Sedimentation problems in river basins

(1)

Sedimentation problems in river basins

Project 5.3 of the

International Hydrological Programme

Report prepared under the

Chairmanship of A. Sundborg

Edited by W. R. White

(2)

The designations employed and the presentation of material throughout the publication do not imply the expression of any opinion whatsoever on the part of Unesco concerning the legal status of any country, territory, city or area or of its authorities, or concerning

the delimitation of its frontiers or boundaries.

Published in 1982 by the United Nations Educational, Scientific and Cultural Organization, 7, place de Fontenoy, 75700 Paris

Printed by

Imprimerie Sodexic, Chartres ISBN 92-3-102014-s

Q Unesco 1982 Printed in France

(3)

Studies and reports in hydrology 35

(4)

Recent titles in this series:

20.

21.*

22.

23.

24.

Hydrological maps. Co-edition Unesco- WMO.

World catalogue of very large floods/Repertoire mondial des tres fortes trues.

Floodflow computation. Methods compiled from world experience.

Water quality surveys.

25.

26.

27.

28.

29.

30.

31.

Effects of urbanization and industrialization on the hydrological regime and on water quality. Proceedings of the Amsterdam Symposium, October 1977/Effets de I’urbanisation et de I’industrialisation sur le regime hydrologique et sur la quahte de I’eau. Actes du Colloque d’Amsterdam, octobre 1977. Co-edition IAHS-UnescojCotdition AISH-Unesco.

World water balance and water resources of the earth. (English edition).

Impact of urbanization and industrialization on water resources planning and management.

Socio-economic aspects of urban hydrology.

Casebook of methods of computation of quantitative changes in the hydrological regime of river basins due to human activities.

Surface water and ground-water interaction.

Aquifer contamination and protection.

Methods of computation of the water balance of large lakes and reservoirs.

Vol. I Methodology Vol. II Case studies

32. Application of results from representative and experimental basins.

33. Groundwater in hard rocks.

34. Groundwater models.

35.

Vol. I Concepts, problems and methods of analysis with examples of their application.

Sedimentation problems in river basins.

* Quadrilingual publication: English - French - Spanish - Russian.

(5)

Preface

Although the total amount of water on earth is generally assumed to have remained virtually constant, the rapid growth of population, together with the extension of irrigated agriculture and industrial development, are stressing the quantity and quality aspects of the natural system. Because of the increasing problems, man has begun to realise that he can no longer follow a "use and discard" philosophy -- either with water resources or any other natural resource. As a result, the need for a consistent policy of rational management of water resources has become evident.

Rational water management, however, should be founded upon a thorough understanding of water availability and movement. Thus, as a contribution to the solution of the world's water problems, Unesco, in 1965, began the first world-wide programme of studies of the hydrological cycle -- the International Hydrological Decade (IHD). The research programme was complemented by a major effort in the field of hydrological education and training. The activities under- taken during the Decade proved to be of great interest and value to Member States. By the end of that period, a majority of Unesco's Member States had formed IHD National Committees to carry out relevant national activities and to participate in regional and international co- operation within the IHD programme. The knowledge of the world's water resources had substan- tially improved. Hydrology became widely recognised as an independent professional option and facilities for the training of hydrologists had been developed.

Conscious of the need to expand upon the efforts initiated during the International Hydro- logical Decade and, following the recommendations of Member States, Unesco, in 1975, launched a new long-term intergovernmental programme, the International Hydrological Programme (IHP), to follow the Decade.

Although the IHP is basically a scientific and educational programme, Unesco has been aware from the beginning of a need to direct its activities toward the practical solutions of the world's very real water resources problems. Accordingly, and in line with the recommendations of the 1977 United Nations Water Conference, the objectives of the International Hydro- logical Programme have been gradually expanded in order to cover not only hydrological processes considered in interrelationship with the environment and human activities, but also the scientific aspects of multi-purpose utilisation and conservation of water resources to meet the needs of economic and social development. Thus, while maintaining IHP's scientific

concept, the objectives have shifted perceptibly towards a multidisciplinary approach to the assessment, planning, and rational management of water resources.

As part of Unesco's contribution to the objectives of the IHP, two publication series are issued: "Studies and Reports in Hydrology" and "Technical Papers in Hydrology". In addition to these publications, and in order to expedite exchange of information in the areas in which it is most needed, works of a preliminary nature are issued in the form of Technical Documents.

The purpose of the continuing series "Studies and Reports in Hydrology" to which this volume belongs, is to present data collected and the main results of hydrological studies, as well as to provide information on hydrological research techniques. The proceedings of sym- posia are also sometimes included. It is hoped that these volumes will furnish material of both practical and theoretical interest to water resources scientists and also to those involved in water resources assessments and the planning for rational water resources management.

