192 DAM OUTLET WORKS
dependability of the gates, it might be better to opt for an ungated spill-
way. Regardless of how reliable gate operation may be, it is often stipu-
lated that the spillway must be adequate to prevent the overtopping of the
dam should one or more gates fail to open.
Although the emphasis in this chapter is on the hydraulics of outlet
works, some of the more general aspects, such as the selection of design
flood, flood routing, freeboard and reservoir sedimentation, have to be
dealt with as well. Throughout this and the next two chapters the terms
cavitation, aeration, and energy dissipation will appear; although the last
two can be treated in relation to specific designs and spillway types, cavita-
tion and its prevention requires a more general discussion before being
dealt with in individual outlet works designs.
Energy dissipators and gates and valves are the subject of the next
two chapters, while models of outlet works are discussed in Chapter 16.
For the treatment of some aspects of the effects of flow over spillways on
water quality see Section 9.1.7.
4.2 The design flood
The selection of the design flood (reservoir inflow) hydrograph is one of
the most important tasks in dam design; it depends on the dam location
and the type of dam and the procedure for its determination. Only the
basic principles for its estimation and selection can be touched upon here;
a survey of methods for the selection of the design flood can be found, e.g.
in ICOLD (1992).
The methods used for the calculation of floods developed from
historical records of maximum observed floods, empirical and regional for-
mulae, flood envelope curves and flood frequency analysis to modern
methods based on rainfall analysis and conversion to runoff. In the UK the
five volume Flood Estimation Handbookpublished by the Institute of
Hydrology (1999) (now Centre for Ecology and Hydrology) largely super-
seded the previous Flood Studies Report (NERC 1993) and placed the
emphasis from generalized regression equations to techniques for transfer-
ring hydrological data from gauged to ungauged catchments. In the USA
the US Army Corps of Engineers Hydrologic Engineering Centre pro-
duced a series of software packages (USACE, 2002) providing methods of
modelling catchments and including reservoir operation simulation.
Methods used for the selection of design floods in different countries
vary (see, e.g., Minor, 1998) and are contained either in recommendations
and guidelines (issued by professional bodies) or even in legislation. In
many – but not all – cases the highest standard uses the PMF – probable
maximum flood – i.e. the flood hydrograph resulting from the probable
maximum precipitation (PMP) and (where applicable) snowmelt, coupled
