Fall ensures that the water runs away. Here is how you convert fall between per mille, percent and cm/m, and what NfA 1992 requires as a minimum for different surfaces.
Introduction
Fall is the slope of the surface. It is expressed in per mille (‰ = mm per m), percent (%) or cm per m. The relationship is simple: 10 ‰ = 1 % = 1 cm/m. Too little fall gives puddles; too much gives discomfort and run-off.
The standard's minimum fall (NfA 1992)
| Surface | Minimum fall | Note |
|---|---|---|
| Paving stones / slabs | 10 ‰ (1 cm/m) | Drainage |
| Cobblestones/setts on gravel | 20–25 ‰ | Subgrade/gravel-near |
| Gravel, roads and yards | at least 25 ‰ | Largest drop 35 ‰ |
| Stairs/steps and landings | 15 ‰ | For drainage |
| Gutters | at least 5 ‰ | No low spots |
| Drainage pipe | at least 3 ‰ | In a straight line |
Suppliers typically recommend more conservatively than the standard's minimum: terrace 15 ‰ (1.5 cm/m) and driveway 20 ‰ (2 cm/m). Note both — the standard's minimum and the industry recommendation — and choose according to the project.
Drainage system's components
Fall directs water somewhere — and there must be something to direct it to. The common drainage components in green systems:
- 01Linear drain (slot drain): a long grate in the surface that collects water along a line — e.g. in front of a garage or at a terrace edge.
- 02Point drainage / catch basin: a single outlet with grate where falls from several directions meet.
- 03Grit trap: a pit with a sedimentation chamber that retains sand and gravel before water flows on — must be able to be emptied.
- 04Fascine: an underground cavity (cassettes or stones) where rainwater is stored and seeps into the ground.
- 05Drain pipe: perforated pipe that collects and drains water from the ground (minimum 3 ‰ fall in straight line, NfA 1992).
- 06Permeable surfacing: joints or stones that let water seep through instead of leading it away.
Sizing of drainage
How much water must the system be able to drain? The runoff from an area is calculated from the size of the catchment, its runoff coefficient and the design rain intensity. On own land, the intensity is determined according to DS 432 or the Wastewater Committee's directives (directive 27 on functional practice, directive 30 on climate factors). A traditional design rain intensity is 140 l/s per hectare.
The runoff coefficient φ expresses what proportion of the rain runs off the surface (the rest evaporates or permeates). Indicative values from municipal design bases (Wastewater Committee practice):
| Surface | Runoff coefficient φ |
|---|---|
| Roof surfaces | 1,00 |
| Dense surface (tiles, asphalt, concrete) | 0,90 |
| Gravel areas | 0,30 |
| Grass reinforcement | 0,40 |
| Green areas / grass | 0,10 |
Example — drainage from a terrace
En 100 m² flisebelagt terrasse: A = 100 / 10.000 = 0,01 ha, φ = 0,90 (tæt belægning), i = 140 l/s pr. ha. Q = 0,90 · 0,01 · 140 = 1,26 l/s. Det er den vandmængde, linjedrænet eller brønden skal kunne tage ved den valgte regn. Skal vandet i stedet nedsives, dimensioneres faskinen efter jordens nedsivningsevne og en klimafaktor (skrift 30).
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