Hydrology Flashcards
hydraulics
- deterministic: know what’s going to happen
- how does water behave
- certainty
Hydrology
- stochastic: don’t know what’s happening
- how much water is there
- uncertainty
interception
water falling on plant leaves and evaporates back into air
Types of flooding
- tidal
- fluvial
- groundwater
- pluvial
- sewers
- man-made structures
Tidal flooding
high tide and high wind speeds
- some degree of predictability
Fluvial flooding
- when capacity of water courses is exceeded
- caused by blockages
- most common natural hazard in NZ
Groundwater flooding
- high groundwater levels
- seasonal: more severe in frozen condtions (water cant sink)
- difficult to prevent
Pluvial flooding
- excessive rainfall (nowhere to go)
- may be predictable
Sewer Flooding
- urban areas
- rainfall intensity > sewer capacity
Man-Made Structures
- failure of dams and reservoirs
Effects of Flooding depend on:
- level of predictability
- rate of onset (e.g. flash flooding)
- speed and depth of water
- duration of flood
- water quality
Consequences of Flooding
- loss of lives
- direct cost (repairs to property and infrastructure)
- indirect costs (loss of access to land)
- damage to environment
- ‘invisible’ costs
Flood Coping Strategies
- adapt (protection of individual properties or communities)
- mitigate ( National and Regional Policies)
- accept
Design Flow Methods
- Flood- Frequency Analysis
- Runoff-Routing Methods
- Rational Method
Flood Frequency Analysis
- long historical record (at least 15 years)
- method for determining max. flow
- more data the better (river developments change relevance of historical records)
Runoff Routing Method
- insufficient historical flow record
- method for determining entire hydrograph
Rational Method
- insufficient historical flow record
- method for determining max. flow
hydrograph
how flood water rises then receeds
current meter
- measures flow
- temporary site
- needs a person on site to measure
- accoustic: doppler effect
Level to flow
- permanent fixed site
AEP
Annual Exceedance Probability
= 1 / return Period
AEP = 1 / P
dye testing
- known quantity of dye
- dilution is measurement of velocity
probability of occurance
= 1 / T
probability of non-exceedance
= 1 - 1 / T
probability of non-exceedance in n years
= (1 - 1/T) ^ n
probability of exceedance in n years
R = 1 - (1 - 1/T)^n
AEP*
= (m - 1) / N
where m = rank,
and N = number of years in flow series
Cunnane Formula (plotting position)
pp = ( m - 0.4 ) / ( N + 0.2 )
where m = rank and N = number of years in flow series
precipitation
any form of water coming from the atmosphere
- rain
- dew
- snow
- hail
- mist (less dense)
- fog (more dense)
water in the atmosphere
- 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
residence time of water in atmosphere
9 days
rainfall distribution
- not evenly distributed
- ## 50 mm near equator
precipitation mechanisms
- supersaturation of air
- condensation of water vapour
- growth of condensation products
- supply of moist air
supersaturation of air
- water holding capacity of air decreases with temperature
- cooling down saturated air can cause supersaturated air
condensation
air at or over saturation point (ice crystals or droplets)
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
lapse rate
α - rate of change of temperature with elevation
α = - (T2 - T1) / (z2 - z1)
hydrostatic pressure law
dP /dz = -ρ(a) g
ideal gas law
P = ρ(a) R(a) T(K)
subcatchment
water droplets in the area will flow into respective stream
catchment
any droplet inside the catchment will flow out the outlet
hyetograph
rainfall graph
hydrograph
river flow
abstraction/losses
- infiltration (into soils)
- evapotranspiration (transpiration and evaporation)
- interception (rain falling on trees or buildings and evaporated - never reaches ground)
- depression storage (puddles)
P(e)
excess precipitation = runoff: anything that cannot be evaporated or infiltrated
I(a)
initial abstraction = 0.2 x S
- relationship between storage and initial abstraction rate
F(a)
final abstraction
CN
curve number: indication of imperviousness
- high number suggests high imperviousness
- low number suggests low imperviousness
curve number determined by
- hydrological soil group
- cover type (concrete vs. dense forest)
- treatment (changing form of slope)
- hydrologic conditions
- antecedent runoff conditions
t (lag)
= 0.6 x t(c)
soil group A
deep sand, deep loess, aggregated silt
soil group B
shallow loess, sandy loam
soil group C
clay loam, shallow sandy loam, high clay/low OM
soil group D
swelling soils, heavy clay, sodic soils
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
