River's End Torrent ^NEW^
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A torrent river, or wave river, is a related concept. Torrent rivers feature wave machines similar to those that are in wave pools; the waves then push riders (who are on rafts, as they are in a regular lazy river) around the river faster than they would be traveling in a regular lazy river. Torrent rivers appear at all of the Schlitterbahn water parks and Aquaventure in Dubai and the Bahamas.
The Kankakee Torrent was a catastrophic flood that occurred about 19,000 calibrated years ago in the Midwestern United States. It resulted from a breach of moraines forming a large glacial lake fed by the melting of the Late Wisconsin Laurentide Ice Sheet. The point of origin of the flood was Lake Chicago. The landscape south of Chicago still shows the effects of the torrent, particularly at Kankakee River State Park and on the Illinois River at Starved Rock State Park.
The Kankakee Torrent was responsible for extensive modification of the Kankakee River and Illinois River river valleys and landforms characteristic of megaflooding. Both the Kankakee River and Illinois River largely follow paths carved out by the torrent, a process that is believed to have taken only days. Most notable today is a region in north-central Illinois known as Starved Rock; while most of Illinois is located on a low-lying plain with little variation in elevation, Starved Rock State Park features several canyons which were created in the Kankakee Torrent. Another, very different, geologic effect left over from the Kankakee Torrent is the existence of "sand prairies". Sand prairies exist where the massive flood waters stopped their movement and deposited large quantities of sand. When European settlers arrived, one remaining sign of these deposits were sand dunes located along parts of the flood's course.
The flood did not occur just once, or all at once. It was a repetitive event, over possibly hundreds of years. In the early years it was doubtless small, but as the years passed it became larger and larger until at some time it reached a maximum, and then, as the Valparaiso glacier receded, it gradually subsided. Before the torrent, the valley of the Kankakee River near the city of Kankakee, Illinois, was neither deep nor broad. It was a wide plain of Marseilles drift, with a small river. The early outflows spread across this plain. At its highest level, the torrent found channels in the Minooka ridge and flowed across the ridge to the drift plain in the west. As the outflows continued, the Marseilles drift plain in the east was eroded. This removed much of the drift and even began eroding into the Silurian dolomite beneath. In the last stages, channels were cut in the bedrock.
This paper aims to discuss the modification of small rivers and the changes in flood protection on the Gürbe River, a tributary of the Aare River. In its upper reach, the Gürbe has the characteristics of a mountain torrent, making flood protection particularly difficult. This article addresses the following questions: (1) Why are small watercourses useful case studies on historical flood prevention measures? (2) What kinds of flood protection measures were implemented on the Gürbe River, and did they correspond to the prevailing protection philosophy? (3) How were protection measures connected to flood events? (4) How has flood protection on mountain torrents changed in the last two decades? (5) How did flood protection measures impact land use?
Despite the advantages of small watercourses for longitudinal studies, historical research has so far mostly focused on the large rivers, especially in densely populated areas (e.g., Bernhardt 2016; Bronar Cafaro 2004; Cioc 2002; Lewis 2005; Lübken 2014; Reynard 2009; White 1995). Flood protection measures on medium and small rivers have been researched, by, among others, Armenat (2012), Deutsch (2007), Heinzmann (2019), Hügli (2007) and Speich (2003). Himmelsbach (2014) analysed and compared the historical floods and flood protection measures on 15 non-navigable tributaries of the upper Rhine River. Studies on the historical flood protection measures on alpine rivers were published by Girel (2008), Gurnell et al. (2009), Hauer et al. (2019) and Hohensinner et al. (2020). Mountain torrents have been an important research object of hydraulic engineering sciences in the last decades since they face major challenges such as the overageing of the existing protective structures (see Stauder 2014). However, the historical perspective has remained sparse: Piton et al. (2016) discussed the French experience of 150 years of torrent control works and compared it to other countries. Göttle (1996) presented an overview over the last hundred years of torrent control in Bavaria, Aulitzky (1994) and Patek (2008) examined the historical torrent control in Austria, Blinkov et al. (2013) in the Balkans, and Jakubis and Jakubisová in Slovakia (2019). Case studies were conducted, amongst others, by Egloff (2016) and Keller (2013).
