Technical Appendix 5
CAESAR modelling results
Technical Appendix 5: CAESAR modelling results
Contents
Page
List of Figures
ii
1.
Reach response to climate change
1.1
The Afon Teifi at Lampeter
1.2
The Afon Teifi at Tregaron
1.3
The Afon Dyfi at Machynlleth
1.4
The River Severn at Caersws (upper reach)
1.5
The River Severn at Caersws (lower reach)
1.6
The River Dee at Corwen
1.7
The River Dee at Bangor on Dee
1.8
Summary of results and implications for future flood
Hazard
1
1
5
9
12
15
19
22
2.
Reach response to land cover change
2.1
Introduction
2.2
Results and discussion
25
25
30
3.
Institute of Hydrology return period flood magnitudes for the
modelled reaches
32
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List of Figures
Page
Figure 1:
Location of the Teifi, DEM of contributing catchment
and modelled reach.
2
Erosion and deposition patterns for runs Climate 1,
2 and 3.
3
Graph of areas inundated for different magnitude
floods, for the present day topography, and the
topography after runs Climate 1, 2, and 3.
4
Figure 4:
Inundation area for 100 m3s-1 flood
4
Figure 5:
Inundation area for 300 m3s-1flood
4
Figure 6:
Location of the Teifi, DEM of contributing
catchment and modelled reach.
6
Erosion and deposition patterns for runs Climate 1,
2 and 3.
7
Graph of areas inundated for different magnitude
floods, for the present day topography, and the
topography after runs Climate 1, 2, and 3
8
Inundation areas for 80 m3s-1 flood for Climate 2
and present day topographies
8
Figure 2:
Figure 3:
Figure 7:
Figure 8:
Figure 9:
Figure 10: Inundation areas for 220 m3s-1 flood for Climate 2
and present day topographies
9
Figure 11: Location of the Dyfi, DEM of contributing catchment
and modelled reach
10
Figure 12: Erosion and deposition patterns for runs Climate 3.
10
Figure 13: Graph of areas inundated for different magnitude
floods, for the present day topography, and the
topography after runs Climate 1, 2, and 3.
11
Figure 14: Inundation areas for 160 m3s-1 flood for Climate 3
and present day topographies.
11
Figure 15: Inundation areas for 400 m3s-1 flood for Climate 3
and present day topographies
12
Figure 16: Location of the River Severn, DEM of contributing
catchment and modelled reach.
13
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Figure 17: Erosion and deposition patterns for runs Climate 3.
13
Figure 18: Graph of areas inundated for different magnitude
floods, for the present day topography, and the
topography after runs Climate 1, 2, and 3.
14
Figure 19: Inundation areas for 100 m3s-1 flood for Climate 3
and present day topographies
14
Figure 20: Comparison of erosion and deposition maps,
and 100 m3s-1 inundation map.
15
Figure 21: Location of the River Severn, DEM of contributing
catchment and modelled reach.
16
Figure 22: Erosion and deposition patterns for runs Climate 3.
16
Figure 23: Graph of areas inundated for different magnitude
floods, for the present day topography, and the
topography after runs Climate 1, 2, and 3.
17
Figure 24: Needs caption
18
Figure 25: Needs Caption
18
Figure 26: Location of the River Dee, DEM of contributing
catchment and modelled reach.
19
Figure 27: Erosion and deposition patterns for runs Climate 3.
20
Figure 28: Graph of areas inundated for different magnitude
floods, for the present day topography, and the
topography after runs Climate 1, 2, and 3.
20
Figure 29: Inundation areas for 200 m3s-1 flood for Climate 3
and present day topographies
21
Figure 30: Comparison of erosion and deposition maps,
and 200 m3s-1 inundation map.
21
Figure 31: Location of the River Dee, DEM of contributing
catchment and modelled reach.
22
Figure 32: Erosion and deposition patterns for runs Climate 1,
2 and 3.
22
Figure 33: Graph of areas inundated for different magnitude
floods, for the present day topography, and the
topography after runs Climate 1, 2, and 3.
