Environmental Management DOI 10.1007/s00267-009-9275-y
History of Bioengineering Techniques for Erosion Control in Rivers in Western Europe Andre Evette Æ Sophie Labonne Æ Freddy Rey Æ Frederic Liebault Æ Oliver Jancke Æ Jacky Girel
Received: 28 April 2008 / Accepted: 11 January 2009 Ó Springer Science+Business Media, LLC 2009
Abstract Living plants have been used for a very long time throughout the world in structures against soil erosion, as traces have been found dating back to the first century BC. Widely practiced in Western Europe during the eighteenth and nineteenth centuries, bioengineering was somewhat abandoned in the middle of the twentieth century, before seeing a resurgence in recent times. Based on an extensive bibliography, this article examines the different forms of bioengineering techniques used in the past to manage rivers and riverbanks, mainly in Europe. We compare techniques using living material according to their strength of protection against erosion. Many techniques are described, both singly and in combination, ranging from tree planting or sowing seeds on riverbanks to dams made of fascine or wattle fences. The recent appearance of new materials has led to the development of new techniques, associated with an evolution in the perception of riverbanks.
This study was part of a research project on river bioengineering funded by the French Ministe`re de l’Ecologie, de l’Energie, du De´veloppement Durable et de l’Ame´nagement du Territoire, Direction Ge´ne´rale de la Pre´vention des Risques. A. Evette (&) S. Labonne F. Rey O. Jancke UR EMGR, Cemagref, 38402 St-Martin-d’He`res Cedex, France e-mail:
[email protected] F. Liebault UR ETGR, Cemagref, 38402 St-Martin-d’He`res Cedex, France J. Girel Laboratoire d’E´cologie Alpine, UMR 5553, Universite´ Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
Keywords Biotechnical engineering Soil bioengineering Streambank stabilization Ecological engineering
Introduction Live vegetation has been used for a very long time, to reduce soil erosion, for stream bank and bed stabilization, or to protect seawalls or sand dunes from the force of water. The term ‘‘biological engineering’’ or ‘‘ingenieurbiologie’’ seems to have been created in 1951 by V. Kruedener, when referring to projects using both the physical laws of ‘‘hard’’ engineering and the biological attributes of living vegetation (Schlu¨ter 1984; Stiles 1991). This term was contracted to ‘‘bioengineering’’ (Schiechtl 1980; Stiles 1988) but ‘‘biotechnical engineering’’ has also been used (Westmacott 1985; Li and Eddleman 2002). These techniques are applied in an ‘‘ecological engineering’’ context, combining both ecological systems and human activity for the beneficial coexistence of both (Bergen and others 2001; Gattie and others 2003). One of the benefits of biological engineering compared to ‘‘hard’’ engineering is its capacity to increase its resistance over time, because plants that form part of these structures (as stakes, layering, plantings, etc.) grow and spread over the soil that they are holding in place. This process provides long-term protection, which is capable of self-regeneration (Stiles 1988). If the vegetation dies, the protection does not last long, and costly repairs are then necessary (Barlatier De Mas 1899). Very often, river bioengineering work combines several techniques, to attain precise objectives corresponding both to environmental factors and mechanical constraints. Indeed, the success of the project depends on the availability of water and light
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for the plants to grow, but it will also depend on its resistance to shear stress during flood events. Shear stress is the average tractive force on a channel (Gray and Sotir 1996), which depends on river channel slope, depth, and more generally on the geometry of river beds. Studies conducted in the laboratory and in the field have shown that a few years after installation some bioengineering works can have a higher permissible shear stress than some mineral works such as rip-rap with large stones (Gray and Sotir 1996; Schiechtl and Stern 1996). These mechanical resistance capabilities of river bioengineering works may explain why they have been used, not only for rivers, but also for torrents. There are numerous bibliographic sources describing early examples of bioengineering. Although the way to use living stakes was described by Columelle during the first century BC in De Rustica, the first references to the use of living material against water erosion date from 1600. In the eighteenth, and particularly in the nineteenth century, several books were published, in French, German, and Italian on soil bioengineering. In the second half of the eighteenth century, faced with the consequences of deforestation, the French, Italian, and Swiss Governments evolved a whole range of measures against erosion in some areas of the Alps, including many that involve soil bioengineering (Hall 2005). In this context, several engineers and foresters from these countries, but also from Austria and Germany wrote interesting guides on soil erosion control of slopes, which proved to be very valuable bibliographic sources that included detailed diagrams. More recently, several authors have returned to the history of bioengineering. For example, the huge bioengineering project in the French Alps to control erosion in torrential catchments during the second half of the nineteenth century has been studied again from an ecological point of view (Vallauri 1998; Rey 2004b). Stream engineering works of the past have been analyzed in France (Girel 1994; Girel 2007; Labonne and others 2007) and in Switzerland (Vischer 2003). Lachat (1999) also commented on early examples of water bioengineering carried out in European French-speaking countries. Schlu¨ter (1984) offered a general overview of biological engineering, not just related to streams but also including polders, hedges, and trenches with many references to German-speaking authors. Stiles (1991) gave more prominence in English to Schlu¨ter’s article by emphasizing the rediscovery of these techniques during the twentieth century. In another article, he described the development of engineering with vegetation in central European countries (mainly in the Alps) since the nineteenth century and its implications for the United Kingdom (Stiles 1988). In a parallel approach, Westmacott (1985) began with English references from the eighteenth century, and then described new developments in these
