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Evaluating the environmental losses and benefits from flooding

Abstract Flood events are natural disturbances with a multitude of environmental benefits. However,

human influence has altered the interaction between floods and ecosystems, such that individual

flood events may have long-term, negative impacts on the environment. Evaluating the effects of

flooding on the environment is difficult due to the inherent complexity of ecological systems and

the associated difficulty of defining ecological outcomes in terms of losses and benefits. The

effects of flooding will vary depending on attributes of the ecosystem in question, past

disturbance history and the timeframe over which assessment takes place. However

environmental evaluation is worthwhile, as it enables a more comprehensive picture of the effects

of flooding with a view towards sustainability. Such an evaluation will alert us to the

environmental, social and economic benefits of a more natural flood regime and the potential

impacts of flood mitigation procedures.

This article presents a framework for conceptualising the environmental benefits and losses from

flooding in the context of natural disturbance cycles and modified environmental systems. Using

this as a basis, an approach is described for evaluating the effects of flooding on ecosystem

condition using environmental indicators. This approach has potential to be incorporated into a

broader social and economic assessment of the losses and benefits incurred by a flood event.


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Introduction

Flooding is part of the natural cycle of many ecosystems and plays an important role in

maintaining ecosystem function and biodiversity (Poff et al 1997). The multitude of

environmental benefits from flooding also benefit society through the continuation of ecosystem

services (Bayley 1995). However, major modification to natural processes and ecosystems

through human activity, has created the potential for flood events to result in long-term,

undesirable changes to the environment (eg Montz and Tobin 1997).

Historically, flood loss assessment has focussed on the direct economic costs of flooding, with

accounting being undertaken for impacts that can be ascribed a value in conventional economic

terms (Thompson and Handmer 1996). The environmental impacts of floods are rarely reported in

flood loss assessments, although there is increasing recognition of the importance of losses that

do not have a direct market value (Bureau of Transport Economics 2001). Also, assessments

rarely consider the potential environmental benefits from events that are solely considered as a

disaster from a human perspective. Accounting for the environmental effects of floods will enable

a more realistic evaluation of the total costs and benefits of flooding; an evaluation that considers

the environmental, social and economic benefits of a more natural flood regime and that alerts us

to the potential costs of flood mitigation procedures.

Before an evaluation of the environmental effects of flooding can be made, it is critical to

establish the impact of flooding on environmental losses and benefits. This is complex, because

while the immediate impression is one of damage and destruction, flood events play an important

role in the maintenance of biodiversity and ecosystem function of riverine systems in the long

term (Poff et al 1997). Multiple factors such as previous disturbance history and ecosystem

attributes will influence the scale and type of effect that a flood event has on the environment, and

the direction of ecosystem recovery (US EPA 1999). Further, the criteria used for assessment and

the timescale over which the assessment is undertaken will both influence the final evaluation of

the extent of environmental losses and benefits.

By assessing the impact of flooding on ecosystem condition, a holist approach can be taken to the

assessment of the environmental effects of flooding. Ecosystem condition is defined here as the

health or condition of an ecosystem to maintain biodiversity and ecosystem function. Because of

the complexity of ecological systems, the assessment of ecosystem condition requires


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identification of sets of attributes that provide general indicators of a complex suite of ecological

conditions (Lindenmayer 2001). Through measurement and monitoring of these indicators over

time, it may be possible to provide an ecosystem scale assessment of whether the impact of a

flood event has “driven” the ecosystem away from or towards the desired state of having a long-

term capacity to maintain biodiversity and function. Although the scope of this document is the

assessment of environmental effects of individual flood events and is not an examination of the

consequences of altered flood regimes, it is critical that flood events are viewed in context of

flood regimes, as they cannot be understood in isolation.

The purpose of this paper is to outline and conceptualise the broad environmental effects of

flooding and to develop an approach to the assessment of environmental losses and benefits

through evaluation of the effects of flooding on ecosystem condition. This paper is divided into

three main sections. Firstly, a conceptual framework is outlined for understanding environmental

losses and benefits in the context of natural disturbance cycles and modified environmental

systems. Secondly, the effects of flooding on five broad ecological communities are described to

outline how these effects vary between ecosystems. Thirdly, broad indicators are suggested for

the assessment of changes in ecosystem condition to determine whether there have been

environmental benefits and losses. By establishing a framework and methodology for assessing

environmental flood losses and benefits, this approach has potential to directly contribute to an

economic valuation of these costs and benefits, through an evaluation of the effect of flooding on

the provision of ecosystem goods and services (eg Haeuber and Michener 1998, Wainger et al

2001). The focus here, is on ecological communities that are components of “riverine

ecosystems;” defined by Arthington (1998) to be an all-inclusive concept of rivers that includes

the source area, river channel, riparian zone, floodplain, groundwater, wetlands and estuary and

features such as threatened species.

