Hydrology And Fluvial Geomorphology Flashcards
1
Q
What is Evaporation?
A
Liquid changes into water vapour, from puddles and streams.
2
Q
What is Transpiration? (3)
A
1) water drawn up from the soil by the plant
3) water leaves the plant as water vapour through tiny pores on the underside of the leaves, known as stomata.
3) it helps plants absorb nutrients from the soil.
3
Q
What is Potential Evapotranspiration (PET)?
A
The amount of evaporation that would occur if an unlimited water source were available and is important in determining water availability for plants.
4
Q
What is River Discharge? (3)
A
1) water that flows to the sea and moves within channels (streams/rivers)
2) is measured to assess the water flow
3) water enters channels as direct channel precipitation or other flows
5
Q
What increases the rate of evaporation? (2)
A
1) hot, dry and windy conditions
2) conditions with larger soil surface area.
6
Q
What are the outputs of the drainage basin system? (3)
A
1) evapotranspiration (ET)
2) potential evapotranspiration (PET)
3) river discharge
7
Q
How is water discharge calculated?
A
Q = AV measured in m3/sec - cumces
Where:
Q = discharge
A = cross sectional area
V = velocity
8
Q
What are the stores in the drainage basin system? (5)
A
1) interception
2) surface water
3) ground water
4) channel storage
5) soil moisture
9
Q
What is interception in the topic of stores in the drainage basin system?
A
When precipitation is caught and stored by vegetation before it reaches the ground
10
Q
What is surface water in the topic of stores in the drainage basin system?
A
When the infiltration capacity is exceeded, water builds upon the surface
11
Q
What are examples of temporary stores of water?
A
Puddles and turloughs
12
Q
What are examples of permanent stores of water?
A
Lakes and wetlands.
13
Q
What is ground water in the topic of stores in the drainage basin system? (3)
A
1) water that has percolated into bedrock
2) is a store of freshwater (like wells and boreholes)
3) it is accessible if dug below water table
14
Q
In extension to ground water, what is Ground Water Recharge? (2)
A
1) the refilling of rock pores as water moves downwards
2) occurs when rate of recharge is greater than rate of abstraction
15
Q
What is Channel Storage in the topic of stores in the drainage basin system? (2)
A
1) all water stored in rivers streams and drainage channels
2) heavy rainfall caused an increase in channel storage.
16
Q
What is Soil Moisture in the topic of stores in the drainage basin system?
A
Water held sub-surface in soil pores
17
Q
What are the characteristics of sandy soils, in the topic of soil moisture? (3)
A
1) large pores
2) permeable
3) quick infiltration rates
18
Q
What are the characteristics of clays, in the topic of soil moisture?
A
1) hygroscopic (they swell when in contact with water)
2) impermeable
3) unstable.
19
Q
What is Soil Moisture Deficit?
A
Available water being used up.
20
Q
What is Soil Misture Recharge?
A
Precipitation exceeds potential evaporation, allowing for some dry pores to refill.
21
Q
What is Soil Moisture Surplus?
A
When soil is saturated, water cannot enter and so it flows over the surface.
22
Q
What is Soil Mosture Utilisation?
A
Evapotranspiration (and other water uses) exceeds precipitation.
23
Q
What is the Wilting Point in the topic of soil moisture?
A
the range of soil moisture content at which permanent plant wilting occurs.
24
Q
What is the the balance of soil moisture?
A
= Precipitation - (run off + evapotranspiration + change in soil moisture).
1) is an important concept in hydrology to understand water movement in ecosystems
25
What is Throughfall in the topic of the above ground flows in the drainage basin system?
leaves and twigs become saturated so water drips from them where precipitation can also fall through gaps in vegetation cover
26
What is Stemflow in the topic of above ground flows in the drainage basin system?
precipitation is intercepted by vegetation then runs down branches and main trunk
27
What is Field Capacity in the topic of soil moisture?
Amount of water held once excess has drained away (saturation point)
28
What is Overland Flow in the topic of above ground flows in the drainage basin system? (2)
1) when soil is saturated, or precipitation exceeds infiltration rate
2) surface runoff occurs where water flows over the surface
29
What is Hortonian Flow in the topic of above ground flows in the drainage basin system? (2)
1) shallow, laminar, fast moving water
2) causes severe soil erosion when precipitation exceeds the infiltration capacity and depression soil capacity.
