2.10.2 Small farm and amenity damsLarge numbers of very small privately owned reservoirs are constructed
for a range of purposes. These include farm dams built to supply local irri-
gation needs and/or to support stock-rearing and amenity dams built to
impound private fishing lakes. Most are very modest in scale, impounded
by small and simple embankment dams, but it is important to recognize
that the same basic principles as regards satisfactory design and construc-
tion practice apply to these dams as much as to the largest.
Low cost is clearly a dominant consideration in the engineering of
small private dams, and in-service supervision and maintenance is likely to
be minimal unless the dam is subject to legislation regarding safety (see
Chapter 7). The design must therefore be simple, conservative and robust.
Translated into practice the following general observations on small dams
of this nature should be noted:
- A homogeneous profile is likely to prove most economic given the
ready availability of soil with a clay content of c. 20–30%. Signific-
antly higher clay contents should be avoided, as they can lead to
problems with shrinkage and cracking. - A simple zoned embankment, employing clayey soil to form an
upstream low-permeability ‘core’ with a base width of approxi-
mately twice the height is advantageous from the point of view of
stability. - Where the local available soils are of higher permeability these may
be used to construct the bulk of the embankment, with a 1 m thick
blanket of clayey soil placed on the upstream face. - Alternatives for the watertight element for a small dam of this type
include a PVC or polyethylene membrane, but these should be pro-
tected by at least 0.6 m of soil. - An embankment with a central 0.6 m thick bentonite diaphragm wall
forming a ‘core’ is a further option. Walls of this nature can be
extended below the base of the dam to form a cut-off, but they are
comparatively expensive to install. - There should be a rubble toe-drain or a base drain under the down-
stream shoulder. - A simple key trench cut-off is satisfactory if a suitably impermeable
clay soil is present at shallow depth. On a more permeable founda-
tion it will be necessary to go considerably deeper to form a cut-off,
e.g. with a bentonite diaphragm wall extending at least 0.6 m into a
suitable low-permeability horizon. - Face slopes should be no steeper than 2.5 : 1.0 upstream and 2.0 : 1.0
downstream. - Overflow capacity and freeboard should be adequate to accept a
design flood with a return period of not less than 100–150 yrs.