Contents preface (VII) introduction 1—37

12.5.2. Spurs
Spurs (also known as groynes, spur dikes, or transverse dikes) are structures constructed in a river transverse to the river flow, extending from the bank into the river (3). Spurs guide the river flow, promote scour and deposition of the sediment where desired, and trap the sediment load to build up new river banks. Spurs are generally made from locally available earth. The

420 IRRIGATION AND WATER RESOURCES ENGINEERING
nose (or head) and the sloping faces of the spurs must be protected against wave action by hand-placed rubble facing. Stone apron is provided to prevent the failure of spurs due to excessive scour at the nose and sides. Spurs are probably the most widely used river training structures and serve the following function in river regulation:
(i) Training a river along the desired course by attracting, deflecting or repelling the flow in the river channel,
(ii) Creating a slack flow with the object of silting up the area in the vicinity of spur, (iii) Protecting the river bank by keeping the flow away from it, and

(iv) Contracting a wide river channel for the improvement of depth for navigation.

Spurs can be used either singly or in series or in combination with other river training measures. The design of spur depends on the following:
(i) River discharge, (ii) Angle of attack, (iii) Sediment load, (iv) Meander length,

(v) Curvature of the river, and

(vi) Upstream and downstream river training measures.
Spur length is usually restricted to less than 20% of the river width to avoid adverse effects on the opposite bank and, at the same time, the spur length is kept longer than 1.5 to 2 times the depth of flow (13). Shorter spur length in deeper rivers induces swirling motion on both the upstream and downstream sides of the spur. This swirling motion may extend up to the adjacent river bank and cause the bank erosion necessitating bank protection measures. The spacing of spurs in a wide river is larger than that in a narrower river for similar conditions. A larger spacing can be satisfactory for convex banks and a smaller spacing is desirable at concave banks. At crossings (i.e., the straight reach between two consecutive bends of a river), an intermediate spacing can be adopted. Spacing between adjacent spurs is generally kept between 2 and 2.5 times the spur length (14). Ahmad (15) has suggested that spurs used for bank protection be spaced at five times their length. However, spurs used in navigation channels are generally spaced at 0.75 to 2 times their length (16). Maintenance of the nose of longer spurs during floods would generally be difficult as has been experienced in the past on the rivers Kosi and Gandak. Moreover, a longer spur would result in relatively higher afflux on the upstream side of the spur and may induce excessive seepage through the spur which may lead to piping and breach in the spur. Such breaches have indeed occurred in the rivers Kosi and Gandak (17). The top width of a spur would be between 3 and 6 m and a freeboard of 1 to

1.5 m above HFL should always be provided in case of non-submerged spurs (14). Slopes on the upstream shank and nose should be 1V : 2H and the slope on the downstream face may be 1V : 1.5H to 1V : 2H (14) (Fig. 12.7). Stone pitching on the slopes of a spur is placed manually as per the standard practice. A graded filter 20 to 30 cm in thickness, satisfying the standard filter criteria should be provided below the pitching (14). A launching apron (Art. 12.5.3.1) should also be provided to protect the stone pitching.

It is always advisable to finalise the spur designs only after conducting model studies. Spurs can be classified as follows:
(i) Classification based on the methods and material of construction : permeable and impermeable.

RIVERS AND RIVER TRAINING METHODS

421

(ii) Classification based on the height of the spur with respect to high flood level : Sub-merged and non-submerged.

(iii) Classification based on the functions : attracting, deflecting, repelling and sedimenting, and
(iv) Special types : Denehy’s T-headed groynes, hockey type, etc.

Flow

30 to

30 to

30 to

2/3 T thick stone pitching

60m

60m

45m

X

High bank or

1.0D

1.5D

2.5 D

T thick stone pitching

embankment

Y

3 to 6 m

Slope 2:1

over graded filter

2/3 T thick stone pitching

No pitching

Slope 2:1 Y

Stone pitching 0.3m thick

3 to 6 m

T thick stone pitching
1.5 D max
1.0D

Plan




Launching apron
1.25 to 1.5 T thick stone pitching
15 to 30 m
X
Graded filter 20 to 30 cm
1.0 To 1.5 m Free board
HFL

2:1
2:1

1.5 D max
LWL

Launching apron in or cement concrete blocks 1.5

3 to 6 m
River bed material fill
2:1
River bed material fill

stone wire crates _{1.25 To 1.5 T} 1.5 1.0 to 1.5 m in two layers