Evolution of River Channels and Floods: A Short- to Long-Term Perspective

9 Evolution of River Channels and Floods: A Short- to Long-Term Perspective

9.1. Introduction The inundation of floodplains occurs when a river experiences overbank flows that consequently cause flooding of the alluvial plain. Several questions are posed in this chapter: at what discharge rate do rivers experience overbank flows, and do they cause damage? Does this rate remain constant across the lifetime of a river, which spans over centuries, or does it vary? If so, why? Finally, are adaptations that have been made to rivers since the beginning of the industrial age responsible for the unstable relationships that exist between inundated rivers and their floodplains, irrespective of the protective works that may have been built? Section 9.2 presents the main principles governing the functioning of a river. Section 9.3 examines long-term relationships between rivers and companies in the second part of the Holocene. This report does not take into account vertical aggradation of river channels in floodplains with very low slope, or deltas with upstream–downstream progression (or progradation); here, accretion increases the relative depth of the river beds, which are perched above the alluvial plain, therefore increasing the risk of switching and overspill of floodwaters. Instead, this chapter will focus on accretion and incision of river channels and floodplains in response to fluctuations that are complex in origin. The above falls within the remit of river metamorphosis, which is relevant to all three dimensions of the alluvial plain, including the vertical dimension, which is of considerable importance to companies exposed to flood risk, located along rivers.

Chapter written by Jean-Paul BRAVARD.

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Section 9.4 considers the evolution of channel profiles, in the knowledge that the issue is often complex, as bankfull capacity may evolve either in a joint or independent way. To simplify this point, we will assume that the altitudinal occurrence of the long profile conditions overbank flows and flooding. This part addresses human-related impacts on flooding, which, in short, are modern adaptations made by mankind to both basins and rivers. Contemporary society has entered a phase of “proactive-prevention”, which generally manifests itself via policies promoting flood reduction at a local level. Success is far from guaranteed due to the fact that scale is often overlooked. To conclude, section 9.5 envisages the impacts of changes across alluvial plains. The impacts are (or used to be) beneficial to stakeholders within land use planning, who favored the impacts. However, companies are becoming increasingly exposed to their negative impacts. Finally, we will address possible strategies that aim to counteract adverse effects of such impacts on both flood processes and conditions. 9.2. General principles of river functioning River hydraulics and morphodynamics have shown that the channel of a river whose balanced floodplains are inundated laterally migrates and constructs forms due to sediment deposition, while eroding the opposite bank during flooding. Lateral erosion occurs at the expense of the alluvial plain, and sometimes in terraces corresponding to steps within ancient fluviatile processes. During the migration process, channels maintain their geometric characteristics (depth and width); fluvial forms are developed due to lateral expansion as a result of large particle deposition (pebbles and sand) as part of bedload transport, predominantly in convexities. Topography of the alluvial plain evolves very slowly via the deposition of very fine particles on its surface, transported in suspension during the flood. They fill channels that have been abandoned due to migration of the active channel and slowly cause the plain to rise, creating long-term regularized topography. This destructive/constructive cycle rejuvenates forms at a time-step that depends on river energy, width of the bottom of the valley and the nature of sediments that are involved in the process; the cycle can range from a few decades to several centuries, or even longer in natural river systems. These principles characterize rivers in dynamic equilibrium. It has been shown that the bankfull stage of a river, i.e. just before overbank flows occur, stands roughly at the same altitude as the topographical level of the alluvial plain, referred to as the floodplain if a hydrological definition of this area is adopted. In temperate regions, bankfull discharge is equal to that of a weak flood, since it corresponds statistically to discharge for which the return period is around 1.2–1.5 years, with a duration of one to several days per year. When discharge exceeds channel capacity, the surplus is

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driven downstream on the surface of the floodplain, regularized to various extents [DUN 78]. When the climate and occupancy patterns of a watershed change, the balance between hydrological characteristics, provision and sedimentary transport are consequently adjusted. Major forms, such as terraces, originate from old alluvial plains that are perched due to incision. Up until 1960–1970, terraces were overwhelmingly considered as levels that dated from the Pleistocene. Studies conducted in the United States and Europe have abundantly shown that lower terraces dating from the Holocene and even ancient times can be found in many valleys subject to variations in hydrosedimentary balance. In addition, the range of situations is extended with direct and indirect impacts of infrastructure built on the channels themselves, either by organized disturbance of channel geometry, or by voluntary or involuntary alteration to sediment transport. 9.3. Holocene metamorphosis and natural dynamics of long profiles in Western Europe The theory of river metamorphosis originates from hydromorphological studies conducted in the United States in the 1950s. The basic principle is that geometry and morphology of river channels at sectional level can auto-adapt themselves to variations in discharge and sediment load within relatively short time-steps. Geomorphologic adjustment concerns, among other variables, the width (w) and depth of the channel (d), sinuosity (Si), slope (S) and the position of the long profile; these variables themselves condition flow velocity, tractive forces and specific energy, this being the movement of material from the bottom and banks of the channel, as such, their morphogenesis. Qualitative equations are as follows [SCH 77, KOZ 77, STA 83]: Ql– < Qsf– induce w–, d+, w/d–, P–, Si+ >> incision and meandration [9.1] Ql+ > Qsf+ induce w–, d+, w/d–, P–, Si+ >> incision and meandration [9.2] Ql– > Qsf– induce w+, d–, w/d+, P+, Si– >> accretion and braiding

