Detention basins designed as usable landscape

A detention basin can serve as usable open space, but only if the inundation frequency, the side slopes and the management regime are designed for people as well as for water. Detention basin amenity value is decided by how often the grass is under water and how easily a mower can get round it, and those two decisions are made at the drainage design stage rather than by the landscape architect afterwards.

The basin that counts towards an open space requirement and the basin that appears on a layout as a fenced hollow are the same feature designed differently.

Dry and wet basins behave differently

A dry detention basin is empty between rainfall events. The base is grass or a wildflower sward, there may be a shallow channel carrying dry weather flow across it, and the storage volume is above the normal ground surface. That is the version with amenity potential.

A wet basin or retention pond holds a permanent body of water, with the storage volume above the permanent level. It has biodiversity and visual value, and it provides better water quality treatment, but it is not usable space. It also brings a safety and maintenance regime of its own.

Deciding between them is partly hydrological and partly about what the site needs. Where open space is short and the drainage volume is large, the dry basin earns its keep twice. Where the scheme has plenty of amenity land and a water quality problem, the wet option may be better.

Side slopes, depth and perceived safety

Three geometric decisions determine whether people will use the space.

Side slopes should be gentle. One in four or shallower reads as landform rather than as an engineering feature, allows a ride-on mower to work safely, and removes the hard break of slope at the top that produces trip complaints. Steeper than one in three and the basin becomes a maintenance liability that nobody mows and nobody enters.

Depth should be shallow and the basin wide rather than deep and compact, which is the opposite of what a volume calculation on its own would suggest. A basin 600 millimetres deep across a large area is usable amenity. The same volume in a 1.8 metre pit is not.

Perceived safety matters as much as actual safety. A basin with a shallow graded edge, no fence, clear sight lines across it and a visible outlet structure feels safe. A basin behind a palisade fence tells residents they should stay out, and they will.

Frequency of inundation and what it allows

This is the central design question, and it should be answered with a table rather than a sentence.

Set out, for each design event, the depth of water, the area inundated and the expected duration. Then say what use each part of the basin supports.

The general pattern is that areas inundated only in the rarer events can carry informal recreation, kickabout space or a mown path, because they are dry for the overwhelming majority of the year. Areas inundated in frequent events hold poor grass, remain soft, and support planting rather than use. The dry weather flow channel and the area immediately around the outlet are wet ground whatever the design.

A single basin can therefore have two or three zones with different uses, and showing that zoning on the landscape plan is far more persuasive than a note claiming the whole basin is amenity space.

An illustrative example: a 0.35 hectare dry basin serving 80 dwellings. The design shows side slopes at one in five, a maximum depth of 700 millimetres, a stone dry weather flow channel across the base, and a table giving inundation area and duration for each design event. The upper two thirds, inundated only in the rarer events, carries a mown kickabout area and a path. The lower third, which is wet more often, is wet meadow planting with no access expected. The management plan gives a cutting regime for each zone and names the management company.

Planting, mowing and management regimes

Match the planting to the inundation zone and accept that the result is not a lawn.

The upper slopes and the rarely flooded base can carry an amenity grass mix cut regularly. The frequently inundated base needs wet-tolerant species and a reduced cutting regime, typically once or twice a year with the cuttings removed. Wildflower meadow treatment works well on the slopes and reduces maintenance cost, but it needs a stated establishment regime for the first few seasons or it reverts to coarse grass.

Access for machinery is a design item, not an afterthought. Provide a maintenance access point from the adopted highway or an estate road, wide enough for the mower and for a small excavator to reach the outlet structure, with a surface that takes the load when the ground is soft.

Then write down who does what and how often, and name the funding route. A management plan without a named party and a funding mechanism is a wish.

Counting basins towards open space

Whether the basin counts towards an open space requirement is a local plan question. Policies and open space standards differ, and many authorities discount land that floods frequently or that has slopes too steep for use.

So make the case with evidence rather than assertion. Submit the inundation table, the zoning plan, the slope sections and the management plan, and state the area claimed as usable with the reasoning. Where the authority applies a published open space standard, set the claimed area against it explicitly.

Expect a negotiation. A reasonable outcome is often partial credit, with the rarely inundated area counting and the wet base not.

Birdstrike and other constraints on open water

Where a site sits within an aerodrome safeguarding consultation zone, which can extend a considerable distance from a safeguarded airport, open water that attracts large birds is a recognised hazard and the airport operator will be consulted.

In those areas, the design response is usually to avoid permanent open water, keep basins dry, avoid shallow margins and islands that gulls favour, and plant to discourage roosting. Raise it early with the operator, because a drainage strategy redesigned after a safeguarding objection is an expensive revision.

Other constraints worth checking before a basin is located: ground conditions and groundwater level, since a basin cut into the water table becomes a pond by accident; existing trees and their root protection areas; and services crossing the proposed footprint.

Evidence that the space is genuinely usable

The package that makes the case:

1. Cross sections at several points giving slope gradients, depth and the top of bank level.
2. A table of inundation area, depth and duration for each design event.
3. A landscape plan showing the use zones, the mown areas, paths and planting.
4. A maintenance access plan showing the route and surface.
5. A management plan naming the responsible party, the funding route and the cutting regime for each zone.
6. A short safety note explaining why no fence is proposed, where that is the case.

That material belongs in the Drainage Strategy and in the landscape submission together, and the hydraulic basis has to be consistent with the site's Flood Risk Assessment. Where the basin forms part of the response to surface water risk, read the mapping carefully in the way described in the note on reading the surface water flood risk map, and keep any property level measures consistent with the note on residual flood risk and resilience measures.

Draft a drainage strategy on PlanScope, then have a drainage engineer and a landscape architect review the basin design together before submission.

This is general guidance, not legal advice; check the position with your local planning authority, the lead local flood authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.