Hillslope Processes

Binghamton Geomorphology Symposium 16

A.D. Abrahams

• Publisher: Routledge
• Year: 2020
• Print ISBN: 9780367464486
• eBook ISBN: 9781000045697
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geography
• Index: Geography

This book, first published in 1986, collects the articles presented to the 16th Binghamton Geomorphology Symposium and is a ground-breaking work in the study of hillslope processes. Hillslope processes are studied in a variety of disciplines other than geomorphology, such as hydrology, pedology, agricultural engineering, civil engineering and engineering geology – the study is truly an interdisciplinary science.

Part I:

General

1

Rates of surface processes and denudation

Anthony Young and Ian Saunders

Abstract

After an initial introduction on the development of studies of rates of slope processes, the present state of knowledge is summarized for rates of soil creep, solifluction, surface wash, solution, landslides, cliff and slope retreat, and total denudation. Other slope processes are noted, with special reference to the activities of animals. Representative rates for denudation as a whole are 50 B (predominant range 10–100 B) for gentle relief and 500 B (predominant range 100–1000 B) for steep relief. Denudation rates reach a maximum in the semiarid and tropical subhumid (savanna) climatic zones, and in montane climates with steep slopes. Problems in the interpretation of the results of process rate studies include sediment transfer paths, the spatial and temporal sampling frames, and relict landforms. Questions of the control of slope retreat, equilibrium and the steady state, time, space, and causality are discussed in the light of process rates. The influence of Man can accelerate denudation by 2, 10, or over 20 times, depending on the type of land use. Recommendations for future process studies are to employ a spatial sampling design; short-term instrumentation and measurement could usefully be supplemented by other independent means of checking the validity of the data, such as estimates of rates of landscape change over geologic time.

Introduction

The aim of this chapter is to provoke discussion on the problems that arise in the interpretation of data on rates of slope processes and slope retreat—that is, change in form as well as processes. Also, on the grounds that most landscapes are made up largely of slopes, we cover rates of total ground lowering, here referred to for convenience as denudation. Particular attention is given to those problems encountered when attempting to adopt a straightforward measure-and-extrapolate procedure.

The chapter begins with some reflections on the historical development of process measurements. This is followed by a summary of the major results achieved to date, based upon a previous review (Saunders and Young, 1983). The remainder of the chapter is directed towards a wider-ranging discussion of the interpretation of the results than was previously attempted, including placing them within the context of some classical questions in the geomorphology of slopes.

An historical perspective

Recognition of the fact that landscapes are not immutable dates back to Hutton and Play fair, although 19th-century geologists had to work hard to combat the alternative hypotheses of Creation or Noachian catastrophism. Lyell (1841, p. 161) had seen that old marine cliffs, no longer undercut, were “reduced to gentle slope.” Scrope clearly set out how surface wash operates: “The direct fall of rain removes particles from the ground surface and carries them away to the lowest accessible levels. The general surface is more or less lowered” (Scrope, 1866, p. 193). Landscape evolution on a massive scale was apparent to the great American geologists who described the West in the closing decades of the 19th-century.

Until the 1950s, however, little was known about the rates of geomorphic processes. A few isolated individuals possessed of natural curiosity had spotted “something that moved” and measured it, mostly in montane periglacial environments in Europe (Schmid, 1925; Morawetz, 1932, Krumme, 1935); but, for the most part, geomorphological discussion of processes was based upon reasoning derived from form: Davis (1892), Fenneman (1908), Gilbert (1909), Lawson (1932), Baulig (1940), and Birot (1949) formed the undergraduate reading of the senior author in the early 1950s. Thus, wash increases in volume downslope and so can carry the same sediment load over a gentler slope and thus forms concavities; but, of course the existence of concavities shows that wash must be predominant! Then there was the indisputable reasoning which led to Horton’s (1945) “belt of no erosion”—indisputable, that is, until Yair (1972) set up wash traps right in the middle of it.

The problem of “the everlasting hills” was more severe than in most other branches of geomorphology. Rivers obviously flow, carry sediment, and from time to time their banks collapse; marine cliffs in soft rocks recede rapidly, whereas glaciers move perceptibly even within the timespan of an expedition. But in most parts of the world, especially the smooth, soil-covered landscapes of western Europe, change on slopes is not apparent in a human lifetime. Both authors experienced the revelation, on their first visits to the Rocky Mountains, that landscapes really are alive. For a time, geomorphology was imbalanced, with a modern, process-oriented textbook on rivers (Leopold et al., 1964) but nothing comparable in other branches.

Michaud began to survey movements of painted stones in the Alps in 1947, Schumm measured ground loss on badlands from 1952 onwards, and Rougerie started wash measurements in the Ivory Coast. Grove recorded the daily movement of a mudflow, and Young put wooden pegs into the ground surface below rock outcrops.

Since that time, instrumentation has been developed to measure all the processes on slopes: soil creep, solifluction, surface wash, landslide movement, and latterly throughflow and solution. The stages in obtaining rates of movement are basically similar whatever the process: (1) develop the instrumentation necessary to record very small changes; (2) measure these changes, usually over a period of 1–3 years; and (3) extrapolate the results in time and space.

