Guidance

Rain gardens are small-scale bioretention features suitable for gardens and small developments. Design principles align with larger bioretention systems (C753 Ch 18).

Siting: Locate near runoff source (e.g., downpipe outlet, patio edge) but min 3-5m from building foundations. Choose sunny/part-shade spot. Avoid steep slopes. Check for underground utilities.

Size & Depth: Size based on contributing catchment area (typically 5-10% of area). Depth usually 300-600mm below surrounding ground level, including ponding depth (100-300mm max) and filter media depth.

Infiltration Check: Assess soil infiltration rate (simple soakage test adequate for small scale). If infiltration poor (<10mm/hr), an underdrain connected to downstream drainage may be needed, or design as fully lined.

Excavation & Layers: Excavate to desired depth. If lined, install impermeable liner up sides. If unlined and infiltration good, may just need filter media. Typical build-up: Ponding area -> Mulch layer (optional, non-floating) -> Filter media (engineered soil mix, often sandy loam/compost blend - specified depth e.g., 300-500mm) -> Drainage/transition layer (optional, gravel/sand) -> Base (subgrade or liner).

Inlet: Design inlet to slow flow and prevent erosion (e.g., splash stones, vegetated channel). Ensure positive flow into garden.

Outlet/Overflow: Provide overflow (e.g., pipe, stone spillway) set at max ponding depth, directing excess water safely away (e.g., to lawn, drain). Underdrain outlet connects at base if used.

Planting: KEY component. Select plants tolerant of both periodic inundation and dry spells. Use mix of species for structure/interest (grasses, rushes, perennials). UK native species beneficial for wildlife (e.g., Yellow Flag Iris, Purple Loosestrife, Water Avens, Sedges). Plant into filter media.

Maintenance (C753 S18.12): Relatively simple. Water plants during establishment. Weed regularly. Inspect inlet/outlet/overflow for blockage, remove debris/sediment. Top up mulch if used. Prune/cut back plants annually as needed.