(6)

Introduction ... Study Programme. ... Land Systems Mapping ... Geomorphological Mapping of the Reservoir Area .... Rainfall, Vegetation and Soil Erosion. ... Evaluation of Present Soil Erosion Processes ... Sediment Transport and Probable Future Sedimentation in the Reservoir ... Main Conclusions from the Sedimentation Studies .... The Use of a Mathematical Model to Simulate Long-Term Changes in Bed Levels in Estuaries ... Introduction ... Great Ouse Estuary ... The Model. ... Results ... The Use of a Scale Model to Study Engineering Problems Associated with the Construction of a Large Dam. ... Introduction ... Description of the Model ... Protection of Cofferdams during Construction ... River Diversion. ... Spillway ... Quantification of Soil Erosion or Sediment Transport Caused by Isolated Storms in a Semi-Arid Region .... Available Data ... Results ... Conclusions ... ... ¹¹⁷

... ¹¹⁷

... ¹¹⁸

... 118

... ¹²⁰

... ¹²¹

. . . . . . . . . . . - .- 121

* . . . - . ..- .- - 121

. . . ..-..-...-. 121

..*---....-... 122

_ _ _ m. _ . ..m .a . . 122

. . ..-.-.-.- .- . 126

. . . ..- . . . 126

. . . *... - - 126

. . . - . . . - 126

___.* . . . . ..- -. 129

.mm ..m..- ..- .- 129

. ..*...-.-. - 129

..-..--...--.. 130

.-...*.- - .- 130

. . . . . ..e _ e.... 133

(9)

Foreword

This technical report on "Sedimentation problems in river basins" was prepared in response to a decision of the Intergovernmental Council of the International Hydrological Programme (IHP) which, in 1975, appointed a special Working Group, under Project 5.3, to this effect.

The terms of reference of the Working Group were to prepare a report on the state of knowledge of the relationships between plant cover, surface runoff, sediment production and deposition and the possibilities of improving the existing sedimentation conditions, the influence of land basin use practices and structural works on sediment production and the influence of structural works on river channel erosion and sedimentation. The report was to provide a description of recommended methods to estimate and predict sedimentation, sediment transport and erosion processes quantitatively, with examples.

The Working Group, which met in three sessions (June 1976, September 1977 and February 19801, was composed of the following experts:

Mr J A Maza (Mexico) Mr E L Pemberton (USA) Mr V V Romanovsky (USSR) Mr A Sundborg (Sweden),Chairman Mr W R White (United Kingdom)

The Food and Agriculture Organization (FAO), the International Association for

Hydrological Sciences (IAHS), the International Association of Hydraulic Research (IAHR), the International Commission on Irrigation and Drainage (ICID) also participated in the meetings of the Group and provided material for the technical report.

In preparing and writing the technical report, the Working Group took into consideration the activities under IHP Project 3.8, "Study of river sedimentation processes" and those carried out by the International Commission on Erosion and Sedimentation of IAHS on measurement methods and predictive techniques.

Final editing of the report was carried out by Mr W R White (United Kingdom).

10

(10)

Introduction

Erosion and sedimentation processes are parts of the geological evolution of landscape.

Erosion of the earth's surface by the action of water, wind, ice and waves has occurred

through the ages. Entrainment, transport and deposition of the material are natural processes visible any time and everywhere. These processes have together through geological time shaped and remodelled the earth's surface with the creation of mountain gorges, river valleys, flood plains, deltas, coastal plains and other landscape elements. The erosive agents considered most significant are rainfall, flowing.water and wind. The actions of waves and of ice or glaciers are limited to restricted environments but most important in coastal regions and glacial environments. Avalanches, landslides, volcanic eruptions, and earthquakes may locally cause abrupt and catastrophic changes of the landscape.

The erosion process by water begins with the initial raindrop impact. Mineral or rock particles are detached and moved a short distance, some are entrained in the flowing water and transported further away. Thus the grains of sediment have started their movement from the mountains to the sea. The particles may be temporarily deposited in a valley flood plain during a flood, then eroded away thousands of years later and swept downstream to be redepo- sited to await the next hop in this sedimentation process. Rivers and river processes play a significant role in landscape evolution. The rivers can be looked upon as immense transportation systems for water and sediments. They evacuate the sediment products from the river basins, thus promoting further weathering and sediment production.

In all parts of the earth and down through the ages, man has had to deal with problems associated with the movement of sediment, whether it be difficulties arising from erosion or deposition on agricultural lands or on lands selected for homesites in cities and towns. In recent years man has become more cognizant and concerned with erosion, transport and deposition of sediments relative to his complete environment. Just how these processes affect

wildlife, fisheries, and the biological life in this environment is becoming of greater concern.

Therefore, the study and knowledge of the complete sedimentation cycle of detachment, entrainment, transportation, deposition, and consolidation of sediments are the important first steps in defining methods for estimating man's activities on sedimentation processes in river basins. It is also important that catastrophic events such as volcanic eruptions and earthquakes be recognized and evaluated because they may suddenly produce severe problems of soil erosion and sedimentation.

The utilization of rivers,their water resources and sedimentary deposits often causes technical and environmental problems. Irrigation schemes, hydropower plants, the withdrawal of water for industrial and domestic uses, the industrialization and urbanization of riverine landscapes change the natural conditions. This may result in accelerated sedimentation

processes, river migration and channel-regime changes, and many other undesirable effects.

