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Table Of Contents
Chapter 5 Working with Hydrology Commands
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5 You can edit and view the data by using the following guidelines:
If you want to} Then}
output the composite CN to a text file click Output.
insert a new row of data for a subarea place your cursor in the row you want to
insert the row above, and click Insert.
delete a row of information place your cursor in the row you want to
delete and click Delete.
6 Click OK.
NOTE
When the Composite curve number displays in the runoff calculator it is
rounded off to the nearest whole number value. This as per instruction in
the TR-55 Manual.
Time of Concentration and Time of Travel
Autodesk Civil Design has calculators you can use to calculate both time of
concentration and time of travel. Autodesk Civil Design uses the methods
outlined in the TR-55 manual for calculating the time of concentration and
travel time and are discussed in detail in Chapter 3, Time of Concentration and
Travel Time, in the TR-55 manual.
These values are used in the TR-55 and TR-20 runoff calculation methods.
Time of concentration (Tc) is the time for runoff to travel from the hydraulically
most distant point of the watershed to a point of interest within the watershed.
Time of concentration influences the configuration of the runoff hydrograph.
Developing (urbanizing) a watershed usually decreases the Tc and increases the
peak discharge rate. Tc is made of up to 3 flow segments: sheet, shallow, and
channel flows.
Travel time (Tt) is the time it takes water to travel (already flowing in shallow or
channel flow) from one watershed subarea through another subarea that is
downstream. Tt can be made up of shallow flow and channel flow. Sheet flow is
not used when calculating Tt because Tt represents the time it takes for water
that is already flowing (from an upstream subarea) to arrive at the point of
outflow of the current subarea. Travel time for a subarea needs to be computed
if drainage from an upstream drainage area is passing through it.
Factors such as surface roughness, channel shape and flow patterns, and surface
slope affect the time of concentration and travel time.