Flyover Drainage in India

ho1856
Jul 30
8 min read

Disclaimer

The topic discussed/information provided by me in this blog is for general informational purposes only. All information in this blog is provided in good faith; however, I make no representation or warranty of any kind, express or implied, regarding the accuracy, adequacy, validity, reliability, availability or completeness of any information in this blog.

This blog intends to bring out mismatches between the actual execution done at site and that mandated in Applicable standards, Specifications, Manuals, Codes etc. in the Drainage of Grade separators i.e. Flyovers/Road Over Bridges (Elevated structures)/Viaducts etc.

Executive Summary

Every monsoon, millions of litres of water fall from India's flyovers directly onto the roads below. Most people assume this is how the structures were designed. It is not.

IRC and MoRTH standards clearly require that rainwater from elevated structures be collected through a proper network of horizontal and vertical pipes and safely discharged into the drainage system. The cost of providing this drainage network is generally included while preparing project estimates and bid documents.

Yet, on a large number of projects, only the drainage spouts are installed, leaving water to free-fall from heights of 5–10 metres onto roads, service lanes and pedestrians below. The consequences are serious—hydroplaning, accidents, rapid pavement deterioration, damage to structures, increased maintenance costs and inconvenience to the public.

The cost of providing a complete drainage system for a typical elevated structure is approximately ₹1 crore. When this system is not implemented, the apparent saving during construction ultimately translates into much larger losses for the public exchequer through repeated repairs, premature deterioration of public assets and avoidable accidents.

This article compares what the IRC codes and MoRTH specifications require with what is actually being constructed on the ground. It raises a simple but important question:

If the cost of proper drainage is already included in the project, why are we still allowing rainwater to fall directly onto the roads below?

When Standards Stay on Paper: The Reality of Flyover Drainage in India

Every monsoon, thousands of motorists drive beneath flyovers where concentrated streams / of water (PANI KI DHHAR) suddenly fall from the deck above. While many consider this normal, it is actually contrary to established IRC guidelines and MoRTH specifications. The issue is not merely one of inconvenience—it is a serious road safety, maintenance and asset management concern.

Drainage of Grade Separated Structures (Flyovers,Road Over Bridges, Viaducts etc.)
In Highway engineering we spend months optimizing structural designs and pavement compositions yet one of the most critical elements for longevity and safety is routinely compromised during execution i.e. Deck Drainage. It plays a vital role in safety of both road users and the structure itself. For grade separator structures like flyovers and viaducts, management of storm water is strictly governed by IRC codes and MoRTH specifications. However, a walk beneath almost any of recently constructed flyovers reveals a stark contrast between standards and ground reality. The most glaring and dangerous manifestation of this mismatch is the ‘free fall’ drainage spout.
What the Codes Demand (The Ideal Scenario)
The guidelines governing bridge and flyover drainage are explicit about how water should be handled to protect both structure and traffic below. Runoff must be collected through deck spouts and channelized into a connected network of longitudinal and vertical downtake pipes embedded in or attached to the piers. Ultimately this water should be safely discharged into nearby drainage system (area’s municipal storm system or rainwater harvesting pits etc.).
Spacing Discrepancies in Theory Vs Standards & Specifications:
While IRC codes generally suggest dense spacing of spouts based on surface area (e.g. 1 spout per 12 sqm on level sections), the widely followed MOST Standard Drawing NO. SD/210 often dictates a staggered spacing of 1 spout per 36 sqm. This internal contradiction sometimes leads to under-designed primary collection on deck itself.
The Ground Reality: The Free-Fall Phenomenon
Despite clear codal provisions against straight drops, the actual practice at many construction sites tells a different story. Instead of seamless, aesthetically integrated network of collector pipes leading to ground, contractors frequently install only the primary deck spouts. These spouts are left protruding a few inches from the soffit of the deck slab, functioning as nothing more than holes that dump concentrated streams of water onto whatever lies beneath.
Why Proper Deck Drainage Matters
Proper bridge drainage is essential because it
- prevents hydroplaning
- protects wearing coat
- prevents corrosion
- prevents deterioration of bearings
- reduces seepage
- increases service life
- improves aesthetics
- prevents nuisance to traffic below
- minimizes maintenance expenditure
Code Vs Actually Execution Summary

