Climate Change and River Management:
What is climate change?
Before you can understand the impacts climate change is expected to have on rivers, a basic understanding of what climate change is, is required. Climate change refers to any long-term change in the statistical distribution of weather patterns over periods of time that can range from decades to millions of years; although it is most commonly used in reference to the increase in the average surface temperature of the earth over the recent decades and that which is predicted in the foreseeable future. Much of the work on climate change and its impacts on rivers is based on estimates as it is very hard to predict the future climate as it is dependent on a wide variety of things - although most importantly carbon dioxide emissions. Despite this uncertainity, central estimates have been produced for changes that are expected to occur within the UK, within the next 30 years.....
- Increase in the average summer temperatures of 3-5 degrees Celcius
- 10-30% increase in the average levels of precipitation experienced during the winter
What is River Management?
River management is any action that includes intentional intervention by humans into the way that river systems funtion. This includes:
- Managing the quantity of water flowing through the channel ---> flooding, droughts, freshwater supply, reservoirs and flow regime modification
- Managing the stability of river channels ---> rivers are unstable and constantly change due to the processes of erosion and deposition and this threatens to cause major, and very costly damage, to infrastructure
- Managing water quality ---> pollution from littering, industry, sewage, sediment and agriculture (eutrophication)
- Managing the physical quality of the river ---> management of ecosystems to maintain and maximise biodiversity
Although managing water quality and biodiversity etc are considered when it comes to river management, its primary aim is to reduce the flood risk. Reducing the flood risk is a big problem for England and Wales as around 4 million people and £200 billion worth of infrastructure are a risk of flooding. At present, even though we spend roughly £800 milliom per year on flood and coastal defences, we experience £1,400 million of damage. Worryingly, this is the damage caused and the costs associated with current sea levels, temperatures and precipitation levels and so the impacts of climate change are likely to provoke a sharp increase in those at risk and the costs involved with reacting to the risks. It has been estimated that, due to the expected increase in the recurrence and magnitude of floods, that the costs of damage done by them could rise from £1,400 million a year to anything up to £20,500 million over the next 70 years if no further action is taken.
So, how is climate change likely to affect the flood risk........
Flood risk is the balance between the conveyance capacity and the amount of water that is actually trying to move through the channel and it is this balance that climate change threatens to disrupt, thereby increasing the flood risk.
Conveyance capacity is, in very simple terms, the amount of water that can fit in the channel and so when this level is exceeded by the actual amount of water trying to fit in the channel, flooding occurs. Due to increased precipitation levels, climate change is predicted to increase the flow. This means that there is more water trying to pass through the river and so a higher probability that this volume will exceed conveyance capacity and therefore result in flooding. A study carried out on a small section of the River Wharfe in Yorkshire, which was based on this idea, concluded that increased flows by 2050, provoked by climate change, would result in a 12.2% rise in the area flooded by a 1 in 0.5 year return flood.
An increase in flow would provoke an increase in sediment and the relationship between the two can be descirbed as being non - linear (for exmaple, in the River Eden, in Cumbria, an increase of 4.2% in precipitation would lead to a 90% increase in the amount of sediment transported). The amount of sediment within a river is further being increased by urbanization. Although urbanization means that there is less bare surfaces, like soil, for sediment to be sourced from; because urbanization produces flashier rivers, as the increase in impermeable surfaces means the water enters the channel quicker; when the water reaches the channel it has more energy and so a greater erosive ability - which results in excess erosion and so therefore more sediment in the river. This decreases the conveyance capacity of the river and so the volume of water that can flow through the channel, before it reaches and exceeds bankfull capacity, is reduced. This relationship between sediment and flow was also tested on the River Wharfe and it concluded that a reduction in the size of the channel, caused by sediment deposition over two years would increase the flood risk by 5.7%.
Rivers are not stable and constantly change which makes it increasingly hard to make predictions in sediment dynamics and how rivers will change in the future due to climate change. To aid the development of rough predictions, cellular catchment evolution models have been created, like CAESAR (Cellular Automaton Evolutionary Slope and River model). CAESAR works by dividing drainage basins into 10m by 10m grid cells and then uses the height above base level and basic information on the river channel and whether or not one is present, to generate a virtual model of the landscape. CAESAR can then simulate the morphological changes in river catchments, on a flood by flood basis, over periods up to several thousands of years. This was used on the River Eden and clearly showed that the predicted increase in flows would increase the sediment yield and this would directly increase the amount of sediment deposited thereby result in a significant increase in the flood risk. So, for example, for the town of the Carlise this means that increased deposition reduces the conveyance capacity and so increases the flood risk.
Climate change will also affect rivers in other ways too:
- More fine sediment will be depoisted in the river beds and thereby smother the natural environment used by fish for spawning
- Increased sediment flow will block out the sunlight and lead to the death of many organisms
- Increase in water temperatures will reduce the concentration of dissolved oxygen in the water and so will kill of many organisms. Also, the warmer conditions will encourage the growth of tropical species which will result in many of our native species being outcompeted
There are solutions to these problems that we are likely to face, like strategic tree planting in the form of tree belts which can be planted to specific places in drainage basins to trap sediment, increase the interception store and therefore reduce the amount of water entering channel and increase the time taken for water to enter the channel. This example of soft engineering is very sustainable as it will continue to work even if climate change provokes increases in flows into a river.