30
What is Channel Flow in the topic of above ground flows in the drainage basin system? (2)
1) movement of water in channels such as streams and rivers
2) channel flow of a river carries water downstream.
31
What is Infiltration in the topic of below ground flows in the drainage basin system?
1) Water is absorbed into the soil or seeps into ground slowly
32
What is infiltration capacity?
Maximum rate that precipitation can be absorbed by soil in given conditions and it is inversely proportional to overland runoff
33
What is percolation in the topic of below ground flows in the drainage basin system?
Slow movement of water downwards through the soil into bedrock under gravity
34
What is throughflow in the topic of below ground flows in the drainage basin system?
Water flows horizontally through soil in natural pipes or pores above the bedrock.
35
What are the factor influencing infiltration capacity? (5)
1) rainfall duration
2) antecedent soil moisture
3) porosity
4) slope angle
5) vegetation.
36
What 2 things does the rate of percolation depend on?
bedrock permeability And bedrock porosity
37
What else is true about certain stones in the topic of percolation? (2)
1) percolation is fast in Carboniferous limestone
2) chalk and sandstone are porous meaning they allow space for water to percolate
38
What is Groundwater in the topic of below ground flows in the drainage basin system? (2)
1) water that has infiltrated the ground, entered the phreatic zone and discharged into the channel
2) is an important source of drinking water for many communities
39
What is the Phreatic zone in the topic of groundwater? (2)
1) part of an aquifer below the water table where all pores are permanently saturated
2) the aquifer in this zone are permeable rocks and sediments that can hold groundwater or allow the water to pass through
40
What is the Baseflow in the topic of the below ground flows in the drainage basin system? (5)
1) where groundwater seeps into the river's bed and contributes to discharge. 2) very slow transfer from bedrock
3) very deep throughflow.
4) process takes anywhere between several months to a couple of years
5) is a crucial component of a river's flow pattern
41
What is the water table in the topic of underground water?
It is the upper layer above the phreatic zone
42
What is the pattern of the water table in the topic on underground water?
It rises and falls depending on the amount of rainfall percolating downwards and the amount of base flow there is at the lower rocks
43
What is the aeration zone in the topic of the water table?
The area of a larger aquifer above the water table where pore spaces among soil particles and rock formations are filled with air
44
What is the condition of aeration zone?
It is seasonally wetted and seasonally dried
45
What are the 4 ways groundwater recharge can occur?
1) infiltration from precipitation
2) seepage through banks/bed of rivers, lakes, puddles and ditches
3) leakage and inflow from adjacent rocks and aquifers
4) artificially through irrigation and reservoirs
46
What are 4 ways groundwater losses can occur?
1) evapotranspiration mainly in low areas
2) natural discharge from seepage and spring flow
3) leakage and outflow from aquifers into aquicludes (areas that hold water but do not release it to supply springs or wells)
4) artificial abstraction (removing)
47
What are aquifers in the topic of underground water? (3)
1) permeable rocks like limestone, chalk and sandstone that contain significant quantities of water
2) water inside moves slowly
3) streamflow is maintained by absorbing and releasing water in dry/wet periods
48
What are springs in the topic of underground water? (3)
1) formed by water flow reaches the surface
2) can be substantial enough to become a source
3) usually located where percolating water reaches an impermeable layer or saturated zone
49
What is the water budget equation in the topic of underground water?
S = P - Q - E
Where:
S = soil storage
P = precipitation
Q = channel flow
E = evapotranspiration
50
What is a hydrograph? (3)
1) Plots river discharge against time
2) shows the river’s pattern
3) used to understand nature of the drainage basin and factors that affect discharge
51
What are the hydrograph components?
The annual hydrograph (river’s pattern) and the storm hydrograph
52
What is a part of the annual hydrograph (river pattern) as a component of the hydrograph?
1) it studies the responses of the river to its environment
2) highlights seasonal characteristics of river
3) biggest influencer is usually climate
53
What is a part of the storm hydrograph as a component of the hydrograph?
It shows the variations of river discharge over a short period of time where both the discharge and rainfall is on the y-axis
54
What is the cumecs as a part of the storm hydrograph?
Is the unit of measure for discharge involving cubic metres per second
55
What is the approach segment as a part of the storm hydrograph?
