Wet Storage – Water Extraction and Irrigation Systems

The following points should be considered where water is extracted from standing water bodies:

• The water level must not be subject to major fluctuations.
• A sewer helps to isolate the water aspiration port.
• The submerged pump has to be securely attached.

In principal, water can be extracted from ground water (if the water table is high). Depending on the water table, a 4-5 metre deep hole is dug and the water accumulation checked. Where appropriate, an extraction sewer (analogue to the one described for flowing water bodies) can be installed.

With a sufficient gradient, an irrigation system can be powered by force of gravity. The required pressure is gained from the difference in elevation of the water body and the storage site. 10 metres difference in elevation create a pressure of 1 bar. Considering a minimum required pressure at the sprinklers and friction loss in the pressure pipe, the difference in elevation should be at least 30 metres water column to ensure failure-free operation. Where the difference - and with it the pressure – is higher, a pressure reducer or a safety valve might be necessary to regulate the pressure.

• Requirement for safe conservation: minimal irrigation quantity of 4 mm per hour or 1 litre per second for 1,000 cubic metres of timber. This amounts to a minimum requirement of 3.6 m³ of water per 1,000 m³ of roundwood per hour (stack height 4 metres). Better: 4-5 m³ water/1,000 m³ roundwood/hour.
• Alternative rules of thumb: approx. 15m³/h per 100 running metres of stacks or per 4,000 m³ roundwood.
• Water demand can also be calculated on the basis of the technical specifications of the installation. Required data are sprinkler diameters, number of sprinklers, and pressure at the last sprinkler (measured with a manometer). Using these values, the water consumption per sprinkler can be found in the table below. Multiplied by number of sprinklers on the site, the water consumption of the whole installation per hour is calculated.

• Suction pressure pumps are suitable. Underwater pumps can be used as well, but are prone to plugging and harder to clean. Hence they are rather suitable for ground water extraction or where a settling pond is used to clarify the water.
• It’s better to calculate the pump output generously if the definite amount of timber is not clear. Giving leeway for higher amounts can save costly retrofitting. Often, wet storage sites are planned too theoretically and have to be enlarged later. Rule of thumb: 1kw pump power per 1,000 cubic metres timber.
• Replacements and repair kits have to be provided for the case of system failure. Buying spare parts ahead is recommendable, especially backup pumps.

• Sufficient and permanent wood moisture has to be provided for at any time. Up to 18 hours of irrigation are necessary in summer (8 – 10 hours in winter).
• Continuous irrigation is better than irrigation in intervals.
• Sufficient moisturisation of the timber is also important in winter. Due to the low relative air moisture the risk of desiccation is high.
• Irrigation has to commence as soon as possible after rolling in. It is even sensible to start irrigation during rolling in or stacking to ensure the bottom logs are moist enough from the beginning.
• Line- or sector-wise control of the system allows for an early start of irrigation.
• The timber should be irrigated continuously during the first 3 – 5 weeks to provide a basic moisture. After that, nightly irrigation can usually be set aside.
• An interval timer for periods of rolling in and out can prevent the system from being left switched off accidentally.
• The system should only be switched off during strong, steady rain over several hours. Never forget to reactivate the system! Therefore it is better to leave the system running in case of doubt.

• Right from the start, a member of staff (several people for large sites) should be seconded to supervise the wet storage site full time on a permanent basis.
• Avoid frequent changes in on-site staff.
• Extensive inspections of the storage are essential. The equipment has to be checked for operability every day and serviced regularly. Check:
  • Operability of the sprinklers
  • Water pressure
  • Water flow rate
  • Operability of the pumps
  • Operability of the valves
  • Piping
  • Barriers
• The following should be clarified up front for potential system failure:
  • Availability of backup pumps
  • Potential to carry out repairs
  • Feeding in water from other water bodies
  • Emergency irrigation with the aid of the fire brigade (only for sites with a capacity of < 50,000 m³)
  • Stock of wearing parts and spares (spare sprinklers, hoses, pipes, magnetic valves)
• Calculating costs for transportation to and from the storage site is part of a complete operating plan. Also include estimated transport capacities.
• People seem to take more care in running the site when the whole forest department is responsible for it. It’s even better to include future customers (or rather wood buyers) right from the start in conception and operation of the site.

Setup and running of the wet storage sites involves risk of accidents. It can’t be ruled out that trespassers enter the storage site. Signs which point out the risks should be put up. These should also include a liability disclaimer.

On-site staff will have to climb onto mossy, slippery stacks to ensure smooth operation of the site. The following points should be considered to make work on irrigated stacks as safe as possible.

• Staff have to be informed about the risks involved in their work and about how to avoid them.
• Install the irrigation system as maintenance-free as possible. Place sprinklers in a way so they’re easy to reach and can be serviced without danger of falling.
• The operator has to provide appropriate protective equipment. Due to high slip hazard, sturdy boots and crampons are required. Rain gear is also necessary.

• The organisational structure should make sure that staff attending the irrigation system leave a note containing location of work, how they are planning to get there and back, and estimated time of return. Where possible, work should be carried out in teams of two.

Other resources that can help make working on wet storage sites safer are:

• Shoulder or bum bag to carry tools and presumably needed spare parts.
• Walking stick with spike for extra support on the logs.
• Proper rain gear for working while the system is running.
• A ladder to climb the stacks (this should be an exceptional case).
• Planks to lay onto the stacks.

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