[9.3]

Ql+ < Qsf+ induce w+, d–, w/d+, S+, Si– >> accretion and braiding

[9.4]

In a fluvial system, sections can react autonomously; widening of one of them can result in alluviation of a downstream section due to upstream–downstream load transfer in a downstream section, coupled with accretion of its water level. The response of a long profile to a set of control variables and its transition to a new

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balance occur over a longer period than that of a section of limited length. Below are two types of well-documented situations where long profile adjustment resulted in impacts to land occupancy of the alluvial plain. These changes concern the Rhone Valley and some of its tributaries on the foothills of the Western Alps. La Tène culture experienced a mainly dry climate after approximately 400 BC. This lasted up to and including the first decades of the Roman Empire. Supposedly, land was heavily used for agriculture, but despite this, sediment transfer in slope channels was reduced by low rainfall and runoff. As such, sediment balance was in deficit from low to high order channels, and the latter were widened (equation [9.1]), the duration of the period enabling large linear extension of morphological changes. In rural areas of the floodplain, inundation became an increasingly rare occurrence, sedimentation was largely reduced to the surface of alluvial plains, and pedogenesis encouraged brown, fertile soils, to develop, benefiting agricultural production and living conditions for residents [BER 03]. These same developments enabled the cities of Vienne, capital of Allobroges, and Lugdunum (Lyon) to build neighborhoods in the old alluvial plain of the Rhone, which during the La Tène, a period of fluvial incision, was perched in low terraces over a meandering river (so a single channel). This was the case for the area situated on the left bank of the Rhone in Vienne, downstream of the Allobroge oppidum and Gallo-Roman neighborhoods on the right bank, which are today occupied by the towns of St-Romain-en-Gal and Sainte-Colombe [FRA O7, BRA 14]; this was also the case for the districts of St. John and St. George, situated on the right bank of the Saône and the peninsula between the Saône and Rhone in Lyon. Floods were rare, and to a large extent, limited to braided channels that slightly incised into the alluvial plain following its development at a time of a morphosedimentary crisis (equation [9.3]), dating back to the Hallstatt culture (first Iron Age, between approximately 800 and 400 BC). The situation prevailed until the Little Ice Age (from approximately 14th Century to mid-19th Century), but short-term, lowintensity crises resulted in the calling into question of this overly simple pattern. One crisis, occurring between 10 AD and the beginning of the 2nd Century AD (chronology is uncertain), was small in size but had far reaching consequences. In this short period affecting a part of the High Empire, archaeologists identified a series of alluvial deposits, sandy-silty in nature, in the ruins of urban buildings, and discovered a high reactivation of paleochannels dating from the First Iron Age. It is without a doubt that Gallo-Roman cities (and some rural settlements in regional alluvial plains) would have experienced a hydrological crisis. It may very well be due to increased precipitation, however various signs cause us to call into question the hydrological effects of geomorphological adjustment; in this case, gravelly alluviation in the Rhone channel due to bedload subsequently transported downstream (bank erosion, increased slope-channel coupling?), also with (limited)

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bedload transfers observed in former braided channels. It is suggested that river changes should be characterized as “incipient metamorphosis”, i.e. expressed over approximately one century, but without having reached equilibrium. In rural areas, floodplains experienced a period of hydromorphy, bearing the marks of surface runoff while habitat returned to the slopes. The human response was to elevate the floodplain (again) using bedload. In Vienne, backfill at the beginning of the first century AD in the ruins of the first warehouses built was almost 3 m thick, and as such was able to support new, better protected imperial warehouses. A relatively similar situation to that of the First Iron Age occurred in the 14th and 15th Centuries and was confirmed in the years 1860–1880. If climatic and hydrological determinants of the previous process remain unknown, documentation of those occurring in the Little Ice Age (LIA) continues to improve, notably with the occurrence of strong summer storms. Material transfer on slopes cleared by dense population were very likely intense, and slope-channel coupling would have been very effective. This resulted in early accretion of torrential channels and waterfront areas in the Alps. Progradation of bedload resulted in the widening of torrential river channels downstream of low order torrents, the creation of flat cross-sections, the systematic expression of braided patterns and aggradation (accretion) of loads in high order rivers within the heart of great Alpine valleys and on the western foothills of the Alps. This dynamic is well-documented in Grenoble (France) where accretion of the Drac (braided pattern) influenced metamorphosis that can be seen in meanders of the Isère River that are clogged with bedload. River cities constitute an excellent method of recording change, since written documentation provides observations of flooding and difficult living conditions. Rich iconography and archaeological excavations helped to reconstruct dynamics of the Rhone River under the Guillotière Bridge in Lyon between its primitive construction in the 13th Century and its widening supported by new arches in the 15th and 16th Centuries [BUR 91]. It is important to note that LIA is not homogeneous. Pichard and Roucaute [PIC 14] showed that periods of very strong floods existed downstream of the Rhone, along with periods of relative remission associated with the periodization of morphodynamic behavior. On a very fine scale, the study of community archives located in Diois, in the southern Pre-Alps, revealed that low-order torrential river channels stored bedload during periods of strong slope-channel coupling (1815– 1823) and were incised in dry periods (1825–1839). In the first case, bridge openings were too narrow; in the second, bridges were adapted to cater for the passage of strong floods. The situation in rural valleys is similar, with reduced grain farming, pastures spread over stony surfaces along the periphery of active braiding forms, and back marshes reactivated by rising waters accompanying rivers. The 18th Century saw the beginning of containment projects, but some communities fought against these works, of which the objective was to improve new land for the benefit of large

