E9 Dynamic Geomorphology

Topic

Dynamic geomorphology studies the physical processes that shape the Earth's landscape: the erosion, transport, and deposition of material, occurring both on hillslopes and within the channels through which this material eventually moves.

On hillslopes, mass movement encompasses a full spectrum of speeds, ranging from slow creep—advancing merely a few millimeters per year—to the catastrophic landslides described elsewhere, which can move at speeds of several meters per second.

Erosion caused by hydrological processes—such as direct raindrop impact, sheet erosion, and rill erosion (previously described in the context of extreme erosion)—is the primary mechanism of degradation in agricultural watersheds; indeed, this hydrological erosion is responsible for the majority of the sediment ultimately transported by rivers.

The rate of fluvial incision—that is, the deepening of the channel into its underlying bedrock—follows the stream power model: incision is proportional to both the watershed's drainage area (acting as a proxy for discharge) and the channel slope, with each factor raised to an empirically determined exponent. This model explains both the characteristic concave shape of river longitudinal profiles and the formation of waterfalls and gorges.

The coupling between hillslopes, the channel, and sediment is the key to this entire system, forming a closed loop: hillslopes supply sediment to the channel, the channel transports and deposits that sediment downstream, and changes in the channel's base level feed back into hillslope erosion—accelerating or slowing it depending on whether the base level rises or falls.

Coastal erosion, meanwhile, involves wave action, the longshore transport of sediment along the coast, and sea-level fluctuations; This littoral sand transport follows an energy-based formula in which the volume of transported sediment scales with the square of wave power; coastal dunes and sand spits are the characteristic landforms resulting from this very process of littoral sediment deposition.

The expected behavior of this dynamic geomorphology includes coastal retreat ranging from one-tenth of a meter to ten meters per year along unprotected coastlines—a rate that accelerates with rising sea levels. Thus, from the continuous mass movement with which this document begins to the coastal retreat with which it concludes—encompassing hydrological erosion, fluvial incision, and slope-channel coupling—dynamic geomorphology ensures that the terrestrial landscape is constantly reshaping itself, from the crest of every slope down to the shoreline where the continent finally meets the sea.

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