Monday, October 15, 2007

ISA HK/China --- HK Tree News (Greening Open Drainage Channels)

*** Our weekly Station Mail is for the information of Station Members only, but Station Mail has given up copyright & can be freely circulated. For administrative reasons, comments from outsiders are usually not entertained & may be circulated within our system locally & overseas. Please note Station Mail is sometimes given in Blog at , although images are usually not attached due to size. ***

Dear Station Members,

Below is a recent set of news regarding greening Open Drainage Channels (ODC) in the old sectors of our city to rid foul smell & provide landscape area for improving the quality of life of nearby residents:

In contemplating about the whole idea, this Station has reservation on just covering the ODC with concrete deck & then try to 'green' the deck, because:

1. There may be more hard surface created which will absorb heat from the sun, than before the construction.

2. If there is going to be extra soft work on deck to cover the created hard surfaces, the soil weight will require a thicker deck at higher cost, & the soft work will lead to heavy maintenance especially from vandalism in the older part of our city. Tree planting would generally require around 0.6 - 1m thick of soil for healthy growth & wind stability for the future mature trees. If planting only dwarf trees, shrubs & groundcovers, the landscape may appear insignificant & immature to show the world, & to justify all the expenses incurred.

3. The temperature of the deck will be the same as ambient temperature. The roots of all plants will be hot in summer & cold in winter. The Buffer Zone of ground temperature can not be provided for anything growing on a concrete deck. This applies to roof planting as well.

Above are just some major points of concern in the greening matter, besides the consideration for storm water collection & sediment removal inside the covered channel.

A possible solution to derive the objective of eradicating smell with landscape provision under low cost, might be the installation of Flood Gate at the estuary of the ODC to convert the ODC into a flowing river implemented with ornamental fishes. At times of low flow, Flood Gate can be raised to increase water level. The slow flowing water can be stirred by circulation pumps & oxygen pumps for sustaining water life so that smell can be reduced to minimum. This is how fish farm is operated in many parts of the world. Water plants can also be installed to add colours & to sustain water creatures. A balanced ecological system will be created in place. Landscape can then be installed on both sides of the converted ODC, with trees, to form a natural setting. In times of storm, Flood Gate can be opened to direct water exit, although some fishes may be lost...

The Station Manager has seen this idea in action in Osaka years ago & illegal angling was happening along the converted channel because it was so attractive. The same idea is also being used everywhere in Holland for navigation & irrigation, although it is to keep water out from the ocean, instead of getting it out from land.

Flood Gate is an option besides decking the ODC which is expensive, time consuming & rather disturbing to the public during construction. Besides, decking may not be as environmentally friendly & may become another Heat Island if not carefully designed with landscape. On the other hand, using Flood Gate would depend upon other factors including pollution control into ODC & also flow rate. Stagnant polluted water will be hard to purify by biological means. Further studies by more parties will be needed to justify its use. Once which system is decided, the landscape can compliment it as it should.

We wonder which system will be chosen at the end & based on what ground ... ?

best regards,

Sammy Au
Station Manager

The ISA Mission - Through research, technology, and education, promote the professional practice of arboriculture and foster a greater public awareness of the benefits of trees. (


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