1.4 Hydrology and fluvial geomorphology: the human impact Flashcards
How do dams and reservoirs impact where water is stored and how it moves
Increase in evaporation due to construction of large dams and holding reservoirs to increase storage of water
Eg Lake Nasser storage reservoir behind the Aswan dam loses up to a third of its water due to evaporation
Decrease in river discharge if water retained by dam – this controls flow and reduces risk of flooding so reduces potential increase in surface storage
Increased percolation as water stored percolates and increases groundwater store and flow as it seeps out of reservoir
Whilst water loss by evaporation may be reduced by using chemical sprays on the water, by building sand-fill dams and by covering the reservoirs with plastic, as the climate warms and rates of evaporation increase with warm air able to hold more moisture, this issue can only get worse.
The impact of this human modification on stores and flows in drainage basins is increasing; number of large dams (more than 15m high) that are being built is increasing rapidly and reaching a level of almost two completed every day. There are over 48000 large dams. With increasing industrialisation and demand for water rising with the rise of the middle classes, the demand for dams is only set to increase.
As rivers are transboundary with complex regimes, building dams causes reduced channel flow downstream and in turn greater water abstraction downstream – the result of this on the Nile is that as a result of the Aswan Dam, hardly any water reaches the Mediterranean.
How does water abstraction impact where water is stored and how it moves
Impacts:
Groundwater refers to subsurface water that is stored under the surface in rocks. It accounts for 97% of all freshwater
Abstraction from groundwater supplies lowers the water table which in turn may lower the base flow in rivers, even causing them to dry up. In coastal areas, it can lead to salt water intrusion of groundwater aquifers affecting fresh water stores
Over-abstraction is common in areas where the demand for water exceeds the amount available during a certain period. This is especially in areas with low rainfall and high population density and in areas where there is intensive agricultural or industrial activity.
Saline intrusion is widespread in the Mediterranean coastlines of Spain, Turkey and Italy where the demand from tourists is a major cause of over-abstraction
In Malta, groundwater can no longer be used for domestic consumption or irrigation because it has been contaminated by saline intrusion.
Irrigation is the main cause of groundwater overexploitation in Italy where overexploitation of the Po River in the region of Milan aquifer has led to a 25m decrease in groundwater levels over the last 80 years
In the High Plains of Texas in the 1930s, the groundwater system was stable and in a state of dynamic equilibrium with long-term recharge (refilling of water pores where the water has dried up or been extracted by human activity) equal to long-term discharge. However, groundwater is now being used at a rapid rate to supply centre-pivot irrigation systems. In under 50 years the groundwater level has declined 30-50m in a large area to the north of Lubbock, Texas.
Evaluation: Groundwater abstraction is essential in some parts of the world and can be managed:
Artificial groundwater recharge is possible as a means of storing water – this stops streams drying up in dry periods and the bore holes can used to extract water when it is in short supply – this is used in southern England
In areas where water supply is low, abstraction for irrigation allows farmers to grow crops. It is also used to allow countries to industrialise.
Also possible to recharge groundwater sources provided sufficient water is available – in permeable areas, water spreading is possible (a form of infiltration and seepage)
In impermeable areas pumping water into deep pits or wells is possible eg used extensively on heavily settled plains in Israel in order to replenish reservoirs and overcome problems of saltwater intrusion
However:
In Europe, groundwater is the main source of fresh water and as 97% of freshwater is stored in groundwater supplies, the scale of this problem is serious.
Ground water abstraction is prevalent across the world, regardless of climate – from Denmark to Saudi Arabia, from Australia to the High Plains of Texas in the US – the scale of the problem is significant.
Solutions are only possible if the country has sufficient water and technology. As climate changes and rainfall patterns shift, this will become increasing problematic and whilst some soil moisture may be recycled by evaporation into atmospheric moisture within a matter of days or weeks, groundwater may not be recycled for as long as 20,000 years – this is a very long term problem. Where recharge is not taking place, groundwater is considered a non-renewable resource.