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techniques in the United States during the twentieth century. In his book ‘‘Earth Repair’’, Marcus Hall (2005) made a thorough historical description of how basin control works developed in Europe in the nineteenth and twentieth centuries. He focused on the restorations in the Italian Alps (Cuneo) and the Rocky Mountains (Utah, USA), compared these actions and placed them in their cultural context. The aim of this article was to complete this bibliography using an historical perspective focused exclusively on bioengineering techniques applied to streams and rivers. We took an inventory, mainly in Western Europe, of the different forms of river erosion control including examples of engineering with vegetation. Bioengineering techniques can be classified according to their level of protection against erosion, from low to intermediate, to high (Li and Eddleman 2002). We listed the works depending on their level of protection. These parameters were directly related to where the work was located in the stream. Thus we first present techniques used to stabilize riverbanks, then those used for water channeling, those applied to riverbeds, and ending with a case study of work done in a river basin that included both methods. To conclude, we compare old and new techniques to show how they have evolved from a technical and social point of view.
Streambank Stabilization Securing Banks with Woody Species In the past, afforestation techniques were used to secure banks and slopes. This included simple planting of trees and shrubs as well as more complex biotechnical methods. In Italy, since the sixteenth century and mainly after Leonardo da Vinci recommended it, planting willows along river banks has become a recognized technique to prevent erosion (Grabecht 1981 in Schlu¨ter 1984). In the same way, in 1735 King Frederick William I of Prussia ordered willow to be planted in order to stabilize the riverbanks of his territory (Stadelmann 1878 in Schlu¨ter 1984). One century later, in France, Dugied suggested planting exotic species such as the Chinese Varnish Tree and White Mulberry, in addition to willows and poplars. These fast-growing and quick-spreading species (by way of root suckers) rapidly formed dense barriers (Dugied 1819). In 1791, Woltmann described an original work designed by Bettoni to both stabilize the river banks and to slow down water flow. Along the banks, perpendicular to the river bed, transverse ditches were dug into which young willow saplings were planted or large willow branches were buried. Soil would accumulate, permitting the trees to establish and form a barrier, thus securing the river bank (Schlu¨ter 1984).
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In 1600, the French agronomist Olivier de Serres, recommended planting cuttings and shoots produced by species such as tamarisk, alder, and various types of willow already well-known for their resistance to flooding (robust rooting and high flexibility) and for their ability to thrive along rivers and in wet habitats. During the next few years the best individuals were selected by eliminating the smallest ones. In 1772, Silberschlag, a German engineer, developed a technique based on planting rooted cuttings, followed by layering; in other words, tree branches still attached to the parent plant were buried in the wet soil in order to produce roots. With this method, dense vegetation could be established along riverbanks. Furthermore, it provided live material for other bioengineering work also requiring surface protection for soil conservation (Schlu¨ter 1984). The afforestation of river banks could also be carried out by planting live branches (Bernard 1927). This material was used both to stabilize the substratum before planting and also to produce natural colonization by way of vegetative reproduction (sprouts, rooted cuttings, and rooted suckers growing from branches in contact with wet soils). Willow branches were arranged perpendicularly, partially overlapping like tiles on a roof, their bases being buried in sediment. Hence, from this bush-mattress construction, rooting was facilitated and the branches could grow rapidly in various directions, the resulting dense bushes securing the slopes. This willow community could also be improved by planting additional species such as alder and ash. In cases where branch layering was used only as soil protection while trees established themselves, they were placed only in damaged sections, with their bases resting on solid foundations so that they could trap alluvial deposits to form a permeable, fertile, and stable substratum for planting (Bernard 1927). Other methods of streambank afforestation combined dead and live material. For instance, in 1774, Scheyer carried out various works designed to protect river banks. One of them consisted of planting willow sprouts in a criss-cross pattern along the bank, with a mixture of clover and grass seed sown in between. On steep slopes and undermined banks he suggested planting rooted willow cuttings and, in order to prevent erosion, to cover the soil around each cutting with branches (Schlu¨ter 1984). August, another engineer quoted by Schlu¨ter, described, in 1792, various techniques used to strengthen river banks and particularly a method using tree stumps, fixed by willow stakes and covered by sandy sediments. In addition, rooted willow cuttings were planted between the stumps and the resulting branches were buried in the sediment deposits (i.e. layering) in order to favor root production and the eventual establishment of new individuals.