The discharge prior to storm
56
What is the rising limb as a part of the storm hydrograph?
Shows quick rise in discharge
57
What is the bank full discharge as a part of the storm hydrograph?
When the channel is full and any further increase in discharge results in a flood
58
What is the peak discharge as a part of the storm hydrograph?
Is the maximum river discharge
59
What is lag time as a part of the storm hydrograph?
The time between maximum rainfall and maximum discharge
60
What is the receding limb as a part of the storm hydrograph?
It is the less steep than the rising limb and shows the discharge decline after peak discharge
61
What is the storm flow as a part of the storm hydrograph?
Is the stream discharge after a rainstorm
62
What is the quick flow as a part of the storm hydrograph?
When the surface runoff reaches the channel quickly
63
What are the 5 steps for the storm process?
Step 1: rain falls on drainage basin in large amounts
Step 2: overland flow occurs as precipitation is greater than infiltration rate thus the rising limb builds to peak
Step 3: after a few hours, overland flow reduces and stops
Step 4: through flow then contributes to discharge and stops floodwaters going down as quickly as they rose
Step 5: base flow takes over and goes back to pre-flood state
64
What are the drainage basin characteristics?
1) size and shape
2) drainage density
3) soil porosity
4) rock type
5) slopes
6) vegetation type
7) land use
65
How does the characteristics size and shape affect the drainage basin? (2)
1) small basins respond quicker thus lag time is reduced
2) river channels in circular basins respond quicker than those in linear basins
66
How does the characteristic drainage density affect the drainage basin? (2)
1) low drainage density causes a long lag time since water has a few paths to take
2) a dendritic (tree like pattern) has a higher density thus it has increased discharge response, greater flood risk and reduced lag time
67
How does the characteristics soil porosity and permeability affect the drainage basin? (3)
1) impermeable surfaces cause greater peak flows due to more overland flow
2) chalks and gravel allow infiltration and percolation
3) clay soils don’t allow infiltration and percolation
68
How does the characteristic rock type affect the drainage basin? (2)
1) impermeable rocks produce a flashier response, lesser lag times and high peak discharge
2) limestone hardly produces a storm hydrograph
69
How does the characteristic slopes affect the drainage basin?
Steeper slope creates more overland thus there is short lag time and higher peak flows
70
How does the characteristic vegetation type affect the drainage basin?
The dense forest vegetation intercepts more thus there is reduced flood response and the effect is the opposite in winter
71
How does the characteristic land use affect the drainage basin?
1) creation of impermeable surfaces (urbanisation) or deforestation increases overland flow
2) increasing drainage density (drains) carries water to rivers quicker thus peak flow increases and lag time decreases
72
What is higher downstream than upstream regarding the topic on channel processes? (5)
1) discharge
2) occupied channel width
3) channel depth
4) average velocity
5) load quantity
73
What is higher upstream than downstream regarding the topic of channel processes? (3)
1) load particle size
2) channel bed roughness
3) slope angle (gradient)
74
What different types of erosion are there? (6)
1) abrasion
2) corrasion
3) solution
4) hydraulic action
5) cavitation
6) attrition
75
What is the process of abrasion? (4)
1) riverbed and bank eroded by river’s load
2) is a mechanical impact produced by debris rubbing on river’s sides
3) it increases with velocity
4) effectiveness depends on energy, hardness and concentration of particles
76
What is the process of corrasion?
The erosive action of particles carried by the river
77
What is the process of solution? (2)
1) the dissolving of rocks (calcium heavy rocks) by removing chemical ions
2) maximum rate is fast flowing, unsaturated streams pass over soluble rocks
78
What is the process of hydraulic action? (3)
1) direct force of air and water on river banks causes chunks of banks to break away
2) eddies in the water compress water into bank cracks
3) the explosion of air bubbles cause cracks to weaken
79
What is the process of cavitation? (3)
1) force of air bubbles exploding
2) with acceleration pressure drops in fluids thus air bubbles to form
3) and bubbles explode and produce tiny water jets cutting at rock
80
What is the process of attrition? (2)
1) collision of sediments that were both particles
2) smaller rounder particles are produced
81
What are the four different load transports?
