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3.3.3 River Training Works

This Section offers an overview of different typologies of hydraulic structures that can be put inpractice along torrent beds for the managing and control of the morphological evolution of streams, inorder to identify appropriate criteria for prevention and mitigation of flash floods. The most importantand widespread standard-typologies of structures can be planned with indication of general technicalaspects are described, highlighting the limits or advantages of each hydraulic structure in relation to itsenvironment-friendly capability which must be analysed in the planning phase. However, thestructures presented do not represent an imposition for technicians working in this sector, but rather achance to analyse hydraulic problems in order to conciliate hydraulic requirements with the ecologicalaspects of river habitats.

Many examples of incorrect and miss management of the hydrographic network have been pointed outin the last decades. Rediscovery of old rearrangement techniques applied in the last centuries and inthe first part of XX century, which are today called bioengineering works, is a cultural heritage thatcannot be ignored by the modern policy management of hydrographic networks. The introduction ofnew bioengineering techniques or the rediscovery of the old ones, sometimes coupled with thetraditional type of interventions, are an extra choice among the applicable solutions for rearrangementof hydrographic networks.

Their employment causes an improvement of the ecological aspects of the river course, favouring, forexample, the reduction of polluting substances in the water because of the filtering action of vegetalspecies and micro-organisms living in the stream habitats. Furthermore the existence of naturalcorridors favour the diffusion and preservation of many different living species within the riparianecosystem.

The contributions brought into the project by each partner have highlighted that the application oftechniques with a low environmental impact, also using vegetal species or rocky material available onthe torrent beds, represents a resource that can easily be used in many circumstances and with lowcosts with respect to the more traditional works.

The management of stream courses, applying traditional interventions, has showed, in many cases, theinefficiency of the adopted solutions. These typologies of intervention have caused the alteration of thedynamic equilibrium of hydrodynamics aspects that cannot be correctly predicted. For example, theincrease of embankments has clearly produced the grow up of maximum elevation of flood peakscausing higher risk conditions for the urbanized areas close to the stream courses. Strongmodifications of the natural hydraulic conditions of long torrent stretches (straightening or lining ofstream courses) have increased the discharge and produced the destruction of natural habitats and themodification of the natural dynamics in many torrent beds.

River training works are fundamental for torrent management in relation to their adaptability to thedifferent typologies of torrent reaches, morphological conditions and hydraulic parameters. They canproduce immediate benefits by removing or reducing the hydraulic problems, as a function of theircorrect fitting in, building techniques and constructive materials used.

Identification of the correct typologies of structures to be put in practice requires hydraulicinvestigations and studies as to guarantee that the planned works, even though they may solvelocalized problems, will not result in negative effects in adjacent torrent reaches or the creation of non-mitigable environmental impacts.

In some cases, it is important to pay attention to the “zero” option, which does not foresee theconstruction of any kind of interventions. For example, the necessity of furnish sediments to avoiderosion of banks can be planned with the elimination or not construction of check dams in unstableslopes (if the urbanistic and hydraulic safety is guaranteed), favouring the creation of controlledlandslides or erosional phenomena.

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The different typologies of hydraulic–forestry structures, described in the following pages, representthe most common and updated interventions to control and optimize the regime of water discharge bylimiting its dynamic energy. They also have the function of reducing solid transport and the naturalprocesses of bed and bank erosion along the water courses, whose uncontrolled evolution can lead togeneral instability of banks and adjoining slopes.

For practical purposes, a standard format of description which includes objectives, structuretypologies, constructive aspects and location, maintenance, environmental impacts and finally, costs,of each typology of intervention has been chosen so that its advantages in relation to the hydraulicproblem being faced can be better evaluated. A series of figures, drawings and schemes have alsobeen inserted in the text, in order to give a synthetic and graphical overview of the different typologiesof hydraulic-forestry structures put in practice in Italy.

The costs expressed in Euros concern only the constructive operations of the structures comprehendingmaterials and labour work. They do not include expenses for the operations of preparation of thebuilding yards and are value–added tax (VAT) free.

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3.3.3.1 Transversal Hydraulic Works

3.3.3.1.1 Check Dam

Objectives

Use of this typology of hydraulic works permits to stabilize the elevation of the bed along rivercourses subjected to significant evolution of their morphological profile.

