Architectural Descriptions of Three Types of Debris Dams
Bar Top Jam (BTJ)
A random accumulation of logs with little vertical stacking characterize BTJs, which form a loose mat deposited on a bar top during receding flows. Although logs in a BTJ are oriented in all directions relative to the depositing flow, most are oblique ... Bar top jams are relatively unstable as they are mobilized at discharges approaching bankfull. Hence they have little appreciable effect on channel morphology.
Bar Apex Jam (BAJ)
The more stable BAJ has a distinctive architecture characterized by three primary structural components: a key member nearly parallel to flow, normal members orthogonal to flow, and oblique members oriented 10-30 degrees to flow ... the key members appear to be invariably a large log with an attached rootwad facing upstream. The deposition of a key member significantly reduces the effective width of flow within a channel. The LWD (large woody debris) that otherwise might be flushed through that portion of the channel is deposited, usually by racking up against the key member and contributing to a further reduction in the effective channel width. Normal members rack up against the key member rootwad orthogonal to flow, whereas oblique members deposit along the flanks of the key member. The sequential deposition of normal and oblique members commonly results in the vertical stacking of five or more interwoven layers. The formation of a jam introduces a local control on channel hydraulics that leads to distinctive changes in channel morphology and riparian forest structure. Stable LWD structures such as the BAJ provide a barrier to high velocity flows, creating sites of sediment aggradation that can lead to floodplain formation. Stable LWD structures also resist channel migration, thereby providing refugia for forest development.
Meander Jam (MJ)
Meander jams become the most common of the stable jams with increasing channel size. Unlike the BAJ, MJs have only two principal structural components: key members and racked members. An MJ has two or more key members that are initially deposited at the upstream head of a point bar and oriented nearly parallel to bankfull flows. Key members usually have rootwads facing upstream and are within approximately one rootwad diameter of one another. Racked members of various sizes accumulate normal to key member rootwads, stacking on top of one another to heights of 6 m or more ... As the river migrates laterally, a stable MJ forms a revetment halting local bank erosion, often measurably compressing the river's radius of curvature and changing the orientation of the flow relative to the jam . These jams eventually armour the concave outer bank of a meander and harbour riparian forest patches proportional in size to the size of the jam.
Source: Adapted from Abbe, T. B. and D. R. Montgomery. 1996. Regulated Rivers:
Research and Management. 12:201-221.
All living systems display autocatalytic behavior, but when it occurs in nonliving systems, it is especially significant and interesting. Ecological engineers should try to take advantage of self-building behavior because it is another way to incorporate free natural energy inputs into a design.
The discussion of self-building behavior brings to mind abstract concepts of self-reproducing machines. John von Neumann (1966) was the first to successfully explore the logic of this ambitious concept (see also Penrose, 1959). He imagined a self-reproducing robotics system and then proved with mathematical logic that it could exist in the form of a cellular automaton, which has been called the von Neumann machine (Sipper et al., 1998). The latest developments in self-reproducing machines are the computer programs of artificial life (Langton, 1989; Levy, 1992) and new directions in robotics based on distributed structures of simple units (Brooks, 1991, 2002; Lipson and Pollack, 2000; Nolfi and Floreano, 2000). Development of autonomous robots (robots that build themselves) is a goal of the latter work which is relevant to self-building behavior. Progress in this field is occurring (Webb and Consi, 2001). Examples of simple robotics that exhibit complex behavior are being built, for example, mobile, insect-like machines that can locate and detonate land mines. One robotics researcher has described these new robots as living machines (Trachtman, 2000) because of their lifelike behavior. This is the term used by John Todd to describe his concept of constructed ecosystems (see Chapter 2). A robot living machine is a nonliving device (technology only) that exhibits lifelike behavior, while Todd's systems are hybrid devices (ecosystems plus technology) that combine living, biological components with nonliving, engineered components. Although the ecosystems of Todd's living machines can reproduce themselves, the associated technology of tanks and pumps cannot. It is interesting to speculate that future ecological engineering development might someday merge and create new forms of living machines with more intimate linkages between the physical machine and the living system. Gastrobots (food-powered robots) have already been built (Wilkinson, 2000) that conceptually could develop mutualistic relationships with species in ecosystems. The 1970 science fiction movie Silent Running presaged this future development with its portrayal of robots managing greenhouse-based lifesupport systems in a space station. Isaac Asimov's (1950) three laws of robotics (protect humans, obey humans, protect yourself) act as guides to robot behavior in his fictional future and might be reprogrammed for the future robotic ecological engineer as protect the ecosystem, serve the ecosystem, protect yourself.
Beavers (Castor canadensis) relate to this discussion because they actively build dams and take advantage of self-building behavior in several ways. Moreover, they have a role in erosion control that may offer a tool for ecological engineers under certain circumstances. Beavers are remarkable animals because of their many building behaviors that create a collection of "artificial" structures and that physically change the landscape. The structures they build include bank burrows, food caches, lodges, and dams. The dams are the most amazing of these structures, as noted by Johnson (1927): "The dam is generally the most conspicuous, and impressive of the beaver's works. The total amount of labor involved is often prodigious. The size may vary from one only a few inches in length and height, damming a tiny trickle,
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