In less than a year, this former stone quarry has been transformed into a tropical garden. The local council wanted to preserve the tags painted on the wall, next to the main entrance gate, in order to maintain the untouched aspect of the place; for phase two of the work, the second part of the garden, which still remains fallow, will accommodate the Heron Tree, a giant tree-like structure built of ironwork and designed by a team from the Voyage à Nantes, a very active group of artists and performers in Nantes. This group created its own graphic designs in the old shipyards situated opposite the garden, on the other side of the Loire. At the exit from the building, the first branch of the tree has already been created. These are the same artists who designed the Grand Elephant that strolls around Nantes and which is three times the size of a real elephant. We often forget this mechanical giant that hit the headlines in Nantes, two or three years ago.
More than 3 kilometres of dripline distributed over the whole garden
The first thing we discovered when entering the garden itself was a magnificent bamboo plantation, and further on, alongside the escarpment, there is a spectacular cascade. The pathway that we followed was bordered with different tropical species: bananas, tree ferns and other flowering tropical plants exist side by side in this exotic garden. In fact, with a microclimate, the garden enjoys the benefit of mild weather, the temperatures being 2 to 3° higher than in Nantes city centre.
“Once the plantations had been created, they added bananas and other tropical plants because it was found that they were not dense enough”, explained Yvon Cessou.
These tropical plants are irrigated with a pressure-compensating and self-cleaning 16mm diam Netafim drip irrigation system, with 2.3 l/h drippers, spaced at 30 cm. A little over 3 kilometres of dripline have been laid over the whole garden. The drip irrigation system is covered with mulch, in order to keep the soil moist and hide the irrigation system, so that the members of the public will not damage the equipment when walking around the gardens.
A special irrigation system has been put in place to water the tree ferns. These are plants that need to be watered in the morning and in the evening, directly to the base of the fern. “For this, we laid out the capillary (feeder) tubes to run the length of the trunk, with a punched dripper positioned in the foliage”, explained Philippe Guilbaut, the site supervisor from Arrosage System, the company that installed the system.
The cascade and the lake operate as part as a closed-circuit system
When we walked further along the path, we came to a bridge constructed above a large reservoir. And up above, alongside the cliff, we could see the cascade, which is spectacular. The jets of water gush out of three spouts equipped with control valves. The water falls into an the artificial reservoir that we can see below us.
The cascades are managed by an automated computer device; there are a number of settings possible: a linear variation in the jets, ranging from the weakest (30 m3/hour) to the strongest (200 m3/hour), or even a random variation in the jets, where the computer decides which sequence to have.
At the foot of the cascade, in the pond, an artificial misting system (TBD) provides mist alternately at the foot of the cascade and under the bridge. “When there is a high flow rate in the cascades, then we don’t activate the mist under the cascade, we activate it under the bridge instead. And, conversely, when there is a low flow rate in the cascades, we activate the mist under the cascade,” explained David Cottineau, electrical automation engineer at Arrosage System. And he added: “We have installed compressors to flush the systems”. In fact, bacteria could grow in the stagnant water of the misting system.
The reservoir, which has a geotextile liner, has a mixture of recovered rainwater and tap water from the city’s network. “In the summer, very probably we will have to top up with tap water because there will not be enough rainwater”, explained Mr. Cessou. The water circulates through a closed circuit. Flowserve 200 m3 circulating pumps connected to variable speed controllers are submerged in the reservoir: one pump sends water up into a supply tank on the cliff top and the other one activates the circulation of the water in the pond. Each of the two pumps has a backup unit so there are four pumps in total.
Before arriving at the pumps, the rainwater undergoes a process of organic (primary) filtration, using a pozzolan system. The pozzolan takes the form of lava stone in containers placed one above the other, next to the pumps. This kind of filtration is entirely natural.
Furthermore, discharge nozzles have have been installed in the pond to give a flowing effect.
If the reservoir becomes too full, an overflow system has been installed to direct the water into the drains.
We continued our walk along the footpath and came across another part of the garden that seemed to be more natural, between the footpath and the cliff. The plant varieties seemed to be more like our own species. Mr. Cessou explained to me: “In this part of the garden, they have retained the species that grow naturally here: brambles and brooms to show what the garden looked like previously”.
The current maintenance operations are centralised in the service room, thus reducing the response time for the operator
Finally, we headed for the service room, which houses the central control of the irrigation system and fountains. Upon entering the building, we noticed that there were 12 systems. “Having 12 irrigation zones has enabled us to distribute the irrigation water as uniformly as possible. This was to simplify the irrigation programming for the operator and distribute the amounts according to the (requirements of) the zones”, explained David Cottineau.
We could see the Rain Bird ESP-LXME irrigation controller on the right, which manages all of the irrigation system on the site. There are also two control cabinets in place: one cabinet on the right for the fountains and the misting control cabinet on the left, both being managed automatically by computers.
“The equipment installed in the service room is an installation that was purpose-built, designed to be easy to use and centralise the current maintenance operations in the service room, thus reducing the response time for the operator”, added Mr. Guilbaut.
An employee from Arrosage System comes to the site once a week to carry out maintenance. “We will start up again in the spring”, explained Mr. Cessou. And he continued: “While waiting for the onset of winter, there will be some protection work to do for the plants. I’d be surprised if we were able to save them all, particularly as planting was a little late in the season, in July”. The project was carried out in accordance with the following timeline: Arrosage System was involved initially to put the water supply, plants and storage systems in place and then the company IdVerde dealt with the planting; once this had been completed, Arrosage System came back a second time to install the drip irrigation system.
On reaching the end of the site, we could see that the garden had been discontinued on the right-hand side. Mr. Cessou added, “This is where they are going to construct the heron tree. And they are going to build a restaurant at the foot of the tree. This second phase of the project should be completed in 1922. There will also be a third phase: a garden will have to be established on the right side of the quarry”.