WEBVTT
1
00:00:01.300 --> 00:00:03.883
(upbeat music)
2
00:00:05.710 --> 00:00:07.330
Croeso, welcome.
3
00:00:07.330 --> 00:00:10.190
We're in Wales to look at flooding and river management
4
00:00:10.190 --> 00:00:13.120
on some of the country's most iconic rivers;
5
00:00:13.120 --> 00:00:17.458
the River Severn, River Rheidol and River Ystwyth.
6
00:00:17.458 --> 00:00:20.320
Across the world, over a billion people
7
00:00:20.320 --> 00:00:22.170
live on river floodplains,
8
00:00:22.170 --> 00:00:26.400
that's over 16% of the world's population.
9
00:00:26.400 --> 00:00:28.850
Most of the food we eat is grown or reared
10
00:00:28.850 --> 00:00:31.580
on these areas of flat, fertile land,
11
00:00:31.580 --> 00:00:35.231
alongside the middle and lower course of rivers.
12
00:00:36.122 --> 00:00:38.100
Floodplains are also home to much
13
00:00:38.100 --> 00:00:40.360
of the world's manufacturing industries,
14
00:00:40.360 --> 00:00:42.110
meaning that flooding in one place
15
00:00:42.110 --> 00:00:44.713
can often have a global knock-on effect.
16
00:00:46.460 --> 00:00:49.140
Floodplains are formed of river sediments
17
00:00:49.140 --> 00:00:52.100
being deposited during flood events.
18
00:00:52.100 --> 00:00:54.570
Flooding on these areas is just as likely
19
00:00:54.570 --> 00:00:57.290
to happen today, as it was in the past.
20
00:00:57.290 --> 00:00:59.810
In fact, in many parts of the world,
21
00:00:59.810 --> 00:01:01.910
floods are becoming more severe
22
00:01:01.910 --> 00:01:05.380
and happening more often, as climate change results
23
00:01:05.380 --> 00:01:08.223
in an increasing number of large storms.
24
00:01:09.850 --> 00:01:12.150
(upbeat music)
25
00:01:12.150 --> 00:01:14.840
Because so much human activity
26
00:01:14.840 --> 00:01:17.340
takes place on river floodplains,
27
00:01:17.340 --> 00:01:20.180
this often means that flooding, erosion
28
00:01:20.180 --> 00:01:23.150
and deposition have to be carefully managed;
29
00:01:23.150 --> 00:01:26.933
in order to protect property, lives and infrastructure.
30
00:01:28.200 --> 00:01:29.800
We're going to look at how river management
31
00:01:29.800 --> 00:01:31.170
was done in the past,
32
00:01:31.170 --> 00:01:33.940
the strengths and weaknesses of those approaches,
33
00:01:33.940 --> 00:01:36.964
and how river management is done today.
34
00:01:36.964 --> 00:01:39.900
(upbeat music)
35
00:01:39.900 --> 00:01:42.620
Traditionally, hard engineering approaches
36
00:01:42.620 --> 00:01:45.070
were used to manage rivers.
37
00:01:45.070 --> 00:01:47.080
Now, rivers flood when the amount
38
00:01:47.080 --> 00:01:49.160
of water flowing down the river,
39
00:01:49.160 --> 00:01:51.750
exceeds the capacity of the river channel,
40
00:01:51.750 --> 00:01:54.860
causing the river to overtop its banks.
41
00:01:54.860 --> 00:01:57.320
On the River Severn, in mid-Wales,
42
00:01:57.320 --> 00:02:01.810
this happened in 1960 and again in 1964.
43
00:02:01.810 --> 00:02:03.750
This caused significant flooding,
44
00:02:03.750 --> 00:02:07.800
here at Newtown, downstream at Welshpool
45
00:02:07.800 --> 00:02:10.270
and on the surrounding agricultural land,
46
00:02:10.270 --> 00:02:13.420
causing significant damage to property.
47
00:02:13.420 --> 00:02:17.030
Following these events, several hard engineering techniques
48
00:02:17.030 --> 00:02:20.673
were put in place to try and prevent flooding in the future.
49
00:02:21.557 --> 00:02:25.370
One way to prevent flooding is to control river discharge,
50
00:02:25.370 --> 00:02:29.586
or the amount of water flowing down the river every second.
51
00:02:29.586 --> 00:02:31.216
(upbeat music)
52
00:02:32.450 --> 00:02:35.530
This is the Clywedog Dam on the River Clywedog,
53
00:02:35.530 --> 00:02:37.680
one of the tributaries of the River Severn.
