- the owners’ resources may be limited;
- small dams are commonly of simple homogeneous section;
- problems of seepage and stability may be accentuated by minimal
profiles;
- the perceived hazard may be relatively low.
Measured against the standards now applied for larger dams, there may
consequently be some moderation in the standards applied in the engin-
eering of many smaller (and essentially older) dams. Examples include
standards employed in specifying the design flood (see Section 4.2 and
Table 4.1), or in the incorporation of adequate seepage control measures.
In the case of the first example, flood estimation for a very limited catch-
ment may of necessity be done on an empirical basis. This may in turn lead
to moderation of the standards applied for freeboard and for overflow
capacity. Engineered seepage control measures, the other example identi-
fied, are critical to a good and safe design, but may be difficult to incorpo-
rate to the desirable degree other than at unacceptable cost. These and
similar problems are usually linked to severe constraints on the resources
available to the owner, compounded in many cases by pressure for early
commissioning of the project. Collectively these and other difficulties,
including underinvestment of time and money in a proper site appraisal, or
problems in construction control, have historically led to significantly
higher incident and failure rates for small privately owned dams (Moffat,
1982).
Minimal standards for the design of satisfactory lesser dams are sug-
gested below:
- side slopes not steeper than 2.5:1.0;
- provision of an underdrain below the downstream shoulder;
- freeboard not less than 1.0–1.5 m;
- an allowance of not less than 8% for settlement;
- vehicular crest access provided with a width of at least 3.0 m; crest
reinforced with geosynthetic if no surfaced road provided;
- adequate bottom outlet capacity, with an upstream control valve;
- good vegetative cover;
- installation of a seepage measuring weir and levelling stations (see
Chapter 7);
- careful selection and placing of fill material;
- careful foundation preparation.
The planning, design and construction of small dams in the UK is
addressed in detail in Kennard, Hoskins and Fletcher (1996). Design of
small flood storage reservoirs is addressed in Hall, Hockin and Ellis
(1993).
104 EMBANKMENT DAM ENGINEERING
