Waterway or length of the work (L) is adopted as given by Lacey’ viz. 83 Q1/2. It is termed as active waterway. An additional span is generally provided to make
up for the loss of waterway in end spans which are partially obstructed by the training works or khadir In railway practice, less twice the width of pierfoundation wells. Overall waterway, L, be guide banks is usually taken as 1.1 to 1.25 L ascertain the waterway required.
2.Length of Guide Bank
The two important consideration for determining the length of guide banks for wide alluvial rivers are (i) Maximum obliquity of the current. In order to avoid heavy river action on the guide bank, the obliquity of flow to the river axis is limited to not more than 300, and (ii) Permissible limit to which the main river channel can be allowed to flow near the approach embankment in the event of the river developing excessive embayment behind the training works. The radius of the worst possible loop is ascertained from the data of the active loops formed by the river.
(a) The upstream length of guide bank is normally kept 1.0 to 1.5 as per IS: 84081976. Spring recommended length 1.1 L and Gales 1.25 L for Q up to 20,000 cumecs and 1.50 L for Q above 20,000 cumecs.
Downstream length. The river fans out on downstream of the work to attain its normal width. The function of downstream length of guide bank is to ensure that in fanning out the river does not threaten the approach embankments on either side of the work or afflux bunds or canal embankments. A short guide bank with sharp curved head is considered suitable for the purpose. The downstream length of guide banks is usually kept 0.25 L to 0.4 L as per I.S. 84081976. Spring recommended 0.1 to 0.2 L, and Gales 0.25 L.
Figure: Gale’s pattern
3. Curved Head and Tail
(a) Circular head. The upstream curved head is designed to guide the flow smoothly and axially to the structures to keep the end spans effective and active.
Upstream head. The upstream curved portion of the guide bank, termed as impregnable head, is provided with a suitable curvature usually of a radius as small as possible consistent with proper functioning of the guide bank. Unless indicated by model studies, the minimum and maximum embankment with a minimum cover of 0.9 m. The seepage gradient in accordance with the nature of soil varies from 4: 1 to 6:1.
Salient parameters of cross section f embankment are given in Table 9.3.
Table : Salient parameters of crosssection
Parameter Marginal embankment Approach embankment Retired embankment Flood embankment
Top width 6 to 9 m 6 to 9 m 6 to 9 m 6 to 9 m
Free board. 1 to 1.5 m above HFL for 1 in 500year flood or above the affluxed water level in the rear portion of the embankment calculated after adding velocity head to HFL corresponding to 1 in 100year flood/ standard project flood at the upstream nose of the bank, whichever higher.
Side slopes. Depending on the nature of river bed material of which they are made and the height of the embankment, slope of 2:1 is generally adequate on river side and 2:1 to 3:1 on rear side.
a.Slope pitching. Embankment is not slope pitching is not necessary, turfing of vegetal cover is provided. Where strong currents are likely to attack, paving of slope with some resistant material is provided
b.River side slope is protected by suitable pitching calculated for 40 per cent of the design discharge. Filter is normally not required except in case approach embankment is heavy.
c.Embankment is not subjected to river current. Turfing is provided. Spurs and other protective measures are provided at salient points.
a.Launched apron. not necessary.
Factors responsible for embankment failure are (i) Erosion of river side by wave wash during very strong current, (ii) Sloughing of the bank slope when saturated with water by floods of long duration, (iii) Sloughing of inland side slope caused by under seepage, (iv) Piping in sublayers due to movement of ground water towards the river, which carries away material with it. Gradual reduction of crosssection of the embankment takes place due to gradual removal of the embankment material with it. Gradual reduction of cross section of the embankment takes place
due to gradual removal of the embankment material through the joints or interstices of the protection movement, (v) Toe of the bank undermined by eddies, currents, etc. followed by a collapse of overhanging material deprived of support, (vi) Overtopping due to a flood of a magnitude greater than the design flood, (vii) Insufficient cross section, leaks and cracks due to shrinkage of soil and ratholes while breaches could result from overtopping of an embankment due to higher than design flood levels being obtained in a river as also due to river erosion. One of the important causes usually is lack of proper maintenance by way not only of annual repairs, but also inadequacy or slackness in watch and timely action during the flood season.