22
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Figure 34:
22
Figure 35:
22
Figure 36: Patterns of erosion and deposition within the
Upper Severn Reach two section, demonstrating
how rapidly patterns of erosion and deposition can
change within a reach.
24
Figure 37: Erosion and deposition maps for Climate 1,
reforested and deforested runs
26
Figure 38: Erosion and deposition maps for Climate 2,
reforested and deforested runs
27
Figure 39: Erosion and deposition maps for Climate 3,
reforested and deforested runs
28
Figure 40: Areas inundated by different flood magnitudes
29
Figure 41: Areas inundated by 120 and 180 m3s-1 floods
29
Figure 42: Detail of erosion and deposition differences
between climate 2 reforest (top) and deforest (bottom)
30
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CAESAR modelling results
1.
Reach response to climate change
This section describes the results from the simulations for the four study
catchments and the reaches. To recap, three simulations were carried out for
each reach/catchment combination for 50 years from present.
1. Climate 1: This is a continuation of present day climate
2. Climate 2: This uses the present day climate record, but increases
winter rainfall magnitude by 20%
3. Climate 3: This increases all rainfall magnitude by 20%
In this section, for each reach, we shall present erosion and deposition maps
from each simulation, as well as data revealing changes in flood inundation
area for each study reach. The flood magnitudes simulated are all in m 3s-1, as
we have chosen to use actual flood values instead of the probability floods
(e.g. 1 in 100 year event etc.). However, graphs are provided at the end of
this appendix (section 3), which allow the reader to compare the floods used
to the probability of flood magnitude as predicted using the Institute of
Hydrology’s Flood Estimation Handbook.
1.1
The Afon Teifi at Lampeter
Figure 2 shows that erosion and deposition patterns following the 50 years of
simulation from runs Climate 1, 2 and 3 are largely similar. There are zones of
incision around the three largest meander loops at A, B and C. For runs
Climate 2 and 3 there is less erosion and some deposition at the first meander
loop (A) which is due to sediment being transported down from the catchment
above, moved by the increased flood sizes caused by climate change.
Climate 2 and notably Climate 3 also have small areas of overbank deposition
just downstream of C. This is caused by increased flood sizes depositing
material outside of the river bank.
Figure 3 plots the results from the HEC-GeoRAS simulations of flood
inundation on the topographies modified by the CAESAR reach model (as
shown in Figure 2). This describes the area inundated for each simulation, as
well as the present day topography over a wide range of floods. These floods
can be related to return frequencies calculated for this reach using the graph
in appendix *. Here, two patterns are apparent. Firstly, all future simulations
result in an increase in the area inundated, with the Climate 2 and 3 scenarios
resulting in a larger area being inundated. These increases are relatively
minor – in the order of 2-8%. Secondly, these increases in inundation area
only occur in medium size floods. The smallest flows ( less than 80 m3s-1)
and the largest flows (greater than 280 m3s-1) are largely unaffected by any
changes in the morphology. This is probably to be expected, as the smallest
flows modelled here barely exceed bankful discharge, and the largest flood
the entire valley floor.
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Figure 1: Location of the Teifi, DEM of contributing catchment and modelled
reach.
Figures 4 and 5 detail where there are differences in flooding between the
Climate 3 (red) and present day (blue) topographies. These show that there
are some areas on the margins that are flooded, and in Figure 5 areas the
other side of the railway line that have been flooded. Some of these changes
have occurred in the upstream section of the reach, where increased
deposition from Climate 2 and 3 will have reduced the channel’s capacity to
convey flood waters, probably resulting in an increase in flooded area for a
given flood event.
In summary, future flood hazard is slightly increased (c.5%) for moderate
floods, more so for runs Climate 2 and 3.
2
A
B
C
3
Figure 2: Erosion and deposition patterns for runs Climate 1, 2 and 3. Flow is from right to left.
Technical Appendix 5: CAESAR modelling results
Figure 3: Graph of areas inundated for different magnitude floods, for the
present day topography, and the topography after runs Climate 1, 2, and 3.