54
00:02:38.870 --> 00:02:41.490
The dam here is used to hold back water,
55
00:02:41.490 --> 00:02:44.170
following heavy rainfall, and this water can then
56
00:02:44.170 --> 00:02:46.800
be gradually released down the river channel,
57
00:02:46.800 --> 00:02:50.570
without flooding urban areas downstream, like Newtown.
58
00:02:50.570 --> 00:02:53.380
During drought conditions the dam can also be used
59
00:02:53.380 --> 00:02:56.310
to maintain a steady flow of water down the river,
60
00:02:56.310 --> 00:02:58.986
which helps maintain river ecosystems.
61
00:02:58.986 --> 00:03:02.670
Clywedog Dam is also used to generate hydroelectric power
62
00:03:02.670 --> 00:03:04.670
and provides a leisure facility
63
00:03:04.670 --> 00:03:08.139
for activities like fishing and sailing.
64
00:03:08.139 --> 00:03:09.522
(upbeat music)
65
00:03:11.330 --> 00:03:13.630
Another way to try and prevent flooding
66
00:03:13.630 --> 00:03:16.550
is to increase the capacity of the river channel,
67
00:03:16.550 --> 00:03:20.240
so that it can hold more water during floods.
68
00:03:20.240 --> 00:03:24.750
Downstream of the Clywedog Dam, back here at Newtown,
69
00:03:24.750 --> 00:03:27.730
this has been achieved in two ways.
70
00:03:27.730 --> 00:03:30.560
The alluvial sediment, the gravels
71
00:03:30.560 --> 00:03:33.420
and the sand that the river transports,
72
00:03:33.420 --> 00:03:36.400
was dredged from the river and taken away,
73
00:03:36.400 --> 00:03:39.190
so that the river channel was deeper.
74
00:03:39.190 --> 00:03:41.750
As a result of the dredging here,
75
00:03:41.750 --> 00:03:44.660
the cover of alluvial sediment, of gravel
76
00:03:44.660 --> 00:03:47.640
on the bed of the river, is actually quite thin,
77
00:03:47.640 --> 00:03:50.180
and in places you can actually see some
78
00:03:50.180 --> 00:03:53.273
of the underlying bedrock come to the surface.
79
00:03:54.560 --> 00:03:57.190 line:15%
Engineers have also built embankments,
80
00:03:57.190 --> 00:04:00.890 line:15%
or artificial levees, along the banks of the river.
81
00:04:00.890 --> 00:04:05.890
So the river channel is deeper, the river banks are higher,
82
00:04:07.470 --> 00:04:08.980
and this means that the river
83
00:04:08.980 --> 00:04:12.622
can carry more water before it begins to flood.
84
00:04:12.622 --> 00:04:15.540
(upbeat music)
85
00:04:15.540 --> 00:04:19.620
Another hard engineering approach to preventing flooding
86
00:04:19.620 --> 00:04:23.470
is to help the water move downstream as quickly as possible
87
00:04:23.470 --> 00:04:27.233
and get the water away from urban areas like this.
88
00:04:28.330 --> 00:04:30.680
This can be achieved by realigning,
89
00:04:30.680 --> 00:04:33.100
or straightening the river channel.
90
00:04:33.100 --> 00:04:36.920
Either by removing gravel bars from the river channel,
91
00:04:36.920 --> 00:04:40.230
so that the water doesn't have to flow around them.
92
00:04:40.230 --> 00:04:43.180
Or by mimicking the natural processes
93
00:04:43.180 --> 00:04:45.202
of river meander cut-off,
94
00:04:45.202 --> 00:04:50.002
by digging a channel between two meander bends.
95
00:04:50.002 --> 00:04:53.654
This creates a shorter and a steeper river channel,
96
00:04:53.654 --> 00:04:55.850
that helps get the water out
97
00:04:55.850 --> 00:04:58.590
of the urban areas as quickly as possible.
98
00:04:58.590 --> 00:05:01.340
Once again, this was done here at Newtown.
99
00:05:01.340 --> 00:05:04.840
So the River Severn used to flow behind me here,
100
00:05:04.840 --> 00:05:06.890
round the back of this car-park,
101
00:05:06.890 --> 00:05:08.830
but during the channelisation works,
102
00:05:08.830 --> 00:05:13.320
it was moved about 200 metres to its present location.
103
00:05:13.320 --> 00:05:16.670
And this means that the water can flow a bit more quickly
104
00:05:16.670 --> 00:05:19.100
through the slightly straighter reach
105
00:05:19.100 --> 00:05:21.523
and the water moves through a bit faster.
106
00:05:22.490 --> 00:05:24.570
Removing other types of roughness
107
00:05:24.570 --> 00:05:28.610
from along the river channel, can also help with this.