Protecting Banks with Fascine and Wattle Fences Living plant materials were also used in more sophisticated structures such as fascines and wattle fences. These structures limited undermining caused by water flows at the bottom of the river bank. They also limited erosion from streaming on the upper part of the river bank by reducing significantly the bank’s slope and creating physical barriers against surface runoff. Fascines were bundles of straight woody sticks tied with a piece of wicker (later with wire) and mostly made of willow. They were generally about 3 m long with a diameter of 30 cm and held by stakes (Forest De Belidor 1730; Scheck 1885). The Romans used fascines to build structures to control water erosion. Similarly, tapestries dating from 28 BC show Chinese workers putting down willow bundles to protect the banks of the Yellow river (Lewis 2000). In 1600, Olivier de Serres, in ‘‘The´aˆtre d’agriculture et Me´nage des champs’’ wrote about using ‘‘bushes attached to stakes and reeds’’ to protect the watersides of fish-ponds from the impact of waves. In 1730, Forest de Be´lidor recommended the use of fascines as ‘‘alternative stone paving’’, to limit undermining at the bottom of locks. Placed on a bed of beaten clay, they were made of oak, ash, willow, or alder. Their dimensions were such as to resist three kinds of hydrological force: the ones at the bottom of riverbeds, maritime flux and reflux, and the flow due to dam release. This author also cited an example by Vauban of such a construction at Gravelines, in 1699. In 1774, Frisi, a hydraulics engineer, visiting Northern Italy and commenting on river engineering works in place, highlighted the advantages of using flexible fascines fixed to the banks versus paved groynes, which were responsible for extensive undermining. Furthermore, he noted that the most developed fascine-work known at that time was practiced in the Netherlands along the River Maas at Rotterdam. In 1885, the German Rudolf Scheck published a detailed technical guide dedicated to fascine works. In this book he described a huge construction of braided fascines called ‘‘Sinkstu¨ck’’, literally: ‘‘sink-piece’’ in German (Fig. 1a). This work could reach dimensions of 6 9 8 9 1 m3 and was composed of fascines and sausages. The latter were similar to fascines but thinner (10–15 cm) and much longer (up to 20 m) (Fig. 1c). The construction was built on an inclined wooden framework that permitted it to slide into the river when finished. The assembly started with a lattice-like underlay of sausages tied together at their intersections (Fig. 1b). The intermediate part was composed of several fascine layers, each layer lying perpendicular to the one below. Finally the overlay was another lattice of sausages with the same dimensions as the underlay. The under and overlays were then tightened with hemp ropes, with the fascine layers in between, thus
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Environmental Management Fig. 1 Braided fascines or ‘‘Sinkstu¨ck’’ (Scheck 1885)
forming a sandwich. The construction could eventually be floated to the riverbank where it was loaded with stones and gravel (Scheck 1885). Wattle fences were made of stakes with flexible branches and twigs woven between them. Stakes used for wattle fences, generally made of live material (willow and black-locust cuttings) of 2–4 cm diameter, were planted 15–30 cm into the ground and 40–80 cm apart, depending on the slope (Arnould 1913). As for fascines, there are also old accounts of the use of wattle fences, with some remnants of wattle fences dating from the beginning of the Merovingian times having been found during an archaeological dig in Paris near the River Seine (Pion 2005). Similarly, woven twigs dating from the seventh century were found along a river in the Jura Mountains (Switzerland) during excavations for the construction of a motorway (Lachat 1999). At the end of the nineteenth century, wattle works were considered as an efficient way of protecting steep naked slopes and areas threatened by water flow (Mathieu 1864; Pontzen 1891; Arnould 1913). In contrast to this, wattle works used to stabilize river beds and secure the banks did not exceed 50 cm in height above ground level in order to avoid the risks of undermining and consequently, loosening the stakes by water run-off (Bernard 1927). In wet soils, live stakes and wattles could develop roots and consequently dense bushes covered the ground. Newly established vegetation thus