FLOOD ROUTING 195
period as during the advanced stages of construction a breach of the unfin-
ished dam could be as destructive as that of the finished structure.
An example of a slightly different approach can be found in the report
of the Committee on Spillway Design Flood Selection (of the Committee on
Surface Hydrology of the Hydraulics Division of ASCE (ASCE, 1988),
which recommends three categories of dams depending on the failure con-
sequences and the level of effort required to select a design flood.
Category 1includes dams ‘where the identification of failure con-
sequences indicates potential for loss of life or other social and economic
losses that unarguably warrant the use of PMF as the safety design flood’.
Category 2 includes dams ‘where the social and economic con-
sequences of failure are not large enough to categorically require the use
of PMF as the safety design flood and thus require a detailed analysis to
determine the safety design flood’.
Category 3includes smaller dams ‘where the cost of construction is
relatively small and the failure damage is low and confined to the owner’.
When comparing the UK guidelines with those used in other Euro-
pean countries Law (1992) concludes that the UK may be oversafe, but
that slackening of guidelines would require more certainty about flood
estimation precision and that community concern and panel engineer
responsibility are unlikely to run risks in a world with non-stationary
climate. Cassidy (1994) further to basic research of hydrological processes,
which could reduce inherent uncertainties, recommends also surveys of
social attitudes to acceptable risks and in economic analysis to take into
consideration the dam owner’s ability to pay for damages resulting from
failure as well as for the reconstruction of the dam.
Thus the difficult selection of a ‘safe’ reservoir inflow design flood
involves complex issues (including moral values) and requires identifying
and evaluating a mix of economic, social and environmental impacts.
4.3 Flood routing
To determine the spillway design discharge we must convert the inflow
hydrograph of the design flood into the outflow by flood routing which, in
turn, is a function of the spillway type, size, and its operation, and of the
reservoir area. This is, therefore, a typical iterative design procedure in
which the outflow at the dam, determining the size (and type) of spillway,
will depend on the inflow and on the spillway type and size.
Reservoir flood routing (which is a special case of general open
channel flood routing – Section 8.6) can be carried out using any of the
established methods (iteration, Puls, Goodrich, initial outflow value)
depending on the size of the reservoir, the time step ∆tchosen, and the
accuracy required.