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3.3.3.2.2 Bank Erosion Protection

Bank protection structures represent a widespread typology of hydraulic works along the river course. Theirconstruction is usually planned along torrent stretches with bank erosive problems that can produceinstability conditions both on natural, artificial banks and on steep slopes. Strong protective structures areusually put in practice for straightening works or planimetrical modification of the river course, or incombination with other hydraulic structures (bridges, dams, etc). In many cases, the construction of strongbank protection structures is required for the defence of urbanized areas or to stabilize landslides.

Construction of this kind of interventions causes a substantial modification of the natural aspect of riverbanks with the partial or complete destruction of the ecological conditions and the reduction of the biologicaldiversification of the riparian habitats.

The traditional bank protection works, concrete walls, cemented stone and brick, play a significant role in themodification of the hydraulic aspect of the discharge values and in the interference in the water dynamics oferosive and depositional phenomena both upstream and downstream.

The construction of protective structures is usually coupled with carrying out transversal hydraulic works, inorder to guarantee their protection against the undermining action of the flowing water.

In the last years, the application of bioengineering techniques has favoured the planning of protectivestructures with a lowered environmental impact. In particular, these have been applied along small-mediumsize basins where not extreme erosive action of the flowing water can be controlled with this kind of softinterventions. Bioengineering criteria are usually applied together with the planning of the transversalhydraulic section with a diversified morphological profile that favours the creation of different riparianhabitats for faunal and floral living species. In particular, the use of wood and rocks favours the revegetationof these kind of interventions.

On the basis of the previous considerations, low environmental impact hydraulic works carried out in someStyrian rivers will be next described also through the use of schemes and figures that exemplify theirapplication in a small-medium size basins.

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CASE 1 - Safen river, in the area of the Spa town Blumau (bank restructuring works)

Figure 3.3.3.2.2 – Safen river: banks structured according to existing natural conditions

• Data relating to the river:

Sea level = approx. 260 mBed slope = approx. 2.0 ‰Bed width = 4.0 – 8.0 mDepth of profile = 2.0 – 3.5 mBed shear stress = max. 70 N/m2

Type = lowland riverCatchment basin = 38.2 km2

HQ100 = 130 m3/s

• Basic characteristics of bank protection measures:

Structured bank footUse of live material (resprouting vegetation)Irregular bank lineRoom for successionEmployment of locally suitable vegetation“Protection strips against water”

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• Basic structural elements:

1 – Base stoneFunction: hinders undermining and scouring, provides a structural element (fish shelters).Employment: securing of bank foot (permanently / temporarily), especially in case ofcourse deviation.Maintenance: occasional adjustments until protective vegetation cover is formed.

2 – Anterior stakeFunction: support element.Employment: securing of bank foot.Maintenance: none.

3 – Sunken fascine rollFunction: to provide mechanical protection of slope toe (150-200 N/m2).Employment: toe stabilization in a number of bioengineering measures such as: fascine walls, fascinerows, etc. Sunken fascine rolls are especially used in cases where the bed shear stress is not too greatand blockstone bank protection would not be suitable.Implementation: placed directly at the relevant sites in the toe. They are tied to wooden stakes withzinc-plated wire. The fascine rolls are bundles mainly consisting of dead and very resistant vegetablematerial and containing stones in their middle to weigh them down. Sunken fascine rolls are placedso that their lower end is perfectly attached to the river floor. Actual bank protection is an immediateresult.Maintenance: none .Ecological evaluation: in water courses where blockstones do not fit into the nature-likehydrological context sunken fascine rolls are a good toe stabilization device, provided bed shearstress allows their use. The dead branches increase the total amount of dead wood in the watercourse. The presence of different materials (wood and stone) provides a variety of surfaces toorganisms living in the water course (zoobenthos, plants).

4 – Wattle workFunction: similarly to a fascine wall, a wattle fence enables to protect banks running vertically in arapid and effective way. Immediately after completion of the works, the wattle fence and the woodpiles become effective and very soon a thick fence of willow brushes is formed.Employment: on water courses with low bed shear stress (50-100 N/m2) and relatively straightcourse.Implementation: after driving wood pickets (10 - 15 cm) into the ground, strong and pliable willowbranches (no fragile willows) are woven horizontally alternately before and behind the pickets so asto form a wattlework. The thicker branch end is always placed on the inside (i.e. facing the slope) sothat enough humidity is always available and no protruding branch can hamper water flow. Branchesshould be placed very closely to one another, so as to prevent soil erosion by the waters. If the watercourse has periods of very poor flow, willow branches can also be woven transversally, so that theirlower end can reach directly into the water. The height of a wattle fence should not exceed 50 cm.Maintenance: in case of soil erosion, the material that has been washed away must be replaced inorder to ensure adequate growth and root development and, as a consequence, proper bankstabilization.Ecological evaluation: optimal sprouting begins from above the mean water level; it is thereforepossible for a variety of structures to develop freely below this level. The great number of habitatsand the connections between water and land are also very interesting from an ecological viewpoint.

