Tải bản đầy đủ - 0 (trang)
1 Introduction: Direct and indirect forms of human disturbance to rivers

1 Introduction: Direct and indirect forms of human disturbance to rivers

Tải bản đầy đủ - 0trang

Geomorphic responses of rivers to human disturbance

processes are in a transition phase. Most reaches

that are either readily navigable or present substantive opportunities for water development programs have already been exploited. In many parts

of the world, regeneration of land cover or forest regrowth has followed the wholesale clearance of

forest cover. Altered water, sediment, and nutrient

fluxes have brought about secondary adjustments

to river forms and processes (e.g., Newson, 1992a;

Liebauldt and Piégay, 2002). Likely future river

character and behavior must be predicted in light

of these societally induced changes.

Human modifications to biophysical attributes

of river systems can be direct or indirect (Park,

1977, 1981; Table 7.1). While most direct modifications are intended, indirect modifications are inadvertent. Direct modifications to the channel bed

and/or banks have typically taken the form of resource development activities (e.g., water supply,

power generation, gravel extraction) or structural

engineering works designed to alleviate the effects

of flooding. Clearance of riparian vegetation cover

and removal of woody debris have generally

accompanied these activities. Indirect human

impacts refer to adjustments brought about as

secondary responses to changes outside the channel that modify the discharge and/or sediment load

of the river. These impacts relate primarily to

changes in ground cover that modify the nature,

balance and interaction of water and/or sediment

fluxes. In general terms, impacts of indirect catchment-scale changes predate those associated with

direct human modifications to river courses.



209



Although these impacts may appear to be less dramatic than direct disturbance responses, their effects are often more ubiquitous and far-reaching,

with considerable lagged and off-site impacts. It is

often exceedingly difficult to differentiate river responses to direct human disturbance at the reach

scale from indirect human impacts at the catchment scale.

Human impacts on river character can only be

reliably interpreted if longer-term controls on

river evolution are understood. Hence, appraisals

of system response to human disturbance must be

made in context of inferred adjustments that

would have occurred under natural disturbance

regimes. Whatever the form of disturbance,

whether a site-specific direct impact such as dam

construction or indirect disturbance such as vegetation clearance, effects can be transmitted long

distances from their source. Ultimately, however,

management must consider cumulative responses

to disturbance, interpreting how these adjustments will shape likely future patterns and rates of

river changes.

This chapter is structured as follows. Direct and

indirect human impacts on river forms and

processes are considered in Sections 7.2 and 7.3 respectively. Spatial and temporal variability in river

responses to human disturbance, and their cumulative nature, are discussed in Section 7.4. Finally,

implications for rehabilitation activities are emphasized, demonstrating how notions of reversible

or irreversible changes are integrated into the river

evolution diagram (Section 7.5).



Table 7.1 Forms of human disturbance to river courses (modified from Brookes, 1994).

Direct channel changes

River regulation

• Water storage by reservoirs and water diversion

schemes (e.g., for irrigation)

Channel modifications

• River engineering. Channelization programs include

flood control works, bed/bank stabilization structures,

and channel realignment

• Sand/gravel extraction and dredging programs

• Clearance of riparian vegetation and removal of

woody debris



Indirect catchment changes

Land-use changes

• Changes to ground cover, including forest clearance,

afforestation, and changes in agricultural practice (e.g.,

conversion of grazing land to arable land and emplacement of

agricultural drains and irrigation channels)

• Urbanization and building/infrastructure construction, including

stormwater systems

• Mining activity



210



Chapter 7

7.2 Direct human-induced changes to river

forms and processes



Schemes that set out to stabilize and regulate river

systems typically endeavor to fix a channel with a

given size and configuration in a given position.

