted by your City Engineer. In either case, you calculated flows are peobably no more "accurate" than plus or minus 50%. Such is the nature of hydrology.

good luck

RE: Isopluvials vs. m&n values

"The coefficients (m and n) have been determined for all major cities for the 2-, 5-,
10-, 25-, 50-, and 100 year mean recurrence intervals (MRI). The coefficients listed
are accurate from 5-minute durations to 1,440-minute durations (24 hours). These
equations were developed from the 1973 National Oceanic and Atmospheric
Administration Atlas 2, Precipitation-Frequency Atlas of the Western United States,
Volume IX-Washington.
With the Region Hydraulic Engineers assistance, the designer should interpolate
between the two or three nearest cities listed in the tables when working on a project
that is in a location not listed on the table. If the designer must do an analysis with a
Tc greater than 1,440 minutes, the rational method should not be used."

good luck

RE: Isopluvials vs. m&n values

Yeah, I originally averaged seattle and tacoma but it came up short when looking at the isopluvial.  I could try some other combinations I suppose.


With the Region Hydraulic Engineers assistance,

Yep.  Guess I'll call again, bet he/she will be thrilled about that.

RE: Isopluvials vs. m&n values


I must ask .

Why are you doing this ?
How "accurate" do you need to be ?
How "accurate" can you possibly be ?
You have said you have a small project, less than 10 acres. You have said detention is not required.
You are using HydroCad to do a Rational Method calculation.

Why do you need a computer to multiply three numbers ?
When you have calculated a design flow, which can only be an estimate of a probability based on a guess (C), a statistic (I), and an area measured from a map (A) you will use it to size a pipe or a few catch basins. If you get within a pipe size you will be doing a very good job for your client and will have done the best anyone can do with a method as unproven and crude as the Rational Method is. Spending hours trying to determine esoteric numbers like "m" and "n" won't make your design any better or save your client any money, or prevent flooding or other environmental disasters.

good luck


RE: Isopluvials vs. m&n values

My purpose is to not get kicked out by the engineer not returning my calls for using incorrect values.  

Somehow I doubt telling him the method he/she requires is not accurate and its all guesswork anyway is going to fly.

If something bad were to happen I would hate to use that logic to explain my failure.

RE: Isopluvials vs. m&n values

King County Storm Drainage.  This is a nightmare - especially the TIR - Days of paper work for a 1 acre site.  Been there too many times - you have my sympathies.

Mike McCann
McCann Engineering

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