 |
| Rational method dialog |
 |
| Dialog for Tc by SCS method |
 |
| Dialog for Tc by TR-55 method |
This command calculates the time of concentration (Tc) by either the TR-55 method, Rational method or the SCS method from A Method of Estimating Volume and Rate of Runoff in Small Watersheds. The Tc value is used in the Hydrograph and Peak Flow commands. Time of concentration is the time required for water to flow from the most distant point in the watershed to the measurement point.
The rational method calculates based on the curve factor, length of flow and average slope. These values are set in the dialog shown. The formula is:
Tc = (1.8 * (1.1 - cf) * sqrt(length)) / (slope ^ 0.33)
The SCS method calculates based on the curve number, length of flow, and average slope. The curve number defaults to the weighted curve number from the Curve Numbers & Runoff routine. When the three inputs are entered, click on Calculate to compute the Tc. Choose Select Flow Line from Screen to use a 3D polyline in the drawing. This sets the length of flow and average land slope. A 3D polyline that models the flow can be created with the Watershed Above Point or Run Off Tracking commands. While reading in the 3D polyline, the Tc is calculated by adding the Tc's for each segment of the polyline. This yields a different and more accurate Tc than using the average slope with the Calculate button.
The TR-55 method divides the type of flow into sheet, shallow concentrated and channel flow. The time of concentration is the sum of the times for the three types. This method also allows for selection of a 3D polyline with precise segment calculations of Tc, for the channel flow portion. The Manning's n for the sheet and channel flow can be chosen from a table by clicking the Select from Table button.
|
| Rational method dialog |
|
| Dialog for Tc by SCS method |
|
| Dialog for Tc by TR-55 method |
Tc by TR-55 method report:
Time of Concentration (Tc) or Travel Time (Tt)
Project: Parking By: TW Date:
Location: West Checked: Date:
Developed
Tc through subarea 1
Sheet flow (Applicable to Tc only) Segment ID: AB
1. Surface description .......................... : Dense Grass
2. Manning's roughness coeff. (n) ............... : 0.240
3. Flow length, L (total L < 300 ft) ............ : 100.0 ft
4. Two-yr 24-hr rainfall, P ..................... : 3.60 in
5. Land slope, s ................................ : 0.010ft/ft
6. Tt ........................................... : 0.296 hr
Shallow concentrated flow Segment ID: BC
7. Surface unpaved
8. Flow length, L ............................... : 1400.0 ft
9. Watercourse slope, s ......................... : 0.010ft/ft
10. Average velocity, V ......................... : 1.60 ft/s
11. Tt .......................................... : 0.243 hr
Channel flow Segment ID: CD
12. Cross sectional flow area, a ................ : 27.00 ft^2
13. Wetted perimeter, Pw ........................ : 28.20 ft
14. Hydraulic radius, r ......................... : 0.96 ft
15. Channel slope, s ............................ : 0.005ft/ft
16. Manning's roughness coeff. (n) .............. : 0.050
17. Velocity, V ................................. : 2.05 ft/s
18. Flow length, L .............................. : 7300.00 ft
19. Tt .......................................... : 0.991 hr
20. Watershed or subarea Tc or Tt ............... : 1.530 hr
Pulldown Menu Location: Watershed
Keyboard Command: flowtc
Prerequisite: None
File Names: \lsp\cntr_grd.arx, \lsp\hydro.dcl