Aspect	Codal Provision (IRC/MoRTH)	Common Site Practice
Discharge Method	Continuous horizontal/vertical downtake pipes to ground drains	Spouts left open at the deck soffit resulting in free-falling water
Discharge location	Safely routed to drainage system as municipal drains or harvesting systems	Directly onto at-grade service roads, medians, or pedestrian zones
Piping Diameter	Large enough to prevent choking with elbow plugs for cleaning	Often undersized PVC pipes that easily clog with road debris

Why Does This Disconnect Happen ?
The root cause of this mismatch rarely stems from a lack of engineering knowledge; it is almost entirely driven by cost and oversight. A single standard MoRTH drainage spout hardware may cost a few thousand rupees. However, installing a comprehensive, code compliant horizontal and vertical down take piping system-especially on along viaduct can add considerable amount to the project cost. During DPR stage, these comprehensive drainage networks are sometimes glossed over to keep project estimates competitive. During execution, if the BOQ doesn’t explicitly mandate and fund the collector pipes, the contractor will simply leave the spouts hanging. Additionally, maintenance play a role. Complex collector pipes require regular cleaning to remove silt and debris. Agencies often avoid them entirely rather than commit to the necessary maintenance schedule, opting for the ‘easier’ (but highly damaging) ‘free-fall’ method.
Some other factors include:
✔ inadequate DPR detailing
✔ BOQ omissions
✔ lack of supervision
✔ absence of inspection checklist
✔ poor maintenance planning
✔ contractor's cost saving
✔ inadequate proof checking
✔ ambiguity between standard drawings and IRC provisions
The Ripple Effect: Safety and Structural Impact
Allowing water to free-fall from a 5 to 10-meter height has severe cascading effects:
i) Road Safety Hazards: A concentrated waterfall hitting an at-grade carriageway creates severe blinding hazards for drivers. For two-wheelers, these sudden streams can cause immediate loss of control, and splashing reduces visibility for everyone.
ii) Accelerated Pavement Distress: Highway engineers know that water is flexible pavement's worst enemy. The continuous, high-velocity impact of free-falling water strips the bitumen binder from the aggregates on the road below. This localized hydrostatic pressure quickly develops into deep potholes right in the middle of active traffic lanes.
iii) Aesthetic and Structural Degradation: Water blown back onto the piers by wind causes unsightly staining, promotes moss growth, and can accelerate the corrosion of concrete reinforcement over time if cover is compromised.
Bridging the Gap
To resolve this, the industry needs to treat drainage networks not as "accessories" but as critical structural components. Client organizations like NHAI and state PWDs must ensure that comprehensive down-take piping is explicitly detailed and funded in the BOQ. Furthermore, strict site supervision must reject any structure where spouts are left to discharge freely over active roadways. Until we strictly enforce what we design on paper, our flyovers will continue to solve traffic problems above while creating safety and maintenance nightmares below.

Detailed Discussion (More relevant to people having Civil Engineering background)

Engineering Consequences

Structural

corrosion of reinforcement
chloride penetration
leakage through expansion joints
deterioration of bearings
carbonation

Pavement

stripping
ravelling
potholes
rutting
pumping

Traffic

skidding
splash and spray
reduced visibility
accidents involving two-wheelers

Environmental

soil erosion
waterlogging
nuisance to pedestrians
uncontrolled discharge

Ambiguity among relevant codes /standard documents

Let’s understand what applicable codes say about drainage of the grade separators. This needs to be discussed as there exists ambiguity among these standards, specifications, manuals, codes etc.

