Lesson 6: Soil moisture and groundwater recharge Flashcards
(88 cards)
1
Q
how is soil formed?
A
in situ weathering
2
Q
difference of soil v sediment
A
soil is formed from in situ weathering while sediments have been transported from place of weathering
3
Q
smaller grains bounded by organic matter forming larger units
A
aggregates / peds
4
Q
determined by the distribution of the size fractions of mineral grains present
A
soil texture
5
Q
blanket of unconsolidated loose deposit covering solid rock
A
regolith
6
Q
how many textural classes are there based on the percentage of sand silt and clay?
A
12 classes
7
Q
why is texture important in pedology?
A
because it helps determine the drainage and fertility characteristics of a soil
8
Q
the percent of void space
A
porosity
9
Q
the ratio of the volume of the voids to the volume of the solids
A
void ratio of soil
10
Q
the mass of the contained water divided by the mass of the solid particles (dry mass of soil).
A
gravimetric water content of soil
11
Q
formula for gravimetric water content
A
φg = 100 (Ww/Ws)
φg -the gravimetric water content (percentage)
Ww -is the mass of the water in the soil (g or kg)
Ws -is the mass of the solid particles (g or kg)
12
Q
symbol for gravimetric water content
A
φg
13
Q
symbol for volumetric water content
A
φv
14
Q
symbol for saturation ratio
A
Rs
15
Q
symbol for dry bulk density
A
pb
16
Q
symbol for particle density
A
pm
17
Q
is the volume of the contained water (Vw) divided by the total volume of the soil (V)
A
volumetric water content
18
Q
is the volume of the contained water (Vw) divided by the volume of the voids (Vv).
A
saturation ratio
19
Q
is the dry mass of the soil particles (Ws) divided by the volume of the sample (V).
A
dry bulk density
20
Q
is the mass of the mineral particles of the
soil (Ws) divided by the volume of the soil particles (Vs)
A
particle density
21
Q
how can soil moisture be measured
A
indirectly by nondestructible means
-one method involves burying small blocks in which electrodes are embedded, and then
- passing an electrical current through the wire.
22
Q
small blocks
A
resistance cells
23
Q
is the depression or the rise of a liquid in a small capillary passage tube having small cross sectional area such as openings in the porous materials.
A
capillarity
24
Q
is the subsurface layer in which groundwater seeps up from a water table by capillary action to fill pores.
A
capillary fringe
25
Pores at the base of the capillary fringe are filled with water due to _______.
tension saturation
26
Capillarity results from 2 force, what are they?
cohesion and adhesion
27
mutual attraction between water molecules
cohesion
28
molecular attraction between water and different soil materials
adhesion
29
effect of cohesion and adhesion
As consequence of these forces, water will rise in small diameter glass tubes to a height above the water level in a large container
30
relate the tube diameter to the rise of water column
the smaller the tube diameter, the greater the rise of water column
31
can provide a means of direct evaporation of groundwater if the water table is close enough to the surface that the it reaches the ground surface.
capillary fringe
32
measuring instrument used to determine water potential (soil moisture tension) in the vadose zone.
Tensiometer
33
describe the tensiometer as an instrument
Device typically consists of glass or plastic tube with a porous ceramic cup and filled with water
34
water in the vadose zone available to growing plants, found in belt of soil water
soil water
35
soil water scope?
Zone extends from land surface to depth of plant roots
36
T or F: top of soil water may be below the belt of capillary fringe.
False. Top of CAPILLARY FRINGE may be below the belt of SOIL WATER
37
Soil moisture at a location varies with changes in ________ and ________.
changes in:
-the amount of precipitation
-evapotranspiration.
38
evapotranspiration is aka?
soil water evaporation
39
when soil moisture content of a layer reaches to the point at which the force of gravity acting on the water is equal to the surface tension, gravity drainage cease.
Field capacity
40
what 3 factors does field capacity depend on?
