Kai Tak River - a new urban green river corridor
By Drainage Services Department (DSD), the HKSAR Government
Kai Tak River and its history
Kai Tak River, previously known as the Kai Tak Nullah, is a major drainage channel in East Kowloon which collects rainwater from its hilly hinterland as well as nearby urban districts including Wong Tai Sin, Tsz Wan Shan, San Po Kong, Diamond Hill, Kowloon City and the Kai Tak Development Area. Altogether, this is a catchment area of approximately 1,080 hectares. The River was first formed in the 1920s and underwent a series of changes following the development of the surrounding urban areas and the old Kai Tak Airport.
The River was constructed with a concrete lining, similar to other nullahs built in Hong Kong at that time, which enabled it to function as an effective rainwater drainage channel. However, due to the rapid urbanisation of the surrounding area and the increasing occurrence of severe weather events, the drainage capacity of the River soon became insufficient. For example, there were severe flooding incidents along Choi Hung Road adjoining the Kai Tak River.
Kai Tak River improvement works in Wong Tai Sin
To mitigate the flooding risk, drainage improvement works have been implemented in stages along the River since 2010.
The Drainage Services Department (DSD) was responsible for 1.1 km of improvements on the long upstream section of the River in Wong Tai Sin at a total cost of about HK$3 billion.
This article focuses on the works undertaken by the DSD.
The Civil Engineering and Development Department was responsible for the remaining 1.4 km long section of the Kai Tak Development Area downstream of Wong Tai Sin.
A series of public engagement exercises were held at the design stage to gather public opinion on the project and to determine the design principles. As a result, three design principles were established:
(1) assigning priority to increasing the drainage capacity of the River;
(2) minimising the river decking; and
(3) revitalising the River into an urban green river corridor.
Improving the drainage capacity
As the River in Wong Tai Sin is situated in a densely populated area bound by heavily trafficked roads and buildings, it was not possible to widen the River to increase its capacity. After considering all the various site constraints, two alternative flood prevention strategies were finally adopted to increase the drainage capacity, namely (1) deepen the riverbed (2) build a box culvert below the existing Choi Hung Road alongside the River.
Previous flooding incidents mainly occurred at the section between Shatin Pass Road and Tai Shing Street. This is the narrowest section of the River with a width of only about 11 m. The two intersections at Shatin Pass Road and Tai Shing Street, where the carriageway structural decks significantly reduce the flow area of the River, were the primary bottlenecks.
In order to increase the drainage capacity for this critical section, a 400 m long box culvert of 6 m wide x 3.2 m deep was designed to run alongside the existing river underneath Choi Hung Road. This would provide an additional flow path to bypass the two bottlenecks. In addition, the main river would be deepened by 1 m.
A numerical hydraulic model was created to analyse the performance of the proposed works. However, the presence of various proposed physical features inside the River, such as greening and landscaping features as well as the turbulences which would occur at the inflow points of side branches including the additional box culvert, created complicated flow effects which were difficult to model in this way.
To tackle this, a 1:50 physical model was built to evaluate the river flow and to assess the hydraulic performance more accurately. This model led to the conclusion that the drainage capacity of the River would be doubled once the proposed works were completed.
Removal of public utilities inside the River
As a result of the urban development in the vicinity, a number of utilities had been laid over and in the River which constricted the water flow and significantly affected the hydraulic performance of the River. To improve the situation, these utilities had to be moved away from the River to maximise the flow. Moving utilities in such a dense urban area with congested underground conditions was however a real challenge for the project team.
Despite the use of trenchless methods involving mechanical tunnelling techniques, considerable time was spent laying new pipes and directing all the utilities away from the River.
Large-scale temporary traffic arrangements
Choi Hung Road adjoining Kai Tak River is a heavily trafficked dual 2-lane carriageway. Closure of any traffic lanes would cause serious congestion and hence all four traffic lanes had to be maintained throughout the construction period.
In order to provide adequate space to construct the box culvert without affecting traffic, portions of the footpath and central divider along Choi Hung Road had to be temporarily utilised for diverting traffic during the construction period. Complicated temporary traffic arrangements, which involved the erection of a massive temporary steel deck above the River to accommodate two traffic lanes, were also put in force near the junction of Choi Hung Road and Shatin Pass Road.
Together with the extensive and complicated utility diversion works, the implementation of these temporary traffic arrangements was extremely difficult, in particular across the two critical road junctions.
Maintaining flow in the River
The River not only collects storm water from its catchment area, but since 1995 has also borne secondary treated effluent from the Shatin and Tai Po Sewage Treatment Works. This amounted to a discharge rate of over 300,000 cu m per day under the Harbour Effluent Export Scheme.