5 – Log cribwall with branchlayers on one bankFunction: the wooden crib provides a very strong supporting element and is able to resist very greatbed shear stress.Employment: this type of bank stabilization measure can be employed also for torrents with irregularflow of water and bed load transport and for banks which are heavily exposed to the action ofsweeping waters.Implementation: to build a log cribwall 18 - 25 strong wood logs are placed and nailed together toform a crib. These horizontal anchor logs, however, are not placed directly and uniformly over oneanother, but alternately. The spaces are filled with willow fascines or stones on the waterside

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whereas behind it, on the inner side, they are filled with earth, as is done with pile walls. Willowfascines have to be slightly covered with earth, too, so that they can sprout above water and take overthe stabilization function after the decaying of the wooden crib. Below the water level wooden cribscan also be filled up with smaller stones and fascines made of dead vegetable material. Never willthe spaces between the anchor logs be filled with branchlayers, since these would be washed awayby the flowing waters.Maintenance: cutting back of vegetation is necessary according to type and growth.Ecological evaluation: the spaces between the anchor logs produce great structural diversity.The useof fascines of dead vegetable material in the area below the mean water level increases the amount ofdead wood and provides extensive surfaces for zoobenthos.

6 – Live brush mattressesFunction: the most effective method of bank stablization in the event of floods has proved to be theuse of brush mattresses with willows. These develop very rapidly into a thick and functional bankvegetation. Mattresses protect river banks like a coat and provide immediate and effectivemechanical stabilization. Owing to their rapid growth, willows are able to promptly reduce flowvelocity and, as a consequence, bed shear stress. After only 1-2 years this type of stabilizationmeasure is able to absorb a bed shear stress value of 300 N/m2 which increases up to 460 N/m2 in thecase of a complete structure including a blockstone embankment.Employment: owing to its high resistance to bed shear stress and in association with proper toestabilization, this type of measure is ideally employed in the case of water courses with higher bedshear stress values as well as for banks heavily subjected to the action of sweeping waters or erosion.The slope, however, should not exceed 1:1 (ideally 1:3).Implementation: willow branches are placed very closely to one another over the whole slope, at anangle with respect to the direction of flow. The cuttings are placed into the water (ideally directlyinto the river bed), so as to guarantee an adequate degree of humidity to the willow branches andprevent withering due to the action of possible underminings in the slope area. The slope shouldpossibly have a regular shape so that willow branches placed on the ground can have optimal contactwith the soil. The willow branches are tied with wire or coconut rope to wood pickets and thencovered with a 3-4 cm thick earth layer. When live brush mattresses are employed toe stabilization iscarried out up to the low water level by means of blockstones, a simple wooden crib or a simplecylindrical gabion. To enhance vegetation variety, rooted deciduous trees (such as alders, ash-trees,etc.) can be added to the willow cover. Live brush mattresses should ideally be planted in lateautumn to achieve faster root development and make sure that, by the following spring, when thesnow starts to melt, willows will be able to exert effective flood protection.Maintenance: maintenance cutting will be necessary according to growth in order to preservecoppice elasticity, which is necessary in case of floods. The thick concentration of willows in thebrush mattress increases competition pressure and, as a consequence, leads to the growth of mainlythin and higher shoots which, in its turn, ensures prolonged conservation of elasticity. When otherdeciduous trees are added, the area around them must be cleared from willows so as to eliminatecompetition with them.Ecological evaluation: The extensive vegetation cover is able to reduce velocity of flow especially inthe case of floods, thereby providing shelter for living organisms. In the summer these slopes offershady, inaccessible and safe habitats.

7 – Cuttings/stakesFunction: firming effect on soil due to the growth of roots; it can be employed between stones aswell; good protection in the long-term.Employment: normally in combination with other measures.Maintenance: containment of growth.

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Sweeping water side – Type “A” measure

Water zone:

1 – Base stone

2 – Anterior stakes

3 – Sunken fascine roll

Wind and Water zone:

4 – Wattle work

7 – Cuttings/stakes

Figure 3.3.3.2.2 a - Sweeping water side (type “A” measure)

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Sweeping Water side - Type “B” measure

Water zone

1 – Base stone

2 – Anterior stake

3 – Sunken fascine roll

Wind and water zone

5 – Log cribwall with branchlayers on one bank

6 – Live brush mattresses

7 – Cuttings / stakes

Figure 3.3.3.2.2 b - Sweeping water side (type “B” measure)

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Gliding Water side - Type “C” measure

Water zone:

6 – Live brush mattresses

Figure 3.3.3.2.2 c - Gliding water side (type “C” measure)

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Gliding water side – Type “D” measure

