8.7.2 Groynes, dykes, vanes, and sillsGroynesare small jetties, solid or permeable, constructed of timber, sheet
piling, vegetation, and stone rubble, etc. They usually project into the
stream perpendicularly to the bank, but sometimes are inclined in the
upstream or downstream direction. The main purpose of groynes is to
reduce channel width and to remove the danger of scour from the banks;
their ends in the stream are liable to scour, with sediment accumulation
between them. As their effect is mainly local, the spacing between groynes
should not exceed about five groyne lengths, but usually is appreciably
smaller; a spacing of about two lengths results in a well-defined channel
for navigation; the larger the ratio of groyne spacing to river width, the
stronger the local acceleration and retardation, and thus the greater the
hindrance to shipping. Details of groyne design are strongly influenced by
economic factors, and a cost–benefit analysis for the determination of their
height, spacing, length, and material is usually necessary; in more import-
ant cases this may have to be backed up by model studies (Chapter 16).
Permeable groynes usually constructed from vegetation (e.g. tree tops)
exert a less severe effect on the flow than solid ones, and by slowing down
the current aid deposition of sediment in the space between them.
Examples of the use of groynes, with some construction details of a
solid groyne, are shown in Figs 8.14 and 8.15. Note the combination of
RIVER IMPROVEMENT 347
RIVERCANAL OLD ALIGNMENTFig. 8.13 Connection of canal and river (after Jansen et al., 1979)