The construction of many check dams in succession along the water course (check dams in tiers)leads to the stabilization of the bed over long distances.

Control of the torrent bed elevation favours the prevention of landslides on natural slopes andartificial banks as well as the carrying out of rearrangement works on slopes involved by activeinstability phenomena.

The employment of these structures is fundamental in solving or mitigating erosional phenomenaand in the control of solid transport, especially in areas subjected to strong erosive action of waterflow (badlands).

These structures, that decrease the morphological gradient of the torrent bed, work towards thereduction of the water velocity during flood events by increasing the concentration time of thehydrographic basins and reducing the flood peak and solid transport capacity of the water flow.

This typology of hydraulic works is often coupled with the construction of bank protections,crossing structures or other typologies of hydraulic works, in order to guarantee their defencefrom undermining and breaking phenomena.

Structure typologies

The preparation of check dams can be made using different materials and constructivetechniques. The application of bioengineering techniques, that agree with the environmental andecological characteristics of the torrent bed, has been the most recent development. The choice ofthe most appropriate check dam structure typology is made as a function of the hydraulicparameters of the water course and also of their environmental and ecological factors. Theplanning phase of these structure must be preceded by an in-depth evaluation of their effects onthe dynamics of the entire water course.

The constructive typologies that are usually carried out in torrent beds are:

- Cemented stones, concrete (Figures 3.3.3.1.1 a and b);

These structures can be located along the entire length of the hydrographic network because oftheir adaptability to different morphological and hydrodynamic conditions of the torrent bed.Their strong rigidity permits to plan check dams with considerable dimensions. A significantenvironmental impact is caused both during the preparation of the building yard and by the finalstructure that cannot be renaturized.

Figure 3.3.3.1.1 a Figure 3.3.3.1.1 b

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- Gabions (Figures 3.3.3.1.1 c and d);

Construction of check dams made with gabions requires a sufficient volume of rocky materialsthat in some cases can be found along the torrent bed. The maximum dimension of thesestructures are bigger than check dams made with stones and wood. These works can reach amaximum height of 9 -10 meters. The adaptability of these structures allows their use along theentire hydrographic network. A significant environmental impact is caused during the preparationof building yard while the final structure permits a rapid renaturization by natural plants orcuttings.

- Wood, rocks and wood (figures 3.3.3.1.1 e and f);

Preparation of these structures needs the construction of a well-organized structure made of rocksand wood (poles with diameter of 10-25 cm). In some cases, the constructive materials can befound and processed near the area where these structures are placed. Their maximum dimensionis conditioned by their rigidity and by the hydraulic characteristics of water course. Their heightdoes not exceed 2 meters while their width depends on the morphological condition of the torrentbed. For this reason they are usually built in the upper portion of the hydrographic network oralong small tributaries with low or intermittent water. Construction of this typology of checkdams reduces the environmental impact in relation to the renaturization capability of theirstructure. The wood species that are more suitable are: larch, chestnut, natural or treated resinousplants. In general, any kind of woods with strong resistance to water that can be found directly inthe area of construction can be used.

Figure 3.3.3.1.1 e Figure 3.3.3.1.1 f

Figure 3.3.3.1.1 c Figure 3.3.3.1.1 d

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- Dry stones (figures 3.3.3.1.1 g, 3.3.3.1.1 h);

Carrying out of these works requires the construction of a well-organized structure made of bigstones in order to resist to the dragging action of water flow. In some cases, the stones can befound near the area where these structures are built.

Rocky check dams are usually built in the upper portion of the hydrographic network or alongsmall tributaries where the intermittent water flow with low discharge favours their durability.The maximum dimension of these works is conditioned by their rigidity and by the hydrauliccharacteristics of the water course. Usually, their height does not exceed 2 meters. Application ofbioengineering criteria can foresee the use steel rope in order to anchor the structure to thesubstratum. Construction of rocky check dams reduces the environmental impact in relation to therenaturization capability of their structure.

Constructive aspects and location

Construction of these works requires careful planning of the foundation site in order to assure thestability and durability of the intervention. The collapse of check dams, caused by theundermining, turning and piping action of the water flow, can produce or increase the erosionalphenomena, solid transport and instability of slopes and river banks.

