Tải bản đầy đủ - 0 (trang)
4 Stage One, Step Three: Assess controls on the character, behavior, and downstream patterns of River Styles

4 Stage One, Step Three: Assess controls on the character, behavior, and downstream patterns of River Styles

Tải bản đầy đủ - 0trang

288



Chapter 9



• can discernible changes in imposed boundary

conditions be isolated in the transitional zones between River Styles? Do transition zones coincide

with tributary–trunk stream boundaries as noted

by changes in contributing area, breaks in slope,

and base level controls along the longitudinal

profile, or geological (lithologic and/or structural)

changes? Do certain River Styles occur in certain landscape units at certain positions in the

catchment?

• can underlying factors that result in changes to

flux boundary conditions be expressed in terms

of changes to the flow and sediment fluxes? Do

cumulative responses reflect a change in stream

power? Can these relationships be quantified?

What has fashioned patterns of sediment transport

and storage along river courses, and their relationships to geomorphic process zones?

Interpretation of controls on river character and

behavior may result in assessments that are catchment (or region) specific. As many factors may conspire to create an opportunity for any given type of

river to be generated, it is considered to be overly

simplistic to quantify controls on the distribution

of individual River Styles in a prescriptive manner.

River character and behavior do not reflect variability along a single continuum in, say, slope,

grain size or valley width. Rather, they reflect a



multivariate continuum with an infinite complexity of associations. Any given set of boundary conditions may result in an array of River Styles, and it

is unlikely that any particular River Style will only

be observed under a “unique” set of conditions

such as a specific lithology on a characteristic

slope set within a particular range of stream power

conditions. Hence, there will be overlap among the

circumstances under which any River Style is observed and associated “typical” patterns in space

and time.

The best way to assess controls on river character and behavior is to determine the conditions

under which all examples of a particular River

Style operate. Differences, similarities, and overlaps in controls among River Styles are analyzed

and interpreted. Just as differing parameters define

the character and behavior of River Styles, so the

relative influence of differing controls condition

the presence/absence and distribution of River

Styles. To interpret the dominant controls on the

character and behavior of each River Style, a comparison is made between River Styles to determine

which controlling parameters are significantly

different. Obviously, depending on the scale of

analysis (e.g., regional versus catchment versus

subcatchment), and the nature of the environmental setting, other measures may be required, such



















Figure 9.14 Stage One, Step Three:

Assessment of controls on the

character, behavior and downstream

patterns of River Styles



Stage One of the River Styles framework



289



as structural geology, drainage density, stream

order, catchment shape, vegetation cover, forms of

human disturbance (e.g., urban attributes, irrigation schemes), etc. Beyond this, an assessment is

made of how various controls interact to dictate

the character and behavior of the River Style.

Finally, any anomalies are explained. Procedures

used to assess controls on the character, behavior,

and downstream patterns of River Styles in a

catchment are summarized in Figure 9.14.



patterns, from those that are distinct. Alternating

patterns of River Styles can be synthesized in this

assessment. This analysis provides a basis to assess the similarity in downstream controls on

River Styles and helps explain river character and

behavior in different subcatchments. A tree-like

diagram is produced showing the tributary patterns of River Styles and how they connect to the

trunk stream (Figure 9.15). The pattern is noted on

the catchment River Styles map (see Plate 9.2).



9.4.1 Determine downstream patterns

of River Styles



9.4.2 Determine imposed boundary condition

controls on river character and behavior along

longitudinal profiles



Downstream patterns of River Styles are determined for all river courses in the catchment, differentiating those that have similar downstream



River Styles boundaries are placed atop the longitudinal profile-contributing area plots, noting



z



b



b



Figure 9.15 Downstream patterns of River Styles in Bega catchment

Four primary downstream patterns of River Styles were identified from 16 subcatchments in Bega catchment:

• those in which headwaters are transitional to long, elongate, bedrock-controlled valleys downstream of the

escarpment (Pattern 1);

• those which have large accommodation spaces at the base of the escarpment in which extensive Holocene fills have

formed (Pattern 2 rivers have channelized fills, Pattern 3 rivers have intact valley fills);

• those in which boulder fans at the base of the escarpment are transitional to bedrock-controlled valleys and the

lowland plain (Pattern 4).