controlled erosion processes and secured the structure (Pontzen 1891). Afterward, deposits accumulated upstream from the wattle-work, the sediment retained water and hence favored the establishment of vegetation. Pontzen recommended the building of wattle structures that tilted obliquely in order to force water runoff to flow more longitudinally along the bank slope. He considered that the ‘‘diamond-shaped’’ construction recommended by his colleague Thie´ry in the case of steep slopes and landslides was the least well adapted because of the concentration of water run-off, which led to the loosening of corner stakes located downstream (Pontzen 1891; Thiery 1891). Finally, in a manual entitled ‘‘Cours de
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Fig. 2 Lattice-pattern Wattle structure (Barlatier de Mas 1899)
navigation inte´rieure’’ and published in 1899, Barlatier de Mas, a French engineer, recommended the use of ‘‘clayonnage en croisillons’’ (a lattice-work construct) for wattle-work structures (Fig. 2). Another pattern for wattle-work structures was sometimes recommended, called ‘‘en e´cailles de poisson’’ (fishscale pattern). This type of construction was built at a slant over the banks and was improved by the presence of small gullies leading run-off water downstream. To reduce erosion, the bottom of each gully was paved with stones or covered with turf (Pontzen 1891; Thiery 1891). After construction, the upper part of these different wattle-work structures could be filled up and planted with willow cuttings, which were increased by layering 1 or 2 years later. In this way a natural defense against undermining processes was obtained (Bernard 1927). In his project involving the correction of an alpine stream called ‘‘Torrent du Bourget’’, Demontzey, a French forest engineer, described a complex system consisting of transverse wattle-fences laid across the riverbed associated with longitudinal wattle-works at the foot of the riverbank. The stakes used in these structures were made from larch and living willow; furthermore cuttings were planted in the space created between two structures (Demontzey 1875). Although fascines were considered more economical and faster to build, they were found to be less resistant than wattle fences, as stated by Depelchin in 1887 (this original
Environmental Management Fig. 3 Fascines and wattle fences (Forest de Belidor 1730)
statement was experimentally disproved by Schiechtl and Stern 1996). Furthermore, wattle fences and fascines were frequently combined (Forest De Belidor 1730; Defontaine 1833; Scheck 1885), wattles often being used to fix fascines to the soil (Fig. 3). These ‘‘soft’’ structures also had the advantage of being cheap compared with ‘‘hard’’ river-training constructions using cobbles, pebbles, and boulders. Nevertheless, the necessary living material had to be to hand to be practical (Thiery 1891). Covering Banks with Grasses and Herbs Another bank protection technique frequently used was to sow grasses. Herbaceous vegetation limited water run-off and increased slope stability by root reinforcement. The method was developed in the middle of the seventeenth century to stabilize river banks. It was also recommended for preparing soils before tree planting, particularly over unstable slopes (many reports have been published about this practice: Surell 1841; Demontzey 1875; Pontzen 1891; Thiery 1891; Barlatier De Mas 1899; Bechmann 1905; Bernard 1927; Bensaadoune and others 2005). Two main techniques could be used to establish herbaceous communities over river banks: seeding and turfing. The technique adopted depended on the substratum, bank slope angle, the possibility of turf extraction in the vicinity, and the availability of manpower. Seeds could be sown broadcast then buried in the soil by superficial harrowing with tree branches pulled along the ground. Seeds could also be sown along furrows dug parallel to the bank curves. Lines of vegetation grown under these
