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Hydrology > River Discharge > Flashcards

River Discharge Flashcards

1
Q

What is river discharge

A

Rivers response to precipitation moderated by the catchment characteristic

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2
Q

What are the two types of hydrograph

A

Flashy or damped

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3
Q

What regulate the hydrograph

A

Regime of rainfall and catchment characteristics

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4
Q

What are the 7 catchment characteristics

A 
Basin shape
Basin area
Drainage density
Basin/channel slope
Vegetation type 
Infiltration
Catchment storage
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5
Q

How does basin shape affect the hydrograph

A

A long catchment means bits are close to outlet but others further away so damped. Circular everything is close

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6
Q

How does basin area affect the hydrograph

A

Increases how much is going down river

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7
Q

How does drainage density affect the hydrograph

A

A lot of channels means water doesn’t travel far on the surface before joining with a river

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8
Q

How does channel slope affect the hydrograph

A

Steep catchment means water will be moving faster compared to a low relief catchment so it’s more flashy

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9
Q

How does vegetation type affect the hydrograph

A

If when it rains a significant proportion never make it to the ground because it’s grabbed by conifers then it will never go into channel so more subdued

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10
Q

How does infiltration affect the hydrograph

A

Roads built to remove logs - compacted surface and tracks provide channels for water to flow down

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11
Q

How does catchment storage affect the hydrograph

A

If there’s an aquifer underground that’s permeable rock like chalk, dampened

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12
Q

What is stormflow percent

A

From hydrograph separation it’s the part above the base flow.

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13
Q

What is the original concept of stormflow vs now

A

Overland flow towards rivers but now it’s all hydrograph (mostly subsurface flow)

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14
Q

What are the three ways to separate out a hydrograph

A

Straight line separation
Hursh and Brater method
Hibbert method

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15
Q

What is hursh and brater method

A

Slope is dependant on catchment area. Groundwater recession to peak time. Then rise at a rate n= 0.8A^0.2
N is days and A is drainage area

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16
Q

What is the hibbert method

A

Slope with a rate of increase of 0.0131m^3/skm^-2/d. Increase line according to this amount everyday with reference of how bit catchment is

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17
Q

What does Hibberts method say about stormflow in Danum

A

It’s 50%

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18
Q

What does hydrograph seperations suggest

A

Precipitation regime is most important

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19
Q

How to measure discharge

A 
Volumetric gauging 
Velocity area methods
Mean section methods 
Dilution gauging 
Structural methods 
Slope area methods
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20
Q

What is volumetric gauging

A

Simplest.
Collect flow or river over time period.
Accurate.
Can’t get under most rivers though.

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21
Q

What are velocity area methods

A

Float gauging - measure velocity of water x cross section of area. Use bouyant object.
current metering - arms low friction bearing so turn will small flow. Got to count each turn and record the turns over time. Can set at 0.6 of depth

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22
Q

Why is float gauging not great

A

Velocity distraction down a river changes, closer to Riverbed is slower because it’s slowing it down. There is a logarithmic decline with that so point of average flow is about 0.6 down from surface

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23
Q

How to correct velocity distribution

A

Reducing it with a curve. Reducing speed of flow to the average

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24
Q

Where is the point of average flow

A

0.6 down from surface of 0.4 up from the bed

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25
Q

What happens in a meandering stream

A

There is a zone of really high flow the move from one side to the other. Huge distribution of velocity o. Would zoom across the fast flowing over estimate the speed of water. No way to correct

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26
Q

What are mean section methods

A

Average velocity over whole channel cross section. Divide channel into series of equally spaced verticals. Areas between vertices known as segments

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27
Q

What happens at each sediment during the mean section method

A

Two rods. Need to be 0.2 and 0.8 measurements to get average. Get velocity at that location with rod, do it to the other V and get average velocity between the two. The area is depth of rod and width. Give us velocity times area for one little area of channel. (Qseg)

28
Q

What are the ways of dilution gauging

A

Constant injection method

Integration (gulp) method

29
Q

When is constant injection method good

A

Steep mountain environment as it’s difficult to get to if river floods or you don’t want to stand on t

30
Q

What is dilution gauging

A

Flux of water particles is equal to discharge of river multiplied by concentration of particles naturally in the river.
Add into river some tracer and the rate at which the tracer is going in is little q.
Could rearrange the equation for bit Q and discharge of river.

31
Q

What is the dilution gauging equation

A

(Q+q)C2=qC1+QCo

Q=q (C1-C2/C2-C0)

32
Q

What is the constantly injection method

A

Concentration starts to go up and then hits a plateau which is the c2 value (conductivity meter). Bottle is sealed with outlet and speed at which water comes out through pipe.

