URBAN
Project funded
by theEU–DGEnvironment
L
andsurfacerelevant
forapplication
Artificial surface
Agriculture land
Forest and semi-natural areas
Wetlands
F
inancialcosts
(C
apital
,
operation
&
maintenance
)
Construction costs for infiltration trenches vary between €70 and €90/m
3
stored vo-
lume, depending on the depth, geometry and underlying soil/geology conditions, and
there will be minor ongoing maintenance costs.
Geotechnical investigations
are required to confirm the land stability and underlying soil/geology conditions.
These may need to be intrusive and require analysis of land contamination to
determine the suitability of infiltration techniques (€0.5-€10k).
D
esign
Infiltration trenches should be 1-2m deep and filled with stone aggregate, with a
sufficiently
high void ratio
. They should have a high-level outfall with a
flow
control device
to accommodate excess runoff. They are restricted to flat sites
(2% max). They should not be used for primary treatment of runoff on brownfield
sites or other pollution hot-spots if the risk of contamination to groundwater is
high, and should only be used in areas with
low sediment loading
, unless
upstream pre-treatment is included.
S
cale
Infiltration trenches are generally
designed to collect and infiltrate runoff
from a small area such as a car-park.
Infiltration trenches may also be applicable for artificial
surfaces in agricultural areas,such as farmyards and roads.
Infiltration trenches are
shallow excavations
filled with rubble or stone. Ideally they should receive lateral inflow from an adjacent impermeable surface. They allow water to
infiltrate
into the surrounding soils from the bottom and sides of the trench. In doing so, they
reduce runoff
rates and volumes and can help replenish groundwater and
preserve base flow in rivers. They are effective at removing pollutants and sediment but must be designed with an effective pre-treatment system.
Case studies: Infiltration trenches inKungsbacka, Sweden
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