Bangladesh flood action plan

The Flood Action Plan (FAP) relies on huge levees along the rivers’ length. At an estimated US$10 x10^9 they could take 100 years to build. Up to 8000 km of levees are planned for the 1600 km of river in Bangladesh. They are not able to withstand the most severe flooding, such as in 1987, 1998 or 2007. The embankments contain sluices to reduce water flow and to control damage caused by flooding.

They are set back from the river, which protects them from constant erosion and is also cheaper to install.

The FAP still has many issues:

  • Increased time of flooding, since embankments prevent back flow into the river
  • River channelization by levees may increase the risk of flooding downstream and in the area between the levees
  • Channelisation will also increase deposition between levees rather than on the floodplain, which has been seen to be an issue in Lousiana during Hurricane Katrina
  • Not enough sluices have been built to control the level of floodwater in rivers. There may be increased damage to land if embankments are breached
  • Sudden breaches of embankments may also deposit deep layers of infertile sand reducing soil fertility and affecting agriculture
  • Compartmentalisation of the drainage basin may reduce the flushing effect of floodwaters to remove pollutants
  • By preventing back flow to the river, areas of stagnant water will be created which may increase the risk of diseases related to water supply
  • embankments may cause wetlands to dry out, losing biodiversity
  • Decreased flooding will reduce the number of fish, which is a major source of protein

The rivers are largely controlled by factors outside the country. Floods in Bangladesh are also not just related to rivers, and the rivers are needed for agriculture and other industries.

The FAP has overall led to social, environmental and economic issues within the Bangladeshi population.

San Gabriel River Flood Controls

The San Gabriel river is situated in California, and runs through Los Angeles. It has the most extensive water control system in the world, in a basin 3,000 km^2.

The Ovens lake is 400km from LA, Sacremento river is 600km away and Parker Dam is 400km away. Between them 300 million m^3 of water is transported to LA daily.

Problems regarding the river- why is flood control needed?

LA is the fastest growing urban area in the USofA. In 1890, LA had a population of 11,000, which was largely made of native American tribes, but by 1990 it had a population of 11 million. In 2015 the population was 3.884 million people.

The headwaters where the San Gabriel starts are in mountains up to 3,000 m high. The valleys are unstable and steep-sided. They are fairly straight with gullying along hillsides; overall they are v-shaped with a steep gradient.

Where the valleys meet with the lowlands below, there are large alluvial fans of depositional material, and there was, previously, large amounts of river braiding.

  • Large population growth
  • Quickly saturated soil in mountains during rain
  • Little vegetation present in the mountains- semi arid environment 
  • Concrete surfaces in town

Past flood

In 1938 there was a flood of the San Gabriel due to a lack of river controls. This was during the economic depression that affected much of the world, so there simply wasn’t funding to help prevent flooding. There was also less money available to build so that structures could withstand flooding.

Control measures

The Los Angeles Flood Control Authority was set up in 1915, after flooding in 1914. The first flood control works started in 1917. There are five main components (listed in order from the headwaters):

  1. Check dams on upper tributaries. These stop debris from the uppermost points in the stream.
  2. Debris dams at exits from the mountains. These collect dirt and sediment carried from higher up. This reduces damage caused by flooding and reduces the chances of there being blocked channels.
  3. Control dams. These control the flow of water downstream. Water is trapped behind and released at a steady rate as water levels lower. The first control dams along the river are 25 km from the source and control water flowing from 500 km^2
  4. Spreading grounds.  Water is absorbed into the soil here- a sort of holding pen is built where water is trapped. These occupy vast areas with only a shallow layer of water, so water readily either evaporates or percolates into the soil. One examples of these is Rio Hondo.
  5. Concrete-lined channels.  These control the direction of travel of the water. A deep channel with a large hydraulic radius is provided, so water travels efficiently down it. As it is very deep, even if water gets past the other measures, such as if there is a storm and a lot of water falls away from the mountains, then there is still a very low chance of flooding. There are over 640 km of concrete lined channel.