Such aspirations reflect concerns for potential impacts of river change, and are typically applied with

little concern for the inherent diversity of the natural world. Change is natural. Innately, the river

will adjust. Once a river is fixed in place, the cost of

keeping it there rises inexorably. Protection of societal, economic, and institutional infrastructure

has ensured that this course of action must be

maintained. However, in many parts of the world,

bold initiatives are being taken to let rivers run free

once more. Equally important are programs to conserve remaining minimally constrained rivers in

their notionally natural state, as far as practicable.

Although various forms of direct human disturbance are typically interlinked in river management programs, individual components are

considered separately in this section. Emphasis is

placed on dams and interbasin transfers (Section

7.2.1), channelization programs (Section 7.2.2),

sand and gravel extraction (Section 7.2.3), and impacts of changes to riparian vegetation cover and

woody debris loading (Section 7.2.4).

7.2.1 Dams and reservoirs

The dominant form of direct human-induced disturbance to river courses reflects schemes that

have endeavored to control and regulate their flow,

and associated concerns for water supply, whether

for agricultural (irrigation), commercial/industrial, or residential purposes. Enormous efforts

have been undertaken to make dry lands wetter

and wet lands drier, ensuring that water is available for human purposes (Cosgrove and Petts,

1990). The clinical efficiency with which engineering programs have achieved this task is, in itself, a testimony to human ingenuity. The extent

of these programs is staggering. The global volume

of freshwater trapped in reservoirs now exceeds

the volume of flow along rivers.

Given the critical importance of security of water

supply for both human consumption and agricultural developments, dam construction has played a

major part in pivotal societal changes, such as the



development of hydraulic civilizations (Wittfogel,

1956). Indeed, dams have been constructed for more

than 5,000 years. The pace of construction quickened dramatically after the Second World War, and

each year more than 200 large dams are completed

(Gregory, 1995). A recent decline in the rate of dam

development and associated water transfer projects

across much of the western world reflects the lack

of remaining reasonable opportunities. This factor,

among other considerations, marks a shift towards

a mature water economy, in which concerns for efficient water use have replaced prospects for generation of further water supply facilities (Smith, 1998).

This situation has yet to be reached across much of

the developing world, where pressures for dam

megaprojects continue to be seen as part of nationbuilding exercises.

Individual dams commonly form part of integrative water supply or hydroelectricity programs,

such as interbasin transfer schemes, wherein

water is transferred across system boundaries,

thereby accentuating water storage and flow in

some systems while diminishing flow elsewhere.

The function of dams may vary markedly, ranging

from water supply facilities to flood control impoundments. In some instances, dams may operate as flow-through structures. A common basis

for all water supply programs, however, is the disruption they place upon patterns and rates of flow.

Flow regulation is inimical to natural variability.

Impacts range across various timescales, whether

measured in terms of instantaneous flow releases

over seconds and minutes, from season to season,

or over annual or decadal timeframes. By definition, flow regulation reduces the extremes of flow,

substantially lowering flood peaks and modifying

base flow conditions. The variability of flow that

drives various geoecological processes is anathema to the regularity of supply that constitutes the

raison d’être for dam construction. Dams not only

disrupt the longitudinal continuity of flow along

rivers; they also act as major barriers to sediment

transfer.

Collectively, disruption to water and sediment

transfer mechanisms impacts directly on river

structure and function both upstream and downstream of the control structure (Figure 7.1). The reduction in channel gradient following elevation of

base level upstream of dams reduces the transporting capacity of flow as it enters the reservoir. This



Geomorphic responses of rivers to human disturbance



211



-



Figure 7.1 Geomorphic impacts of dam construction on river character and behavior

Dam construction traps sediment in a delta, creating an accumulation zone at the entrance to the reservoir (point A).