1. the specific retention
2. evaporation depth
3. unsaturated permeability characteristics curve of the soil
41
happens when the remaining moisture is too tightly bound to the soil particles
wilting point
42
texture with highest wilting point, lowest?
clay and sand respectively
43
Capillarity fringe ______ as groundwater is being recharged
rises
44
Time of movement of infiltrating water depends on what 2+ factors?
1. thickness of unsaturated zone and
2. the vertical unsaturated hydraulic conductivity
also topographic conditions and geology
45
Humid vs Arid regions in terms of recharge rates
arid regions have less recharge rates than humid regions
46
average annual rainfall of these areas:
1. siargao
2. davao city
3. arid regions
1. more than 2000mm/year
2. 1000-1500mm/year
3. less than 300mm/year
47
what is the relationship of rates of evapotraspiration and water table level?
higher rates of evapotranspiration means a lower water table, usually in warm areas
48
what other factors affect recharge rates
hydraulic conductivity of upper layers, precipitation, level of water table, thickness of zone of aeration
49
Some slopes are inherently _______ than others and are therefore more prone to mass wasting
LESS STABLE
50
As the slope increases, so too does the _______ component acting _______ to the slope, which, in turn, allows more of the rock’s weight to be directed in the ________ direction. When gravitational force becomes _______ than the _________, ___________ can occur.
As the slope increases, so too does the GRAVITATIONAL component acting PARALLEL to the slope, which, in turn, allows more of the rock’s weight to be directed in the DOWNSLOPE direction. When gravitational force becomes GREATER than the FRICTIONAL FORCES, DOWNSLOPE MOVEMENT can occur.
51
4 factors that may trigger mass wasting
1. slope (steepness)
2. nature of slope material: angle of repose, oversteepened slopes
3. climate
4. vegetation
5. earthquakes
6. volcanic activity
52
example of rocks that are inherently so strong and homogeneous that they are able to form stable cliffs
granitic rocks of Yosemite Valley, California.
53
explain how loose sediments are more prone to mass wasting that hard lithologies
frictional forces within loose or unconsolidated sediments are usually lower than solid rock, making sediment more prone to mass wasting and less able to form vertical slopes.
54
is the steepest angle at which a sloping surface of
unconsolidated sediments is stable
angle of repose
55
typical range of angle of repose
typically less than 35 degrees
56
Large angular fragments generate greater _________, and therefore are capable of ________ compared to _________.
Large angular fragments generate greater FRICTIONAL FORCES, and therefore are capable of MAINTAINING STEEPER SLOPES compared to SMALL, WELL-ROUNDED SEDIMENTS.
57
Slippage can occur along ________, ______, ______ and _______ that are inclined in ________ direction as the slope
bedding planes, foliation planes, fractures and fault surfaces inclined in the SAME DIRECTION as the slope
58
On _______ hillsides the component of ______ operating the direction of slope is _______, which, in turn, _______ the potential for _________.
On STEEPER hillsides the component of GRAVITY operating the direction of slope is GREATER, which, in turn, INCREASES the potential for MASS WASTING.
59
example of how natural oversteepened slopes can cause mass wasting
An common example:
- the undercutting of stream banks
- due to the natural migration of stream channels
= results to destabilizing/overhanging of riverbanks.
= River valleys become wider over time
60
undercut stream banks
cutbanks
61
example of how human induced oversteepened slopes can cause mass wasting
flat surfaces are required for the construction of roads, buildings, and parking lots
- means that material must be excavated from hillsides
- in order to create a leveled surface
= weakens the slope significantly
= greatly increases the potential for mass wasting
62
The long-term average weather for a region is defined as
Climate
63
2 reasons why climate is an important factor in slope stability
1. because it ultimately determines how and when precipitation falls.
2. determines the types of vegetation we see blanketing the various slopes
= influences the fraction of rain or snow that infiltrates into the subsurface
64
describe how vegetation can affect slopes
1. fairly dense vegetation tends to stabilize slopes plant roots
- help bind together loose particles of rock and sediment.
