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)
Estimating Runoff Chapter 2 Curve Number Method
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TR-55 Estimating Runoff

• Urban Impervious Area Modifications
• Connected Areas
• Runoff flows directly into drainage system
• Runoff flows to drainage system over pervious
  area as concentrated shallow flow
• Unconnected Areas
• Runoff flows over pervious area in sheet flow

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TR-55
Figure 2-2 Selecting CN
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TR-55 Estimating Runoff

• Curve Number Method
• Table 2-2a ? Urban CNs
• Assumes connected areas
• Assumes average impervious area

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TR-55 Estimating Runoff

• Curve Number Method
• Table 2-2a ? Urban CNs
• Figure 2-3 ? If assumed impervious area is not
  applicable

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Example 2-1

• Given
• Dyer County, TN
• 250 Acres
• 175 Ac Type C
• 75 Ac Type B
• 25 Yr / 24 Hr Event
• Rainfall 6
• Pasture Good
• Find CN Q

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TR-55 Estimating Runoff

• Figure 2-3 Example
• ½ acre lot, HSG B, 25 impervious
• Table 2-2a ? CN 70
• If 20 impervious Pervious CN 61
• Figure 2-3 ? Composite CN 68

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Example 2-1

• Given
• Dyer County, TN
• 250 Acres
• 175 Ac Type C
• 75 Ac Type B
• 25 Yr / 24 Hr Event
• Rainfall 6
• Pasture Good
• Find CN Q

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TR-55 Estimating Runoff

• Curve Number Method
• Table 2-2a ? Urban CNs
• To determine CN when all or part of the
  impervious area is unconnected
• Figure 2-4 ? impervious lt 30
• Figure 2-3 ? impervious gt 30

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TR-55 Estimating Runoff

• Figure 2-4 Example
• ½ acre lot, 20 impervious (75 Unconnected),
  Pervious CN 61
• Figure 2-4 ? Composite CN 66
• If all of the impervious area is connected
• Figure 2-3 ? Composite CN 68

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Example 2-1

• Given
• Dyer County, TN
• 250 Acres
• 175 Ac Type C
• 75 Ac Type B
• 25 Yr / 24 Hr Event
• Rainfall 6
• Pasture Good
• Find CN Q

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WinTR-55
Example 2-1 Land Use Details
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WinTR-55
Example 2-1 Weighted CN 70
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WinTR-55
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Example 2-3
Find CN Q
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Example 2-1

• Given
• Dyer County, TN
• 250 Acres
• 175 Ac Type C
• 75 Ac Type B
• 25 Yr / 24 Hr Event
• Rainfall 6
• Pasture Good
• Find CN Q

74

• Pasture
• Good
• HSG B

Pervious CN 61
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Example 2-1

• Given
• Dyer County, TN
• 250 Acres
• 175 Ac Type C
• 75 Ac Type B
• 25 Yr / 24 Hr Event
• Rainfall 6
• Pasture Good
• Find CN Q

82

• Pasture
• Good
• HSG C

Pervious CN 74
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Example 2-4
Find CN Q
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78

• 25 Impervious
• 50 Unconnected
• Pervious CN 74