Construction of an RCC Overflow Weir
Step-by-Step Construction Methodology, Hydraulic Design Example, Stability Checks, QA/QC and Site Engineering Guidelines
A practical technical guide for civil engineers, contractors, site engineers, consultants and students involved in river and water-retaining structures.
1. Introduction
An overflow weir is a hydraulic structure constructed across a river, stream or channel to raise the upstream water level, regulate flow, divert water or create a small storage/ponding effect.
An RCC overflow weir generally consists of a concrete overflow body founded on competent strata, together with upstream and downstream floor/protection systems, cut-off arrangements, abutments, wing walls and energy dissipation measures.
The hydraulic performance of a weir cannot be considered independently from its foundation and downstream protection. A properly designed overflow section must safely pass the design discharge while controlling afflux, uplift, seepage, downstream velocity, hydraulic jump and scour.
A weir is not merely a concrete wall across a river. It is a complete hydraulic system consisting of the overflow body, foundation, seepage control, energy dissipation, river training and erosion protection.
The numerical values used in the example below are intended for demonstration and educational purposes. Actual construction drawings must be based on project-specific hydrological, hydraulic, geotechnical and structural investigations and must be checked and approved by the competent design authority.
2. Contents
- Site Survey and Investigation
- River Diversion and Cofferdam
- Excavation and Dewatering
- PCC Foundation / Levelling Course
- Reinforcement Work
- RCC Construction
- Upstream and Downstream Faces
- Abutments and Wing Walls
- Energy Dissipation
- Finishing and Curing
- Removal of Temporary Diversion Works
- Hydraulic Design Example
- Stability Checks
- Concrete Quantity Calculation
- QA/QC Requirements
- Construction Safety
- Common Construction Mistakes
- Site Engineer Checklist
- Important Codes and References
3. Step 1 – Site Survey and Investigation
The first and most important stage is understanding the river, its catchment, foundation and flood behaviour.
Major investigations
- Topographical survey of the river and surrounding area.
- Longitudinal river profile.
- Cross-sections at suitable intervals.
- High Flood Level (HFL) assessment.
- Normal water level and tailwater assessment.
- Design flood determination.
- River-bed material investigation.
- Geotechnical investigation of the foundation.
- Groundwater and seepage investigation.
- Assessment of upstream and downstream scour.
Never finalize the weir foundation level merely from an existing river-bed level. Foundation excavation must continue until the specified competent stratum is encountered and accepted by the Engineer/Geotechnical Engineer.
4. Step 2 – Diversion of River Flow
Construction normally requires the river flow to be temporarily diverted away from the working area.
Typical arrangements
- Temporary diversion channel.
- Pipe/box diversion for small streams.
- Upstream cofferdam.
- Downstream cofferdam.
- Temporary pumping arrangement.
- Staged construction of the weir.
The diversion system must be designed considering the expected construction-period flood and the consequences of overtopping.
A properly designed permanent structure can still be damaged if the temporary cofferdam or diversion arrangement fails during construction.
5. Step 3 – Excavation and Dewatering
- Set out the approved foundation limits.
- Excavate to the required founding level.
- Remove loose, organic and unsuitable material.
- Trim the foundation surface.
- Remove standing water.
- Provide suitable dewatering arrangements.
- Obtain foundation approval before concreting.
Foundation acceptance should include
- Required founding level.
- Rock/soil type and quality.
- Absence of loose pockets.
- Absence of soft seams where prohibited.
- Clean and stable foundation surface.
- Approved treatment of joints, cracks or cavities.
6. Step 4 – PCC Levelling Course
A plain cement concrete levelling course is generally provided to create a clean, uniform and accurately levelled working surface.
- Clean the foundation.
- Set the required levels.
- Place PCC of specified grade and thickness.
- Compact and finish properly.
- Check level and dimensions.
- Protect the surface until subsequent work begins.
