TRAINING PROGRAMME ONENGINEERING DESIGNS - CANAL STRUCTURES

GENERAL DESIGN PRINCIPLES

Aqueducts, Canal Syphons & Escapes

BY ROUTHU SATYANARAYANA CHIEF ENGINEER (Retired.) FORMER ADVISOR, GOVERNMENT OF A.P

Cross Drainage Works-Aqueducts 1. Aqueducts: Classified as: type-I, type-II and type- III depending up on the arrangement of canal passing over the Stream/drain, Type-I: Structures come under this type where the canal continuous over the stream with its normal earthen section including the banks and earthen slopes. The HFL of the stream shall be lower than the bottom level of the canal trough.Type-II: Structures where the canal continues over the stream but the outer banks are replaced by outer walls.Type-III: The canal banks will be discontinues over the stream and the canal water is carried by a masonry or concrete, RC C Box, Pipe of suitable section. The Service and inspection Tracks may be continuous or discontinues. Generally the canal section is flumed and head loss is accounted for. 2. Viaducts: Similar to type-III aqueduct except the length of the structure is very large compared to the stream or there is no stream/drain existing in the valleys joining the two sides of the structure. 3. Syphon Aqueduct: The HFL of the stream/drain will be lower than the under side of (bottom level) the canal trough . If the HFL of the stream is above the canal bed it is called syphoned.

Cross Drainage Works-Aqueducts 4. Under tunnel (barrel/ pipe)/ Syphon Aqueduct:Stream discharge carried in the barrel if required by depressing stream bed level to make the headway below the canal. Canal section carried over barrel as it is with head walls to replace the outer slopes of canal partially/ fully. The under tunnel flowing full under pressure is called syphon aqueduct. 5. Buttress type under tunnel:Steam discharge carried in the barrel if required by depressing the stream bed level to make the head way below the canal and canal discharge carried in the flumed trough (Trapezoidal or rectangular). The abutments and piers raised to the bottom of trough in case of Trapezoidal shape. For continuity of inspection path, a bridge and for non inspection path foot bridge will be provided. 6. Super passage:Stream discharge carried in the trough normally rectangular shape (1 or more bays) with vertical clearance over F.S.L. of canal. For continuity of inspection path bridge and for non inspection path foot bridge will be provided separately. 7. Canal siphon:Canal discharge carried below the stream by depressing the canal bed to make the headway. For continuity of inspection path a bridge will be provided separately.

CD Works Level Crossings 8. Level Crossing:In this type of works the drainage water and canal water are allowed to intermingle with each other .A Level crossing is provided when large canal and a huge drain approach at the same level.An Inlet and out let for the canal and an escape for the drainage or vice versa are provided. Perennial drainage discharge can be used in the canal supplies.

9. Inlets and Out lets:Provided at exceptional cases.

Cross Drainage Works-Aqueducts Design Criteria :Hydrology of the drain or stream.Hydraulic design of The stream or drainThe hydraulic deign of the canalStructural Design. Design of sub structure Design of super structure

Cross Drainage Works-Aqueducts

Basic Data : Site plan with net levels at 10m intervals and contours and duly marking the flow direction of the canal and the stream.Hydraulic particulars of the canalLS of the stream covering 500m on u/s and d/s with levels at 10m to 20m intervals and CSs at centre line and at 10m, 25m, 50m, 100m, 200m, 300m, 400m, and 500m on /s and d/s sides.Levels on the CS to be 3m, to 5m, in the gorge portion and 10m, intervals on the flanks up to 50m beyond HFL mark on the ground.Catchment area plan of the seam/drain on the topo sheet for Catchment up to 2.5 Sq..m and the CA to be traversed on ground for Catchment less than 2.5 sq..m. Computation of Maximum Flood Discharge (MFD) of the stream/drain. and the HFL/ MFL are to be marked on ghe LS & CSs and cross checked with the Observed MFLs (OMFL).Trial Pit (TP) particulars (Bore logs) taken up to hard strata, for a minimum depth of 2m below ground level or drain bed level or canal bed level for shallow foundations and up o 1.33R below maximum scour level on the centre line of the structure, (and) at least one on either side or as decided as per the filed conditions along the centre line and One each on u/s and d/s side.Safe bearing capacity of the strata may be obtained and furnished.

Hydraulic data:Canal Width of road way and class of IRC loading.Head loss provided. The Stream or drain:Allowable afflux.Nature of bed material and value of n

Cross Drainage Works-Aqueducts

Cross Drainage Works-Aqueducts I. Hydrology of the drain:Computation of maximum flood discharge and the MFL II. Hydraulic design:Water way/Vent way:Vertical Clearance:Free Board:Crust Level of the Road way or a Bridge: Afflux:Depth of Scour:Mean Depth of Scour (d):Maximum Depth of Scour or Designed Depth of Scour (D or R):UpliftExit gradient. Loss of Head (Energy Loss): Joints: III. Structural Design:Super structureSub structure

Aqueducts - Hydrology I. Hydrology of the Drain/Stream: 1. Compute the designed flood of the stream from catchments area plan using any one of the empirical formulae or by the flood frequency method, SPF, PMF.