Primarily following codes/ standards are referred regarding drainage of elevated structures (in simple words flyovers):

(I) IRC: SP:50-2013

(II) IRC: SP:84-2019

(III) IRC: SP:42-2014

(IV) IRC: SP:90-2010

(V) Standard drawings: Std Plans Drags.(MOST) DRG. NO. SD/210)

What these codes/ standards actually say are briefed below:

(I) IRC: SP:50-2013 (Cl. 9.1) & as per IRC: SP:84-2019 (Cl. 6.8.2.2): -

The rain water on flyover, which has to be above road is drained through either down take pipes or pipes embedded in the piers and ‘Efficient drainage be ensured by providing drainage spouts connected to horizontal and vertical pipe system such that the water from the structure does not fall on the road. (Whereas in actual practice it is not followed in most cases)

(II) IRC: SP:84-2019 (Cl. 6.8.2.3) & IRC: SP:90-2010 (Cl. 14.2), Typically water spouts are provided at the kerbs at the rate of 1 No. per 12 sqm of the surface in level portions and 1 No. per 15 sqm of the surface area on gradients.

(III) IRC: SP:42-2014 (Cl. 9.8) standard practice was to provide drainage spouts at a spacing of 3 m on both edges of a two-lane bridge.

(IV) Standard drawings of bridge super structures have also mention of spacing between drainage spouts. As per Std Plans Drgs. (MOST) DRG. NO. SD/210)

(V) IRC: SP:84-2019 (Cl. 6.8.2.4) Drainage fixtures and downspouts shall be of rigid. Corrosion resistant material not less than 100 mm as the least dimension and shall be provided with suitable cleanout fixtures.

Let’s put it in tabular form for direct comparison and more understanding as below:

As per IRC: SP:42-2014	As per IRC: SP:50-2013	As per IRC: SP:84-2019	As per IRC: SP:90-2010	As per Std Plans Drgs. (MOST)
	9.1 Drainage of Flyovers and Bridges The entire rain water on the carriage way of flyover shall be drained though efficient piping network of rainwater of the area. Caution shall be exercised not to allow straight drop of water from flyover to road surface below, which results in disruption of traffic and damage to road pavement. The rain water on flyover, which has to be above road is drained through either down take pipes or pipes embedded in the piers into series of network of rainwater harvesting system of the area and overflow can then be directed to the drains. All such disposal systems shall blend well and aesthetically pleasing.	6.8.2.2 Efficient drainage of the deck structure shall be ensured by providing a suitably designed drainage arrangement consisting of drainage spouts connected to horizontal and vertical pipe system such that the water from the structure does not fall on the road, does not stagnate over the road or at entry and exit points of grade separated structure and is discharged into the draining system of the area. Care must be taken that the pipes are taken down in such a way that they are aesthetically pleasing.
As per IRC:SP:42-2014	As per IRC:SP:50-2013	As per IRC:SP:84-2019	As per IRC:SP:90-2010	As per Std Plans Drgs. (MOST)
9.8 Spacing of Drainage Spout In the[A1] olden days the standard practice was to provide drainage spouts at a spacing of 3 m on both edges of a two-lane bridge. But the inlets were of the same diameter as the drain pipes.		6.8.2.3 Typically water spouts are provided at the kerbs at the rate of 1 No. per 12 sqm of the surface in level portions and 1 No. per 15 sqm of the surface area on gradients. Water spouts are connected to runner pipe of suitable diameter (minimum 100 mm) on either side of roadway and taken down by down-take pipes at pier & abutment locations.	14.2 Typically, water spouts are provided at the kerbs at the rate of 1 No. per 12 sqm of the surface in level portions and 1 No. per 15 sqm of the surface area on gradients. Water spouts are connected to runner pipe of suitable diameter (minimum 100 mm) on either side of roadway and taken down by downtake pipes at pier & abutment locations.	DRG. NO. SD/210 Typically, water spouts are provided at the kerbs at the rate of 1 No. per 36 sqm of the surface. (Generally followed in Practice)

Actual Practice: It has been noticed that the above provisions are not being followed true to its spirit, causing damage to structures/ roads, inconvenience to commutes and losses to the Nation itself at last.

The following photographs downloaded from Street views of Goggle map represents the practice at sites.

A Technical Contradiction That Needs Clarification

Current IRC publications recommend one drainage spout per approximately 12 sqm of deck area on level sections, whereas the commonly adopted Standard Drawing SD/210 indicates one spout per 36 sqm. This inconsistency can result in different interpretations during DPR preparation and execution. A harmonized guideline from IRC or MoRTH would eliminate ambiguity and promote uniform practice.