Why do sports fields need subsoil drainage?

A sports field is a drainage asset with grass on it. Everything about how many hours it can be used comes back to how fast water leaves the profile.

What goes wrong without it

**Surface water sitting after rain,** which closes the field for days rather than hours.

**Compaction,** because play on a saturated surface destroys the soil structure, and compacted soil drains even more slowly. It is a cycle that accelerates.

**Turf failure,** since roots in a waterlogged profile die and the species that survive are the ones nobody wants.

**Wear concentrating** in the areas that do drain, which produces bare patches in the high traffic zones.

The result is a field that is unusable for a large part of the winter season, which is exactly when it is most in demand.

The components

**Subsoil drainage,** a grid of perforated pipe in drainage aggregate, at a spacing and depth set by the soil type and the design rainfall event. Closer spacing in heavy clay, wider in sand.

**A drainage layer or amended profile** above it, so water can reach the pipes rather than perching on a clay horizon.

**Sand slits or gravel bands** where a retrofit cannot practically regrade the whole field, connecting the surface to the subsoil system.

**Surface grade,** because subsoil drainage does not remove water the surface never let in. A crown or a cross fall sheds the bulk of a heavy event and the subsoil system handles the rest.

**An outfall** that works, connected to the stormwater system with the capacity to receive the flow, which is where a retrofit most often fails. A perfect field drainage system discharging into a blocked pit is not a drainage system.

That connection is the part designed with the site's wider stormwater, per [stormwater and drainage on live sites](/answers/stormwater-and-drainage-on-live-sites).

Natural turf and synthetic are different problems

**Natural turf** needs the profile to drain while still holding enough moisture and nutrient for the grass, which is a balance rather than a maximum. The establishment and the grow-in period after construction decide whether it works, which is the subject of [turf establishment and landscape maintenance periods](/answers/turf-establishment-and-landscape-maintenance-periods).

**Synthetic turf** drains vertically through the carpet into a free draining base, which means the base and the drainage under it are the whole system. A synthetic field over an inadequate base holds water under the carpet, and that is not a surface problem that can be fixed from above. The installation detail is in [how is an artificial turf sports field installed](/answers/how-is-an-artificial-turf-sports-field-installed).

Irrigation and drainage are one design

A field that drains well needs water put back on it in summer, and the two systems occupy the same ground.

Which means the irrigation mains, the laterals and the heads are coordinated with the drainage lines rather than installed afterwards and cut through them. Both are set out together, which is the coordination in [irrigation systems on sports fields](/answers/irrigation-systems-on-sports-fields).

Where water is harvested for irrigation, the drainage system is a source, which connects to [rainwater tanks and reuse on public sites](/answers/rainwater-tanks-and-reuse-on-public-sites).

Retrofitting a field in use

Most of this work is on existing fields that a club and a school both rely on.

So the sequence is set by the seasons: construction in the window between seasons, with enough time for grow-in before play resumes. A field returned to use before the turf has established is a field that will be back in the same condition within a season.

Which makes the programme the critical item rather than the construction, and it is planned the way the holiday window works are planned in [how do contractors deliver works during school holidays for a day 1 term handover](/answers/how-do-contractors-deliver-works-during-school-holidays-for-a-day-1-te).

What to check on an existing field

Where the water sits, how long it stays, where the outfall is and whether it is clear, and whether the surface still has its original grade after years of topdressing and wear.

Those four answers usually identify whether the problem is the surface, the profile or the outfall, and the three have very different costs.

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