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# has the exfiltration units changed in 7.1?

## has the exfiltration units changed in 7.1?

(OP)
Hi.

Call my crazy, but wasn't exfiltration always modeled in ft/min?

The current exfiltration page is asking for in/hr.

The current HELP function associated with Exfiltration has a few examples of exfiltration rates, but those appear to be the old units of ft/min  (could even be ft/hr - I'm real hazy on the old units.)

The current HELP page also has a "formula" to use in converting a Perc Rate (min/in) to V.  The explanation of that formula mentions a factor of 12, but the example formula has a factor of 60 in it.

Obviously I can make the conversions as needed, I am just needed clarification on the units the exfiltration calculation is using.

Craig Bailey

___
Craig T. Bailey, PE
http:\\www.bailey-associates.com

Replies continue below

### RE: has the exfiltration units changed in 7.1?

(OP)
I stand corrected - the HELP page example exfiltration velocities are listed "in/hr."  Which is the same unit as is in the Exfiltration pond-outlet page.

I still think the units have changed though.

Possibly what is throwing me off is: .83 in/hr is listed for sand.
.83in/hr(1hr/60min)=0.0138 in/min

(0.0138in/min)^-1 = 72 min/in.

72 min/in for a sand is very slow.

NHDES (New Hampshire Department of Environmental Services) lists Group A sandy soil (Gloucester, Hinkley for example) as 2 min/in.

In central NH, If I take a sandy-loam perc rate of 10 min/in I get an exfiltration velociy of 6 in/hr (sorry to possible confuse the example.)

I realize NH is just one governmental body, does anyone have any other typical exfiltration rates they have used in the past? I suppose we'd have to key them to the Hydrologic Soil types, as the SCS "Soil Names" are variable region to region, and they are only valid in the U.S.A.

___
Craig T. Bailey, PE
www.bailey-associates.com

### RE: has the exfiltration units changed in 7.1?

In HydroCAD 7.1, the infiltration rates are expressed as inches/hour (or mm/hour).  These are the same units commonly used to describe rainfall intensity, and are easier to comprehend than feet/minute values used in earlier versions.

Note: Previous project files will continue to produce the same exfiltration results.  But the exfiltration rates are now entered and reported in inches/hour, as labeled.

Design values will vary widely.  But remember that perc rate tests do not necessarily predict how a large pond will exfiltratate over many hours.  The sample values listed in HydroCAD help are examples from several sources including http://services.eng.uts.edu.au/~simonb/Switch%20site/Design/detailed%20design/Soil%20Infiltration%20Rates.htm

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