SCS TR-55 Urban Hydrology for Small Watersheds Time of - - PowerPoint PPT Presentation

▶
scs tr 55
SMART_READER_LITE
LIVE PREVIEW

SCS TR-55 Urban Hydrology for Small Watersheds Time of - - PowerPoint PPT Presentation

Oct 25, 2023 167 likes •326 views

SCS TR-55 Urban Hydrology for Small Watersheds Time of Concentration Presented by: G. Windsor Tracy, PLS March 2016 Presented by: G. Windsor Tracy, PLS 30 years experience Licensed in 1993 10 years self employed Performed

slide-1
SLIDE 1

SCS TR-55

Urban Hydrology for Small Watersheds

Presented by: G. Windsor Tracy, PLS March 2016

Time of Concentration

slide-2
SLIDE 2
  • G. Windsor Tracy, PLS
  • 30 years experience
  • Licensed in 1993
  • 10 years self employed
  • Performed Hydraulic & Hydrologic Studies for various

size projects

Presented by:

10/3/2017 2

slide-3
SLIDE 3

Introduction

  • Time of Concentration
  • Travel time ( Tt ) is the time it takes water to travel

(distance) from one location to a Point of Interest (POI) in a watershed.

  • Tt is a component of time of concentration ( Tc )
  • Time of concentration ( Tc ) is the sum of Tt values for

the various consecutive flow segments:

  • Tc = Tt1 + Tt2 + Tt3 + ….. [eq. 3-2]

10/3/2017 3

slide-4
SLIDE 4

10/3/2017 4

  • Time of Concentration
  • Tc influences the shape and peak of the runoff

hydrograph.

  • Urbanization usually decreases Tc, thereby

increasing the peak discharge.

  • Tc can be increased as a result of:
  • ponding though inadequate drainage

systems

  • reduction of land slope through grading
  • Minimum Tc is 0.1 Hours
slide-5
SLIDE 5

10/3/2017 5

  • Other Factors effecting Time of Concentration
  • Surface roughness
  • Smooth – Concrete
  • Moderate – Grass
  • Rough – Woods
  • Channel Shape
  • V shape vs. Trapezoid
  • Slope
  • Flat vs. Steep
slide-6
SLIDE 6

10/3/2017 6

  • Travel Time Computations
  • Water moves through a watershed as:
  • Sheet flow
  • Shallow concentrated flow,
  • Open channel flow
  • Or some combination of these.
slide-7
SLIDE 7

10/3/2017 7

  • Travel time ( Tt ) is the ratio of flow length to flow

velocity: Tt = L / 3600V [eq. 3-1]

  • Where:
  • Tt = travel time (hr)
  • L = flow length (ft)
  • V = average velocity (ft/s)
  • 3600 = conversion factor from seconds to hours.
  • Time of Concentration must be in HOURS
slide-8
SLIDE 8

10/3/2017 8

Demonstration of paved vs. unpaved in time

  • f concentration
slide-9
SLIDE 9

10/3/2017 9

  • Sheet Flow
  • Flows over plane surfaces
  • Usually occurs at or near the headwaters of the drainage

area

  • Sheet flow is limited to 300 feet in maximum length by

TR55

  • Important as sheet flow is the most sensitive component
  • f the TC.
  • Manning’s Roughness Coefficient
  • Ground surface slope
slide-10
SLIDE 10

10/3/2017 10

slide-11
SLIDE 11

10/3/2017 11

  • Shallow Concentrated Flow
  • More concentrated flow than sheet flow
  • Begins after a maximum of 300 feet of sheet flow, flow

becomes shallow concentrated flow

  • Travel time ( Tt ) is :

Tt = L / 3600V [eq. 3-1]

  • Where:
  • Tt = travel time (hr)
  • L = flow length (ft)
  • V = average velocity (ft/s)
  • 3600 = conversion factor from seconds to hours
slide-12
SLIDE 12

10/3/2017 12

  • Average velocity is determined in figure 3-1
slide-13
SLIDE 13

10/3/2017 13

  • Open Channels
  • Determined by Manning’s equation or

water surface profile information

  • Average flow velocity is usually determined

for bankfull elevation

slide-14
SLIDE 14

10/3/2017 14

  • Geometric Elements of Channel Sections
slide-15
SLIDE 15

10/3/2017 15

  • Trapezoidal Swale Example
  • Given:
  • Base 10 feet
  • Depth is 4 feet, flow is 3 feet
  • Side slopes are 4:1
  • Calculate

to determine velocity

  • Given:
  • Concrete Swale
  • Slope = 3%
  • Length = 1000’
  • Calculate Tc