Figure 4: Inundation area for 100 m3s-1 flood; Figure 5: Inundation area for 300
m3s-1flood
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1.2
The Afon Teifi at Tregaron
The erosion and deposition patterns for the Tregaron reach are quite different
from the Lampeter reach. For runs Climate 1,2 and 3 there is mild deposition
at the upstream section, followed by extensive erosion after the second main
meander loop (as highlighted in Figure 7). Interestingly, this erosion is far
more widespread in the Climate 2 simulation. This may be caused by
enhanced flood magnitudes increasing erosion, but due to the seasonal
nature of Climate 2 runs no more sediment is deposited from the catchment
upstream, whereas in the Climate 3 run more sediment is input from
upstream.
This erosion has a significant impact on the area flooded as described in
Figures 8, 9 and 10. The graph (Figure 8) shows that all future topographies
simulated by Climate 1, 2 and 3 drastically reduce the area flooded. Similar to
the Lampeter reach, the largest differences are in the medium sized floods,
but unlike Lampeter there are ubiquitous reductions in flood inundation area.
The largest difference is with Climate 2, where the large amounts of channel
incision identified in Figure 7 have increased the river’s capacity to convey
flood waters, preventing it from spilling over bank. This is apparent in Figure
9, where a significant area of floodplain is no longer inundated after the
Climate 2 run. As flood sizes increase, this area does become inundated (as
shown in Figure 10) but a further area downstream does not flood, again due
to the channel incision from the Climate 2 run.
In summary, for the Teifi at Tregaron, all simulations significantly reduce the
inundation area by up to 35% for small and medium sized floods. Climate 2
causes widespread incision in the lower 2/3 of the reach reducing flood
hazard for all modelled flood events. Climate 3 also incises, reducing flood
hazard, but also causes some deposition (compared to Climate 1) so lower
magnitude flood hazard is reduced, but larger magnitude flood hazard is the
same.
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Figure 6: Location of the Teifi, DEM of contributing catchment and modelled
reach.
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Figure 7: Erosion and deposition patterns for runs Climate 1, 2 and 3. Flow is
from left to right.
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Figure 8: Graph of areas inundated for different magnitude floods, for the
present day topography, and the topography after runs Climate 1, 2, and 3.
Figure 9: Inundation areas for 80 m3s-1 flood for Climate 2 and present day
topographies.
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Figure 10: Inundation areas for 220 m3s-1 flood for Climate 2 and present day
topographies.
1.3
The Afon Dyfi at Machynlleth
Erosion and deposition is also apparent on the Dyfi simulations. Here in
Figure 12, patterns are largely similar between runs Climate 1, 2 and 3 (so we
present only those results from Climate 3), but areas of erosion and
deposition are accentuated for the climate change runs. These patterns show
incision at the top of the study reach, and two areas of deposition in the
middle and lower middle sections of the reach as indicated by the red ovals.
The chart of inundation area (Figure 13) shows that there is a general
increase in flood inundation area, with Climate 2 and Climate 3 having a
greater impact. This is shown in Figures 14 and 15, where the red areas
indicated the difference between the Climate 3 and present day topographies.
Again, these changes can be largely associated with erosion and deposition
patterns, with increases in area flooded corresponding to areas of deposition
identified in Figure 12.
In summary, the Dyfi at Machynlleth displays a similar response to the Teifi at
Lampeter, with an overall increase in flood hazard due to climate change.
These changes are also most prominent in medium sized floods, and whilst
the Climate 1 and 2 runs create up to a 6% increase in inundated area, the
Climate 3 run increases flooded area by up to 15%. This is caused by
aggradation (deposition) on the channel bed.
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Figure 11: Location of the Dyfi, DEM of contributing catchment and modelled
reach.
Figure 12: Erosion and deposition patterns for runs Climate 3. Flow is from
right to left.
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Figure 13: Graph of areas inundated for different magnitude floods, for the
present day topography, and the topography after runs Climate 1, 2, and 3.
