River Management Flashcards
All 20 cards in this deck
How does prolonged or heavy precipitation increase flood risk?
Rain falls faster than it can infiltrate, so the soil becomes saturated and water reaches the river quickly as surface runoff, raising discharge.
How does impermeable rock in a drainage basin affect flood risk?
It increases flood risk – water cannot infiltrate, so it runs over the surface and reaches the river quickly.
How does urban land use increase flood risk?
Concrete and tarmac are impermeable, and drains and gutters carry rainwater straight to the river, so peak discharge is higher and lag time shorter.
True or false? A drainage basin with steep slopes has a longer lag time than one with gentle slopes.
False. Steep relief speeds up surface runoff, so water reaches the river sooner and lag time is shorter.
What is lag time on a flood hydrograph?
The time between peak rainfall and peak discharge.
What do the rising limb and the falling (receding) limb of a flood hydrograph show?
Rising limb – discharge increasing as water reaches the river; falling limb – discharge decreasing back towards base flow.
What is the difference in shape between a flashy hydrograph and a subdued hydrograph?
Flashy: short lag time, steep rising limb and high peak discharge. Subdued: long lag time, gentle limbs and a low peak discharge.
Give one benefit and one cost of using dams and reservoirs to manage flooding.
Benefit: river flow is controlled and water is stored, and the reservoir can supply water and generate HEP. Cost: very expensive, and land, settlements and habitats are drowned behind the dam.
Give one benefit and one cost of channel straightening.
Benefit: water flows faster and more directly out of the area, reducing local flood risk. Cost: flood risk increases downstream where the water arrives quickly, and habitats are lost.
Give one benefit and one cost of embankments.
Benefit: raised banks increase the channel's capacity so it can hold more water, and they are relatively cheap. Cost: they look unnatural and flooding is severe if they are breached.
Give one benefit and one cost of a flood relief channel.
Benefit: excess water is diverted around a settlement, lowering discharge in the main channel. Cost: expensive to build and maintain, and flood risk may be moved downstream.
Give one benefit and one cost of flood warnings and preparation.
Benefit: people can move possessions upstairs or evacuate, so fewer deaths and less damage. Cost: they do not stop the flood, and some people do not receive or act on the warning.
Give one benefit and one cost of flood plain zoning.
Benefit: building is restricted on the highest-risk land, so damage and surface runoff are reduced. Cost: it limits land available for development and does nothing for buildings already there.
Give one benefit and one cost of planting trees in a drainage basin.
Benefit: interception and uptake by roots reduce runoff, lengthening lag time and lowering peak discharge; cheap and good for habitats. Cost: less land available for farming or building.
Give one benefit and one cost of river restoration.
Benefit: returning the river to its natural course and flood plain slows the flow, stores flood water and improves habitats, with little maintenance. Cost: land must be given over to flooding, and local flooding still happens.
Why was the Jubilee River flood management scheme (Maidenhead, Windsor and Eton) required?
Around 5,500 homes and businesses on the Thames flood plain were at serious risk of flooding, as in the 1947 floods.
What strategy was used in the Jubilee River flood management scheme?
An 11.6 km man-made flood relief channel that diverts high flows of the Thames around Maidenhead, Windsor and Eton and returns them downstream.
Give one social issue caused by the Jubilee River scheme.
It increased flood risk downstream – Wraysbury and Datchet flooded in 2003 – causing conflict between protected and unprotected communities.
Give one environmental issue of the Jubilee River scheme.
Algae and weed growth in the slow-flowing channel harmed water quality, and the weirs and banks were damaged by erosion (though new wetland habitats were created).
True or false? On a flood hydrograph, peak discharge happens at the same time as peak rainfall.
False. Peak discharge comes after peak rainfall, because water takes time to reach the river — the gap is the lag time.