Oh, Dam

This is a great video that addresses a point I discussed in an earlier post - the perils of dam building on rivers in the face of climate change induced discharge/runoff changes. The video addresses the likely impacts on humans, as well as alternatives. One of the main messages is that we really need to carry about proper hydrological and climatic evaluation before building dams for short term gain.

In The Bleak Midwinter

It’s Boxing Day, the turkey sandwiches are flowing, the family arguments are reaching a near Shakespearian conclusion, and the drinking is continuing.  As usual, Father Christmas had a successful flight the other night, and it seems appropriate in this festive blog post to focus on river discharge in the Arctic, Father Christmas’ homeland.

Examination of 1936 to 1999 river discharge data by Peterson, et al. (2002) shows a 7% increase in average annual discharge of freshwater from the six largest Eurasian rivers that flow into the Arctic Ocean. This change was correlated with changes in global mean surface air temperature, and the authors concluding that these changes could have a significant effect on thermohaline circulation in the Atlantic Ocean in the future. Quite as to why this increasing trend has occurred is unclear, however McClelleand, et al. (2004) suggest four possible causes of this change: 1. Increased moisture transport to higher latitudes in a warming climate; 2. Dam construction and operation; 3. Permafrost thaw; and 4. Increasing forest fires. Through a combination of observation and monitoring, they suggest that dam construction, permafrost thaw and forest fires are all unlikely to be responsible for changes in discharge, sensitivity analysis showing that the changes required to enact current changes in discharge are unrealistic. Ultimately it is suggested that increased moisture transport to higher latitudes from global warming is the most likely conclusion, though there is great uncertainty. 

This hypothesis is echoed by the work of Wu, et al. (2005) who examine human influence upon changing Arctic discharges.  In running a complex climate model to simulate arctic flows (HadCM3), they show that a model not forced with anthropogenic factors does not produce the increases in discharge that have been observed in the Arctic.  Increasing higher latitude precipitation as a result of anthropogenic warming is the most likely factor to have forced increases in Arctic discharge. It is very important we try and quantify these changes - a shut down in thermohaline circulation in the North Atlantic would have catastrophic effects upon our world’s climate.

At least Santa is surviving so far.

You'll Manage

The last post talked about restricting building on floodplains as a way to manage climate change induced changes in discharge, the idea being that a natural and free flowing river is better suited to withstanding increases in discharge. A natural channel can move dynamically and adjust flows to buffer against negative impacts of increased discharge (Palmer, et al. 2008). However, the big problem is that most rivers no longer exhibit flow regimes in line with historic flow regimes, such as been the influence of humans with the construction of infrastructure such as dams and levees. For example, Poff, et al. (2007) conclude that dams have changed flow regimes in rivers across the USA – important for both humans and ecosystems.  Clearly, even without human interventions, rivers have a point at which they can no longer deal with large increases in discharge, but our own human interventions are making it worse.

We can’t have it both ways – we can’t cause increases in discharge from climate change, and then prevent natural processes that might mitigate this change from occurring. When I talk about human intervention, I mean many different things: urbanisation on a floodplain, building dams/reservoirs, and excessive water withdrawals/additions. All of these things alter floodplains such that rivers are less able to buffer and adapt to changes in discharge. These changes leave humans at risk through increased flood risk and contamination of water supplies used for consumption. Changes in yearly flow regimes can also have impacts on agricultural production, particularly in irrigation fed agriculture.

Ecosystems are also at risk too. Though they are clearly biased towards certain conclusions, the WWF (2004) and their beautifully named ‘Dam Right’ initiative identified 21 river basins at risk of severe ecological degradation, the cause being multiple (six and over) large dams either constructed, planned, or under construction within these basins. Palmer,et al. (2008) have shown that it is likely basins impacted by dam or extensive development will suffer greater changes in discharge and water stress in the face of climate change, as opposed to naturally free flowing rivers – see Figure 1.

Figure 1 - Relative percentage (%) change in discharge in large river systems around the world, from present to 2050, under two different climate models. From Palmer, et al. (2008).

We change the climate, and we also change the land. Maybe we aren’t so far from a hydrological ‘Anthropocene’.

A Long Wey to Go

It is time to diverge from pure theory, and try to illustrate a few points with a case study.  In my last post, I talked about the River Wey (a tributary of the Thames) and how urbanised floodplains have exacerbated flooding along some parts of the river. I have been studying this river in some detail for the last 6 months, and it illustrates some of the points I have been talking about so far.

The UKCP09 climate projections are often used in thinking about the potential effects of climate change on water resources in the UK (Christierson, et al. 2012). These projections suggest a variety of changes, but the general pattern is an increase in evapotranspiration and the intensity of rainfall events. For the majority of its length, the Wey is bounded by ample natural floodplain, making any flooding slow moving and of a slow intensity – however the channel is more constrained in some urban areas and flooding can be faster moving and intense (EnvironmentAgency, 2010).

The Wey flooded considerably in some urban areas during late 2013/early 2014. Wang, et al., (2012) have suggested that the weather leading to recent flooding on the River Wey may be associated with a pattern of increasing storm intensity in the North Atlantic, consistent with patterns of climate change. Data suggests that the highest flows since the extreme floods of September 1968 (a 1 in 1000 year event) were recorded on the River Wey (Met Office and Centre for Ecology and Hydrology, 2014). This is a very large change in discharge and runoff, likely forced by climate change. At different points along the river, thousands of homes and businesses were affected by floodwater. 