A conceptual framework of the environmental losses and benefits associated with flooding

It is important to ensure that environmental assessment and evaluation of flooding is embedded

within a holistic framework that recognises the value of ecological integrity; i.e. the condition or

"health" of an area. Native species and ecosystems have intrinsic value, as well as being of value

to humans through the provision of ecosystem goods and services such as the purification of air

and water; production of food and fibre; and fulfilment of peoples spiritual, cultural and

intellectual needs (Daily 1999, CSIRO 2001, Salzman et al 2001). The framework developed here


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(Figure 1), takes biodiversity and ecosystem function to be the overarching valued elements of

the environment against which we can assess environmental losses and benefits. Flood events that

cause changes in ecosystem condition that contribute to the long-term viability of the plants and

animals living in flood prone communities and to ecosystem function have environmental

benefits. Degradation in the long term of either diversity or function is counted as an

environmental loss, and individual flood events are viewed in this context. Environmental losses

and benefits are presented in the lower horizontal axis of the framework diagram (Figure 1).

INSERT FIGURE 1

Floods are characterised by five critical components: the magnitude of discharge, the velocity of

the discharge, the duration of the flood, timing or seasonality of the event and the frequency of

the disturbance. Together, these elements make-up what is known as the flood regime. Large

flood flows maintain ecosystem productivity and diversity through processes such as transport

and cycling of nutrients sediments and organisms between river channels and flood plains (Poff et

al 1997, Baldwin and Mitchell 2000), and habitat creation and maintenance of channel structure

through scouring fine sediments and washing organic matter out onto floodplains (Cullen et al

1996, Poff et al 1997). The organisms of flood prone systems are adapted to this disturbance,

with many species of fish and macroinvertebrates requiring flood events to complete their life

cycle (Junk et al 1989). In light of this multiplicity of environmental benefits from flooding, most

research and policy has focussed on the environmental degradation resulting from flood

mitigation and changes to environmental flows (eg Cullen et al 1996, LWRDDC 1997,

Arthington 1998) rather than the potential negative impacts of individual flood events.

Individual flood events in combination with human activities such as vegetation clearance, river

regulation and intensive agricultural production can result in long-term, negative effects on the

environment. Increased levels of human activity in a region may increase the likelihood of the

spread of pollutants (Montz and Tobin 1997) and invasive organisms (Howell and Benson 2000),

transfer of nutrients and sediments (Moss et al 1992) and fish kills (McKinnon and Shepheard

1995, McKinnon 1997). Changed flood regimes due to flood mitigation practices (Crabb 1997,

Sparks et al 1998), urbanisation (Novotny et al 2001) and global warming in the future (Smith

and Ward 1998) may also result in individual flood events that have negative impacts on native

biota (eg Quinn et al 2000) and the chemical and physical structure of flooded ecosystems.


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The top horizontal axis of the framework diagram (Figure 1) represents the degree of human

activity in the flooded region in terms of modified landscapes and altered flood regimes.

Increased activity drives the environmental effects of individual floods towards greater, long-term

environmental losses.

The time-scale over which losses and benefits are assessed is also a critical component of this

framework. In the short term an individual flood event may appear to be an ecological disaster,

with unsightly deposition of sediments and debris, death and injury of plants and animals, and

even local species extinctions. However, in the long term, flood events that are part of the natural

flood regime will ensure the long-term viability of the plants and animals adapted to flood prone

environments and the functioning of these ecosystems. It is essential that a time frame is

established for the monitoring and evaluation of environmental losses that identifies a period

deemed acceptable by society over which natural recovery can take place. If the system has not

recovered within this period, a loss can be counted. Time is represented as the vertical axis on the

framework diagram (Figure 1), with environmental losses or benefits changing over time through

ecosystem recovery or further environmental degradation.

Because the effects of flooding on the environment will differ between different ecological

communities, assessment of the impacts of flooding must be tailored for each system accordingly.

The role of flooding in five broad ecological communities is outlined below to illustrate the

diversity of ecological responses to flood events.

The effects of flooding on different ecological communities

The communities discussed in this document are the river channel, riparian zone, floodplain,

wetland and estuary. Strictly speaking, the riparian zone and wetlands are included in the

floodplain, however they have been treated separately in this document, due to the different

ecologies of each. There are many other ecological communities affected by flooding that are

beyond the scope of this paper for consideration. These include: embayments, inshore lagoons,

littoral zones, outer reefs, inland lakes, coastal wetlands, saltmarshes, mangrove swamps and

freshwater swamps. For each community discussed the environmental losses and benefits from

flooding are described.