Water zone

4 – Wattle works

Wind and Water zone

4 – Wattle works

Figure 3.3.3.2.2 d - Gliding water side – Type “D” measure

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CASE 2 – Lafnitz river, in the reach between Neudau and Burgau (securing of given conditions –bank with protection side-dam)

• Data relating to the river:

Sea level = approx. 270 mBed slope = approx. 1.8 ‰Bed width = 3.0 – 14.0 mDepth of profile = 2.2 – 4.3 mType = lowland river (“river with dam banks”)Catchment basin = 50.8 km2

HQ1 = 45 m3/sHQ100 = 155 m3/s

• Basic characteristics of bank protection measures:

Securing and protection of given conditionsStructured bank footUse of live materialIrregular bank lineEmployment of locally suitable vegetation

• Additional basic structural elements in respect to Case 1:

8 – Brushlayers (“living brushes and combs”)Function: they promote siltation and are able to protect small bank segments affected by lateralerosion. By growing, the root mattress which is thus formed prevents soil erosion.Employment: this type of measure is not resistant to high bed shear stress and is therefore oftenemployed in the spaces between more laborious and resistant live siltation constructions.Implementation: Living brushes: a large number of loose willow branches is planted very thicklyinto the soil, at an angle to the direction of flow. Fascine bundles can also be used in place of theloose willow branches. Living combs: single cuttings of approx. 1 m in length are planted in rows atan angle to the direction of flow and at a depth of 70 cm into the ground. The distance between thecuttings must be about twice or three times the section of the cuttings themselves. The result is a sortof comb, offering lower resistance than living brushes. These types of construction are very easy andquick to carry out.Maintenance: in case of excessively rapid siltation these types of construction will have to berenewed since willows take root only sporadically if completely covered.Ecological evaluation: in the free spaces between the living brushes or combs there is enough roomfor other seeds, carried over by the wind from surrounding areas, to sprout. This means that in time abank vegetation cover will grow which is suitable for local conditions.

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Sweeping water side with dam along the banks – Type “E” measure

Water zone:

1 – Base stone5 – Log cribwall with branchlayers

Wind and water zone:

7 – Cuttings / stakes

Protection dam:

8 – Brushlayers (“living brushes”)

Figure 3.3.3.2.2 e - Sweeping water side with dam along the banks (type “E” measure)

Figure 3.3.3.2.2 e1 – Local protection of the dam (type “E” measure)

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CASE 3 – Pöllau Saifen in Pöllau (Creation of transverse structures)

• Additional basic structural elements in respect to Case 1 and 2:

9 – Crib groynes with cuttings (Living groynes)Function: groynes are structures built near the banks and transversally with respect to the direction of flow.As a difference to guide banks, they limit the stream only at specific points and are generally used at largerrivers (10 m broad) to protect the banks and repair eroded segments. According to their height with respect tothe flow level, base flow level, mean flow level and high flow level groynes can be distinguished.Considering the angle with respect to the direction of flow, perpendicular, upstream looking (inclined orattracting) and downstream facing (declined or fending) groynes can be distinguished. Inclined groynes, builtat an angle of 75-80° against the direction of flow, are particularly effective bank protection measures in thatthey divert water towards the middle of the river. Along the segments between the groynes, known as groynebays, suspended sediments, bed load material and debris are deposited. The distance between one groyne andthe next one should never be longer than 1.5 - 2.5 times the length of the groynes themselves, otherwise thecurrent could reach the banks and damage the slopes.Employment: in case of extremely varying water level the construction of a mean flow level groyne could bein order, which would prove very valuable from an environmental point of view.Furthermore, groynes can be employed for the protection and repair of bank erosions. The type of groyne tobe chosen depends on local water level and bed shear stress.Implementation: according to the extent of the stress to be borne and to the raw materials that are locallyavailable, different types of groynes can be built out of different materials:• log crib groynes with cuttings• gabion groynes with cuttings• branchpacking groynes• tree spur groynes• fascine groynes• wood pile groynes with tree stumps• log cribwall groynes• gravel groynes with brush mattresses, etc.Maintenance: coppice maintenance must be such as to keep the plants’ elasticity in view of possible floodevents.Ecological evaluation: Groyne bays are of great ecological significance because they provide still waterareas used by fish as spawning and growth places and, as a consequence of natural succession, they becomethe habitat of a variety of plant and animal species. If groynes are built alternately on both sides at a distancefrom one another of 5-7 times the measure of the bed width, meanders are formed in low water whichcontribute to enhance the presence of living organisms in the water course. The construction of a base flowlevel groyne improves living conditions in the water and upgrades water quality.

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Transverse structure – Type “F” measure

Water zone:

9 – Groynes with cuttings

Figure 3.3.3.2.2 f – Top view of stone groyne with cutting (type “F” measure)

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Figure 3.3.3.2.2 f 1- Cross section of stone groyne with cutting (type “F” measure)

Figure 3.3.3.2.2 f 2 – Front view of stone groyne with cutting (type “F” measure)