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ENCN242 EXAM > Hydrology > Flashcards

Hydrology Flashcards

1
Q

hydraulics

A
  • deterministic: know what’s going to happen
  • how does water behave
  • certainty
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2
Q

Hydrology

A
  • stochastic: don’t know what’s happening
  • how much water is there
  • uncertainty
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3
Q

interception

A

water falling on plant leaves and evaporates back into air

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4
Q

Types of flooding

A
  • tidal
  • fluvial
  • groundwater
  • pluvial
  • sewers
  • man-made structures
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5
Q

Tidal flooding

A

high tide and high wind speeds

- some degree of predictability

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6
Q

Fluvial flooding

A
  • when capacity of water courses is exceeded
  • caused by blockages
  • most common natural hazard in NZ
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7
Q

Groundwater flooding

A
  • high groundwater levels
  • seasonal: more severe in frozen condtions (water cant sink)
  • difficult to prevent
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8
Q

Pluvial flooding

A
  • excessive rainfall (nowhere to go)

- may be predictable

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9
Q

Sewer Flooding

A
  • urban areas

- rainfall intensity > sewer capacity

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10
Q

Man-Made Structures

A
  • failure of dams and reservoirs
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11
Q

Effects of Flooding depend on:

A
  • level of predictability
  • rate of onset (e.g. flash flooding)
  • speed and depth of water
  • duration of flood
  • water quality
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12
Q

Consequences of Flooding

A
  • loss of lives
  • direct cost (repairs to property and infrastructure)
  • indirect costs (loss of access to land)
  • damage to environment
  • ‘invisible’ costs
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13
Q

Flood Coping Strategies

A
  • adapt (protection of individual properties or communities)
  • mitigate ( National and Regional Policies)
  • accept
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14
Q

Design Flow Methods

A
  1. Flood- Frequency Analysis
  2. Runoff-Routing Methods
  3. Rational Method
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15
Q

Flood Frequency Analysis

A
  • long historical record (at least 15 years)
  • method for determining max. flow
  • more data the better (river developments change relevance of historical records)
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16
Q

Runoff Routing Method

A
  • insufficient historical flow record

- method for determining entire hydrograph

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17
Q

Rational Method

A
  • insufficient historical flow record

- method for determining max. flow

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18
Q

hydrograph

A

how flood water rises then receeds

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19
Q

current meter

A
  • measures flow
  • temporary site
  • needs a person on site to measure
  • accoustic: doppler effect
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20
Q

Level to flow

A
  • permanent fixed site
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21
Q

AEP

A

Annual Exceedance Probability
= 1 / return Period
AEP = 1 / P

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22
Q

dye testing

A
  • known quantity of dye

- dilution is measurement of velocity

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23
Q

probability of occurance

A

= 1 / T

24
Q

probability of non-exceedance

A

= 1 - 1 / T

25
Q

probability of non-exceedance in n years

A

= (1 - 1/T) ^ n

26
Q

probability of exceedance in n years

A

R = 1 - (1 - 1/T)^n

27
Q

AEP*

A

= (m - 1) / N
where m = rank,
and N = number of years in flow series

28
Q

Cunnane Formula (plotting position)

A

pp = ( m - 0.4 ) / ( N + 0.2 )

where m = rank and N = number of years in flow series

29
Q

precipitation

A

any form of water coming from the atmosphere

  • rain
  • dew
  • snow
  • hail
  • mist (less dense)
  • fog (more dense)
30
Q

water in the atmosphere

A
  1. 5 % of water in atmosphere is water vapour
    - most abundant greenhouse gas
    - influences cloud formation which influences reflection of radiation
    - temp would be approx - 19oC without water vapour blanket
31
Q

residence time of water in atmosphere

A

9 days

32
Q

rainfall distribution

A
  • not evenly distributed
  • ## 50 mm near equator
33
Q

precipitation mechanisms

A
  • supersaturation of air
  • condensation of water vapour
  • growth of condensation products
  • supply of moist air
34
Q

supersaturation of air

A
  • water holding capacity of air decreases with temperature

- cooling down saturated air can cause supersaturated air

35
Q

condensation

A

air at or over saturation point (ice crystals or droplets)

36
Q

growth and precipitation

A
  • condensation light enough to form cloud ( 20 micro m)
  • to reach earth, need drops > 0.1 mm + cloud thickness > 1200 m
  • atmosphere can hold only 50 mm water at 20oC
37
Q

lapse rate

A

α - rate of change of temperature with elevation

α = - (T2 - T1) / (z2 - z1)

38
Q

hydrostatic pressure law

A

dP /dz = -ρ(a) g

39
Q

ideal gas law

A

P = ρ(a) R(a) T(K)

40
Q

subcatchment

A

water droplets in the area will flow into respective stream

41
Q

catchment

A

any droplet inside the catchment will flow out the outlet

42
Q

hyetograph

A

rainfall graph

43
Q

hydrograph

A

river flow

44
Q

abstraction/losses

A
  • infiltration (into soils)
  • evapotranspiration (transpiration and evaporation)
  • interception (rain falling on trees or buildings and evaporated - never reaches ground)
  • depression storage (puddles)
45
Q

P(e)

A

excess precipitation = runoff: anything that cannot be evaporated or infiltrated

46
Q

I(a)

A

initial abstraction = 0.2 x S

- relationship between storage and initial abstraction rate

47
Q

F(a)

A

final abstraction

48
Q

CN

A

curve number: indication of imperviousness

  • high number suggests high imperviousness
  • low number suggests low imperviousness
49
Q

curve number determined by

A
  • hydrological soil group
  • cover type (concrete vs. dense forest)
  • treatment (changing form of slope)
  • hydrologic conditions
  • antecedent runoff conditions
50
Q

t (lag)

A

= 0.6 x t(c)

51
Q

soil group A

A

deep sand, deep loess, aggregated silt

52
Q

soil group B

A

shallow loess, sandy loam

53
Q

soil group C

A

clay loam, shallow sandy loam, high clay/low OM

54
Q

soil group D

A

swelling soils, heavy clay, sodic soils

55
Q

CN limitations

A
  • based on average conditions
  • does not account for rainfall intensity or duration
  • I = 0.2S is based on agricultural catchments
  • snowmelt/frozen soil runoff
  • low accuracy for low precipitation
  • only direct runoff
  • if weighted CN

ENCN242 EXAM (2 decks)

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