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has the exfiltration units changed in 7.1?

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.

Thanks in advance,

Craig Bailey

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

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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