conditions acted as small living dams that controlled water run-off and erosion (Surell 1841; Thiery 1891; Cabianca and Ferrari 1919; Bernard 1927). Sometimes small turfs of grass were directly deposited in holes made in the bank soils. This technique was repeatedly used with success in the Ubaye valley (Southern French Alps) to restore the banks of a small mountain stream called Riou-Bourdoux (Thiery 1891; Bernard 1927). Grass turfs could also be used to restore vegetation over flat areas and gentle slopes. These pieces of soil and grass were extracted from surrounding fields and fixed with stakes to the ground, which was thus paved with a living layer of soil and plants. On steep slopes, turfs could be stacked in order to create a more stable stratum or they could be lined up in ditches dug in the bank. An economical method used in the past was to line up the turfs along the sides of 1–3 m square patches, then sow grasses and herbs in the free space within the plots (Pontzen 1891; Barlatier De Mas 1899; Bernard 1927). It should also to be noted that Olivier de Serres had already recommended, back in 1600, in the case of pond dams, to build two walls of stacked turfs ‘‘cut like building stones’’ and filled with wet and packed clay. Methods Combining Re-Vegetation Techniques and Traditional Rip-Rap Revetment Practices Bioengineering could be used to complement practices using stone and rock pavements (rip-rap revetments). The technique called ‘‘perre´’’, ‘‘perre´s a` plat’’ or ‘‘placages’’ corresponded to the construction of a superficial pavement
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Fig. 4 Rock pavement on an upper bank (Defontaine 1833; Barlatier de Mas 1899)
of flat stones fixed to the ground by vegetation (Fig. 4). Another form of joint planting involved the construction of a thicker pavement made of rounded rubble stones called ‘‘teˆtes de chien’’ (dogs head) and willow cuttings planted between the stones, each live cutting developing roots that fixed the stones to the ground and bound them together. It was necessary afterward to manage the vegetation so that it remained at the shrubby bush stage, without a main trunk to reduce the risk of erosion. In fact, the development of trees was also to be avoided in order to maintain access for towing and other riverbank activities. Along the river banks these structures were generally limited to the annual flood level, i.e. 2 or 3 m above the low-flow level (Defontaine 1833; Barlatier De Mas 1899).
Water Channeling Bioengineering techniques have been used to channel or guide the direction of streams and rivers and prevent their wandering, or on the contrary, direct them toward divagation fields. They can also be used to protect riverbanks and reclaim fields that have been damaged by floods. In order to reduce the River Rhine’s velocity, Forest de Be´lidor proposed, as early as 1730, the construction of fascine groynes, also called ‘‘tunes’’. A century later, Defontaine (1833) recommended them as dams to close off the ‘‘unwanted branches’’ of this river. These constructions combined a fascine mat with a stone bed and were crossed by thick willow stakes anchored in the banks. They were simple to build, economical and efficient, but for Defontaine, they were not considered to be sufficiently sustainable (Forest De Belidor 1730; Bourdet 1773; Defontaine 1833; Scheck 1885). Fascine groynes were suitable for slow-moving rivers with low bedloads, but not for mountain torrents and rivers (Surell 1841). ‘‘Saucissons’’ (Fig. 5) often used to channel rivers, could also be used as groynes. They were frequently used during the nineteenth century, in the upper part of the Rhine River basin. These sausage-like constructions, with
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Fig. 5 Riverbank protection with a system of two parallel ‘‘saucissons’’ placed on an alluvial fan (Mathieu 1864)
diameters of 1–1.2 m, were made of soil, stones, or sand wrapped inside branches. They were tightened every meter with wire. Depending on their function, they were used alone or in groups of three piled up saucissons, on the alluvial fans of alpine torrents, along banks, or at the base of dams (Mathieu 1864; Scheck 1885; Vischer 1988). Biotechnical works have also been used to channel rivers under fast-flowing conditions. For instance Dugied described, in 1819, dykes made of gabions and fascines successfully created along the rapid alpine Asse River. In general, they were composed of conical gabions made of 5 m long willow, placed side by side, with their points oriented to the river, on a layer of bushes. These gabions were progressively buried with tons of stones, then willow and poplar cuttings were inserted in the spaces across the brush layer. During the first flood, any free spaces were filled with clay. Then further willow or poplar cuttings were planted between the gabion points for re-sprouting. During the next flood, water no longer crossed the construction, whose strength was then reinforced each year.