2. during unusually large rainstorms or rapid snowmelts, dense vegetation will increase infiltration ( since it reduces the ability of surface water to move downslope)
- excessive infiltration adds significant weight to a slope
- reduces friction through higher pore pressured
65
2 ways seismic waves affect slopes
1. as the seismic waves pass along the surface,
= the least stable slopes will tend to fail
BECAUSE the ground vibrations suddenly reduce the frictional forces within the slope materials
2. seismic waves may also cause surface materials to liquefy
= immediately destabilize a slope
=trigger a mass wasting event
66
how does rain/snow affect cause mass wasting
1. Rain or melting snow will infiltrate
= cause subsurface void spaces to become saturated
= weight of the water in the saturated zone causes the fluid/pore pressure within the voids to increase
= reduces the friction between the solids
= Downslope movement occurs when the frictional forces
become less than the gravitational force in the slope direction
67
what are the 4 types of causes of mass wasting?
1. geological
2. morphological
3. physical
4. human
68
geological causes of mass wasting
Weak material
* Sensitive material
* Weathered material
* Sheared material
* Jointed or fissured material
* Adversely oriented mass discontinuity (bedding, schistosity,
etc.)
* Adversely oriented structural discontinuity (fault, unconformity etc.)
* Contrast in permeability
* Contrast in stiffness (stiff, dense material over plastic material)
69
morphological causes of mass wasting
*Tectonic or volcanic uplift
* Glacial rebound
* Erosion of slope toe
* Erosion of lateral margins
* Subterranean erosion (solution, piping)
* Deposition of load on the slope or its crest
* Vegetation removal
70
physical causes of mass wasting
*Intense rainfall/ Prolonged exceptional precipitation
* Rapid snow melt/ Thawing/ Freeze-and-thaw weathering
* Earthquake/ Volcanic eruption
71
human causes of mass wasting
*Excavation of the slope or its toe
* Deposition of load on the slope or its crest
* Drawdown (of reservoirs)
* Deforestation
* Irrigation
* Mining
* Artificial vibration
* Water leakage from utilities
72
how to identify landslides: 6 indications
1. Vegetation cover: intact/disrupted
2. Presence of cracks or scars
3. Disturbance of the drainage pattern
4. Excessive soil moisture or abrupt soils moisture changes. 5. Terrain form: hummocky, niche-lobe sequence
6. if there is growing or tilting of trees, and electrical posts
73
what are the 9 morphological diagnostics of mass wasting?
1. concave-convex
2.semicircular niches
3.back tilting of slope faces
4.hummocky relief
5. formation of cracks
6.steepening of slopes
7.vegetational clearances on steep scarps
8. irregular linear clearances along the slope
74
is associated with landslide niches and associated deposits.
The sequence of a concave and a convex part of the slope
75
is related to retrogressive sliding
A step-like morphology
76
are associated with the head part of a slide with the outcrop of the failure plane
semicircular niches
77
how are semicircular niches recognized on photographs?
as a light toned scarp, with small light curved lineament
78
indicates rotational movement of slide blocks and appears as oval or elongated depressions with imperfect drainage conditions
Back tilting of slope faces
79
Irregular slope morphology. Micro-relief associated with shallow movements or small retrogressive slide blocks
hummocky relief
80
how is hummocky relief recognized in aerial photos?
the coarse surface texture of hummocky terrains stand
in contrast with smooth surroundings
81
an indication for recent mass wasting activity
The formation of new cracks
82
occurs with sliding and toppling movement
crack formations
83
how do crack formations appear?
They appear as lineaments more or less parallel to the existing scar
84
can indicate the presence of a landslide scar. This type of slope anomaly is clearly visible on aerial photographs
steepening of slopes
85
coincides with morphological steps. Headscarps and steps in a slide body often show this
Vegetational clearances on steep scarps
86
how do vegetational clearances on steep scarps show on photos?
On photographs this shows as light-toned elongated areas at the crown or on the body of the mass movement.
87
what causes the irregular linear clearances along the slopes?
1. Flows and avalanches strip parts of the slope from vegetation; leaving the flowpath uncovered.
2. Slip surface of translational slides as well are usually uncovered.
88
what do irregular linear clearances along the slopes look like?
The denudated areas are showing light tones, often with linear pattern in direction of movement