The understanding of the erosion and sedimentation processes requires a basic knowledge of some fundamental concepts and features. Many Of the patterns of erosion visible on the earth's surface can be defined in quantitative, often mathematical terms. Channel gradients, for instance, are comparable in different environments as they are functions of several

physical parameters such as water discharge, sediment load and sediment characteristics. The patterns of river meandering and of drainage channels created by rainfall-runoff are to some extent constant and predictable. These similarities occurring in natural stream channels of

(11)

water working on lands lead to a quantitative approach, whereby application of physical principles and mathematical relationships can be used for estimating some of the natural sedimentation processes in river basins. These same principles and relationships with modification factors can also be applied for estimating sedimentation changes directly

resulting from man's activities, taking into account the effects of vegetation and animal life.

To help achieve and establish a more uniform understanding of the processes, a list of definitions of terms associated with erosion and sedimentation is presented in Appendix 7.2 as used throughout this report.

The main objectives of this report are:

(1) To define the effects of man's activities on erosion and sedimentation processes in river basins, including activities such as land use and irrigation and drainage practices, and also the effects of reservoirs and dams, river control works, and interbasin transfers of water.

(2) To inform about the state of knowledge of the cause and effect relationships between plant cover, surface runoff, sediment production and deposition.

(3) To inform about possibilities for the amelioration of the existing sedimentary conditions.

(4) To describe methods for the estimation and prediction of changes in erosion and sedimentation processes following man-made changes in the river basin. This includes changes in the watershed as well as in the river channels.

Although it is recognized that sediments may play an important role in the transport of nutrients and contaminants from the land surface into water bodies and that accelerated erosion has important implications for water pollution from non point sources, the scope of this report has been restricted to a consideration of the physical processes of sedimentation.

It is intended to give general information for planners of water resources and land use and for ecologists and will be of interest to hydrologists and hydraulic engineers. The text is comprehensive but necessarily superficial in many respects. The use of mathematical

formulae and descriptions has been limited to the minimum in the main text. To illustrate sedimentation processes and methods of prediction case studies with worked out examples are included in the report. For detailed treatment of the problems discussed in the report the reader is referred to textbooks and manuals mentioned in the bibliography.

12

___--._-_ .-_I____-~-

(12)

1 Influence of man’s activities on sedimentation processes

1.1. INTERFLUVIAL AREAS

1.1.1. Introduction

Erosion is the detachment of soil and rock fragments or particles from their initial resting place by water and other geological agents such as wind, waves and ice. Thus erosion is a natural geological process that has modelled the earth's surface through geological ages.

This natural process is often referred to as geological erosion.

Different types of man's activities - agricultural practices, forestry, grazing, road and building construction, etc. - tend to affect the erosion processes, often considerably accelerating the rate. The erosion process induced by man is normally called accelerated erosion or soil erosion.

In most inhabited regions of the world erosion and sedimentation processes are strongly affected by man. In many environments soil erosion induced by man is the predominant erosion process, whereas the natural geological erosion is of secondary importance only.

The relation on the global scale between geological erosion and man-induced soil erosion is not very well known. Some scientists have estimated the present rate of erosion to be about two and a half times the rate before man started to affect the landscape on a large scale

(Focus on Environmental Geology 1976, p 162). This figure must, however, still be considered as rather hypothetical. In many smaller study areas detailed investigations have been made that give a reasonably correct idea about the relative importance of the two types of erosion.

A striking example of the influence of man on erosional processes is given by Trimble (1974) in a study in the Piedmont area, USA. At the time of the European settlement in the 1700s geological erosion was slight and man-induced soil erosion was practically nil. After the clearing and cultivation of uplands, especially in the latter part of the 1800s and early 19OOs, gullies were formed, slopes were severely eroded, channels and ponds were filled with sediments, and fertile bottomlands became back-swamp land. In the middle of the 1900s soil conservation measures resulted in reduced upland erosion. As a result of the decreased sediment yield the stream channels degraded, but new sedimentation problems were created downstream, in reservoirs and estuaries. The development is schematically illustrated in figure 1.1.

Similar landscape and soil deterioration can be observed today in many regions all over the world, often in their earlier stages of evolution. In recently cultivated areas the soil erosion process is just in the beginning of its activity.

The importance of land use and vegetation cover can be demonstrated by the results of soil erosion tests on ground with different vegetation covers in Tanzania (figure 1.2, redrawn from Staples 1938). The test results in the diagram are not representative for all areas with similar types of vegetation, but they are still indicative of the large variations between different environments and different types of land use.. A careful evaluation of the effects of changes in land use and different agricultural practices is a necessary step in every soil erosion and sedimentation study.

(13)

Fig. 1.1 Generalised evolution of the Piedmont landscape 1700-1970. A. At the time of European settlement. Note hickory forest scattered with pine. B. After the clearing and erosive cultivation of uplands. Note gullies, valley fill of recent sediments, and back-swamp areas. C. After the checking of erosion. Regrowth of forests mostly pine, drained back-swamps, incised stream, conservation practices.

Redrawn from Trimble, 1974.

Fig. 1.2

14

0 tons 0.4%

UNGRAZED THICKET SOIL LOST BY EROSION,

TONS PER ACRE WATER LOST RY RUNOFF

w

PERCENT OF RAINFALL EXPLANATION :

MILLET

BARE FALLOW

26.0%

50.4 7.