Figure 14: Inundation areas for 160 m3s-1 flood for Climate 3 and present day
topographies.
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Technical Appendix 5: CAESAR modelling results
Figure 15: Inundation areas for 400 m3s-1 flood for Climate 3 and present day
topographies
1.4
The River Severn at Caersws (upper reach)
Figure 17 indicates a highly varied pattern of erosion and deposition in this
reach. There are clear zones of erosion and deposition, with incision at the
upper part, an area of deposition, followed by erosion, and another large area
of deposition followed by general incision towards the last third of the reach.
Also of note are several areas of overbank deposition (indicated) even in
areas where there has been channel incision. Interestingly, this demonstrates
one of the feedbacks of river systems, whereby they can increase their own
capacity through the creation of flood levees and the incision of the channel.
Figure 18 interestingly reveals a general decrease in flood hazard for runs
Climate 1, 2 and 3 and Climate 2 and 3 appear to give near identical results.
Again, this decrease is most prominent for medium sized floods, but unlike the
Lampeter and Machynlleth reaches, there is still a significant reduction for the
largest floods.
However, closer inspection of the areas inundated (Figure 19) reveals a
complex pattern. Here there are areas where the Climate 3 inundation area is
greater than present day, and also areas where the present day area is
greater than Climate 3. These correspond closely with the areas of erosion
and deposition, with a reduction in inundated area related to incision and an
increase to deposition. In comparison to the other simulated reaches this
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Technical Appendix 5: CAESAR modelling results
shows how the floodplain can evolve to change the pattern of flooding, in
quite different ways, within less than a km of each other.
Figure 16: Location of the River Severn, DEM of contributing catchment and
modelled reach.
Overbank Deposits
Figure 17: Erosion and deposition patterns for runs Climate 3. Flow is from left
to right.
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Figure 18: Graph of areas inundated for different magnitude floods, for the
present day topography, and the topography after runs Climate 1, 2, and 3.
Figure 19: Inundation areas for 100 m3s-1 flood for Climate 3 and present day
topographies
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Erosion and
reduced
flood hazard
Deposition and
increased flood
hazard
Erosion and
reduced
flood hazard
Figure 20: Comparison of erosion and deposition maps, and 100 m3s-1
inundation map.
1.5
The River Severn at Caersws (lower reach)
Similar to the reach immediately upstream, the lower Caersws reach displays
different patterns of erosion and deposition. The upper half is largely
erosional, with the channel incising up to 1.5m in places. The mid section is
clearly depositional (Figure 22) and this is followed by a small erosional
section close to the end of the reach. In this reach there are substantial areas
of over bank deposition, as highlighted by the blue circles in Figure 22.
The chart of inundation area (Figure 23) has been plotted in a slightly different
way from previous reaches, and this reflects the slightly more complex
response of this reach. Changes in total inundation area are relatively slight,
with a c.10% increase in inundation areas around small/medium floods (100140 m3s-1). Like the Lampeter and Machynlleth reaches there is more
difference in medium sized floods and little in large. Interestingly, Figure 23
shows that for all flood sizes Climate 3 has very similar inundation areas to
the present day topography, whereas Climate 1 simulations vary the most.
This may be due to increased flood magnitudes in the Climate 3 scenarios
eroding more sediment, thus decreasing inundation area relative to Climate 1.
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Technical Appendix 5: CAESAR modelling results
Figure 21: Location of the River Severn, DEM of contributing catchment and
modelled reach.
Figure 22: Erosion and deposition patterns for runs Climate 3. Flow is from left
to right.
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4000000
3500000
Present Day
3000000
Climate 1 (continuing present day)
Climate 2 (+20% winter rain)
Climate 3 (+20% annual rain)
Area Inundated (m 2)
2500000
2000000
1500000
1000000
500000
600
560
540
500
460
440
400
360
320
300
280
260
240
220
200
180
160
140
120
80
100
60
40
20
0
Size of flood (m 3s -1)
Figure 23: Graph of areas inundated for different magnitude floods, for the
present day topography, and the topography after runs Climate 1, 2, and 3.