Suspended load sediments drape the former channel at this point, which now lies beneath the reservoir. At point B,

immediately downstream of the dam, reduced bedload and increased erosive potential of the “hungry river” have

induced bed incision following dam closure. A slot channel has been produced, and the bed has become armored. Inset

floodplains that line the compound channel have been colonized by dense, rapidly growing weeds. The original

floodplain is increasingly decoupled from the channel because of changes to the flow regime and morphological

adjustments. The channel at point B has become an area of net sediment loss following dam closure. Sediments

released from this zone have accumulated downstream at point C, where the channel has contracted through the

formation of lateral bars. Accelerated rates of bedload sediment supply to this reach cannot be sustained from

upstream because of the armoring effect at point B, and reworking of sediments is likely. These effects are

progressively propagated further downstream. Off-site impacts of dam construction may include incision of tributary

streams and altered morphodynamics at the coastline.



promotes delta development at the backwater

limit, reducing the water storage capacity of the

reservoir. Although aggradation takes place rapidly initially, its upstream extent may be limited or

long delayed, dependent upon sediment supply

conditions (Leopold and Bull, 1979). Reservoirs

make excellent sediment traps, commonly retaining more than 90% of the total load and the entire

coarse fraction (i.e., all bedload sediment and all

or part of the suspended load). In large reservoirs,

trap efficiency is commonly greater than 99%

(Williams and Wolman, 1984).



Downstream impacts of dam construction reflect lowered flood peak magnitudes and marked

reductions in sediment load (Williams and

Wolman, 1984). Reduction in sediment concentration, coupled with a reduction in high flows, can

reduce the total sediment load to a fraction of

predam values. The impact of sediment reductions

on downstream channels can vary widely, depending on the amount of reservoir storage, the dam

operations, and the location of the dam relative

to sediment sources (Brandt, 2000; Pitlick and

Wilcock, 2001).



212



Chapter 7



Water releases have the energy to move sediment but little or no sediment load is available to

them. This “hungry water” is able to expend its energy on erosion of the channel bed and banks

(Kondolf, 1997). Typically, the channel incises, and

decreases in bankfull cross-sectional area of over

50% are not uncommon (e.g., Petts, 1979;

Andrews, 1986). Where there is no sediment supply immediately downstream of a dam, and bed

materials are relatively fine-grained, bed degradation may be experienced (Williams and Wolman,

1984). In extreme circumstances, basal scour may

potentially undermine the structure itself

(Komura and Simons, 1967). In general terms,

downstream channel contraction and degradation

continue until development of bed armor or reduction in the energy slope stabilizes the channel.

Under conditions of notable degradation, progressive reductions in slope and increases in channel

roughness may modify hydraulic conditions such

that the rate of degradation diminishes. An initial

decrease in channel width following dam closure

may be followed by a widening phase as the bed becomes armored and relatively more resistant

(Williams and Wolman, 1984; Xu, 1990, 1996).

However, if impacts on the flow regime are sufficiently dramatic, such that substantive flows no

longer occur, no incision may be observed regardless of bed material texture (Kondolf, 1997).

Alternatively, channel capacity may be reduced as

fine-grained bars and berms are deposited at channel margins. This may be accompanied by a change

in channel planform, such as an increase in channel sinuosity or a decrease in channel multiplicity.

If vegetation encroachment occurs, channels may

become increasingly stable (e.g., Kondolf, 1997;

Erskine et al., 1999; Steiger et al., 2001).

Bed incision and channel narrowing inevitably

entail a timelag, as materials are removed and/or

redeposited. Changes to sediment transfer regimes

following dam closure are manifest over timeframes ranging from 10 to over 500 years (Petts,

1984). Since many dams were constructed in the

twentieth century, it may be a century or more before the river adjustment process is fully realized,

especially in downstream reaches. Exponential

decay functions used to describe incisional responses of channels to dam closure are far from

uniform. Around 50% of the total change may be



achieved in the first 5% of the adjustment period

(Knighton, 1998).

Large distances may be required before the river

regains, by boundary erosion and tributary inputs,

the same sediment load that it transported prior

to dam construction, and in some instances it

may never do so (Pitlick and Wilcock, 2001).

