Environmental Guidelines for River Management Works

ENVIRONMENTAL GUIDELINES FOR RIVER MANAGEMENT WORKS FOR THE STANDING COMMITTEE ON RIVERS AND CATCHMENTS 8571(F1) Published for the Standing Committee on Rivers and Catchments by the Department of Conservation & Environment Edited by Georgina Katsantoni Designed and Typeset by Dead Set, Fitzroy Printed by Victorian Government Printing Office ISBN 07306 2062 X © 1990 GUIDELINES Gu IDELINES FOREWORD INTRODUCTION TO RIVER MORPHOLOGY AND ECOLOGY 1 RIVER MORPHOLOGY 1 RIVER ECOLOGY 6 GENERAL ENVIRONMENTAL GUIDELINES 11 STREAM MORPHOLOGY 11 PROTECTING THE STREAM ENVIRONMENT 12 RECREATION 14 GUIDELINES Guidelines Foreword The Department of Conservation and Environment is involved in river management activities throughout Victoria. Most river management works are undertaken by statutory river management authorities which receive funding through the Office of Water Resources. The Department recognizes its responsibility to ensure that stream works take adequate steps to protect or enhance the aquatic and streamside environment. This report is one in a series prepared for the Standing Committee on Rivers and Catchments to assist government agencies and other authorities involved in river management. It provides a basic introduction to river morphology and ecology and describes the environmental effects of river management works. It also brings together a number of case studies and suggests ways in which major types of river management works should be carried out to protect stream environments. Horrie Poussard Convenor Standing Committee on Rivers and Catchments GUIDELINES Introduction to River Morphology and Ecology When Europeans settled in Victoria less than River morphology two centuries ago, these natural instabilities were This introduction to river morphology briefly regarded as incompatible with the way the land was describes and explains the processes which have to be used. European land uses altered the 'graded' shaped the way in which Victorian stream systems equilibrium and introduced new instabilities into the have evolved through time and the way in which system. The magnitude of these new instabilities these stream systems have responded to external varies throughout the State and even within river influences such as land use changes and stream basins but, as a generalisation, the effect of European management activities. settlement on the stability of Victorian streams has been severe. Drainage basin morphology Tune scale perspectives Stream stability Viewed on a human time scale, many stream The majority of streams in Victoria are dynamic instabilities are regarded the cause of severe alluvial streams characterised by a capacity to alter environmental degradation. Viewed from the their own boundaries. Variations in the balance perspective of a time scale in the order of hundreds between sediment supply and sediment transporting to thousands of years these changes may be perceived capacity, both along the length of a stream and as fluctuations within a more or less steady state. within a cross section, cause the channel to alter its Fig. 1 (see page 2) schematically illustrates four slope, depth, width, shape or location. For most of time scales. The vertical axis in each case is a their lengths these alluvial streams flow through bed notional elevation at a point in the drainage system, and bank material which was initially carried there or it could represent other variables such as sediment and deposited by that stream or its ancestors. Given transport or size composition of bed material. the right conditions, the stream will be able to move Fig. 1(a) represents a time scale in the order of that material once more. days. Generally speaking, within this time scale, the Over thousands of years, these processes drainage basin will appear to be in a state of static produced 'graded' stream systems. Most streams had equilibrium. Static equilibrium over time periods of adopted a grade, shape and plan-form which 1 day is generally observed in all but the most balanced the variable inputs of water and sediment. unstable of Victorian streams. Only during major The concept of stream stability does not imply erosion events (corresponding to the episodic events static stability in either the long or the short term. in Fig. 1(c)) will it be possible to observe changes Short-term instabilities (such as scouring a bank or occurring 'before your very eyes'. filling or creating a deep hole) reflect short-term Fig. 1(b) represents a time scale in the order of variations in water and sediment inputs. 