6.2: Unit Hydrograph

Question 1

What is effective rainfall?

Answer 1

The portion of rainfall that contributes to quick flow, a.k.a. ‘excess rainfall’

Question 2

What is direct runoff?

Answer 2

Direct runoff is the flow of water as a result of rainfall

Question 3

What are the 5 methods of base flow separation?

Answer 3

1. straight-line method
2. fixed base method
3. variable slope method
4. master baseflow recession curve method
5. matching strip method

Question 4

What is the difference between constant loss and proportional loss?

Answer 4

Constant loss: set amount of loss over the whole time

Proportional loss: percentage loss per interval

Question 5

What is the constant loss equation (Ø method)?

Answer 5

∑Q∆t = A∑(Ri-Ø)∆t
Excess rainfall = ∑Q∆t/A = ∑(Ri-Ø)∆t
Constant loss rate = Ø < Ri

Question 6

How do you do the constant loss method by inspection?

Answer 6

When given the excess rainfall, draw a horizontal line across the hyetograph that has the excess rainfall above it. The corresponding value on the Y-axis gives the Ø value. Be careful with units as the data is given per time period

Question 7

What is the T-hour unit hydrograph?

Answer 7

The hydrograph resulting from 1mm of rainfall evenly distributed from a storm with uniform intensity and duration of T hours.

Question 8

What are the 5 assumptions of the unit hydrograph?

Answer 8

• excess rainfall & effective runoff only
• linear response of catchment
• even distribution of rainfall
• superposition applies
• time invariant

Question 9

What are the steps to derive a unit hydrograph?

Answer 9

1. separate baseflow from hydrograph (gives the direct runoff hydrograph)
2. calculate excess runoff (constant loss method)
3. determine the duration of excess rain from hyetograph
4. divide runoff hydrograph by magnitude of excess rain depth [mm] i.e. obtain hydrograph for 1mm

Question 10

How do you apply superposition to a unit hydrograph?

Answer 10

Multiply the unit hydrograph by the rainfall at each time. Start each set of multiplication at the time of that particular rainfall. Take the sum of the flow at each time step to produce the direct runoff hydrograph. Add in base flow if applicable

Question 11

When is a synthetic unit hydrograph used?

Answer 11

When the catchment is ungauged i.e. there is no discharge data

Question 12

What are the 3 features of the synthetic unit hydrograph?

Hint: base, height, area

Answer 12

1. peak discharge, Qpeak - the height of the triangle
2. duration, T - the base of the triangle
3. volume of runoff = area of catchment [m2] x 0.001m - area of the triangle

Question 13

What are the advantages of the TUH?

Answer 13

• simple measure of catchment effects on excess rainfall
• gives reasonable flood estimation
• STUH can be constructed without data

Question 14

What are the disadvantages of the TUH?

Answer 14

• not suitable for nonlinear catchments
• unreliable for non uniform excess rainfall distribution
• difficult to derive from complex storms

Question 15

What does Ø represent? What is its units?

Answer 15

Constant loss rate [mm/interval]

Question 16

On a combined graph of rainfall hyetograph and direct runoff hydrograph, what does each represent?
Hint: constant loss method equation

Answer 16

The area of the hyetograph represents the ∑(Ri-Ø)∆t

The area under the hydrograph curve represents ∑Q∆t