1) traction
2) saltation
3) suspension
4) solution
82
What affects the rate of erosion? (6)
1) amount and weight of load
2) velocity
3) gradient
4) hardness of geology
5) ph level
6) human impacts increasing erosion
83
What is the process of traction? (2)
1) large particles of rolled along the river by force of water
2) large particles all/most of time on the riverbed
84
What is the process of saltation? (3)
1) gravel and small stones hop along riverbed
2) fast eddy picks the small stones up
3) slower eddy cause stones to fall back down
85
What is the process of suspension? (2)
1) silts and clays are held up by water
2) there is a cloudy appearance in the water especially close to the mouth
86
What is the process of solution?
Calcareous rocks are dissolved
87
How does load vary?
With velocity and discharge
88
When is the load calculated?
When the bank is full
89
What is capacity in the topic of load transport?
The greatest amount of load that can be carried
90
What is competence in the topic of load transport?
The diameter of the largest particle that can be transported
91
What is a part of the Hjulstrom curve? (3)
1) deposition
2) sedimentation
3) flocculation
92
What is deposition?
Occurs when there is a reduction in energy often in river mouths, estuaries and deltas
93
What causes energy to reduced as part of the topic of deposition? (5)
1) gradient reduces
2) friction increases
3) load increases
4) water volume decreases
5) water flows on the side of a meander
94
What is sedimentation?
Occurs when sediment is dropped from still water
95
What is flocculation? (2)
1) where charged ions in sea water allow clay particles to group and settle
2) it leads to the development of mudbanks where water becomes brackish close to the sea
96
What is critical erosion velocity?
The lowest velocity need to pickup particles that are small enough to carry (entrainment)
97
What is mean settling velocity?
The velocity needed to drop particles from suspension
98
What is true about clays regarding the Hjulstrom curve?
1) they have high entrainment values due to their cohesive nature
2) they remain in suspension if velocity is 0
99
What is true about gravel regarding the Hjulstrom curve?
It has a high entrainment value due to their weight
100
What is true about sands regarding the Hjulstrom curve?
They are easier to pick up
101
What is true about velocity regarding the Hjulstrom curve?
Velocity for transport is always less than the velocity required pick up (entrain) the particle
102
What is true about velocity regarding the Hjulstrom curve?
The velocity for transport is always less than the velocity required to pick up (entrain) the particle
103
What is true about velocity and friction in the topic of river flow?
Velocity is affected more by friction than gradient
104
What are the different ways we can measure friction? (2)
1) bed roughness
2) hydraulic radius
105
How do we use bed roughnesses to measure friction? (2)
1) the equation is:
N = R to the power of 2/3 x S to the power 1/2 / V
Where:
R = hydraulic radius
S = channel gradient
N = friction
2) the higher the value of friction, the rougher the riverbed
106
What is the thalweg as a part of the topic on river flow?
The imaginary line of the fastest velocity down a stream
107
What is laminar flow? (3)
1) water flows in sheets parallel to the riverbed
2) no eddies or meanders
3) common on smooth surfaces
108
What is turbulent flow? (4)
1) water closest to bed/banks is slowed by friction and is overtaken by thalweg
2) turbulence created
3) water closest to banks eddies towards the banks
4) water close to bed eddies towards bed
109
What is helicoidal flow? (2)
1) horizontal turbulence produces a corkscrew motion
2) thalweg moves laterally from bank to bank and also vertically from surface to bed during one rotation
110
What are the 3 different channel types?
1) straight channel
2) braided channel
3) meandering channel
111
What does a straight channel consist of? (3)
1) has a sinuosity (ability to curve easily/flexibly) of smaller than 1.5 (sinuosity of 1 is perfectly straight)
2) is rare because thalweg will still move from side to side due to helicoidal flow
3) normally artificial
112
What does a braided channel consist of? (4)
1) channel is divided by islands or bars
2) islands are vegetated but bars are unstable and not vegetated
3) when discharge is reducing (hence velocity), sediment is plentiful and bars form (goes from coarse to fine sediment)
4) with reduced discharge, river must split to go around the bar
113
What are braided channels formed with? (4)
1) steep gradient
2) coarse material
3) easily erode-able bank
4) highly variable discharge
114
What does a meandering channel consist of?
channel slope, discharge, helicoidal flow and load combine to a situation where lateral erosion causes meandering and is not a result of obstacles
115
What are the different landforms that can be identified? (13)
1) meanders
2) river cliffs
3) point bars
4) oxbow lakes
5) pools
6) riffles
7) waterfalls
8) gorges
9) potholes
10) bluffs
11) floodplains
12) levees
13) deltas
116
What are meanders?
a pronounced bend in the course of a river
117
How do meanders form? (3)
1) pools and riffles cause the thalweg to deflect
2) where the thalweg is at its fastest, erosion occurs and deposition is at its slowest
3) over time, bend in the river is created
118
What is a river cliff as a part of the topic on meanders?
