Vegetation (Floating River Vegetation) - Office of Public Works
Vegetation (Floating River Vegetation) - Office of Public Works
Vegetation (Floating River Vegetation) - Office of Public Works
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Fig. 8 Weed cutting blade operated from a boat (photo courstesy <strong>Office</strong> <strong>of</strong> <strong>Public</strong> <strong>Works</strong>)<br />
The removal <strong>of</strong> silt deposits from channels brings about the removal <strong>of</strong> nutrient enriched<br />
soils and root material <strong>of</strong> aquatic vegetation. Through the removal <strong>of</strong> vegetation and root material<br />
the growth capacity <strong>of</strong> plants is reduced. Extraction <strong>of</strong> root and aerial material also reduces the<br />
quantity <strong>of</strong> plant biomass degraded in watercourses during winter months. Therefore the Phosphate<br />
and Nitrogen quantity released into the water column as a consequence <strong>of</strong> plant tissue breakdown<br />
results in less nutrient availability for the following season growth. In such cases the drainage<br />
maintenance regime may be spread out over a couple <strong>of</strong> years.<br />
Use <strong>of</strong> hydraulic machinery with a weed cutting bucket attached enables weed control to be<br />
performed without disturbing the riverbed. The bucket <strong>of</strong> the hydraulic machine can be used to cut<br />
weeds just above the sediment layer. On exiting the water the weed cutting bucket with slits in the<br />
back curve collects the vegetation and allows excess water to drain from the bucket. Weed material<br />
is deposited on the adjacent channel bank. Cutting and removal <strong>of</strong> aquatic vegetation will firstly,<br />
reduce the plants ability to grow by eliminating plant photosynthetic capacity. Secondly, the initial<br />
regrowth after cutting, which is dependent on energy stored in plant roots, will produce weaker<br />
plant growth and also deplete plant energy reserves for the forthcoming season.<br />
Aquatic vegetation cut using the weed cutting boat brings about a slightly different dynamic.<br />
The weed is cut above the channel bed but unlike the weed cutting bucket <strong>of</strong> the hydraulic machine,<br />
the boat bucket is not designed to collect cut vegetation. Some or all <strong>of</strong> the cut vegetation therefore<br />
floats on the water surface until it re-enters the water column through degradation <strong>of</strong> plant material<br />
or sinks to the bottom channel bed. All nutrients are not removed from the water column and are<br />
therefore readily absorbed by the regrowth. As weed cutting occurs in the summer months when<br />
water levels are lowest and the oxygen capacity <strong>of</strong> the system is reduced (compared to winter<br />
months), therefore the uncollected cut vegetation could be favouring opportunistic aquatic flora.<br />
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