Embed Size (px)
Citation preview
39
Section A4Erosion treatment techniques
40
CONTENTS PAGE
T1 Log and rock bed control 45
T2 Log/timber V-weirs 48
T3 Rock chutes 52
T4 Boulder plunge pools 54
T5 Drop structures 57
T6 Rock beaching 59
T7 Hard linings 66
T8 Rock mattress lining 70
T9 Rock gabion walls 73
T10 Brushing 75
T11 Battering and revegetation 78
T12 Retards 80
T13 Groynes 83
T14 Snag management 85
41
Erosion treatment technique # T1 LOG AND ROCK BED CONTROL
Description
A log retaining wall and rock cascade built across the bed of the creek to control an upstream-progressing erosion head.
The log retaining wall is built across the full width of the creek bed and immediately downstream of the erosion head. Fill is placed upstream of the wall to the level of the upstream bed. The retaining wall holds the fill in place to control the progress of the erosion head and the rock cascade on the downstream side slows the flow and prevents downstream bed and bank erosion.
Variations/Alternatives
Steel sheet piling Log/timber V-weir Rock chute Boulder plunge pools
Applications
Used to control the upstream progress of an erosion head in the bed of small to medium sized creeks.
Limitations
Availability of suitably sized rock. Bed material needs to be suitable for driving poles.
Advantages
Does not require skilled labour.
Disadvantages Logs will eventually rot.
Cost
Moderate
Duration
Medium term – depends on suitable rock and type of logs.
For a detailed discussion on this technique, refer to Section B2
42
Equipment and materials
excavator (trench excavation and general earthworks plus lifting and placing logs) pile driver (or hammer attachment for excavator) posts and logs geotextile filter fabric rock
Construction method 1. Where necessary cut bank ramp to access bed and stockpile material for use in the restoration of
the creek bank. Minimise disturbance to the creek bank and never cut bank ramp into the outside bank of a bend.
2. Grade upstream and downstream bed to a level surface (at different elevations) with invert in centre.
3. Excavate trenches in bed and banks (see attached figures) and stockpile material for fill. 4. Lay geotextile fabric on downstream bed and banks, in trench and upstream of trench, leaving
enough fabric to cover the back of the logs. 5. Cover fabric on bed with 50 mm of sand. 6. Drive posts into downstream trench at 1m spacing. 7. Cut logs to length and drop in behind posts but in front of fabric. 8. Wire each log to the posts. 9. Fill in any space behind the fabric with sand or gravel and tamp down as each log is placed. Do
not fill behind top two logs. 10. Lay fabric on bed and in trench upstream of log retaining wall. 11. Lay rocks of sufficient size and grading [1] on the sand upstream and downstream of the log
retaining wall. The rocks forming the cascade should be at a slope of 1 vertical to 8 horizontal and should finish flush with the top of the log retaining wall. Lay rocks to encourage the stream to flow towards the centre of the creek bed. Ensure voids between large rocks are filled with rock of progressively smaller size and that the final mass is well interlocked with minimal protruding rock faces.
12. Spread topsoil on disturbed banks and re-establish vegetation consistent with existing vegetation upstream and downstream.
13. Control any other erosion heads downstream of the site using this technique or other appropriate techniques.
[1] Consult hydraulics specialist for selection of rock size and grading.
Maintenance
Inspect works at regular intervals during first year of operation to check for removal of rocks from rock cascade (by either flood flows or human interference). Replace rocks as necessary.
43
Figure T1.1 Typical layout of a log and rock bed control system [Source: Riverwise Brochure – Log and Rock Bed Control and Road Crossing On Small Intermittent Flow
Stream, Land and Water Conservation, NSW Government]
44
Erosion treatment technique # T2 LOG/TIMBER V-WEIR
Description
A log retaining wall built across the bed of the creek to control an upstream progressing erosion head.
The log retaining wall is built across the full width of the creek bed and within a trench excavated across the creek bed adjacent to the erosion head. Fill is placed in the trench behind the wall to the level of the upstream bed. The retaining wall holds the fill in place to control the progression of the erosion head. The scour hole downstream of the erosion head is left to function as a stilling pool for stream flows cascading over the logs.
Variations/Alternatives
Steel sheet pilingLog and rock bed control Rock chuteBoulder plunge pools
Applications
Used to control the upstream progress of an erosion head in the bed of small to medium sized creeks.
Limitations
Bed material needs to be suitable for driving poles.
Advantages
Provides habitat diversity by leaving a scour hole.
Disadvantages Logs will eventually rot away.
Cost
Moderate
Duration
Medium term – depends on type of logs.
For a detailed discussion of this technique, see Section B2..
45
Equipment and Materials
excavator (trench excavation and general earthworks plus lifting and placing logs) pile driver (or hammer attachment for excavator) posts and long logs, preferably treated hardwood for maximum durability high strength or reinforced geotextile filter fabric
Construction Method
Where necessary cut bank ramp to access bed and stockpile material for use in the restoration of the creek bank. Minimise disturbance to the creek bank and never cut bank ramp into the outside bank of a bend. Excavate trench in bed and banks and stockpile material for fill. Drive posts into trench. Cut logs to length and drop between posts. Wire each log to the posts. Fill and tamp down behind logs. Place fabric at 45-degree angle down from the top of logs, fill and tamp down on upstream side of fabric. Refill bank access ramp, spread topsoil on disturbed banks and establish appropriate vegetation. Control any other erosion heads downstream of the site using this technique or other appropriate techniques.
