Time of Concentration - Lag/CN Method 1

[hydroponder]

Anybody have any good ways of determining the average slope of a catchment area for determining the time of concentration?

How about other good methods for determining time of concentration?

 

[ptmoss]

As none of my CAD programs perform that function for me I just pick off similar looking areas from the plan, get their area and slope, and do a composite slope (i.e. pro-rate the areas). I don't get too wound up in perfection but I try to be as diligent as possible with it.

There's a whole host of methods for figuring Tc. I see a lot of people using the SCS method, and I used to use that one quite a bit, but now I generally use the lag/CN method.

I think as long as you're consistent and use sound judgement then it's use whatever you're most comfortable with. Though I suppose you don't want to use the FAA method on a hilly, woodsy site ;-)

 

[SMIAH]

I do that with ArcGis.

Average-slope used to be calculated by hand...
Making a grid (example 100 m x 100 m) and :

Sb = (Nh+Nv) x Int. / (Lh+Lv)
Where
Sb = Average Slope
Nh = Nb of times that an horizontal line cut a contour line
Nv = Nb of times that a vertical line cut a contour line
Int. = Interval of the contour lines
Lh = Length of the vertical and horizontal lines

Underestimating the slope for flat watershed.

 

[RWF7437]

Or, you can measure it directly using flourescine (sp?) dye and flows provided by an open hydrant or a garden hose, and a stopwatch.

Or, you can use the 10% and 85% points along the stream thalweg and a good topo map..

If your hydtology model is NOT calibrated, you may want to do a sensitivity analysis to see how much difference a change in Tc makes in your results.

good luck

 

[psmart]

You can calculate the average land slope from a contour map using the following equation:

C I
Y = -----
A

where
Y=Average land slope (rise/run)
C=Total length of all contour lines [feet]
I=Contour line interval [feet]
A=Total land area [sq-ft]

To determine the total contour length, measure the length of each contour line (with a measuring wheel or equivalent electronic version) and add them up. Include all contour lines within the subcatchment, including any closed contours.

If you're using HydroCAD, this procedure is provided on the Lag/CN screen. For more information see "Ponds - Planning, Design, and Construction" by NRCS.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Ryb01]

Came across this spreadsheet some time ago. Caculates multiple times of concentration based on various methods. Hope this helps

 

[hydroponder]

Thanks to everyone. I ended up creating separate surfaces for each watershed in AutoCAD and viewing the extended statistics, which calculates average slope. I was trying to stay away from measure contour lengths...too time consuming.

 

[Drew08]

The NRCS Soil Surveys list typical slopes for each soil type in the muaggatt table downloaded from the NRCS Soil Data Mart. Also, the second capitol letter in each soil type abbreviation refers to the slope where generally speaking: A=0-2% B=2-5%, C=5-8%, D=8-12%, E=12-17% and F=12-30%. You can find the average slope using an area weighted average the same way CN is estimated using the HSG.

Accuracy may be +-5% but is within the accuracy of the method, which is not particularly sensitive to this variation in slope anyhow.