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landowners. Annual flooding is caused by fluvial accretion and, as perhaps was the case in the Upper Rhone River, an auxiliary of socially disadvantaged groups; effectively deprived of any personal land, they rely on communal land to survive. Free grazing and hay harvesting, while limited, were guaranteed each year due to alluvium deposits, replenished in the back marshes of the Rhone during lengthy summer flooding. Morphological and social balance was not broken, as floods upstream were seen to be beneficial for cities downstream. Very high volumes of water retention had very favorable impacts on flood peaks and their transmission downstream. Following the flood of 1856, the 1858 Act was introduced, prohibiting the containment of plains upstream of major French cities, such as Paris and Lyon. The beneficiaries of flooding were small rural areas, which later extended to include river cities, and this principle has not changed, even if it has been poorly applied. This is one of the unknown effects of the Little Ice Age, which pushed river responses such a long way toward channel accretion that it unexpectedly gave floodplains new value and durability. 9.4. Changes to long profiles and flood conditions as a result of impacts to channels: windmills, dikes and extractions For more than two centuries, rivers around the world, particularly in formerly industrialized regions, have been affected by development or exploitation of river resources on a scale previously unknown. In most cases, facilities have sought to protect goods and people located in floodplains (dams and reservoirs), exploit sand and aggregate resources, and use water as a resource (mills, dams and reservoirs designed for drinking water supply and agricultural irrigation). In presenting the construction of such works in chronological order, we suggest that the age of the works conditions (at least to a certain extent) the age of the impacts related to their existence, and therefore their relative importance insofar as the restoration of balance (static or dynamic) is performed over periods ranging from decades to centuries. This presentation will take on an analytical perspective to start, on the understanding that a single valley may have witnessed construction of several types of works, generally producing synergistic impacts. The works mentioned above alter both water and material flows, geometric variables of the channels and lastly, long profiles, the subject of this chapter. It goes without saying that human activities also affect watersheds, as they cause external variables such as liquid and solid flow to change; these changes operate interactively with changes to internal variables that are expressed across river sections. We will go on to see that changes in long profiles result more frequently in deepening of river channels, but that this response is far from systematic.

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9.4.1. Mills Mills are the oldest form of flow control designed to produce kinetic energy. The creation of a waterfall designed to operate a wheel requires the construction of a weir in the river. This weir may be equipped with valves that are open during the flood, but it generally increases the depth of floodwaters on the floodplain. Floodwaters are stimulated by alluvium deposits made over several centuries, a process that may eventually offset increases to the water level. This was the age of “hydraulic control”, in the valley floors of Normandy, for example, where a dense technical system was implemented in the Carolingian period. There was a mill every 1,300 m by the end of the 18th Century, along a practical linear river with a combined length of 3,900 km, not counting 1,350 km of artificial reaches that supplied the works [LES 15]. 9.4.2. River channel containment Dikes protecting floodplains from inundations date back a long way, even as far as the 12th Century in France, as is the case for banks of the Loire, or the Camargue. Containment of high-energy rivers that were more complex to control (particularly in Alpine valleys) began in the 16th Century and cut through cities. It was systematized from the end of the 18th Century with the aim to protect land and to ensure the development of means of transport (roads first, then railways). The following example, among many others, illustrates why engineers suffered setbacks. Approved in 1780, reconfigured in 1813 and, in 1820 before works began, “general” containment of the Arve failed, and would remain a failure throughout the 19th Century. In addition to financial difficulties, the main reason behind the failure was the lack of knowledge of river dynamics, which at the time was approached from a strongly empirical perspective. If longitudinal dikes were effective due to the fact that they encouraged evacuation of bedload in sections with steep slope, they also caused infilling of all sections of the channel with reduced slope (plains of Sallanches and Bonneville). This was followed by increases to the water level, “filtering” toward neighboring land covered by marshes, and dangerous overbank flows inciting strong local opposition [CHO 25]. Similar responses were recorded in high-energy floodplain streams, and responses recorded in the Rhine River between Basel and Mannheim became infamous. Even though General J. Le Michaud d’Arçon’s (1784) project appeared to make the most sense, since he proposed to gather the discharge in a series of welldeveloped sinuosities in order to reduce slope and therefore energy, the more radical project proposed by engineer J.G. von Tulla was chosen. Works began between 1817 and 1876 and gathered together previously dispersed water into multiple channels in one single rectilinear channel; the length was shortened by 32 km (14%)