How does urbanisation impact where water is stored and how it flows
Urbanisation is the increase in the proportion of the population living in urban areas. It is usually accompanied by urban sprawl as the % of land area covered by urban areas increases.
Impact:
leads to a huge reduction in evapotranspiration due to the lack of vegetation.
Removal of trees and vegetation to facilitate urban growth drastically reduces evapotranspiration and interception whilst increasing surface run off but reducing surface storage (short term storage through throughfall and stem flow).
During the initial construction of the urban settlements, decreased infiltration, throughflow, percolation and lowered groundwater levels are common as surface gets paved over with impermeable concrete and the ground is compacted reducing porosity.
As groundwater levels subside, base flows in rivers (the water that is fed into the river by groundwater seepage) also decreases and storm flows (excess from throughflow and overland flow) increase.
As continued and complete urbanisation continues, there is decreased porosity and permeability. Discharges in rivers can reach higher peaks after rainfall events and water travels more quickly to the river as overland flow.
Storm drains and channel improvements may reduce surface storage as water is channelled away quickly rapidly over impermeable surfaces into drains and gutters. This reduces overland flow in the longer term but leads to problems downstream with increased discharges in the river along its course. The reduced surface storage will also decrease evapotranspiration rates
Evaporation may actually increase as warmer temperatures caused by anthropogenic (human caused heat eg air con, central heating, industry, vars etc) and higher specific heat capacity of building materials in urban areas actually increase air temperatures (urban heat island effect) · Urbanisation can also impact on groundwater abstraction as water is taken from aquifers for an increasing population with increasing demands eg Jakarta, Indonesia
Evaluation:
Urbanisation is on the increase – the UN says that by 2050 it will have reached 68% of the world’s population. 90% of this increase will take place in Asia and Africa and will impact stores and flows in areas where water is often scarce which may lead to even more groundwater abstraction.
How does deforestation impact where water is stored and how it moves
Removal of trees and vegetation to facilitate urban growth and farming drastically reduces evapotranspiration and interception whilst increasing surface run off but reducing surface storage (short term through throughfall and stem flow) and reduces the time lag so that channel storage in rivers is much higher leading to risk of flooding.
Infiltration rates are reduced – infiltration is up to 5 times greater under forest compared with grassland. This is because the forest trees channel water down their roots and stems. With deforestation there is reduced interception – infiltration capacity on bare soils is 0-10mm/hr v 50-100mm/hr on vegetated soils as vegetation otherwise produces better drainage through roots acting as pipes and percolines and better soil fertility encouraging earth worms etc which aerate soil and increase porosity. Bare soils also lead to rainsplash which may create an impermeable crust on the surface. Machinery used to deforest also increases soil compaction due to machinery so porosity is decreased, reducing infiltration and leading to greater overland flow
Population growth in Nepal, in the Himalayas, has led to forests being cut down to provide fuelwood and terraced fields for agriculture. Overgrazing of the deforested land has led to over-grazing and washing of sediment overland with increased run off and this has caused river channels to lower their channel capacity. The local rivers are tributaries of the Ganges which have seen increase in flood risk further downstream in Bangladesh as a result of deforestation. Illegal logging by the Taliban in the KPK valley in Pakistan was also partially blamed for the 2010 floods since interception and infiltration were reduced, overland flow sped up transfers to rivers whilst sediment washed off slopes and logs stored in the rivers, reduced the channel capacity increasing the risk of overtopping and floods.
The province of Yunnan in China saw mass deforestation in the 1960s because of the need for land to produce food for a growing population. This led to an increase in flooding of the Yangtze due to higher peak discharges in 1998 which saw the government introduce an afforestation programme although the positive impacts on stores and flows was longer term
Evaluation: Scale of the problem: According to Global Forest watch there was a 9.7% decrease in tree cover from 2000 to 2019 · Some countries are more affected than others
How does afforestation impact where water is stored and how it flows
Afforestation is believed to have the opposite effect as deforestation with increased evapotranspiration and interception and so on.
