The Flood Hydrograph Flashcards
What is discharge?
The volume of water passing a certain point in a river
What is the equation for discharge?
Discharge = cross sectional area (m2) x velocity (meters per second)
What are the characteristics of a flash hydrograph?
Short lag time and high peak
Basin size of a flashy hydrograph?
Small drainage basins often lead to rapid water transfer
Drainage density of a flashy hydrograph?
A high drainage density speeds up water transfer
Rock type of a flashy hydrograph?
Impermeable rocks encourage rapid overland flow
Land use of a flashy hydrograph?
Urbanisation encourages rapid water transfer
Relief of flashy hydrograph?
Steep slopes lead to a rapid water transfer
Soil water of a flashy hydrograph?
Saturated soils encourage rapid water transfer
Rainfall intensity of a flashy hydrograph?
Heavy rainfall may exceed the infiltration capacity of vegetation, and lead to overland flow.
Rock type of a low hydrograph?
Permeable rocks encourage a slow transfer by groundwater flow.
Land use of a low hydrograph?
Thick vegetation (forest) intercept precipitation, holding the water on its leaves; slowing the movement of rainwater to the ground and so to river channels. Water is also lost to evapotranspiration from vegetation - reducing how much gets to the river.
Soil water of a low hydrograph?
Dry soil soaks up water and slows down its transfer
Define lag time.
The time between peak rainfall and peak discharge.
Define peak discharge.
The point on a flood hydrograph when river discharge is at its greatest.
Define storm flow.
Discharge resulting from storm precipitation involving hot tub overland flow, throughflow and groundwater flow.
Define storm hydrograph.
A graph of the discharge of a river over the time period when the normal flow of a river is affected by a storm event.
Why are storm hydrographs important?
Because they can predict how a river might respond to a rainstorm. This can help to manage the river.
On a storm hydrograph, what happens when happens when all of the storm water has passed through?
The river returns to its base flow
Explain drainage basins as a factor to make a hydrograph flashy.
Drainage basins with steel sides tend to have flashier hydrographs because water flows more quickly on sleep slopes, whether as throughflow or overland flow and so gets to the river more quickly.
Explain drainage basins as a factor to make hydrographs subdued.
- drainage basins underlain by sandstones have a subdued hydrograph because they water soaks into the porous rock.
- THICK VEGETATION cover the drainage basin and intercepts precipitation, holding water on the leaves. This slows the movement of rainwater to the ground and so to the river channels. Water is also lost to due to evaporation and transpiration from the vegetation - this subdues the storm hydrograph, increasing lag time and reducing peak discharge.
- there’s more but see notes
Typically discharge is greater in winter than winter due to decreased temperatures and subsequently less evapotranspiration. But why might peak discharge be in spring?
Spring maximum discharge caused by snow melting in the mountains.
Why might discharge increase more so in October in America?
Increasing rainfall from extra-tropical cyclones increase discharge.
In the U.K., why does discharge decrease throughout the spring?
As plants take up more water and leaves grow, evaporation increases.
Precipitation of a flashy hydrograph?
Heavy storms with a lot of water entering the drainage basin over a short time = higher discharge.
Rain rather than snow
Precipitation type of a subdued hydrograph.
Snow rather than rain because snow takes time to melt before the water enters the channel.
But when there’s rapid melting of snow the peak discharge could be high.
Human factors leading to flashy hydrograph?
- deforestation reduces interception rates, allowing rainwater to hit the surface directly. The lack of vegetation roots reduces the infiltration rate into the soil. This leads to rapid overland flow. Deforestation also exposed the soil to greater rates of erosion, which leads to sedimentation of the channel. This reduces the bankfull capacity of the river and leads to a greater chance of flooding
- large number of animals on small areas compact soils leading to overland flow
- urbanisation
Human factors leading to a subdued hydrograph?
- afforestation increases interception and infiltration which slows down the progress of water to the river channel and subdues any changes in discharge
- agriculture - ploughing breaks up the top soil and allows greater infiltration, subduing hydrographs.
- terracing on hillsides
- water abstraction reduces the base flow so more water must reach the channel before it reaches bankfull capacity