Is the steep side on the outside of a meander bank where erosion is at its strongest and downwards
119
What are point bars as a part of the topic on meanders?
Are deposits of sediment on the inside of the meander bend where the thalweg is at its slowest and rising
120
What is a oxbow lake in the topic of meanders?
A narrowing of the bend neck caused by erosion with the thalweg
121
How do oxbow lakes form? (3)
1) after flood event, neck of river is breached
2) meander is cut off with more deposition
3) oxbow is created
122
What are pools?
Deep sections developed where erosion dominates
123
What are riffles?
Shallower sections of the riverbed where sediment has been deposited therefore there is low velocity and turbulent flow
124
What do pools consist of? (3)
1) high velocity
2) dominant laminar flow
3) negative gradient
125
What do riffles consist of? (3)
1) low velocity
2) turbulent flow
3) steep positive relief gradient
126
What is true about pools and riffles alike? (2)
1) Once pools and riffles have developed, there is a helicoidal flow that forms due to the regular spacing
2) the different gradients in pools and riffles creates variations in subcritical flow thus erosion or deposition occurs
127
What is a waterfall?
a river or other body of water's steep fall over a rocky ledge into a plunge pool below
128
What is one way waterfalls form?
Key word: resistant rock (3)
1) river spills over gradient change
2) more resistant rock is on top of less resistant rock
3) splashback undercuts rocks by processes of abrasion
129
What is another way waterfalls form?
Key word: rejuvenation (3)
1) by rejuvenation where a Knick point is present
2) plunge pool removes support for overhanging so collapses
3) causes upstream migration (water moves up)
130
What is a gorge?
A deep, steep sided valley caused by water fall retreat
131
How do potholes form? (3)
1) turbulence swirls pebbles around a depression in the river’s bed
2) sides widened and deepen as pebble erodes the cavity
3) eddying get initiated
132
How do potholes form? (3)
1) turbulence swirls pebbles around a depression in the river’s bed
2) sides widened and deepen as pebble erodes the cavity
3) eddying get initiated
133
What are rapids?
It is an upper course feature with a steep gradient where the riverbed is rocky and irregular and turbulent flow is present
134
What are bluffs?
Old floodplains that erode and leave terraces
135
How do bluffs forms?
The meanders erode the edge of terrace which creates a line of steep slopes
136
What is a floodplain?
Flat land made up of alluvium (loose clay, silt, sand or gravel deposited by running water) next to the river where it rises during the floods since fine silt is deposited
137
What else happens during flood events regarding floodplains?
Back-swamps can be created and the riverbed can raise if discharge is low and sediment is deposited
138
How do levees form?
1) banks burst following a flood event and wetted perimeter increases
2) increased friction reduces velocity
3) coarse material is deposited first around banks
4) finer material deposited second moving across the flood plain
139
How do deltas form? (5)
1) sediment is deposited where the river meets a standing body of water due to loss of energy
2) clay particles flocculate and deposit
3) bottomset beds (fine materials built out by turbidity currents) form first
4) foreset beds (coarse material carried seaward by rolling/saltation) form second
5) topset beds (fine material built by distributaries) form last
140
What 3 different deltas can be formed?
1) arcuate
2) cuspate
3) bird’s foot
141
What is an arcuate delta?
A fan shaped delta where longshore drift occurs
142
What is a cuspate delta?
A pointed delta formed by two opposing currents
143
What is a bird’s foot delta?
They are formed when still sea allows each distributary to build in any direction
144
What are the different catchment flow modifications?