Evaluation: The province of Yunnan in China saw mass deforestation in the 1960s because of the need for land to produce food for a growing population. This led to an increase in flooding of the Yangtze due to higher peak discharges in 1998 which saw the government introduce an afforestation programme although the positive impacts on stores and flows was longer term
Evaluation – however …
The impact of trees on interception and overland flow and peak discharges will vary with type eg coniferous trees intercept more than deciduous ones in the winter as they lose their leaves and less in the summer.
Following afforestation. Sediment yields in the River Severn catchment were seen to have increased by 4 times indicating greater overland flow.
Believed that afforestation in the shorter term has negative impacts because new sapling trees leave little ground cover of vegetation and bare soil is left for tractor access routes and fire and wind breaks.
Bare earth leads to rainsplash and low infiltration rates of only 0-10mm/hour whereas on vegetated ground, infiltration rates can reach 50-100 mm/hour
But….
Despite the short term negative impacts on stores and flows, after only 5 years, the amount of erosion and sedimentation in the Severn Valley catchment had declined suggesting that in the longer term, afforestation will indeed have the opposite effects of deforestation .
What are are the causes of flooding
The main causes of floods is the excessive input of precipitation into the river basin, resulting in the flooding of floodplains usually in the lower catchment areas. Lower catchment areas can also be affected by storm surges from the ocean or sea, as strong onshore winds blow water onto a shoreline and up rivers and onto their deltas and floodplains – often during hurricane/cyclone events, as in Bangladesh.
Human activities can influence flooding by changing the physical conditions within a river catchment area so that overland flow is increased, allowing for a much more rapid response to a rainfall event that could potentially lead to flooding. Activities such as deforestation and urbanisation, especially on floodplains adjacent to river channels, may cause increased overland flow at the expense of slower baseflow. Similarly, the channel capacity may be reduced by channel straightening or the concreting of the channel bed and sides.
What are the impacts of flooding
Worldwide, floods were responsible for 84% of all disaster-related deaths between 2000-5, and for 65% of disaster-related economic losses between 1992 and 2001.
Globally, floods accounted for 31% of the 9632 natural disasters that occurred in the 20th century. The 4035 major floods during the period 1900-2012 killed 9.9 million people and affected another 3.6 billion people to some extent, and caused economic losses totalling $550 billion.
In Asia, 1625 of these flood related disasters (40% of total disasters worldwide) resulted in 6.8 million deaths (98% of deaths worldwide) and caused $330 billion in economic losses.
Other flood-related losses include damage to ecosystems, land, and water quality degradation, and an increased incidence of waterborne diseases, such as cholera. Flood damage in Asia in the 20th century was estimated at 60% of global economic losses due to floods.
From 1965 to 2011, total economic losses due to floods in Asia showed an upward trend which may be attributed to the greater frequency of floods, the acceleration of economic development in flood-prone areas, or, more usually, both.
What is hard engineering
Hard Engineering = construction of artificial structures to control flooding
· Building of dams & reservoirs – to control discharge of river
· Raising levees
· Channel straightening or deepening – to accommodate increased flows
· Diversion spillways or land drainage
What is soft engineering
Soft Engineering = working with natural processes & features rather than controlling them artificially
· Flood abatement – changing land use patterns to modify speed/amount of run-off (afforestation, terracing, changing farming practices, planning restrictions on building to reduce run off etc.)
· Land use zoning & land use planning
· Wetland and river-bank conservation
· River restoration
What are forecasts and warnings
Forecasts & warnings - These allow the adoption of behavioural strategies – people will adjust their lifestyles and take personal responsibility for the hazard risk
Long-term prediction: recurrence intervals (10-100 year floods); catchment models using satellite photography (to understand how drainage basin will respond to predicted rainfall event) & warnings
Short-term prediction: monitoring approaching rainfall events & warnings
Need to make sure these are communicated to people and they understand the risks so they can move animals, furniture etc
They need to have an emergency plan of what to do if these are issued
They should take out insurance
How does flood prediction work
· Long term measurements of individual rivers can determine frequency of floods, their magnitude and their recurrence interval ie how often a flood of that size will occur. These recurrence intervals are usually based on 10, 50, 100 and 500 year events.
