The Geomorphic Machine

An understanding of geomorphology begins with hydrology. In very dry or very cold environments other factors are also required, but here the focus is on the more-or-less humid environments where human population density is highest. A minimodel of the hydrologic balance is shown in Figure 3.2. Precipitation is a source or input of water storage, while evapotranspiration, runoff, and infiltration are outputs. The energetics of this model are critical but straightforward. Movements of liquid water have kinetic energy in proportion to their velocity, and the storage of water has potential energy in proportion to the height above some base level. The energetics of hydrology drive geomorphic processes and create landforms.

In humid environments geomorphology involves mainly erosion, transport, and deposition of sediments. The action of these processes has been metaphorically referred to as the "geomorphic machine" in which hydrology drives the wearing down of elevated landforms (Figure 3.3). Leopold's (1994) quote for the special case of rivers given below describes this metaphor:

FIGURE 3.3 A machine metaphor for geomorphology. (From Bloom, A. L. 1969. The Surface of the Earth. Prentice Hall. Englewood Cliffs, NJ. With permission.)

The operation of any machine might be explained as the transformation of potential energy into the kinetic form that accomplishes work in the process of changing that energy into heat. Locomotives, automobiles, electric motors, hydraulic pumps all fall within this categorization. So does a river. The river derives its potential energy from precipitation falling at high elevations that permits the water to run downhill. In that descent the potential energy of elevation is converted into the kinetic energy of flow motion, and the water erodes its banks or bed, transporting sediment and debris, while its kinetic energy dissipates into heat. This dissipation involves an increase in entropy.

The machine metaphor is especially appropriate in the context of ecological engineering and brings to mind John Todd's idea of the living machine (see Chapter 2). In fact, vegetation regulates hydrology and therefore controls the geomorphic machine described above. For example, the role of forests in regulating hydrology is well known (Branson, 1975; Kittredge, 1948; Langbein and Schumm, 1958). Perhaps the most extensive study of this action was at the Hubbard Brook watershed in New Hampshire. This was a benchmark in ecology which involved measurements of biogeochemistry and forest processes at the watershed scale (Bormann and Likens, 1979; Likens et al., 1977). It was an experimental study in which replicate forested watersheds were monitored. One was deforested to examine the biogeochemical consequences of loss of forest cover and to record the recovery processes as regrowth occurred. The forest was shown to regulate hydrology in various ways by comparing the deforested watershed with a control watershed that was not cut. Deforestation increased streamflow in the summer through a reduction in evapotranspiration, changed the timing of winter streamflow, reduced soil storage capacity, and increased

Impact of Deforestation

Impact of Deforestation

Response Deforestation
FIGURE 3.4 Sequence of watershed responses to deforestation, based on the Hubbard Brook experiment. (From Likens, G.E. and F.H. Bormann. 1972. Biogeochemical cycles. Science Teacher. 39(4):15-20. With permission.)

peak streamflows during storms. The summary diagram of the deforestation experiment illustrates an increased erosion rate (Figure 3.4) and thus the connection between the ecosystem and landform. Soil bioengineering systems are designed to restore at least some of this kind of control over hydrology and geomorphic processes.

To further illustrate the geomorphic machine, the three main types of erosion in humid landscapes are described below with minimodels. Emphasis is on geomorphic work, so other aspects of hydrology are left off the diagrams. In each model, erosion is shown as a work gate or multiplier that interacts an energy source with a soil storage to produce sediments.

Upland erosion is shown in Figure 3.5. Initially, precipitation interacts with soil in splash erosion. Vegetation cover absorbs the majority of the kinetic energy of rain drops, but when it is removed or reduced in agriculture, construction sites, or cleared forest land, this initial form of erosion can be significant. Sheet and rill erosion occur as the water from precipitation runs off the land. Various best management practices (BMPs) are employed to control runoff and the erosion it causes as will be discussed later.

Channel erosion is shown in Figure 3.6. Stream flow, which is runoff that collects from the watershed, is the main energy source along with the sediments it carries.

FIGURE 3.5 Energy circuit model of the types of upland erosion.
FIGURE 3.6 Energy circuit model of stream channel erosion.

