SELECTION OF CREST HEIGHT, P
Large heights are uneconomical and create upstream flooding, whereas
small heights create frequent non-modular (drowned) flow conditions. The
limiting modular ratio, H 2 /H 1 for the Crump weir is 0.75 (Water Resources
Board, 1970) and a design modular flood of around 1/2 to 1 year return
periods is considered as the norm. From the given information on the
flood flows in the river, the 1-year return flood is around 5 m^3 s^1 (log–log
plot between Q and T). The limiting modular ratio suggests that
H 1 1.33H 2 and, choosing a crest height of 0.7 m (as first trial), the curve Q
versus 1.33H 2 is plotted on Fig. 8.25, which intersects the QversusH 1 plot
atQ9.4 m^3 s^1. This corresponds to a 3.3-year return period flood, which
is far in excess of the design norm. Hence select a lower value of P.
A crest height of 0.6 m produces a modular limit flood of 4.6 m^3 s^1
with a return period of 1.1 years. This is satisfactory and hence a crest
height of 0.6 m is adopted for the design.
CHECK FOR SPECIFICATIONS
- Non-modularity: the modular limit discharge of 4.6 m^3 m^1 corres-
ponds to H 2 0.29 m and H 1 0.39 m (Fig. 8.25). Thus H 2 /H 1 0.74
and is satisfactory.
356 RIVER ENGINEERING
Fig. 8.23 Flow chart for the design of ‘Crump weir’