Downstream degradational impacts following

dam closure may extend over hundreds of kilometers, at rates extending up to tens of kilometers per

year (e.g., Galay, 1983; Williams and Wolman,

1984). Variability in response reflects pattern of

flow releases and bed material characteristics. Offsite impacts of flow regulation can be especially

pronounced at the interface with other landscape

compartments. Downstream-progressing degradation along the trunk stream can induce upstream-progressing degradation along tributaries,

promoting accelerated deposition at tributary confluences (e.g., Howard and Dolan, 1981; Petts,

1984; Brierley and Fitchett, 2000). The effects of

river regulation tend to diminish with distance

downstream, as nonregulated tributaries make an

increasing contribution to the flow. In some instances, the pattern and rate of morphodynamic

interactions may be altered a considerable distance from the control structure, as exemplified by

accelerated erosion and shoreline recession at the

coastal interface (e.g., Kashef, 1981; Kondolf, 1997;

Brierley and Fitchett, 2000).

Dams also induce changes to thermal regimes,

water quality, and biogeochemical fluxes, impacting on habitat availability and viability along the

trunk stream and secondary channels (e.g., van

Streeter and Pitlick, 1998). Altered base flows and

associated adjustments to the water table may

result in the loss of refugia in isolated pools, increased predation by terrestrial animals, changes

to riparian vegetation cover, and increased outbreaks of algal blooms.

Many dams no longer fulfill the purpose for

which they were initially constructed. Indeed, in

many instances, rates of sedimentation were underestimated, such that infilling of the reservoir

compromised or negated some of the core functions of the dam prior to its completion. New, larger structures may make the original structures

redundant. This realization, along with increasing

awareness of the geoecological consequences of



Geomorphic responses of rivers to human disturbance

dams, has resulted in many calls for dam removal,

and cautious steps are underway to make this

happen. An array of geoecological consequences

will accompany dam removal, relating to altered

flow and sediment fluxes, and accompanying

changes to river structure and function. These concerns require careful planning to appraise the longterm viability and sustainability of dams and other

forms of control structures along river courses. In

rare instances, collapse of dams may lead to devastating consequences, in societal and geoecological

terms.

7.2.2 Channelization programs

As many settlements are established along valley

floors, concerns for flood control and hazard reduction to support infrastructure development have

prompted calls for the training of river courses.

These issues, along with widespread efforts at

drainage improvement, erosion prevention, and

maintenance of navigational arteries are the

primary purposes of channelization practices

(Brookes, 1988). Most streamlines in urban and

peri-urban areas have been channelized via concrete lining and piping of flow. Swampy areas have

been extensively drained for agricultural purposes.

A range of structural measures can be applied to

stabilize channel bed and bank conditions (Table

7.2). In contrast to dam construction, which essentially represents a point disturbance with off-site

impacts, channelization activities are applied over

varying lengths of river.

Initial endeavors at river clearing and engineering date back to the Roman era (e.g., Herget, 2000).

However, systematic channelization programs

that set out to address human concerns for navigation and flood control only began in earnest in the

seventeenth century (Brown, 1997; Petts, 1989).

Since then, channelization programs have extensively modified tens of thousands of river kilometers (Brookes, 1985). These activities typically

transform a heterogeneous system into a homogeneous one, with resulting loss in the complexity

of instream geomorphic structure and associated

changes to flow interactions and habitat availability (e.g., Rhoads and Herricks, 1996; Toth, 1996).

Artificial cutoff and realignment programs may

increase the efficiency with which the channel is



213



able to convey flow and sediment in the short

term, initially enhancing prospects for flood control and navigability. However, local adjustments

to bed slope trigger secondary adjustments that

may have negative and very costly consequences.

For example, levee construction deepens flows,

potentially increasing rates of bed erosion (James,

1999). Since channelization involves manipulation of one or more of the dependent hydraulic

variables of slope, depth, width, and roughness,

feedback effects promote adjustments towards

a new characteristic state (Brookes, 1988).