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between Basel and Lauterbourg and 51 km (37%) between Lauterbourg and Mannheim. Von Tulla aimed to incise the river in its alluvium and to reduce flooding, in addition to improving navigation. The incision reached 7 m in 75 years in Istein, a few kilometers downstream from Basel, where a limestone bar that was posing problems for navigation was exhumed from quaternary alluviums. In total, the incision and strengthening of farm levees strongly reduced flooding in the plain of Alsace; a reorganization of the plain was also carried out at the expense of wetlands in order to benefit agriculture [CIO 02]. Holocene evolution Evolution post-correction Post-Würmien profile Subactual profile before corrective works Profile in 1960

Istein Bar

Section

Upstream section (braided)

Middle section (braided and anostomosed)

Downstream section (anastomosed and (Meanders) emerging meanders)

Figure 9.1. Evolution of the Rhine’s long profile in the Alsace–Baden section after containment, implemented in the 19th Century (source: [SCH 16])

Such examples are numerous in valleys with dammed rivers, particularly in the Alps and in their foothills, where strong slopes and artificial narrowing of channels results in excess energy. Up until 1860 (with the exception of 1792–1815), the Isère’s upstream course, or the Combe de Savoie, belonged to the Kingdom of Piedmont-Sardinia. The course from Isère to Grenoble, or Grésivaudan, was French. The Government of Turin pushed to gain land on braided channels and floodplains that were very humid and unsanitary. This led them to build a “non-submersible” dike from Isère, which was contained in a broad channel measuring 112 m upstream of the confluence of the Arc, and 132 m downstream (1829–1845). The upstream slope of the Isère in the Combe de Savoie was narrowed between the dikes (1829– 1848); the channel was widened upstream of the altered section (2–5 m) and accreted downstream (+1.50 m). Hydromorphy worsened in plains adjacent to alluvium sections and the resulting unsanitary conditions were offset by a systematic policy implemented by the State of Piedmont-Sardinia: this was in the form of a “golène” containment designed with dike openings to ensure siltation of the plain by floods loaded with highly concentrated suspended sediment. On the other hand, sections incised due to impact contributed to the development of xeric vegetation,

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which is still present today. In addition, the Isère, in Grésivaudan, was subjected to accretion coupled by an increase in flood levels, and by the same undesirable effects of humidity and unsanitary conditions. The channel was contained between 1845 and 1870, however it was overly narrowed (112 m), which only caused unsanitary conditions to worsen. Drainage of the floodplain, relative drying of gains and the development of extractions enabled the process to be reversed [BRA 93]. This relatively recent occurrence demonstrates the magnitude of changes recorded over a 55 km linear within a 50-year period. The result is a floodplain with strong ecological diversity, particularly created due to the nature of the soils, as well as the degree of hydromorphy experienced [GIR 10]. Objectives of works Construction works Stream 1st degree channel Floodplain impacts

2nd degree impacts

Flood protection

Border

Accretion Flooding, stagnation

Fine aggredation

Malaria

Flood protection

Construction works

2nd degree impacts

Spacing between dikes: 112 m Spacing between dikes: 132 m Plugged basin Drainage network

'Non-submersible' Development of basins and siltation (1845-1865) containment

Objectives of works

Stream 1st degree channel impacts Floodplain

Unsanitary conditions of plain (1845-1865)

'Non-submersible' containment

Upstream impact Accretion, flooding, stagnation

1 and 2

Clean-up, cultivation

Unsanitary conditions of plain

Drainage

1' and 2'

Accretion Stagnation Malaria

Clean-up, cultivation

Figure 9.2. Right: Location of containment works in the Combe de Savoie and the Grésivaudan. Left: Graph showing the multiple impacts of their construction. Figures 1 and 2 show the first two objectives and works carried out in order to achieve them in the Combe de Savoie upstream of the Isère (1825–1860); in light gray, first degree impacts, and in dark gray, secondary impacts (negative: malaria positive: agriculture). Figures 1' and 2' show the same objectives as in the Grésivaudan. Identical to the previous ones (with the addition of drainage), they were necessary due to the very negative impacts downstream of work carried out in the Combe de Savoie; these impacts were delayed, occurring a few decades later (1845–1880) (from [BRA 93])

In summary, longitudinal dikes prevent the channel from correcting itself by increasing its sinuosity. Contracting the channel using dikes increases tractive force,