1) deforestation
2) afforestation
3) urbanization
4) grazing
5) abstraction
6) channelization
7) reductions in industrial activity
8) water storage
145
How does deforestation impact the catchment flow modifications? (5)
1) reduces evapotranspiration
2) increases surface runoff
3) reduces lag time
4) less surface storage
5) high peak discharge
146
How does urbanization impact the catchment flow modifications? (5)
1) creation of impermeable surface reduces infiltration and increases overland flow
2) sewage systems and storm drains get water to main channel quicker than throughflow
3) lag time reduces
4) flood peaks increase
5) building on floodplains reduces flood space thus flood waters rise higher
147
How does grazing affect the catchment flow modifications? (3)
1) ploughing increases infiltration
Contrast:
1) heavy machinery causes soil compaction (soil particles press together, reducing pore space) thus reduced infiltration results in higher peak discharge
2) less evapotranspiration occurs than forested area
3) water logging/salination occurs with poor drainage
148
How does channelisation affect the catchment flow modifications? (3)
1) increases hydraulic radius of a channel
2) shorter lag time
3) higher flood peaks
149
How does over abstraction impact the catchment flow modifications? (3)
1) drying up of rivers
2) falling water tables
3) saltwater intrusion in coastal areas
150
How do reductions in industrial activity impact the catchment flow modifications? (5)
1) old springs re-emerge
2) surface water flooding
3) basements flood
4) leakage into tunnels
5) reduced slope stability
151
How does water storage impact the catchment flow modifications?
Building dams helps with flood/drought control, irrigation and hydroelectric power however they can lose water, salinization occurs and ground water changes
152
What are the physical causes of river floods? (6)
1) heavy, persistent rainfall (deep weather depressions)
2) rapidly melting snow and ice
3) impermeable soil and bedrock
4) coastal storm urges
5) lack of vegetation
6) disaster (natural or dam failure)
153
What are the human causes of river floods? (5)
1) urbanization like impermeable surfaces, storm drains, channel restrictions from bridges
2) floodplain developments increase risk
3) engineering that obstructs the channel
4) mechanized farming
5) poor/inappropriate farming practices
154
What are the impacts of river floods? (3)
1) deaths, damage and disruption
2) death toll higher in LICs
3) costs higher in HICs
155
What are the two flood prediction methods?
1) recurrence interval
2) flood risk maps
156
What is recurrence interval as a part of flood prediction?
Is how often, on average a flood of a certain size is likely to occur
157
What else is true about the recurrence interval? (3)
1) a 100 year flood is one expected to occur every 100 years on average
2) plotting a graph of flood magnitude against recurrence interval shows when a certain size flood is likely to occur (best fit line used)
3) flood history in area can be checked
158
What are flood risk maps?
they show where the river is likely to flood depending on if flood risk is severe or moderate
Severe = 1 - 75 years
Moderate = 76 - 200 years
159
Which areas are at most risk for floods? (4)
1) low - lying parts of active floodplains
2) small basins subject to flash floods
3) areas below unsafe dams
4) low - lying inland shorelines
160
What are the different flood prevention methods? (6)
1) forecasting and warning
2) loss sharing
3) hard engineering
4) hazard resistant design
5) land use zoning
6) soft engineering
161
What is forecasting and warning as a method of flood prevention? (6)
1) use of weather satellites
2) have an emergency plan
3) radio/internet communication
4) rain gauges
5) river discharge gauges
6) computer models that compare new data with history
162
What is loss sharing as a method of flood prevention?
Disaster aid and insurance
163
What is hard engineering as a method of flood prevention? (5)
1) work against natural processes
2) construct dams and leaves
3) straighten the channel
4) creaste reservoirs
5) build diversion spillways
Contrast:
Normally fixes local problem but causes more up/downstream
164
What is hazard resistant design as a method of flood prevention? (4)
1) adjust buildings to reduce losses
2) sandbags and seal door/windows are used
3) moving off lower floors
4) flood gates get installed on individual houses, example, Yarm on the River Tees
165
What is land use zoning as a method of flood prevention?
By moving or avoiding to build on flood prone areas and allowing flooding to happen on the floodplain
166
What is soft engineering as a method of flood prevention?
1) working with natural processes
2) flood abatement (flood water movement management) decreases amount of run-off
3) afforestation
4) contour ploughing
5) removing sediment
6) flood diversion - allowing areas to be flooded and not built on
167
What is appropriate floodplain use as a part of soft engineering? (4)
1) working from the channel outwards
2) protect wetlands, rough grazing land (where animals can be easily removed from)
3) protect parks and leisure areas
4) protect houses and critical buildings such as hospitals
