Swale Design Spreadsheet

What is a Swale?

A swale is a shallow, vegetated channel designed to slow, convey, and infiltrate stormwater runoff. Sometimes referred to as a bioswale. Concrete Swales are possible where infiltration is not available or desired. And Permaculture Swales can be used where sloping sites are too steep for a traditional drainage swale.

Unlike a conventional drainage ditch, a swale is engineered to manage water sustainably by encouraging infiltration, filtering pollutants, and reducing peak flows before the water reaches downstream systems.

Swales are commonly used alongside roads, car parks, housing developments, and open spaces. Their gentle side slopes and planted surfaces help spread water, trap sediments, and support natural treatment processes. Depending on the design, a swale may function as:

- A conveyance feature that moves water safely across a site

- An infiltration system that allows water to soak into the ground

- A treatment device that improves water quality through vegetation and soil interaction

- A temporary storage area that attenuates peak flows

Key benefits include reduced runoff, improved water quality, enhanced biodiversity, and alignment with SuDS (Sustainable Drainage Systems) requirements.

Common Swale Design Methods

Swale design varies depending on site conditions, regulatory requirements, and the intended hydraulic function. Several established methods are widely used:

• - Hydraulic Conveyance Design

Engineers size the swale to safely convey runoff from a chosen design storm. This includes calculating flow depth, velocity, and freeboard using Manning’s equation and selecting appropriate side slopes and channel width.

• - Infiltration Swale Design

Where soils allow, swales are designed to infiltrate water into the ground. This requires soil infiltration testing, sizing the base width, and ensuring the swale drains within an acceptable timeframe.

• - Check Dams and Energy Dissipation

Small check dams are added to slow water, increase infiltration, and reduce erosion. This method is common on sloped sites.

• - Vegetation‑Based Treatment Design

Swales can be planted with grasses, wildflowers, or wetland species to improve pollutant removal. Designers consider plant selection, soil depth, and maintenance needs.

Dry vs. Wet Swales

• - Dry swales

drain fully between storms and rely on infiltration.

• - Wet swales

include an underdrain or maintain a shallow water table, providing treatment even in poorly draining soils.

Each method must consider rainfall intensity, soil type, groundwater depth, longitudinal slope, maintenance access, and erosion control to ensure long‑term performance.

How Does the CivilWeb Swale Design Spreadsheet Work?

Firstly the spreadsheet calculates the quantity of rain water that the swale will be required to handle. The spreadsheet is pre-loaded with rainfall data for all parts of the UK, so the user simply needs to input the location, runoff coefficient, return period and climate change factor. The spreadsheet uses this info to calculate the rainfall intensity, which is simply multiplied by the catchment area to give the total volume of water for all possible design storm events.

The Australian and Malaysian versions of the spreadsheet use a simple rainfall intensity to calculate the rainfall volumes and flow rates. All versions of the spreadsheet (UK, Australian and Malaysian) are delivered together, there is no need to purchase multiple versions.

Next the user can input the infiltration rate expected from the underlying soils beneath the swale. This enables the spreadsheet to calculate the outflow from the swale through infiltration through the underlying soils.

Finally the user can input the dimensions of the swale to be analyzed. This includes the shape of the channel, width, side slopes, length, gradient and Manning roughness coefficient.

With all these inputs made, the spreadsheet cleverly analyses each potentially critical design storm event and calculates the inflow into the swale and uses this to calculate the depth of flow and the maximum flow velocity through the swale. The spreadsheet also checks these values to ensure that they are in accordance with the relevant standards. This ensures that the swale cannot present any dangers during operation which is important as they are often used in heavily populated areas and housing projects where fast or deep flows could potentially endanger life during storm events.

In addition to the depth and flow calculations, the spreadsheet next calculates the total flow into the swale, the infiltration out of the swale through the underlying soils and therefore the water storage volume required. This is compared with the storage volume available calculated from the swale shape and dimensions.

With all checks completed, the user can clearly see whether the proposed swale is acceptable and in accordance with the relevant design standards or whether it needs to be adjusted.

Why Use a Swale Design Spreadsheet?

A swale design spreadsheet helps engineers manage the many variables involved in sizing, checking, and documenting the system. Because swales combine hydraulic, infiltration, and ecological

functions, a spreadsheet ensures calculations are accurate, consistent, and easy to update.

- Automated calculations for flow depth, velocity, and channel capacity

- Quick comparison of different storm events or swale geometries

- Scenario testing for vegetation types, slopes, and soil conditions

- Clear documentation for planning submissions and SuDS compliance

For developers, engineers, and local authorities, a spreadsheet becomes a practical tool that reduces design time and increases confidence that the swale will perform effectively in real‑world conditions.

Get Started Today

Save hours of repetitive calculation time. Whether you’re working on a major project or a small renovation, this tool will give you clarity, accuracy, and confidence.

Buy the CivilWeb Swale Design Spreadsheet now for only £19.99.

Or alternatively big savings are available with our Swale and Filter Strip design bundle. Get both spreadsheets for only £29.99.