Hydrology

Question 1

How is hydrology different from hydraulics?

Answer 1

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

Question 2

What is interflow?

Answer 2

Lateral movement of water within unsaturated soil

Question 3

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

Answer 3

Precipitation
Surface water inflow
Groundwater inflow

Question 4

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

Answer 4

Surface water outflow
Groundwater outflow
Evapo-transpiration

Question 5

How is a catchment defined?

Answer 5

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

Question 6

How is flow direction algorithmically found?

Answer 6

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

Question 7

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

Answer 7

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

Question 8

What are the main two ways of measuring precipitation?

Answer 8

Storage gauges
Tipping bucket recorders

Question 9

Where should a rain gauge be sited?

Answer 9

Level ground
Partially sheltered
Representative of the average for the area

Question 10

What is the name of a graph of rainfall intensity?

Answer 10

A hyetograph

Question 11

How does the isohyet method map rainfall?

Answer 11

By drawing contours on a map according to recorded rainfall data

Question 12

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

Answer 12

Very simple
Very inaccurate

Question 13

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

Answer 13

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

Question 14

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

Answer 14

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

Question 15

What 3 factors affect the rate of evaporation?

Answer 15

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

Question 16

How is the evaporation rate defined (equation)?

Answer 16

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

Question 17

What is Tp?

Answer 17

The dew point

Question 18

What is lv?

Answer 18

Latent heat of vaporisation

Question 19

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

Answer 19

Er = Rn / lv * ρw

Question 20

What is Rn?

Answer 20

Net radiation

Question 21

What is a lysimeter?

Answer 21

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

Question 22

What are the two ways of modelling evapotranspiration?

Answer 22

Not limited by vapour transport rate
Not limited by energy supply

Question 23

How is porosity (ω) defined?

Answer 23

Volume of void space / volume of rock

Question 24

How is porosity of a large rock sample determined?

Answer 24

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

Question 25

How does the formation of igneous rock affect porosity?

Answer 25

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

Question 26

How is effective porosity different from regular porosity?

Answer 26

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

Question 27

What is the equation of effective porosity?

Answer 27

Volume of through-flowing water / volume of rock

Question 28

How do you measure rock volume not including voids?

Answer 28

Crush it into a powder

Question 29

What is recharge water?

Answer 29

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

Question 30

How is piezometric head expressed?

Answer 30

h = p/ρg + z

Question 31

How is pressure head expressed?

Answer 31

H = p/ρg

Question 32

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

Answer 32

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

Question 33

What is Darcy's Law for saturated flow?

Answer 33

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

Question 34

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

Answer 34

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

Question 35

Define the 4 "Aqui-" terms

Answer 35

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

Question 36

How can unsustainable use of aquifers affect the local environment?

Answer 36

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

Question 37

What is the probability of A & B occuring?

Answer 37

P(A) * P(B)

Question 38

What is the probability of A or B occuring?

Answer 38

P(A) + P(B)

Question 39

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?

Answer 39

The 3 ways are mutually exclusive, so sum each probability

Question 40

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

Answer 40

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

Question 41

How are probability and return period related?

Answer 41

If time period = T, probability = 1/T

Question 42

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

Answer 42

Integrate it between values of concern

Question 43

What are the steps for creating a cumulative probability function?

Answer 43

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)

Question 44

What distribution can often approximate the cumulative probability graph?

Answer 44

Gumbel distribution

Question 45

How do you perform the Theisson method for rainfall analysis?

Answer 45

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

Question 46

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

Answer 46

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

Question 47

What is the probability of k floods in n years?

Answer 47

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