Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 176

gravel to the channel, and concerns about the impact on the gravel supply were raised in response to
recent proposals to expand bank protection along the river.
Fig. 3.61 A bottom-dump scow loaded with a sand-gravel mixture is dumping gravel onto the Rhine River bed
downstream of the Iffezheim Dam to mitigate the incision of the river (after Kuhl, 1992).
In many cases, the root causes of incision are changes in runoff or sediment generation at the
watershed scale. In many European rivers, afforestation and small dam construction in the late 19th
century reduced the delivery of sand and gravel to the channel, causing incision, which was later
exacerbated by gravel mining and further dam construction (Bravard et al., 1997).
In the cases where the incision has proceeded so far that it cannot be reversed, or where the factors
causing the incision cannot be controlled, the most practical approach may be to accept the incision and
simply mitigate the effects as best as possible. Because of the environmental effects of lowered water
tables, efforts have been made to restore alluvial water tables even if the bed elevation cannot be restored.
Along the Gardon River, France, an impermeable dike was buried at depth in the floodplain to dam
groundwater, and to restore moisture conditions in the riparian floodplain. In the riparian forest along the
Rhone River, a permeable canal has been proposed (Fruget and Michelot, 1997; Stroffek et al., 1996) to
supply the forest with water.
In the mountain rivers where the bed surface contains large blocks and boulders the natural resistance
against bed erosion is relatively high. Large, isolated boulders induce local energy losses. If the big
concrete components are selectively added to the bed surface, then the natural resistance can be increased
by taking into account the contribution of the components, which are already in place. For extreme flows
this contribution may be insufficient, and the components with individual masses of 10 to 40 t are required.
As a result, they have to be manufactured on the spot. While the fact that these elements are made of
concrete may not please environmental purists, the application of heavy concrete elements rather than
high check dams leaves the low flow conditions in an almost natural state. It is argued that this technique
is also a major contribution to river restoration. Such techniques set up hidden training structures which