7. Step 5 – Reinforcement Work
Reinforcement shall be fixed strictly according to the approved structural drawings and bar bending schedule.
- Verify reinforcement diameter.
- Verify spacing.
- Verify lap lengths.
- Verify development lengths.
- Provide specified concrete cover.
- Use approved chairs/spacers.
- Ensure reinforcement remains stable during concreting.
- Provide construction joints exactly as detailed.
8. Step 6 – RCC Construction
RCC should be placed in planned lifts and layers to avoid cold joints, honeycombing and segregation.
Typical sequence
- Check reinforcement and formwork.
- Check embedded items and waterstops.
- Verify concrete grade and approved mix.
- Check slump/workability as specified.
- Place concrete systematically.
- Compact using suitable vibration equipment.
- Maintain specified lift thickness.
- Finish the exposed surface.
- Begin curing immediately after finishing.
For massive RCC sections, temperature rise, thermal cracking and construction-joint planning should be specifically addressed in the method statement.
9. Step 7 – Upstream and Downstream Faces
The upstream and downstream faces determine the hydraulic behaviour and structural geometry of the overflow body.
Upstream face
- Maintain the specified profile.
- Provide smooth concrete finish where required.
- Ensure proper transition to the upstream floor.
Downstream face
- Maintain approved slope/glacis geometry.
- Ensure smooth hydraulic transition.
- Provide adequate downstream energy dissipation.
- Protect against cavitation, erosion and scour where applicable.
10. Step 8 – Abutments and Wing Walls
Abutments and wing walls connect the weir with the river banks and guide the flow through the intended waterway.
- Retain adjoining earth.
- Guide the river flow.
- Prevent bank erosion.
- Prevent water from bypassing the structure.
- Provide stable transition between river bank and structure.
11. Step 9 – Energy Dissipation and Scour Protection
Water flowing over a weir possesses substantial kinetic energy. If this energy is not safely dissipated, excessive downstream velocity can cause bed erosion, undermining and structural damage.
Possible arrangements
- Stilling basin.
- Hydraulic jump basin.
- Chute blocks.
- End sill.
- Staggered baffle blocks where appropriate.
- Concrete apron.
- Riprap or stone pitching.
- Launching apron.
- Downstream protection works.
The appropriate arrangement depends on discharge, head, tailwater, foundation, bed material and hydraulic-jump characteristics.
12. Step 10 – Finishing and Curing
- Repair approved surface defects.
- Remove laitance where necessary.
- Maintain specified geometry.
- Provide adequate curing.
- Protect fresh concrete against impact and flowing water.
- Maintain construction joints properly.
Curing is particularly important for hydraulic structures because surface cracking can increase permeability and reduce long-term durability.
13. Step 11 – Removal of Temporary Diversion Works
Temporary works should be removed only after the permanent structure has achieved the required strength and the Engineer confirms that the structure is ready to receive river flow.
- Remove temporary cofferdam systematically.
- Avoid sudden uncontrolled release of water.
- Remove debris from the river channel.
- Check downstream protection.
- Inspect all exposed surfaces.
- Restore the river channel where required.
14. Step 12 – Completed Weir
Before commissioning, carry out a final inspection of the entire hydraulic structure.
- Verify crest level.
- Verify structure dimensions.
- Inspect concrete surfaces.
- Inspect downstream apron.
- Inspect abutments and wing walls.
- Check seepage-control arrangements.
- Check downstream protection.
- Remove construction debris.
- Record as-built levels and dimensions.
