Wadis, water and mud-bricks EarthCache

Wadis

Wadi (Arabic: وَادِي, romanized: wādī, alternatively wād; Arabic: وَاد, Maghrebi Arabic oued, Hebrew: וָאדִי, romanized: vadi, lit. 'wadi') is the Arabic term traditionally referring to a river valley. In some instances, it may refer to a wet (ephemeral) riverbed that contains water only when heavy rain occurs.

Wadis are located on gently sloping, nearly flat parts of deserts; commonly they begin on the distal portions of alluvial fans and extend to inland sabkhas or dry lakes. In basin and range topography, wadis trend along basin axes at the terminus of fans. Permanent channels do not exist, due to lack of continual water flow. They have braided stream patterns because of the deficiency of water and the abundance of sediments. Water percolates down into the stream bed, causing an abrupt loss of energy and resulting in vast deposition. Wadis may develop dams of sediment that change the stream patterns in the next flash flood.

Wind also causes sediment deposition. When wadi sediments are underwater or moist, wind sediments are deposited over them. Thus, wadi sediments contain both wind and water sediments.

Modern English usage differentiates wadis from canyons or washes by the action and prevalence of water. Wadis, as drainage courses, are formed by water, but are distinguished from river valleys or gullies in that surface water is intermittent or ephemeral. Wadis are generally dry year round, except after a rain. The desert environment is characterized by sudden but infrequent heavy rainfall, often resulting in flash floods. Crossing wadis at certain times of the year can be dangerous as a result.

Wadis tend to be associated with centers of human population because sub-surface water is sometimes available in them. Nomadic and pastoral desert peoples will rely on seasonal vegetation found in wadis, even in regions as dry as the Sahara, as they travel in complex transhumance routes.

The centrality of wadis to water – and human life – in desert environments gave birth to the distinct sub-field of wadi hydrology in the 1990s.^[1]

Wadi Sediments and sedimentary structure

Wadi sediments may contain a range of material, from gravel to mud, and the sedimentary structures vary widely. Thus, wadi sediments are the most diverse of all desert environments.

Flash floods result from severe energy conditions and can result in a wide range of sedimentary structures, including ripples and common plane beds. Gravels commonly display imbrications, and mud drapes show desiccation cracks. Wind activity also generates sedimentary structures, including large-scale cross-stratification and wedge-shaped cross-sets. A typical wadi sequence consists of alternating units of wind and water sediments; each unit ranging from about 10–30 cm (4–12 in). Sediment laid by water shows complete fining upward sequence. Gravels show imbrication. Wind deposits are cross-stratified and covered with mud-cracked deposits. Some horizontal loess may also be present.

Deposition

Deposition in a wadi is rapid because of the sudden loss of stream velocity and seepage of water into the porous sediment. Wadi deposits are thus usually mixed gravels and sands. These sediments are often altered by eolian processes.^[2]

Over time, wadi deposits may become „inverted wadis,“ where former underground water caused vegetation and sediment to fill in the eroded channel, turning previous washes into ridges running through desert regions.

Aflaj/Falaj

The Aflaj (Arabic أفلاج, DMG aflāǧ; plural of Falaj / فلج / falaǧ; also known as Falaj, Faladj; Aflaj, Afladj) constitute an irrigation system in Oman and the United Arab Emirates, which enables year-round water supply in the rain-scarce desert states. This system is unique among the Arab states. A selection of five sites of the Aflaj irrigation system has been part of the UNESCO World Heritage since 2006,^[3] and the associated traditions, knowledge, and skills were included in the UNESCO list of Intangible Cultural Heritage of Humanity in 2020.^[4]

There are three types of Falaj:

• The Aini (Arabic: العينية(, which are directly fed from the sources.
• The Ghaili (Arabic: الغيلية(, which receive their water from a wadi, and channel it mostly through open canals to the destination.
• The Daudi (Arabic: داوودية(, which start as groundwater in underground wells and channel this water through tunnels up to 20 meters deep with minimal slope over kilometers-long paths to the surface. During construction, watercourses are first identified by exploratory drilling, then shafts are dug, which are later connected and led to the farmland. The length of the underground facilities can be up to ten kilometers. This ensures the regular supply of fresh water with minimal evaporation losses. – In Iran, these underground channels are referred to as Qanat.^[3]
   

Al Khashbah

The site of Al-Khashbah is located in the Shamal Al-Sharqiya governorate, about 18 km to the northeast of the modern city of Sinaw at the southern foothills of the Al-Hajar Mountains. It extends more than five kilometres east-west and up to two kilometres in a north-south direction and is the largest known 3rd millennium BC site in the region. Al-Khashbah is characterized by at least nine monumental round buildings, so-called towers. The site is mentioned in the literature early on as it features with Building IV a unique structure that in contrast to all other 3rd millennium BC towers has a rectangular and not a round ground plan.^[6]

Around Al Khashbah, you can find many rock paintings,^[7] for example, at waypoint 2. You are welcome to visit the monolith, but please:

This is an archaeological site and a historical testimony of Omani culture, which is why you must NOT climb on the rocks under any circumstances!

The abandoned mud-brick settlements of central Oman

In the centre of many cities in Central Oman there is a mud-brick settlement that has been abandoned since the 1970s as part of the country’s modernization processes. More recently, the mud-brick settlements have been at the centre of conflicting interpretive processes. Scientists and national institutions warn of the decline of this unique cultural heritage, while local populations have little interest in their active use, but do not want to forego the settlements either. The abandoned settlements are visible structural representations of the past, while, at the same time, occupy uncertain spaces in the present and future.^{[8 ]}

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