Hydraulic empire

Summary

A hydraulic empire, also known as a hydraulic despotism, hydraulic society, hydraulic civilization, or water monopoly empire, is a social or government structure which maintains power and control through exclusive control over access to water. It arises through the need for flood control and irrigation, which requires central coordination and a specialized bureaucracy. Often associated with these terms and concepts is the notion of a water dynasty. This body is a political structure which is commonly characterized by a system of hierarchy and control often based on class or caste. Power, both over resources (food, water, energy) and a means of enforcement such as the military, is vital for the maintenance of control. Civilizations A developed hydraulic civilization maintains control over its population by means of controlling the supply of water. The term was coined by the German-American historian Karl August Wittfogel (1896–1988), in his book Oriental Despotism: A Com

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https://en.wikipedia.org/wiki/Hydraulic_empire

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Hydro-morphological processes through permeable sediment traps at mountain rivers

Sebastian Schwindt

Sediment traps are used for the protection of urban settlements at rivers in mountainous regions. These structures aim at the retention of sediment in the case of hazardous floods, but existing sediment traps tend to retain sediment also when the discharge is not hazardous to the downstream urban regions. This excessive retention of sediment causes an interruption of the river continuum that may lead to channel incision and the morphological depletion of downstream reaches. Another problem is the remobilization of formerly deposited sediment during a flood, which is addressed here in terms of the unwanted flushing of sediment traps. This research project aims at the development of sediment traps which are permeable up to a certain flood, but not susceptible to unwanted sediment flushing. Typical sediment traps consist of a retention area upstream of a barrier or check dam equipped with openings. The barrier can trigger the retention of sediment in the deposition area either by hydraulic control or by mechanical control. The hydraulic control leading to deposition is achieved by check dams with one or more openings constricting the flow vertically and/or laterally. Improved formulae for the estimation of the discharge capacity of such constrictions have been experimentally obtained for rough, turbulent upstream flow conditions with bed load under varying channel slopes. The constriction-induced head loss and reduction in the bed load transport capacity based on the bed shear stress are analyzed as a function of the upstream flow depth and discharge. The experiments show that the flushing of upstream sediment deposits may occur at open-crested slit check dams or close-crested slot check dams, but only when the latter are overtopped. The mechanical control leading to sediment retention is achieved by screens with vertical bars. The horizontal space between the bars corresponds to the characteristic grain size of traveling bed load. The required bottom clearance under such screens was optimized here in view of the possibility of bed load transfer for small (flood) discharges on the one hand, and the ensured clogging of the screen for high (flood) discharges on the other hand. This optimum bottom clearance height was found to be 1.75 times the characteristic grain size that is transported during floods. Once the bar screen was clogged, the unwanted sediment flushing could not occur anymore. However, the clogging depends on the estimation of the characteristic grain size. The experimental study shows that the combination of mechanical and hydraulic control structures provides a reliably working solution for permeable sediment traps. Smaller bed load-laden discharges can pass unhindered through such combined barriers. For higher discharges, the hydraulic control causes backwater which reduces the influence of the characteristic grain size on the clogging of the bar screen. Moreover, the bar screen prevents unwanted sediment flushing through the hydraulic control. The implementation of a guiding channel across the retention area is introduced and was experimentally verified as being a pertinent structural tool for improving the eco-morphological flow continuum. Finally, the design of a permeable sediment trap is described based on an optimal interaction between a guiding channel and a barrier combining the mechanical control by a bar screen and hydraulic control by a slot check dam.

EPFL2017

In alpine regions, villages are often situated near alluvial fans, which are fed by mountain rivers transporting high sediment load during floods. In the past, these mountain rivers on the alluvial fans have often been channelized near urbanized areas, where the bed and banks were lined with large sawed granite or gneiss blocks laid into concrete layer. Some of these channels have still a mobile bed, which is fixed with transversal sills against erosion. These channels have a high discharge and sediment transport capacity as long as no sediment deposits occur. The latter may be triggered by constrictions as they occur at bridges or by backwater effect when the channel enters the main river in the plain. In order to avoid sediment deposits in the channel, which can create dangerous overtopping with catastrophic consequences in the nearby urbanized areas, sediment traps have been installed upstream of the alluvial fans with the purpose to retain bed load during such critical floods. Practical experience with most of these sediment traps have shown that they retain already sediments for quite low and not yet dangerous floods. Thus, for such conditions the morphology of the river downstream becomes impoverished since traveling bed load creating gravel bars during frequent floods is missing. Furthermore, it has been observed in many cases that unwanted flushing of sediment traps occurred during the flood where they should safely retain sediments. In his research, Dr. Sebastian Schwindt developed a new concept of sediment traps, which are permeable for bed load transport up to floods which are frequent and not yet dangerous for the downstream reach, but which retain safely sediment for hazardous floods. With systematic laboratory experiments, it could be revealed that a guiding channel implemented across the deposition area upstream of the barrier, having a combination of mechanical and hydraulic control structures, provides a reliably working concept for permeable sediment traps. Combined mechanical-hydraulic control is achieved by an appropriate orifice or slot in the barrier, which is equipped with an especially designed bar screen in front of it. Smaller bed load-laden discharges, which correspond to the bank-full discharge of the guiding channel, can pass unhindered through such combined barriers. For higher discharges, the hydraulic control at the orifice or slot provokes a backwater, which reduces flow velocities in the upstream guiding channel, causing of the channel banks followed by bedload deposition. The latter, especially in the presence of large boulders, lead to a mechanical blocking of the bar screen which can then prevent unwanted sediment flushing with increasing filling of the deposition area. As a basis for the new sediment trap concept, Dr. Schwindt could give design recommendations about the required bottom clearance of the bar screen and the spacing between the vertical bars, as well as the size of the orifice or slot in the barrier and the function of the guiding channel capacity.

EPFL-LCH2017

Hydro-morphological processes through permeable sediment traps at mountain rivers

Sebastian Schwindt

EPFL2017

EPFL-LCH2017