15. Hydraulic Design Example – RCC Overflow Weir
Consider the following simplified example for understanding the basic overflow-discharge relationship.
| Parameter | Value |
|---|---|
| Design discharge, Q | 25 m³/s |
| Net/river width, L | 20 m |
| River bed RL | 100.00 m |
| Proposed crest RL | 102.00 m |
| Weir height above bed | 2.00 m |
| Concrete grade | M20 – illustrative only |
| Reinforcement | Fe500 – illustrative only |
| Foundation | Hard rock – illustrative assumption |
15.1 Basic Overflow Equation
For a simplified broad-crested overflow condition, the discharge can be expressed in the form:
where:
- Q = discharge over the weir
- C = coefficient of discharge
- L = effective length of crest
- H = effective head over crest
For the illustrative calculation, assume:
Therefore:
In a final design, the coefficient of discharge and effective length must be established using the applicable hydraulic design procedure, including approach velocity, crest geometry, contraction effects and other relevant conditions.
15.2 Upstream High Flood Level
For the simplified example:
15.3 Storage/ponding Depth
16. Illustrative Weir Section Dimensions
| Component | Illustrative Dimension |
|---|---|
| Weir height | 2.00 m |
| Crest/top width | 0.60 m |
| Base width | 2.50 m |
| Effective length | 20.00 m |
| Upstream face | Vertical |
| Downstream face | Sloping |
| PCC thickness | 0.15 m |
| Upstream cut-off | Illustratively 1.0 m deep × 0.50 m thick |
| Downstream cut-off | Illustratively 1.2 m deep × 0.50 m thick |
| Downstream apron | Illustratively 5.0 m long |
17. Illustrative Concrete Quantity Calculation
For preliminary quantity estimation, individual components can be calculated separately.
| Component | Illustrative Calculation | Quantity |
|---|---|---|
| Weir body | (0.60 + 2.50) / 2 × 2.00 × 20 | 62.00 m³ |
| PCC levelling course | 3.50 × 0.15 × 20 | 10.50 m³ |
| Upstream cut-off | 0.50 × 1.00 × 20 | 10.00 m³ |
| Downstream cut-off | 0.50 × 1.20 × 20 | 12.00 m³ |
| Downstream apron | 5.00 × 0.30 × 20 | 30.00 m³ |
| Illustrative Total | 124.50 m³ | |
18. Simplified Stability Check
An overflow weir must be checked against overturning, sliding, excessive foundation pressure and other applicable failure modes.
18.1 Weight of Weir
Assume a simplified cross-sectional area:
For one metre length and an assumed concrete unit weight of approximately 24 kN/m³:
18.2 Hydrostatic Water Pressure
For a simplified vertical water face:
For H = 0.81 m and γw ≈ 9.81 kN/m³:
The corresponding line of action is approximately H/3 above the base for a triangular pressure distribution.
18.3 Uplift Pressure
Uplift can be a major component of the stability analysis of hydraulic structures. The actual uplift diagram should be obtained from the seepage analysis and the adopted drainage/cut-off arrangement.
For permeable foundations, seepage, exit gradient, uplift and piping must receive particular attention.
18.4 Factor of Safety Against Sliding
Depending on the foundation and design methodology, resistance may include friction and permitted cohesion/shear-friction contribution. The values must be based on appropriate geotechnical parameters and applicable design criteria.
18.5 Factor of Safety Against Overturning
The actual check should consider all relevant load cases and not just the simplified water pressure shown in the introductory example.
19. Important Load Cases
A professional weir design should consider appropriate combinations such as:
| Load Case | Typical Considerations |
|---|---|
| Normal condition | Normal upstream and downstream water levels. |
| Design flood | Maximum design discharge and corresponding water levels. |
| Low-flow condition | Minimum tailwater/upstream conditions where relevant. |
| Uplift condition | Seepage and foundation uplift. |
| Sediment condition | Hydrostatic and sediment effects where applicable. |
| Earthquake condition | Applicable seismic loading and hydrodynamic effects. |
| Construction condition | Temporary water levels and partial construction stages. |
20. Foundation and Seepage Considerations
Foundation safety is one of the most critical aspects of a weir.
For rock foundations
- Assess rock quality.
- Remove weathered and loose rock.
- Treat open joints and cavities as specified.
