Waterway and crest level at alluvial zone

1. Length of waterway (total)

The regime width of the river is found from Lacey's wetted perimeter

$$P = 4.75\sqrt{Q}$$

where $Q$ is the maximum flood discharge (m³/s). The total waterway of the headworks is taken as about $L = P$ (up to $1.2P$ where needed), but not more than the natural river width. The clear (effective) waterway is $L$ minus the total thickness of the piers, and the afflux produced (usually 0.5-1.2 m) must be checked to be within the permissible limit.

Distribution: the undersluices occupy about 10-20% of $L$ (sluice bays of 5-8 m); the rest is the weir. The waterway of the canal head regulator is found from its discharge formula for the canal full supply discharge, with the head equal to the pond level minus the regulator crest level.

2. Crest levels

• Pond level $=$ canal full supply level (FSL) $+$ losses through the head regulator (0.3-0.5 m), so that the canal can draw its full supply.
• Weir crest level is found in two ways and the lower value is adopted:
  • from the flood: crest $=$ HFL after afflux $-$ head $H$ needed to pass the design flood, where $H$ comes from $Q = C L H^{3/2}$ ($C \approx 1.7$);
  • from ponding: crest $=$ pond level $-$ height of shutters (0.9-2.0 m) if shutters or gates are used (or crest $=$ pond level for a weir without shutters).
• Undersluice crest level $=$ average (low-water) river-bed level at the site, about 1.2-2.0 m lower than the head regulator crest.
• Head regulator crest level $=$ undersluice crest $+ (1.2$-$2.0)$ m; it must be above the sluice crest to keep the bed silt away from the canal.

HFL (after construction) $=$ HFL (before construction) $+$ afflux, and the head over the weir and sluices is checked with the discharge formulae.