Geomorphic response times following the emplacement of river engineering works depend on

the type of works installed and the extent to which

they alter flow and stream power, sediment supply, and vegetation cover. The time taken to attain

this new characteristic state may be anything up to

1,000 years (Brookes, 1988).

Channelization may induce instability not only

in the “improved” reach but also upstream and/or

downstream (Figure 7.2). Impacts are particularly

pronounced in response to channel slope modifications or straightening programs that increase local

bed steepness and hence erosive potential (e.g.,

Winkley, 1982). The character of bed level adjustment depends on reach position relative to the area

of maximum disturbance (Simon, 1989a, b, 1992,

1994). This zone acts as a fulcrum with net degradation upstream and net aggradation downstream.

Degradation tends to be at a maximum immediately upstream of the area of maximum disturbance, as the upstream progression of headcuts

accentuates unit stream power. This effect declines in severity upstream. Rapid upstream

progression of headcuts may compromise the integrity of infrastructure such as bridges (Brookes,

1989). Incision and basal erosion increase bank

height and bank angle until a critical state is

reached, promoting mass failure and channel

widening. Bed incision and subsequent channel

expansion may increase channel capacity by several hundred percent (Brookes, 1989, 1994; Jaeggi,

1989). Over time, armoring may increase resistance, inhibiting further bed erosion. Degradation

and widening provide an effective means of energy

dissipation as systems adjust to channelization

(Simon, 1992). Bed level change (aggradation or

degradation) may alter the biological and chemical



214



Table 7.2 Geomorphic impacts of channelization procedures (modified from Knighton, 1998, p. 312).

Methods



Purpose



Description



Impacts

Gradient is steepened as flow follows a shorter path. Flow velocity and

transport capacity are increased. Degradation ensues, progressing

upstream as a headcut. Bed and bank erosion increase sediment load

to the reach downstream, ultimately flattening its slope and promoting

aggradation.

Widening reduces velocity and unit stream power, thereby lowering sediment

transporting capacity, promoting bench deposition.



Flood protection (flow

evacuation);

infrastructure

development



River is shortened artificial

by cutoffs



Resectioning

(overwidening)



Increase conveyance

capacity to reduce

overbank flooding

Flood protection, confine

floodwaters, maintain

irrigation channels



Widening and/or

deepening of the

channel

Raise channel banks,

increasing channel

capacity



Channel stabilization

and bank protection

works



Control bank erosion



Clearing and

snagging



Aid flood passage and

navigation capacity



Use of structures such as

paving, gabions, steel

piles, subaqueous

mattressing, dikes, and

jetties

Removal of obstructions

from the river



Dredging



Maintain navigable

channels



Weir and lock

emplacement



Regulate slope for

navigation



Levee and floodwall

construction



Sediment removal from

the bed to deepen the

channel, especially

along the thalweg in

lowland reaches

Channel spanning

structures



Reduces floodplain inundation and hence sedimentation rates,

inducing profound changes to wetland ecosystems. May “trap”

floodwaters in extreme events. Alternatively, concentration of flow may

promote bed incision.

Alters channel width and roughness components, with secondary

implications for bed incision and subsequent sediment release, thereby

adjusting channel bed slope. May promote sedimentation adjacent to

the bank, potentially increasing flooding if channel capacity is reduced.

Decreases resistance and increases flow velocity, thereby promoting

bed degradation, subsequent widening, and marked increase in

channel capacity.

May promote degradation through lowering of base level, enabling

knickpoints to migrate upstream, thereby contributing additional

sediment to the dredged reach. Deepening may also promote bank

collapse and promote upstream-progressing degradation within

tributaries.

Alter bed slope, reducing conveyance of sediment. Modify river

structure, promoting elongate pools in place of hydraulic diversity.



Chapter 7



Straightening

(realignment)



215



Geomorphic responses of rivers to human disturbance

z

-



Figure 7.2 Geoecological impacts of

channelization (modified from

Corning, 1975)

Channelization of a meandering

river transforms a channel with a

diverse array of habitats and

significant bed heterogeneity into a

uniform, homogeneous system.

