flood hydrograph

Question 1

what is a river regime

Answer 1

the variability in its discharge throughout the course of a year in response to precipitation, temperature, evapotranspiration and drainage basin characteristics

Question 2

how does precipitation create a flashy hydrograph?

Answer 2

• Heavy short showers can cause rivers to rise quickly and burst their banks, these would have a very short lag time and high peak discharge. Soil reaches field capacity quicker resulting in surface runoff.
• Raindrop sizes bigger than pores in soil increase overland flow.
• If precipitation is stored as snow on the ground and temperatures rise, the snow will melt causing high rates of overland flow and a steep rising limb and high peak discharge.

Question 3

how does precipitation create a subdued hydrograph?

Answer 3

Long steady prolonged rainfall will produce rivers which rise slowly but can flood, these produce hydrographs with longer lag times and generally lower peak discharges.

Likely to infiltrate into soil and percolate into rock resulting in a slower flow back into the river resulting in longer lag time and low peak discharge

Question 4

how does temperature and evapotranspiration create a flashy hydrograph?

Answer 4

• When temperature warms and snow melts many days’ worth of precipitation can end up in rivers and cause flooding leading to a flashy response
• Lower rates of evapotranspiration, likely to see a flashy response
• Warm air holds more moisture so potential for higher discharge in warm areas

Question 5

how does temperature create a subdued hydrograph?

Answer 5

When temperature cools and snow freezes it prevents flooding leading to a slower response

Question 6

how does antecedent soil moisture create a flashy storm hydrograph?

Answer 6

Wet - if reaches soil capacity the next rainfall will cause surface runoff

Question 7

how does antecedent soil moisture create a subdued hydrograph?

Answer 7

Dry - water infiltrates, no surface runoff

Question 8

how does drainage basin size and shape create a flashy hydrograph

Answer 8

Steep slopes tend to reduce the amount of infiltration of water into the ground, this water can then flow quickly down to rivers as overland flow reaching the river quicker. In addition, steep slopes also cause more through flow within the soil. Both can raise river levels.
Drainage basins more circular in shape lead to more flashy hydrographs because each point in the drainage basin is roughly equidistant from the measuring point of the river
- Boscastle Flood 2004

Question 9

how does drainage basin size and shape create a subdued hydrograph?

Answer 9

Gentle slopes or flat land allow water to penetrate into the soil and increase lag times and reduce peak discharges

Question 10

how does drainage density create a flashy hydrograph

Answer 10

Higher drainage density = more water can collect quickly from within the basin. This reduces lag times and increases peak discharges.

Question 11

how does drainage density create a subdued hydrograph

Answer 11

Lower drainage density means less water can collect quickly from within the basin. This increases lag time and decreases peak discharges

Question 12

how does porosity and impermeability of rocks create a flashy response

Answer 12

Impermeable soils and rocks such as clay or shale do not allow water to infiltrate, this forces water to run off reducing river lag times and increasing flood risk.
Low porosity - more surface flow

Question 13

how does porosity and impermeability of rocks create a subdued response

Answer 13

Permeable rocks allow water to infiltrate into them. If permeable rocks allow water in through cracks, fissures and bedding planes but not through their pores they are said to be pervious
Porous rocks allow water to penetrate into their pores such as sandstone.

Question 14

how do slopes create a flashy hydrograph

Answer 14

Steep slopes tend to reduce the amount of infiltration of water into the ground, this water can then flow quickly down to rivers as overland flow. In addition, steep slopes also cause more through flow within the soil.

Question 15

how do slopes create a subdued hydrograph

Answer 15

Gentle slopes or flat land allow water to penetrate into the soil and increase lag times and reduce peak discharges

Question 16

how does vegetation type create a flashy hydrograph

Answer 16

In winter - fostidious trees lose their leaves, interception decreases so more likely to have a flashy response

Question 17

how does vegetation type create a subdued response

Answer 17

Summer - leaves on trees so more interception therefore likely to have a slower response

Question 18

how does land use create a flashy response

Answer 18

Urban areas - flashy responses because impermeable surfaces e.g. concrete
deforestation - reduces interception rates allowing rainwater to hit the surface directly - reducing infiltration and exposes soil to greater rates of erosion leading to sedimentation of river channel reducing the bankfull capacity leading to greater chance of flooding

Question 19

how does land use create a subdued hydrograph

Answer 19

Rural areas like having foresty areas means more interception, less likely the ground is saturated, slow response
afforestation
ploughing - breaks top soil = greater infiltration
terracing on hillsides - stops movement of water downhill
grass crops increase infiltration