Hydrograph

Question 1

What is a hydrograph?

Answer 1

Graph showing change in the discharge if a river over a period of time

Question 2

What does a hydrograph represent?

Answer 2

How a catchment responds to rainfall

Question 3

What is river runoff?

Answer 3

Surface water in rivers

Question 4

What are the four ways that rainfall contributes to river runoff?

Answer 4

1) Overland surface runoff
2) Interflow (subsurface runoff)
3) Base flow from groundwater
4) Rainfall on river channel

Question 5

When does infiltration excess runoff occur?

Answer 5

When rainfall intensity is greater than ground surface infiltration rate.
Occurs during high intensity rainfall events

Question 6

When does interflow occur?

Answer 6

If subsoil infiltration rate is lower than the top soil infiltration rate, infiltration excess runoff occurs below ground surface

Question 7

When does saturation excess runoff occur?

Answer 7

When rainfall intensity is less than soil infiltration rate but prolonged rainfall saturates the soil (due to large soil particles) and no more water can be held and excess water is released into groundwater and river channels

Question 8

What is direct runoff? Why is this term made?

Answer 8

The combination of overland surface runoff and interflow runoff
Made since these runoffs travel much faster than groundwater

Question 9

What are hydrograph’s components?

Answer 9

Rising limb, recession limb, and peak flow

Question 10

What is required to calculate the total runoff volume?

Answer 10

Flow event separation, each rain event has own curve

Question 11

When can Qt equation for runoff flow be used?

Answer 11

If recession part of runoff curve is dominated by groundwater supply

Question 12

What does Q0 represent in the Qt equation for runoff discharge and where can it be set?

Answer 12

Discharge at time zero which can be set anywhere in the lower part of the recession limb

Question 13

How can we find total runoff volume for recession limb?

Answer 13

W = integral of Qt between inf. and 0
W = kQ0

Question 14

How can we find total runoff volume for both direct runoff and base flow for an event?

Answer 14

Runoff Volume = A_shaded + k(Qb - Qa)

A_shaded = area of rising limb and peak
Qb = flow from recession limb curve
Qa = flow from river without rainfall event

Question 15

As time goes by from the peak flow, how does the runoff dominance vary?

Answer 15

Dominance of surface and interflow decreases and groundwater takes over

Question 16

What is each part of hydrograph dominated by?

Answer 16

Total recession curve = direct runoff
Recession = base flow

Question 17

What does N symbolise?

Answer 17

Number of days from the peak flow in which ground water takes over dominance from direct runoff.

Question 18

How do we divide baseflow and direct runoff?

Answer 18

A = start of event
B = peak of event
C = N days after peak
D = end of event
Line AC separates event
ABC = direct runoff
ACD = base flow

Question 19

If not separating events how can direct runoff and base flow be separated?

Answer 19

Drawing a horizontal line from where rising limb begins.
Below line = base flow
Above line = direct runoff

Question 20

What is effective rainfall?

Answer 20

Part of total rainfall that contributes to direct runoff

Question 21

What is total effective rainfall equal to?

Answer 21

Direct runoff volume

Question 22

How do we calculate the losses?

Answer 22

Total rainfall - Effective rainfall

Question 23

What are the 4 methods to calculating effective rainfall?
What are assumptions of each?

Answer 23

1. Index Method
   ~ Rainfall losses are constant with time
2. Initial and Continuing Losses
   ~ Initial losses attributed to interception from vegetation and land surface storage (due to dry land) and then losses becoming constant with time
3. Proportional Losses
   ~ Same proportion of total rainfall is lost to evaporation, infiltration etc.
4. Soil Moisture Accounting Scheme
   ~ Model which accounts for soil layers and soil moisture providing effective rainfall related to surface runoff, interflow and base flow.

Question 24

What is a unit hydrograph used for?

Answer 24

Modelling direct runoff with effective rainfall

Question 25

What is a unit hydrograph?

Answer 25

Catchment flow response to a unit (1cm) of effective rainfall occurring over a given duration

Question 26

What is assumed for a unit hydrograph?

Answer 26

a. effective rainfall is uniformly distributed over whole catchment and duration b. direct runoff process is linear in superposition and proportionality c. rainfall runoff process is stationary i.e. system behaves the same regardless of season

Question 27

How do we compute the hydrograph resulting from a unit hydrograph and baseflow measurements?

Answer 27

Using a table with the following headings: Time (h) Unit Hydrograph Measurement (m3/s) a. R1 * U(t) (R1 = effective rainfall of event 1 in cm) b. R2 * U(t-1) (R2 = effective rainfall of event 2 in cm) c. Base Flow (m3/s) Total Flow (m3/s) = a + b + c Total Volume (m3) = Total ^ * unit time of hydrograph Plot total flow with time

Question 28

How do we calculate how much flooding will occur?

Answer 28

Subtract maximum flow limit from all total flows and sum the total flows multiplied by the unit time of the hydrograph

Question 29

How can be estimate a unit hydrograph?

Answer 29

Linear system - R . U = Q Using a computer program the least squares can be used to estimate unit hydrograph from effect rainfall and direct runoff (which are found from selecting a historical event and back calculating to find effective rainfalls)

Question 30

How can we convert a unit hydrograph from one duration to another?

Answer 30

Using an S-Curve

Question 31

How is S-Curve derived?

Answer 31

Assuming continuous rain and summing all the unit hydrograph ordinates

Question 32

How do we derive a T hour 1 cm unit hydrograph from a S-curve?

Answer 32

Using Q(dt, t) in equation sheet t = current time of S-Curve (1 hour usually) dt = T hour unit hydrograph required

Question 33

When is a synthetic unit hydrograph used?

Answer 33

When a catchment is ungauged and there are no rainfall or runoff data to develop a unit hydrograph model

Question 34

What are equations for the time to the peak (Tp) and peak flow (Qp) estimations for a synthetic unit hydrograph?

Answer 34

Tp = 3.1 L^(0.23) S^(-0.25) I^(-0.18) PHI^(1.57) Qp = 31.62x10^3 A^(0.96) Tp^(-1.07) L = river length S = main channel slope I = percent of impervious area PHI = channel roughness and impervious area coefficient A = catchment area

Question 35

What is the shape of a synthetic unit hydrograph?

Answer 35

Triangular and symmetric

Question 36

What does the assumptions of linearity in superposition and proportionality mean for a unit hydrograph?

Answer 36

If rainfall doubles runoff doubles. Any runoff from a later time can be added to previous runoff.