While rapid conveyance of flow and

bed materials is achieved, ecological

attributes of the river may be

fundamentally compromised.



functioning of channel and floodplain zones.

Geomorphic responses to bed incision exert a significant impact on hydraulic and habitat conditions along river courses, the lateral connectivity

of channels and floodplains, and vertical changes

to substrate conditions and associated hyporheic

zone processes. Drops in water-table levels may

bring about detrimental effects to agriculture on

adjacent alluvial lands.

Greater concentration of flow within the channel may accelerate the transmission of flood waves

and accentuate flood peaks, relative to the period

prior to channelization (e.g., Wyzga, 1993, 1996).

Not only are effects of flood hazard transferred

elsewhere, their extent and consequences may be

exaggerated! Effects of flood alleviation and land

drainage via combinations of channel widening,



dredging, and straightening are particularly

marked on rivers that transport significant

amounts of bed material load, as they are able to respond very quickly to imposed changes. On rivers

with low bed material loads, responses are less dramatic. Transfer of excess load to reaches downstream of the area of maximum disturbance may

result in accelerated channel aggradation and/or

bank accretion, reducing channel capacity. This

secondary response progressively works its way

upstream over time. In areas of reduced velocity

that promote the deposition of sediments, response times for stabilization and channel contraction may be enhanced by revegetation of

sediment stores. Increased flow resistance promotes accelerated rates of sedimentation.

Changes to the nature and extent of riparian vege-



216



Chapter 7



Table 7.3 Geomorphic influence on riparian vegetation recovery patterns following channelization. Modified from

Hupp and Simon (1991). Reprinted with permission from Elsevier, 2004.

Stage



Geomorphic attributes



I: Premodified



II: Construction

III: Degradation



IV: Threshold



V: Aggradation



VI: Recovery



Aggrading bed and banks, meandering

channel, low convex, upward banks,

minimal mass wasting

High gradient straight channel, linear

banks

Active bed degradation, minimal mass

wasting, linear banks

Active bed degradation, active mass

wasting, concave upward banks,

severe instability

Aggrading channel bed, mild mass

wasting, significant bank accretion,

multiple thalweg, diverse bank

forms

Meandering channel, low banks and

gradient, convex upward banks,

general mild aggradation,

point bar development



tation cover may exert a significant role in channel

recovery (Table 7.3; Hupp and Simon, 1991; Hupp,

1992).

Although engineering works generally result in

a reduction of the sediment flux, local areas may

experience accelerated rates of erosion and sediment transfer associated with river bed scouring,

bank erosion, and increased tributary sediment

supply (e.g., Bravard et al., 1999). Accentuated

sediment loads may result in build-up of deposits,

especially in lowland basins. Generation of sediment slugs may diminish habitat availability, presenting barriers for fish passage. If these responses

impede human activities, dredging may be undertaken to maintain a navigable channel. Such actions not only act against the natural depositional

tendency of the river, they may also trigger bed

level instability, promoting the generation of headcuts that may extend some distance upstream.

7.2.3 Gravel/sand extraction

Gravel and sand extraction can take the form of instream (wet mining) where sediment is extracted

from instream bar and bed surfaces, or open flood-



Ecological attributes



Duration



Mature, diverse riparian communities,

100% cover



Stable



All woody vegetation removed, 0% cover



Short <1 yr



Channel adjustments exert little influence

on woody vegetation, which is

generally high and dry, 100% cover

Bank failure removes most woody species,

herbaceous weeds present,

0–5% cover

Initial active revegetation at same site

as initial bank accretion, 10–50%

cover



1–3 yr



Diverse bank vegetation growing down

and into water, 90–100% cover



Stable



5–15 yr



50 yr



plain pits. Instream mining may involve extensive

clearing, diversion of flow, stockpiling of sediment, and excavation of deep pits (Kondolf, 1994,

1997; James, 1999). In many instances, sediment

extraction has been applied without due regard for

sustainable rates of bedload transport (i.e., replenishment), such that the bed and floodplain have effectively been mined. By removing sediment from

the channel, the preexisting balance between sediment supply and transport capacity is disrupted.