Only 2% of the rainwater within the San Gabriel basin ever reaches the sea. The rest percolates into permeable rocks, evaporates, or is used by people.

The dams have to be emptied after rain to remove the debris. The debris is either dumped elsewhere or used as aggregate for engineering.

The Whittier Narrows are an example of a site where a single dam has blocked two tributaries.

One of the large dams is the Santa Fe dam, which is 7 km long and contains an area of water of 445 hectares.

Issues with the management

  • Less sediment reaching the coast.
  • Beaches are not being built up
  • Removes natural attraction of the beaches
  • Removes natural shoreline protection from the beaches, putting beach-side properties at risk.

 

(Image Sources: https://en.wikipedia.org/wiki/San_Gabriel_River_(California) https://dpw.lacounty.gov/wmd/watershed/sg/ http://pubs.usgs.gov/wri/wrir034279/wrir034279.pdf https://www.kcet.org/shows/departures/the-other-river-that-defined-la-the-san-gabriel-river-in-the-20th-century )

 

River Mole part II

(Read part I First: https://deigmologyblog.wordpress.com/2016/05/06/river-mole-part-i/)

Factors affecting flood risk

  • Most of bedrock is impermeable, largely Wealden clays and greensand; 60% is “low permeability”
  • Basin shape- the river has two “sections” to it’s catchment basin. The upper river is mostly circular while the lower river is mostly long and thin. This shape is exacerbated by the Mole gap at Dorking, where the river has to pass through a narrow gap in chalk hills. During flooding, water can become trapped there, worsening the impacts of the flooding on the town. Circular basins result in higher and shorter flood peaks.
  • The Mole has higher relief than most of Southern England, with maximum elevation of 265m at Leith Hill. The Downs from Ranmore to Box Hill are 100m taller than the valley below.
  • Precipitation- the Mole has a fairly modest 750mm/pa.
  • Gatwick airport- only a narrow channel is available for water to flow through
  • Prevalence of urban areas. Urban areas mean more concrete, which is impermeable.
  • Effluent discharge- The water is normally treated and fed into the Mole. Some effluence comes from other catchment areas.

Flood defense schemes:

Upper Mole

  • Upper Mole alleviation scheme: Protects urban areas n upper river- £15 million Environment Agency project
  • Upgrading of flood retention at Clay lake
  • Constructing a higher dam wall at Tilgate Lake (adding 2.5m onto the height)
  • River Mole redirected at Gatwick Airport, running underneath it
  • Flood ponds at Gatwick. Latest pond can store 180,000 m^3 of water, supposedly halving flood risk. Water is tested and has its quality improved to high standards before being allowed back into the river, to ensure heavy metals from the planes being washed off do not enter the river.
  • Worth Farm

Worth Farm is located near Crawley and the M23 motorway. An embankment dam 6.5m tall has been built along the farm. No water is stored in the reservoir through most of the year, and the land can be used for farming, mostly of cattle.

During floods, the water in the Mole that would normally just flow under the embankment is captured and held, creating a reservoir. Over a few days, this water slowly drains out.

Water level can rise by 3.5m behind the embankment, which is predicted to occur 20% of years, while reaching peak capacity is predicted to happen once every 200 years on average.

Lower Mole

  • Weirs (normally natural-looking features involving using branches to redirect and limit the flow of water) and bank defenses at Molesey built in 1968.
  • Barrage preventing discharge directly into the Thames, reducing any potential impacts (unlikely to be much!) upon London
  • Redeveloping natural local wetlands
  • Moors Project managed by Surrey Wildlife Trust
  • Creating Water Meadows. Reinserting natural environments, and vegetation, which can absorb the impact of flooding and intercept some water to reduce the volume continuing to move downstream
  • Flood warning scheme

Factors affecting management schemes

  • If flooding worsened after installations of defenses elsewhere, tensions are created between towns
  • Hard engineering requires frequent repairs
  • Natural replacements of the environment can be expensive and take a while to become cost effective
  • Hard engineering is expensive to build
  • Locals in Leatherhead blame Molesey defenses for the flooding in their town
  • Hard engineering can affect an area’s aesthetic appeal.

