Contents preface (VII) introduction 1—37

13.7.3. Upstream and Downstream Loose Protection
To protect a weir structure from the adverse effects of scour at the upstream and downstream ends of the structure, some additional measures in the form of cement concrete blocks over loose stones are needed.
Just beyond the upstream end of the impervious floor, pervious protection in the form of cement concrete blocks is provided. The blocks should be of adequate size so as not to get dislodged. For barrages in alluvial rivers, the blocks of size 1500 mm × 1500 mm × 900 mm are used (2). The size of the blocks should, obviously, be larger in case of barrages in the boulder reach of a river. The length of the upstream block protection is kept approximately equal to the depth of scour below the floor level. The blocks are laid over loose stones.
The pervious block protection is provided beyond the downstream end of the impervious floor as well. The cement concrete blocks of size not less than 1500 mm × 1500 mm × 900 mm

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are laid over a suitably designed inverted filter. The space of about 75 mm between the adjacent blocks is packed with gravel. The length of the downstream block protection is kept approximately equal to 1.5 times the scour depth below the floor level.

The graded inverted filter should approximately conform to the following criteria (2):

	D15 of filter	≥ 4	≥		D15 of filter
D	of river bed material	D		of river bed material
15					85

The filter may be provided in two or more layers. The grain size curves of the filter material and the bed material should be approximately parallel.
A toe wall of masonry or concrete should also be provided at the end of the inverted filter to prevent the filter from getting disturbed. The wall should extend up to 500 mm below the bottom of the filter.
Beyond the block protection on the upstream and downstream of a weir on permeable foundation, launching aprons of loose boulders or stones are provided so that these stones can spread uniformly over the scoured slopes. These stones should not weigh less than 400 N, and should always be larger than 300 mm in size. In case of non-availability of suitable material for launching aprons, cement concrete blocks or stones packed in wire crates are used. The quantity of stone provided in launching aprons should be adequate to cover the slope of scour holes (ranging from 2 (H ) : 1 (V) to 3 (H) : 1 (V) for alluvial rivers and 1.5 (H) : 1 (V) for boulder rivers) with a thickness of 1.25 T where T (i.e., thickness of pitching) is to be obtained from Table 13.3.