More on this topic in the overview article Retaining and storing water in forests: a key to adapting to climate change
Permeable gravel “bars” in the structure of the forest road: infiltration trenches (Photo: FVA BW/Puhlmann)
Effect of infiltration trenches
Like classic pipe culverts, infiltration trenches channel the water from the ditches on the upper side of the forest roads into the forest stand on the lower side. However, this slows down the runoff, and some of the water seeps into the ground directly underneath the infiltration trench. This leads to reduced and slower runoff on the downhill side, which reduces the risk of channel formation and renewed acceleration of runoff. The increased infiltration under the road structure and on the downhill side of the infiltration trench also increases soil moisture and promotes infiltration into deeper layers.
Requirements
Infiltration trenches are particularly recommended for gently sloping roads with a longitudinal gradient of up to 6 %. To ensure that infiltration trenches remain functional for a long time, the inflowing water should be largely free of suspended solids. To achieve this, space is required on the upper slope side before the infiltration trench for a sufficiently large sedimentation basin in which the incoming water is slowed down and some of the suspended solids are deposited. Infiltration trenches can be driven over without any problems and can bear the required payloads if constructed appropriately. They thus do not restrict normal use of the forest road.
Planning
It makes most sense to construct infiltration trenches when road culverts are being renewed anyway in the course of road maintenance, when it can be done at little additional cost. As a rule, the installation of infiltration trenches does not require authorisation under water legislation. However, if there is a surface water body on the lower side of the infiltration trench and a direct inflow of water from the infiltration trench is to be expected, the water management authority must be consulted. It is also advisable to consult the nature conservation authority to ensure the avoidance of the spawning season and other sensitive periods for amphibians during maintenance work on the sedimentation basins.
Infiltration basin on the valley side (Photo: FVA BW/Puhlmann)
Structural implementation
Like pipe culverts, infiltration trenches are installed at an angle to the road axis, and with a gradient of 3 to 5 %. To achieve this, the existing material is removed over a length of at least two metres and a depth of approximately one metre. The bottom 75 cm at least is filled with coarse crushed stone (grain size 10 to 20 cm), followed by an approx. 10 cm thick layer of gravel (grain size approx. 3 to 6 cm) and finally approx. 10 cm of road construction material. If necessary, a separating filter fleece layer can be integrated so that material from the base course does not wash into the coarse aggregate. If possible, the sedimentation basin should be placed in the roadside ditch on the upper side of the road, a few metres before the infiltration trench, in order to separate it spatially from the infiltration trench inlet and thus further reduce sediment input. The bottom of the sedimentation basin should be below the level of the infiltration trench. To increase the diversion of the runoff towards the infiltration trench, the roadside ditch is closed off below the infiltration trench with an earth wall. In the case of high flow rates, it is advisable to create an infiltration basin on the valley side. Its level should be a maximum of 0.5 metres below the trench outlet to prevent scouring.
Infiltration trench structure (image: Office B612 GmbH)
Maintenance
No reliable empirical values are available as yet on the service life of infiltration trenches. Regular dredging or digging out of the sedimentation basins with an excavator is necessary to ensure their functionality. High sediment loads can quickly clog the infiltration trench, particularly in the case of steep gradients, rapid runoff, soils prone to erosion and large quantities of leaf litter in the roadside ditches. The frequency of maintenance measures depends heavily on the site conditions. The sediment input should thus be checked at an early stage after construction and especially after heavy runoff events.