6 - Classic Hydrology Flashcards
(104 cards)
1
Q
What is hydrology?
A
The scientific study of the movement, distribution and management of water.
Includes the water cycle, resources and drainage basin sustainability.
2
Q
What is hydrometry?
A
River flow movement
3
Q
Why measure river flow for flooding?
A
For the design of flood defences.
4
Q
Why measure river flow for the water industry?
A
To measure natural runoff in rivers for potable, industrial and irrigation use.
5
Q
Why measure river flows for the power industry?
A
Dtermines the water available for hydroelectric generation and steam turbines.
6
Q
Why measure river flows for waste treatment?
A
Determines safe discharge from combined sewer overloads
7
Q
Name some of the velocity-area methods for measuring river flow:
A
Current metering, pitol tube, acoustic dopple velocimeter
8
Q
What hydraulic structure can be used to measure river flows?
A
Measurement weirs.
9
Q
What is the mass balance method of measuring river flows?
A
Dilution gauging.
10
Q
What is a remote sensing method of measuring river flows?
A
Ultrasonic and electromagnetic.
11
Q
What is uniform river flow?
A
When the velocity pattern in a constant cross-section does not change in the direction of flow.
12
Q
If the depth of flow decreases, what happens to the velocity?
A
It increases to maintain discharge
13
Q
In a velocity profile, at the solid boundary, what does the velocity need to reach?
A
u = 0m/s
14
Q
What is subcritical flow?
A
When Fr < 1
15
Q
What is supercritical flow?
A
When Fr > 1
16
Q
What are the two main methods that can be used to gauge the flow of a river?
A
The discharge and the stage, H (m)
17
Q
What is the ‘stage’ ?
A
The water level relative to some datum, often a fixed point on the bank.
18
Q
What is the most accurate method that discharge can be obtained?
A
By using the velocity-area method.
19
Q
What is the velocity-area method?
A
Cross section of river bed is surveyed and considered constant.
20
Q
What is the simplest method to determine the velocity of flow?
A
By timing a float over a known distance.
21
Q
From the ‘float’ method, what is the only thing obtained?
A
The surface velocity.
22
Q
What are the 2 correction factors applied for the ‘float’ method?
A
0.7 for < 1m deep rivers, and 0.8 for > 6m deep rivers.
23
Q
Why is the ‘float’ method only recommended as an estimate of the velocity?
A
Because the float moves around the surface.
24
Q
What is ‘horizontal’ sampling, where are the rakes placed?
A
Vertical rakes spaced at intervals no greater than 1/15th of the channel flow
25
From theoretical studies, where does the mean velocity on a vertical line in a river occur?
At about 0.6 of the river depth, h below the water level.
26
What does a current meter display?
The velocity
27
What does 'ADCP' stand for?
Acoustic Doppler Current Profiler
28
What do ADCPs transmit and recieve?
Sound waves
29
How can you use ADCPs to measure the velocity of a river?A boat
A boat.
30
For very wide rivers, where do the gaugings have to made from?
The ADCP is moved from one bank and tyo the other, stopping at various points to measure the bed profile and velocity at those points.
31
How is a cableway used to measure velocities?
The current meter is hoisted across the river and lowered
32
Calculation of discharge from the velocity and dpeth measurements can be made from the 'mean section method' how is this doen?
Averages of the mean velocities and depths are multiplied by the depth and width.
33
What are some reasons for asymmetry in a flow channel?
Curved stretch, obstruction, differences in the roughness of the channel.
34
If you have a compound channel, what do you do in order to find the hydraluic radius?
Split them up, and find the hydrualic radius for each section.
35
To calculate the discharge in the channel, you multiply the velocity by the cross sectional area of the channel. How is the velocity found?
Through manning's equation
36
Is manning's n higher when there is little vegetation or more?
More vegetation results in higher manning's n
37
If there is a rocky bed, what does this do to manning's n?
Increases the value of manning's n
38
The water level at a gauging station is measured with respect to a datum, give 2 examples of what this datum could be?
A local bench mark, or the crest of a weir or bottom of a flume.
39
What is a river flow staff gauge?
A permanents graduated staff fixed vertically to the river at a stable point.
40
What is the accuracy of a river flow staff gauge?
Plus or minus 3mm
41
When are river flow crest guages used?
In the absense of a continuous level recorder, they are very simple.
42
What does a river flow crest gauges consist of?
A 50mm diameter steel tube perforated near the bottom, and closed at the top with holes under the lid to let air escape.