For this reason, the planning phase of these interventions must foresee the carrying out of bankprotection structures, foundation planks, etc.. Protection of check dams in their downstream partusually requires the fitting in of structures (auxiliary dams, torrent bed covering with stones orconcrete, etc.) in order to dissipate the hydraulic power of the water fall.

The strong energetic environment where these hydraulic works are usually constructed requiresthat the technical execution must be carefully planned in advance to resist to the hydraulicstrength of the water flow. For this reason, some typologies of check dams made with dry walls,wood and stones, etc., can only reach a maximum dimension that is a function of the technicalcharacteristics of the constructive materials, morphological aspects of the bed and hydraulicaspects of the water flow.

The construction of wood, dry stone or gabion check dams permits to assure, in situations ofinstability conditions of their foundation site, a greater durability due to the flexibility of thestructure and the possibility of settling of foundations. The renaturization of these structuresassures a stronger resistance against the hydraulic stress of flowing water.

Check dams made of concrete and cemented stones can be constructed in larger scales andlocated along the entire torrent or river length because of their superior adaptability to thehydraulic and morphological characteristics of the water course.

Figure 3.3.3.1.1 hFigure 3.3.3.1.1 g

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Figure 3.3.3.1.1 i

Maintenance

The durability of these hydraulic works depends on their constructive techniques and it is closelylinked to the maintenance and renovation operations as to guarantee stability conditions of thestructures and thus preservation of the hydraulic stabilization of the water course.

In the last years, the maintenance and renovation of check dams have been sometimes carried outwith the preservation of the ecological characteristics of the water course through the substitutionor modification of traditional structures with other typologies with an increased ecological value(e.g. ramps or works made with stones and wood).

Environmental impacts

These hydraulic structures, transversal to the water course, cause a sensible environmentalimpact both during the constructive and the operation period.

Check dams made of wood, stones and wood and dry walls permit to minimize the impact ofthese structures in relation to their better insertion in a natural environment subjected torevegetation.

They may represent a physical barrier for the diffusion of the ichthyic species along the watercourses. For this reason, along the torrent or river stretches with a perennial or semi-periennalwater flows, or where the morphological hydraulic conditions are suitable, these structures shouldbe constructed with a low height or adapted, in order to guarantee the going back up of the greatpart of the ichthyic species living in the stream.

Carrying out of ramps, lateral corridors, etc. is thus a necessary integration since the planningphase of new transversal works or restoration of the existing ones.

Construction of ramps with blocks placed or anchored to the ground permit the creation of zoneswith different energy that favour the overcoming of significant slopes by fish especially during thereproducing period (Figure 3.3.3.1.1 i).

In many cases, these transversal structures cause the destruction of the natural riparian habitatsin the upstream portion of the water course because of the sedimentation of the transportedmaterials that destroy the morphological variation of the natural torrent bed.

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Costs• Check dams with wood and rocks: 150-190 Euros per m3;• Check dams with dry wall: 50-70 Euros per m3;• Check dams with gabions: 110-125 Euros per m3;• Concrete check dam: 180-200 Euros per m3.

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3.3.3.1.2 Sill

Objectives

This typology of hydraulic works, with transversal orientation to water flow, assures the preservationof the river bed elevation. These structures are carried out along river stretches with a medium-lowgradient subjected to bank and bed erosional phenomena that may cause instability conditions of thenatural and artificial embankments and of the other existing hydraulic works. Their construction isoften coupled with other hydraulic structures to avoid their undermining and erosion of the foundation(bridges, walls, etc.).

The purpose of this typology of hydraulic works is the same of check dams but, in relation to themedium-low gradient of the stream where they are usually carried out, sills have a more limited height.

Structure typologies

Sills, in relation to the low energy condition of the river course where they placed, can be built withmany constructive typologies and different materials. The choice of constructive typology is alsoconditioned by the morphological and ecological aspect of streams.

The constructive typologies usually applied are:- Concrete, cemented stones (Figures 3.3.3.1.2 a and b);

This typology of hydraulic works is widespread along torrent courses, in particular, in the lowerportion of hydrographic network, characterized by a limited topographic gradient, in relation to theirconstructive facility. Rigidity of concrete or cemented stone sills favours the carrying out of widthstructures in all morphological conditions. Construction of these works does not cause a highenvironmental impact during the construction of the building yard, nor does the final structures due totheir limited height.

