Transcript Floods and disturbances

Floods and disturbances in aquatic communities
Outcomes
Discuss
the practices that exacerbate floods, and flood control
measures
Describe the effects of floods, particularly on aquatic
communities
Use ‘disturbance ecology’ as a theoretical framework to assess
the extent and severity of a disturbance
Floods as catastrophes
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Variations in stream flow lead to floods and droughts.
Floods are natural earth processes, amongst earthquakes,
volcanoes, fires, drought etc.
Why do we see an increase in flood incidence?
Less mortality, but more damage due to:
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changes in land use patterns (overconfidence in flood control)
urbanisation
overpopulation
changes in magnitude and frequency (climate: Bangladesh)
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Definitions and terms
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River discharge is simply a measure of the amount of water moving
down a channel past a given point per unit time (m3 s-1). It is related to
stream width, depth, current velocity and roughness of the substrate.
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Flood- when stream flow exceeds bankfull discharge and water
spreads out onto adjacent land or floodplain
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During rainfall events, flood hydrographs are used to measure base
flow, the time taken to reach peak discharge, and the time taken to
achieve base flow when rainfall ceases.
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Floodplain- should not be considered as dry land that is damaged, but
a natural extension of the riverbed that is less frequently used.
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Causes
• INCREASE IN STREAM DISCHARGE (proximal cause)
• snow melt
• arterial drainage, waterway improvement both act to reduce time of
rise to peak discharge.
• Increase in rainfall
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Major features of floods:
• the flood hydrograph
• relationship between time and flood severity
overhead
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Response time related to
• catchment size
• catchment shape
• gradient
• vegetation
• soil and permeability
Floods as disturbances in aquatic systems
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What is a disturbance?
A relatively discrete event that in time that is characterised by a
frequency, severity and intensity that lies outside a predictable
range for the system (Resh et al 1988)
• focuses on physical phenomenon
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Any relatively discrete event in time that through its frequency,
severity or intensity lies outside a predictable range for the
system, and that removes organisms and opens up space or other
resources that can be utilised by individuals of the same or
different species (Townsend 1989)
• focuses on physical and biological
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Intermediate Disturbance Hypothesis
Factors affecting species richness/diversity over
large spatio-temporal scales
•Time
•Environment
•favourability (mean conditions)
•stability (variance in conditions, little fluctuation)
e.g. stable climate, Abysal ocean depths
•Disturbance (Grime and Connell): rainforests,
various vegetation types, coral reefs
•Spatial structural heterogeneity
•Area
•Productivity
•Competition/Predation
(Further info: Chapter 8 of Giller: Community
Structure and the Niche, available in library)
The disturbance regime is influenced by a number of
factors
 Magnitude. The intensity or strength of the disturbing
force, it consists of two main components:
• Intensity- a measure of the strength of the disturbing force
• Severity- a measure of the damage caused
• The physiological and morphological characteristics of organisms
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Frequency.
Number of disturbances per unit time.
Separate
terms are used for the average frequency of disturbance at
the local and the regional spatial scales.
• Random point frequency
• Regional frequency
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Different types of rivers have different frequencies of flood
The disturbance regime- contd
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Predictability (mentioned in definition) results from three main
processes:
• events that occur with a constant probability e.g. snow melt, winter rain
• predictable cycles in climate or weather (storms, temperature, rain, El
Nino, glacial periods)
• biological processes with predictable cycles e.g. production/
accumulation of biomass
• predicatability in space (upper vs lower river)
If a flood is very predictable, it may not be a disturbance!
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Areal extent.
• The absolute and relative size of the disturbed area, and the shape of
the disturbed area, have an important effect on recolonisation.
• E.g. hurricane vs thunderstorm, climate and catchment
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Larger scale disturbances are rarer in occurrence.
Effects of floods on physical system
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There are a number of beneficial aspects for the ecology of both the
river and lowland system (Giller 1996):
 Nutrient and energy transfer
 Provision of nursery areas for fish
 Fertilisation of the floodplain
 Creation and maintenance of specialised habitats
 Creation of patchiness (e.g. the Amazon lowland forest).
Substrate and o.m. in riffle areas (shallow fast-flowing and steep)
dislodged
severe flooding scours the stream bed removing vegetation/animals
removal of sediments to depths between 20cm and 2m
In pools and glides (slow and low slope) large amounts of sediment
redeposited
Effects on biota
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Case and netbuilding, locomotion, territoriality, respiration,
Dislodgement and downstream transport
Mortality
Impact may be related to life stage (..predictability..)
• e.g. greatest impact on fish is on eggs (buried in substrate) or fry,
and loss of habitat or food resources
Usually, recovery can take in the region of months
Catastrophic floods:
• Yoshino, Japan (typhoon) 1959: 32g/ 0.25m2 ; 1964 0.5g/ 0.25m2
• see overheads
Recovery and resistance to floods
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Adaptations to strong flow
life-history strategies (and timing of flood)
recolonisation: fastest when areal extent small at the usual time of year
upstream areas, flight from downstream areas
REFUGIA
Flood plain (especially for fish, in lowland floodplains)
Hyporheic zone]
Flow refugia (see handout)
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(modern) Flood Alleviation
Realises that physical simplification of river system makes
catchment more vulnerable to flood (WHY?), and less
likely to recover (WHY?)
use the floodplain to dissipate energy
physical complexity of the river system spreads the risk
and avoids simultaneous catastrophes.
Summary
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Floods are natural phenomena that form part of the normal part of
many aquatic systems. The impacts of floods will depend on several
factors, the more important of these being the size of the disturbed
area, the nature of the river system, and the timing, frequency and
duration of the flood.
Human activities are continuing to alter the rate at which river water
enters stream channels, and these alterations become most apparent
during heavy rainfall and normal flood events. These activities
include (after Giller 1996):
Drainage
Afforestation development (preparation, growth, harvesting)
Dams
Channelisation
Dredging and straightening of streams
Flood alleviation schemes.