xvii
14.8Summary
409
following reintroduction of gravel during a process
known as substrate replenishment. The success of the
project relied upon determining the correct bed material
particle size distribution so that the material did not
undergo large-scale removal during the first large flood but
still remained sufficiently mobile so that pools and riffles
would be created. Another concern was that the perme-
ability of the gravels did not result in the complete disap-
pearance of water during low flows; to combat leakage the
gravel was laid down in layers, consolidated and some
finer material added to create less permeable layers. From
these two examples of river restoration the direct and
paramount importance of an understanding of fluvial
geomorphology is evident.
Reflective questions
➤Based on your knowledge of fluvial
geomorphology, do you think you could design a
river to be in equilibrium with a given flood regime
and sediment loading?
➤What are the main problems associated with traditional engineering approaches to river management?
➤Why is morphological diversity in river channels important to the flora and fauna?
14.8 Summary
This chapter has focused on fluvial processes, the linkage
between processes and landforms, and natural and
human-induced channel change. It has also examined the relevance of fluvial geomorphology to the needs of river management in the twenty-first century.
Catchments vary in the amount of water and sediment
they carry. This variation is a function not simply of catch- ment size but also of local topography, land management, geology, soils, vegetation, and so on. Catchments with high drainage densities are likely to have high peak flows and a flashy regime. River channel networks can be described and classified in a number of ways including the stream order systems of Horton and Strahler. The size and shape of river channels can be described in terms of channel cross-section and planform which may vary over short distances. River channel planform ranges from braided and anastomosing to meandering and straight river channels. River cross-sections are not uniform and water and sediment flows through a cross-section vary. At river bends a circulatory pattern of flow (helicoidal flow) is superimposed on the downstream movement of the water.
Water within a channel is subject to gravity and fric-
tional forces. Steep, smooth channels will have a high
average water velocity. A similar channel with a channel
boundary consisting of boulders, and thus high frictional
forces, will have a slower average water velocity, even
though the turbulence of the water may give the impres-
sion of fast-moving water. Mean water velocity in an open
channel can be estimated using the Manning equation
which accounts for channel slope, hydraulic radius and
channel roughness. For a given cross-section the way in
which water velocity, depth and width increase with a rise
in water level is known as hydraulic geometry. Stream
power is a key parameter in determining rates of erosion,
sediment transport and instability. Slight changes in
velocity can significantly affect potential stream power.
For a particle to be entrained from the stream bed or
bank, a threshold has to be passed whereby a critical
velocity or shear stress exceeds the frictional forces that
resist erosion. This is dependent upon channel slope,
particle size and shape, and immersed weight in relation
to the bed shear stress and fluid kinematic viscosity.
However, processes such as imbrication may also play a
role. Deposition and cessation of bed load movement for
an individual particle occur when velocity falls below crit-
ical conditions. Hydraulic sorting occurs under these
conditions. Sediment can be carried as bed load or as
suspended load and bed load transport is almost entirely a
function of flow volume, velocity and turbulence. Particles
roll, slide or saltate along the bed.
Bed morphology can vary depending on bed material
while the nature of a channel’s boundary materials may
➤
Chapter 7Soil and the environment
196
HOW ALUMINIUM
INFLUENCES SOIL ACIDITY
As clay minerals weather and break
down, the aluminium in the octahe-
dral layer is released into the soil
solution, where it either reacts with
water or is adsorbed onto the
exchange sites of negatively
charged clay minerals. Al
3➤
ions are
adsorbed in preference to all the
other major cations. The influence
that aluminium has on soil acidity is
itself dependent on the acidity of the
soil. At pH less than 5, aluminium is
soluble and exists as Al
3➤
. When Al
3➤
enters the soil solution it reacts
with water (it is hydrolysed) to
produce H
➤
ions:
Al
3➤
➤H
2O AlOH
2➤
➤ H
➤
(7.3)
Thus the acidity of the soil
increases (pH falls). In soils with a
pH of between 5 and 6.5, aluminium
also contributes H
➤
ions to the soil
solution but by different mechanisms,
as aluminium can no longer exist as
Al
3➤
ions but is converted to
aluminium hydroxy ions:
Al
3➤
➤OH
AlOH
2➤
(7.4)
AlOH
2➤
➤OH
Al(OH)
2
➤(7.5)
aluminium
hydroxy ions
3
3
3
hydroxy aluminium ions have been
converted to gibbsite ((AlOH)
3), which
is insoluble and cannot be adsorbed
by the negative clay minerals as it has
no charge. The general relationship
between soil pH and the composition
of cations held on the exchange sites
of clay minerals is presented in
Figure 7.22. In a neutral soil the
exchangeable cations that dominate
the cation exchange sites are the
base cations, whereas in an acidic soil
aluminium and hydrogen ions domi-
nate the exchange sites.
These hydroxy aluminium ions act as
exchangeable cations, just like Al
3➤
,
and are adsorbed by the clay
minerals. They are in equilibrium with
hydroxy aluminium ions in the soil
solution, where they produce H
➤
ions
by the following reactions:
AlOH
2➤
➤H
2O Al(OH)
2
➤ ➤ H
➤
(7.6)
Al(OH)
2
➤➤ H
2O Al(OH)
3➤ H
➤
(7.7)
In soils where the pH is above 7,
Ca
2➤
and Mg
2➤
dominate the
exchange sites and most of the
3
3
BOX 7.4
100
75
50
25
0
38 7654
Soil pH
Per cent cation adsorption capacity
Bound H and Al
Exchangeable
H
+
Exchangeable
base-forming cations
(Ca
2+
, Mg
2+
, K
+
, Na
+
)
Al(OH)
n ions
Exchangeable
Al
3+
Figure 7.22Relationship between soil pH and the cations held on the exchange sites of
colloids, including clay minerals. (Source: Brady, N.C. and Weil, R.R., The Nature and
Properties of Soil, 13th Edition. © 2002. Adapted by Permission of Pearson Education,
Inc., Upper Saddle River, NJ)
breakdown, solubility and possible movement from the soil
to surface waters and groundwaters. For example, many
heavy metals become more water soluble under acid condi-
tions and can move down with water through the soil to
aquifers and surface waters (see Chapter 15).
There are 16
essential elementswithout which green
plants cannot grow normally. The availability of these essen-
tial nutrients for plant uptake is greatly influenced by soil
pH (Figure 7.23) as are the number, species and activities of
soil organisms. All 16 essential elements must be present in
the correct proportions as too little or too much of any
element will result in symptoms of nutrient deficiency or
toxicity. On the basis of their concentration in plants they
are divided into
macronutrients(carbon, oxygen, hydrogen,
nitrogen, phosphorus, sulphur, calcium, magnesium, potas-
sium and chloride) and
micronutrients(iron, manganese,
zinc, copper, boron and molybdenum).
Strongly acidic soils (pH 4–5) usually have a low or
reduced supply of the macronutrients, particularly calcium,
magnesium, potassium, nitrogen, phosphorus and sulphur,
Reflective Questions
encourage further thought
about topics under discus
sion
and can be used to consoli-
date learning.
Chapter summariesrecap and reinforce the
key point
s to take away from the chapter. They
also provide a useful revision tool.
Multiple Choice Questions
on the Companion Website give you the chanc
e to check
what you have learned and get instant feedback.
PHYSICAL GEOGRAPHY AND
ENVIRONMENTAL ISSUES IN ACTION
Six categories of boxed features explore and illustrate
topics and conc
epts through real world examples.
Fundamental principlesoffer further explanation of core
concept
s.
Guided tour
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