Inside is a removable rod that retains the highest water mark from a floating granular cork.
43
What is the main disadvantage of river flow crest gauges?
Minor peaks over a short period may be missed.
44
Historically, a float and counterweight pass over a pulley on a continuously rolling piece of paper, what is the modern method?
Digital devices can be used to remotely sense water levels.
45
How is a stilling well used to measure river depths?
Hollow tube is positioned vertically adjactent to the water channel, preventing wind, waves and debris from effecting the gauage.
An intake tube allows water to flow into the stilling well from the water channel, so that the level within the tube is always identical to the level within the channel/
46
Is it easier to measure stage or discharge?
Stage
47
Are conditions in a natural channel stable for long periods of time?
Rarely.
48
After flood events that may have altered the river bed banks, what needs to be repeated?
Stage-discharge relationships.
49
What are the three ways that stage-discharge relationships can be presented?
Through a rating curve, rating table or a rating equation
50
What is the most common modern method of representing a stage-discharge relationship?
Through using a rating equation.
51
In a stage discharge rating curve, what does it mean if the points do not lie on a smooth curve, what is it due to?
Either a small nearby waterdall is acting as flow control is drowned out by higher flows, or the channel is compound.
52
What does a hyetograph represent?
The distribution of rainfall intensity over time.
53
When rain falls on the earth, why does it start to move?
Due to gravity.
54
How does some rainwater enter the atmospher?
Through evapotranspiration
55
During a storm, how does water flow downhill?
As surface runoff
56
Due to a storm lots of the water runs downhill as surface runoof, what does this cause?
A massive discharge into rivers.
57
What is 'groundwater runoff' ?
Water that enters the groundwater system during a storm.
58
What is the time of concentration?
The longest time taken for the rain at some point in the catchment to reach the measuring point in the river or drainage system.
59
What does the Rational method only really work for?
Small, relatively homogeneous catchments.,
60
What is 'interception'?
The retention of rainfall by vegetation
61
Once the vegetation is saturated, what happens to the rainfall?
It runs through the soil
62
What is the interception loss for impervious areas?
Small and generally neglected.
63
What is 'depression storage'?
Storage that accounts for rainwater that becomes trapped in small 'depressions' on the surface (puddles)
64
What are the 2 types of intital losses?
Interception and depression storage
65
When is the only time intitial losses are deducted from the rainfall depth?
At the start of the storm only,
66
What is evapo-transpiration?
The vaporsation of water from plants and open water bodies.
67
In the UK in summer, what is the average evapo-transpiration rate?
2 to 3 mm per day.
68
What does infiltation represent?
The process of rainfall passing through the ground surface into the pores in the soil.
69
What does the magnitude of infiltration depend on?
Soil type and structure, initial moisture content .
70
When does runoff occur in terms of infiltration?
When the rainfall intensity exceeds the infiltration capacity of the soil.
71
What is the effective rainfall intensity?
The continuing loss intensity is subtracted from the net rainfall.
The continuin loss intensity is the infiltration rate.
72
What does a hydrograph describe?
The time history of the flow in a river due to a rainfall event.
73
What are the axis on a hydrograph?
Rainfall intensity on the y axis, and time on the x-axis.
Or flow on the y-axis and time on the x- axis.
74
On a hydrograph which displays the rainfall intensity in blocks of time, what are the two things it shows?
The actual rainfall and the effective rainfall.
75
In a hydrograph, between points A and B, what occurs ( for flow vs time) ?
A rising limb, peak and inflection point.
And a dotted base flow at the bottom.
76
What is the 'lag' in a hydrograph?
The time between the start of the rising limb and the peak.
77
Do initial and continous losses contribute to the hydrogrpah?
No
78
When will the increase in discharge in a hydrograph coincide with the first tranche of effective rainfall?
If it is close to a river, if not there is a delay.
79
What is the 'baseflow' ?
The portion of the streamflow thay contains groundwater flow and flow from other delayed sources ( infiltration, upstream reservoir ).
80
What are some of the factors that have an effect on the shape of the hydrograph?
Catchment area
Slope
Roughness
Storage
Drainage intensity
Channel length
81
What is the relationship of discharge and area for a hydrograph?
Larger area results in a larger discharge
82
What is the relationship of discharge and slope on a hydrograph?
Larger the slope, the higher the peak over a shorter time.
Runoff is quicker
83
What is the relationship of discharge and roughness on a hydrograph?
