- Design freeboard should be a minimum 1.0 m and crest width not
less than 2.5 m.
- Outlet pipework should be surrounded in concrete and laid in trench
in the natural foundation with an upstream control valve.
- Allowance for long-term settlement should be a minimum 10% of
dam height.
- Simple protection to the upstream face can be provided by rubble
and suitable aquatic vegetation.
- Operational drawdown rates should be kept to less than 1 m/week.
- Provision should be made for measuring seepage flows.
- Good vegetative cover should be encouraged.
The design of small dams and farm dams is described in Stephens (1991).
2.10.3 Flood control banks and dykes
Structures of this nature differ from conventional embankment dams in a
number of important regards:
- they are of modest height but of very considerable length, with con-
sequent variation in soil type and consistency of construction stan-
dard;
- they are not generally ‘engineered’ in the sense of being a form of
embankment dam;
- operation as a dam is occasional only; this may result in leakage asso-
ciated with shrinkage and cracking of more cohesive soils;
- the section is minimal, with steep slopes;
- they have poor resistance to local overtopping at low spots;
- they are low-profile, at least until a catastrophic flood event;
- they are prone to damage by root growth, by burrowing animals and
by animal tracking;
- construction is invariably ‘lowest cost’;
- flood banks do not normally have a core;
- surveillance and maintenance is frequently minimal.
With the foregoing general points of difference in mind it is suggested that
flood bank designs should follow the guidelines listed below.
- side slopes no steeper than 2.0:1.0; flatter if subject to wave attack or
to animal tracking;
- crest width adequate for access, including light plant, and not less
than 3.0 m in width;
- settlement allowance not less than 10%;
- good vegetative cover;
- careful selection and use of soil types;
106 EMBANKMENT DAM ENGINEERING