Results of soil erosion tests on ground with different vegetation cover

3 at Mpwapwa, Tanzania. Annual average of two years records in erosion plots of 50 m area of red sandy loam soil on a pediment slope of 3.5O gradient. The grass cover effectively prevented loss of soil and water. Data from Staples 1938 (from Rapp-Berry-Temple, 1973).

(14)

1.1.2 Rural Environment 1.1.2.1 Land use and planning

As has already been illustrated in figure 1.1 the rate of soil erosion is closely related to the type of land use. It is generally accepted that the erodibility of land is influenced more by land management than by any other activity involved. Both land use in general and crop management in particular are of great importance for the progressing erosion processes. There-

fore, a proper land use planning is one of the most important procedures to avoid devastating soil erosion.

A proper land use planning aims at the selection and development of the ground for different purposes to accomplish an optimal use of all land resources without serious environmental effects, e.g. soil erosion. A programme for land use planning normally involves a land capability survey or classification which shows the suitability of the land to be used for different crops and crop managements in particular. A land capability classification includes studies of many different parameters, for instance thickness of soil cover, soil texture, permeability, soil erosion conditions and other soil characteristics.

The analyses of the data from the land capability studies may result in specific program- mes for different activities as:

1 Soil and water conservation

2 Watershed and catchment management 3 Water pollution control

4 Grazing control and vegetation restoration 5 Wildlife conservation

6 Restoration and reclamation research etc.

1.1.2.2 Agricultural practices

Soil erosion by water or wind is facilitated by the lack of vegetation. A cover of trees, bushes, grass or other vegetation prevents the rain from beating the ground, reduces splash erosion, increases infiltration and decreases surface run-off. Therefore, the clearing of natural vegetation and bringing land under cultivation will inevitably increase the risk of soil erosion by water or wind or both.

Compared to natural conditions most agricultural practices imply a substantial reduction of vegetation cover, at least during part of the year, including loss of surface litter and content of humus. In its original state the content of humus is related to climatic factors:

temperature and humidity. High temperatures and arid conditions are unfavourable for the main- tenance of high humus content. Especially in tropical and subtropical regions with low rainfall, cultivation tends to reduce the content of humus, and the erodibility is increased.

Agricultural practices will also have other effects on soil and ground surface, e.g.

physical changes of the soil, including destruction of soil aggregates - which makes the surface more erodible by wind and water - sealing of the surface by silt and clay particles, compaction of the soil and the creation of new drainage channels for surface run-off. Together with the reduction of retarding vegetation all these effects will contribute to the lowering of infiltration capacity, the increase of surface run-off and the concentration of the flow in space and time. The result is subsequent increase in raindrop erosion, sheet erosion, rill and gully erosion, in some regions also accompanied by wind erosion.

There are great variations in the erodibility of agricultural land. Erosion depends upon what crop is grown, but also on how it is grown. The density of vegetation and the state of nutrition are important. Maize (corn), tobacco and cotton often leave great parts of the ground bare and open for splash erosion, sheet wash and rill erosion, especially when the plants are non-mature. Only during the growing season or, strictly speaking during its latter part, are the fields properly protected against erosion.

The gradient of slope directly influences the flow velocity of run-off and consequently also its erosivity. In East Africa, for instance, there are two types of areas, which are particularly susceptible to erosion (Rapp 1975):

1 Deforested mountains with cultivation of steep slopes.

2 Semi-arid savanna lands and'other drylands with sparse vegetation cover at the end of the dry season.

In these two critical types of environment the main human over-exploitation activities leading to soil erosion and loss of productivity are generally summarized under the following practices:

(15)

1 Overgrazing 2 Overcultivation

3 Excessive collection of firewood, and large-scale charcoal burning 4 Excessive burning of grassland, woodland or forest.

One of the more spectacular forms of man-induced soil erosion is gully erosion. In some areas gullies may assume over-whelming forms and proportions. Their importance in terms of damage to agriculture should, however, not be over-emphasized, as they often appear in regions of moderate agricultural significance. Their main impact on the conditions in the river-basins may often be the large quantities of sediments yielded to the headwater streams;

As discussed in chapter 4 there are several methods to control erosion on cultivated land, both mechanical protection works such as terraces, diversion drains and channels, and biological methods such as the use of grass strips, contour cultivation, crop rotation, etc. (see figure 1.3).

1.1.2.3 Forestry

Due to the protection by the leaf canopy and by the ground cover of litter and vegetation forests and woodland are normally characterized by low surface run-off, high infiltration rates, and insignificant soil erosion. The forest soils often have a relatively porous structure, which facilitates groundwater recharge. However, when the forest cover is removed, the hydrological conditions change, and the risk of soil erosion and mass movement may increase to critical levels (see figure 1.4).

Forests are cleared for many purposes: cutting for pulp and paper industries, for saw- mills and board factories, wood-cutting for local building purposes, production of charcoal, collecting local fuel-wood for heating and cooking, clearing of forests, woodland and bush to extend or improve cultivation and grazing on rangeland, cutting trees or tree-branches for fodder. In critical areas deforestation and local clearing should always be done with the greatest caution to avoid soil erosion (see figure 1.5).