Stabilization of River and Torrent Beds Methods used to stabilize river and torrent beds were mainly described by authors from the nineteenth century. At that time, erosion control in the Alps was of major concern, leading to numerous constructions. Several of them were located in gullies (little catchments areas) with strong erosion, where floods were rare but violent, and where substrates were highly erodible. Different kinds of bioengineering were used to stabilize these riverbeds. As early as 1811, Francesco Focacci, in his manual on the control of torrents, suggested planting different kinds of willow and poplar at the start of the floodplain to stop the coarser sediment and to reduce the flow velocity. Brush mattresses were simple bioengineering structures made by placing branches so that they faced upstream in torrent and river bed, the base of the branches being held with stakes (Mathieu 1864; Thiery 1891). Brush mattresses acted by reducing the flow velocity and filtering the
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Fig. 6 Dams made of crib walls (Thiery 1891) Fig. 8 Dam made of wattle fences (Thiery 1891)
Fig. 7 Dam made of fascines (Demontzey 1875)
sediment. Dams were also built to stabilize the long profile of stream beds, and avoid regressive erosion. Strong dams have been built in the form of crib walls in Switzerland (Fig. 6). The first to be built was located in the Roetzligrund torrent and measured 10 m high by 40 m wide. It was the biggest construction of its type listed at the end of the nineteenth century. It was composed of layers of piled up trees, their branches turned to face upstream and then buried in the alluvium. Transverse pieces with their extremities buried in the banks anchored each layer (Duile 1826; Thiery 1891; Bernard 1927; Vischer 2003). Dams made from both fascines and wattle fences were described in a book printed in London by Edmondson in 1780 (Westmacott 1985). Dams made of fascines were barriers perpendicular to the stream and made of wooden stakes behind which cuttings were piled up (Forest De Belidor 1730; Depelchin 1887; Thiery 1891; Bernard 1927). These constructs also had the objective of trapping fine sediment inside their mixed branches (Fig. 7). Wattle fences were simple woven panels of cuttings installed vertically and perpendicular to the direction of flow. The biggest dams made from wattle fences (Figs. 8 and 9) could measure 20–30 m long and 1.5 m high. They were placed perpendicular to the torrent axis and were made of rough wooden stakes (oak or larch), planted every meter. The stakes, between which cuttings were interlaced, were
Fig. 9 Wattle fences in the Echarina torrent in the Alps (Kuss 1903)
tied at the top by longitudinal poles made of oak or chestnut (Thiery 1891; Bernard 1927). Demontzey described, in 1875, a huge project in the Bourget torrent. The construction, located between two dams, was aimed at decreasing the risk of floods. It included transverse and longitudinal wattle fences, with oblique rows of willow cuttings on the banks. The transverse fences were placed every 5 m on the sediment deposits upstream of the dam. To limit undermining, the height of the wattle fences did not exceed 0.5–0.6 m and their bases were consolidated with stones. Maintenance work was required on these constructions every year. Demontzey recommended these wattle fences as they avoided the need for stone paving downstream of the dams. However, they were not appropriate for streams that carried coarse sediments (Demontzey 1875). The techniques previously cited could be combined or reinforced with other vegetative methods (Fig. 10) or with
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Fig. 10 Dams made of crib walls and fascines (Thiery 1891)
civil engineering operations. Thus, dams could be consolidated with gabions (rock-filled baskets) or wattle fences (Bechmann 1905). It was also possible to insert ranks of fascines between layers of wood, even if this decreased its resistance. Wooden cross-pieces separated each layer and a foundation of fascines, added at the bottom of the construction, protected it against undermining (Thiery 1891). It was also possible to increase the resistance of wattle fences by adding different elements. Thus, these wattle fences could be preceded, 1.5 m upstream, by a row of stakes attached to longitudinal poles. The construct would be buried and cuttings planted upstream, to protect it against floods. These wattle fences could also be surmounted by longitudinal beams embedded in the banks (Thiery 1891).
