Floodplain channels: their distribution, morphology and formation
Catherine E. Millington and Dr David A. Sear
[email protected]
1. Introduction & study area
[email protected]
4. Formation
3. Morphology
Within forested floodplains geomorphological processes are strongly modified by live and dead vegetation. However, interactions
between the vegetation, water and sediments are poorly understood. Few remaining natural floodplain forests exist in Britain in
which to observe these interactions, although some reaches of rivers in the New Forest, southern England (Fig 1), remain seminatural. Furthermore, other reaches which were channelised in the 1800s and the 1960s have recently been restored under the
LIFE III restoration programme (see www.newforestlife.org.uk).
The rationale for this research based in the New
Forest is that a greater understanding, and the
development of a conceptual model of how
geomorphological processes operate on seminatural forested floodplains will provide a guideline
for what processes to expect to occur on the
floodplain of successfully restored reaches. The
conceptual model can be adapted and further
developed through monitoring the processes that
actually occur post restoration.
~
a) Floodplain channels in relation to the main channel
Observations made along semi-natural
reaches suggest the presence of particular
suites of landforms on the floodplain, in
particular floodplain channels, that owe their
formation to interactions between the
vegetation, water and sediments. This
research focuses on the distribution,
morphology and formation of such floodplain
channels.
Reach 1
Woody vegetation
Debris dams
Debris dam
LWD
residence
time
Overbank flow
Tree throw
Roots
line of
floodplain
channel
profile
2. Distribution
a) Catchment scale
Debris dams
Reach 3
Figure 1 Location of study area
b) Reach scale
From field observations and detailed maps constructed from tape
measurements, four different types of floodplain channel
distribution were observed at the reach scale. The first type may
occur in the absence of debris dams, but the other three have
only been observed in the presence of in-channel, hydraulically
effective (or ‘active’ Gregory et al., 1985) debris dams.
Sediment
deposition
N
20
m
Reach 2
Channel
morphology
Floodplain
surface
morphology
line of
floodplain
channel
profile
0
The distribution of floodplain channels was recorded on a
1:10,000 base map during a walk-through survey of the
catchment. The stream was split into reaches based on
changes in geomorphological character, and the presence
or absence of floodplain channels was recorded in each
reach. The results were displayed in ArcView (Figure 2)
Main channel
Floodplain
In the walk-through survey of the Highland Water and the Black Water, many reaches with floodplain channels were identified. In selected
reaches drawings were made of the floodplain channel planforms in relation to the main channel and then digitized in ArcView. Detailed
long profiles of the main floodplain channels were surveyed.
N
0
10
m
Flow concentration
& acceleration
Highland Water
1
Black Water
2
Flow erosivity
Floodplain
erodibility
Debris dam
line of
floodplain
fp-ch-profile
channel
profile
N
m
0
1. Shallow floodplain channels,
only flowing during floods, appear
across meander bends even in
the absence of debris dams in the
main channel.
Ober Water
5m
b) Planform types
5
2. Multiple channels, flowing
during high flows and floods,
are found on the floodplain
adjacent to some hydraulically
effective debris dams.
1
Floodplain channels
2
3
5
4
Feature
Process
Characteristic
Figure 3 Conceptual model of floodplain channel formation
Floodplain_c hannels_present
3
Floodplain_c hannels_absent
Unsurveyed channel network
4
4. Conclusions
known_channelisation
catchment_boundary
0
Floodplain surface
scour
From the drawings of the floodplain channels, five main planform types were identified:
Legend
§
Overbank flow
frequency &
magnitude
500 1,000
2,000 Meters
Figure 2 Distribution of reaches with floodplain channels
ƒ The presence of floodplain channels at the catchment scale is favoured by a high debris
dams frequency, high main channel sinuosity, and small channel capacity and entrenchment
ratio.
Table 1
Channel sinuosity *
2
Channel capacity (m )
(wxd)
Number of debris
dams/100m
Entrenchment ratio
(floodprone
width/bankfull depth)
Floodplain channels present Floodplain channels absent
Mean
Standard dev. Mean
Standard dev.
1.29
0.185
1.10
0.0848
3.44
2.67
4.35
3.37
1.54
1.13
1.12
1.21
3.50
3.104
3.59
*Means are significantly different at the 0.05 probability level
Only for channel sinuosity was the difference between the means
significant at the 0.05 level. The means of the other variables all
show a trend in the direction expected: reaches with floodplain
channels had a higher number of debris dams and lower channel
capacity, both of which allow water to spill onto the floodplain more
easily; and these reaches also had lower entrenchment ratios.
