Determining Rational Runoff Coeficient

[jugglerbri]

Howdy all,

We are in the process of designing the storm system for a cemetary and we've run into a little unusual situation. What we have is a large area that will have pre-placed crypts. These crypts are basically a large concrete box that will be buried 18" deep. My question is, will the standard runoff coeficient for grassy areas still apply even if there is a concrete slab 18" down?

tia,

Brian

 

[n3rei]

Brian,
I would suggest you forego the traditional Rational Method and in this case use the TR55 method. In TR55 you can downgrade your soils for compaction. The results will be very similar to the rational method except you will have more control over the end result. By all means check against the Rational method. You can use standard time to inlet eg. 0.1 min or 0.15 min. and make changes easier than in the Rational. Use Grass range short for cover. But you will need a Soils Map book USGS type and Hydrologic soils type chart. If not available make your best judgement as to the type of soil a,b,c,or d you down grade in this order an existing a to b, an existing b to c, c=c d=d. Let me know how it turns out for you.
Charlie

 

[PECPESC]

You can also use a formula developed by Dr. Ron Rossmiller of Iowa university. It uses SCS CN values, Imperviousness, Recurrance Interval, Intensity & Antecedant Moisture condition to compute C.

 

[cvg]

to answer your question, no - standard grassy area coefficients should not be used!

shallow soils should be downgraded all the way to "D". 18" depth is very shallow soil and to be conservative you would want to assume antecedent moisture conditions are high.

Therefore there will be very little moisture loss into the soil. It will likely all run off.

Otherwise, just use the rational method and assume C = 0.90. Chuck
cgopperton@stantec.com

http://www.stantec.com/

 

[jugglerbri]

I appreciate all the replies.

cvg, a C of 0.90 seems rather high. Paved areas have a C of 0.89. In this part of the country, we have high intensity, short duration storms. Would the TR55 method work for these types of storms? I've heard of that method but never used it. The municipalities around here use the Rational Method, UDSWMM (Urban Drainage Storm Water Management Model) or CUHP (Colorado Urban Hydrograph Procedure). Would it be safe to assume the standard C value for that type of grass if the time of concentration for the basin is less than the time it would take for the grass to become saturated?

Again, thanks for all the help.

Brian

 

[CarlB]

You didn't mention the percent coverage by area of these vaults. If it was a continuous slab and saturated, then C could be as high as 0.9. But if the vaults are less than 50% of the area, and soils not likely to be saturated, then I think infiltration/runoff would be similar to normal grassed areas.

Your antecendent moisture condition may be determined more by irrigation practices than by climate!

Just my thoughts,
Carl

 

[cvg]

In Maricopa County (Phoenix) - we use the following range of C values:

pavement .75 - .95
gravel .66 - .88
commercial .55 - .95
industrial .60 - .95
LPC 0.10 - 0.50

all values are limited to .95 max
doesn't include streets and alleys

c values tend to increase with higher return periods and with higher land slopes. Therefore the higher numbers would be used for 100 year returns and/or over 5% slope

note that for lawns/parks/cemetaries, the assumption is that it is 80% grass

i would use a high c value for the crypt areas and then average it with the lower value for the rest of the cemetary to come up with a weighted value. Chuck
cgopperton@stantec.com

http://www.stantec.com/

 

[gibfrog]

The classical rational method does not have an antecenant moisture condition but it does assume that the soil has the capability to absorb all the rainfall that does not runoff. The rational method also increases the pervious area runoff coefficient for higher rainfall/recurrance events.

If you assume a sandy profile with a 20% available void area (mildly compacted and well irrigated) and a 2%-7% slope for a 0.2 rational runoff coefficient. A 24 hour/10" rainfall would nominally infiltrate 8" (not corrected for higher recurrance interval). 18" of sand with 20% available void area has a maximum infiltration capability of 3.6" inches. 1-(3.6"/10&quot = 0.64 rational coefficient for a 10" rainfall event.
The green areas not over crypts would be treated normally. as the drives and headstones/mounmuments would be.

For the SCS type methods, TR55, SBUH, etc.. I would first assume an antecendant moisture condition in terms of runoff infiltrated, like 1". Then, based upon the available soil void area, I would determine the total potential infiltration in 18" of soil less the 1" moisture antecedant condition. I would write a little table, with potential CN's versus rainfall's. I would compare these CN's with D values for good grass areas and use the maximum CN values. (I may use an average of C+D, depending on the native soil classification)

If you want more detail, then ask and I will get out a book. However, you must provide your soils data [name, type (sp,sc,etc..) scs class], slope data and irrigation policy. Clifford H Laubstein
FL Registered PE 58662