So, in conclusion, climate change is likely to increase the flood risk by increasing the flow and thereby the sediment yield of the river whilst causing degradation of the physical habitat and water quality. This means that more needs to be done, and perhaps a different approach taken, to try and reduce the flood risk - so, if you are interested in a career related to river management and flood control there are likely to be lots of available jobs, in this field, in the near future!
Coastal Management in Bridgewater Bay:
What is the coast?
The coast is the region of mutual interaction between terrestial and marine environments and is extremely valuable in terms of as a defence, a habitat and a resource (industry and tourism). 50% of the worlds population live within 2km of the coast mainly due to the above reasons, its historical importance, for trade, climate, food and the flat and fertile land associated with it and so coasts are therefore important to manage and with rising sea levels and growing populations, the need to do so is only increasing.
Sea level rise is not a new thing though and has happened throughout history......
- 18,000 years ago the last glacier was at its maximum extent and so sea level was 140m lower than at present
- Initially the melting of the ice was rapid and so sea level rose rapidly but it gradually started to slow
- 4000 years ago sea level reached its present day level
In terms of the South West, we are very prone to coastal flooding as much of the land lies below sea level already (and I think, that this area will increase not only due to rise in sea level but also isostatic readjustment in Scotland which is causing this part of the world to sink slightly). This means that coastal management is required to ensure that those living here are protected.
The question; would you rather live behind a sea wall or a sand dunes system was asked - you can make your own mind up on which you would prefer - but the class I was in was split. Many said that they would rather be protected by a sea wall but were they wrong........
Firstly, lets go over the basics of sand dunes and beaches. Beaches and dunes are linked and wind energy can modify beaches to create specific landforms like sand dunes which are created by aeolian sediment transport.
Different factors are required for the formation of sand dunes including:
- an area to accomodate them
- onshore winds
- sediment supply
- shallow sloping beach
- high tidal range
Sand dunes are such a good coastal defence as, unlike a sea wall, they are not a static sea defence (although due to human influences provoking coastal squeeze they are being forced to be more static than they would like) and so change to new conditions. New such conditions can occur due to storm surges. Storm surges are generated by tropical storms as the low pressure system and winds associated with them, provoke a large, but temporary, rise in sea level. This is why they have the capacity to cause extensive flooding over coastal lowlands. The average storm surge in the UK, adds an additional 1.45metres in height to that of the high tide and therefore much of the UK, especially around this part of the country, is suseptible to flooding. If you then factor in the possilbe impacts of climate change on sea levels and the frequency and severity of storms, this is likely to become more and more of a prominant issue in the not too distant future.
In relation to sea level rise, the IPCC predict a :-
- 0.10m - 0.30m rise by 2030
- 0.50m rise by 2050
Beaches and sand dunes, imparticular, are crucial for the protection of the of coasts over the next 100 years and are by far the most sustainable option. Not only do they cost alot less than many hard engineering options, they create rare and vital habitats that provide a home for a diverse range of species, both plants and animals, but they are also, and perhaps most importantly, able to adapt to changes in conditions that we are going to face in the not too distant future. The development of dune systems sometimes needs a helping hand. For example, in Bridgewater Bay, the beach is becoming very muddy and consists of very fine grain which is not ideal for sand dunes and so beach nourishment is required to feed the dunes. However this is significantly less costly than building a huge sea wall. What option for coastal management further in the future than the next 100 years is best is unclear. Urbanization, the encroachment of developments onto the coastline and our attempts to fix sand dunes into there location means that sand dunes will not offer a sustainable and effective coastal defence option for ever. Depsite the fact that they can adapt to changes in the energy levels and height of the sea/waves, they are unable to adapt to us changing surfaces and restricting their movement. Coastal squeeze is one of the largest threats to sand dunes and their development as the building of housing developments and sea walls etc is preventing sand dunes from migrating, which allows them to adapt to new conditions. Instead anthropogenic influences on the sand dunes have the potential to turn the dunes into an almost static form of defence which threatens to halt their development and lead to their destruction. This is why land use management is more crucial than ever (this implies to rivers aswell). With the risks of coastal flooding and extensive erosion being so high, is it really a good idea to be building new developments right along the coast or by waterways. For example, think about Hinkley Point.........
So, in conclusion, opting for natural sea defences like sand dunes are by far the most sustainable option in the near future as they are able to adapt and cope with changing conditions but this approach needs to be accompanied by landuse management to further reduce the risk of flooding and lengthen the time for which sand dune systems can offer a sustainable and effective method of managing and reducing the risk of coastal flooding.
This is just a summary of what we got taught and so only really covers the basics and is no way near as good as actually participating in the lectures yourself - but unfortunately many of you were unable to attend and so you will have to put up with my explanation instead..........
I hope the revision is going well - if there is anything I can do to help, via this blog, please don't be afraid ask. I have had lots of requests surrounding the topic of rejuvenation in rivers and, as it is a tricky topic which took me a long time to get my head around, I am going to try and write a summary post about either later tonight or tomorrow...........