· The return period or recurrence interval is a statistical estimate of the average time interval before a flood of a certain magnitude recurs.
· As a rule, the higher the magnitude event, the longer the return period.
· Analysing flood recurrence intervals – done by analysing data of past flood events and working out how often a flood of a particular magnitude will occur in a specified time span by plotting flood heights against average time intervals
· This can be useful tool in reducing the impacts of flooding – can feed into flood action plans, landuse zoning and regulation
Evaluation:
· However, can cause complacency/ are often confusing to residents – eg Cumbria in North of England experienced two 1 in 100 year event floods in 2009 and 2015
· Catchment modelling may also be done by computer modelling but there are limitations because historic data is not always available and if it is, it may not be reliable because data collection practises have changed over time
· Prediction is also less successful in small drainage basins which respond quickly to rainfall events and create flash floods. These are especially common in arid and semi-arid areas.
· Landuse will affect return periods – urbanisation especially so any changes need to be updated in the flood recurrence interval calculation
· Historical records are also becoming unreliable guides for return periods caluculations due to climate change.
How does forecasting and warning work
· Most widely used strategy to help reduce impact of flooding.
· Uses satellite imagery, rainfall radar, rain gauges and discharge recorders and accurate weather forecasting to provide an early warning system for the potential flooding of a drainage basin. This can be shared via social media, radio, tv etc so people prepare and protect themselves or evacuate.
· Advances in weather satellites and use of radar for forecasting has increased accuracy – this is especially important in places like the UK where floods are associated with deep depressions (low pressure systems) that roll in from the Atlantic and create long lasting rainfall events over a very large area.
· Satellite imagery can be updated on a 15 minute basis and allow approaching weather systems to be identified and can allow for tropical storms to be tracked and their course be predicted
Eval:
· However, despite improvements in flood forecasting and warning in the 1980s and 1990s in terms of weather satellites and the use of radar for forecasting, over 50% of households in England and Wales have less than 6hrs of flood warning time. This reduces effectiveness in terms of preparing for floods in terms of evacuation and removing valuables from water’s way.
· Does not prevent floods Also still errors and limitations
· LICs in general have less effective forecasting although Bangladesh is an exception – most floods start in the Himalayas so authorities have 72 hours warning.
–> Improvements could be made in terms of:
Improved rainfall and snow pack estimate and better and more effective forecasts
Better gauging of rivers
Better sharing of information especially across transboundary rivers
Eval:
· One issue with prediction and forecasting is that extreme events are infrequent.
· Many urban areas are equipped to deal with floods that happen annually or once every ten years.
· They are less well-equipped to deal with low-frequency, high magnitude events that have a recurrence interval of once every 50 or 100 years.
· In Bangladesh for example, monsoonal floods regularly cover 20-30% of the flat delta. However, in extreme floods, almost half the country may be submerged eg in 1988 46% of the land was flooded and over 1500 people died. In Pakistan, the abnormal rainfall and monsoon rains amounted to double the 50-year average rainfall total. Although it was known that 4-10 cubic km of water would pass through the Taunsa, Guddu and Sukkur Dams – there was poor anticipation of the scale of the flood and no measures were taken to mitigate the effects further downstream.
· Pakistan is also limited in its prediction and forecasting – it does not cover the whole of the Indus river basin – Swat valley did not receive any warnings in 2010, people caught unawares which accounts for the high loss of life and property in this area. The Indus is a transboundary river with part of its catchment in neighbouring countries. Cooperation with these is limited so that key information regarding precipitation input and river discharge from the upper catchment areas is insufficient and delayed.
How do levees work:
Levees are the most common form of river engineering designed to prevent flooding. They increase the capacity of the channel to reduce the risk of discharge exceeding bankfull and overtopping. These may be constructed some distance away from the main channel to increase channel capacity further. They may also divert and restrict water to lower value land thereby diverting the flood elsewhere.
· Over 4500km of the Mississippi have levees.
