Rainwater management and LAR

LAR stands for local management (or handling) of rainwater. Instead of directing all rain to the sewer, water is delayed, infiltrated and evaporated locally on the site. This relieves the sewer system, reduces the risk of flooding during heavy rain and is today often a requirement for new facilities. For the landscape gardener, LAR has become a core point: paving must drain water, and green areas must be able to receive it.

• 01Infiltration: the water seeps into the soil through infiltration trenches, rain gardens, permeable surfaces or grassed areas — requires suitable, permeable soil.
• 02Delay: the water is temporarily retained (reservoir, basin, green roof) and released slowly, so the sewer and recipient are not overloaded.
• 03Evaporation and uptake: planting and green roofs evaporate water and absorb it in the plants — contributes to water balance and to cooling and biodiversity.

• 01Permeable surfaces: stones or joints that let water seep through the surface and base layer down into the soil.
• 02Rain gardens: depressions with well-draining growing medium that collect and allow rainwater from roofs and surfaces to seep down.
• 03Soakaways: underground reservoirs (stone-filled or plastic cassettes) where rainwater is collected and seeps down.
• 04Swales and ditches: grass-covered depressions that transport, delay and allow water to seep down.
• 05Infiltration on grass: surface runoff is directed onto a grass area that infiltrates it.
• 06Green roofs and detention basins: retain and delay water before it is discharged.

LAR systems are dimensioned so they can handle a rainfall with an agreed recurrence period without backing up water more often than allowed. The basis is the Wastewater Committee's design method and the Pipe Centre's instructions:

• 01The Wastewater Committee's LAR dimensioning note (2015, version 2.0) with associated spreadsheet uses a statistical time series as the basis for calculation.
• 02Climate factor is applied to future rainfall (Wastewater Committee Report 30), so the system also holds in a wetter climate.
• 03Combined rainfall events are accounted for by adding approx. 20% to the reservoir volume (Wastewater Committee's method).
• 04Rørcenter's/Teknologisk Institut's instructions on infiltration and LPS collect the rules for design, dimensioning, execution and operation.
• 05Gentagelsesperioden aftales med myndigheden — faskiner dimensioneres ofte for en mindre regn end regnbede, der kan tage en større hændelse (fx T = 10 år).

All infiltration rises and falls with the soil. Sandy soil infiltrates quickly; clayey soil almost not at all. Before a seepage system is dimensioned, the soil's infiltration capacity (the hydraulic conductivity) is determined — either from the soil type or by a seepage test on site, where you measure how quickly the water level drops in a filled test pit. If the infiltration capacity is too low, retention is chosen over infiltration.