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or specific power, causing incision often coupled with accumulation of the diked section downstream, with slope reduction being a possible adjustment. 9.4.3. Aggregate extraction The extraction of significant amounts of bedload goes back to the 19th Century, a period where the reloading of pathways and construction of embankments took place in order to lay railway tracks, for example. The second half of the 19th Century saw generalized usage of “aggregate” to manufacture concrete for construction works, and became heavily used in the 1950s in the river channels that flowed close to cities experiencing rapid demographic expansion. The majority of materials were used for non-noble purposes (embankments, for example). From the 1950s onwards, rivers in Northern Italy were strongly sought after for construction of a dense motorway network, and universities on the plain of the Pô became known for their pioneering studies of extraction-related impacts. In Germany, rivers at the Bavarian foothills were also strongly affected during this same period. In the basins of the Rhone, the Arve and the Fier as well as tributaries of the Rhone downstream of Lyon were overexploited to fulfill similar objectives (embankments for highways and power plants, construction, etc.). These practices exploded in developing countries, especially in order to facilitate production of industrial platforms and artificial islands in Southeast Asia, for example in Singapore. The market value of aggregate resources was often high enough to justify extraction of substantial and excessive volumes, which were and continue to be extracted. Coastal rivers paid a high price, particularly due to modifications made to cross-sections and the generally widespread reduction of floodplain inundation. The Gave de Pau’s alluvial plain (Atlantic Pyrenees region) is a good example of the institutional organization that led to extraction in fluvial environments across the Béarn, a small region in Southwest France. Fluvial extraction and alteration are not the result of spontaneous and disorderly interventions, but the result of wellreasoned policy. Causes of extraction were directly linked to research carried out on the reduction of flood-related risks. By the late 1960s, the management in charge of the Atlantic Pyrenees region consulted with State authority to organize the extraction of aggregate in stream channels to enable the region to benefit as much as possible from favorable economic impacts. The chosen method was to intercept a significant amount of solid flux descending from the Pyrenees, and to extract much more than the natural flux in a bid to lower the river channel, controlled by weirs and bank defenses, by several meters. A management strategy was drawn up by an engineering office with experience in the field. Lowering floodwater levels and narrowing braided streams would have positive impacts, the reduction of floods enabling the acquisition of agricultural land and space for transportation routes, business and urban and peri-urban growth in the town of Pau. The report published

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by [SOG 74] addressed to the Atlantic Pyrenees Departmental Equipment Directorate stipulated that this situation (i.e. braiding) had been accepted by residents, although it contradicts all land development of the “Saligue” alluvial forest (on the subject of shifting of the Gave) and “this is clearly beneficial, and is an important argument in favor of extraction development” (on the subject of incision of the Gave, which had already reached 3 m over the period of 1921–1973, of which 1 m in 1973 alone). Economic sectors may have benefited from these public policies, notably public works. However, the negative impacts cannot be ignored. First of all, no one was aware of impoundment conditions of the floodplain for the 100-year flood; due to absence of an up-to-date model, the flood of 1952 was the most recent to be modeled. In addition, the Ministry of the Environment criticized the ecological impacts caused by groundwater depletion, as a result of rivers incising their bed, in the mid-1980s, particularly the degradation of the “Saligue” alluvial forest; the loss of a part of the Gave’s groundwater resources represents a significant economic loss that will one day be quantified. Economic interests preferred at the end of the 1960s continue to take precedence today, and riverbed gravel mining remains the official policy governing the supply of aggregate in the region, despite strong beliefs in favor of a policy integrating the forest. On a completely different geographical scale, that of a very large river, the Mekong provides an exemplary case study. Like practically all of Asia’s coastal rivers today, the lower Mekong was considered to be a practically inexhaustible resource, or at the very least, endlessly exploitable by riparian countries. Sand, and to a lesser extent, fine gravel, were required by Singapore who were extending their artificial islands, and by the Vietnamese for construction. Extraction was locally justified due to impacts considered to be positive in the improvement of navigation conditions, and on flooding conditions of the capital, Cambodia, Phnom Penh, built on the Mekong River’s right bank close to the fluvial diffluence, that fills and empties the Tonlé Sap Lake in each monsoon season. On the same subject, in 2012 the Cambodian Prime Minister, Hun Sen, stated to the press: “We should use the river to save the river.... We must think of the River as a whole. If we do not solve these issues, we do not know what will happen in the future”. A certain vision of integrated planning. A part of the Mekong River’s mean yearly flow (350 km3) fostered an exceptional fluvial-lake ecosystem, as well as fishing conditions. Suspended sedimentary flow, that for decades has been estimated at 150–170 millions tons of silt and clay per year (sand is measured in small quantities), seemed to ensure sustainable extractions, accretion of new neighborhoods in Phnom Penh, and comfortable income for some. Two recent studies conducted for WWF Greater Mekong and IKPM (Mekong River Commission) enabled progress to be made in the

