SMALL DAMS PETITS BARRAGES

These spillways can be lined or unlined. Normally the following problems are identified for
existing by-wash spillways.
7.2.2 Spillway Capacity
Inadequate flood handling capacity not meeting design standards is normally caused by
underestimating the design flood, not acknowledging the effect of the upstream approach
channel on the hydraulic gradient or not ensuring that the dam was constructed with its total
freeboard over the entire crest length of the dam. The hydraulic control must be correctly
defined for the design flow by determining the hydraulic flow and energy lines based on the
correct sectional information. Furthermore, the design floods associated with the classification
shown in section 5.2 can be used as a guide.
7.2.3 Backward Erosion Control
Backward erosion of a spillway channel occurs due to fast flowing water over soil and
weathered rockfill materials. An example is shown in Figure 7.1.
Fig. 7.1 – Backward erosion of the by-wash Spillway of Toleni Dam in South Africa
This may lead to undermining of the hydraulic control structure e.g. a concrete sill or a
weir if that structure is also founded on erodible material. Backward erosion can be prevented
by providing a control structure e.g. concrete weir founded on a solid unerodible foundation.
Erosion can be controlled by limiting the depth and the velocity of the water depending on the
type of gravel or soil medium between grasses in the case of grass lined channels. In arid
areas care must be taken not to use grass as the grass will die and provide no resistance to
erosion i.e. it will not be effective. A “better practice” is to protect the soil with a liner or rockfill
as described in the following sections. The erodibility of spillways in unweathered to
moderately weathered rock can be evaluated in accordance with water power and the rock
strength using the chart shown in Figure 7.1 (Van Schalkwyk, 1994). The following formula
applies:
P = ρw * Q1:100/b * s
Where
P = Power per unit area (kW/m 2 )
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