Hydrology Flashcards
1
Q
hydraulics
A
- deterministic: know what’s going to happen
- how does water behave
- certainty
2
Q
Hydrology
A
- stochastic: don’t know what’s happening
- how much water is there
- uncertainty
3
Q
interception
A
water falling on plant leaves and evaporates back into air
4
Q
Types of flooding
A
- tidal
- fluvial
- groundwater
- pluvial
- sewers
- man-made structures
5
Q
Tidal flooding
A
high tide and high wind speeds
- some degree of predictability
6
Q
Fluvial flooding
A
- when capacity of water courses is exceeded
- caused by blockages
- most common natural hazard in NZ
7
Q
Groundwater flooding
A
- high groundwater levels
- seasonal: more severe in frozen condtions (water cant sink)
- difficult to prevent
8
Q
Pluvial flooding
A
- excessive rainfall (nowhere to go)
- may be predictable
9
Q
Sewer Flooding
A
- urban areas
- rainfall intensity > sewer capacity
10
Q
Man-Made Structures
A
- failure of dams and reservoirs
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
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
13
Q
Flood Coping Strategies
A
- adapt (protection of individual properties or communities)
- mitigate ( National and Regional Policies)
- accept
14
Q
Design Flow Methods
A
- Flood- Frequency Analysis
- Runoff-Routing Methods
- Rational Method
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)
16
Q
Runoff Routing Method
A
- insufficient historical flow record
- method for determining entire hydrograph
17
Q
Rational Method
A
- insufficient historical flow record
- method for determining max. flow
18
Q
hydrograph
A
how flood water rises then receeds
19
Q
current meter
A
- measures flow
- temporary site
- needs a person on site to measure
- accoustic: doppler effect
20
Q
Level to flow
A
- permanent fixed site
21
Q
AEP
A
Annual Exceedance Probability
= 1 / return Period
AEP = 1 / P
22
Q
dye testing
A
- known quantity of dye
- dilution is measurement of velocity
23
Q
probability of occurance
A
= 1 / T
24
Q
probability of non-exceedance
A
= 1 - 1 / T
25
probability of non-exceedance in n years
= (1 - 1/T) ^ n
26
probability of exceedance in n years
R = 1 - (1 - 1/T)^n
27
AEP*
= (m - 1) / N
where m = rank,
and N = number of years in flow series
28
Cunnane Formula (plotting position)
pp = ( m - 0.4 ) / ( N + 0.2 )
| where m = rank and N = number of years in flow series
29
precipitation
any form of water coming from the atmosphere
- rain
- dew
- snow
- hail
- mist (less dense)
- fog (more dense)
30
water in the atmosphere
99. 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
residence time of water in atmosphere
9 days
32
rainfall distribution
- not evenly distributed
- 50 mm near equator
-
33
precipitation mechanisms
- supersaturation of air
- condensation of water vapour
- growth of condensation products
- supply of moist air
34
supersaturation of air
- water holding capacity of air decreases with temperature
| - cooling down saturated air can cause supersaturated air
35
condensation
air at or over saturation point (ice crystals or droplets)
36
growth and precipitation
- 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
lapse rate
α - rate of change of temperature with elevation
| α = - (T2 - T1) / (z2 - z1)
38
hydrostatic pressure law
dP /dz = -ρ(a) g
39
ideal gas law
P = ρ(a) R(a) T(K)
40
subcatchment
water droplets in the area will flow into respective stream
41
catchment
any droplet inside the catchment will flow out the outlet
42
hyetograph
rainfall graph
43
hydrograph
river flow
44
abstraction/losses
- infiltration (into soils)
- evapotranspiration (transpiration and evaporation)
- interception (rain falling on trees or buildings and evaporated - never reaches ground)
- depression storage (puddles)
45
P(e)
excess precipitation = runoff: anything that cannot be evaporated or infiltrated
46
I(a)
initial abstraction = 0.2 x S
| - relationship between storage and initial abstraction rate
47
F(a)
final abstraction
48
CN
curve number: indication of imperviousness
- high number suggests high imperviousness
- low number suggests low imperviousness
49
curve number determined by
- hydrological soil group
- cover type (concrete vs. dense forest)
- treatment (changing form of slope)
- hydrologic conditions
- antecedent runoff conditions
50
t (lag)
= 0.6 x t(c)
51
soil group A
deep sand, deep loess, aggregated silt
52
soil group B
shallow loess, sandy loam
53
soil group C
clay loam, shallow sandy loam, high clay/low OM
54
soil group D
swelling soils, heavy clay, sodic soils
55
CN limitations
- 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