However, the differences between the means of these three
variables were not significant at the 0.05 level. This could either be
due to an incorrect hypothesis, although this is doubtful because
the trends are towards the direction expected; or due to the
difficulty of surveying an inherently complex system with huge
variability within reaches (shown by large standard deviations), that
creates a high level of subjectivity in the survey technique.
4.72
ƒ Five different types of floodplain channel planform have been identified, based on the
number of entry and exit routes to and from the floodplain.
ƒ Floodplain channel long profiles are vary irregular and often contain deep scour holes, which
are possibly the result of varying hydraulic conditions caused by roots crossing the floodplain
channels.
c) Long profiles
These four types of distribution could be different stages along a
continuum of floodplain channel development. As a debris dam builds
up and becomes more hydraulically effective, floodplain channels
increase in number and size, until one or more develop
characteristics similar to the main channel and share its flow. It is
expected that the next stage would be a highly developed floodplain
channel capturing the majority of flow from the former main channel,
resulting in abandonment and infilling of the main channel.
Development of multiple anastomosing channels could also be
expected, however they have not been observed in this study area,
possible because the system has insufficient energy to support them.
The whole process, however, could be abandoned at any time due to
debris dam removal, either naturally by high flows or through
anthropogenic interference. Furthermore, debris dams may build up in
floodplain channels causing them to re-route and not remain in one
location long enough to develop into larger channels.
ƒ Four different types of floodplain channel distribution were observed at the reach, ranging
from single, shallow floodplain channels to multiple floodplain channels of varying depth with
some displaying characteristics similar to the main channel. It is proposed that the four types of
distribution lie on a continuum of floodplain channel development.
5. Network of floodplain
channels bifurcating and
re-joining.
The floodplain channels generally had a meandering planform, although the number of entry routes to the floodplain and exit routes
back to the main channel varied. Most channels occurred on meander bends, although some were found in relatively straight
sections of river where a debris dam was present in the main channel. This implies that meander bends and debris dams play an
important role in controlling the location of floodplain channels, although other factors subsequently cause variability in their
planform.
ƒ The morphology of floodplain channels does not appear to vary systematically through the
catchment.
Floodplain channel profiles are irregular. Within the channels deep scour holes often exist that may contain stagnant water for much of the
flood season. It is hypothesised that these scour holes are caused by differential erosion rates within floodplain channels where exposed
tree roots cross the flow pathway, creating intensely varied hydraulic gradients. As channels are cut into the surface, roots become
exposed creating a mini waterfall and hence increase shear stress and erosion immediately downstream, scouring out the hollows and
generally promoting an irregular channel profile. However, in other channels floodplain material is removed, leaving fully exposed roots that
further complicate the local hydraulic environment.
Reach 1
Reach 2
Floodplain channel profile
1.5
Arbitrary elevation
(m)
Summary statistics
4. Multiple channels of varying
depths, including some with similar
characteristics to the main channel,
e.g. bars, a gravel bed, similar
channel dimensions, and flowing
throughout the year, occur on the
floodplain adjacent to very large, (old
?) hydraulically effective debris
dams.
4. Multiple entry points to
floodplain and multiple exit
points back to main
channel.
3. Single entry point to
floodplain and multiple
exit points back to main
channel.
2. Multiple entry points to
floodplain and single exit
point
back
to
main
channel.
1
0.5
0
0
5
10
15
20
25
Distance (m)
30
35
40
45
Floodplain channel profile
1
0.5
0
0
5
10
15
Distance (m )
20
25
30
ƒ
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
5
10
15
Distance (m)
20
A conceptual model of floodplain channel formation has been proposed:
The main cause of the channels appears to be scour resulting from the presence of a large,
hydraulically effective debris dam in the main channel. The debris dam causes ponding of flow
upstream which increases the frequency and magnitude of flow onto the surrounding floodplain.
Once on the floodplain, water is concentrated between trees and debris accumulations. Where
this concentrated flow exceeds the critical threshold for erosion of cohesive floodplain soil, scour
initiates channel formation. At the point of re-entry into the channel, water surface slope
increases and headcutting can be initiated.
Floodplain channel profile
Reach 3
1.5
Arbitrary elevation (m)
3. Multiple channels of
varying depths, some flowing
throughout most of the flood
season,
occur
on
the
floodplain adjacent to large,
hydraulically effective debris
dams.
1. Single entry point to
floodplain and single exit
point back to main
channel.
Arbitrary elevation (m)
Figure 2 shows that floodplain channels are present in
some areas but not in others. Based on field observations,
it was hypothesised that the following variables were likely
to influence floodplain channel distribution: frequency of
debris dams; main channel sinuosity; channel capacity; and
entrenchment ratio. These variables were recorded for
each geomorphic reach.
The reaches were grouped
according to whether channels were present or absent, and
the means of each variable from both groups were
compared using students’ t-tests.
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