Swales and filter strips Construction guide

Swales and filter strips Construction guide
Swales & Filter Strips
Construction Guide
STORMWATER DEVICE INFORMATION SERIES
SWALES & FILTER STRIPS Construction Guide
What are swales and filter strips?
STORMWATER DEVICE INFORMATION SERIES
Swales, also known as bioretention, filter or infiltration strips, are broad, grass channels used to
treat stormwater runoff. They direct and slow stormwater across grass or similar ground cover
and through the soil. Swales also help filter sediments, nutrients and contaminants from incoming
stormwater before discharging to downstream stormwater system or waterways. Some swales have
liners to direct filtered runoff, or rocky linings to slow fast flows. Swales are simple to maintain and
can fit well in urban design.
Fig.1 Swale in Waitakere City
SWALES & FILTER STRIPS Construction Guide
Seven key components of swales and filter strips
1. Inflow points
3. Channel base
Low flow path, may have gravel or rip
rap reinforcing to prevent erosion.
5. Plants and soil
Grass or other low lying plants in
permeable soil for filtering stormwater.
d
rip
st
ge
ed
rb
k
cu
ar
rp
te
ot
sl
Stormwater flow
entry, via pipe outlet
or surface runoff.
STORMWATER DEVICE INFORMATION SERIES
ai
n
in
g
so
il
ca
dr
7. Flow controls
w
el
l
Check dams or spreaders, used
to slow and spread flows to
improve filtering. Often used
on swales with slopes over 5%.
2. Side slopes
Total channel width. Slope less than
3:1 for mower access and to prevent
scour.
4. Underdrain (if present)
6. Outlet
Usually perforated pipe, buried
under channel to capture filtered
flow and connected to stormwater
system.
Discharge point for filtered
stormwater.
SWALES & FILTER STRIPS Construction Guide
STORMWATER DEVICE INFORMATION SERIES
Fig.2 Waitakere City Hospital car
park swale
• Slotted kerbs not too close to edge of swale.
• Sign post in centre of swale are compromising underdrain.
Fig.4 Manawa Wetland vegetated swale
• Native vegetated swale following contours and overland flow paths.
• Check dams used to slow flow and allow for filtration.
Fig.3 Henderson Valley Road
– newly constructed road side swales
• Geotextile used to protect catchpit (swale outlet) during construction.
• Dish channels for each driveway controls flows and allow high flows to
pass above.
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SWALES & FILTER STRIPS Construction Guide
Construction Sequence
Swale design will usually be specifically detailed in construction plans.
The following outlines the general swales construction sequence.
STORMWATER DEVICE INFORMATION SERIES
1. Excavate
3. Connect
• Remove material to form channel.
• Form to levels on construction plans with excavator and blade. Side slopes
not to exceed 3:1 horizontal to vertical slope. Base of channel usually
minimum 600mm to allow mower access, or maximum 2m wide.
• Do not compact ground at base of channel, as it acts as filter for flows.
• Construct flow collection inlets (may be catchpits, manholes) and
connect to stormwater system.
• Construct outlets (may be concrete or rock sill, catchpit or vertical riser pipe with grate or scruffy dome) and connect to specified
stormwater system. (see Figure 6)
• Set sills at elevation specified on plans to prevent flooding.
• Cover all collection points until swale construction is complete.
4. Install underdrain (if present)
Fig.5 Excavation, preparation of side
slopes to form channel
Fig.6 Construction of inlets with
erosion protection
• Excavate trench for underdrain in base of channel.
• If specified, lay liner or geotextile around trench and pin in place.
• Lay gravel bed (minimum 50mm).
• Lay underdrain pipe, (usually perforated such as Novaflow) covered with geotextile barrier or filter sock.
• Connect underdrain to outlets as detailed on construction plans,
making a water tight seal.
• Fill underdrain trench with gravel bedding to bottom of channel,
place geotexile overtop of metal and then cover with topsoil.
2. Check fall
Use laser, survey or spirit level to check slope of swale length is not greater
than 3:1, or as detailed in construction plans.
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SWALES & FILTER STRIPS Construction Guide
Construction Sequence Cont...
STORMWATER DEVICE INFORMATION SERIES
5. Install flow controls
7. Remove sediment controls and restore
• If slope of swale length is steeper than 5% gradient, flow controls are required,
for instance, rocks and check dams. (See Figure 3 dish channel driveway built
as check dams)
• Check construction plans for details of what controls should be fitted and where.
• Once planting completed, remove sediment/dewatering controls.
• Restore surrounding areas to original condition, removing excess
materials and regrassing.
• Establish and uncover collection inlets.
6. Sow and plant
• Place 100mm minimum topsoil across swale length and channel width.
• Topsoil must be free of clay to allow free draining.
• Sow grass densely (and plant native plants, if specified) as detailed in
construction plans. Sow to cover entire swale length and width.
• If specified, plant native plants in positions detailed on plans.
• Fence off until grass established, and water regularly early on to establish
dense grass growth.
Fig.7 Preparation of top soil layer
(minimum 100mm) for planting
Fig.8 Constructed swale. Picture
shows curb cuts as inlets, established
dense grass and plants (North
Harbour Stadium)
Fig.9 Dewatering control measures
(also helps to control sediments) at
the outlets during construction
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SWALES & FILTER STRIPS Construction Guide
STORMWATER DEVICE INFORMATION SERIES
Quick checks
Avoid
Plant grass densely to give complete coverage without bare patches.
Confirm design specification before construction. Changes during
site development may mean levels and locations need revision.
Do not overcompact soil during construction – soil needs to be free-draining.
Construct swale banks at slope less than 3H:1V.
Check fall along length of swale before installing underdrain.
Do not mix clay into topsoil mix.
Do not construct swale when ground is wet, especially on clay sites.
Do not block inflows with woody or large plants.
Install flow controls if slope of swale length is more than 5% gradient.
Make sure kerbs and other street edge features allow for sheet flow
across and into swale.
Do not drive over swale during construction to avoid soil compaction,
fence off if necessary.
Keep collection points covered until grass is established.
Block or divert any new or existing inlets and outlets from swale
during construction.
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