- GREENSPACE: FRIENDS OF AVIS
Minutes and a Report about Events at the Avis Dam Sluice Gates in January 2006
Below a brief layout of the events on 21 and 22 January 2006 are provided. Any comments and suggestions are welcome and should be addressed
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Minutes of a Meeting held on Saturday 21 January 2006 – prior to opening of the Sluice Gates (Report A. Speiser)
A meeting between representatives of Friends of Avis and the CoW was held on Saturday 21.01.06 at 16:00h at the dam wall. The minutes below were mailed to the ExCo of Friends of Avis for further distribution.
Dirk Reed (CoW)
Horst Schommarz (CoW)
Kai Lundt (KWC)
timing of the sluice gate opening:
1. Friends of Avis FoA raised concerns of the opening as too much water might be lost. Experiences gathered two years ago showed that the possible threat exists that branches, etc. might jam the gate while closing.
2. FoA emphasised that the dam is used by a number of people for recreational purposes - the current high water level allows for water sports which is otherwise during the rest of the year hampered, e.g. depth
1.It is in the management plan of the sluice gate that it has to be tested once a year when water is reaching the dam wall. This should happen btw August and October but water levels didn't allow for that - as water is currently in the dam they like to do it now.
2. Flood damage control down stream as water can be released controlled. If max height is reached and dam spills over this is not possible.
3. Dirk Reed promised that the gates will only be opened minimal and as soon as the water is released from the chamber under the walkway they will start with refilling to ensure that as less water is lost as possible. He agreed that the threat exists that branches might jam the gate! Future plans to ensure that branches, etc. is kept away from the gates itself were discussed btw Kai, CoW and FoA. A written proposal will be submitted to the CoW during the next week.
4. Dirk Reed invited us to be present and included some FoA names on the list to be allowed to enter the parking area.
Security during opening:
1.Nobody, except CoW officials and people on name list are allowed to enter the area behind the second river crossing.
2. Grader will be on stand-by to ensure that access is allowed as soon as the water has gone done.
We, FoA, made it clear that we not happy with the decision, got backing from Kai Lundt as he doesn't see the necessity of the timing either. I further mentioned that we as the lessees should have been contacted beforehand as mentioned in our lease contract. Dirk agreed and I hope we will work better in future.
Minutes of events re opening of the Sluice Gates (Report P. Müller)
Sunday 22 January 2006 11:00 - FoA was present to ensure that the pump was working so that the sluices could be closed at a faster rate than waiting for it to happen automatically.
During the course of the day the sluices were opened 3 times, each time with the intention that sticks and branches getting stuck in the sluices would be flushed out. Sticks and branches getting stuck prevented the gates from closing completely, leading to a continuous gentle flow of water from the dam (the same happened in 2004, and was solved when the diving club removed obstructions on the dam side of the sluices).
All three attempts to flush the gates and to seal them tight failed, because branches got stuck.
The dam lost approximately 3cm to 6 cm of water on Sunday.
FoA was very keen to stop this steady loss of water and grateful to the diving club for solving the problem to large degree.
Monday 23 January 2006 in the morning – WCC Storm Water Management engineers opened the gates completely with the intention to flush out sticks and branches once more. When the gates were closed, branches were still stuck as with all attempts before.
The rush of water down the Klein Windhoek river caused much disruption of traffic in the Klein Windhoek valley.
Monday 23 January after 5:00pm – Members of the diving club cleared sticks from the dam side of the sluice and managed to stem the flow to a trickle, which was acceptable to FoA. By this time the level in the dam had dropped by a further approximate 30 cm.
Conclusions after events on 22 and 23 January:
Testing of the sluice gates needs to be done only once, because no matter how often or how wide the sluices are opened, sticks will always get stuck.
Diving and removing the sticks from the Dam side is the most effective way of closing the gates as much as possible.
Opening the gates completely causes a massive flood wave down the river and disrupts traffic in the Klein Windhoek valley. It does not flush out the sticks.
The rate of flow and therefore the amount of water flowing down the river can be managed by releasing water from the flotation tanks in small doses i.e. opening the gates a little bit at a time and for a number of times over a protracted period.
If the level of the dam has to be dropped for safety reasons or for flood management reasons, this can be done in a controlled manner by applying #3 above.
Tuesday 24 January in the morning – WCC Storm Water Management engineers informed FoA of their intention to:
· Open the sluices at 11:00am on Wednesday morning to let out water from the dam until the level in the channel stood at 2m. The reasoning was that in case of heavy rain in the catchment areas of the Avis and Michaelis rivers, the dam could act as a reservoir for run off in the Avis River, while the Michaelis was given a chance to subside. The WCC intended to drop the level by a further 1m to 1.5m so that the remaining water would be 2m deep in the channel adjacent to the sluice gates.
· Bulldoze a path on the western side of the sluices from the scout House to the sluices, so that they could bring a crane in to lift the western sluice completely. The reason given for lifting the sluice gate completely was to inspect whether damage had been done to the rubber seals. Why WCC wanted to inspect seals at this point in time was not explained.
The path was duly created on Tuesday during the course of the day.
It was much appreciated that the engineers kept FoA informed about their intended actions. This bodes well for future co-operation.
Tuesday 24 January evening approx. 6:00pm – Massive rain in the catchment area of Avis dam caused the dam to fill up very rapidly. Just before it overflowed, WCC technicians set the swimmers which trigger the opening of the sluices so that the western (and tested sluice gate) opened first (at 10cm above the sluice) and the second at 35cm above the sluice.
The dam did overflow and the western gate opened successfully, releasing a massive amount of water down the Klein Windhoek River. By this time the Michaelis River had completely subsided, so that the concept of the Avis dam being an initial catchment for floods from the two rivers was proven to work properly. The water released by the full opening of the gate did however result in the Klein Windhoek River running exceptionally high. The extent of damage down stream is not known.
Tuesday 24 January evening approx. 8:00pm – A log got stuck in the sluices, preventing proper automatic closure and the holes which are designed to fill the sluice tank were blocked with debris, which prevented them from filling up properly and thereby further prevented the sluices from closing properly. The result was that water was released from the dam all night at a considerable rate until later in the morning when the WCC managed to close the gates again, by clearing the holes mentioned above. The Klein Windhoek River was impassable at 7:30 in the morning causing considerable traffic problems for people going to work that morning.
The dam lost 1.5 to 2m of water during the night.
The dam lost about as much water as the engineers had intended it to loose from 11:00 onwards on Wednesday morning. It took more than 12 hours of rapid and destructive flowing throughout the night to drop by 2m or more.
Traffic in the Klein Windhoek was seriously disrupted on Wednesday morning.
It almost certainly may have been possible to curtail the violent release of water when the sluices opened automatically on Tuesday, and the resulting destruction down the river, if the sluices had been opened manually before it overflowed at 6:00pm.
The gates could have been managed throughout the night, especially with regard to unplugging the holes in the tank, which allow new water in to weigh the tank down. The big branches which prevented full closure for a while could have been flushed out by manually opening the gates.
The gates could have been closed to reduce the flow by Wednesday morning and would not have caused disruption of the traffic if a pump had been deployed to speed up closure of the sluice.
The fact that the sluices are manually manageable can be used to control damage and disruption.
If the Engineers want a controlled release of water to drop the dam level by 2m, and cause less disruption in the Klein Windhoek Valley, it would take a longer time than anybody had expected (probably 2 days of relatively rapid flow and a managed reduction of the flow early in the morning, at lunch time and at 5:00pm when traffic is at peak levels).
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