River discharge (fact sheets 9,10,11)

Question 1

Storm hydrograph

Answer 1

A river’s response to precipitation moderated by catchment characteristics

Question 2

Stages of a storm hydrograph

4

Answer 2

rising stage, peak, recession limb, falling stage

Question 3

‘Time to peak’

Answer 3

the time from the centroid of the rainfall event to the hydrograph peak

Question 4

Catchment characteristics that affect the shape of the river hydrograph
7

Answer 4

basin shape, basin area, drainage density, basin or channel slope, vegetation type, infiltration, catchment storage

Question 5

Basin shape
(Catchment characteristics that affect the shape of the river hydrograph)

Answer 5

elongated catchments have a more damped hydrograph

Question 6

Basin area
(Catchment characteristics that affect the shape of the river hydrograph)

Answer 6

double the area, double the hydrograph peak

Question 7

Drainage density
(Catchment characteristics that affect the shape of the river hydrograph)

Answer 7

length of river channel per unit catchment area,
an increase in drainage density gives an increase in the channel intercepting subsurface water, giving increased flashiness

Question 8

Basin or channel slope

Catchment characteristics that affect the shape of the river hydrograph

Answer 8

increase the slope, increase the rate at which water moves through the catchment

Question 9

Vegetation type
(Catchment characteristics that affect the shape of the river hydrograph)

Answer 9

conifer forest evapotranspires greater volumes of water than does grassland, and therefore may damp hydrographs

Question 10

Infiltration

Catchment characteristics that affect the shape of the river hydrograph

Answer 10

low infiltration surfaces increases river flashiness

Question 11

Catchment storage
(Catchment characteristics that affect the shape of the river hydrograph)

Answer 11

eg: chalk aquifers have a high storage and, therefore, damp hydrograph response

Question 12

how can the flashiness of a river be characterised numerically?

Answer 12

hydrograph separation

Question 13

methods of hydrograph separation

3

Answer 13

1. straight-line separation
2. the Hursh and Brater method
3. Hibbert method (most common)

Question 14

The Hibbert method for separating the hydrograph

Answer 14

a separation line with a slope increasing at a rate… (?)

Question 15

What is river discharge measured in

Answer 15

cumecs

Question 16

Spot discharge

Answer 16

river discharge at one point in time

Question 17

Ways to measure river discharge

3

Answer 17

1. volumetric gauging
2. float gauging
3. current metering

Question 18

Volumetric gauging, in order to measure spot discharge

Answer 18

only possible with small river flows or indirectly with the velocity-area methods float gauging and current metering

Question 19

Float gauging, in order to measure spot discharge

Answer 19

involves measuring the velocity of a neutral buoyancy object (eg: orange) and multiplying this by the average cross-sectional area of the river

Question 20

Why should float gauging be reduced by a coefficient?

in order to measure spot discharge

Answer 20

because the objects float close to the river surface, which is faster than the average velocity of the water profile

Question 21

Current metering
(in order to measure spot discharge)

Answer 21

the rotation of a current meter’s impeller gives the local water velocity following application of a calibration equation, measurements should be made at difference depths to cope with vertical velocity

Question 22

How can the transverse distribution of water velocity be characterised?

Answer 22

dividing the channel cross section into segments

Question 23

What are the edges of segments called when dividing the channel into cross sections?
and their use?

Answer 23

verticals,

the locations at which the measurements are to calculate profile-average velocity should be made

Question 24

Two methods to calculate discharge for each segment?

Answer 24

‘mean section method’ or ‘mid section method’

Question 25

When is current metering not an accurate method of river gauging?

Answer 25

when the river is very shallow

Question 26

When should dilution gauging be used?

Answer 26

when the river water is very shallow

Question 27

What method is dilution gauging based on?

Answer 27

the two component mixing method

Question 28

Two approaches to dilution gauging?

Answer 28

1. constant injection method

2. integration/ gulp method

Question 29

Constant injection method

Answer 29

a known tracer concentration is trickled into a river at a fixed rate

Question 30

Integration/ gulp method

Answer 30

Involves the addition of a known volume and conc. of a tracer into the river as a single ‘slug’

Question 31

Instead of measuring spot discharge, how can continuous traces of river discharge be measured?

Answer 31

the structural method

Question 32

The height of water in the river using the structural method

Answer 32

stage

Question 33

Rating curve
(structural method)

Answer 33

the relationship between a stage and several spot discharge measurements

Question 34

Hydraulic drop

Answer 34

eg: river moves over a boulder and goes from a subcritical flow to a supercritical flow

Question 35

Hydraulic jump

Answer 35

supercritical to subcritical flow

Question 36

How is an accurate trace discharge calculated?

Answer 36

building an artificial hydraulic drop

Question 37

Example of an artificial hydraulic drop

2

Answer 37

weirs and f,lumes

Question 38

How do weirs generate a hydraulic drop?

Answer 38

the water falls from a stllling pool

Question 39

How do flumes generate a hydraulic drop?

Answer 39

use a constriction

Question 40

How to measure peak discharge during a flood when only max. height is known/ river gauges are flooded

Answer 40

slope area methods

Question 41

Most common slope area method

Answer 41

Manning equation

Question 42

Three types of stream classification

Answer 42

Ephemeral rivers
Intermittent channels
Perennial channels

Question 43

Ephemeral rivers

Answer 43

Only flow during flood events

Question 44

Example of an ephemeral river

Answer 44

desert wadi

Question 45

Intermittent channels

Answer 45

Will be dry for one or more seasons

Question 46

Example of an intermittent channel

Answer 46

headwater channels

Question 47

Perrenial channels

Answer 47

Flow all year

Question 48

How is the return period of extreme events calculated?

Answer 48

using an annual maximum series,

i.e. a data series of the max. daily discharge recorded during each year

Question 49

Flow duration curve (FDC)

Answer 49

a graphical method to show whether a river has a variable flow or relatively uniform flow