However, Figure 24 reveals that this may not be the case. As per the
upstream Severn reach, the patterns of erosion and deposition generate
several different changes in flood inundation area within the same reach. The
inundation area chart (Figure 23) shows negligible changes in flooded area
between present day and Climate 3, yet Figure 23 shows a marked reduction
for the upper area (where there has been erosion and incision) and an
increase in the middle section (where there was deposition). Therefore, these
gains and losses in area are to some extent balancing each other out. Again,
this illustrates how the behaviour of a reach can change dramatically (3 times
here) within a reach. Furthermore, it would seem that overbank deposits do
not seem to make a significant difference to overbank inundation patterns, as
the highlighted areas of overbank deposition have had little impact on local
inundation changes. Finally, a very large flood (600 m3s-1) inundates the entire
valley floor, with very little difference in inundation area.
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Figure 24: Needs caption
Figure 25: Needs Caption
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Technical Appendix 5: CAESAR modelling results
1.6
The River Dee at Corwen
The River Dee at Corwen shows a different response to the climate changes.
Here Figure 27 shows a distinct trend in erosion and deposition that is found
in all three simulations. There is widespread incision in the upper third of the
reach, and in channel deposition across nearly the whole of the lower two
thirds.
This has an interesting impact on the areas inundated with Figure 28 showing
that for small and medium sized floods, the area inundated is generally
increased, yet for the largest floods there is a small decrease. This is due to
the split in erosion and deposition across the reach as is evident in Figure 29
and 30. In Figure 29 the upper incisional section shows a significant decrease
in flood hazard, as shown by the increased blue areas. Downstream, the blue
lines of the present day flood area lie within the red areas showing a
substantial increase in flood area caused by the Climate 3 run. Figure 30
shows this in comparison to the erosion and deposition patterns, and the
relatively similar areas inundated are probably due to increases offsetting
decreases in inundation area. If we were to measure changes either side of
the line shown in Figure 30, then we may get quite different results.
Figure 26: Location of the River Dee, DEM of contributing catchment and
modelled reach.
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Figure 27: Erosion and deposition patterns for runs Climate 3. Flow is from left
to right.
Figure 28: Graph of areas inundated for different magnitude floods, for the
present day topography, and the topography after runs Climate 1, 2, and 3.
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Figure 29: Inundation areas for 200 m3s-1 flood for Climate 3 and present day
topographies
Figure 30: Comparison of erosion and deposition maps, and 200 m3s-1
inundation map.
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1.7
The River Dee at Bangor on Dee
The River Dee at Bangor on Dee, is the most insensitive reach of all of those
presented in this report. Figure 31 shows that there is very little difference in
inundation area between the three runs and present day conditions. There are
small differences as shown in Figure 34, and as with previous reaches, these
can be linked to areas of erosion and deposition as indicated in Figure 32.
Exact reasons for relatively small changes are not clear, though this reach is
the most downstream of all reaches studied here, and it is possible that its
lowland location may be a factor.
Figure 31 Location of the River Dee, DEM of contributing catchment and
modelled reach.
Figure 32 Erosion and deposition patterns for runs Climate 1. Flow is from left
to right.
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6000000
Present Day
Climate 1 (continuing present day)
5000000
Climate 2 (+20% winter rain)
Climate 3 (+20% annual rain)
Area Inundated (m 2)
4000000
3000000
2000000
1000000
0
40
60
80 100 120 140 160 180 200 240 280 320 360 400 440 480 520 560 600
Size of flood (m 3s-1)
Figure 33 Graph of areas inundated for different magnitude floods, for the
present day topography, and the topography after runs Climate 1, 2, and 3.
Figure 34 Images if different flood inundation extents for a 160 cumec flood
between present day and climate 3 scenarios.