Representative examples of each were chosen to explain the controls on the character, behavior, and downstream

pattern of River Styles in the catchment.



290



Chapter 9



Figure 9.16 Controls on the downstream pattern of River Styles along the Bega River

Significant catchment areas drain from the uplands landscape unit where the steep headwater River Style occurs. The

longitudinal profile has a distinct step in the escarpment zone where the gorge River Style is formed. Beyond the

escarpment, the longitudinal profile has a relatively smooth, concave-upwards form. Associated with this progressive

downstream change in slope, there is progressive downstream widening of both the channel and the valley through

the base of the escarpment, the rounded foothills and the lowland plain landscape units. At the base of the escarpment,

the low sinuosity boulder bed River Style occurs. This occurs where slopes are high and flow exits from the

escarpment zone. Large boulder fans have been deposited. There is a lack of a valley constriction along the

downstream margin of the base of escarpment landscape unit. This prevents the accumulation of valley fills at the

base of the escarpment along these valleys. Instead bedload materials are transferred through the system, until they

reach the lowland plain where they accumulate.

Along the majority of the Bega trunk stream, the confined valley with occasional floodplain pockets River Style

occurs. As catchment area increases and discharges increase with the inputs from numerous tributaries, gross stream

power along this River Style progressively increases. Peaks occur where bedrock steps occur in the longitudinal profile

(e.g., at ~ 60 km where Kanooka knickpoint is located). It is not until the valley widens and slope decreases

significantly around the Wolumla Creek confluence that the transition to a low sinuosity sand bed River Style occurs.

This transition coincides with the start of the lowland plain landscape unit and a drop in the gross stream power. No

partly-confined valleys are found along this pattern of River Styles in Bega catchment.



Stage One of the River Styles framework



291



-



z



Figure 9.17 Controls on the downstream pattern of River Styles along Wolumla Creek

Most subcatchments in Bega catchment drain directly from the escarpment and have smooth concave-upward

longitudinal profiles, with a gentle break in slope at the base of the escarpment. In these tributaries, where the uplands

landscape unit is absent, the channelized fill River Style is formed at the base of the escarpment. These laterallyunconfined valley settings are formed under a particular set of catchment boundary conditions. Broad, asymmetrical

valleys are formed downstream of a gentle break in slope. The downstream margin of these valleys is characterized by

either a significant narrowing of the valley or a bedrock step which gives the valley a funnel shape. Large

accommodation spaces store material behind these constrictions. The formation of these laterally-unconfined valley

settings at the base of the escarpment is a direct result of interactions between escarpment retreat and valley-sidewall

expansion (Fryirs, 2002). When infilling, these valleys are characterized by the intact valley fill River Style. When

cutting, these valleys are characterized by the channelized fill River Style. In 8 of the 10 subcatchments that display

this downstream pattern of River Styles, only two remain intact (i.e., contain the Intact valley fill River Style at the

base of the escarpment).

In the rounded foothills of these subcatchments, the confined valley with occasional floodplain pockets and the

partly-confined valley with bedrock-controlled discontinuous floodplain River Styles extend to the trunk stream.

Unlike the classical downstream sequence of channel geometries and process zones along long profiles, streams along

this pattern of River Styles have large, laterally-unconfined valleys with wide, deep channels at the base of the

escarpment. These channels are mixed load in composition, with sands and muds accumulating on the channel bed.

These are transitional to narrower, shallower channels in the confined and partly-confined valley settings in the

middle and lower sections of the catchment which act to effectively transfer bedload materials through to the lowland

plain.