The River had to maintain operations at all the times even during the dry season to cater for the continuous flow of treated effluent. However, as most of the improvement works were carried out inside the River, the flow had to be temporarily diverted. A cofferdam was then constructed to facilitate the works, even though during rainy season, storm water might still overflow into the cofferdam. Therefore, the river works could only be carried out at full swing during the dry season. It appeared that a significant reduction of working efficiency in the rainy season could not be avoided.
One of the biggest challenges was to avoid creating higher flood risks to the vicinity during construction. It means that the original drainage capacity of the River had to be maintained throughout the construction period.
Since placing temporary works inside the River would reduce the flow and affect the drainage capacity, additional drainage capacity had to be created by removing obstacles inside the River or commissioning the box culvert in stages, before carrying out any other works.
In order to further reduce the flood risk during rainy season, removable water gates were installed upstream, allowing water to flow into the works area in case of heavy rainstorms.
Whenever a heavy rainstorm was predicted, work would have to stop. The working efficiency was drastically lowered particularly during the rainy season.
Early rainstorm alert system
In order to improve the efficiency of the works during rainy season, an early rainstorm alert system was developed to manage and process data obtained from the Hong Kong Observatory's (HKO) nowcasting system, the Short-range Warning of Intense Rainstorms in Localised Systems (SWIRLS). This enabled a more accurate and timely localised rainfall forecast to be used for planning works inside the River in a safe, explicit and deliberate manner.
The SWIRLS computes the direction and moving speed of rain areas based on two successive radar scans.
After analysing the motion of the radar echoes over the concerned area, the system extrapolates the radar echoes according to the information collected. With this technique, a more precise location of rain areas and the respective rainfall intensity in the coming hours can be deduced.
The project team deployed a data agent to further process the data received from SWIRLS and formulated a user-friendly interface to present the forecast rainfall intensity in a grid format which covered the whole river catchment.
When rainfall intensity was expected to exceed 30 mm per hour (equivalent to an Amber Rainstorm Warning signal) in over 60% of the River catchment area in the coming two hours, the system would automatically send an instant notification to the project team, who would then immediately arrange to evacuate workers and construction plants within the River, and remove the water gates when appropriate to maintain the drainage capacity. Meanwhile, Shatin Sewage Treatment Works would also be notified to temporarily suspend the discharge of effluent into the River.
In this way, the working time inside the River was effectively optimised compared to the previous working arrangement.
Transforming the River into the first urban green river corridor in Hong Kong
Apart from improving the hydraulic performance of the River, the DSD also took the opportunity to integrate the concept of revitalising the water by injecting various greening and ecological elements into the River to turn it into an urban green river corridor.
In order to fulfill public aspiration for a green river corridor, it was essential to select and incorporate suitable and sustainable revitalisation elements for the River.
The plants, for example, had to be able to sustain the unique and special environment of the River as its flow characteristics would vary significantly under different weather conditions.
During fine weather, the water would mainly be the treated effluent from Shatin and Tai Po Sewage Treatment Works which had considerable levels of salinity. On the other hand, during adverse weather, storm water collected from the catchment could be vigorous and might flush the plants away. The plants selected therefore had to be robust enough to survive the varying levels of salinity and against strong river flows.
To ensure the plants could survive in such highly variable circumstances, trial planting of over a hundred species was conducted at a very early stage of the project.
An off-site trial was conducted at the Shatin Sewage Treatment Works to study the survivability of different species of plants in different levels of treated effluent. Several species with better growth results were then selected for the on-site trial at the River to see if they could also withstand heavy water flow conditions. During the trial period, heavy rainstorms, including one in which a Black Rainstorm Warning was hoisted, all the plants were immersed under water and subjected to vigorous water flows for a period of time.
Parthenocissus dalzielii (異葉爬山虎), Scaevola taccade (草海桐) and Clerodendrum inerme (苦郎樹) were finally selected as they had proven to be the most robust and could adapt to the River environment best.
Three planting systems were adopted: roadside planters, artificial rock planters and submerged planters.
Except for an area at Tung Kwong Road where a section of old masonry wall had been preserved, roadside planters were provided at the top of the riverbank. To further enhance the aesthetics the upper part of river wall, draping plants were chosen. After conducting planting trials of several draping plant species, Bougainvillea spectabilis (簕杜鵑) which blossoms twice a year was chosen. Its vivid colour of the flowers would create a unique and thematic border for the River.