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1.8
Summary of results and implications for future flood hazard
There is a varying response between catchments with the future runs causing
increases in flood inundation area of between 5 and 15%. Notably, there are
larger, and significant decreases in inundation area in the Teifi at Tregaron
and the Upper Severn reaches. Some reaches (Upper Severn, Tregaron,
Corwen) display significant increases and decreases in inundation area within
the same reach. These changes are strongly linked to erosion (decrease in
area) and deposition (increase in area) within and around the river channel.
Incision increases the depth and thus capacity of a channel to convey flood
waters, making it less likely to flood therefore decreasing the area inundated.
Deposition or channel aggradation has the opposite effect, decreasing the
capacity of the channel increasing the likelihood that the banks will be over
topped and a flood will occur.
Interestingly, reaches where there was a decrease in flood hazard showed
that this decrease occurred across the whole range of floods modelled. This is
intuitively correct, as increasing channel capacity through erosion will reduce
inundation area across all flood sizes. Conversely, deposition and
corresponding increases in flood inundation area will not affect the largest
floods, as they are still restricted by the main valley wall – as shown for the
Dyfi in Figure 15. As incision appears to impact all flood events, possibly this
is the effect that may be most significant.
Furthermore, overbank deposits do not seem to make a significant difference
to overbank inundation patterns. It is apparent from the two Severn reach
simulations that the main cause of change in inundation area is changes to
the channel capacity. This may be due to the relatively short duration of these
simulations in comparison to the times taken to develop floodplains (1000’s of
years) as well as low levels of suspended sediment found in these systems.
The complexity of reach response (Figure 36) , and its capability to vary
rapidly within even a few kilometres of the reaches modelled here illustrates
both the complexity of fluvial response, as well as the capability of the
CAESAR model to simulate such dynamics. It also shows that it is dangerous,
if not futile to attempt to generalise river behaviour through a reach. Each
section of a river needs to be studied and modelled individually, as its
behaviour is governed by the sections immediately upstream and
downstream. This point is reinforced as these results are from a wide variety
of river reaches, in a range of settings (upland, piedmont and lowland) yet all
give different results, it is not possible to generalise the reach response
according to its location or context.
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Figure 36: Patterns of erosion and deposition within the Upper Severn Reach
two section, demonstrating how rapidly patterns of erosion and deposition can
change within a reach.
Yet general trends found across the reaches may have important implications,
as some reaches are largely erosional, others are depositional. For example,
the Corwen reach exported very little sediment during the simulations, overall
it accumulated sediment. This represents an interesting situation as the
sediment feed from the catchments above Corwen was very low, largely due
to the trapping effects of Lake Bala. As it is a depositional reach, the flood
hazard would increase greatly if the volumes of sediment input from the
upstream catchments were to increase, therefore this reach is sensitive to
environmental change, yet not changing as the inputs are so low. A
contrasting example is the Upper Severn reach, where significant volumes of
sediment were exported or eroded from the reach. Interestingly, the upstream
catchments input large volumes of sediment into this reach, suggesting that
the reach is quite resistant to environmental change as it can move these
volumes of sediment through the reach rapidly. Both these reaches are well
suited or adjusted to the inputs from the catchments upstream. This
fascinating balance and the sensitivity of the reach may be more complex
than outlined here, especially given the intra reach variability highlighted in the
previous paragraph.
A primary aim of this project was to determine whether climate change would
cause the morphology of river reaches to change, which would then cause a
change in flood hazard. The results presented here show that there can be
significant changes in channel morphology over 50 simulated years, and that
these changes can have a large effect on flood hazard (increasing it by up to
15% and decreasing by 33%). However the changes are not necessarily all
due to climate change. Most of the simulations presented above show
substantial changes by continuing with present day climate, though several
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Technical Appendix 5: CAESAR modelling results
runs do show increased or enhanced changes with increases in precipitation.
This raises two important questions, firstly are these changes due to the
inherent dynamics of river systems and secondly are these changes due to
climate change? These results show that both factors are important, but the
question is somewhat redundant, as this study shows that flood hazard can
change substantially and rapidly due to either factor.