The rougher the channel, the lowerer the peak is over a longer time
84
What is the relationship of discharge and drainage density?
Higher the drainage density, the higher the peak is
85
What are the two types of flood prediction recommended?
Unit hydrographs and statistical methods.
86
What are the two types of catchment used for flood prediction?
Gauged and ungauged
87
What is a 'gauged' area?
An area withy lots of rainfall and runoff data
88
What is an 'ungrauged' area?
An area without the data
89
Suppose we want to make short term predictions of flow in a river after an extreme rainfall event, what is the unit hydrograph?
The surface runoff resulting from the effective rainfall in a unit of time.
90
What is the coordinates of the runoff hydrograph directly proportional to?
The amount of effective rainfall.
91
If 2 successive amounts of rainfall each fall in T hours, what is the runoff hydrograph?
The sum of the two componet hydrographs, the seconnd being lagged by 'T' hours
92
When is the only condition that the unit hydrograph can be used?
If the initial and continuing loses are taken into account - the effective rainfall must be used.
93
For gauged areas with long records, greater than 25 years. What can be used to determine the magnitude of any flood event with a return period, T ?
Frequency analysis
94
What is the return period, T when analysing a flood event?
The average interval between occurences of floods that exceed the magnitude of flow.
95
The flood estimation handbook introduced the concept of the median annual maximum flow, QMED. What is this?
The flow that is exceeded on average, every other yar.
96
What is the probability of QMED?
1 in 2
97
What is QMED formally known as?
The middle ranking value in the series of annual maximum floods.
98
By replacing a histogram with a PDF, what does this allow?
Prediction of flood events beyond the limit of our data records.
99
What does Gringorten's plotting positions method predict?
The probability of non-exceedance of each event by ranking the events.
100
In Gringorten's method, how are the flood events ranked?
i=1 for the lowest flow, i = N for the highest flow
101
What are the annual maximum flows an example of?
Extreme value series.
They are not well described by a normal distribution.
102
What does the generalised logistic function relate?
The flow in the river, to the probability of non-exceedance of that flow, F
103
What are the conventional moments of distribution?
The mean, variance and skewness.
104
You are asked by your line manager to quote for 3 design solutions for a flood defence scheme for the river Erewash in the Sandiacre area around Station Road (B5010, figures). Outline the approximately 10 main steps to be able to quote
Have a look in the Hydraulic Design lecture notes and follow-up sources such as the Flood
Estimation Handbook.
Conduct online/newspaper searches about past flood events in this area.
Field surveys to take stage measurements and pictures of critical sections, estimate some
Manning friction coefficients and the river slope.
Define the return period of the flood event for which you design the flood defence scheme.
Typical would be Q100. You may use a safety factor to account for the effects of climate
change.
You need to find out how many m3/s Q100 corresponds to at the design location. The National
River Flow Archive would be an obvious first source: data.https://nrfa.ceh.ac.uk. You are
lucky as there is the Erewash at Sandiacre gauging station with daily flow data measurements
since 1965 in the design location. Without this gauging station you would have to make some
sort of assumptions, e.g. look at a gauging station close by and scale the flow based on the
catchment area/properties complemented with some flow measurements and/or use empirical
rainfall-run-off models. You would most likely consult with a hydrologist. In the detailed
study you would likely run numerical simulations (you would estimate how much time would
be needed for this to be considered in the quotes).
This data since 1965 (approximately 50 years) do not necessarily contain the Q100 value, so
you need to apply one of the statistical method to extrapolate this data series. You may apply
the Gringorten’s Plotting Positions or the Generalised Logistic Function as you heard about
them in the lecture.
Next you need to know which water level Q100 corresponds to in the critical sections. The
Erewash at Sandiacre gauging station may help to some degree if there are some stage-Q
values given. You could extrapolate this stage by taking the extra flow rate between the
maximum measured one and Q100 into account. If you want to do this more accurately or
there is no gauging station, you could get very rough estimates with the Manning equation (as
a rough estimate, there may not be uniform flow). Again, in the detailed study you would likely
run numerical simulations (to be considered in the quotes) and you would need bathymetric
and topographic data for this (e.g. from laser scans in the field).
You would make drawings of different designs (flood walls, embankments, flood retention area,
etc.).
You would have to check weather these solutions are working for Q100, so you would do some
rough hand calculations with new configurations (to be replaced by computer simulations later
10 ↑ Table of Contents
Hydrology Class Exercises with Solutions
on).
You would now put together the costing and present all in the quotes.