There are numerous reports on erosion hazards and accelerated sediment production from areas with logging activities and deforestation. Megahan (1975) reported that sediment production per unit area in a watershed with logging activities in Idaho, USA, equalled over 150 times the undisturbed sediment production. Most sediment was produced by surface erosion from new roads and road construction works, intended to facilitate logging operations.

In areas of subsistence agriculture wood-cutting for house-building and fuel is often limited to a region within easy walking distance from the villages. Within this radius there is also a large demand for tree and bush vegetation for grazing, foraging, etc. The area is also exposed to concentrated trampling by man and animals. The effect may be disastrous

'erosion, particularly on steep slopes and where, for other reasons, erodibility is high. De- struction of tree and bush vegetation may lead to irreversible damages on soil and vegetation.

Clearing of thorny bush vegetation is sometimes necessary in dry regions to maintain

grass growth and a maximum of food for grazing herds. Provided that conditions for grass growth are favourable and that overgrazing is prevented, cutting may decrease topsoil erosion rather than promoting it.

In tropical grasslands burning is a common method to avoid bush encroachment. Natural fires or fires lit for hunting purposes are also normal occurrences. Generally speaking, burning of trees, bushes and old grass does more harm than benefit, and most soil scientists

condemn the method. There may, however, be differences in effects depending upon the season, type of soil, vegetation, etc. A short-term effect is the fertilizing of the soil by the ash.

On the other hand, the amount of humus and litter is considerably reduced, which may have serious, negative effects on soil structure, infiltration conditions, soil moisture and resis- tance against soil erosion. Consequently, burning practices very often involve the risk of greatly accelerated soil erosion.

1.1.2.4. Grazing

The effects of grassland practices on hydrological conditions and soil erosion depend on climate and grazing intensity. Mismanagement of grassland is particularly serious in semi-arid areas, where over-grazing may cause considerable changes in run-off and may lead to an increased erosion activity.

Grazing practices are influenced by the environmental characteristics of the region, such as long-term, or seasonal variations in climatic elements, availability of water and fodder for the animals, etc. The number of people dependent upon grazing animals is another important factor.

16

-_-...--.

(16)

(17)

(18)

Population density may vary within wide limits. In central Australia a rancher may have 10 000 to 1 000 000 hectares at his disposal, whereas, in climatically similar environments in Tunisia, Algeria or Syria, a cattle-owner has to earn his living from 10 hectares or less of non-irrigated land (Le Houerou 1976).

With special regard to the relatively intensively grazed areas of the East African savannas and of the dry land bordering the Sahara desert, the following observations may be of interest (see figure 1.6).

In the dry season animals are often bound to live and graze within short walking distance from a water hole or any other water available. In this season grasses and other herbs are dry and poor in nutrition. Therefore, browsing from bushes and low tree-branches is of great importance, and supplementary fodder is often provided by cutting down branches from trees. During extremely dry years - especially during sequences of dry years - all types of vegetation may be more or less completely destroyed by grazing, browsing, foraging and con-

centrated trampling. Water and wind erosion may increase to critical levels.

In the wet season temporary surface water is usually available in most areas. The animals can move to the most favourable grazing grounds, where grass is abundant. During this season vegetation and soil are not so often destroyed by grazing animals. However, areas that have been destroyed during the preceding dry seasons are now susceptible to heavy rainstorms with severe splash, wash, rill and gully erosion (see figure 1.7). It has been observed that the destruction of soil and vegetation is often an irreversible process, which has transformed vast regions into sterile areas with naked rock, gravelly surfaces or drifting sand.

Such grazing practices, which are a natural way of meeting the seasonal shortages of water and fodder, thus create erosion by wind and water, and an increased sediment production.

Some parts of the eroded soil material will be deposited on alluvial fans and on muddy flats, other parts will be discharged into river channels. Much of the finegrained material and the humus will be blown away from the area, whereas parts of the sand will form fields of sand- dunes with drifting material.

Even if the processes described are typical of semi-arid areas inhabited by nomads, grazing by cattle, sheep or goats, together with wood-cutting, may induce serious soil erosion also in regions with permanent rural settlements.

Fig. 1.7 Fast, concentrated runoff, eroding and spoiling the road between Iringa and Mawand, central Tanzania. Photo L.Stromquist, 1976.

(19)

1.1.3 Urban and industrial environment 1.1.3.1. Mining

Mining operations often give rise to a tremendous increase in the activity of erosion and sedimentation processes. Surface strip mining of coal and shales, in particular, has involved significant hydrological and sedimentological problems in many regions of the world. Also excavation of sand and gravel in open pits and dredging of material from river beds and from lake shores or bottoms can be included in the same problem complex.

Surface mining includes the removing of the topsoil, rock and other strata that cover the mineral or fuel deposits, and the exploitation of these deposits. Surface mining offers many advantages over underground mining, but may cause considerable effects on surface and subsurface water conditions and on sedimentation processes.

