flood hydrograph Flashcards
what is a river regime
the variability in its discharge throughout the course of a year in response to precipitation, temperature, evapotranspiration and drainage basin characteristics
how does precipitation create a flashy hydrograph?
- 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.
how does precipitation create a subdued hydrograph?
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
how does temperature and evapotranspiration create a flashy hydrograph?
- 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
how does temperature create a subdued hydrograph?
When temperature cools and snow freezes it prevents flooding leading to a slower response
how does antecedent soil moisture create a flashy storm hydrograph?
Wet - if reaches soil capacity the next rainfall will cause surface runoff
how does antecedent soil moisture create a subdued hydrograph?
Dry - water infiltrates, no surface runoff
how does drainage basin size and shape create a flashy hydrograph
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
how does drainage basin size and shape create a subdued hydrograph?
Gentle slopes or flat land allow water to penetrate into the soil and increase lag times and reduce peak discharges
how does drainage density create a flashy hydrograph
Higher drainage density = more water can collect quickly from within the basin. This reduces lag times and increases peak discharges.
how does drainage density create a subdued hydrograph
Lower drainage density means less water can collect quickly from within the basin. This increases lag time and decreases peak discharges
how does porosity and impermeability of rocks create a flashy response
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
how does porosity and impermeability of rocks create a subdued response
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.
how do slopes create a flashy hydrograph
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.
how do slopes create a subdued hydrograph
Gentle slopes or flat land allow water to penetrate into the soil and increase lag times and reduce peak discharges