· However, these may be breached eg August 29 2005 Hurricane Katrina caused breaches in over 50 different places submerging 80 % of New Orleans city. Most levees failed due to water overtopping them but some failed when water passed underneath the levee foundations causing the levee wall to shift and resulting in catastrophic sudden breaching.
· There are 6000km of levees in Pakistan which cover the critical stretches of the river and these are the main flood defence infrastructure.
Eval:
· However, the height of these levees remains arbitrarily at 1.8m which is higher than the previous flood levels observed in the basin but repair of these remote levees has remained a challenge and remote monitoring of the channel shape and depth is key to the future effectiveness of these to ensure that their heights are adequate
· Sedimentation has also reduced their effectiveness because it affects channel shape and capacity in Pakistan especially due to high and variable discharges caused by glacial melt and significant erosion of the alluvial geology. Deforestation has further affected channel capacity since heavy rainfalls create sheet wash and removal of sediment in areas void of vegetation since interception and infiltration capacity are reduced.
–> Bangladesh: There are over 10,000km of levees yet following a 1 in 100 year flood in 1998 which was affected by heavy rainfall in an El Nino year, 50% of the country was affected by floods with upto 3m of water which lasted 67 days. High tides caused the rivers to back up so breaching levees.
Eval:
· River engineering work can’t totally prevent floods and may actually exacerbate it by leading to false sense of security and by decreased channel capacity by increased sedimentation. People may feel safer as a result and be encouraged to build closer and closer to floodplains – this increases the risk but also urbanisation exacerbates the flood risk by creating impermeable surfaces and from the faster transfer of water to rivers via drains, sewers etc.
· Impacts may also be felt for a longer time since flood waters cannot drain back into the channel and levees may simply protect one area at the expense of others further downstream as flood waters are passed onwards. In the case of the Pakistan Floods in 2010, it was argued that the authorities in the Sindh province deliberately breached the major dams and barrages, and dykes to relieve the pressure of floodwaters and released water onto surrounding farmland.
In Pakistan, 1410 spurs (walls constructed to divert flow and flooding away from towns, roads etc) have been built since 1960
How does channel straightening/dredging work?
Channel straightening/dredging increases channel capacity and speeds flow of water.
Channel straightening is where meanders are taken out of a river by building a cut-through. This straightens out the channel and increases the flow rate of the river, the result is water drains downstream faster and no longer breaches the banks which prevents flooding in the area.
Eval:
· However, it can result in areas downstream flooding that did not previously flood. Wildlife habitats may be affected and also the increased velocity may increase erosion downstream which can lead to greater sedimentation
However, improvements in preventing flooding in one location may have knock on effects further downstream which may include greater sedimentation so raising the river bed, erosion, or increased in discharge. These are also not used that often so channels get clogged with weed so capacity actually drops over time despite enlargement. Generally believed now that holding back water in upland areas and as surface storage eg on farms, as lakes or in peat bogs is more effective that speeding up flow as this transfers problem downstream and also causes water to arrive quickly as the surges did in Pakistan
How do flood relief channels/diversion spillways work:
Diversion spillways are channels that divert water away from the river channel, this reduces discharge and the risk of flooding.
The water is usually diverted around important areas, protecting them from the consequences of flooding.
The water is either channelled so that it re-joins the same river further upstream or directly into a different river.
Some diversion spillways have gates which are used to control the release of water from them.
These are appropriate when it is too difficult or costly to modify the existing channel eg flood relief channels around Oxford and the Intercepting channel eg Jubilee River which bypasses Windsor and is only used during times of flood to divert part of the flow away from a town or agricultural land. As these are not used all the time, weeds may grow in the channel, reducing its capacity when it comes to flood
Disadvantages
Due to an increase in discharge there is an increased risk of flooding downstream of the point where the diverted water re-joins the river (or another river).
In extreme conditions the diversion spillway may not be able to cope with the amount of water entering the channel, this can lead to the surrounding area (which has doesn’t normally flood) being flooded causing even greater problems in the area.