If the rainfall event that caused these flows has been influenced by climate change, we need to act sooner rather than later, or this level of flooding could become a common occurrence. Things need to change on a number of scales; all the way through from local building on the River Wey’s floodplain to global emissions of greenhouse gases. Herein lies the problem – there are so many different actors involved, that just solving changes in discharge caused by climate change along a small catchment is very difficult.

Wall to Wall

The news today is dominated by the headline ‘Treasury unveils £2.3bn for (flood) schemes to protect homes’. In the light of recent flooding during late 2013/early 2014, this may seem a prudent move by the government. As we have seen, climate science and hydrological modelling suggests discharge and runoff (and thus flood regimes) are going to change in the face of climate change. Reynard, et al. (2001) carried out hydrological modelling based on climate change scenarios for the largest rivers in the UK and predicted increases in the frequency and magnitude in the flooding of these rivers. Interestingly, they find that land use is the largest influence (bar climate) on the flood regimes; with an increase of forest in a catchment flooding is less severe, but an increase in urban surfaces increases both the frequency and magnitude of flooding. Clearly, land use is hugely important for addressing future riverine flooding in the UK. 

It may sound overly cynical, but the announcement made by the UK Treasury today is little more than political spin – the money is not new. The Committee on Climate Change noted that these new schemes (in the Thames Estuary, Oxford, Somerset) are perhaps missing the point, and that this money should maybe go into managing existing defences. Hard engineering seems to be the overriding strategy in the new schemes, and this likely reflects the link between flood disasters and the demand for public policy (Johnson, et al. 2005). After the floods last winter, the government wants to appear strong on the issue to the electorate – what better way than to allocate money to good old fashion hard engineering? I have no doubt these schemes will have been properly researched and will protect some properties, but hard engineering is unlikely to be a long term answer to changes in flood regimes.

As Reynard, et al. (2001) have suggested, land use is a very important issue that we should be focusing on, in the form of catchment management, and preventing overdevelopment on floodplains. Rivers with wide, natural floodplains have a far greater natural capability to deal with flooding. For example, take the River Wey (Surrey), a tributary of the Thames that experienced some severe flooding last winter. The majority of the river has ample natural floodplain, and such severe flooding did not occur in these areas. However, there are a few pinchpoints where the floodplain has been built on. Urban surfaces like concrete and tarmac have no ability to absorb water, and thus flooding occurred heavily at these parts of the river – see the picture of Guildford car park under many feet of water.

Urbanised foodplain = underwater.

Today’s new funding is mostly a load of spin, and seems to very focused on making a political point, and trying to win the hearts of the electorate before the next election. Such flood projects have their place, but long term we need to focus on why we have made property so susceptible to flooding, and why we are allowing changes to the hydrological cycle to occur. If you don’t want to be flooded, don’t live on a floodplain.

Closer to Home

Having been talking a lot at a huge, global scale, it’s time to turn things on their head and focus in on the national scale. Dear old blighty (the UK, for the uninitiated) naturally has large banks of hydrological research, given the strength of our academia. The question is, what is going to happen to river flow, runoff and flooding in the future? Will we still be able to sit near the local river in driving rain on a ‘summers’ day, eat poor quality ice cream and have a family argument?

In the early 2000s, Defra advised that research in the UK should take account of possible climate change by modelling a 20% increase in peak flows in the next 50 years. Back in 2004, Reynard, et al. (2004) carried out hydrological modelling using UKCIP02 climate scenarios (UKCIP09 are now the most recent) and concluded that all but the most extreme increases in flow across major catchments in the UK would be within this 20% boundary. This would suggest the 20% peak flow increase is a useful precautionary value in the face of great uncertainty of the effect of climate change on river flow. Kay, et al. (2009) identify the variety of sources of uncertainty in hydrological prediction: future greenhouse gas emissions; Global Climate Model (GCM) structure; downscaling from global to regional model level; the structure and parameterisation of hydrological models, and the natural internal variability of climate. Clearly, this is not an easy process with any level of great surety. Both Kay, et al. (2009) and Prudhomme, et al. (2003) concur that the largest uncertainties are attributed to the type of GCM that is used, suggesting predictions for the future should be made using a variety of GCMs under a range of climate scenarios.

With respect to likely effects of climate change on rivers in the UK, Wilby, et al. (2008) suggest a connection between the seasonality of flood and the seasonality of climate in larger catchments; the peak of annual rainfall is followed by annual peak of discharge, as with the River Thames. For smaller catchments, large flow events tend to be more flashy in nature and occur directly after the heaviest rain, whatever time of the year – future changes in precipitation intensity could have implications for flows and flash flooding in such catchments. Prudhomme,et al. (2003) model the largest increases of peak flow in the UK (using a variety of GCMs) to be in northern England and Scotland, with a 0.13% increase in flood magnitude each year, compared to 0.04-0.05% in southern catchments of England. While some of this may be related to catchment morphology, there is a clear geographical signal of climate change across the country. 

After all the global scale examination, we see that even an island as small as the UK will likely be differentially affected by changes in discharge and flood magnitude as a result of climatic change. At least we’ll still have the weather to moan about.