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development of sediment balance, taking into account extraction volume that was previously unknown, and actual sediment transport, without doubt affected by construction of dam reservoirs on the Lancang, the Chinese Mekong, since 1994 [KOE 12, BRA 13]. The actual sedimentary flow would be less for clay loam parts (effect of upstream retention) with an annual flow of 77 Mt/year, but higher for sandy parts, for which bedload or saltation transfer had never been measured (1–4 Mt/year). Estimates concerning extraction downstream of the Mekong River from China, which were based on declarations, and as a result significantly underestimated compared to reality, gave a total of 34.5 million m3 or equally 56–57 million tons per year. Practical impacts were that sand extraction was much higher than the natural flux, the river experienced bed degradation upstream of its delta, its connection with the Tonlé Sap, and nutrient supply was reduced. Furthermore, groundwater subsided and the huge floodplain that formed the alluvial plain of Cambodia and the Mekong delta were less able to maintain the fertility of agricultural land, the preservation of coastal sand ridges and fertility of ocean waters. 9.5. Changes to long profiles and flooding conditions due to dam reservoirs Dams are designed to retain all or some of the bedload and suspended load by reducing velocity of impounded water. A river’s long profile may experience multiple changes [BRA 00]. 9.5.1. Backfilling of impoundments One of the first impacts, both sedimentary and hydraulic, is the partial backfilling of impoundment and, in some cases, accretion upstream of the impoundment due to deposition of bedload in the hydraulic eddy formed during a flood (this is due to reduced flow velocity upstream of impounded sections). As a result, greater frequency of river floods and increased hydromorphy are generally observed, which may constrain agriculture upstream of impoundment, even if deposition of suspended matter partly offsets this process due to siltation of the floodplain [BRU 86]. On the other hand, rising water levels and their stabilization at a high level, which results in increased hydromorphy of soils on the alluvial plain, can have beneficial environmental effects, such as promoting plant formations linked to the presence of floodwaters, as well as an increasing the denitrifying effects that hygrophilous vegetation provides. Deltas formed by river tributaries transformed into impoundments present similar problems. For example, on the Hoang Ho River (North China), sedimentation in the Sanmexia impoundment (1960) also affected its right bank tributary, the Ho Wei, which converged with the

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stream channel at the top of the impoundment, near the city of Tongguan. The dam was not equipped with bottom valves to enable the evacuation of sediments. In the first 2 years following the dam’s initial operation, deposits in the Sanmexia impoundment reached 1.7 Gt in 1960, 43% of the reservoir’s capacity. The length of the area affected by the deposition and upwelling along the Wei Ho River was estimated at approximately 135 km, and accretion at over 4 m. Changes made to the flushing procedure at the Sanmexia Dam partially solved the problem, but extensive dredging of the accreted channel and erection of huge dikes along the Wei Ho River were required to achieve this [WU 07, ZHE 15]. 9.5.2. Widening channels downstream of dams Widening of channels and increasing the hydraulic section downstream of the dam are often one of the secondary impacts of sediment retention and bedload deficit downstream. The development of these impacts can be curbed in accordance with new hydrology activated by the dam (reduction and modulation of outflow). They can also be curbed by (i) lateral or vertical erosion downstream of the dam and transfer of materials in accreted sections, (ii) sediment inflow from loaded tributaries that can no longer be carried downstream by the main river and (iii) massive sedimentary transfers related to flushing, such as the process carried out at the Sanmexia Dam; hydromorphological impacts recorded downstream of dams are particularly complex in terms of filling of the floodplain. Geomorphological changes depend on variable liquid flow, sedimentary input and on transport capacity of the flow; they are modulated in their extension downstream and implementation speed of the adjustment [BRA 00]; on the one hand, the cross-section is disrupted across variable distances, and on the other hand, flood hydrology is itself impacted by frequently organized retention in the impoundment (although this is not systematic). Each flood event must be considered on a case by case basis, depending on the combination of local variables and their individual dynamics. 9.5.3. Case studies The Upper-Rhone in Chautagne, France, downstream of Geneva, with annual module of 350 m3/s, provides a good illustration of the impacts of erosion downstream of a modestly sized European river. In European rivers, evolution is so old that phenomena induced by dam construction are rarely “pure”, as can be the case in “newer” or developing countries. Dikes were installed on the left bank of the Rhone toward 1780 in order to contain braided accretion and to protect large alkaline marshes on Chautagne. Dikes were installed on the right bank to protect a railway track (1858). Hydroelectric works were constructed on the river between 1870 and 1925, but they only managed to block a small amount of bedload in the