Extensive mining operations intercept the natural stream channels and change run-off and erosion processes within the river basins. Ephemeral streams may be diverted, resulting in local scouring and deposition. Spoil banks built up under contour stripping operations are often steepsided and consist of easily eroded material with only thin plant cover or no vegetation at all. Heavy rains may result in disastrous erosion and serious sedimentation

problems.

Surface mining may influence the quality of water and soil and through these also animal life and plants. Chemical pollutants often have toxic effects. If sulphuric minerals are mined the acidity of the spoil bank materials may for instance be lethal to many plants (pH <

4.0). Physical pollution with sediments is most serious in hilly country with steep slopes - and in environments with high-intensity rainstorms. Research conducted in Kentucky, USA, showed that "sediment yields from coal strip-mined lands can be as much as 1 000 times that of undisturbed forest. During a four-year period, the annual average from Kentucky spoil banks was 27 000 tons per square mile while it was estimated at only 25 tons per square mile from

forested areas" (Focus on environmental geology 1976).

In arid and semi-arid areas erosion and sedimentation problems caused by mining operations are normally less severe than in other regions, due to the fact that the vegetation is also sparse under undisturbed conditions, and rainstorms are infrequent. When heavy storms do occur, however, the effects may be considerable, and large quantities of sediment may be discharged from mining sites, spoil banks and access roads.

Strip mining operations should be carefully planned to avoid hydrological, sedimentological and other environmental problems. Guy (1977) has proposed the following steps to be included in the planning:

1 Estimates of erosion within and sediment yield from catchments whose streams may flow across or adjacent to the proposed mining area.

2 Designs for channels which will prevent erosion and deposition of streams diverted from the mine area.

3 Estimates of erosion on and sediment yield from reclaimed areas.

4 Estimates of erosion, transport, and deposition in the different segments of re-established stream channels across mined areas.

5 The sediment impacts on nearby water bodies, both during and after mining operations.

1.1.3.2 Road and building construction

Areas without any protection by vegetation are always more susceptible to high erosion losses than vegetated areas. This is particularly true for areas that have been reworked and remodelled, and where the natural sedimentological balance has been disturbed. Thus erosion and sediment production can be expected to be exceptionally high on road banks, road ditches and construction sites in general. The sediment production normally reaches its maximum during the earlier stages of the construction period.

There are several reports of exceptionally high sediment production, when road construction works are in progress. The causal relationship between operational and environmental factors and sediment yield is, however, very vague. This is mainly the result of the unpre- dictability of the erosion processes caused by different construction operations. Careful planning of the operations is necessary in order to keep the sedimentation processes under control.

As artificial slopes are often created by remodelling the terrain during construction operations, the stability of the new slopes and of the earth masses moved may be poor or critical. Not only surface erosion related to rain splash and run-off will occur, but also different mass movement processes, e.g. slides, slumps, debris flows, falls and slippages, etc. These

20

(20)

processes may contribute significantly to the quantities of sediment discharged into the neigh- bouring river channels. Thus the construction operations may affect and control the future

development of the sedimentation processes in the river basin for a long period.

1.1.3.3. Urban development

Urban areas often produce sediments at a much higher rate than rural environments. Sediment yields of as much as 20 000 to 40 000 times the yields from natural or undisturbed areas have been reported (Becker and Mulhern 1975).

Most sediment is delivered during construction stages, especially when vegetation and surface soils are temporarily removed. Construction works may drastically increase erodibility and decrease slope stability. But also older sections of an urbanised area can produce considerable sediment amounts, often mixed with chemical and biological pollutants.

An example of changing erosion conditions is given by Wolman (1967), who described the variation of the sediment yield during periods of different land use practices and urbanisation for an area near Washington, D.C. The development is illustrated in figure 1.8.

Another example of the changing sediment concentration in a stream, draining an area undergoing a cycle from natural through constructional to stabilised conditions, is shown in figure 1.9 (after Guy 1965).

Erosion and sediment production in areas of urban development often do more harm to downstream areas than to the erosional sites themselves. Drainage channels may be filled with sediment and their discharge capacity decreased. Water and sewage systems may be affected.

The natural or artificial adaptation to new water-sediment discharge relations may cause considerable problems.

Erosion and construction-site sediment yields can be effectively reduced by appropriate measures. From a study in Maryland, USA, it is reported that sediment yields from active construction areas were reduced by 60-80 percentage during a period of 8 years. "Some of the reduction was attributed to reducing slopes on construction sites and increasing distances between construction sites and stream channels, however, most of the reduction was the result of improvements in the design and implementation of sediment-control measures. These included limiting grading to reduce the amount of land open at any one time, planting of temporary vegetation and application of mulch to protect exposed soils, construction of diversion berms and stabilized water-ways to reduce erosion on critical slopes, and use of large sediment basins to trap eroded sediment on site" (Proc.Sed.Conf. 1976).