292



Chapter 9



their relation to each valley setting and landscape

unit (see Figures 9.16 and 9.17). These imposed

boundary condition controls define the potential

range of variability of a River Style. For each River

Style, the range of valley widths, valley slope, and

contributing areas are determined, and presented

as part of a “controls” table. The transition zone

between River Styles is related to catchment-scale

boundary conditions (e.g., geological boundaries),

associated landscape units, and the catchment

position (and contributing area). By definition,

shifts in River Styles are evident whenever

changes in valley setting occur. These transitions,

in turn, are commonly associated with downstream changes in landscape units, reflecting landscape history. Considerable overlap may exist in

these imposed boundary conditions, as a range of

River Styles can be formed under similar sets of

these conditions.



tion zones) and associated sediment transport

regime (i.e., bedload, mixed load, or suspended

load system) are plotted beneath these curves. An

estimate is made as to whether each reach is sediment supply- or transport-limited. In general,

rivers in confined and partly-confined valley

settings act as sediment transfer reaches, as sediments are readily flushed. Rivers in laterallyunconfined valley settings tend to be accumulation zones, unless the reach responds to disturbance by reworking its sediment stores, thereby

acting as a sediment source zone.

This analysis provides the basis to interpret

process responses of each reach to imposed and

flux boundary condition controls, which are major

determinants of river character and behavior. A

summary representation of the range of controls

for all River Styles in Bega catchment is presented

in Figure 9.18.



9.4.3 Determine the flux boundary condition

controls on river character and behavior along

longitudinal profiles



9.5 Overview of Stage One of the River

Styles framework



Stream power provides a guide to the energy

regime at differing positions along a river. Outputs

required to generate stream power include contributing area and a running average of slope.

When combined with the catchment areadischarge relationships derived in Stage One, Step

One, discharge estimates can be extracted for the 2,

5, 10, 50, and 100 year events and plotted as a continuous data set along the longitudinal profile.

When discharge is combined with slope, gross

stream power is generated (Reinfelds et al., 2004).

In the River Styles framework, the gross stream

power curve is superimposed onto the longitudinal profile-contributing area plot for each

subcatchment. If possible, unit stream power estimates are derived for each River Style, recognizing

explicitly that the range of estimates may vary

markedly for differing reaches (reflecting variability in slope or upstream catchment area, or the geomorphic condition of the reach; see Chapter 10).

Generally, only one recurrence interval relationship (e.g., 1 in 2 year event) is depicted, but all values are presented in a summary table of controls on

the character, behavior and pattern of River Styles

(Table 9.10). The distribution of geomorphic

process zones (i.e., source, transfer, and accumula-



The baseline survey of River Styles integrates

catchment-scale controls on rivers with reachbased assessments of river character and behavior

through use of a nested hierarchical approach.

Classification of River Styles is based initially on

valley setting. For differing settings, variable sets

of parameters including channel planform, the assemblage of geomorphic units, and bed material

texture are used to define River Styles, emphasizing distinguishing attributes in River Styles trees.

Proformas are completed for each River Style.

Analysis of catchment-scale linkages and boundary conditions aids determination of the controls

on river character and behavior for each River

Style.

The following products are produced in Stage

One of the River Styles framework:

• regional setting chapter;

• River Styles tree that is specific to the

catchment;

• catchment-wide map showing the distribution

of River Styles;

• River Styles proformas, annotated crosssections, annotated geomorphic unit planform

map and photographs for each River Style in the

catchment;



Table 9.10 Controls on river character and behavior in Bega catchment.

River style



Valley

width (m)



Unit stream power (Wm-2)



Catchment

area (km2)



Formative/bankfull

recurrence interval (years)



1 in 2



1 in 5



1 in 10



1 in 50



1 in 100



0.02

0.04–0.08

0.005–0.03

0.020–0.028

0.03

0.004–0.006



40

10–40

<300

200

100

60–240



>20

0–135

<20

<20

>50

100–1000



180

685

100

3

70

100



415

1730

125

4

90

130



500

2305

440

25

390

390



270

2530

1020

70

900

640



390

2770

1140

100

1190

730



N/a

N/a

>100

N/a



>100



0.005–0.029



20–80



20–325



165



210



680



1270



1520



2–50



0.010

0.005–0.012



150

40–210



<30

30–200



3

95



4

120



25

410



70

820



100

1030



N/a

10–50



0.002–0.0008



100–650



500–1840



30



35



95



220



280



5–10



Note

Gorges are found on high slopes in confined settings that generate high stream powers. This acts to flush materials efficiently through the escarpment zone of the catchment.