Changes in morphology caused by climate change, or by the continuous
action of rivers can cause a significant change in the flood inundation area
(+15% to -33%) and this factor is largely ignored by all conventional modelling
studies. If we wish to simulate flood hazard accurately, then we need to either
model changes in morphology, or use the potential rates of change in
inundation determined from studies like this one to create suitable ranges for
error and uncertainty.
2.
Reach response to land cover change
2.1
Introduction
Six further runs were carried out in order to assess the potential impacts of
land cover change on flood hazard. These simulations were carried out on the
Corwen reach of the River Dee, as discussed in 1.8 this section is largely
depositional, and may be especially sensitive to any changes in upstream
sediment delivery due to land cover changes.
Three runs had already been carried out for this reach (climate 1, 2 and 3)
and in order to simulate changes in land cover, the ‘m’ value in the
hydrological model was altered. ‘M’ controls the rate at which the peak of the
flood hydrograph arrives and decays, as well as the magnitude of the flood
itself. Its use within the CAESAR model is documented at greater length in the
report by Coulthard and Jones (2002). The ‘m’ value used in the runs
described in section 1.6 was 0.015, and this was altered to simulate a
forested environment (0.02) and a less forested or deforested environment
(0.01). The effects of changing this value are first impacted upon the
simulated catchment hydrology. For example, reducing the ‘m’ value to
simulate deforestation increases the size of a flood event for a given rainfall
magnitude as well as increasing the ‘flashiness’ or speed at which the flood
occurs. For the catchment simulations above the reach model, this has the
impact of increasing flood sizes and (usually) increasing the volumes of
sediment discharged by the catchment, and thus into the reach. Contrastingly,
increasing the ‘m’ values reduces flood magnitudes and normally reduces
sediment yields. Therefore, the changes applied to the CAESAR model affect
the hydrology rather than making (for example) the surface of the landscape
easier to erode. This resulted in six runs, climate 1 forested, climate 1
deforested, climate 2 forested, climate 2 deforested, climate 3 forested and
climate 3 deforested.
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Figure 37: Erosion and deposition maps for Climate 1, reforested and
deforested runs
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Figure 38: Erosion and deposition maps for Climate 2, reforested and
deforested runs
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Figure 39: Erosion and deposition maps for Climate 3, reforested and
deforested runs
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1800000
1600000
1400000
Inundation area /m2
1200000
1000000
800000
Clim 1 deforest/area
Clim 1 reforest/area
Clim 2 deforest/area
Clim 2 reforest/area
Clim 3 deforest/area
Clim 3 reforest/area
600000
400000
200000
0
0
50
100
150
200
250
300
350
400
Discharge /cumecs
Figure 40: Areas inundated by different flood magnitudes
Figure 41: Areas inundated by 120 and 180 m3s-1 floods
30
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Technical Appendix 5: CAESAR modelling results
2.2
Results and discussion
Figures 37 to 39 show that there are patterns of erosion and deposition that
are largely similar to the previous simulations of the reach at Corwen, with
erosion in the upper section and deposition in the lower - as in Figure 30.
However, patterns appear more pronounced, and with closer inspection there
are significant differences. These are apparent in the influence on the area
inundated (Figures 40 and 41). Here there is a marked reduction in the area
inundated for the Climate 2 and 3 deforested scenarios. All of the other runs
(present day, Climate 1 reforested and deforested, Climate 2 and 3
reforested) are largely similar, with similar erosion and deposition patterns
(Figures 37 to 39).
The reduction in inundated area is significant. Figure 40 indicates that for an
increase in flood magnitude from a 20 to 120 m 3s-1 flood for most scenarios
there is an increase in inundate area from 170 000 m3 to 450 000 m3, a 165%
increase. Whereas for the Climate 2 and Climate 3 deforested runs there is an
increase from 170 000 m3 to 260 000 m3 inundated, only a 53% increase.