Artificial rock planters
To simulate a natural river bank, artificial rocks with hidden soil pockets were provided at the river bank to support plant growth. They were scattered along the maintenance walkways of the River. The openings of soil pockets were narrowed so that shrubs and climbers could emerge from the cracks of the rocks.
Climbers clinging to the riverbank would eventually spread their aerial roots over the surface of the river wall, which effectively increases greening coverage and provides vertical greening effects from the lower part of riverbank.
Apart from the roadside planters and artificial rock planters, submerged planters for aquatic plants, including native mangrove species, were provided in the wider section of the River to create a wetland habitat.
The planting species include Crinum asiaticum (文殊蘭) and Aegiceras corniculatum (桐花樹), which were already well-tested in trial planting. The submerged plants introduce both greening and naturalistic effects without impeding the discharge of the storm water. They also provide potential habitats for young fish and amphibians.
Aquatic ecological elements
Fish shelters and flow deflectors, formed by artificial rocks to simulate a natural river bed and bank, were provided at scattered locations along the River.
Every piece of artificial rock is unique. All of them were tailor-made on site to different sizes ranging from 300 mm - 1,500 mm in diameter, with different shapes, textures and detailing. The artificial rocks not only enhance the natural look and feel of the River, they also vary the speed and direction of the river flow, thus creating habitats for fish. The artificial rocks also help to prevent the water from flushing away the submerged plants.
By introducing sustainable greening and ecological elements, the River has been transformed into a vibrant green river corridor. The revitalised River with its simulated natural environment also fosters biodiversity by providing a habitat for wildlife. The number of birds and fish seen in and by the River has increased remarkably compared to the numbers around the original concrete-paved drainage channel which had no ecological or landscaping measures.
Going the extra mile
The River is a major drainage channel which is susceptible to adverse weather and the risk of sudden increases in flow and water level. According to past records during heavy rainstorms, the water level could increase by 1 m in just a few minutes. It is not practical to allow the public to enter the river channel itself for safety reasons.
Nevertheless, the DSD has gone one step further to enhance the neighbouring environment along the River, and enable the public to enjoy the view of the River from various safe vantage points.
Beautification works were undertaken at Morse Park (Park No 1). Construction of viewing platforms as rehabilitation works were also carried out at various footbridges across the River. The living environment has been improved through enhancing the townscape of the area, providing a scenic and leisurely place for the enjoyment of the public, as well as fostering closer connection with adjacent areas.
Beautification works at Morse Park
Morse Park (Park No 1), although located next to the River, used to be surrounded by tall fences. The physical connection between the River and the park was cut off.
With the aim of enhancing the living environment of the community and improving the connectivity between the River and the adjacent amenity areas, beautification works were carried out at Morse Park (Park No 1) using a barrier-free design. The design concept was to converge Morse Park with the revitalised River by removing all the fences. A cascade-type water feature was also introduced at the park entrance to emphasise the concept of the interaction between people and water. Morse Park (Park No 1) now blends seamlessly with the River and the living environment in the vicinity has been enhanced.
Featured viewing platform and footbridge rehabilitation
Near the bus stop opposite to Tung Tau Estate, a featured viewing platform with an iconic shelter structure was constructed adjoining the River. The shelter, which has a glass canopy above, is formed by two arches representing the old and current alignments of the River. With various landscaping features and sitting areas in the viewing platform, people are able to take a break from the busy environment to enjoy the scenery of the River.
Footbridges across the River were also rehabilitated to enhance the townscape of the area. The footbridge at Tung Tai Lane was temporarily relocated to facilitate the river construction works. The DSD also constructed a brand new footbridge which provides more space for pedestrians with enhanced landscape features.
With the significantly improved drainage capacity of the Kai Tak River, flooding risk in Choi Hung Road has been effectively mitigated. The Kai Tak River also enhances the living environment of the entire surrounding area through the provision of a green river corridor with improved connection to adjacent areas.
In addition to increasing the drainage capacity of the river channels, the DSD has also delivered on a higher goal to realise the concept of revitalising water bodies by incorporating green and eco-conservation elements into channel and river improvement works.
The Kai Tak River clearly sets a new benchmark for revitalising river channels in Hong Kong.
This article was prepared by the Drainage Services Department, the HKSAR Government.
The scenery of the Kai Tak River after the improvement works
The catchment area of the Kai Tak River (approximate 1,080 hectares)
Two flood prevention strategies : deepening of riverbed and the construction of a box culvert
Numerous utilities across the river were present before improvement works
Birds in the River
Revitalised the Kai Tak River