3. Factors influencing river regime characteristics (Boscastle in bold)…
3. Factors influencing river regime characteristics
(Boscastle in bold)
Deep soils and abundant vegetation: increased evaporation rates and transpiration rates from plants.High infiltration and dominant through flow
Semi arid :warning:
short but high intensity rain. low infiltration and lots of overland flow.
Convectional rainfall occurred, 50mm of rain in the space of 2 hours. another two hours the flood waters were full, drastically decreasing lag time. storm took place in early afternoon, followed by a flash flood
Extreme cold weather :warning:
frozen ground, cannot infiltrate
European mountain rivers have high water period during July and august when glaciers melt rapidly.
Tropical seasonal rainfall (monsoon)
high evapotranspiration in summer because it rains the most.
permeable rocks (chalk, sandy soils) :forbidden:
absorb water easily so surface run off is rare
impermeable rocks (granite, clay) :warning:
closely packed soils so rainwater cannot infiltrate, reaches river quickly.
Dominated by shales and slate that break down weathering to give clay rich soils. Previously high rainfall left the bedrock extremely saturated and permeable
Pervious rocks (limestone) :forbidden:
water can pass through joints, and pours rocks like chalk have spaces in between rock particles
low density :warning:
less interception and transpiration, more overland flow. Smaller biomass storage
Flora in the upper valleys were dominated by grassland
high density :forbidden:
increased interception and infiltration.Lander biomass storage. low peak flow.
for example a mature deciduous woodland
compacts soil and removes vegetation
increases surface run off
development of road and car park decreased the river channels capacity. Boscastle previously built on the rivers flood plain. Water piled up behind cars trapped under a bridge, making a huge surge of material when this gave way.
reduce travel distance, channel water directly
increases infiltration and percolation
less interception so rain reaches the ground faster. increased surface run off.
infiltration rates are 67 times higher under trees compared to grassland.
also delays discharge because snow can be stored for months
Hoover Dam and Glen Canyon Dam now store two years of discharge in lake mean and two other resoviors, which has evened out the discharge/flow.
drainage basin size
round shaped basin :warning:
water has less distance to travel
River Valency was very small (30km2) and close to neighbouring streams.
elongated basin :forbidden:
more tributaries for the water to travel through
steep valleys :warning:
particularly vulnerable to summer downpours and both the main river and tributaries flow in steep sided valleys beneath a plateau rising to some 160m in height. Considerable gradient means less infiltration therefore slow transfer via through flow groundwater. Quicker overland surface runoff.
wet= increased ground saturation :warning:
dry=infiltration higher :forbidden: