ABSTRACT
Looking back on the history of navigation on River Rhine, the morphologic conditions within the stretch between Mainz and Bingen (Rhine-km 493–529) continuously caused a great effort to achieve reasonable fairway conditions.
Located at the lower end of the Upper Rhine, the stretch shows the least slope of the free flowing Upper Rhine with cross section widths up to 900 m and mean water depths during the effective discharge concerning morphodynamics of about 4 to 5 m. Sediment supplied from the upstream part of the Upper Rhine mainly tends to deposit within the stretch. Until the beginning of river regulation, these conditions led to a dynamic, braided river morphology between Mainz and Bingen, coming along with moving shoals and obstacles hindering navigation.
Against this background, the article exemplifies past, current and potential future river engineering measures such as river regulation and sediment management within the stretch, aiming at the improvement of navigational conditions. By means of morphodynamic analysis of the stretch, the morphodynamic responses to the mentioned anthropogenic activities and other changing conditions such as potential climate induced changes of hydrological conditions are shown. The findings serve as a base for the adaptation and the development of further measures concerning sediment management and river engineering.
An indispensable element of sediment management within the stretch is the bed load trap located nearby Mainz Weisenau (Rhine-km 494.3–494.46). The bedload trap was implemented in 1989 with a width of 250 m, a length of 160 m and a depth of 1.5 m related to the surrounding bed level. The measure aims at the reduction of bedforms such as dunes passing the stretch between Mainz and Bingen, as bedforms are dynamic obstacles within the fairway and thus difficult to locate in order to be dredged. Up to the year 2014, the bedload trap was emptied 28 times with a dredging volume of about 1.99 million m3 of sediment. 27% of the material was supplied downstream of the bedload trap, whereas 73% was removed from the system. The reduction of dunes occurring within the stretch could be reached successfully by this measure. Nevertheless, comparisons of echo soundings of the last two decades show a trend towards bed degradation downstream of the bedload trap. The current magnitude of degradation indicates a modification of the dredging practice aiming at an increased sediment input into the downstream stretch. Ongoing investigations are focusing on the impact of modified dredging rules and geometries of the bedload trap on the further development of bed levels downstream of the measure.
Furthermore, examples of current morphological phenomena occurring locally between Mainz and Bingen are presented. One phenomenon of interest is a scour occurring in the right-hand branch of the bifurcation between Eltville and Ostrich (Rhine-km 512.5). The coarse upper layer with a thickness of about 0.2 m up to 0.3 m, mainly consisting of gravel and cobbles is eroded locally, exposing the subjacent Tertiary clay. Detected for the first time in the year 2004, the scour dimension increased continuously up to a volume of about 2300 m3 and a depth up to 1.7 m below the surrounding bed level in 2014. Potential countermeasures to stabilize the affected riverbed are subject of current investigations.
Looking from present to future, the potential development of bed levels up to the year 2100 within the stretch between Mainz and Bingen is shown. The underlying studies, based on 2D-morphodynamicnumerical long-term simulations were conducted to quantify the potential effort necessary to maintain the today’s fairway conditions even under changed climatic conditions.
