Storm Hydrographs Flashcards
(9 cards)
What is river discharge and how is it calculated
Volume of water passing any one point in a given time (cumecs)
Q=A x V Q=discharge A= Cross sectional area V= Velocity
What is a storm hydrograph
Shows river discharge over a given period of time
Hydrograph used to identify flood risks in the drainage basin
Key terms: Baseflow, rising limb, peak discharge, falling limb
Baseflow- normal day to day discharge if the river. Consequence of groundwater seeping into the channel
Rising limb- rapid increase in water level due to surface runoff
Peak discharge- highest level, lag time= peak rainfall —> peak discharge
Falling limb- discharge decreases and river level falls. Gentler gradient as overland flow has been discharged.
HYDROGRAPH PROFILES: what are the characteristics and environment associated with flat hydrographs
Characteristics: Environment:
Gentle limbs Permeable soil and rocks
Low peak discharge Forestry
Long storm flow
Long lag time
Significant base flow
Flooding unlikely
HYDROGRAPH PROFILES: what characteristics and environment is associated with flashy hydrographs?
Characteristics: Environment:
Steep limbs Impermeable soil and rocks
High peak discharge Urbanisation of surfaces
Short storm flow
Short lag time
Small base flow
Flooding likely
FACTORS AFFECTING BASIN DISCHARGE AND THE STORM HYDROGRAPHS: Nature of Precipitation and Basin Size and shape
- Short intense rainstorms= rapid surface run off= steep rising limb
-Extreme temps- frozen or baked ground= rapid surface run off
-snow on ground= store= long lag time and shallow rising limb
-once thaw sets= steep rising limb
LARGE BASIN= more precipitation= more surface run off= more discharge
-longer lag time (water longer distance to travel to reach main river)
ELONGATED BASIN= water takes longer time reaching the river
-lower peak flow + longer lag time than a circular one
FACTORS AFFECTING BASIN DISCHARGE: relief (slope) and underlying soil and geology
Water flow= fastest down steep slope= steeper rising limb and shorter lag time
Water moves quickly as surface run off as infiltration will not occur quickly enough
Gentle slope= infiltration= more surface transfers= water slows down reaching channel= ‘flat’ hydrograph
Permeable/ porous rock absorbs water that has infiltrated= less overland flow
Porous= rapid infiltration= little overland flow= shallow rising limb
Impermeable= water can’t pass through= end up in rivers + soil becomes soggy= quicker water flow into rivers= steeper rising limb
FACTORS AFFECTING BASIN DISCHARGE: Drainage Density and previous conditions
Amount of river channel within drainage basin area
On surface rain will drain rapidly if it can reach channel quickly
Higher drainage density= more efficient basin drainage= ‘flashy’
State of basin before storm event eg recent rainfall= saturated soil or period of drought= depleted groundwater stores
FACTORS AFFECTING BASIN DISCHARGE: Vegetation cover and urbanisation
Will have high Evapotranspiration output= reduced precipitation transferred by river. Type of vegetation will have impact (summer deciduous trees intercept 60% more than in winter)
Water more likely to be taken up by tree roots for growth
New Artificial surfaces= more flashy hydrographs
Replacement of solid with less permeable surfaces eg tiled roofs, tarmac roads and concrete pathing
Urban environments provided with highly engineered drainage systems eg guttering and down pipes