There is a broad range of activities in the urban environment that may result in drastically increased sediment yields. As soon as the activity involves land development, erosion may be expected. Examples are the construction of housing, schools, shopping centres, office buildings and factories, the development of transportation and communication networks such as highways, streets, roads, railroads and bridges, the erection of energy facilities such as power plants and transmission lines, the construction of water structures such as dams, aque- ducts, canals, and flood-control measures, and the establishment of recreation projects such as campgrounds, parking lots and different multiple use developments.

tons/km2 1000

CONSTRUCTION 7

n

FOREST CROPPING WOODS AND URBAN

GRAZING

Fig. 1.8 A sequence of land use changes and associated sediment yield, beginning prior to the advent of extensive farming, continuing through a period of urban construction, and finally extending into the subsequent phase of the urban landscape. Based on experience in the Middle Atlantic region of the United States (after Wolman, 1967).

(21)

100000 wm

I I I I I I I I I

60000 -

z

; 40000- 2

z 5 10000 E f

y 6000 8 z 4000 4 z

0 .

,2 . .

/ / 0 : 000 9

/

/ ^{O ‘a}

l 0

0 3”

/

. l

BEGINNING ^/’

. /

OF / .

CONSTRUCTION,/ .

MAXIMUM CONSTRUCTION

ACTIVITY

. obrcrvsd 0 computed

.

END. 0 OF . CONSTRUCTION 0

0

.

IY37 1958 1959 1960 1961 1962

Fig. 1.9 Mean sediment concentration of storm runoff from an area of residential construction at Kensington, Maryland, USA, 1957-1962 (from Guy, 1965).

Both local conditions and type of construction activities vary within extremely broad limits. Therefore, the planning engineer is faced with a most difficult task, in trying to assess the probable erosion and sedimentation problems associated with a particular construction project. Extreme care should be used in environments with considerable risk of erosion, and the construction activities should be preceded by careful data collection and evaluation of risk factors.

Data on site conditions should include certain hydrological information, for instance on surface and subsurface drainage aspects, topographical and geological characteristics, soil conditions, extent of vegetative cover and climatic conditions. Information on construction activities should include the area1 extent and the nature of the ground surface disturbance, the kind of equipment and the number of people involved, and the time schedule of events.

Methods for the acquisition of data and the prediction of the effects of man's activities are discussed in chapter 3.

1.1.4 Sedimentation processes and ecological changes

As has been stressed, sedimentation processes are to a large extent dependent upon environmental factors and natural processes - climate, geology, topography, vegetation, animal life, etc.

(see figure 1.10). In addition, man's influence has nowadays become most important.

An ecological approach to environmental problems emphasises the interrelations between physical and biological processes. Man's interference with his environment is of particular importance. The accelerated rate of technical development has drastically increased the impact of human activities, especially during the last few decades.

We are dependent upon the natural resources - energy, minerals, soils - to maintain agriculture and industry and the production of food and technical articles. The pollution and deterioration of air, water and soil is a problem that must be solved in order to maintain the quality of the environment.

Much attention has been paid to the imbalance between the accelerated growth of human population and the world's limited and perhaps decreasing soil resources for food production.

The deterioration of the soil is closely linked to processes such as leaching, salinization, soil erosion and sedimentation. These processes are largely influenced by changes in precipi- tation, run-off, vegetation, land use etc.

Man's impact on the soil condition is mainly caused by the effects of intensive land use.

Many of these effects involve irreversible changes of the soil mantle by erosion and sedimen-

22

_.-- ._-... .^ ._. -__-_

(22)

(23)

(24)

tation. The degradation of soil is often combined with and partly caused by a more or less complete destruction of the natural vegetation.

The excess production of sediments in the headwater areas by accelerated soil erosion leads to an increase in the yield of material to rills, streams and rivers, ponds and reservoirs (see figure 1.11) Measures taken in one part of the basin will in most cases influence conditions in other regions, and problems arising in one specific area may be solved only by applying suitable practices in some other place. The whole river basin should be looked upon as a compound unit (cf.Sundborg 1964 and 1979).

The clearing of forests and natural bush and grass vegetation may not necessarily lead to a serious acceleration of erosional processes, even if it may have irreversible consequen- ces for the natural flora and fauna. With careful management the quality of soil for agricultural production may be maintained and even improved. In most cases when the topsoil is eroded or covered by unproductive gravelly and sandy sediments, the development is primarily caused by improper land management, including inappropriate reduction of vegetation, plant detritus and humus.

As has already been mentioned there are many traditional forestry, grazing and agricultural practices that may cause harmful erosion and sedimentation - from timber-logging to tse-tse fly control. Erosion control may mean some restrictions in present practices.

To-day there are efficient technical methods available for specific conservational purposes. Relevant facts about soils and their properties have been collected. However, the complicated nature of erosional and sedimentational processes needs further detailed studies.

There is a great demand for surveys and investigations when specific projects are planned.

The most urgent need, however, is to increase the awareness and the understanding of the complex ecological relations and processes, in order to promote a better application of present conservation techniques in the long-term planning of water and soil resources.

1.1.5 Arid and semi-arid environment

Arid and semi-arid regions are especially sensitive to exploitation measures and to changes in land use practices. In a report on desert encroachment the situation in arid land was characterized in the following way: "One third of the earth's surface is arid or semi-arid land. The territories of half of the world's nations lie partly or wholly in dry regions. Utilization of the primary dry land was ecologically tolerable for thousands of years. It is only in recent times that overexploitation has occurred - often in combination with climatic fluctuations - as a result of increasing population densities and social disturbances among the people in- habiting arid lands.

