Contents preface (VII) introduction 1—37

Enlarged section XX
0.3 m thick stone pitching

1 to 1.5 m free board

HFL

2:1	1.25 T

G.L.

1.0 max Launching apron in stone wire crates or

cement concrete blocks 1.5 1.5 1.0 m in two layers
Enlarged section YY
Note : See Fig. 12.10 and Eq. (12.7) for meanings of D and T.
Fig. 12.7 A typical impermeable spur

422 IRRIGATION AND WATER RESOURCES ENGINEERING
A series of permeable spurs reduces the flow velocity between the spurs which results in deposition of the sediment carried by the river water. Such spurs are, therefore, more suitable for rivers carrying heavy sediment load. In rivers carrying clear water, these spurs dampen the erosive strength of the current and thus prevent local bank erosion. Experience has indicated that permeable spurs are more effective than solid spurs for regulating the river course or protecting the banks and levees, especially in a sediment-laden river flow (3). Further, flow through the permeable spurs does not change abruptly (as it does in passing around a solid spur) and, hence, does not cause serious eddies and scour holes. Permeable spurs can be either submerged or non-submerged. These spurs are relatively cheap, but, are not strong enough to resist shocks and pressures from debris, floating ice, and logs and are, therefore, unsuitable for the upper reaches of a river (3).
Usually, permeable spurs are either tree spurs or pile spurs. A tree spur has a thick wire rope of about 25 mm diameter. This wire is firmly anchored to the bank at one of its ends and tied to a heavy buoy or concrete block at the other end (Fig. 12.8). Leafy trees with strong stem and branches are tied to the main rope by subsidiary ropes through holes drilled in the tree stem. The trees should be packed as closely as possible. When trees become heavy due to entrapped sediment, they sink.

Tripod

To	anchor

To anchor

Fig. 12.8 A typical tree groyne
Pile spurs are constructed (3) by driving piles of timber or RCC or sheet piles up to about 6 to 9 m inside the river bed. These piles are 2.4 to 3 m apart and form at least two to three rows. Each row of piles is closely inter-twined either by brushwood branches or by horizontal railings. The upstream row is braced to the downstream row by transverses and diagonals. The space between the rows of piles is filled by alternate layers of 1.8 m thick brushwood weighted by 0.6 m thick boulders or sand bags. The filling should not be completely of stone since the spur is intended to be permeable to start with. Deep scour holes developed at the nose of these spurs do not cause danger because stones from the face of the spur fall into the scour hole and create a blanket which prevents undermining.
Impermeable or solid spurs are constructed as either rockfill or earth-core embankment armoured with a scour resistant surface. These can be made to attract or repel the flow away from the bank along the desired course. The side slopes vary from 1V:1H to 1V:5H depending on the material of construction (5). The nose of the spur is usually flat with a slope of 1V:5H. Spurs used in river training for navigation are generally kept straight. But, other shapes of spurs, such as hockey spurs and T-shaped, have also been used. Shapes of scour holes for different types of spurs have been shown in Fig. 12.9. Obviously, T-head spur (first constructed by Denehy at the Okhla headworks on the Yamuna river and, hence, also known as Denehy’s

RIVERS AND RIVER TRAINING METHODS

423

spur) requires stone apron protection against scour for relatively small area and, therefore, is most economical. T-head spurs have also been effectively used on the Ganga river at the Narora headworks.

Spurs meant for contracting the river channel are generally oriented with their axes normal to the current. Sometimes, spurs are oriented to point upstream, the advantage of which is that during the flood the flow is directed towards the centre of the river. Thus, the strong currents are kept away from the flood plains and flood dikes. Such spurs, pointing upstream, are also called repelling spurs. On the other hand, spurs pointing in the downstream direction attract the flow towards the bank and are, therefore, known as attracting spurs. A repelling spur produces a more desirable curvature to the flow downstream, leading to pronounced deposition (5). Besides, such a spur has large stagnation region on the upstream side and is, therefore, able to protect a greater length of the bank than that protected by the attracting spur. The repelling spur is usually inclined at 5° to 20° (to the line normal to the bank) in the upstream direction.

Silting
Straight spur	Hockey spur

Inverted hockey spur				T-Head spur