Catherine asks
<Can any help with the range of gradient for a leat please
Using contours, the Dukes Low Watercourse at Grassington is 1:199 (10 m
in 1.99 Km), the Dukes High Watercourse is 1:545 (6m in 3.27 Km) and the
extension to Yarnbury is 1:338 (4m in 1.35 Km) and the leat in Dibbles
Gill at Appletreewick is 1:580 (5m in 2.90 km).
Too steep a leat would give problems with scouring, and too flat a leat
would give problems with silting. I have a very distant memory that an
optimal gradient for salt glazed pipes was in the region of 1:130 (no
doubt we have a civil engineer who will confirm/correct that). At
Grassington, before the water got into the Yarnbury leat it had gone
through three reservoirs (in a line) and along the Low Watercourse - so
it was probably fairly clean.
The Appletreewick leat took its water from the beck immediately below
the dam of the fairly large Grimwith reservoir.
Regards,
Mike
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