Concrete Drainage Swale Detail

Concrete drainage swale detail refers to the engineered layout, dimensions, and construction layers of a concrete-lined swale used to convey surface water runoff. In the UK, these are often used in highways, industrial sites, and retrofit SuDS schemes where erosion resistance and low maintenance are critical.

The CivilWeb Swale Design Spreadsheet can save hours of repetitive calculations while ensuring compliance with CIRIA C753 SUDS Manual in the UK, CSIRO 1999 in Australia or MSMA in Malaysia, reliable accuracy and even optimisation tools which can save money on materials during the construction phase.

🚧 What Is a Concrete Drainage Swale?

A concrete drainage swale is a shallow, channelised structure lined with concrete to convey stormwater runoff. It may be cast in-situ, precast, or formed using bituminous concrete. These swales are typically used where:

- Flow velocities are high

- Vegetation is impractical

- Maintenance access is limited

- Erosion risk is significant

They are common in:

- Highways and motorways

- Industrial yards

- Car parks

- Retrofit schemes with limited space

📐 Concrete Swale Geometry & Dimensions

Parameter	Typical Range	Notes
Top Width	0.6–2.0 m	Wider for higher flows
Base Width	0.3–1.0 m	Flat or V-shaped
Depth	150–450 mm	Based on flow volume
Side Slopes	1:3 to 1:6	Steeper if lined with concrete
Longitudinal Slope	1–5%	Use check dams if steeper
Surface Finish	Broomed or trowelled	For slip resistance

Cross-section types:

- V-shaped: Efficient for low flows

- Trapezoidal: Easier to construct and maintain

- U-shaped (precast): Modular and rapid install

🧱 Concrete Drainage Swale Detail: Layer Breakdown

1. Subgrade Preparation

- Excavate to design depth

- Compact native soil

- Apply geotextile if required

2. Sub-base Layer (100–200 mm)

- Material: MOT Type 1 or Type 3

- Purpose: Load distribution and frost protection

- Compaction: 95% MDD minimum

3. Concrete Lining (100–150 mm)

- Type: C25/30 or C30/37 concrete

- Reinforcement: Optional mesh or fibres

- Finish: Broomed for slip resistance

- Joints: Expansion joints every 6–10 m

4. Edge Restraints

- Options: Concrete haunching, kerbs, or flush edging

- Purpose: Prevent lateral movement and erosion

5. Inlets & Outlets

- Inlet: Curb cut or grated inlet

- Outlet: Headwall, pipe, or soakaway

- Overflow: Optional pipe or spillway

🛠️ Construction Sequence

1. Excavation & Subgrade Prep

• - Excavate to swale profile
• - Compact and level subgrade

2. Install Sub-base

• - Lay and compact granular material
• - Check levels and falls

3. Formwork & Reinforcement

• - Install timber or steel formwork
• - Place mesh or fibre reinforcement

4. Pour Concrete

• - Use ready-mix or site-batched concrete
• - Vibrate and finish surface

5. Curing & Jointing

• - Apply curing compound
• - Cut joints to control cracking

6. Edge Restraints & Landscaping

• - Install kerbs or haunching
• - Backfill and tidy edges

🧪 Hydraulic Performance

Concrete swales are designed for conveyance, not infiltration. Key hydraulic considerations include:

- Flow velocity: Up to 3 m/s with concrete lining

- Capacity: Based on rainfall intensity and catchment area

- Erosion resistance: High due to rigid lining

- Overflow provision: Critical in urban settings

Use CIRIA C753 and Highways England DMRB for design guidance.

📋 Example Specification Table

Component	Specification	Purpose
Sub-base	MOT Type 1, 150 mm	Load support
Concrete	C25/30, 100 mm	Durable lining
Reinforcement	A142 mesh or fibres	Crack control
Side Slopes	1:3	Stability
Inlet	Curb cut with apron	Flow entry
Outlet	Headwall with pipe	Flow exit

🧼 Maintenance Requirements

Concrete swales are low-maintenance but require periodic inspection:

Monthly

- Remove debris and litter

- Check for ponding or blockages

Annually

- Inspect joints and surface condition

- Reapply curing compound if needed

- Check edge restraints

🎯 When to Use a Concrete Swale vs Vegetated Swale

Feature	Concrete Swale	Vegetated Swale
Flow Capacity	High	Moderate
Erosion Resistance	Excellent	Moderate
Water Quality Treatment	Low	High
Maintenance	Low	Moderate
Aesthetic Value	Low	High
Infiltration	None	Possible

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