to either divert or detain urban runoff. For example, pipe outlets are screened and drainage lines directed away from closed-end canals on Patterson Lakes canal estates, Victoria. Enrichment from the excessive application ¡ of garden fertiliser, from hydromulching during the final stage of canal development and from the release of human waste from boats and marinas appears to be common to many canal estates. Release of pollutants such as oil and petrol from boat fuelling facilities and the leaching of heavy metals from boat antifouling paint is associated with marinas located within or at the entrance to canal estates. Polycyclic aromatic hydrocarbons have been found concentrated at boat marinas on canal estates in Victoria (Nuttall, 1987). In many cases, water quality in closed-end canals proves to be inferior to the parent water-body, resulting in a fall in standards such as turbidity, suspended-solids and chlorophyll-a during ebb tides. Canals constructed on river mouths are vulnerable to catchment runoff, particularly in urbanised or industrialised areas where various biodegradable and non-degradable pollutants enter the water. Even where low flow river diversion is undertaken to protect water quality standards on canal estates (for example Dandenong Creek is diverted from the Patterson Lakes estuary developments on Port Phillip Bay) flood flows carrying a significant amount of pollutants in the first flush enter the original course of the river.
A
10.11.86 2m 20m
A
16.1.87
mobile silt
SEDIMENTATION On-site runoff during heavy rain or aerial fall during hot, dry winds causes silt and clay from canal construction to enter the parent water-body and other completed waterways. Stormwater runoff, particularly from developing catchments, carries large amounts of inorganic and organic particles which wash into the estuary and settle out in the slack water of canal estates (Figure 3). 1986
1987
2.0 -+--'----J'--.,__.....__.__.__.___,___,J.__,._.....__.__.____.___,___,
,
~fl;:::==:::::::::=.=------~1 s_o <..::::::::::: ,_ :_2
1975
1.5
19861
19861 ,
~
rJj
......
GI
1.0
GI
E
:r: I-
0.5
Cl.
w
C
0
_ _ _ _ _ _ _ _ closed-end canals _________ estuary _______________ entrance to canals Figure 3. Water clarity in Patterson Lakes canal estates, Victoria (measured over a 16 month period using a Secchi-disc).
The result is that bottom sediments in canal estates have an increased silt and clay content when compared with the parent water-body. Bottom sediments collected from McMillan Strait, Gippsland Lakes contained a higher percentage of coarse material such as sand and shell than sediments from the Riviera Harbours canals estates being developed on the strait. At times, runoff from Riviera Harbours carried a plume of turbid material into McMillan Strait which eventually settled on the coarser sediments causing a localised but measurable increase in the silt-clay fraction. There are several important environmental effects from this sedimentation pattern. Firstly, tidal canals undergo profile changes caused by the deposition of suspended solids (Figure 4). The problem is intensified by boat movement reworking and redistributing loose, unstable sediments and by the erosion of beaches by anglers using sand pumps to collect bait. Organisations who are responsible for maintaining the waterways as navigable channels usually respond to this problem with suction or mechanical dredging , even though it is accepted that this affects the ecological stability of aquatic organisms. Suction dredging on the Nerang River canal estates resulted in a marked influx 16
WATER December, 1987
B
t 1975
~c
Figure 4. Changes in canal profile at Patterson Lakes estates, Victoria at three sites over 1 year (Site A) and 10 years (Sites B and C).
of opportunistic species ab le to utilize areas of freshly-exposed sediments to the disadvantage of the established fauna (Saenger and Mclvor, 1975). Also, benthos initially adapted to living in coarse sediments of the parent water-body, may find difficulty colonising the fine-textured material of tidal canals. Turbid water restricts light penetration for submergent aquatic plant photosynthesis with a consequent reduction in habitat diversity and food supply for estuarine animals . Organisms living in or on the bottom of canals are smothered by the deposition of finegrained particles which also fill up the interstices between coarser material. This not only affects the cryptic fauna and flora but results in higher temperatures and oxygen depletion within the sediments because of the reduction in pore space. Pollutants brought downstream in catchment runoff readily bind to clay particles, flocculate in saltwater and settle on the canal bottom. Bioturbation can return the pollutants to the sediment surface from underlying layers where they are available for biotic uptake or re-entry to the water column.
BIOLOGICAL PROBLEMS Biting midges (Culicoides spp.) have become a problem on intertidal beaches of canal estates in the subtropical zone of New South Wales and Queensland. Often misnamed sand flies, biting midges are pests of both people and livestock. The species C. moles/us has assumed major pest status on residential canals and is found on all beaches which extend up from mean tide level