Infiltration Blanket Design Spreadsheet

Infiltration Blanket Design: Purpose, Methods, and Spreadsheet Tools

A well‑designed infiltration blanket plays a quiet but crucial role in modern stormwater management. It helps developments handle rainfall sustainably, reduce runoff, and protect downstream networks. The sections below walk through what an infiltration blanket is, how engineers typically design one, and why spreadsheets have become a practical tool for getting the calculations right.

What is an Infiltration Blanket

An infiltration blanket is a shallow, engineered layer designed to absorb, store, and infiltrate stormwater into the underlying soil. It typically sits beneath landscaped areas, car parks, sports fields, or lightly loaded surfaces and acts as a distributed infiltration system rather than a single concentrated soakaway.

Most infiltration blankets are built using:

• A permeable surface layer such as turf, gravel, or permeable paving
• A granular sub‑base with high void space to temporarily store water
• A geotextile layer to prevent fine particles from clogging the system
• Native soil beneath, which ultimately receives the infiltrated water

The blanket spreads water over a wide area, allowing it to soak into the ground slowly and naturally. This reduces peak runoff, supports groundwater recharge, and helps developments meet SuDS (Sustainable Drainage Systems) requirements.

Key benefits include:

• Lower surface runoff and reduced flood risk
• Improved water quality through natural filtration
• Distributed infiltration that avoids overloading a single point
• Compatibility with green spaces and low‑impact design

Common Infiltration Blanket Design Methods

Designing an infiltration blanket requires balancing hydraulic performance, soil infiltration capacity, and structural requirements. Several methods are commonly used:

Infiltration Rate Assessment — Designers conduct percolation or infiltration tests to determine how quickly the native soil can absorb water. This value drives the required blanket area and depth.

Storage Volume Calculations — The sub‑base layer is sized to temporarily store stormwater from a chosen design storm (e.g., 1‑in‑10‑year or 1‑in‑30‑year rainfall event). Engineers calculate the void volume needed to prevent surface flooding.

Hydraulic Loading Method — The blanket is sized so that the inflow rate from rainfall does not exceed the soil’s infiltration capacity. This ensures the system drains within an acceptable timeframe.

Layered System Design — Some blankets use multiple layers of different aggregate sizes to improve filtration, structural strength, and long‑term performance.

Overflow and Bypass Provisions — Where infiltration alone cannot manage extreme storms, designers include overflow pipes or connections to downstream drainage systems.

Each method must consider soil type, groundwater depth, rainfall intensity, land use, and maintenance requirements to ensure long‑term reliability.

How Does the CivilWeb Infiltration Blanket Design Spreadsheet Work?

Firstly the spreadsheet calculates the quantity of rain water that the infiltration blanket 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.

Next the user can input the infiltration rate expected from the underlying soils along with the infiltration blanket area and the material porosity which the spreadsheet can determine the total storage volume and outflow rate.

With these calculations done, the spreadsheet cleverly analyses each potentially critical design storm event and calculates the depth of infiltration blanket material required to achieve the volume of rainwater storage required. The user can then simply pick the desired storm event value or use the worst case for the design thickness.

A further check is finally made on the time of emptying to ensure that the infiltration blanket drains quickly enough to make space for any subsequent storms.

Why Use an Infiltration Blanket Design Spreadsheet?

An infiltration blanket design spreadsheet helps engineers manage the many variables involved in sizing and checking the system. Because infiltration performance depends on soil tests, rainfall data, storage depth, and void ratios, a spreadsheet ensures calculations are accurate, consistent, and easy to update.

Advantages include:

• Automated calculations for storage volume, infiltration rate, and drain‑down time
• Quick comparison of different storm events or soil conditions
• Built‑in checks for groundwater levels, soil permeability, and structural limits
• Scenario testing for alternative materials or blanket depths
• 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 infiltration blanket 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 Infiltration Blanket Design Spreadsheet now for only £19.99.

Or alternatively big savings are available with our Infiltration Blanket and Permeable Pavement design bundle. Get both spreadsheets for only £29.99.