SMALL DAMS PETITS BARRAGES

� The elements for surface drainage are usually applied at dams higher than 6 m. At
lower dams, the fundamental condition of a good drainage is a good and maintained grassing
over of the downstream slope or an effective and stabile fortification of the upstream slope.
� For the design of the surface drainage, knowledge of the filtering environment (mutual
relations regarding the permeability of the dam and the background) is important. It is
unnecessary to use the surface drainage in a case, when a permeable sandy dam is founded
upon a permeable (sandy or gravelly) and more than 3 m thick background.
� At dams higher than 10 m the slopes are usually divided by berms, which in a case of
downstream slope use to be 5 ÷ 10 m vertically from each other, are min. 1,5 m wide (better 3
m) and have a longitudinal inclination 1 ÷ 2 %. The berms restrict the flow velocity of rainfall
waters on the slopes and contribute to the protection of the dam surface against the water
erosion.
� From the dam crest, upon which a road surface is solidificated by asphalt, blocks etc.,
the rain-fall waters are drained to both abutments of the dam by a surface-water sewerage
system.
5.9.2 Possible Mechanical Solution of the Surface Dewatering
� Upon the lower part of the downstream slope and to the area of downstream toe, into
a partly digged out adjoining downstream area (ditch-shaped), it is possible to put layers of
permeable materials in a configuration of a inverted filter (Figure 5.10).
Figure 5.10 – Configuration of a inverted filter.
By this modification the surface water will be drained into a downsloped permeable
element. The construction is suitable for earthy or earthy-sandy homogenous dams for
territories with mild winters (without a risk of freezing), with a dam height approx. to 10 m.
� The formation of longitudinal strengthened open ditches, draining gravitationally
the rainfall water running upon the grassy downstream slope along the downstream toe to the
channels from outlets or from spillways. For to limit the expansion of energy of the flowing
water it is convenient to place the dewatering ditches or rills also upon the berms, as long as
the dam has them. The waters are then safely deflected into the downstream area upon a
slope with a strengthened slope in the dam abutment.
� In case of concerns for a local damage of the construction by a concentrated rain-fall
water at other dam types (especially made of concrete and masonry) it is possible to utilise
common elements of a surface-water sewerage system (e.g. plumber’s downcomers etc.).
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