Filter Strip Design Spreadsheet

What is a Filter Strip?

A filter strip is a gently sloping, vegetated surface designed to slow down stormwater runoff and remove pollutants before the water enters a downstream drainage feature such as a swale, basin, wetland, or watercourse. Unlike a channel, a filter strip is broad and shallow, encouraging water to spread out into a thin sheet rather than concentrating into a flow path.

Filter strips are commonly installed along:

• Roads and highways
• Car parks and driveways
• Industrial yards
• Pathways and landscaped areas
• Watercourse edges

Their vegetation—often grasses or meadow mixes—helps trap sediments, absorb nutrients, and promote infiltration. By slowing the water and filtering it through soil and plant roots, filter strips improve water quality and reduce the hydraulic load on downstream systems.

Key benefits include:

• Reduced sediment and pollutant transport
• Lower peak runoff entering SuDS features
• Enhanced biodiversity and landscape value
• Low maintenance and long service life
• Compliance with SuDS and water‑quality regulations

Common Filter Strip Design Methods

Designing a filter strip involves ensuring that runoff spreads evenly, flows slowly, and has enough contact time with vegetation and soil to achieve meaningful treatment. Several established methods are widely used:

Sheet Flow Design — The filter strip is graded to maintain shallow, uniform sheet flow. Designers calculate the maximum allowable slope and length to prevent water from concentrating into rills or channels.

Hydraulic Residence Time Method — The strip is sized to ensure runoff remains on the surface long enough for sediment deposition and infiltration. This often involves calculating flow velocity using Manning’s equation.

Vegetation‑Based Treatment Design — Plant selection is tailored to the pollutants expected. Dense grasses are effective for sediment removal, while deeper‑rooted species can improve infiltration and nutrient uptake.

Pretreatment and Flow Spreading — Features such as level spreaders, kerb cuts, or perforated edges help distribute water evenly across the strip. This prevents erosion and improves treatment efficiency.

Infiltration‑Enhanced Design — Where soils allow, filter strips are designed to infiltrate a portion of the runoff. Soil testing determines infiltration rates and the required strip width.

Each method must consider rainfall intensity, contributing catchment area, soil type, slope, vegetation type, and maintenance access to ensure long‑term performance.

How Does the CivilWeb Filter Strip Design Spreadsheet Work?

Firstly the spreadsheet calculates the quantity of rainwater that the filter strip 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.

Finally the user can input the dimensions of the filter strip to be analyzed. This includes the width, gradient and Manning roughness coefficient of the proposed channel.

With all these inputs made, the spreadsheet cleverly analyses each potentially critical design storm event and calculates the inflow into the filter strip 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.

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

Why Use a Filter Strip Design Spreadsheet?

A filter strip design spreadsheet helps engineers manage the many variables involved in sizing and checking the system. Because filter strips rely on hydraulics, vegetation performance, and soil infiltration, a spreadsheet ensures calculations are accurate, consistent, and easy to update.

Advantages include:

• Automated calculations for flow velocity and depth
• Quick comparison of different slopes and rainfall events
• Scenario testing for alternative layouts or catchment sizes
• 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 filter strip 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 Filter Strip 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.