Maintenance
Inspect works regularly, particularly after flooding. Replace any rotten logs or logs damaged by flows.
46
Figure T2.1 Typical layout of a log V-weir
[Source: Riverwise Brochure – Log/Timber V-Weir, Land and Water Conservation, NSW Government]
47
Erosion treatment technique # T3 ROCK CHUTES
Description
A relatively short and steep section of creek bed armoured with rock to form a chute that transfers stream flow from a higher to a lower elevation without erosion.
Variations/Alternatives
Grouted boulder drop structures (usually preferred) Log and rock bed control Log/timber V-weir Boulder plunge pools
Applications
Control the upstream progress of an erosion head in the bed of small to medium sized creeks.
Limitations
Availability of rock suitably sized rock.
Not suitable at, or near, bends or major expansions and contractions.
Advantages
Does not require skilled labour. Flexible solution that can be maintained easily. Environmentally sympathetic.
Disadvantages
A high failure rate - usually because of poor rock selection and construction techniques.
Cost
Moderate to high – can be high if creek has wide bed.
Duration
Long-term – depending on suitability of rock, construction technique and supervision. Grouted boulder drop structures are recommended for long-term stability.
For a detailed discussion of this technique, see Section B2.
48
Figure T3.1 One possible layout of a rock chute [Source: McLaughlin Water Engineers, Ltd, 1986]
49
Erosion treatment technique # T4BOULDER PLUNGE POOLS
Description
Large diameter rocks (boulders) are placed in the creek bed and up the banks to control an upstream progressing erosion head. The boulders are placed so they line the scour hole immediately downstream of the erosion head, providing a hard bed to control further progress of the erosion head.
The boulders are placed so they pull the flow to the middle of the creek bed and away from the banks.
Variations/Alternatives
Grouted boulder drop structures Log and rock bed control Log/timber V-weir Rock chutes
Applications
Control the upstream progress of an erosion head in small to medium sized creeks.
Limitations
Availability of suitable rock sizes.
Advantages
Does not require skilled labour. Easily and quickly constructed. Environmentally sympathetic – restores riffle and pool system.
Disadvantages
Boulders might not be readily available.
Cost
Moderate
Duration
Long-term – will depend on suitability of rock and regular maintenance.
For a detailed discussion on this technique, see Section B2.
50
Equipment and Materials
excavator (general earthworks and boulder placing) rock (boulders) geotextile filter fabric
Construction Method
1. Where necessary cut bank ramp to access bed and stockpile material for use in the restoration of the creek bank. Minimise disturbance to the creek bank and never cut bank ramp into the outside bank of a bend.
2. Excavate bed and banks as shown on the construction sketches on the following page. 3. Lay fabric over all areas to be covered with boulders. 4. Spread a layer of gravel or suitably sized rock over the fabric. 5. Place suitably sized boulders [1] to the bed and banks as shown on the construction sketches on
the following page. 6. Fill bank access ramp, spread topsoil on disturbed banks and re-establish vegetation consistent
with existing vegetation upstream and downstream. 7. Control erosion heads downstream of the site using this technique or other appropriate
techniques.
[1] Consult hydraulic specialist for selection of appropriate boulder size.
Maintenance
Inspect works at regular intervals. Replace or reposition boulders as necessary.
51
Figure T4.1 Boulder plunge pool layout [Source: Riverwise Brochure – Boulders, Land and Water Conservation, NSW Government]
52
Erosion treatment technique # T5 DROP STRUCTURES
Description
Stabilised, vertical or near-vertical structures usually consisting of concrete, rock, sheet piling or timber. Typical drop structures include log and rock cascades, log or timber V-weirs, rock chutes, grouted boulder chutes and vertical concrete steps. Sloping, non-stepped structures are generally preferred, even in minor streams, to avoid disturbing aquatic corridors (fish migration paths).
Drop structures are built across the full width of the channel immediately downstream of a migrating erosion head.
Variations/Alternatives
Reinforced concrete Grouted boulders Loose rock chutes Sheet piles Treated timber and logs Rock-filled gabions and mattresses
Applications
Used to control upstream migration of an erosion head in the bed of small to medium sized creeks.
Limitations
Availability of suitable materials (rock sizes) and construction access.
Limited life span in highly mobile and/or erosive (sandy) creek systems.
Advantages
Good short-term stability. Some designs exhibit good long-term stability.
Disadvantages
Many designs have poor aesthetics. Some designs are major interruptions to aquatic corridors. Loose rock structures can exhibit instability problems. Some construction materials can introduce weed control problems. Public safety problems.
53
Cost
Moderate to high depending on the design, construction materials and access.
Duration
Medium to long-term depending on the design, construction materials and the stability of the watercourse.
For a detailed discussion on this technique, see Section B2.