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River channel communities

The river channel is a well-defined, high-energy ecosystem. Many ecological benefits stem from

the lateral connections between rivers and floodplains that occur during flood events (Fairweather

and Napier 1998). Overbank flows promote nutrient cycling (Baldwin and Mitchell 2000),

inoculate rivers with carbon, algae and microorganisms and give riverine organisms access to

floodplain resources, stimulating spawning and recruitment (eg Rowland 1989, Sparks et al

1998). Flooding may also stimulate germination and re-establishment of aquatic macrophytes

(Bendix and Hupp 2000). New habitat for aquatic organisms may be created through flood events

through changes to stream cover and structural complexity due to the transfer of rocks, sediments

and organic matter and the introduction of woody debris or snags (Wallace and Benke 1984). An

increase in snags is also beneficial as they maintain friction in watercourses, helping to slow the

system down and create a range of flow conditions (Gippel et al 1996).

Flooding may also negatively impact on river channels. Erosion, which is exacerbated by flood

flows, may occur at greater rates than would have occurred naturally because of altered flood

regimes and land clearance (Abernethy and Rutherford 1999). Changes to surface water quality

may also be profound during a flood event (Hart et al 1987, 1988). Water quality is generally

poorer in catchments where there has been extensive land clearance, as floodwaters flush

increased sediment and nutrient loads into river systems (Moss et al 1992, MDBC 2001a). Flood

events can raise turbidity levels by as much as twenty-fold (Shafron et al 1990) through erosion

and algal growth in response to increased nutrients. This may in turn impact on aquatic organisms

through increases in particulate matter that clog fish gills, and decreases in light levels, which

results in decreased photosynthesis and water temperatures (Jolly et al 1996). Another water

quality issue is de-oxygenation, which may be exacerbated in land dominated by exotic plants

that are intolerant of extended inundation (McKinnon and Shepheard 1995, McKinnon 1997).

Run-off from pastures and urban areas may carry chemical pesticides and herbicides, nutrients

from fertilisers and microbiological contamination (Montz and Tobin 1997). Poor water quality

may result in fish kills (Gehrke 1991) and impact on other aquatic biota.

Environmental losses and benefits in terms of exotic species are complex in river ecosystems.

Exotic species such as the floating weed Giant Salvinia (Salvinia molesta) can be flushed out of

rivers in cyclonic floods and scouring may remove shallow rooted exotics such as Para Grass

(Brachiaria mutica), however, if these plants disperse and become established further

downstream, broader environmental benefits would be negated. Flood events may also play a role


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in the release of exotic fish species from outdoor ponds into river systems, but interestingly

studies have shown that floods may disadvantage exotic fish, through aiding recruitment of

indigenous species (Puckridge et al 2000) or through directly reducing populations of introduced

fish species relative to indigenous species (Meffe 1984).

Riparian zone

The riparian zone is defined here as the vegetated area that grows immediately alongside rivers

and streams. Riparian vegetation has a number of important ecological functions, including

organic and nutrient inputs into the river system, stabilisation of riverbanks (Abernethy and

Rutherford 1999), provision of instream habitat through fallen logs and root systems (Rutherford

et al 2000) and habitat for fauna that exist in the riparian zone (O’Reilly et al 1999).

Flood events contribute to the erosion of river banks; a natural process that even occurs in areas

that have good cover of native vegetation and is believed to be an important component of

maintaining river health, as areas erode and are recolonised by vegetation (Poff et al 1997).

Landscape change and altered hydrological regimes have accelerated these processes and flood

events may exacerbate rates of erosion to the extent that the systems are unable to recover before

the next large flow (O’Reilly et al 1999), particularly on unstable riverbanks where there has

already been vegetation clearance (Abernathy and Rutherford 1999, LWRDDC 2000). A

particular issue in cleared landscapes is that unimpeded floodwaters move much faster than in a

floodplain with natural levees, and floodwaters may erode back through riverbanks, in a process

known as avulsion. Destruction of riparian vegetation by floods is an issue in terms of reduction

in size and connectivity of habitat, and decrease in structural complexity of riparian zones. These

impacts will be of much greater importance in regions that have a narrow riparian zone such as

dryland river systems where there is limited potential for re-colonisation by surrounding

vegetation (E. Coleman, Queensland Department of Natural Resources, personal

communication). Flooding may also contribute to the degradation of riparian vegetation condition

through dispersal of propagules of exotic species and provision of nutrient rich conditions that

enhance growth of invasive weeds (Howell and Benson 2000).

Floodplain

The floodplain occupies the land adjacent to the river channel, and may be defined in

geomorphological terms as the alluvial surface constructed by a river under current environmental

conditions (Dunn and Leopold 1978) or in hydrological terms as the area with a specified annual


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probability of flooding, such as 1 in a 100 years. A gradient exists between the river and the outer

edges of the floodplain, where species are adapted to varying degrees and frequencies of

inundation (Junk et al 1989). Benefits to floodplains from flooding are largely a result of the

lateral connections of floodplains to rivers (US EPA 1999, Baldwin and Mitchell 2000), as

described in the previous section. Common biotic responses to flooding include breeding of

waterbirds (MDBC 2001b), emergence and hatching of aquatic invertebrates and tadpoles

(Boulton and Lloyd 1992, Nielsen 1998, MDBC 2001b) and establishment of vegetation (Bendix

and Hupp 2000).