High stream powers are also generated along the confined valley with occasional floodplain pockets and the partly-confined valley with bedrock-controlled discontinuous

floodplain River Styles. While found on lower slopes, their position in the catchment ensures that large discharges are generated in these midcatchment locations. Given their

bedrock-controlled character they too act to efficiently flush sediment to the lowland plain. The lowest stream powers in the catchment are generated in the intact valley fill and

floodout River Styles which remain unchannelized and effectively dissipate energy over valley fill surfaces. These River Styles act as large sediment sinks.



Stage One of the River Styles framework



Steep headwater

Gorge

Channelized fill

Intact valley fill

Low sinuosity boulder bed

Confined with occasional

floodplain pockets (trunk)

Confined valley with occasional

floodplain pockets

(tributaries)

Floodout

Partly-confined valley with

bedrock-controlled

discontinuous

floodplain

Low sinuosity sand bed



Valley

slope



293



z



Figure 9.18 Summary controls on the character and behavior of River Styles in Bega catchment

This figure summarizes controls on the character and behavior of River Styles in Bega catchment. These controls

include slope and valley confinement. Each reach is placed within its landscape context through analysis of landscape

units. Reprinted from Brierley and Fryirs (2000) with permission from Springer-Verlag GmbH & Co. K.G. 2004.



v



-



Figure 9.18 Continued



296



Chapter 9



• longitudinal profile diagrams with associated

assessment of controls for a representative example of each downstream pattern of River Styles in

the catchment;

• table of controls for all River Styles in the

catchment.

Baseline information derived from analysis of

catchment-wide river character and behavior pro-



vides a geomorphological baseline for comparing

like-with-like, ensuring that reaches of the same

River Style are used in analyses of river condition

and recovery potential, as outlined in Chapters 10

and 11.



C H A PT E R 1 0

Stage Two of the River Styles framework:

Catchment-framed assessment of river

evolution and geomorphic condition

Many rivers no longer support valued native species or sustain healthy ecosystems that

provide important goods and services.

LeRoy Poff et al., 1997, p. 769



10.1 Introduction

Completion of Stage One of the River Styles

framework produces a catchment-wide analysis of

differing geomorphic types of river. Inevitably,

reaches of any given type do not have a uniform

character and behavior. Inherent diversity in river

forms and processes may be evident at the local

scale, such that a mapped River Style represents a

summary sense of character and behavior at the

reach scale. In many instances, these differences

may reflect variability in the geomorphic condition of differing sections of a reach of a given River

Style, as induced by human disturbance (whether

direct or indirect, at-a-site or off-site). In this chapter, a set of procedures with which to appraise the

geomorphic condition of rivers is documented.

This represents Stage Two of the River Styles

framework.

Measures of geomorphic river condition record

deviations from a natural or expected state in any

given reach (i.e., how human disturbance has altered river character and behavior). In this book,

river condition is defined as a measure of the capacity of a river to perform functions that are expected for that river within the valley setting that

it occupies (Table 10.1). The further a reach sits

from its reference condition, the poorer its geomorphic condition. When appraised in terms of the

capacity for adjustment that is appropriate for the

given boundary conditions (i.e., the reference condition), each reach is placed into a good, moderate,

or poor condition category.



To frame the assessment of geomorphic river

condition, human-induced changes to river forms

and processes must be viewed in context of the inherent evolutionary tendencies of the system. In

applying a generic set of procedures in each field

situation, elements of subjectivity are encountered. So long as limitations are recognized, they do

not present an insurmountable problem. Indeed,

much is to be gained by thinking through and discussing these issues. Regardless of the challenges

faced, assessments of river condition (or health)

constitute an integral part of the river management

process, providing a critical platform for environmental decision-making and associated actions.