33
Q

What so integration (gulp) method

A

Background concentration, mixed concentration of tracer and in this case we mix with a known volume of water that we throw straight into channel and it mixes with flow in channel and you get a breakthrough curve. Arrival of tracer downsreqmZ

34
Q

How to work out integration method

A

Volume of tracer in x concentration divides by area of curve. Trapezium rule.

35
Q

Critical characteristic of a good water tracer

A 
Chemically conservation (won’t bind on sediments)
Non reactive
High water solubility
Relatively non toxic 
Measured in field 
Cheap
36
Q

How does salt tracer get measured

A

By electrical conductivity

37
Q

What happens to salt in large rivers

A

Dilutes and becomes undetectable so need a better trace r

38
Q

What could we use that’s better than salt as a tracer

A

Radioactive tracer

Fluorescent tracer

39
Q

What are structural methods

A

Continuously measure levels in river but combine with spot discharge measurements to get rating curve

40
Q

What is the rating curve

A

Relation between river level (stage) and spot discharge

41
Q

What do we need when doing structural methods

A

One energy state. Usuallt there’s two which is a problem for a head discharge rating - prevents shooting flow at stage measurement location -> add fixed hydraulic drop so place we measure can never flip.

42
Q

What is the point of critical flow

A

Point at which a river flips between the two regimes - hydraulic drop and hydraulic jump. Difficult to measure - usuallt just measure upstream. Characterised with froude number

43
Q

What is flow less than critical called

A

Subcritical

44
Q

What is flow more than critical called

A

Super critical

45
Q

What are the creations of a large hydraulic drop

A

Weir from fall. Rectangle thin plate.

Flume from constriction

46
Q

Problem with weirs

A

Sediments accumulate and structure won’t work properly. Could use a sediment trap upstream of weir .

47
Q

Why are flumes good

A

They use a construction in channel but allow sediment to pass. Width construction generates hydraulic drop

48
Q

What do we need with flumes or weirs

A

Water level recording device - could be a laser one. stilling well reduces chart noise. Stilling pool stops waves moving through the weir.

49
Q

What are slope area methods

A

Sometimes we have floods that destroy monitoring stations. Under those circumstances we can get an estimate of peak discharge using slope area methods but this is very approx rule

50
Q

Examples of slope area methods

A

Trash line after flood (max height floor trash lifted) surveyed channel cross section up to line and slope of trash running in a down stream relation. Slope of trash line = slope or water-surface during the flood.

51
Q

What is discharge equal to in slope area methods

A

Proportionate to the slope on the water surface (S)

52
Q

What is river discharge proportionate to in turbulent flow

A

Slope on the water surface strictly to the slope on the energy line raised to the power of 0.5. Can use this to get manning equation and estimate peak flow

53
Q

What is A in manning equation

A

Cross sectional area of flow in channel

54
Q

What is R in mannings equation

A

Cross section area divided by wetted perimeter

55
Q

What is n in manning equation

A

Mannings roughness coefficient. Calculated during normal flow. The rough bed slopes down the flow and effect relationship between water level and slope perimeters and discharge. Get rougher estimate of peak flow.

56
Q

What are the three stream classifications

A

Ephemeral stream
Intermittent stream
Perennial steam

57
Q

What is the ephemeral stream

A

Flows only during storms

58
Q

What is intermittent stream

A

Seasonally dry

59
Q

What is a perennial stream

A

Flows all year

60
Q

What is the flow duration curve

A

Method used for characterising the regime of the river.

61
Q

How to derive the flow duration curve

A

Comparing the logged river to adjacent catchment that’s never been disturbed to see if there’s a difference that’s apparent in flashiness of river

62
Q

How to plot the flow duration curve

A

Number of days that have flows within that range to cumulate the frequency from high flow to low flow.
Present the cumulative frequency as a percentage where we have percentage time of the here the flow is equal to or exceeded the first column on the left hand side

63
Q

What scale do we need to use in the flow duration curve

A

Log-log otherwise it hugs the x-axis - becomes more like a straight line that can be used to compared with another catchment

64
Q

What do relatively flat flow duration curves have

A

Relatively uniform flows indicating high amounts of storage like deep groundwater

65
Q

What do highly variable flow duration curves suggest

A

Steep catchments of impermeable geologise

66
Q

Flow duration curve timeline for Baru catchment (disturbed) and W8S5 (undistburbed)

A

FDC slope (range 30-70%) less steep and less flashy.

67
Q

Why does a disturbed catchment not make the hydrograph more flashy

A

You have a lot more flow left in river because of less evaporation during times of low flow. Low flows are brought up but it doesn’t really affect high flows so you end up with a more uniform flow regime post logging

Hydrology (9 decks)

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