Although floodplain scour and sediment deposition are a natural phenomenon during floods,

transportation and deposition of sediments during flood events will be increased in floodplains

that have been extensively cleared. This will have negative impacts on the physical structure and

biota of these systems. The increase in the magnitude of flows due to urbanization has resulted in

the enlargement of floodplains (O’Reilly et al 1999), and as a consequence ecosystems that are

not adapted to flooding may be inundated and severely impacted. Also, prolonged inundation

during a flood event may result in mortality of floodplain plants (Chapman et al 1982).

A number of serious environmental weeds that become invasive in floodplain ecosystems in

Australia are known to spread via floodwaters such as Olive Hymenachne (Hymenachne

amplexicaulis), and the Giant Sensitive Weed (Mimosa pigra) (Calvert 1998) and post-flood

conditions may also be ideal for the establishment of weedy species such as the Coffee Bush

(Leucaena leucocephala), which germinates profusely after a flood event from seed lying

dormant in the soil (Calvert 1998).

Wetlands

Wetlands are highly productive systems (Boon et al 1990) that contribute to floodplain

ecosystems through their role in the export of organic matter and organisms back into the river

channel, assimilation of nutrients, adsorption and filtration of pollutants, and flood attenuation

(De Laney 1995). Wetlands also provide important nursery grounds for fish (Welcomme 1992).

Floods are integral to the ecology of wetlands, with flooding being essential for the maintenance

of wetlands and associated species (Quinn et al 2000). The depth, duration and timing of each

flood event is critical for the dispersal, reproduction and establishment of the distinctive biota of

wetlands. Even small changes in the rate of flooding, can impact on wetlands and alter species


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composition and abundance (LWRDDC 1997). Flood events may also negatively impact on

wetlands through scour, sediment deposition and oil and pollution contamination.

Estuaries

Estuaries have been defined as “semi-enclosed coastal bodies of water where salt water from the

open sea mixes with freshwater draining from the land” (Audit, 2000). Major effects of flooding

on estuaries include large plumes of suspended sediments, nutrients, phytoplankton and possible

pollutants and a resulting increase in turbidity. There also maybe changes in phytoplankton

community composition in response to nutrient pulses from the flood event (Grice et al 2000).

Changes in water quality may affect seagrass communities, which are impacted by changes in

light penetration through decreased water clarity. The large influx of freshwater into the estuarine

environment may also have a serious effect. Because seagrass meadows provide critical habitat

for numerous fauna, including fish and shellfish, and are the major food source of turtles and

dugongs, flooding will affect the many species that utilise seagrass beds for food and habitat.

Sediment levels of flood plumes will be greatly increased in catchments with substantial land

clearance and erosion, particularly gully and streambank erosion (Caitcheon et al 2001).

Summary

Although the effects of flooding on ecosystem processes differ widely between ecological

communities, there are broad classes of effect on key ecosystem processes that are common

between a number of ecosystems. These are summarised in Table 1, and have been used as a

basis for development of environmental indicators of ecosystem change due to flooding, which

are outlined in the next section.

INSERT TABLE 1

Evaluating the environmental losses and benefits from flood events

The following is a brief outline of a range of environmental indicators that are appropriate for use

in the assessment of the effects of flooding on ecosystem condition and the associated

environmental losses or benefits. These indicators have been selected through the process of

identifying how floods drive change in key ecosystem processes and hence ecosystem condition.


9

A critical issue for the assessment process is that in most cases there will be no baseline data

available for an appraisal of flood effects. Ideally an assessment using a BACI (Before, After,

Impact Assessment) would be undertaken, however such a design will not be possible unless

comparative data is available from programs such as those undertaken for water quality

monitoring. Comparison with control areas unaffected by the flood event may also be possible in

some instances. However, in general the only means of assessing the impact of flooding on the

environment will be through qualitative assessments by individuals familiar with the system

under investigation. Potential experts that could contribute to an assessment include individuals

on River Trusts and Catchment Management Authorities, university academics, council

environment officers, land managers, and organisations such as state agencies, the Great Barrier

Reef Marine Park Authority and the CRC for Freshwater Ecology. The emphasis in selection of

these indicators has been on fairly coarse measures that could be used in a rapid appraisal, in a

similar manner to the rapid appraisal method used in Victoria for the assessment of impacts of

flood management activities (DNRE 2000). However, more complex indicators are also

discussed.

