Water
Management Plan, Lot 3, South West Highway Whitby
4
4.0
Pre & Post Development Scenarios
Pre Development Scenario
The site to be filled is not high quality agricultural land. Whilst the soils are good loams
they contain a proportion of stone and rock. They are sloping and are constrained in
area between two drainage lines. There is no local surface or groundwater available in
sufficient quantities for irrigated landuse. The slopes and small area make the land
unsuitable for cultivated agriculture, and not suitable for perennial horticulture. It has
Low to Very Low agricultural capability.
Currently there are low amounts of runoff on the proposed landform raising site, due to
the high ground vegetation and roughed clay topsoil surface. Any runoff that does
leave the area is collected in the winter creeks running to the south and west of the
site.
There are no apparent sedimentary runoff issues or flooding issues in the proposed
landform raising area. The area is well drained by the winter creeks bordering the site,
and the large elevation differences across the site prevent water from excessively
pooling.
Post Development Scenario
The water runoff of the site will be similar in
the post development scenario, than as it
was prior to development. The final level of the land will be 3 metres higher across the
whole side, aside from some gentle batter sloping along the edges of the site to ‘marry
in’ to the existing levels.
Topsoil that was collected from the surface prior to the clay infilling of the site shall be
respread across the site, so that the existing vegetation can be restored. In addition,
tree planting will take place along the southern and western sites of the site for visual
management purposes.
Initially, the topsoil respread shall be contoured across the site to limit the potential for
sediment to enter the winter creeks. However, once the topsoil vegetation gets a
foothold
over the clay respread, sediment issues are expected to be at a minimum.
As the landform raising will occur in segments not exceeding 2.5 hectares before
revegetation and tree planting will take place, runoff from sparsely vegetated clay will
be minimised. Having the material placement only taking place in the summer months
will also minimise the risk of excessive sediment runoff into the nearby creeks.
SJS TRIM - IN12/17340
OCM122.9/01/13
Water Management Plan, Lot 3, South West Highway Whitby
5
5.0 Flood Levels and Displacement Impact
Flood Levels
Stormwater calculations show that approximately 12,000,000
litres of water is predicted
to fall onto the 10.3 hectare site within a 24 hour period in the event of a 1 in 100 year
rain storm. Given all runoff water is either soaked into the ground or captured by the
winter creeks that surround the area, it is expected that flood levels after the landform
raising will not change from the existing situation.
Water infiltration into the clay during construction may have an impact on the structural
stability of the landform raising. This issue is dealt with separately in the geotechnical
report. Given there is no structure to be placed on the clay, the clay only needs to be
capable of holding its own weight in a (potentially) rain sodden state.
Displacement Impact
The flood displacement impact on surrounding areas as a result of the landform raising
is effectively nil. The pre-development contours of the land will be maintained post-
development, aside from the 1:10 slope battering down of the edges of the site. As a
result, the volume of water fed into the winter creeks surrounding the site will not
change as a result of the landform raising.
SJS TRIM - IN12/17340
OCM122.9/01/13
Water Management Plan, Lot 3, South West Highway Whitby
6
6.0 Impacts on Surface drainage and Water Quality
Impacts on Surface Drainage
Surface drainage will be impacted initially, with the natural surface flow being diverted
around the clay placement on the lower reaches of the site. Spoon drains and contour
channels will be installed during the works duration to ensure water flows are
minimised through the earthworks area.
Once the clay placement has been completed in a section, the topsoil respread will
occur over the affected area. Topsoil contours will then be
installed over the newly
constructed works to minimise the sediment flow from the freshly placed topsoils.
Tree planting along the southern and western edges of the site will also assist in
breaking down any excessive water flows across the site.
Once a good coverage of vegetation is established over the topsoil, the surface
drainage flow should return to a similar state prior to development of the site.
Refer to Water Management diagrams at the end of this document for illustration of the
proposed water flows at various stages of construction.
Impacts on Water Quality
Water runoff quality will be impacted in areas where water runoff is adjacent to freshly
placed clay. In these areas, sediment will inevitably be picked
up with the water flows
on the site.
It is expected that sediment runoff will settle after about 3-4 years, where vegetation
will stop the further erosion of the earthworks. However, water management of the site
will deal with the likely sediment runoff through the following measures:
•
Installation of a sediment trapping basin at the lowest extents of the works,
which will be sufficiently sized to handle a significant runoff event. Water from
here is then released back into the winter creek
•
Installation of contour channels around the earthworks which will collect and
sediment impacted water and divert it to the sediment trapping basin
•
Installation of spoon drains around the upper reaches of the site to prevent
runoff from infiltrating the freshly placed
earthworks from the top
•
Installation of topsoil contours along the completed sections of landform raising,
to prevent further erosion of the new earthworks
•
Programming of the works to occur in lots of no greater than 2.5 hectares
before revegetation works commence, so as to avoid large catchment areas
impacting the water runoff and overwhelming the sediment trapping basin
•
Strategic placement of tree replanting in areas of high runoff flow, to slow down
water travelling at high speeds and creating further sediment and erosion
By implementing
the above measures, the water management of the proposed
landform raising on Lot 3 South West Highway Whitby can be sufficiently conducted
with negligible impact to neighbouring properties, the amenity of the site and the
environment.
SJS TRIM - IN12/17340
OCM122.9/01/13