5.3 - Storm Hydrographs

Question 1

What is a storm hydrograph

Answer 1

A storm hydrograph is a graph showing variation in discharge (volume) of water at a given point over a short period of time, normally an individual storm ( few days)

Question 2

What is the rising limb

Answer 2

The rising limb is the increase in river discharge, not necessarily instantaneously after precipitation

Question 3

What is the peak discharge

Answer 3

The peak discharge is the maximum discharge, it occurs after a delay after maximum precipitation has occurred

Question 4

What is lag time

Answer 4

Lag time is the time delay between peak discharge and precipitation

Question 5

What is the falling limb

Answer 5

The falling limb shows the decline in discharge after peak flow as storm precipitation levels decrease

Question 6

What is bankfull discharge

Answer 6

Bankfull discharge occurs when a rivers water levels reach the top of its channel. Any further increase will result in flooding of surrounding land

Question 7

What is the normal ( base) flow

Answer 7

The normal base flow is the contribution to river flow from long term storage sources such as permeable rocks, areas of bog and marsh and peat covered moorlands

Question 8

What would a flashy storm hydrograph look like

Answer 8

-a flashy storm hydrograph would have a short lag time , high peak discharge and steep rising limb

Question 9

What would a subdued hydrograph look like

Answer 9

A subdued hydrograph would have a long lag time and low peak discharge

Question 10

What size of drainage basin would be most likely to produce a flashy storm hydrograph and why

Answer 10

Small basins would be most likely to produce a flashy storm hydrograph as water will reach the channel rapidly as it has less distance to travel

Question 11

What shape and relief type of drainage basin would be most likely to produce a flashy storm hydrograph

Answer 11

• A circular shaped basin would be most likely to produce a flashy hydrograph as it will take less time for water to reach the channel as all extremities are equidistant from the channel
• Steep sloped basins will produce a flashy storm hydrograph as water flows rapidly downhill and reaches the channel quickly

(Note - an elongated Basin will cause a subdued hydrograph as water will take a long time to reach the channel from the extremities of the basin)

Question 12

What soil type will produce a flashy storm hydrograph

Answer 12

• clay soils and thin soils will cause a flashy storm hydrograph as clay should have a low porosity and the grains in the soil swell when they absorb water , so water infiltrates slowly.
• Thin soils also just become saturated quicksand

(note Sandy soils will produce a subdued hydrographas Sandy soils have high porosity so water can infiltrate effectively)

Question 13

What type of land use will cause a flashy storm hydrograph

Answer 13

-Urban surfaces have more impermeable surfaces e,g roads so there is more run off, and there are more drains that carry water rapidly and directly to the river ( increasing discharge)

Question 14

What type of evapotranspiration rates will cause a flashy storm hydrograph

Answer 14

Low evapotranspiration rates will cause a flashy storm hydrograph as low rates will mean that there are fewer losses from the drainage basin system and thus will cause an increase in discharge to the river channel

Question 15

What type of drainage density will cause a flashy storm hydrograph

Answer 15

A large number of surface streams per km2 means more storm water will reach the main channel more quickly

Question 16

Which type of precipitation duration will cause a flashy storm hydrograph

Answer 16

Prolonged precipitation duration will cause a flashy storm hydrograph as if the precipitation is prolonged then the water table rises and the soil becomes saturated, causing surface run off, which travels rapidly to the river channel

Question 17

What type of natural vegetation will cause a flashy storm hydrograph

Answer 17

Thin grass will cause a flashy storm hydrograph as it intercepts little water and there is little loss by evapotranspiration so water reaches the main channel rapidly

Question 18

What rock type will produce a flashy storm hydrograph

Answer 18

Impermeable rocks will produce a flashy hydrograph as water cannot percolate into the rock , increasing surface run off into rivers

Question 19

What type of snowmelt will cause a flashy hydrograph

Answer 19

Fast snowmelt will cause a flashy hydrograph as meltwater cannot inflitrate into the frozen ground , so it flows rapidly over the surface into the river channel