The hydrological regime The effects of recurrent disturbances are additive, with each disturbance modifying the effects of

subsequent disturbances, and the ability of biological populations to survive and regenerate after a

particular disturbance event, determining population response to the next disturbance (Bradstock

1999, Puckridge et al 2000, Rutherford et al 2000). Hence, the effect of an individual flood event

can only be understood in the context of previous flood events, and the five components of the

flood regime should be recorded during an assessment process and where possible compared to

historical hydrological data, to determine deviation from a natural flood regime. This will enable

an assessment of potential for the flood event to cause environmental losses and benefits and will

be useful for future assessments. Arthington (1998) has reviewed holistic approaches to the

assessment of environmental flows and these have potential application for the assessment of a

changed flood regime. Also, the CRC for Freshwater Ecology is currently developing a

hydrological index that will provide a useful tool in this regard.

Habitat loss and fragmentation

Extensive damage to habitat may occur through floods, however, ecological recovery can be

rapid in unmodified ecosystems where flooding is a natural, dynamic attribute (Poff et al 1997).

The process and degree of habitat loss and fragmentation through flooding will differ between

ecosystems; and so assessments of habitat loss must vary, depending on the system under Evaluating the environmental losses and benefits from flooding

10

investigation. A useful measure of changes in instream habitat would be to record changes in

stream cover condition, using a simple rating system, such as that outlined in Shepherd and

Siemon (1999). Instream cover is comprised of leaf litter, branches, rocks and vegetation.

Riparian habitat loss and fragmentation through stripping of vegetation can be measured in terms

of the change in width and continuity of vegetated stream length, relative to cover prior to the

event. Assessment should be made for the canopy, middlestorey and the understorey. Similarly,

changes in the cover of wetland and floodplain vegetation should also be recorded. Seagrass loss

is a common measure of estuary health (Grice et al 2000) and the distribution and depth of

seagrass cover may be an important indicator of the magnitude of flooding impacts on estuarine

systems.

Physical change

Visual estimates of bank erosion and avulsion should be recorded and monitored after a flood

event. Such estimates commonly form a component of Rapid Bioassessment Protocols for habitat

assessment (Barbour et al 1999) and a similar approach could be used for assessment of flood

impacts. Photographic records are an effective tool for this assessment. Re-establishment of

vegetation and stabilisation of streambanks would be appropriate measures of ecosystem

recovery. Sediment deposition is also a strong driver of physical change to flooded systems,

particularly within river channel and floodplain environments. Although techniques are available

to monitor the extent of deposition of new sediments (eg Phillips and Marion 2001), these are

complex and time consuming.

Research suggests that natural disturbances maintain structural complexity of ecosystems and that

this complexity promotes plant and animal diversity (Hansen et al 1991) through provision of a

greater variety of niches. Flood events will influence the structural complexity of affected

ecosystems, particularly in-channel environments, through erosion and transport of large woody

material and sediments (US EPA 1999). A Rapid Bioassessment Protocol for physical habitat

assessment in streams (Barbour et al 1999) could be undertaken after a flood event to describe

change in the structural complexity of lotic systems. This system scores physical parameters such

as in-stream cover, substrate quality, channel quality, riparian quality, and pool/riffle quality. The

structure of physical microhabitats within streams, such as snags and rocks, will be profoundly

affected by flood events. Visual assessment of changes in the density of snags can also be made

following a flood event, during periods of low flow. Although important, measurement of


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changes in rocks would be time consuming and measurement of changes in biota that utilise these

habitats would be a more simple assessment of associated losses and benefits of changes in

physical habitat.

The Index of Stream Condition (ISC), uses indicators of bank stability, bed aggradation and

degradation and the abundance of snags to develop a subindex of physical form (Ladson et al

1999). The ISC is used by Catchment Management Authorities in Victoria, to capture information

about the physical condition and habitat of the stream channel and has potential for application to

the assessment of flood impacts on ecosystem condition.

Water Quality

The core measures of water quality that should be considered after flooding are nutrient

enrichment, turbidity and chemical and biological pollution. Because natural background water

quality varies in response to factors such as climatic variation, geological development and soil

composition, the assessment of changes in water quality would ideally be compared to paired

reference sites or regional least-impacted reference sites (Bartošová 2000).

Nitrogen and phosphorous are two of the most significant nutrients limiting plant growth (Udy et

al 2001), and hence are the most important to measure in an assessment of condition of aquatic

environments. Measures of concentration should be given as nutrient loads (amount per unit time)

to increase reliability (Fairweather and Napier 1998). Thresholds for various aquatic ecosystems

will need to be developed to determine whether the changes will have a negative impact.

Turbidity is a measure of water clarity and reflects the presence of suspended matter or soluble

compounds such as silt, clay, and algae (MDBC 2001a) with measurement commonly made in

Nephelometric Turbidity Units (NTU). Because turbidity levels are highly variable within and

between different water-bodies, it is difficult to define a limit that is reasonable for all aquatic

systems (Jolly et al 1996). The Australian Water Quality Guidelines for Fresh and Marine

Waters recommend that the seasonal mean NTU should not vary by more than 10% (Walker and

Reuter 1996).

