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Hydraulics and Hydrology 4 > Hydrology > Flashcards

Hydrology Flashcards

1
Q

How is hydrology different from hydraulics?

A

It applies to processes which occur upstream of the point where detailed hydraulic analysis can be performed

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2
Q

What is interflow?

A

Lateral movement of water within unsaturated soil

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3
Q

What are the three mechanisms of water volume in soil increasing?

A

Precipitation
Surface water inflow
Groundwater inflow

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4
Q

What are the three mechanisms of volume of water in soil decreasing?

A

Surface water outflow
Groundwater outflow
Evapo-transpiration

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5
Q

How is a catchment defined?

A

The area of all points that can possibly contribute surface water to a particular river

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6
Q

How is flow direction algorithmically found?

A

The greatest difference between a cell and it’s 8 neighbours in a 9x9 square cell

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7
Q

How can cell algorithms predict where a flow channel will form?

A

By finding the cells with the most other cells flowing into them

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8
Q

What are the main two ways of measuring precipitation?

A

Storage gauges
Tipping bucket recorders

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9
Q

Where should a rain gauge be sited?

A

Level ground
Partially sheltered
Representative of the average for the area

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10
Q

What is the name of a graph of rainfall intensity?

A

A hyetograph

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11
Q

How does the isohyet method map rainfall?

A

By drawing contours on a map according to recorded rainfall data

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12
Q

What are the key advantage and disadvantage of an arithmetic average of rainfall?

A

Very simple
Very inaccurate

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13
Q

What are the key advantage and disadvantage of an Theisson polygons?

A

Improved accuracy for non-uniform rain gauges
Do not amount for topography

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14
Q

What are the key advantage and disadvantage of the isohyet method?

A

Most accurate
Highly time consuming - must be redrawn for every point in time

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15
Q

What 3 factors affect the rate of evaporation?

A

Energy supply for vaporisation
Ability for Vapor to move away from the surface
Supply of moisture on the surface

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16
Q

How is the evaporation rate defined (equation)?

A

E = -dh/dt where h is depth of water

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17
Q

What is Tp?

A

The dew point

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18
Q

What is lv?

A

Latent heat of vaporisation

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19
Q

What is the equation for rate of evaporation, assuming all energy goes into vaporisation?

A

Er = Rn / lv * ρw

20
Q

What is Rn?

A

Net radiation

21
Q

What is a lysimeter?

A

A large block of representative soil and vegetation, sealed at the sides and base by watertight barriers and a pipe to measure percolation

22
Q

What are the two ways of modelling evapotranspiration?

A

Not limited by vapour transport rate
Not limited by energy supply

23
Q

How is porosity (ω) defined?

A

Volume of void space / volume of rock

24
Q

How is porosity of a large rock sample determined?

A

A Representative Elementary Volume (REV) is investigated and taken to be equivalent in porosity to the rest of the rock

25
Q

How does the formation of igneous rock affect porosity?

A

Intrusive rock has low porosity
Extrusive rock can have very high porosity, but some, like basalt, form detached columns which individually have low porosity

26
Q

How is effective porosity different from regular porosity?

A

It describes only the volume of pores through which fluid can flow

27
Q

What is the equation of effective porosity?

A

Volume of through-flowing water / volume of rock

28
Q

How do you measure rock volume not including voids?

A

Crush it into a powder

29
Q

What is recharge water?

A

Water that infiltrates into the unsaturated zone and is not then lost due to evapotranspiration and will reach the water table

30
Q

How is piezometric head expressed?

A

h = p/ρg + z

31
Q

How is pressure head expressed?

A

H = p/ρg

32
Q

What hydraulic head term is not included in piezometric head and why?

A

u^2/2g
Because flow velocity in porous rock is very slow

33
Q

What is Darcy’s Law for saturated flow?

A

Q = AKΔh/L
Where Δh is from top of water to bottom of rock, L is simply depth of rock

34
Q

What is K in Darcy’s Law? What is its equation?

A

Hydraulic Conductivity
K = kρg/μ
k is called intrinsic permeability

35
Q

Define the 4 “Aqui-“ terms

A

Aquifer - Rock which is saturated with water and permeable such that extracting water is economically viable
Aquitard - A less permeable bed
Aquiclude - A bed with almost zero permeability
Aquifuge - Rock that is saturated but neither contains or transmits much water

36
Q

How can unsustainable use of aquifers affect the local environment?

A

Removal of fluid can cause the rock to crush in on itself, permanently reducing porosity and causing subsidence

37
Q

What is the probability of A & B occuring?

A

P(A) * P(B)

38
Q

What is the probability of A or B occuring?

A

P(A) + P(B)

39
Q

If it can only flood once per year, there are 3 ways that 2 floods can happen in 3 years. How do you find this as a probability?

A

The 3 ways are mutually exclusive, so sum each probability

40
Q

What does
(n)
(k)
mean in terms of probability?

A

n!/((n-k)!k!)

41
Q

How are probability and return period related?

A

If time period = T, probability = 1/T

42
Q

How do you make use of a probability density function in hydraulics?

A

Integrate it between values of concern

43
Q

What are the steps for creating a cumulative probability function?

A
  1. Create a series of yearly maxima over n years
  2. Rank them with 1 as the largest and n the smallest
  3. Cumulative probability is 1-(rank/n)
44
Q

What distribution can often approximate the cumulative probability graph?

A

Gumbel distribution

45
Q

How do you perform the Theisson method for rainfall analysis?

A

Draw lines connecting each rain gauge to all adjacent gauges
Bisect each of these lines and join to form a polygon
Weight gauge’s measurement by area of polygon

46
Q

Why is k used as well as K in Darcy’s Law components?

A

K is dependant on rock and fluid properties
k is only dependant on rock properties

47
Q

What is the probability of k floods in n years?

A

(n)
(k) P^k (1-P)^(n-k)

Hydraulics and Hydrology 4 (3 decks)

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