Along the 300-meter landscaped stretch downstream of the Zhongzheng Liu bridge, the Water Resource Bureau emphasized the use of on-site natural features to further enhance water quality by reducing diffuse pollution from stormwater runoffs. The channel was widened by eight meters on each side by taking over highway space on each bank. The concrete embankments previously flanking the canal were replaced with gentle grass slope reinforced with geocell mats. 159 new trees were planted with a variety of native plants like Chinese fringe tree (Chionanthus retusus spp.), flamegold rain tree (Koelreuteria elegans spp.), Himalayan ash (Fraxinus griffithii spp.), sweetgum (Liquidambar styraciflua spp.) and tassel tree (Garrya elliptica spp.) as well as some native water willows (Salix mesnyi spp.). Some of the existing trees were incorporated to the new design, while 38 were relocated. A total of 3,083 square meters of new green space were created, allowing half of the riverbank to become water permeable.
A range of sustainable urban drainage techniques (also known as “low-impact development” stormwater management) were incorporated to the riparian landscapes thus created including raingardens, infiltration strips, bioswales and permeable paving. Underground rainwater storage tanks were incorporated along the riversides providing a sustainable water supply for landscape irrigation. Perforated drainpipes were laid under the river course to help adjust the amount of water during the high and low water level period.
This first phase of the Liuchuan canal remediation and improvement project has been hailed as a success in light of the wide range of benefits achieved. Water quality has significantly improved. In the upstream reach, between the Chongde Liu and Zhongzheng Liu bridges, the River Pollution Index (RPI) decreased from 7.25 to 3.5, meaning the water quality changed from “severely-polluted” to “lightly-polluted”. Within the 300-meter landscaped section, the RPI decreased from 8 (“severely-polluted”) to 3.75 (“moderately-polluted”). This is the result of the sewer interception and treatment process that was established upstream and of the sustainable drainage techniques that were implemented. Measurements have shown that these landscape-based solution to address diffuse pollution resulting from stormwater runoffs remove 69% of the suspended solids, 67% of the phosphorus and 24% of the nitrates that would otherwise enter the canal in this location. The Liuchuan canal no longer smells bad and its waters have become clear and clean enough to sustain fishes and damselflies, which are now routinely sighted by those who enjoy a stroll along the water edge.
The project has also withstood the test of flash flooding, with all plantings coping well while being submerged for short time periods. From a management point of view, reliance on an automated system to warn users of the Riverwalk of an impending flood has proven an effective approach to guarantee people’s safety at a fraction of the cost of relying on on-site staff.