A basic assessment of the presence of pollutants can be made during a flood event through

identification of water with an unpleasant odor of an oily sheen on the surface (Shepherd and

Siemon 1999). More detailed measurement can be expensive and labour intensive, because there


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are approximately 70,000 toxic substances that may affect water quality and impact on biota

(Fairweather and Napier 1998). Further, such a large number of potentially polluting chemicals

makes it prohibitive to set guidelines for their measurement (Fairweather and Napier 1998). It is

recommended that potential sources of chemical pollution during a flood event be identified

rather than undertaking ad hoc environmental sampling. Overflow from tailings dams used for

storage of toxic heavy metal wastes is a particularly serious issue that should be monitored and

assessed. Other potential sources of pollutants include: sewage, septic and industrial tanks, buried

petroleum tanks, barrels, drums and containers, household hazardous waste and solid waste

(Montz and Tobin 1997). Techniques are available to map the extent of sewage plumes (eg Grice

et al 2000), however these require considerable time and resources and are not strongly

recommended for use in post flood assessment.

The Sustainable Rivers Audit in Australia (Cullen et al 2000) aims to develop a water quality

index that incorporates total phosphorous, electrical conductivity (salinity), turbidity and pH, and

this may have potential in the future for the monitoring of the impacts of flooding on water

quality.

Nutrient flux

A technique that uses microbial exo-enzymes can provide a measure of nutrient flux between

river and floodplain systems. This technique can be adapted for use as an indicator of biofilm

activity to determine rates of bacterial carbon processing and track responses of the microbial

community to periods of connection between rivers and floodplains (Hillman et al 2001).

However, this technique is complex and may be time-consuming.

Introduction of exotic species

Flood events influence the distribution and abundance of exotic species through processes of

disturbance and dispersal. Changes in the number and abundance of plant and animal exotic

species that are dispersed by floodwaters, or are affected by flooding in some way, should be

measured for both terrestrial and aquatic ecosystems. Both increases and decreases in number and

abundance of species are important to report, as they correspond to environmental losses and

benefits respectively. Changes in abundance of ecologically significant exotic species could be

assessed in terms of whether species populations had moved into a different abundance category,


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such as: Abundant, Frequent, Occasional or Rare (eg Shepherd and Siemon 1999). Another

simple but important measure of changes in exotic species would be an assessment of changes in

exotic flora relative to indigenous flora, measured as change in cover of canopy, middlestorey and

understorey plants (eg Shepherd and Siemon 1999) and fauna measured as change in approximate

numbers or density of individuals relative to numbers for functionally similar indigenous taxa.

Biological indicators of ecosystem condition

Because the tolerances of biota to environmental conditions varies between taxa, the presence or

absence of particular species or functional groups can be indicative of the condition of an aquatic

system and provide a surrogate measure of system health (Cranston et al 1996, Karr and Chu

1999). In fact biological monitoring may in fact provide the most integrated measure of river

condition (Karr 1999). Because aquatic taxa such as invertebrates, fish and aquatic plants, are the

“end users” of water management they are routinely used in water quality monitoring

(Fairweather et al 1998). Microbial indicators also have great potential for assessment of river

health (Veal 1997). Aquatic systems that are stressed often show reductions in diversity as

sensitive taxa disappear and pollution-tolerant taxa predominate (Rapport 1991). Taxa with

narrow and specific tolerances make the most reliable indicators (Cranston et al 1996), with fish

and macroinvertebrates being perhaps the most advanced biotic measures available (Cullen et al

2000).

Although it is not feasible to assess impacts of flooding on all species, it is particularly important

that changes to populations of rare or threatened species is recorded to aid in their management

and conservation. Assessment could simply be a record of whether species populations had

moved into a different abundance category, such as that described above for weed species. Fish

kills would be another appropriate measure of environmental impacts as kills may result from

changes in water quality during flooding (McKinnon and Shepheard 1995, McKinnon 1997). The

size of the kill (measured as number of fish per unit area or unit length of waterway) and

approximate number of individuals and number of species (Fairweather and Napier 1998) would

be key measures. Recording changes over time in fisheries production in marine regions affected

by flooding, may also be a simple way of capturing information on environmental losses and

benefits from flooding.

Macroinvertebrates are the taxonomic group with perhaps the most advantages for the assessment

of the condition of riverine systems, in terms of their functional importance, their availability and Evaluating the environmental losses and benefits from flooding

14

wide distribution, and the ease of sampling (Cranston et al 1996). Data on macroinvertebrate

community structure, combined with predictive modelling, is used in a system known as

AUSRIVAS (Australian River Assessment System) to assess the biological health of Australian

rivers (Barmuta et al 1998). This system has great potential for assessment of changes in

ecosystem condition and ecosystem recovery from a flood event. The basic methodology of

AUSRIVAS is to compare the aquatic macroinvertebrate fauna collected at a site, to model

predictions of the fauna expected to occur at the site in the absence of environmental stress, such

as pollution or habitat degradation. The AUSRIVAS program utilises a standard operating

protocol and provides a standardised set of field sampling sheets, which could be incorporated

into this approach to streamline assessments of flood losses and benefits. The composition and

diversity of fish fauna is used as an indicator of estuary health (Blaber 2001) and for monitoring

river health in an Index of Biotic Integrity (Harris and Silveira 1997). However, there are several

difficulties in using fish to monitor water quality, including a lack of sampling protocols and

knowledge about the water quality tolerances (Cranston et al 1996).