River Mole part I

Much of the area is on impermeable clays, and is mostly flat. Because of the characteristics of the area, flooding can last for 3 days after any storms are gone.

Types of flooding

  • Drainage capacity can be overwhelmed by the water, leading to surface water flooding.
  • Blocked gullies and surcharging of outfall pipes can cause flooding by major roads.
  • Groundwater flooding can occur when groundwater rises above the ground surface, which is heavily effected by the local geology. In surrey this is most common in chalk-based areas, such as the north downs.

River Mole floods, December 2013- January 2014

Although naturally being a small river,the Mole has the potential to cause major disruptions due to having several major transport routes, namely Gatwick airport, East Surrey Hospital, the M25 and M23, and the London-Brighton railway housed (at least partially) within it. Below are the specific impacts of the 12/13-01/14 floods:

  • Gatwick airport power failure; delays with baggage handling on 24/12/13; 100 flights delayed or cancelled; thousands left stranded or abandoned as rails disrupted too.
  • Power cuts; 100 homes in Merstham (Near Reigate) and Sidlow left without power for 3 days.
  • Leatherhead crematorium; closed due to flooding.
  • Burford Bridge Hotel, Dorking and Ye Olde Six Bells, Horley; among various businesses submerged and closed during the flooding.
  • Plane Damage: Damage to planes in Redhill Aerodrome (South-East of Reigate)- 71mile per hour winds measured on Kinley in the North Downs.
  • Road closures and rail closures; two closures of the M24 at Mickleham, A217, A23 around Horley and Leatherhead. Creates “islands” making the job of the emergency services very difficult.
  • Flanchford bridge, Reigate, damaged by the waters, and some bridges in the area are still yet to be repaired.
  • Flooding of hundreds of residential properties; 40 in just Fetcham underwater for 15/12/13.
  • Morrisons in Reigate, and other businesses elsewhere, flooded.
  • Damage to telephone land-lines in Brockham village.
  • Cars swept away, or people stranded in stalled cars.
  • People stranded in buildings, including 27 at the Burford Bridge Hotel.

Management

Governmental/Local Council level:

  • A long term drainage strategy is being developed, including sustainable drainage systems
  • The Environment Agency is implementing flood management strategies along the Lower Thames.
  • Insurance policies by the government (and energy companies) to recompensate families with losses from the floods.Similarly, management schemes are being funded, such as property level management schemes.
  • New technologies being created and implemented to better understand the flood risks.

Environment Agency:

  • Engineered components: Three flood diversion channels and improvements to weirs (naturally-designed barriers over the top of a river to slow or redirect some of the water flow).
  • Floodplain management component: working more closely with local authorities to ensure future developments account for flood risks, routes for flood flow and potentially diversion channels.
  • Community-based flood protection measures: Flood warning services and property-level protection.

The Gatwick flooding scheme and other factors will be mentioned elsewhere as this post is already long.

(Part II here: https://deigmologyblog.wordpress.com/2016/05/10/river-mole-part-ii/)

(Image sources: https://rgsweather.com/2014/01/04/flooding-on-the-river-mole-surrey-causes-and-management/ https://en.wikipedia.org/wiki/River_Mole I would highly recommend the http://www.rgsweather.com site – and not just because it’s run by one of my geography teachers- it is genuinely a very good site!)

River Tees

1-2c_tees_drainage_basin

The River Tees is located in the North East of England.

The Tees’ source is located in the Pennines close to Cross Fell, at 893 metres above sea level, where roughly 1200mm rainfall occurs annually, although some areas have 2000mm. It is 100km long.