Within the context of the exponential growth in geomorphology as a whole, this procedure gave rise to a steadily growing stream of published rates of slope processes, from some 10 per year in the 1960s to over 40 per year in the 1980s. Three times we have attempted to summarize and compare such reports, for total denudation (Young, 1969), and for slope processes, slope retreat, and total denudation (Young, 1974; Saunders and Young, 1983). On the last occasion our report covered 419 publications and was by no means complete. Further updatings of this nature will probably have to be made for individual processes or climatic environments.

The present state of knowledge

Design of Procrustean bed

Figures 1.1 and 1.2 present a highly generalized summary of available data on rates of surface processes on slopes, slope retreat, and total denudation. These are derived from the corresponding figures in Saunders and Young (1983), to which reference can be made for details.

The environments from which these data originate are extremely diverse, including variations in climate, vegetation, rock, soil type, and slope angle. In an attempt to derive consistencies from this diversity, some gross simplifications have been made. Vegetation cover is assumed to correspond to climatic type, thus eliminating vegetation as an independent variable. Soil type is also excluded for lack of sufficient data; it would certainly be worthwhile for someone to attempt a summary of process rates in relation to the FAO or other soil classification, for certain relations would clearly emerge, such as faster creep on expanding clays, rapid surface wash on impermeable clays. Rock type is grouped into limestones (for chemical solution records only), unconsolidated rocks, and all other rocks.

Having thus for the most part eliminated three environmental factors, the main ways in which the data are classified are by climatic type and slope. Climates are grouped into eight broad classes as shown in Table 1.1. Tropical subhumid (sometimes called tropical “wet-and-dry” corresponds to savanna vegetation (cerrado in South America) and tropical humid to rain forest. We have been taken to task for grouping polar with montane climates on grounds of climatic differences of importance to processes, such as the high diurnal temperature range of mountains. However, the mountains in which periglacial geomorphologists often work are in high latitudes. Through lack of data, arid climates are omitted from all except one figure.

Table 1.1 The eight broad climatic classes.

Climatic class	Abbreviation in Figures 1.1 and 1.2	Approximate Köppen equivalent
polar/montane	P/M	E
temperate maritime	Tm	Cfb, Cfc
temperate continental	Tc	Df
Mediterranean	Med	Cs
semiarid	S-A	BS
arid	Arid	Bw
subtropical humid	ST	Cfa
tropical subhumid	TrS	Aw
tropical humid	TrH	Af, Am

Finally, slopes are divided into “gentle” and “steep.” Steep covers individual slopes over 25° and mountainous or steeply dissected areas in which such slopes are frequent. Gentle is short for ’gentle to moderate’ slopes, and also covers river basins or other area which possess a wide range of slope angles. It is realized that the above are generalizations which have been made because the aim of the present chapter is to identify broad trends. Readers who seek more specific detail may refer initially to Tables I–X in Saunders and Young (1983) and from there back to the 419 primary sources.

For measurements of ground loss and slope retreat the Bubnoff unit B is employed, where 1 B = 1.0 mm per 1000 years, equivalent to 1.0m³/km²yr and, in terms of rock (not soil) mass, 0.026 t/ha yr.

The way in which the ranges have been derived from the original data is explained in Figure 1.1. We should like to stress that there is no element of a priori reasoning in these diagrams–that is, no adjustments or interpolation for what the results ought to be. They are wholly based on the observed records, with ranges omitted where data for particular climate/slope combinations is insufficient.

Processes

Soil creep Measurements of soil creep have shown that movement near the surface in temperate maritime climates is typically 0.5–2.0 mm/yr (Fig. 1.1A). In temperate continental climates, probably because of the more severe ground freezing in winter, it ranges up to 15 mm/yr. More data are needed from the deep soils of the tropical humid zone. No doubt clays creep faster than sandy soils, especially if montmorillonitic. Several studies have found that wet sites move much faster than dry, a fact which destroys the relation of rate with slope angle. This means, too, that creep may remain as fast on gently sloping conca...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Dedication
• Preface
• Acknowledgments
• List of Tables
• Contributors
• Part I General
• 1 Rates of surface processes and denudation
• Part II Hydraulic processes
• 2 Evaluation of Horton's theory of sheetwash and rill erosion on the basis of field experiments
• 3 Erosion processes and sediment properties for agricultural cropland
• 4 Plant cover effects on hillslope runoff and erosion: evidence from two laboratory experiments
• 5 Sediment movement in ephemeral streams on mountain slopes, Canadian Rocky Mountains
• 6 Sediment movement and storage on alpine slopes in the Colorado Rocky Mountains
• 7 Solute movement on hillslopes in the alpine environment of the Colorado Front Range
• 8 Hillslope hydrology models for forecasting in ungauged watersheds
• 9 Hillslope runoff processes and flood frequency characteristics
• 10 A two-dimensional simulation model for slope and stream evolution
• Part III Gravitational processes
• 11 Controls on the form and development of rock slopes in fold terrane
• 12 Influence of scree accumulation and weathering on the development of steep mountain slopes
• 13 Flow behavior of channelized debris flows, Mount St. Helens, Washington
• 14 Dynamics of slow landslides: a theory for time-dependent behavior
• 15 The morphology and mechanics of large-scale slope movement, with particular reference to southwest British Columbia
• 16 Processes leading to landslides in clay slopes: a review
• 17 Hollows, colluvium, and landslides in soil-mantled landscapes
• 18 Relative slope-stability mapping and land-use planning in the San Francisco Bay region, California
• Index