A characteristic of arid land is the great variety in the environment. It is true that a common feature is the irregularity of the water supply available for man, but generally the environmental variations lead man to exploit the resources in many different ways. The wide variety of conditions and problems in arid and semi-arid areas complicates development. Since Unesco launched the first arid zone programme 25 years ago, scientific and technological research has greatly advanced man's capacity to make use of dryland resources. On the other hand, the overexploitation mentioned above has resulted in increased desertization, which may counteract efforts at enhancing the quality of life in dry lands" (Brink 1976); see also figure 1.12).

The main limiting factors for development in arid and semi-arid areas are low precipi- tation, exceptional variations and contrasts of rainfall in space and time, insufficient soil moisture and limited availability of certain soil nutrients.

Even if the present situation of man is discouraging in many dry regions, arid land will be of utmost importance in the future, also from a global standpoint, as a significant reserve of natural resources. Solving the problems of development of dry areas without serious environmental effects should be an interdisciplinary task that requires an over-all knowledge of ecology and environment and the efforts of people from many fields of specialization.

"It is said that we have adequate knowledge of the processes causing desertization and of the technology of restoration of desertized areas, but our knowledge of the structure and functioning of dry ecosystems is generally poor. Research has an important role to play in helping us to find ways not only of restoring desertized areas but also of improving and con- verting them into ecologically adapted systems of higher productivity. More information about the linkage between the ecological and socio-economic systems is urgently needed. Even the very best desert technology is useless if it cannot be adapted to social and economic reality"

(Le Houerou and Rapp 1976).

(25)

1.2 WATER COURSES 1.2.1. Introduction

Sediment transported in the river channel or water course can be directly traced back to its external source, i.e. sediment from either the slopes of the upstream interfluvial catchment area, or the upstream river channel and flood plain. At any particular site on the water course the production of sediment in the channel represents an internal source dependent on the alluvium and other material available within the channel flood plain.

Man's activities in the upstream river basin will have an impact on the external source of sediment supply to a particular site. These impacts are described in the interfluvial area in Section 1.1. Other activities by man at or near a particular site on a river will impact the internal sources. Thus changes in water discharge or sediment transport from the upstream external sources or the internal sources caused by man may drastically affect the stability of a channel with changes in channel formation and development.

1.2.2 Changes of flow regime and sediment input

Channel formation is a self-regulating process. The regulation is realised through the transporting capacity of the stream channel determined both by the parameters of the flow and by the parameters of the sediments transported by it. (Karaushev 1960 and 1972, Graf 1971).

If the transporting capacity of the stream channel is not realised, i.e. it is under- loaded by sediments, erosion of the bed will occur. This erosion, if not inhibited by armour- ing, will cause an increase in depth of the flow, and a decrease in current velocity and the transporting capacity of the flow. This will again cause a decrease in the intensity of erosion until the transporting capacity matches the sediment supply and erosion ceases.

The rate of channel change and the time required for its completion depend on both the basic characteristics of the flow and channel, and on the regime of water and sediment discharges. The length of time involved may be a few hours or stretch over a number of decades.

Within the framework of all channel processes sediment discharge depends on the hydraulic conditions, and channel changes result from a difference between the amount of sediments entering the channel and the transporting capacity of the flow within the particular reach under consideration.

When sediments discharge into the river from the interfluvial areas they represent an external influence on channel processes. This influence is only marginal in importance for fine materials. If the channel receives more (or less) coarse sediments a change in the processes occurs to transport the changed amount of sediments, and continues until dynamic equilibrium is attained. Therefore the channel formation processes are determined not only by the water and/or sediment discharges but by the ratio between actual sediment discharge and the transporting capacity of the flow. Channel changes may thus be intensified by natural and man made changes of water and sediment discharge.

The magnitude of channel change depends on the difference between the amount of sediment entering the river reach, the transporting capacity of the flow, and the nature of the material composing the river bed. Depending on these conditions channels may be referred to as either stable or unstable.

For example, A V Karaushev proposed the following dynamic classification of river channels:

1 - stable channels, channels composed of stable material and subject only to slight erosion even in case of a considerable depletion of the sediment supply.

2 - degrading channels, unstable channels composed of erodible sediments and characteristic of river reaches with some underloading of the flow by sediments. (The Las Vegas Wash as an example, see figure 1.13).

3 - aggrading channels, unstable channels with excessive sediment inflow or reduced transporting capacity. (Payette River with backwater of Black Canyon reservoir, see figure 1.14).

4 - dynamically stable channels, channels in which the transporting capacity and the sediment inflows are in long term equilibrium. (Regime channels).

Discharges of water and sediments in river channels are characterised by a great natural variability. The variability of sediment discharge in rivers is closely connected with the water regime of rivers and conditions at the watershed.

Seasonal and annual variations of sediment discharge may be considerable, particularly in arid and semi-arid areas and in cold mountain climates.

The ratio between the transporting capacity of the channel and the sediment inflow may change in time with the river regime. Seasonal changes of river regime may be characterised

26

(26)

(27)

Sedimentation problems in river basins