DESARROLLO DE LAS CARACTERÍSTICAS DEL NIÑO POR EDADES

For natural channels in alluvial beds and having undefined banks, effective linear waterway can be determined from some accepted rational formula. One such formula as per I.R.C. for regime conditions is given below :

Linear waterway W =

Where Q = Design maximum discharge in m3/s

C = A constant. Usually 4.8 for regime conditions but may vary from 4.5 to 6.3 according to local conditions.

2.2.5 SCOUR DEPTH

When the velocity of stream exceeds the limiting velocity, which the erodable particles of bed material can stand, the scour occurs. The normal scour depth is the depth of water in the middle of stream when it is carrying the peak flood discharge.

The probable maximum depth of scour to be taken for the purpose of designing foundations of abutment and piers shall be estimated after considering all local conditions. If possible the soundings for depth of scour shall be taken in the vicinity of bridge site during or immediately after the flood but before the scour holes had time to silt up appreciably. Allowance shall be made for increased depth resulting from

(a) The design discharge being greater than flood discharge.

(b) The increased velocity due to obstruction to flow caused by construction of bridge. (c) The increase in scour in the proximity of piers and abutments.

Theoretically the scour can be estimated as below. This method is applicable for natural channel flowing in non-coherent alluvium.

Q_b = Discharge in cumecs per width.

K_sf = The silt factor for representative sample of bed material obtained up to the level of deepest anticipated scour. =

d_m = Weighted mean diameter of bed material in mm. The discharge per meter width (Q_b) shall be maximum of :-

(i) The total design discharge divided by effective linear waterway between abutments.

(ii) The value obtained taking into account any concentration of flow through a portion of the waterway assessed from the study of the cross section of river. However these, modification may be applied

Q C 3 1 sf 2 b sm _K Q 34 . 1 d scour of depth Mean ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ = m d 76 . 1

for bridge length more than 60m. The unit discharge (Q_b) for a high level bridge is obtained by dividing the total discharge by effective linear waterway. For submersible bridges the unit discharge should be worked out by considering two layers.

(1) Bed to R.T.L. (2) R.T.L. to H.F.L.

In case of submersible bridges, the scour depth and afflux calculations are to be done simultaneously and involve trial and error procedure. To provide for adequate margin of safety, the foundation shall be designed for a larger discharge which should be a percent as mentioned below over design discharge. (IRC:78-2000 clause 703.1) The discharge worked out by Empirical Formula be increased by

Catchment up to 3000 sq.km. - 30 % 3000 - 10000 sq.km. - 30 to 20 % 10000 - 40000 sq.km. - 20 to 10 % More than 40000 sq.km. - 10 %

The value of K_sf for various grades of bed material is given in Table 2.2.

TABLE 2.2

Value of silt factor (K_sf) for various bed materials.

Sr. No. Bed Material Grain size in mm Silt factor(K_sf)

1. Silt Fine 0.081 0.50 Fine 0.120 0.60 Fine 0.158 0.70 Medium 0.233 0.85 Standard 0.323 1.00 2. Sand Medium 0.505 1.25 Coarse 0.725 1.50

Mixed with fine bajri 0.988 1.75

2.2.6 MAXIMUM DEPTH OF SCOUR FOR FOUNDATION DESIGN

The maximum depth of scour below the highest flood level (H.F.L.) shown in fig 2.5 .shall be estimated from value of mean depth of scour (d_sm) in following manner :

For the design of piers and abutments located in a straight reach and having individual foundations without any flood protection work.

(i) In the vicinity of pier - 2.00 d_sm

(ii) Near abutments - 1.27 d_sm when approach are retained. - 2.00 d_sm when approaches are washed off. (iii) Raft foundations - 1.00 d_sm (with u/s & d/s protection aprons)

For the design of protection to raft foundations, shallow foundations or flood protection the scour depth should be considered as follows:

(i) In a straight reach - 1.27 d_sm. (ii) At a moderate bend - 1.50 d_sm. (iii) At a severe bend - 1.75 d_sm (iv) At a right angled bend - 2.00 d_sm

These above scour values can be suitably increased if actual observation data is available on similar structures in the vicinity. In the following abnormal conditions, special studies should be undertaken for determining maximum scour depth for the design of foundations.

H.F.L.

BED LEVEL

IS 1.27 Dsm FOR ABUTMENTS 2 Dsm IN THE VICINITY OF PIERS

MAXIMUM SCOUR LEVEL.

FIG.2.5 MEASUREMENT OF MAXIMUM SCOUR DEPTH. WATER PRESSURE DIAGRAM

(i) Bridge located in a bend of the river involving a curvilinear flow or excessive shoal formation. (ii) Bridge located at a site where deep channel in the river hugs to one side.

(iii) Bridge having very thick piers inducing heavy local scour. (iv) Where the obliquity of flow in the river is considerable.

(v) Where a bridge is required to be constructed across a canal, or across river downstream of storage works, with the possibility of the relatively clear water inducing greater scour.

(vi) Bridge in the vicinity of the dam, weir, barrage or other irrigation structures where concentration of flow, aggradations/degradation of bed, etc., are likely to affect behaviour of structure.

If a river is of flashy nature and the bed does not lend itself readily to the scouring effect of floods, the formula for d_sm given above shall not apply. In such cases the maximum depth of scour shall be assessed from actual observations.

For bridges located across streams having bouldary beds the formula given in above paragraph may be applied with a judicious choice of values for Q_b and K_sf and results may be compared with the actual observations at site or from experience on similar structures nearby and their performance.

2.3 VERTICAL CLEARANCE

It is the height from the design highest flood level with afflux of the Channel to the lowest soffit point of the bridge superstructure.

Clearance shall also be provided according to navigational or anti obstruction requirement. Where these considerations do not arise, vertical clearance in case of high-level bridges shall be as follows :

Discharge Minimum Vertical Clearance

(m3/s) (in mm) Up to 0.3 150 0.3 to 3.0 450 3.0 to 30 600 30 to 300 900 300 to 3000 1200 Above 3000 1500

In structures with metallic bearings, no part of the bearing shall be at a height less than 500mm above affluxed design highest flood level.

2.4 AFFLUX

When a bridge is constructed, the abutment and pier structures as well as approaches on either side cause the reduction of natural waterway area. The contraction of stream is desirable because it leads to tangible saving in the cost especially of alluvial streams whose natural surface is too large than that required for stability. Therefore to carry maximum flood discharge within bridge portion, the velocity under the bridge increases. This increased velocity gives rise to sudden heading up of water on the upstream side of stream. This heading up phenomenon is known as afflux. Fig.2.6 shows the afflux at bridge site.Greater the afflux, greater will be the velocity under downstream side of the bridge and greater will be depth of scour and consequently greater will be the depth of foundations required.

Afflux should be as small as possible and generally shall not exceed 0.6m. Where the floods spread over the banks is large, use of average velocity for calculating the afflux will give an erroneously low afflux. In such cases, the velocity in the main channel/ compartment should be used. The permissible afflux will be governed by the submergence effects on adjoining structures, fields etc. on upstream side.Afflux is calculated by the following formulae:

(a) Afflux at H.F.L. by Molesworth formula (In case of high-level bridge)

Where V = Mean velocity in m/s

BED

UP STREAM _{P I E R} DOWN STREAM V

H.F.L.

AFFLUX