2. Compute the MFL in the stream by step by step method by trial and error and verify with observed MFL.

3. For drains with discharge > 150 cumecs and canals with discharge > 30 cumecs detailed study is to be conducted in respect of Catchment area and computations of HFD/SPF or PMF.

Aqueducts - Hydrology I. Hydrology of the Drain/Stream: Formulae for computation of maximum Flood Discharge:_____________________________________________________________ S.No.Type of CanalCatchment Area (CA) in M in Sq. Miles----------------------------------------------------------------------------------------------------------Up land Areas Deltaic Tracts --------------------------------------------------------------------------------------------------------------------------------------------------------1. Main CanalDickenss formula, Rye's formula Q = CM 3/4 Q = CM 2/3 C=1400 for CA500Velocity in the barrel up to12 to13 ft/sec-------------------------------------------------------------------------------------------------------------------------------2.Branch CanalQ=CM 2/3 Q > 500 c/sC=1000 and Velocity

CD Works - Aqueducts Water way / Vent way and the Lay out: Design the vent way of the stream/drain limiting the velocities in the drain, Keeping in view the Laceys wetted perimeter limiting the fluming ratio to 60% to 80% and velocity

Transition walls : Transition walls to be provided at either ends keying 600mm in to the earth banks both for drain and canal.Compute the TELs of the canal starting from d/s side end of the canal transition up to u/s side transition of the canal (Designed canal section on either side) and verify that the head loss and the velocities are with in the permissible limits.Finalize the widths of the inspection tracks foot paths on either side if required. Design the tail channel and the approach channel of the stream.Compute the TELs in the stream limiting the velocities, and permissible afflux etc.Draw flow diagrams both for the stream and canal.Compute and design foundation levels considering scour depths.Compute and design the barrel and the floors of the stream for uplift pressure.Check for exit gradient. (cont)

CD Works - Aqueducts

CD Works - Aqueducts Lay out :Preferably a straight reach. The carrier canal and the drain shall be at right angle crossing. Proper training works for the drain and suitable protection works like turfing, pitching and launching aprons etc., Expansion joints, Contraction Joints and construction joints.

AqueductsHydraulic Design II. Hydraulic Design:Vertical Clearance:It is the vertical distance between the HFL of the stream and the under side of the canal trough including afflux.

S. No. Designed flood in Cumecs Minimum Vertical Clearance in mm 1. < 3450 2 Between 3 and 30 600 3. Between 30 and 300900 4. Between 300 and 3000 1200 5. 3000 and above 1500

Free Board:It is the vertical distance between the HFL/FSL to the top of embankment/TBL in case of stream and canal respectively.Crust Level of the Road way or a Bridge: The TBL of the canal or the crust level of the road way or the natural ground level which ever is higher.Afflux:It should be restricted to the value which should not cause serious bed scours or submergence.It is the rise in water level on the upstream due to an obstruction to the flow of drain or canal.It is computed using the Rational formula, Orifice formula or Empirical formulaIt is the vertical distance measured from HFL or FSL to the underside of trough, including afflux.

AqueductsHydraulic Design

Scour:Mean Scour Depth:Mean scour depth is the depth (d) below HFL or FSL in m d = 1.34[q2 /f]1/3 Where, q = Discharge per meter width with or without concentration of flow in cumecs, f = Layces silt factor expressed as f = 1.76 (d m )1/2dm = average grain size

Designed Scour Depth (Dor R):Straight reaches for individual foundations without floor protection

In the vicinity of pier2.00 dNear abutments1.27 d approaches retained2.00 d scour all roundFor floor protection works, for raft foundations and shallow foundations

In straight reaches1.27 dAt moderate bends1.50 d Aqueducts Hydraulic Design

At sever bends1.75 d m At right angle bends2.00 dStructures and earth work connections:Uplift:Uplift under floor of the barrels and under the u/s and d/s side floors caused by the seepage flow from the canal when it is running full and the drain is dry or vise versa may be accounted for the design using the Khoslas theory and the thickness checked for adequacy. Cut off walls may be provided on either ends.For reducing the uplift and exit gradient pucca floor should be provided for in the canal bed in adequate length u/s and d/s side with cut off walls at the ends.

Exit gradient. The rigid structure and the flexible earth work shall be properly connected and checked for exit gradient (GE).GE = H/d[1/ () 1/2]Aqueducts Hydraulic Design

Loss of Head (Energy Loss): The losses are at inlets (h1) and outlets (h2), at bends elbows (h3), losses due to transitions (h4) and losses due to skin friction (h5). The sum of losses (H) shall be sum of all the losses H = h1 + h2 + h3 + h4+h5

Joints: Bell Mouth on U/S side:Cut and ease waters:Water Stops: Bearings:Miscellaneous works:

Aqueducts Hydraulic Design

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