Lecture 9 pt 1

Return Periods of Extreme Events

Tomorrow's practical exercise focuses on understanding return periods of extreme events. The goal is to understand how return periods are determined and their limitations. When media outlets report events like floods as 'one in a thousand year event,' it's often just an estimation due to limited observational records. For example, in Australia, records for rivers like the Brisbane River typically span about 90-95 years. This short observational window makes it difficult to accurately determine the characteristics of a one in 500 or 1000-year event.

Wrapping Up Arid Landscapes

Today's lecture concludes the discussion on landscapes, landforms, and processes in arid lands. The lecture on cold climate environments and landscapes will be presented in the last lecture of the semester, along with an exam brief. The exam is already written and under review.

Rainfall Patterns and Aridity Classifications

Rainfall patterns across Australia show that aridity increases rapidly away from the coast. Different classifications of aridity exist, such as semi-arid, arid, and hyper-arid. Quaternary scientists sometimes refer to landscapes becoming "more arid" based on pollen records, but it's important to quantify what that means, such as by identifying specific vegetation assemblages associated with different rainfall regions.

Climatology and Reference Periods

A climatology of a region typically relies on 30 years of observations to encompass natural climate oscillations, such as the Indian Ocean Dipole, El Niño Southern Oscillation (ENSO), and variability in the Southern Annular Mode (SAM). When analyzing climatological data, it's important to consider how the reference period may have changed over time and the influence of global warming on climatological averages.

Agents of Geomorphic Change in Arid Landscapes

Geomorphic change refers to changes in the physical attributes and properties of the Earth's surface. In arid landscapes, key agents of change include:

Aeolian Processes

Wind plays a crucial role in the entrainment, transport, and deposition of sediment. These are referred to as aeolian processes. Aeolian processes in arid environments are intertwined with fluvial processes.

Fluvial Processes

Fluvial systems in arid environments are often ephemeral, with streams and lakes that are dry for extended periods. However, water action is significant for eroding sediments, eroding valleys, transporting sediment, and creating alluvial fans or valley trains of sediment.

Arid environment landforms are a product of the combination of both aeolian and fluvial processes. For instance, the Channel Country in Far Western Queensland experiences significant flooding due to monsoonal rains in the Diamantina River system. The landscape is shaped by alternating wet and dry periods.

Lake Eyre and Inland Drainage Basins

Lake Eyre is a large, internally draining basin covering about 1,400,000 square kilometers. It fills episodically, with water primarily sourced from Queensland. Floodwaters move through major river systems like the Thompson River, Cooper Creek, Diamantina River, and Georgina River, eventually reaching Lake Eyre. The lake's episodic filling is measured in cubic kilometers of water.

Classifying Arid Landscapes: Shield and Platform Deserts

One classification scheme for arid landscapes is shield and platform deserts, which are typical of Australia. These areas are located in tectonically stable regions without active fault zones or mountain building. They are characterized by vast stable plains, which may be surmounted by remnants of old mountain ranges. These landscapes are dominated by erosional cut surfaces, resulting in large, shallow basins like the Lake Eyre Basin.

The present form of these systems is largely a product of geomorphically active systems in the geologic past. Remnants of old mountain ranges may be present. In simplified form, old mountain ranges are worn down and infilled with valley systems, creating large alluvial aprons. Relic or inactive fault signs may also be present. More erosion-resistant rock can be seen surmounting the landscape.

Scarp Retreat

A scarp is a sharp break in the surface of the landscape, often caused by erosion. Scarp retreat is when this sharp break erodes and moves backwards. Water flowing over the surface erodes the scarp, causing it to retreat. More resistant rock will be exposed as the feature scarp retreats.

Pediments and Lateral Planation

Material that breaks off from remnant ranges is transported down into valleys, forming pediments. A pediment is a smooth, planar, concave upward erosion surface. It typically slopes down from the foot of a highland area and is graded to a local or more general base level. These features are thought to result from lateral planation of a river, where the river erodes across the valley floor. Rivers oscillate backwards and forwards across the valley, which erodes material and cuts down. Pediments may also have formed in more humid conditions and later exposed by erosional processes.

Alluvial Fans

Alluvial fans form between high relief areas in arid environments. Due to the lack of vegetation, water rushes off mountain surfaces quickly during rainfall events, picking up loose material and transporting it down into lower relief areas. This material is deposited in a fan shape as the river oscillates across the surface. Coarser material is deposited on the fan, while finer material (silts and clays) is deposited in the valley floor, forming clay pans.

Dry Streams (Wadis or Washes)

Dry streams in arid environments, also known as wadis or washes, are ephemeral, with water flowing through them only for short periods. Their hydrographs are spiky. A hydrograph is a time series of discharge from a stream. In arid environments, hydrographs show a very rapid increase in discharge during rainfall events, leading to flash flood conditions. This contrasts with the more prolonged flooding seen in environments like the Channel Country.