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run up to 1943 (Verbois) and 1948 (Genissiat); works along the Fier, a major left bank tributary, also had relatively little effect. Maximum of the impacts on bedload is due to gravel extraction in tributaries and in the river. By the time the hydropower facility by-passing Chautagne (Motz Dam and Anglefort plant) was completed in 1980, the channel had already been incised, shrunk and the bottom was paved with torrential blocks. Implications of containment consisted of a theoretical elevation of floodwater depth, but this was largely offset by incision, enabling factories to be built on the floodplain located upstream of the left bank. Total interruption of input of bedload to the “Old Rhone” (section by-passed by an 8 km stretch of power canal) from 1980 onwards resulted in significant reactivation of incisions in fragile sections and a heavy reworking of the long profile of the bottom during the 100-year flood of February 1990. On the one hand, the channel was further incised upstream of the old Rhone, and on the other hand, pebbles accumulated 4.5 km downstream of the Motz Dam, in an area of low specific power (in a wider section); this accumulation caused floodwaters to rise in the channel along with water in the marshes, cutting across the Turin-Lyon train line. This impact was partially offset by dredging and rising of materials under the dam, but the old dike upstream was under threat. In order to contain the morphological evolution of a high-energy river while attempting to avoid changing flood conditions requires constant vigilance, coupled with quick and costly offsetting, without which balance is achieved [KLI 98]. In China, the Yangtze River is a remarkable example of the impacts experienced downstream of the Three Gorges Dam. Partially filled in 2003 and completely filled by 2009, the reservoir of the Three Gorges Dam altered both the hydrological and sedimentary cycle downstream of the Yangzi River (Jingjiang). On the one hand, the “artificialized” flow shows a certain amount of regularization of the seasonal cycle, with an increase in discharge during periods of low water levels notably during winter months (0.20–2.10 m), and a reduction in flood discharge in monsoon and autumnal periods (0.70–3.35 m). On the other hand, reduction of sedimentary input in the downstream section due to the impact of retention caused by impoundment (65–85% of the solid discharge), resulted in immediate incision of the sandybottomed channel of the Yangtze River (loss of one billion m3 from 2002 to 2010). Systematic comparisons of cross-sections between 2002 and 2013 showed that bankfull widths experienced little change due to coating of the embankment, but also showed that the stream channel increased in depth from 1.0 to 1.60 m [XIA 16]. From 2009 to 2012, incision of the channel cut seasonal winter and spring water levels in half; it also reduced the blocking effect the river had on lateral lakes (Lake Dong and Lake Dongting-Hu) in the spring as well as the potential benefit of rising water levels in winter [WAN 93]. Flood management in the Jingjiang was highly dependent on Lake Dong being filled by riverine floods (a vast complex of river channels and lakes acting as natural flood storage areas located on the right bank of the Yangtze River, between Wuhan and Ychang). Filling of the lake took place naturally, and it is now controlled by three spillways. In just 6 years (2008–2014),

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the reduction of the river’s water levels during flooding at equal flowrate (due to incision) reduced discharge drifting downstream toward the lake in Upper Chang Jiang; in addition, sedimentary deposits impacted spillway sections and helped reduce derivated flow. Moreover, agricultural pressure on the bottom of the Lake Dong in dry season damaged the effectiveness of this natural storage area. The relative increase in flood discharge and depth in the Yangtze River (at equal flowrate) increased pressure downstream. Disruptions recorded in the channel of the Yangtze River are expected to continue, and attenuating and delaying effects on floods are expected to reduce further [LI 16]. 9.6. Conclusion: numerous implications for floodplains Wyzga et al. [WYZ 16] highlights that incision of a channel must be coupled with lowering of water levels at bankfull stage, so that the process of incision can be fully qualified. River incision increases channel capacity and reduces the frequency and extent of overflow and volumes stored in the floodplain; on the other hand, the same parameters increase downstream of the diked section, i.e. cause flood risk to increase. Incision has multiple ecological effects that are well identified but complex in terms of priority, as the economic benefits of incision generally seem to take precedence over other considerations [BRA 97]. Local facilities are justified in terms of reducing flood risk or improving economic development, but these justifications fail to take into account negative impacts that occur downstream. Occasionally, public policies advocating integration across basins are flawed. In contrast, the impacts of large hydropower facilities, such as the examples provided of the Hoang Ho and Yangtze rivers, have been underevaluated and/or denied, and we are now discovering the magnitude of problems to be solved, as was previously the case in the United States. 9.7. Bibliography [BER 03] BERGER J.-F., “Les étapes de la morphogenèse holocène dans le Sud de la France”, in VAN DER LEEUW S., FAVORY F., FICHES J.-L. (eds.), Archéologie et systèmes socioenvironnementaux. Etudes multiscalaires sur la vallée du Rhône dans le programme ARCHAEOMEDES, Monographies CRA 27, Paris, 2003. [BRA 00] BRANDT S.A., “Classification of geomorphological effects downstream of dams”, Catena, vol. 40, pp. 375–401, 2000. [BRA 14] BRAVARD J.-P., “La Vienne antique et le Rhône”, in PROVOST M. (ed.), Carte Archéologique de la Gaule, Vienne, vol. 38, no. 3, pp. 39–59, 2014. [BRA 97] BRAVARD J.-P., AMOROS C., PAUTOU G. et al., “River incision in south-east France: morphological phenomena and ecological effects”, Regulated Rivers Resources and Management., vol. 13, pp. 1–16, 1997.