Therefore, the combination of deforestation with increased rainfall magnitudes
of Climate 2 and 3 scenarios results in a dramatic reduction in flood risk. This
is apparent across all flood sizes, with the reduction in area inundated after
the Climate 2 deforested run ranging between 60% (medium floods) and 15%
for the very largest floods. The difference this makes is quite noticeable as
shown in Figure 41.
A
B
E
E
D
E
C
Figure 42: Detail of erosion and deposition differences between climate 2
reforest (top) and deforest (bottom)
31
Technical Appendix 5: CAESAR modelling results
As with the climate change scenarios, these alterations in inundation area are
closely linked to patterns of erosion and deposition within the channel and
floodplain. Figure 42 highlights some of these areas. Here there is far less
deposition in the deforested simulation at points A and B, as well as more
channel erosion or incision at point C.
Again, this pattern of erosion and deposition is not straightforward. It might
well be expected that there is reduced deposition at A and B, and with
increased erosion at C, due to the larger flood magnitudes in the deforested
simulations eroding more material and depositing less. However, there is
more deposition in the deforested simulation at point D. This could be caused
by increased sediment yields from the catchment upstream causing excess
deposition here, though it is interesting that most of this sediment is actually
removed from the reach. Therefore, despite more sediment being added to
the reach due to deforestation and climate change, even more material is lost
from the reach due to the larger magnitude of the floods.
The Climate 2 deforestation simulation also causes extensive channel
widening as highlighted by the blue circles labelled E in Figure 41. This is an
interesting example of how the model has responded to increased flood
magnitudes by increasing its capacity to carry flood flows. This is exactly the
response we would expect to find in natural channels. This, along with the
decreased deposition at A and B, and incision at C leads to the significant
decrease in area inundated for a given flood. This decrease is obviously
shown in Figure 41. An interesting check would be to see whether the same
return frequency flood, given the deforested conditions, would produce the
same inundation area.
These results reveal an interesting sensitivity in the catchments and reach
modelled here. The results for the present day, Climate 1 deforested and
reforested, Climate 2 and 3 reforested simulations are nearly identical, with
little change in the total area inundated. It appears that there is a threshold
response in these simulations, whereby above a repetition of a particular flood
size there is a significant change - or step change in how the reach responds.
It crosses a threshold above which it deepens and widens in order to
accommodate the increased flows. Such thresholds are not uncommon in
fluvial geomorphology, and a vital role of modelling exercises such as this
would be to identify what size these threshold events are. This would provide
vital information for catchment managers in order to take preventative or
remedial measures.
32
Technical Appendix 5: CAESAR modelling results
3.
Institute of Hydrology return period flood magnitudes for the
modelled reaches
Flood magnitude frequency curve for the Corwen reach
600
500
Discharge /cumecs
400
300
200
100
0
1
10
100
1000
Return period /years
Flood magnitude frequency curve for the Bangor on Dee reach
700
600
Discharge /cumecs
500
400
300
200
100
0
1
10
100
Return period /years
33
1000
Technical Appendix 5: CAESAR modelling results
Flood magnitude frequency curve for the Dyfi reach
500
450
Discharge /cumecs
400
350
300
250
200
150
100
1
10
100
1000
Return period /years
Flood magnitude frequency curve for the upper Caersws reach
180
160
140
Discharge /cumecs
120
100
80
60
40
20
0
1
10
100
Return period /years
34
1000
Technical Appendix 5: CAESAR modelling results
Flood magnitude frequency curve for the lower Caersws reach
400
350
Discharge /cumecs
300
250
200
150
100
50
0
1
10
100
1000
Return period /days
Flood magnitude frequency curve for the Roundabout reach
600
500
Discharge /cumecs
400
300
200
100
0
1
10
100
Return period /years
35
1000
Technical Appendix 5: CAESAR modelling results
Flood magnitude frequency curve for the Tregaron reach
200
180
160
Discharge /cumecs
140
120
100
80
60
40
20
0
1
10
100
1000
Return period /years
Flood magnitude frequency curve for the Lampeter reach
400
350
Discharge /cumecs
300
250
200
150
100
50
0
1
10
100
Return period /years
36
1000