Figure 3.3.3.1.2 a Figure 3.3.3.1.2 b

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- Gabions (Figures 3.3.3.1.2 c and d);

Sills made with gabions can take on in many different hydrodynamic conditions. Their limited heightallow for a considerable width. Sometimes, building of gabions can be carried out with the rockymaterials available along the torrent beds. Use of gabions, in relation to their capability to berenaturized, favours the ecological fitting of their structure in natural riverine habitats.

- Blocks, blocks anchored to ground, wood and rocks (Figures 3.3.3.1.2 e through h );

In relation to their constructive facility, this typology of sills are built mostly in the uppermountainous portion of the hydrographic network or in sites with morphological difficulties. Rockysills cause a limited environmental impact because of their easiness of renaturization that favours theecological fitting. The wood species most suitable are: larch, chestnut, natural or treated resinousplants. In general, any kind of woods with strong resistance to water that can be found in the area ofconstruction can be used.

Sills made of concrete are up to now the ones most commonly used because they present a minimumconstructive difficulty. The environmental sensitiveness of the last years has driven to the planning and

Figure 3.3.3.1.2 g Figure 3.3.3.1.2 h

Figure 3.3.3.1.2 c Figure 3.3.3.1.2 d

Figure 3.3.3.1.2 e Figure 3.3.3.1.2 f

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construction of other typologies of structures made with natural materials in order to reduceenvironmental impact inside the torrent bed.

Sills built with rocks can be put in practice both using blocks of rock with a dimension and weightsufficient to resist to the tensile strength of the water flow during the maximum flood events andthrough the construction of anchor points with steel cables that fix blocks to the rocky or alluvialsubstratum. Furthermore, the use of gabions or rocks and wood favours the hydraulic fitting alongtorrent beds with strong morphological modifications in relation to the flexibility of their structures.Constructive aspects and location

The stability of the foundation site (torrent bed and banks), in relation to the resistence of thestructures to the turning, undermining and piping action caused by the water flow, must be verified.

Usually, these works are built together with bank protection structures (made in the upstream torrentstretch) to guarantee durability of their anchorage, favour directing of water flow and prevent theundermining of the embankment. Furthermore, the hydraulic energy of the torrent course where theseworks are built assumes that their structures are able to resist to collision, wear and tear action causedby the solid transport flow.

These hydraulic works are usually carried out along river stretches with a low morphological gradientwhere the preservation of torrent bed elevations does not require the construction of check dams.Maintenance

The durability of these hydraulic works is directly linked to the maintenance that must be periodicallyput in practice in order to guarantee the global stability conditions of their structures. Therefore, theirperiodical control is a necessary operation in order to identify damages of structures, plan thenecessary renovation action and guarantee their effectiveness.

Breaking and unanchorage of these works caused by water flow produce or increase bed and/or bankerosional phenomena that can signify damages to long torrent stretches.

In the last years, maintenance and renovation actions have been directed towards the respect of theecological aspects of streams also with the substitution or modification of traditional structures withnew typologies made with natural materials (rocks, wood) that are integrable with the natural torrentenvironment.Environmental impacts

Carrying out of these hydraulic structures, even though they have a transversal direction with respectto the water flow, does not cause a significant environmental impact in relation to the low height of thestructure above the bed elevation and, in cases where the stream has a permanent water flow, to thecomplete or partial submersion by the water level.

The use of concrete for construction of these works does not eliminate the environmental andlandscape impacts of the structure located along torrent beds. The construction of different typologiesof hydraulic sills using natural materials (wood, rocks, gabions) permit to guarantee a goodenvironmental insertion.

Furthermore, the structures made of rocks and wood allow to create morphological variations of thetorrent bed with the formation of new and different ecological habitats for the faunal species.Costs

• concrete: 170-185 Euros per m3;• blocks of rock anchored to ground: 120-130 Euros per m3;• blocks of rock: 55-65 Euros per m3;• wood and rocks: 110- 120 Euros per m3;• gabions Euros 120-135 per m3.