After its source at an altitude of 1685 m above sea level on the Alp Obernünenen in the Gantrisch region, the water flows as a small rivulet through the conical headwaters for a few hundred metres. Afterwards, the Gürbe River runs through a steep gorge. Due to the soft rock in this area (predominantly flysch and molasse), the river and its tributaries cause huge amounts of bed load that the river discharges during high water since flysch is easily eroded and water impermeable. Moreover, the area is highly unstable and therefore prone to landslides. After the gorge, the slope lowers significantly, and the water deposits the transported debris on a large alluvial fan. With the steep slope, the small catchment area of only 12.1 km2, the strongly varying runoff and the temporarily high sediment discharge, the upper reach of the Gürbe River fulfils the criteria of a mountain torrent (cf. Loat and Meier 2003).
The end of this transition zone marks the beginning of the lower reach of the Gürbe River. From here, the river flows about 20 km through the flat valley floor. The valley opens in the area of Belp, where the Gürbe River finally enters the Aare River. The tributaries in the lower reach are smaller and less dangerous than the torrents in the headwater, but these affluents can cause considerable damage since they swell quickly after heavy rainfall.
About two-thirds were caused by thunderstorms, a bit less than a quarter by long-lasting precipitation, and just ten percentage by a combination of precipitation and snowmelt. Most floods occurred during summer (seasonal distribution: DJF: 5, MAM: 7, JJA: 47, SON: 15, N/A: 4). This analysis reveals a dissimilarity in the flooding in the river's upper and lower reaches.
Old maps and regulation plans provide important sources for the reconstruction of the river's environmental conditions before regulation (Black 1997; Horst 2008; Schenk 2003). In some stretches the Gürbe River flowed as a braided river, in others it meandered. As was common practice in the Middle Ages and Early Modern times, the main flood protection strategy was to avoid hazardous areas (Vischer 2003). Only in areas where the locational advantages outweighed the disadvantages of the flood hazard or where were no other possibilities existed did people try to protect themselves and their properties. The early hydraulic engineering measures were mostly done in urban areas, where the water served many different purposes such as sanitation, transportation, power supply, fishing, or waste disposal. These water uses frequently required interventions in waterways which were often contrary to flood protection measures (Longoni and Wetter 2019). Landowners and users mostly carried out small-scale measures, often creating problems downstream (Schmidt 2000; Vischer 2003).
Only three settlements in the Gürbe valley were on the river: Belp near the river's mouth and Blumenstein and Wattenwil on the alluvial fan. Settling on alluvial cones was popular despite the considerable hazard of floods and debris flows since the soils were more fertile and less swampy than the ones in the valley floors, making them better not only for agriculture but also for building. To protect the houses and the arable land from the water, longitudinal and transverse structures such as sills were built (Egger 1958).
Except for the aforementioned villages, all other settlements and all major roads were situated on the flood-safe hillsides. The wet areas with poor-quality soils in the flood plain were used as common lands as was typical in traditional agriculture (Pfister 1995). Accordingly, people only minimally used the valley floor for activities like pastures, peat cutting or reed harvesting. Crops were only planted in elevated areas (Graffenried 1761). Despite the limited land use, some old maps and documentary sources indicate that riparian communities had already built small hydraulic structures to protect their fields from the Gürbe River. Wooden training structures, dams and fascines (bundles of brushwood used protecting the banks of streams from erosion) amended and stabilised the watercourse and sills made of timber and stone levelled the gradient. However, all early efforts were uncoordinated and therefore only effective for a short time (Bericht des Regierungsstatthalters, 26.11.1832). In the 1730s, people even straightened a longer river stretch with the intention of facilitating rafting (Hartmann 1752). Maps from later decades suggest that this straightening did not last. In the steep upper reaches, no hydraulic structures were built yet. Although check dams had been tested in Tyrol in the sixteenth century, the gorges of the European torrents generally remained unobstructed until the second half of the nineteenth century (Duile 1826; Schnitter 1992).
The implementation of the river regulation and the torrent control was possible due to political changes in the Canton of Bern, where the Gürbe was located, during the first decades of the nineteenth century. In the context of political transformations and state-building processes, hydraulic engineering laws were enacted from the 1830s onwards. The canton started to subsidise hydraulic engineering projects and the implementation of large-scale projects therefore became feasible. 2b1af7f3a8