Typical responses include lowering of the

streambed, local increases in slope and flow velocity upon entering the pit, and adjustments to

channel geometry (Figure 7.3). Once bed armor is

destroyed, enhanced bed scour may generate headcuts in oversteepened reaches, and hungry water

erodes the bed downstream (Peiry, 1987; Kondolf,

1997).

As geomorphically effective sediment transporting events are infrequent in many gravel-bed

rivers, instream mining activities may operate for

several years without obvious effects upstream or

downstream (Kondolf, 1998a, b). However, responses may be manifest during high flows many

years later. Headcuts may propagate upstream for



Geomorphic responses of rivers to human disturbance



217



-



Figure 7.3 Geomorphic impacts of

instream gravel mining

(a) In the preextraction condition, the

river’s sediment load and the force

available to transport sediment are

continuous through the reach. (b)

Excavation of an instream pit breaks

the bed armor and instigates a headcut

at the upstream end of the pit.

Initially, the pit traps sediment,

interrupting the transport of sediment

through the reach. Downstream, the

river retains the capacity to transport

sediment but has no sediment load. (c)

Headward extension of the headcut

acts to maintain bed surface slope.

Hungry water erodes the downstream

end of the pit, as incision expands both

upstream and downstream. (d)

Sediments released following the

upstream progression of the headcut

and associated channel expansion

partially infill the incised and

expanded trench of downstream zones

in the form of bars and benches. This

results in a compound channel form.

Modified from Kondolf (1994) and

reprinted with permission from

Elsevier, 2003.



kilometers on the main river and tributaries, potentially undermining bridges and weirs and exposing aqueducts, gas pipelines, and other utilities

buried in the bed. Incision may also be accompa-



nied by coarsening of bed material, as smaller,

more mobile fractions are transported first.

Undercutting of banks promotes channel expansion. Planform changes may ensure, typically en-



218



Chapter 7



tailing the adoption of a low sinuosity, single channeled river. Enhanced rates of downstream sediment delivery may promote channel aggradation

and instability. These adjustments alter the availability and viability of aquatic habitat, groundwater levels, and riparian vegetation associations.

Removal of sand and gravel via floodplain mining also represents a nonrenewable exploitation of

resources. However, if managed effectively, with

clear separation of geomorphic activity from the

channel zone, pits may be stabilized and left open

once mining activities are completed, creating

habitat for local flora and fauna in large open-water

ponds. However, if not carefully managed, the pit

may be captured by the channel, resulting in upstream and downstream propagation of incision

and consequent bed coarsening, channel widening, and destabilization of the banks (e.g., Kondolf,

1997).

7.2.4 Geomorphic responses of rivers to

clearance of riparian vegetation and removal

of woody debris

As noted in Chapter 3, river change is brought

about when the preexisting balance between impelling and resisting forces along a reach is unsettled. Adjustments that modify the water and/or

sediment regime reflect changes to either the driving forces that promote change, or the resisting

forces that inhibit change. Riparian vegetation and

woody debris have key roles in various feedback

linkages that influence channel capacity, hydraulic roughness, channel slope, sinuosity,

sediment transport rates, bank strength, and floodplain evolution. Riparian vegetation cover and the

loading of woody debris are perhaps the most readily manipulated form of channel resistance.

Whether induced by environmental (climatic)

changes, bushfire activity, or as a consequence of

human activity (mechanical removal or grazing),

changes to riparian vegetation cover are likely to

bring about significant adjustments to river structure and function. Although human disturbance to

riparian vegetation represents a direct human action, its consequences have been largely unintended.

