Time of Concentration - Lag/CN Method
Time of Concentration - Lag/CN Method
(OP)
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?
How about other good methods for determining time of concentration?





RE: Time of Concentration - Lag/CN Method
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
RE: Time of Concentration - Lag/CN Method
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.
RE: Time of Concentration - Lag/CN Method
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
RE: Time of Concentration - Lag/CN Method
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
www.hydrocad.net
RE: Time of Concentration - Lag/CN Method
RE: Time of Concentration - Lag/CN Method
RE: Time of Concentration - Lag/CN Method
Accuracy may be +-5% but is within the accuracy of the method, which is not particularly sensitive to this variation in slope anyhow.