- Provide dental concrete where required.
- Provide consolidation/contact grouting where specified.
- Ensure proper interface between rock and concrete.
For permeable/alluvial foundations
- Analyse seepage.
- Check uplift.
- Check exit gradient.
- Provide suitable cut-offs.
- Provide upstream/downstream floors.
- Provide filters/drainage where required.
- Check piping and undermining.
For alluvial reaches, hydraulic design should follow the applicable provisions of IS 6966 Part 1 and project-specific design criteria.
21. Scour and Erosion Protection
Scour protection must be designed based on the expected hydraulic conditions and river-bed characteristics.
Potential failure mechanisms
- Local scour downstream of the structure.
- General bed degradation.
- Bank erosion.
- Undermining of apron.
- Backward erosion/piping.
- Damage due to hydraulic jump instability.
Depending on site conditions, protection may include concrete floors, stone pitching, riprap, launching aprons, cut-offs and energy dissipation structures.
22. QA/QC Requirements
22.1 Concrete Quality Control
- Approved concrete mix design.
- Approved source of cement and aggregates.
- Water quality verification.
- Admixture compatibility checks.
- Slump/workability checks.
- Cube strength testing.
- Batching records.
- Pour cards.
- Temperature monitoring where required.
- Curing records.
22.2 Reinforcement Inspection
- Diameter verification.
- Bar spacing.
- Lap length.
- Development length.
- Cover.
- Bar cleanliness.
- Anchorage.
- Chairs and spacers.
22.3 Survey Control
- Foundation RL.
- PCC RL.
- Crest RL.
- Upstream/downstream profiles.
- Apron levels.
- Cut-off levels.
- Wing-wall alignment.
23. Construction Joints
Construction joints must be planned before concreting begins.
- Do not create random cold joints.
- Locate joints at approved positions.
- Prepare old concrete before subsequent placement.
- Remove laitance.
- Clean and roughen the joint as specified.
- Use waterstops where detailed.
- Ensure proper bonding treatment.
24. Construction Safety
River Safety
- Flood warning system.
- Emergency evacuation plan.
- Safe access routes.
- Continuous weather monitoring.
Excavation Safety
- Stable excavation slopes.
- Safe access.
- Dewatering control.
- Protection against collapse.
Concrete Safety
- Safe pump operation.
- Proper lifting arrangements.
- Electrical safety.
- PPE compliance.
Plant Safety
- Excavator exclusion zones.
- Crane lifting plans.
- Equipment inspection.
- Trained operators.
25. Common Mistakes to Avoid
| Mistake | Why It Is Dangerous |
|---|---|
| Constructing without proper foundation investigation | May cause excessive settlement, sliding, seepage or undermining. |
| Ignoring construction-period floods | Can result in cofferdam failure and loss of partially completed work. |
| Insufficient downstream protection | May cause severe scour and structural undermining. |
| Changing reinforcement at site | May compromise structural capacity and durability. |
| Improper concrete vibration | Can cause honeycombing, voids and weak zones. |
| Inadequate curing | Can increase shrinkage cracking and reduce durability. |
| Incorrect crest level | Can change upstream water level and hydraulic performance. |
| Ignoring uplift | Can significantly reduce stability against sliding and overturning. |
26. Do's and Don'ts
✓ DO
- Follow approved drawings.
- Verify all important levels.
- Maintain proper survey control.
- Inspect reinforcement before each pour.
- Maintain concrete quality records.
- Monitor river conditions.
- Maintain proper curing.
- Record as-built dimensions.
✗ DON'T
- Don't bypass foundation approval.
- Don't alter design dimensions without approval.
- Don't permit uncontrolled water entry.
- Don't compromise concrete compaction.
- Don't omit downstream scour protection.
- Don't remove temporary works prematurely.
- Don't ignore seepage.
- Don't rely only on visual inspection.