The Case of the Diois and Baronnies Mountains in the Southern French Prealps River channel responses to changing sediment supply have been a management issue since the nineteenth century in the French Alps. Bioengineering work to regulate coarse sediment deposits from hill slopes was applied early in upland catchment areas. During the nineteenth century, the challenge was to act against the aggradation of wide and active braided channels and the associated increases in flood risk across the floodplain. This situation was triggered by the accelerated erosion on hillsides due to both climatic and anthropogenic forces, their respective influences being an important research question (Bravard 2002; Lie´bault and others 2005). Accelerated erosion was not exclusive to the Southern Prealps: most upland environments in France have been affected. Laws were therefore enacted in 1860, 1862, and 1884 to carry out an ambitious program of soil conservation in the French mountains, known as the RTM scheme (‘‘Restauration des Terrains en Montagne’’: mountain land restoration). Planned reforestation of eroded hillsides and
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the construction of engineering work in active gullies and torrents were considered at that time as the best remedial strategy for reducing flooding and erosion risks that threatened the socio-economic development of mountain territories (Surell 1841). The French forest and water administration at that time was put in charge of purchasing land highly degraded by erosion to carry out reforestation, turfing and torrent-control measures, including bioengineering techniques. In the highly degraded Diois and Baronnies mountains, records from the National Forest Office archives have been analyzed to make an inventory and to reconstruct the chronology of the RTM work. This study shows that RTM work was mostly carried out between 1863 and 1917 and ninety percent of the investment was made during this period. Seventy-six RTM zones were delimited and purchased, of which *15,600 ha were reforested, mostly by planting exogenous black pine (Pinus nigra), *15,940 cut stone check-dams and *92,720 bioengineering structures (fascines and wattle fences, Fig. 9) were constructed along tributaries and *760 km of headwater channels were stabilized with brush mattresses (Lie´bault and Pie´gay 2002). The technique of cut stone check-dams was preferred to bioengineering works in boulder-bed headwater channels where it was easy to get supplied with big stones. Bioengineering measures were used as a complementary technique of erosion-control in gullies entrenched in soft rock terrains (marls and alternating marl and limestone sequences) where big stones are less accessible. At that time, the work was done manually and it was easier to transport branches and wooden stakes than boulders to remote and steep gullies. Reconstructing the chronology of the RTM work allows us to characterize how bioengineering structures were used in the overall erosion-control strategy. The example of the Upper-Droˆme River catchment area, in the upland part of the Diois mountains, shows a typical chronological sequence of the different RTM measures (Fig. 11). The first step was to construct cut stone check-dams in the main channels of small mountain catchment areas, where it was easy to find boulders. Once the main channels were controlled, the second step was to deploy bioengineering dams along low-order tributaries, generally cut into soft rock terrain. The last step involved planting trees on hillsides and making brush mattresses on sediment deposits located behind check-dams, wattle fences, and fascines. Bioengineering techniques were thus envisaged as a complementary way to prevent erosion along small upland channels, in order to improve the success rate of hillside reforestation projects. The present-day situation is very different from the one that prevailed at the end of the nineteenth century. Most of the rivers draining the Southern French Prealps are now
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Fig. 11 The chronology of erosion control works conducted in the Upper Droˆme catchment area
starved of sediments and affected by channel incision (Bravard and others 1997). This situation has come about due to the cumulative effect of gravel mining, channel
embankment and reduced sediment supply from upland catchments. This has led managers to reconsider erosioncontrol works and to adopt a new strategy that takes into
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Fig. 12 Wattle fence to protect the Droˆme riverbank a few years after its construction (Photo taken in 2007)
account the conservation of the sediment transport continuity better. For example, in the water management scheme of the Droˆme River basin a program was adopted in 1997 of erosion-control works only on eroded slopes undercut in marly terrains. These slopes produce mainly fine sediments that are rapidly flushed downstream and which don’t play a major role in controlling river channel morphology. It has been also proposed to replenish incised river reaches with coarse sediments that accumulate behind big check-dams. Despite these recommendations, it appears that the use of bioengineering methods for erosioncontrol on hill slopes has been abandoned since the 1960s (the last intervention dates back to 1976) because the erosion of marls is not a big concern in the Droˆme River basin, as it is not equipped with hydroelectric dams or reservoirs. In the present-day context, bioengineering work is restricted only to destabilized river banks that threaten housing or roads (Fig. 12) and are considered as an alternative technique to traditional civil engineering methods.
