CWR4101: Hydrology

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CWR4101 Hydrology
Mr. Jeff Earhart Curve Numbers http/classes.cec
s.ucf.edu/cwr4101/shagen
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NRCS Curve Number Method(Derivation)
Apply continuity (perform a mass balance) to a
rainfall event

• where P cumulative precipitation
• R cumulative rainfall excess
• Ia initial abstraction
• F cumulative infiltration

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NRCS Curve Number Method(Derivation)

• Consider two quantities that describe
  infiltration characteristics
• F cumulative infiltration that actually occurs
• S storage that could occur (potential maximum
  storage)

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NRCS Curve Number Method(Derivation)

• Consider two quantities that describe rainfall
  excess
• R cumulative rainfall excess that occurs
• P-Ia Potential rainfall excess

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NRCS Curve Number Method(Derivation)

• Hypothesis ratios of two actual to two potential
  quantities are equal to

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NRCS Curve Number Method(Derivation)

• Solve the ratio equality (previous page) for F

Substitute this into the original mass balance
(continuity) equation
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NRCS Curve Number Method(Derivation)

• Solve for R, which is the cumulative rainfall
  excess

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NRCS Curve Number Method

• Based on empirical results Ia 0.2S, which
  allows us to write the expression as

Note R 0 when P lt 0.2S
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NRCS Curve Number Method

• S for SI units

• S for BG units

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NRCS Curve Number Method

• where
• R cumulative rainfall excess
• P cumulative precipitation
• S function of a curve number, CN

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NRCS Curve Number Method

• How do we define the curve number, CN?
• 1. Establish soil type
• Textbook page 500, Table H.1 and Soil Survey of
  Orange County
• 2. Specify watershed type and land cover
• 3. Determine curve number
• Tables 5.7 5.10, e.g., NRCS Reports TR-55 and
  TR-20

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NRCS Curve Number Method Separate vs Combined
Approach

• Not Directly Connected Impervious Area (NDCIA)

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Combined Approach

• Method 1 Composite Curve Number, CCN
• where
• CNi curve number for sub-region i
• Ai area for sub-region i
• ? Ai total area

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Separate Routing Approach

• Method 2 Separate Routing
• Step 1. Calculate R for impervious

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Separate Routing Approach

• Step 2. Include Rimp with P to produce an
  adjusted precipitation for pervious areas

(Note mistake in book Example Pr. 5.7.)
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Separate Routing Approach

• Step 3 Calculate the rainfall excess for
  pervious areas

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Separate Routing Approach

• Step 4 Compute the total rainfall excess over
  the total area

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Separate Routing Approachwith DCIA
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Infiltration

• Hortons Equation

where f(t) infiltration rate L/T fc
final rate of infiltration L/T fo initial
rate of infiltration L/T K recession
constant 1/T t time T
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Hortons Equation for Infiltration
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Infiltration

• Hortons Equation, total volume, F