108
00:05:28.610 --> 00:05:31.240
Like getting rid of some of the riparian vegetation,
109
00:05:31.240 --> 00:05:33.220
so for example, some of the trees
110
00:05:33.220 --> 00:05:35.063
that grow along the riverbank.
111
00:05:35.900 --> 00:05:38.430
Or by getting rid of the deep pools
112
00:05:38.430 --> 00:05:41.480
and shallow riffles that you would normally expect to see
113
00:05:41.480 --> 00:05:43.510
on the bed of a natural river,
114
00:05:43.510 --> 00:05:47.683
creating a smooth, graded riverbed profile.
115
00:05:48.990 --> 00:05:53.010
All of these measures help the water flow more quickly
116
00:05:53.010 --> 00:05:56.781
through these sections and away from vulnerable areas.
117
00:05:56.781 --> 00:05:58.103
(upbeat music)
118
00:05:59.453 --> 00:06:01.570
Erosion and flooding along riverbanks
119
00:06:01.570 --> 00:06:04.370
is also prevented by reinforcing the banks,
120
00:06:04.370 --> 00:06:07.115
using different hard engineering structures.
121
00:06:07.115 --> 00:06:09.980
So this riprap is made up of large boulders,
122
00:06:09.980 --> 00:06:12.510
and it's resistant from erosion from the water.
123
00:06:12.510 --> 00:06:14.490
It's been placed here to protect
124
00:06:14.490 --> 00:06:16.760
and strengthen the riverbanks.
125
00:06:16.760 --> 00:06:18.770
Other hard engineering structures,
126
00:06:18.770 --> 00:06:21.820
such as block revetments and gabion baskets
127
00:06:21.820 --> 00:06:23.260
are also used in the same way,
128
00:06:23.260 --> 00:06:26.404
to prevent erosion and strengthen riverbanks.
129
00:06:26.404 --> 00:06:28.987
(upbeat music)
130
00:06:30.260 --> 00:06:32.760
This concrete mattressing beneath my feet,
131
00:06:32.760 --> 00:06:34.200
has been engineered here,
132
00:06:34.200 --> 00:06:36.780
to strengthen and protect the riverbed.
133
00:06:36.780 --> 00:06:39.690
So you can see behind me, that this mattressing extends
134
00:06:39.690 --> 00:06:41.360
up onto the side of the river,
135
00:06:41.360 --> 00:06:43.220
so it's not only protecting the bed
136
00:06:43.220 --> 00:06:45.123
of the river, but also its banks.
137
00:06:46.170 --> 00:06:48.520
Groynes are also used along riverbanks,
138
00:06:48.520 --> 00:06:50.700
to divert water away from the banks
139
00:06:50.700 --> 00:06:52.967
and to encourage deposition.
140
00:06:52.967 --> 00:06:55.627
Traditionally, hard engineering approaches
141
00:06:55.627 --> 00:06:59.230
are used in combination to fix the river channel in place
142
00:06:59.230 --> 00:07:01.450
and to prevent future flooding.
143
00:07:01.450 --> 00:07:03.980
These different hard engineering approaches can
144
00:07:03.980 --> 00:07:07.560
be very effective in preventing river flooding and erosion.
145
00:07:07.560 --> 00:07:10.890
However, they can be very expensive.
146
00:07:10.890 --> 00:07:13.660
Based on recent Environment Agency reports,
147
00:07:13.660 --> 00:07:15.900
dredging a river channel can cost anywhere
148
00:07:15.900 --> 00:07:20.340
from £5 to £75 per cubic metre of sediment removed.
149
00:07:20.340 --> 00:07:23.170
This can add up to over £50,000,
150
00:07:23.170 --> 00:07:26.600
just to dredge one kilometre of a river channel.
151
00:07:26.600 --> 00:07:28.670
Artificial levees and embankments,
152
00:07:28.670 --> 00:07:32.840
generally cost thousands of pounds to build just one metre.
153
00:07:32.840 --> 00:07:37.810
Gabions are reported to cost £1,216 per metre,
154
00:07:37.810 --> 00:07:40.580
and hard reinforcements of river beds and banks,
155
00:07:40.580 --> 00:07:44.740
is reported to cost £1,075 per metre.
156
00:07:44.740 --> 00:07:46.945
All these structures have ongoing costs,
157
00:07:46.945 --> 00:07:50.520
as they require regular maintenance and repairs.
158
00:07:50.520 --> 00:07:52.550
Towards the end of the 20th century,
159
00:07:52.550 --> 00:07:54.570
the negative impacts of these techniques
160
00:07:54.570 --> 00:07:56.123
began to be recognised too.
161
00:07:57.373 --> 00:07:59.956
(upbeat music)