Given that each River Style records the character and behavior of reaches that operate within an

equivalent set of boundary conditions, comparison of reaches of the same River Style provides an

ideal basis to assess river condition. Geomorphic

river condition is appraised in context of the capacity for adjustment of the River Style and the degree

to which contemporary measures of geomorphic

structure and function for the reach have moved

away from the reference condition. As geomorphic

structure and function, and associated adjustments, are predictable for each River Style, a platform is provided with which measures of river

condition can be evaluated in a consistent yet flexible manner, ensuring that appropriate criteria

are measured to make this determination. River

condition is appraised in terms of channel planform, channel geometry, bed character, and the

geomorphic unit assemblage along a reach.



298



Chapter 10



Table 10.1 Definition of terms used to describe the geomorphic condition of a reach.

Term

River condition



Capacity for adjustment

Degrees of freedom

Relevant geoindicators



Desirability criteria



Natural river (Natural

reference condition)



Expected reference

condition



Irreversible geomorphic

change



Good condition



Moderate condition



Poor condition



Definition in the River Styles framework

A measure of the capacity of a river to perform functions that are expected for that river within the

valley setting that it occupies. The contemporary geomorphic state of a reach relative to a

“natural” or “expected” reference condition of the same type of river. Assessment of river

condition requires an understanding of:

• the spatial distribution of river types

• how those rivers behave

• river dynamics (i.e., river evolution), and

• forms, extent, and impact of human disturbance, including an appraisal of whether this change

has been irreversible.

Morphological adjustments brought about by the changing nature of biophysical fluxes that do not

record a wholesale change in river type.

The ability of differing components of a river system to adjust, measured in terms of bed character,

channel attributes, and planform attributes.

Parameters used to interpret and explain system structure, function, and condition for each degree

of freedom. “Relevant geoindicators” provide a reliable and relevant signal about the condition

of a reach. The geoindicators measured are River Style specific.

Assessment of the appropriateness of each relevant geoindicator for each River Style. A question is

posed for each geoindicator to produce a set of desirability criteria to identify a reference

condition and assess the geomorphic condition of a reach.

A “natural” river is dynamically adjusted so that geomorphic structure and function operate within

a capacity for adjustment that is appropriate for that type of river, given the prevailing boundary

conditions. A “natural reference condition” is considered to be a river that is operating in the

absence of human disturbance. Changes to this “intact” or “predisturbance” condition are

considered to be reversible.

A prehuman disturbance reference condition is largely irrelevant for many river systems. Hence,

expected reference conditions are identified against which the geomorphic condition of a reach

is assessed. Three types of expected reference condition differentiate among situations in which

the reach has been:

• reversibly altered by human disturbance;

• irreversibly altered by indirect human disturbance;

• irreversibly altered by direct human disturbance.

A wholesale shift in the geomorphic unit structure, planform, and bed material texture, such that

the river operates in a fundamentally different manner to its former state with no prospect of

return over a 50–100 year timeframe. This transition in the behavioral regime marks the adoption

of a different type of river.

River character and behavior are appropriate for the River Style given the valley setting and withincatchment position. Geomorphic structures are in the right place and operating as expected for

the River Style. These reaches have a near-natural potential for ecological diversity and associated

vegetation associations.

Certain characteristics are out-of-balance or inappropriate for the River Style. Localized degradation

of river character and behavior is typically marked by modified patterns of geomorphic units. Key

geomorphic structures are in the wrong places. Locally anomalous processes are occurring. In

general, these reaches have poor vegetation associations and/or cover.

River character is divergent from the natural reference condition. Abnormal or accelerated

geomorphic behavior occurs. Key geomorphic units are located inappropriately along the reach,

and processes are out-of-balance or anomalous. These reaches generally have low levels of bank

vegetation and/or are weed infested. If fundamental threshold conditions are breached,

irreversible geomorphic change would transform the reach to a new River Style.



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

4 Stage One, Step Three: Assess controls on the character, behavior, and downstream patterns of River Styles

Tải bản đầy đủ ngay(0 tr)

×