Caution must be applied when using biological indicators, because there are no universal

indicators that can be applied at all geographic locations. Also, there is considerable argument as

to whether indicator species can act as surrogates for determining the responses of other taxa to

the same ecological conditions (Lindenmayer et al 2001). Problems will arise when there is a

limited understanding of causal relationships between the species responses to changed ecological

conditions and when there is a lack of knowledge about indicator sensitivity and the magnitude of

the effect size that is detectable with the chosen indicator (Lindenmayer et al 2001).

The evaluation process This broad list of environmental indicators described above are summarised in Table 2 and

indicators that are too complex for use in a rapid appraisal are identified. This list is by no means

definitive or prescriptive. Instead it outlines classes of environmental indicators that can be used

in a post-flood assessment and that can be monitored over time, to assess the extent of ecosystem

recovery from the flood event.

INSERT TABLE 2

In an environmental assessment of the impacts of flooding, simple ratings could be given for each

indicator selected and these could be weighted depending on their perceived importance. These

will need to be qualitative, where no comparative data is available. Finally a score of the overall Evaluating the environmental losses and benefits from flooding

15

impact could be calculated to assess whether the flood event has driven the ecosystem toward

benefits or losses in terms of changes to ecosystem condition. Depending on the indicators

selected and the resources and expertise available for the assessment process, the summary of

environmental effects of flooding will vary. Thresholds, standard protocols and methodologies

could be developed through a more detailed analysis of the indicators outlined in this document

and through discussion between experts in the field. An index could be developed for each

ecological community that provides an integrated measure of changed environmental condition,

following an approach similar to that of the Index of Stream Condition. Alternatively, a scorecard

of ecosystem condition could be presented, such as that used to evaluate freshwater and estuary

health in the Moreton Bay Ecosystem Health Monitoring Program (Grice et al 2000, 2001; Smith

et al 2001).

Monitoring of indicators will be necessary to assess ecological recovery or further degradation,

and hence environmental losses and benefits in the long term. It is recommended that an

assessment be undertaken shortly after floodwaters subside, another at 6 months and a third

assessment at least two years after the event. Scores for each assessment could be compared after

the second and third assessment to determine whether there has been environmental recovery,

increased degradation or no change; particularly in terms of instability and rates of erosion of

soils, change in diversity of threatened species and invasion by exotic species.

Because the indicators outlined in this paper have been developed through identification of the

major effects of flooding on key ecosystem processes, assessment of the effects of flooding on

ecosystem services is a logical step and would in turn enable an economic evaluation of the

environmental effects of flooding. An ongoing study on ecosystem services in the Goulburn-

Broken Catchment, Australia (CSIRO 2001) has identified the maintenance of healthy waterways

as a highly ranked ecosystem service for industries such as dairying, fruit and grapes, grazing,

forestry, food processing, water production and recreation. Because floods have a broad influence

on the health of waterways, there is great potential for evaluating the economic cost of flooding

on these industries. Direct economic benefits of flood events could also be evaluated, such as the

deposition of nutrient rich topsoil in floodplains (Bayley 1995) and maintenance of wetlands that

mitigate the severity of flood events (Salzman et al 2001). Value should be based on each

ecosystem’s capacity to provide goods and services, the quality and quantity of these goods and

services and the demand for the services where they are produced (Wainger et al 2001). It is

hoped that as more research is undertaken on how to measure ecosystem condition and to value


16

ecosystem services (eg Costanza and Folke 1997) that we may further our understanding of the

economic value of more natural flood regimes and adjust our flood mitigation strategies

accordingly.

Conclusion

The focus of this paper has been on building an understanding of the effects of individual flood

events on ecosystem condition and discussing how this can be assessed. The approach outlined

here will enable a standard methodology to be developed for environmental considerations to be

incorporated into more traditional flood loss assessments. This is an excellent starting point for

developing a methodology for the economic evaluation of environmental impacts of floods. By

translating the ecological effects of flooding into effects on ecosystem services, an economic

assessment of environmental losses and benefits could be undertaken. Consideration of the

environment in flood loss assessments will better place land managers and policy makers to have

a more sustainable approach to flood loss mitigation.


17

Acknowledgments The author gratefully acknowledges the helpful advice and comments from the following people: Prof. John Handmer, Prof. Barry Hart, John and Elaine Ridd, Oliver Percovich, Yung Chee, Dale Watson and Dr Jann Williams.