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[BRA 13] BRAVARD J.-P., GOICHOT, GAILLOT S., “Geography of sand and gravel mining in the Lower Mekong River”, Echogéo, vol. 26, pp. 2–18, available at: http:// echogeo.revues.org/13659, 2013. [BRA 99] BRAVARD J.-P., KONDOLF G.M., PIEGAY H., “Environmental and societal effects of channel incision and remedial strategies”, in DARBY S.E., SIMON A. (eds), Incised River Channels, Wiley, New York, 1999. [BRA 93] BRAVARD J.-P., PEIRY J.L., “La disparition du tressage fluvial dans les Alpes françaises sous l’effet de l’aménagement des cours d'eau (19-20e siècle)” in DOUGLAS I., HAGEDORN J. (eds), Geomorphology and Geoecology. Zeitschrift für Geomorphologie, Supplt Bd, Stuttgart, 1993. [BRU 86] BRUK S., Méthode de calcul de la sédimentation dans les réservoirs, UNESCO, Paris, 1977. [BUR 91] BURNOUF J., GUILHOT J.-O., MANDY M.O. et al. (eds), Le pont de la Guillotière. Franchir le Rhône à Lyon, Documents d’Archéologie en Rhône-Alpes no. 5, Circonscription des Antiquités Historiques, 1991. [CHO 25] CHOLLEY A., Les Préalpes de Savoie et leur avant-pays, Armand Colin, Paris, 1925. [CIO 02] CIOC M., The Rhine. An eco-biography, 1815-2000, University of Washington & Weyerhauser Environmental Books, Seattle & London, 2002. [DUN 78] DUNNE T., LEOPOLD L.B., Water in Environmental Planning, W.H. Freeman, New York, 1978. [FRA 07] FRANC O., VEROT-BOURRELY A., BRAVARD J.-P., “Géographie et géo-archéologie du site de Lyon”, in LE MER A.-C. & CHOMER C. (eds), Lyon, Carte archéologique de la Gaule, 69/2, Acad. des Inscriptions et Belles lettres, Paris, 2007. [GIR 10] GIREL J., “Histoire de l’endiguement de l’Isère en Savoie : conséquences sur l’organisation du paysage et la biodiversité actuelle”, Géocarrefour, vol. 85, no. 1, pp. 41–54, 2010. [KLI 98] KLINGEMAN P.K., BRAVARD J.-P., GIULIANI Y. et al., “Hydropower reach bypassing and dewatering impacts in gravel-bed rivers” in KLINGEMAN P.C., BESCHTA R., KOMAR P. et al. (eds), Gravel Bed Rivers in the Environment, Water Resources Publications, Littleton, 1998. [KOE 14] KOEHNKEN L., Discharge Sediment Monitoring Project (DSMP), 2009-2013. Summary & analysis of results, Final Report, Phnom Penh, MRC-IKPM, 2014. [KOZ 77] KOZARSKI S., ROTNICKI K., “Valley floors and change in channel patterns in the north Polish plain during the late Würm and the Holocene”, Quaestiones Geographicae, no. 4, pp. 51-93, 1977. [LES 15] LESPEZ L., VIEL V., ROLLET A.J. et al., “The anthropogenic nature of present-day low energy rivers in western France and implications for current restoration projects”, Geomorphology, no. 251, pp. 64–76, 2015.

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[LI 16] LI Y., YANG G., WAN R., DUAN W. et al., “Quantifying the effects of channel change on the discharge diversion of Jingjiang Three Outlets after the operation of the Three Gorges Dam”, Hydrology Research, vol. 47.51, pp. 161–174, 2016. [PIC 14] PICHARD G., ROUCAUTE E., Sept siècles d’histoire hydroclimatique du Rhône d’Orange à la mer (1300-2000). Climat, crues, inondations, Presses Universitaires de Provence, Aix-en-Provence, 2014. [SCH 16] SCHMITT L., HOUSSIER J., MARTIN B. et al., “Paléo-dynamique fluviale holocène dans le compartiment sud-occidental du fossé rhénan (France)”, Revue Archéologique de l’Est, vol. 42, pp. 15–33, 2016. [SCH 77] SCHUMM S.A., The Fluvial System, Wiley, New York, 1977. [SOG 74] SOGREAH, Etude générale du Gave de Pau entre Coaraze et Orthez, Thesis, DDE Pyrénées Atlantiques, Pau, 1974. [STA 83] STARKEL L., “The reflection of hydraulic changes in the fluvial environment of the temperate zone during the last 15,000 years”, in GREGORY K.J. (ed.), Background to Palaeohydrology, New York, 1983. [WAN 13] WANG J., SHENG Y., GLEASON C.J. et al., “Downstream Yangtze River levels impacted by the Three Gorges Dam”, Environmental Research Letter, no. 8, 044010, pp. 1–9, 2013. [WU 07] WU B., WANG G., XIA J., “Case study: delayed sedimentation response to inflow and operations at Sanmenxia Dam”, Journal of Hydraulic Engineering, no. 133, pp. 482–494, 2007. [WYZ 16] WYZGA B., ZAWIEJSKA J., RADECKI-PAWLIK A., “Impact of channel incision on the hydraulics of flood flows: Examples from Polish Carpathian rivers”, Geomorphology, no. 272, pp. 10–20, 2016. [XIA 16] XIA J., DENG S., XU Q., ZONG Q. et al., “Dynamic channel adjustments in the Jingjiang Reach of the Middle Yangtze River”, Nature Scientific Reports, vol. 6, no. 22802, p. 10, 2016 [ZHE 15] ZHENG S., WU B., THORNE C.R. et al., “Case Study of Variation of Sedimentation in the Yellow and Wei Rivers”, Journal of Hydraulic Engineering, no. 141, p. 10, 2015.