Loose Boundary Hydraulics

Arved J. Raudkivi

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000150506
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

This text looks at sediment transport, two-phase flow and loose boundary hydraulics which are some of the names used to identify problems of interaction between fluid flow (water or air) and its boundaries that may be non-cohesive (alluvial) or cohesive.

CHAPTER 1

Introduction

Sediment transport, two-phase flow, loose boundary hydraulics are some of the names used to identify problems of interaction between fluid flow (water or air) and its boundaries that may be non-cohesive (alluvial) or cohesive. Unlike in the classical hydraulics the boundaries can change their geometry and roughness with changing flow conditions. Some of the material from the boundaries may be entrained into the flow or sediment may be added to the flow, e.g. by tributaries, as suspended matter. Since the changing boundaries are the central feature of most flow processes in nature, the term loose boundary hydraulics is used here. A special case is the transport of granular material in pipelines.

The crucial characteristic of all loose boundary problems is the interaction between the fluid and sediment, that is, the erosion and sediment transport problems cannot be treated in isolation from hydro- or aerodynamics. Sediments form a passive medium that only reacts to the applied forces. The interactions involve topics of fluid dynamics that have many unsolved problems of their own, like turbulence, boundary layers, diffusion and, in particular, hydrodynamics of the surf zone at the coastlines.

The annual precipitation on land surfaces (excluding Antarctica) amounts to about 108 × 10³ km³ of which about two-thirds evaporates, but some 36 to 37 × 10³ km³ becomes runoff. This water not only maintains life but also shapes the landscape. It is therefore natural that the sediment transport problems are conventionally associated with land erosion, rivers and coastlines. However, the same physical concepts apply to many industrial and chemical processes. The emphasis here is on the naturaJ processes of land erosion and sediment transport from mountains to the sea. These processes shape the landscape and coastlines, affect the water quality, wild life, agriculture, forestry, navigation, flood control, hydro-power, recreation, and fishing. Many aspects of the fluid-sediment interactions are sciences on their own right, like sedimentology, morphology, etc.

Sediment particles are derived from breakdown of older deposits, volcanic eruptions, chemical precipitation and biological secretion. Of short term interest are mainly particles eroded as solids from land. These become detached from bedrock by weathering and glacial activity. Estimates from the sediment content of Antarctic glaciers delivered to the sea suggest that continental glaciers are the world’s greatest producers of sediments. Weathering is the other major producing agent, which also acts to further break down the rock fragments. On the global scale, the amount of solids delivered to the sea is estimated to be of the order of 14 × 10⁹ tonne per annum, plus an additional 4 × 10⁹ tonne per annum of dissolved solids. This averages to about 100 tonne/km² per annum, but only a fraction of this is transported by the river systems directly to the sea. Most of it is deposited at intermediate locations, where it may rest for very long periods. Local variation in sediment yield may be substantial. For example, the western slopes of the Southern Alps in New Zealand are subject to rainfalls, which may yield 0.5 m in 24 hours or about 10 m per annum. Local sediment yields are of the order of 15,000 tonne/km². Much of it is in rock and landslides that constitute special problems. Erosion is accompanied by deposition. Erosion and deposition, together with orogenic processes, shape the earth. The trend is for all soil and water to move to the lowest possible elevation, which for soils ultimately means the ocean deep. However, the indications are that, at present, the rates of orogeny (formation of mountains) are much greater than the average rate of erosion.

Sediments in mountains are very poorly graded, in particular, in the land/rock slides. These also dump large quantities as ‘point load’ into rivers. From mountains to the sea the sediments vary substantially in size, grading and properties with distance travelled. The grain-size distribution becomes narrower and the sediment finer as the distance from mountains increases. There may be local ‘discontinuities’ due to the inputs from tributary streams.

The earlier concepts of sediment fining with distance were based on wear and breakdown; the Sternberg exponential decay relationship of particle weight. This was followed by models of selective transport and sorting. Yet, in a channel in equilibrium, where sediment input equals output, sorting alone cannot lead to an overall reduction of grain size with distance, since ultimately all material entering must leave. Clearly, both processes have to be involved.

The streams and rivers are transport systems. The material originates from the erosion of land. The soils of the top layers of our lands are one of the most valuable natural resources and soil erosion means a loss of a valuable asset. Therefore, the aim should be to reduce soil erosion. Retention of soil will lead to improved agricultural, forestry, etc. production. It also means reduced quantities of sediment to be transported by rivers, reduce...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• PREFACE
• LIST OF SYMBOLS
• 1 INTRODUCTION
• 2 SEDIMENT AND FLUID PROPERTIES
• 3 THRESHOLD OF PARTICLE MOVEMENT
• 4 SAND TRANSPORT BY AIR
• 5 GEOMETRY OF FLUVIAL CHANNELS
• 6 RESISTANCE TO FLOW
• 7 SEDIMENT TRANSPORT
• 8 STABLE CHANNEL DESIGN
• 9 EROSION AND DEPOSITION
• 10 COHESIVE SEDIMENTS
• 11 COASTAL ZONE
• 12 TRANSPORT IN PIPELINES
• APPENDIX
• REFERENCES
• AUTHOR INDEX
• SUBJECT INDEX