Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 172

Construction of bed sills is sometimes accomplished by simply placing the rock along the streambed
to act as a hard point to resist the erosive forces within the degradation zone. In other situations, a trench
may be excavated across the streambed and then filled with rock. A critical component in the design of
these structures is ensuring that there is a sufficient volume of non-erodible material to resist the general
bed degradation, as well as any additional local scour at the structure. This is illustrated in Fig. 3.56,
which shows a riprap control structure designed to resist both the general bed degradation as well as any
local scour that may be generated at the structure. In this instance, the riprap section must have sufficient
mass with an acceptable thickness to fill and protect the anticipated scour hole depth.
Fig. 3.56 (a) A riprap control structure with sufficient launch stone to handle anticipated scour and (b) the riprap
structure in response to bed degradation and local scour
In the case of the sloping riprap drop structures used by the Denver Urban Drainage and Flood Control
District, an impervious clay fill is used in conjunction with a lateral cutoff wall (McLaughlin Water
Engineers, Ltd., 1986). This design is illustrated in Fig. 3.57. A significant feature that distinguishes the
sloping riprap structure from the other structures is the pre-formed, rock protected, scour hole. A scour
hole is a natural occurrence downstream of any drop whether it is a natural overfall or a human-made
structure. The lateral extent of the scour hole must also be considered to ensure that it does not become
so large that the structure is subject to being flanked. With many simple grade control structures in small
stream applications, very little, if any, attention is given to the design of a stilling basin or pre-formed
scour hole. The erosion is allowed to form the scour hole. However, at higher flow and drop situations, a
pre-formed scour hole protected with concrete, riprap, or some other erosion resistant materials is usually
required to dissipate energy and to eliminate uncertainties in the size and location of the resultant scour
hole. This scour hole serves as a stilling basin for dissipating the energy of the plunging flow.
Weirs are widely used to control channel incision. In effect, weirs act as artificial bedrock outcrops,
boulder clusters, or large organic debris in the channel bed. They do not solve the incision problem
downstream in cases of sediment starved flows or increased shear stress resulting from channel
confinement; rather they physically control the downward cutting of the riverbed in one reach. As shown
in Fig. 3.58, the bed profiles of the Giffre River in France measured in 1912 and 1988 show how weirs
control the downward cutting of the riverbed. Such solutions may induce fish habitat degradation by
blocking migration and increasing reaches with very low slopes. In some cases, installation of weirs in an
incising channel can improve aquatic habitat, partially restoring some of the habitat values lost in the
process of channel incision. However, their greatest utility is probably the prevention of upstream
incision.
DeBano and Schmidt (1989) also illustrated that weirs can raise the water table and favor riparian
vegetation recolonization in a desert area (Fig. 3.59). On Twentymile Creek, Mississippi, U.S. weirs