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Table 1: Summary of ecological processes that define ecosystem function and the effect of flooding on these processes. Attributes of a flood event that are the key drivers of ecological change include velocity, duration, timing, frequency, velocity, temperature, and transfer and dispersal of nutrients, biota and pollutants. Surrounding land-use and river management practices will also have an influence.

Key Ecological Processes That define ecosystem function (Adapted from US EPA 1999)

Summary of broad effects of flooding Environmental losses (-) and benefits (+)

Critical Habitats Pattern and Connectivity of Habitat Patches

“+” Re-establishment of aquatic, riparian and floodplain vegetation

“-” Loss & fragmentation of vegetation (aquatic macrophytes. riparian, seagrasses)

Structural Complexity

“+” Increase in snags “+/-” Erosion, stripping, sedimentation and avulsion

Nutrient Cycling “+” Transfer of nutrients & organic matter between floodplain & stream

Stabilising processes “+/-” Erosion & deposition of sediments Purification Services

“+” Recharge of ground aquifers “+/-” Increased turbidity, sedimentation & nutrients

loads in freshwater systems “+/-” Flood plumes “-” Chemical & biological pollution

Biotic Interactions Population Dynamics Genetic Diversity

“+” Reproduction, dispersal & establishment of indigenous species

“+” Exploitation of new food & habitat resources. “-” Death, injury & local extinction of indigenous

species through direct action of flood waters, decreased water quality, altered habitat & food resources

“-” Dispersal of exotic species Hydrological Patterns Natural Disturbance Regime

Because floods are a component of hydrological pattern and the natural disturbance regime, assessment of the effect of flooding on these processes is circuitous. Rather, assessment of the effects of modified hydrological patterns and natural flood regimes on other ecosystem processes described above, is important.


23

Table 2: Indicators for assessing changes in ecological condition, as defined by ecological processes. Complex indicators that may not be appropriate for a rapid appraisal are identified (*).

Ecological Processes Indicators

Critical Habitats Pattern and Connectivity of Habitat

Patches

- Change in width and continuity of riparian vegetation

- Change in stream cover condition

* Distribution and depth range of seagrass beds

- Re-establishment of vegetation (ecosystem recovery)

Structural Complexity

- Change to in-stream cover, channel quality, riparian quality, pool/riffle quality

- Visual estimate of change in snag density Nutrient Cycling * Microbial exo-enzyme technique Stabilising processes - Visual estimates of bank erosion and slumping, stripping,

landslides, gully incision

- Re-establishment of vegetation (ecosystem recovery)

- Extent of sediment deposition Purification Services

- Increased nutrient loads (N and P)

- Increased turbidity (NTU)

- Chemical and biological pollutants – identified from potential sources

- Extent and duration of turbid flood plumes Biotic Interactions Population Dynamics Genetic Diversity

- Germination and establishment of indigenous plant species

- Change in numbers or abundance of significant species

- Size of fish kills

- Change in fisheries production

- Change in number and abundance of exotic fish species

- Change in number and abundance of exotic weed species Broad indicators of ecosystem condition * Biological indicators including macroinvertebrates, fish and

microbes

Hydrological Patterns Natural Disturbance Regime

- 5 components of flood regime and comparison with hydrological record.


24

Figure 1: Conceptual framework for understanding environmental losses and benefits from flooding

Evaluating the environmental losses and benefi

Short Term

Long Term

Flood event

HighEnvironmental lossesEnvironmental benefitsHigh

• Spawning• Dispersal• Seed set

HighLow

• Introduction of Exotics• Reduced water quality (pollutants & acidification)

Long-term viability of ecosystems,species

and populations

Maintenance of ecosystem function

Recovery (th

rough re-

establish

ment, re-co

lonisation)

Degree of Human activity •River/floodplain management (eg dams, levee banks, channel modifications, floodplain modification, irrigation)• Land clearance/agriculture• Urbanisation

Reduced ecosystem function

Extinction of species & ecological communities

Recovery

•Death and injury of individual plants & animals

•Erosion•Sedimentation

• Nutrient transferShort Term

Long Term

Flood event

HighEnvironmental lossesEnvironmental benefitsHigh HighEnvironmental lossesEnvironmental benefitsHigh

• Spawning• Dispersal• Seed set

HighLow

• Introduction of Exotics• Reduced water quality (pollutants & acidification)

Long-term viability of ecosystems,species

and populations

Maintenance of ecosystem function

Recovery (th

rough re-

establish

ment, re-co

lonisation)

Degree of Human activity •River/floodplain management (eg dams, levee banks, channel modifications, floodplain modification, irrigation)• Land clearance/agriculture• Urbanisation

Reduced ecosystem function

Extinction of species & ecological communities

Recovery

•Death and injury of individual plants & animals

•Erosion•Sedimentation

• Nutrient transfer

ts from flooding

25

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