When rainwater moves across large areas—particularly roads, paving and other hard surfaces—it can travel quickly towards the lowest point. This concentrated runoff can overwhelm traditional drainage systems when it reaches a restriction such as a gully, pipe or outlet.

In Sustainable Drainage Systems, or SuDS, features such as rain gardens and swales slow, store and manage surface-water runoff. One of the main constraints is the rate at which water can infiltrate or percolate into the soil.

To maximise the soil’s capacity to manage water, a SuDS feature needs sufficient surface area and depth for runoff to pond temporarily. This allows infiltration to take place gradually rather than forcing all the water into the ground immediately.

The space provided above the design water level is known as freeboard. It provides an additional allowance before water reaches the top of the SuDS feature, an adjacent footpath, a building threshold or another sensitive level.

Why Is Freeboard Needed in SuDS?

Stormwater calculations are based on assumptions about rainfall, runoff, infiltration, storage and discharge rates. In practice, actual site conditions can vary.

More intense rainfall, restricted outlets, sediment build-up, construction tolerances or reduced infiltration can all cause water levels within a rain garden, swale or other SuDS feature to rise higher than expected.

Freeboard can provide an additional margin of safety for these uncertainties. It can help to:

• Reduce the risk of water entering buildings.
• Protect utilities and other sensitive infrastructure.
• Accommodate temporary increases in water level during extreme rainfall.
• Improve resilience where outlets become restricted or sediment accumulates.
• Maintain safe access around SuDS features.
• Support clearly defined exceedance routes.

Current SuDS guidance reinforces the principle that the freeboard allowance should reflect both the uncertainty in achieving the required level of protection and the potential consequences if flooding occurs.

What Should Designers Consider When Assessing Freeboard?

There is no single freeboard depth that is appropriate for every SuDS feature. The required allowance should be established through the drainage and flood-risk design for the individual site.

Designers should consider:

• The design storm event and applicable climate-change allowance.
• The expected depth and duration of temporary ponding.
• The proximity of buildings, roads, utilities and access routes.
• Outlet capacity and the risk of blockage.
• Ground conditions and infiltration performance.
• Construction tolerances.
• Long-term sediment accumulation.
• Public safety, particularly where SuDS features are located beside footpaths.
• The route water will follow if the design capacity is exceeded.

Freeboard should therefore be considered during the early stages of site design. It should not be added after the landscape, drainage, road and building levels have already been fixed.

Early coordination allows SuDS to manage runoff effectively while also supporting planting, biodiversity, amenity value and attractive public spaces.

Explore Greenleaf Ireland SuDS solutions

Why Early SuDS Coordination Matters

The level of a rain garden or swale must be coordinated with surrounding roads, kerbs, paving, footpaths, building thresholds, drainage outlets and utilities.

The inlet must allow runoff to enter the SuDS feature, while the planting and soil levels must be low enough to provide temporary ponding space. At the same time, the top level and freeboard allowance must help prevent uncontrolled overflow towards vulnerable areas.

Designers should also identify a safe exceedance route so that, when the capacity of the SuDS feature is exceeded, water is directed away from buildings and other sensitive infrastructure.

The key takeaway

Freeboard is a simple but important part of safe and resilient SuDS design.

By providing a controlled allowance above the design water level, freeboard helps accommodate uncertainty, protect vulnerable assets and manage exceedance during exceptional rainfall.

Good SuDS design is not only about how much water a system can store. It is also about understanding how water enters the feature, how long it may pond, how it infiltrates and where it will go if the normal design capacity is exceeded.