Unit Hydrograph

Intro to Unit Hydrograph Presented by: Dr. Michael Horst Assistant Professor of Water Resources The College of New Jersey

Unit Hydrograph Theory • Sherman (1932) • What is Unit Hydrograph Theory – Given two evenly distributed rainfall events over an entire watershed – The response hydrographs of the watershed will have similar characteristics – The only difference will be in the magnitude of the flows

Unit Hydrograph Theory Storm Hydrograph (4 inches) 400 350 300

Flow

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Unit Hydrograph Theory Storm Hydrograph (4 inches vs 2 inches) 400 350 300

Flow

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Time

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Unit Hydrograph Theory Hydrograph Characteristics 400 350 300

Flow

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Unit Hydrograph Theory • The unit hydrograph is the response of the watershed to 1 unit of excess runoff distributed uniformly over the entire watershed – 1 inch (English units) – 1 mm (Metric units)

Unit Hydrograph Theory Unit Hydrograph vs Storm Hydrographs 400 350 300

Flow

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Significance of Unit Hydrograph • Watersheds response to a given amount of excess precipitation is just a multiplier of the unit hydrograph • Use unit hydrograph as a basis to determine the storm hydrograph from any given rainfall distribution

Example • Given the following rainfall distribution Time

Precipitation

1

0.5

2

3

3

1.5

4

0.2

• The watershed will respond as follows

Example Incremental Storm Hydrographs 500

400 Time

Precipitation

1

0.5

2

3

3

1.5

4

0.2

Flow

300

200

100

0 0

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35

Example Incremental + Final Storm Hydrograph 500

400

Flow

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0 0

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Unit Hydrograph Derivation • A unit hydrograph is derived from historical rainfall and runoff data • The volume of water produced by the storm (area under the hydrograph curve) divided by the area of the watershed equals depth of excess precipitation • The ordinates of the storm hydrograph are divided by this depth to obtain the unit hydrograph • Timing must be taken into consideration (S-curve technique to adjust timing)

What if there is no Historical Data?!?! • Synthetic Unit Hydrograph – Snyder (1938) – Clark Instantaneous Unit Hydrograph (1945) – SCS Dimensionless Unit Hydrograph (1957)

• Each method uses the principles of Unit Hydrograph theory

Synthetic Unit Hydrographs • Time to Peak • Lag Time – centroid of excess precip to peak of hydrograph

• Time of Concentration – end of excess precip to inflection point on recession limb

• Time to Base • Watershed Area

Synthetic Unit Hydrographs • Use watershed characteristics to compute various parameters – Time to peak = f(slope, LHL, Lca CN, etc.) – Peak flow = (Constant * Area)/Time – Time to base

Synthetic Unit Hydrograph Synthetic Unit Hydrograph 600

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Flow

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0 0

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Synthetic Unit Hydrograph • Predetermined shape distribution of hydrograph

Synthetic Unit Hydrograph Synthetic Unit Hydrograph 600

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Flow

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