Water Lesson 6: Storm Hydrographs Flashcards
Define storm hydrograph
Shows variations in a river’s discharge at a specific point over a short period of time (usually before or during a storm)
Define normal (base) flow
The contribution to river flow from long term storage sources such as permeable rocks, areas of bog and marsh and moorlands.
Define rising limb
The increase in discharge in response to surface runoff and through flow from a rainfall event until peak flow is reached
Define peak discharge
The maximum discharge by a stream or river in response to a rainfall event
Define bankfull discharge
Occurs when the water level in a river reaches the top of its channel. Any further increase would result in flooding of the surrounding land
Define falling limb
The decline in discharge that occurs after peak flow. This is usually less steep than the rising limb because through flow is being released slowly into the channel
Define lag time
The difference in hours and minutes between the time of maximum precipitation and peak discharge. Lag time will vary heavily based on human and physical factors. Rivers with a short lag time have higher peak discharges and are more prone to flooding
Define storm runoff
The part of the river flow derived from the immediate rainfall event. The most rapid transfer of water occurs overland and via throughflow
Define approach segment
The discharge of the river before the storm (antecedent flow rate)
What are the 2 types of storm hydrograph?
Flashy and subdued
Characteristic of flashy storm hydrograph?
Short lag time and high peak discharge
Characteristic of subdued storm hydrograph?
Long lag time and low peak discharge
Drainage basin size of flashy hydrograph?
Small basins - reach channel rapidly and has a shorter distance to travel
Drainage basin size of subdued hydrograph?
Large basins - longer to reach channel and longer distance to trvael
Drainage basin size of subdued hydrograph?
Large basins - longer to reach channel and longer distance to travel
Drainage basin shape of flashy hydrograph?
Circular basin - less time to reach channel - extremities equidistant from channel
Drainage basin size of subdued hydrograph?
Elongated basin - water will take a long time from extremities
Drainage basin relief of flashy hydrograph?
Steep slopes - water flows rapidly downhill to channel
Drainage basin relief of subdued hydrograph?
Gentle slopes -water can infiltrate into the ground and travel slowly
Soil type of flashy hydrogrpah?
Clay soils and thin - low porosity and swell when they absorb water - water infiltrates slowly - saturated
Soil type of subdued hydrograph?
Sandy soil and thick - high porosity so water can infiltrate- deep soils mean greater infiltration
Rock type of flashy hydrograph?
Impermeable - water cannot percolate - greater surface runoff
Rocky type of subdued hydrograph?
Permeable - water percolates increasing groundwater stores
Drainage density of flashy hydrograph?
High drainage density - surface streams means storm water reaches channel faster
Drainage density of subdued hydrograph?
Low drainage density - few surface streams - slow process
Natural vegetation of flashy hydrograph?
Thin grass - intercepts little water and little loss by evapotranspiration - more water in channels
Natural vegetation of subdued hydrograph?
Forest and woodland - intercepts water and high evapotranspiration - less water to channel
Land use of flashy hydrograph?
Urban - impermeable surfaces and drains to carry water to rivers rapidly
Land use of subdued hydrograph?
Rural - vegetated surfaces intercept water and allow infiltration
Precipitation intensity of flashy hydrograph?
High intensity - rain falls faster than infiltration capacity -large surface runoff to channels
Precipitation intensity of subdued hydrograph?
Low intensity - water can infiltrate soil and travel slowly
Precipitation duration of flashy hydrograph?
Prolonged -water table rises and soil is saturated - surface runoff occurs
Precipitation duration of subdued hydrograph?
Short duration -most infiltrated into soil and travel slowly to channel
Snowfall of flashy hydrograph?
Fast snowmelt - cannot infiltrate into frozen ground - flows rapidly to channel
Snowfall of subdued hydrograph?
Slow snowmelt - ground thaws with snow so meltwater infiltrates soil and rocks
Evapotranspiration of flashy hydrograph?
Low rates - fewer losses from system increases discharge
Evapotranspiration of subdued hydrograph?
High rates - high losses reduce discharge
Describe green roofs
Vegetation cover planted over a waterproof membrane on flat roofs
Describe infiltration basins
Shallow depressions dug out to delay runoff and increase infiltration
Describe permeable pavements
Gaps are put in between pavement slabs to delay runoff
Describe rainwater harvesting
Collecting rainwater from roofs to be recycled e.g. for watering plants in the garden
Describe soak away
Channels dug out to disperse surface water into the ground