 

In the upper course of the river is a famous waterfall known as “High Force” as well as a gorge accompanying it. High Force is 21m high, and has been formed over millions of years. The top of the waterfall is made from a very tough rock called “Winstone”. Below the winstone lie layers of limestone, sandstone and shale. The river erodes rapidly through the rocks at the bottom, due to their being relatively soft, and eventually, once a large enough overhang has been created, the rocks collapse downwards under gravity, and create a sheer rock face for the waterfall to flow down. 20m^3second^-1 runs across the winstone rock.

High Force also creates potholes nearby. Smaller rocks are rotated by the river’s flow after being trapped in depressions in the rock and eventually wear downwards to make round, deep holes.

 

Close to Yarm, the Tees forms large meanders, which have formed ox-bow lakes. Flooding has caused the formation of levees.

During the last ice age, water was locked up on land, meaning that sea levels were lower. When sea levels rose, the river was rejuvenated, meaning that it tried to achieve a natural long profile again (of a long and mostly gentle curve from the source to the mouth). The adjustments created knick points, most notably High Force, which is eroding back to form a new long profile, but this process has also caused an estuary to be made as the sea flooded the original mouth of the river.

Parts of the estuary area are SSSIs (Sites of Special Scientific Interest) (which are carefully managed due to holding unique ecosystems such as seal sands), or are just generally important for the local wildlife, notably migratory birds and seals.

The flat estuary area is also attractive to large industries due to the flat land, and the fact that the river is quite wide, allowing container ships to their premises easily.

 

The Tees takes large meanders through its middle course. Directly, it is 30km from Darlington to Teesmouth, but along the river it is 75km. Several miles have even been cut off from the original course to shorten boat journeys.

 

The Tees is used for water-sport, for the protection of wildlife habitats, for farming in the fertile alluvial soil, and for big industries, as well as for residential use.

Cow Green Reservoir: (Upper course). The highest reservoir in the area. The river supplies water (high water quality) and also helps flood control.

Sheep Farming: (Upper course): The land in the upper course is too steep for machinery and too acidic for crops, so is instead used for sheep.

Tourism and conservation: (Upper course). The moorland, High Force, shooting etates, and a nature trail at Windybank Fell all atract visiots, as do the Pennine Way, and the rural villages of the area. Tourism provides jobs and helps stimulate the economy, but also causes congestion, litter, and overcrowding.

Tees Barrage: (Lower course). Opened in 1995 for sport and flood control.

Seal Sands: (Lower course). The mudflats around the river mouth are very important for seals and for migratory birds.

Urban and Industrial use: (Lower course). Large towns such as Stockton and Middleborough and Teesdale support large industries such as chemical industries, ships, steel-making and engineering.

ICI: (Lower course). Petrochemical industries based at Billingham and Wilton are placed to receive North Sea oil.

Shipbuilding has been replaced by oil platform construction.

 

bowesfield-site-aerial-pic2Management

The management of the Tees has various aims:

  • Reducing flooding
  • Improving the water supply
  • Improving the water quality
  • Improving navigation
  • Providing recreational opportunities

Reservoirs, such as Cow Green and Grassholme have been built.

In the 19th century, cut-offs were built around Stockton to straighten the river, and flood protection schemes have been built recently at Yarm. There is also a water sports complex at the Tees barrage.

Yarm used to be a huge port town, enclosed by a meander. However, it was difficult for boats to travel 18km inland on the Tees. Commerce was broken off by a bridge closer to the sea at Stockton. The Victorians saw too many meanders and built a 3km long straight channel.

The estuary used to be a marsh area but is now almost entirely artificial. Heavy industries reside on the reclaimed estuary mud. There is space and access to open water, so it is used by North Sea oil and petrol companies, and by a nuclear power station. 90% of the raw materials for these are carried from outside because the channel is deep enough for the them.

 

(Image sources: https://co-curate.ncl.ac.uk/river-tees/ http://www.banksgroup.co.uk/new-residential-development-plans-approved-for-unique-bowesfield-park-site/)

(PS: I promise that I won’t do so much explaining of processes once I’ve explained it once!)