The system itself is depicted as a set of concentric storages: the bank soils contain the channel volume, which contains the stream water, which contains suspended sediments. Movement of water through the system erodes bank soils and simultaneously increases channel volume. The term for output from the system is discharge, which includes the stream water and the sediment load that it carries through advection. The behavior of this system is covered by the subdiscipline of fluvial geomorphology. Velocity of stream water is of critical importance since it is a determinant of kinetic energy and erosive power. A typical relationship for velocity is shown below (Manning's equation; see also Figure 3.22):

where

V = mean velocity of stream water R = mean depth of the flow S = the stream gradient or slope n = bottom roughness

Size, mm

FIGURE 3.7 Complex patterns of sediment behavior relative to current velocity in a stream environment known as the Hjulstrom relationship. (Adapted from Morisawa, M. 1968. Streams, Their Dynamics and Morphology. McGraw-Hill, New York.)

Size, mm

FIGURE 3.7 Complex patterns of sediment behavior relative to current velocity in a stream environment known as the Hjulstrom relationship. (Adapted from Morisawa, M. 1968. Streams, Their Dynamics and Morphology. McGraw-Hill, New York.)

Thus, velocity is directly proportional to depth and gradient and inversely proportional to roughness. This relationship will be explored later in terms of design of soil bioengineering systems.

The work of streams and rivers depends on velocity according to the Hjulstrom relationship, which is named for its author (Novak, 1973). This is a graph relating velocity to the three kinds of work: erosion, transportation, and sedimentation, relative to the particle size of sediments (Figure 3.7). Sedimentation dominates when particle sizes are large and velocities are slower, transport dominates at intermediate velocities and for small particle sizes, while erosion dominates at the highest velocities for all particle sizes. Based on this relationship, particle sizes of a stream deposit are a reflection of the velocity (and therefore the energy) of the stream that deposited them.

Fluvial or stream systems develop organized structures through geomorphic work including drainage networks of channels and landforms such as meanders, pools and riffle sequences, and floodplain features. Vegetation plays a role in fluvial geomorphology by stabilizing banks and increasing roughness of channels.

Coastal erosion is modelled in Figure 3.8. The principal energy sources are tide and wind, which generates waves. River discharge is locally important and, in particular, it transports sediments eroded from uplands to coastal waters. Coastlines are classified according to their energy, with erosion dominating in high energy zones and sedimentation dominating in low energy zones. Inman and Brush (1973) provide energy signatures for the coastal zone with a global perspective. Wave energy is particularly important and it is described below by Bascom (1964):

The energy in a wave is equally divided between potential energy and kinetic energy. The potential energy, resulting from the elevation or depression of the water surface,

FIGURE 3.8 Energy circuit model of coastal erosion.

FIGURE 3.8 Energy circuit model of coastal erosion.

advances with the wave form; the kinetic energy is a summation of the motion of the particle in the wave train and advances with the group velocity (in shallow water this is equal to the wave velocity).

The amount of energy in a wave is the product of the wave length (L) and the square of the wave height (H), as follows:

where w is the weight of a cubic foot of water (64 lb).

Geomorphic work in the coastal zone builds a variety of landforms including channels and inlets, beaches, dunes, barrier islands, and mudflats. Vegetation is an important controlling factor in relatively low energy environments but with increasing energy, vegetation becomes less important, and purely physical systems such as beaches are found.

While early work in geomorphology focused on equilibrium concepts (Mackin, 1948; Strahler, 1950; Tanner, 1958), more recently nonequilibrium concepts are being explored (Phillips, 1995; Phillips and Renwick, 1992), such as Graf's (1988) application of catastrophe theory and Phillips' (1992) application of chaos. This growth of thinking mirrors the history of ecology (see Chapter 7). Drury and Nisbet (1971) provided a comparison of models between ecology and geomorphology, indicating many similarities that have developed between these fields. Like ecosystems, geomorphic systems can be characterized by energy causality, input-output mass balances, and networks of feedback pathways. They therefore can exhibit nonlinear behavior and self-organization as described by Hergarten (2002), Krantz (1990), Rodriguez-Iturbe and Rinaldo (1997), Stolum (1996), Takayasu and Inaoka (1992), and Werner and Fink (1993). Cowell and Thom's (1994) discussion of how alternations of regimes dominated by positive and negative feedback can generate complex coastal landforms is particularly instructive and may provide insight into analogous ecological dynamics. While these developments are exciting and can

stimulate cross-disciplinary study, it is somewhat disappointing that geomorpholo-gists have written little about the symbiosis between landforms and ecosystems. Knowledge of both disciplines and how they interact is needed to engineer and to manage the altered watersheds of human-dominated landscapes. Workers in soil bioengineering are developing this knowledge and probably will be leaders in articulating biogeomorphology to specialists in both ecology and geomorphology.

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