Discussions and Conclusions Among the large variety of techniques discussed in this article, some have been completely abandoned, others are
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still being used unchanged, and still others have evolved significantly. Although the use of vegetative engineering to stabilize river and torrent beds has sometimes shown very good results (Fig. 13), it has also shown its limits (see photos in Vallauri 1998). These techniques are still effective in small gullies where the outflow is temporary (Rey 2004a), but they cannot resist in environments where the flow is continuous, with high sediment transport. Now only ‘‘hard’’ engineering techniques are used to stabilize the beds of rivers and torrents. Furthermore, with the introduction of fossil fuel, improved transportation and greater mechanization, has made large structures of stone or concrete much easier and cheaper to build. This type of work has expanded enormously since the Second World War, providing quick and solid solutions to erosion problems and the protection of socio-economic installations. For example, rip rap with concrete to protect riverbanks, or sediment retention dams (deposition areas) to stop floods at the outflow of torrential river basins have been very successful. Another advantage of civil engineering work is its low manpower requirements. So, with the sharp increases in manpower costs during the twentieth century, bioengineering methods have become too expensive and thus less attractive alternatives. That explains why some of the very sophisticated bioengineering techniques of the past (Fig. 11), involving a lot of manpower, have gone into decline. This increase in labor costs has sometimes been associated with the scarcity of suitable materials, as in United States after about 1910 (Przedwojski and others 1995). Maintenance work, even if recommended, is seldom carried out and reinforcement techniques using cuttings or layering have been abandoned for several reasons. Conversely, the fixed stumps suggested by August in 1792, were very similar to the cabled large woody debris used in United States in the second part of the nineteenth (Thompson and Stull 2002) and at the beginning of the twentieth centuries (Taylor 1929), and are still promoted in technical guides about riverbank bioengineering (McCullah and Gray 2005; Allen and Leech 1997; Zeh 2007). Furthermore riverbank protection techniques, such as wattle fences, fascines, brush layering, or brushwood groynes are still frequently used in France (Lachat 1994; Adam and others 2008), in Italy (Sauli and others 2006), and in Central Europe (Przedwojski and others 1995; Schiechtl and Stern 1996; Faber 2004). These techniques have also been developed all over the world, for example, in America (Gray and Sotir 1996; Lewis 2000; Petrone and Preti 2008) and in Asia (Barker and others 2004; Wu and Feng 2006). Even if nowadays these bioengineering techniques are used in quite a similar way as in the past, new technologies
Environmental Management Fig. 13 La Valle di Mezzo (Rivoli Veronese) during construction and 5 years later, the plantation of willow, alder and locust trees has been successful (Di Tella 1912)
have made their implementation much easier. For instance, mechanical excavator and chain saw often replace shovel and saw. Stakes that were initially beaten with a sledgehammer can now be easily knocked into the ground by mechanical means. Sowing grass is still done following the two same main techniques: seeding and covering the ground with turfs. However, seeding is sometimes mechanized using a hydro seeder, which throws a mix of hydraulic mulch, water, fertilizers, and seeds onto the slopes (Gray and Sotir 1996; Schiechtl and Stern 1996). To cover the ground with turf nowadays we may also use ready made rolls of turf (Gray and Sotir 1996; Zeh 2007). The appearance of new materials such as iron mesh, geotextiles, geogrids, or plastic geocells has also led to the development of new techniques. For instance, in the past, the protection of soil from erosion just after the work was done, was assured by willow brush layering or braided fascines. However, today we would use biodegradable geotextiles instead. Furthermore, lattice-work wattle fences (Fig. 9) correspond geometrically and functionally to geocell structures. These new techniques are easier to implement using synthetic materials, which are also quicker and less expensive to produce. These materials are now generally included in combined works associated with planting, cutting, or seeding. We should also note that the current use of helophytes (often in coir rolls), although not cited in the old literature on riverbank protection, has been cited as having been used to reinforce flood protection dams in Holland since the thirteenth century (Williams 1990).
Bioengineering techniques may also have been used in the past for functions other than soil erosion. For example, tree-planting and cabled large woody debris on riverbanks (both mentioned above) have been used in United States from the second half of the nineteenth century, with the aim of improving habitat quality in in-stream fisheries (Thompson 2005). Old bioengineering techniques also had recognized secondary uses, which were useful for riverside residents. Thus, willow was widely used for basketry, but also as a source of salicin (an aspirin ancestor) extracted from Salix bark (Kuzovkina and Quigley 2005). The Chinese Varnish Tree, planted to protect riverbanks, could also be used for building frames, and the fruit of White Mulberry trees was fed to pigs and its leaves to feed silk-worms (Dugied 1819). Field Elm was recommended for anchoring sediments behind dams, but was also a very good cattle feed (Thiery 1891). The disappearance of these secondary uses of riparian woods explains, at least in part, the current disinterest of riverside residents and river decision makers for maintaining riparian woods. Voit, suggested back in 1820 that trees should be planted in Bavaria on riverbanks, not only to reinforce the soil but also to improve the quality of the landscape (Schlu¨ter 1984). In 1841, Surell wrote that imitating nature was a guarantee of success for bioengineering works. Today, river bioengineering is also a useful tool for restoring the degraded ecological habitats of riverbanks (Bentrup and Hoag 1998; Wu and Feng 2006; Adam and others 2008) and in landscape restoration projects (De Santoli and others 2007).
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Environmental Management Acknowledgments We warmly thank Thomas Spiegelberger, Maryse Berlandis and Vito Bacci (Cemagref Grenoble) for their translation of articles into German and Italian, and Nicole Sardat for her work on the figures. We sincerely thank Daniel Vischer for providing images and for his authorization to publish them. We are also very grateful to two anonymous referees who helped to improve the quality of the manuscript.
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