As widespread disturbance to riparian zones occurred before records were kept across much of the

world, insights into the nature, pattern, and rate of



river adjustments to clearance of riparian vegetation and removal of woody debris remain inferential, rather than directly proven. Regardless of

environmental setting, contemporary river morphodynamics across most of the globe have adjusted to conditions in which riparian vegetation and

woody debris are either absent or highly altered.

Variability in geomorphic response to vegetation

removal reflects the type of river and the role

played by riparian vegetation and woody debris as

determinants of form–process associations, the

inherent capacity of a river to adjust, withincatchment position (and related scalar considerations), and the sequence of driving factors (i.e.,

floods) that promote change (e.g., Hupp and

Osterkamp, 1996; Gurnell et al., 2002).

River responses to clearance of riparian vegetation and/or woody debris are likely to be greatest in

those settings where vegetation exerts greatest influence on river morphology, namely sand-bed alluvial rivers. For example, the presence of an intact

riparian forest maintained a low capacity, slowly

meandering sand-bed channel throughout the

Holocene, at least, along various river courses in

southeastern Australia (Brooks and Brierley, 2002).

However, removal of riparian vegetation and

woody debris resulted in catastrophic incision and

channel expansion within a matter of years

(Brooks et al., 2003). In their intact state, there was

sufficient inherent roughness in these riparian

landscapes that thresholds for geomorphic change

were virtually unattainable and geomorphologically effective floods were unable to bring about

river metamorphosis. Even following major

floods, there was sufficient capacity for geomorphic recovery, such that a characteristic state

was maintained. However, once the inherent resilience of these valley floors was altered by wholesale clearance of riparian forests, accompanied by

the removal of woody debris, subsequent events

initiated channel metamorphosis. In one river system, direct human disturbance to the riparian vegetation cover tied to a desnagging program brought

about a 700% increase in channel capacity, a 360%

increase in channel depth, a 240% increase in

channel slope, and a 150-fold increase in the rate of

lateral channel migration within a few decades

(Brooks et al., 2003; Figure 7.4). Once triggered, the

enlarged channel capacity not only reduces channel resistance; it also increases energy concentra-



219



-



Geomorphic responses of rivers to human disturbance



Figure 7.4 Geomorphic changes following clearance of riparian vegetation and removal of woody debris along the

Cann River, Victoria, Australia

Clearance of riparian vegetation and removal of woody debris induced dramatic changes to the geomorphic structure

of the Cann River in East Gippsland, Victoria (see text; Brooks et al., 2003). Partial clearance of riparian vegetation had

occurred by 1919, but a desnagging program in the decade prior to 1971 was primarily responsible for channel

metamorphosis. The near-instantaneous reduction of vegetative roughness elements lowered threshold conditions

that determine bed level stability and critical bank height, such that the channel became highly sensitive to change.

Flood events that brought about minor perturbations under intact vegetation conditions were much more

geomorphologically effective under altered boundary conditions. Exceedance of threshold conditions brought about

fundamental shifts in river character and behavior via incision, straightening, and channel expansion. The

progressively enlarging channel increasingly concentrated flow energy at flood stage. The channel became

increasingly decoupled from its floodplain.



tion. The capacity of woody debris to increase

roughness and stabilize instream sediment may

significantly enhance geomorphic river recovery

following disturbance (e.g., Cohen and Brierley,

2000).

In general terms, loss of riparian vegetation increases bank erosion and promotes channel widen-



ing and shifting, or bed degradation. Removal of

floodplain vegetation can cause the water table to

drop leading to secondary salinization (Burch et al.,

1987). Piping, tunnel, and gully erosion may damage infrastructure such as roads, railways, and

bridges. Alternatively, influxes of exotic vegetation can smother a channel bed, inducing excess



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

1 Introduction: Direct and indirect forms of human disturbance to rivers

Tải bản đầy đủ ngay(0 tr)

×