100 — 1000 years. Within this time scale fluctuations Natural processes may be manifest as gradual of drainage basin variables become apparent. Never- changes to the stream system occurring, sometimes theless fluctuations are observed to be on either side imperceptibly, over a period of years. Sudden and of a more or less steady state, and the drainage basin dramatic channel changes may occur as the result of appears to be in steady state equilibrium. Steady a rare or catastrophic natural event such as flood, state equilibrium is apparent in many Victorian bushfire or landslip, or as the culmination of a streams and is typified by a meandering system. In gradual progressive change, eventually reaching a such a system, fluctuations of bed level will occur as threshold condition (channel avulsion, for example). the result of major and moderate flood events, or the Such natural instabilities are present in alluvial progression of meanders and associated riffles and streams regardless of human interference. Over time, pools through the system, with a period in the order these mechanisms have created the floodplain and of centuries. channel characteristics evident in the present day Fig. 1(c) represents time scales in the order of stream system. 1 000 000 years. Within this time scale progressive (a) Static Equilibrium (c) Dynamic Metastable Equilibrium 1 co e5P6\5° 1 00 - 1 000 years (6) Steady State Equilibrium (d) Progressive Change Figure 1. Perspectives on timescales changes or trends become apparent. Superimposed When sediment supply is less than sediment on the trends are episodic events where major transport capacity (Fig. 2(a)) all available sediment changes occur over relatively small time periods. is flushed out of the system and remaining bed This has been termed dynamic metastable material and bank material is generally erosion equilibrium. Episodic erosion is typified by major resistant. deepening episodes moving through the stream When net sediment supply is approximately systems. An excellent example would be Bruthen equivalent to net sediment transport capacity (Fig. Creek in the Yarram area where a major incision has 2(b)) in a stable and natural flood plain system, occurred and is progressing upstream. It is likely that sediment will be moved without net scour or there has been an increase in episodic erosion events deposition. Of course, storage and scour will occur in in the last two centuries as a result of human some places and temporary storage also occurs as the influence. (Note that one of the causes of result of variations in flow and sediment inputs, but fluctuations in the 100— 1000 year time scale is an there is no net accumulation or depletion. In ongoing complex response to such an episodic meandering reaches of many Victorian streams, event.) where sediment supply is locally out of balance with Fig. 1(d) is the traditional geomorphic view the sediment transporting capacity, then deposition originally proposed by Davis and represents time or erosion will occur (Fig. 2(c)). scales in the order of tens of millions of years. At this When sediment supply from upstream is greater scale the fluctuations and episodic events disappear than the ability of the stream to transport sediment into the general trend of progressive change within deposition occurs. The result is frequently a braided the erosion cycle. stream or the formation of alluvial fans or deltas. Continuing deposition makes these inherently Fluvial system concepts unstable morphologic forms. The sediment Fluvial systems can generally be divided into three deposition zone is represented in Victoria by the geomorphic zones. Fig. 2 is a simplified and idealised lower reaches of main stem tributaries and also by representation of the fluvial system. the lower reaches of those streams which drain The basic characteristics of each of the three directly or via lakes to the sea. Examples include the zones in Fig. 2 can be explained by comparing the lower Avon, Tambo and Snowy Rivers in Gippsland amount of sediment available to the stream with the and numerous alluvial fans flanking the north amount of sediment that the stream has the capacity western slopes of the Strathbogie Ranges or the to transport. Beechworth hills in north-east Victoria. 2 Sediment production zone Sediment transfer zone. i Sediment deposition zone. (al Sediment supply less than sediment transport capacity. (b) Sediment supply approx. equivalent to sediment transport capacity. (c) Sediment supply greater than sediment transport capacity. Stream Profile Figure 2. Idealised fluvial system The upper zone, termed the sediment production zone is typical in mountain regions. Within this zone the stream flows on bed rock or through very large bed material (pebbles and gravels) and the presence of smaller alluvial material is rare. In Victoria, where rates of denudation are very Low by world standards, the term sediment production zone is somewhat misleading. Large lengths of Victorian rivers belong to the sediment transfer zone. Within this zone, the stream typically flows through flood plains and terraces of alluvium. It would generally have a meandering character and point bars in the stream channel. In the sediment deposition zone net deposition occurs, and the stream is likely to be braided. Examples include alluvial fans and deltas. Complex response in stream systems alluvium. Rejuvenation of this stream is initiated, causing the stream to incise (Fig. 3(b)). Rejuvena- A complex series of reactions may be triggered by a tion could be caused by, for example, a downstream single event within the drainage basin.

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