27. RCC Weir Site Engineer Checklist
| Activity | Check |
|---|---|
| Survey and setting out | ☐ Alignment ☐ Centreline ☐ Levels ☐ Coordinates |
| Foundation | ☐ Founding level ☐ Stratum ☐ Cleaning ☐ Approval |
| Dewatering | ☐ Pumps ☐ Standby pump ☐ Discharge arrangement |
| PCC | ☐ Grade ☐ Thickness ☐ Level ☐ Finish |
| Reinforcement | ☐ Diameter ☐ Spacing ☐ Cover ☐ Laps |
| Formwork | ☐ Line ☐ Level ☐ Plumb ☐ Stability |
| Concrete | ☐ Mix ☐ Slump ☐ Temperature ☐ Cubes ☐ Vibration |
| Construction joints | ☐ Location ☐ Surface preparation ☐ Waterstop |
| Curing | ☐ Method ☐ Duration ☐ Protection |
| Hydraulic profile | ☐ Crest RL ☐ Face profile ☐ Apron levels |
| Energy dissipation | ☐ Basin ☐ Blocks ☐ End sill ☐ Protection |
| Final inspection | ☐ Dimensions ☐ Levels ☐ Defects ☐ As-built drawings |
28. Important Indian Standards and Technical References
| Reference | Subject |
|---|---|
| IS 6966 (Part 1):1989 | Hydraulic design of barrages and weirs – Guidelines – Alluvial reaches. |
| IS 11130:1984 | Criteria for structural design of barrages and weirs. |
| IS 7720:1991 | Criteria for investigation, planning and layout for barrages and weirs. |
| IS 11150:1993 | Construction and maintenance of concrete barrages. |
| IS 12892:1989 | Safety of barrage and weir structures – Guidelines. |
| IS 7349:2012 | Barrages and weirs – Operation and maintenance guidelines. |
| IS 9461:1980 | Guidelines for data required for design of temporary river diversion works. |
| IS 10084 Series | Criteria/design guidance for diversion works including cofferdams. |
| IS 14955:2001 | Guidelines for hydraulic model studies of barrages and weirs. |
| IS 456 | Plain and reinforced concrete – general structural requirements, where applicable. |
| CWC / CWPRS Publications | Hydraulic structures, energy dissipation, seepage, river hydraulics and model-study guidance. |
Always verify the latest applicable edition, amendments and project-specific technical specifications before using any standard for detailed design or construction. For alluvial barrages and weirs, IS 6966 Part 1 is specifically concerned with hydraulic design, while structural and safety aspects require additional standards and project-specific criteria.
29. Professional Engineering Summary
The successful construction of an RCC overflow weir depends on the integration of hydrology, river hydraulics, geotechnical engineering, structural engineering, concrete technology and construction management.
The most important principle is that the weir should be treated as a complete hydraulic structure rather than an isolated RCC element.
Catchment → Design Flood → River Hydraulics → Crest Level → Overflow Capacity → Afflux → Foundation → Seepage/Uplift → Stability → Energy Dissipation → Scour Protection → Structural Design → Construction → Inspection → Commissioning
Failure at any one of these stages can compromise the performance of the entire structure. Therefore, field execution should always follow the approved drawings, specifications, method statements, inspection and test plans and directions of the competent Engineer.
30. Conclusion
An RCC overflow weir is a relatively compact structure but involves complex interaction between water, concrete, foundation and river sediments. Correct crest geometry alone is not sufficient. Foundation stability, uplift, seepage, energy dissipation and downstream scour are equally important.
A properly constructed weir should provide the required upstream water level and discharge capacity while safely passing flood flows and protecting the downstream river bed and banks.
"Never construct a hydraulic structure only to the dimensions shown on a drawing. Understand the hydraulic purpose of every level, slope, cut-off, apron, joint and protection arrangement before execution."
Posts
0 Comments
If you have any doubts, suggestions , corrections etc. let me know