TROY

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GEM 4409HydrologyFall 2005

• TROY
• UNIVERSITY

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Technical Release (TR-55)
Urban Hydrology for Small Watersheds
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Technical Release (TR-55)
Time of Concentration Travel Time Chapter 3
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Technical Release (TR-55)

• Time of Concentration (Tc)
• Time it takes water to travel from the
  hydraulically most remote point to the point of
  interest
• Time of Travel (Tt)
• Time it takes water to travel from one point to
  another
• A component of Tc

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Technical Release (TR-55)

• Urbanization Tc
• Urbanization usually decreases Tc and increases
  peak flow
• However, Tc can be increased as a result of
• Ponding behind inadequate drainage systems
• Slope reduction through grading

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Technical Release (TR-55)

• Factors Affecting Tc
• Surface Roughness
• Development decreases surface roughness
• Channel Shape Flow Patterns
• Development reduces overland flow lengths
• Slope
• Development may increase or decrease slope

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Technical Release (TR-55)

• Types of Flow
• Sheet Flow
• Shallow Concentrated Flow
• Open Channel Flow

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Technical Release (TR-55)

• Computing Tt
• Travel time is the ratio of flow length to flow
  velocity

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Technical Release (TR-55)

• Computing Tt

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Technical Release (TR-55)

• Computing Tc

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Technical Release (TR-55)

• Sheet Flow lt 300 feet

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Technical Release (TR-55)
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Travel Time

• Shallow Concentrated Flow
• Average Velocity
• Figure 3-1
• Appendix F
• For slopes lt 0.005
• Calculate Tt with Eq. 3-1

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Travel Time

• Shallow Concentrated Flow
• Appendix F (slope lt 0.005)

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Technical Release (TR-55)

• Open Channel Flow

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Technical Release (TR-55)

• Example 3-1
• Given
• Dyer County, TN
• 2-Yr, 24-Hr rainfall 3.6
• A-B Sheet flow, dense grass, s0.01, L100
• B-C Shallow concentrated flow, unpaved,
• s0.01, L1400
• C-D Channel flow, n0.05, a27ft2,pw28.2,
  s0.005, L7300
• Find
• Tc at D

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Example 3-1

• Worksheet 3 Project Data

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Example 3-1

• Sheet flow

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Example 3-1

• Shallow concentrated flow

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Example 3-1

• Open Channel flow

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Example 3-1 WinTR-55 Solution
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Example 3-1 WinTR-55 Solution
Slight Change
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Technical Release (TR-55)
Graphical Peak Discharge Method Chapter 4
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Graphical Peak Discharge Method
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Graphical Peak Discharge Method

• Input Requirements
• Tc
• Drainage area (mi2)
• Rainfall distribution (I, IA, II, III)
• 24-hr rainfall
• CN

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Graphical Peak Discharge Method

• For a selected rainfall frequency, the 24-hr
  rainfall (P) is obtained from Appendix B
• CN Total Runoff (Q) are computed (Ch 2)
• Weighted CN must be gt 40 for this method
• Obtain Ia from Table 4-1
• Compute Ia/P

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Graphical Peak Discharge Method

• Given CN Find Ia
• Table 4-1

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Graphical Peak Discharge Method

• Find qu
• Exhibit 4 (4-I, 4-IA, 4-II, 4-III)

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Exhibit 4-I
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Exhibit 4-IA
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Exhibit 4-II
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Exhibit 4-III
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Graphical Peak Discharge Method

• Pond/Swamp Adjustment

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Graphical Peak Discharge Method

• Example 4-1
• Compute the 25-year peak discharge for watershed
  described in examples 2-2 and 3-1.

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Example 4-1

• qp 345 cfs