On November 24 the NW IAA held it's annual field day in Moree covering the construction and management of storages. The format for the field day included a morning of presentations from industry personnel plus the civil and mining industries as well.They discussed various aspects including construction, management, remedial action, location, soil surveying and losses. The day concluded with a field trip to examine the management and repair of 2 storages in the Gwydir District.

Peter Smith, can you give us a rundown of just background of the Irrigation Association?

The IAA is an organization set up to bring irrigators together as an Association, we think we have got a better chance of getting things done if we can group together irrigators across the board, broadacre like up here or turf people, recreational small irrigators, small horticultural producers and the like we get together to pool our resources and in terms of the things that we offer it is information exchange is the biggest single thing, there is training, there is magazine that comes out quarterly, there is a monthly emailed newsletter and a good interaction between all irrigators and technical people.

What type of things are you covering today?

Well, we want to cover an outline of what the issues are, the extent of what people have been experiencing, we want to outline how to overcome those and we want to introduce some input from other industries. We have brought in a couple of guys who are experienced in broader civil engineering, particularly the mining areas to give us a few clues on what they do to minimize the problems so we wanted to deal with the issues in increasing detail as the days progressed and bring in some new thinking and then we are going to go out a field trip to see some storages that have had problems and see how they may have been overcome and we have got the advantage today in fact of having one of those being breached by an excavator which is there so we can see what the dam wall looks like inside the dam wall.

You have got about 30 – 40 people here today, what were your major aims of today?

We wanted to cover the issue of storages which is a big issue, a big item in broad scale irrigation. It’s not only a huge capital investment it’s a major management component of the big farms for a variety of reasons. There has been an increase in the number of failures. There is always some failure rate but over the last few years, the last couple of seasons in particular, there has been a noticeable increase in failure rates and we suspect that this is associated with the drought but it is an issue that needs to be dealt with because it is such a major investment in such a significant management component of the big farms. So today we want to address the issues that were associated with the failures and also how to remediate that and perhaps avoid it if possible and how to manage the situation would be good.

Peter, I know it may be a bit of a rushed question but what do you think are some of the key points of today? Peter, if people want to get more information, where can they get this information from?

They can come to us, we have got most of the talks on hand from today, that’s to me, Chairman of the Association at NSW DPI Tamworth or from the others on our Committee, particularly Peter Taylor at SMK Consultants at Moree. They all have the information on hand, failing that, we can put people in contact with the speakers and some of the other Consultants around that have been here today, themselves try to increase their knowledge and their ability to service the client and perhaps some of those problems have developed due to swelling and drying of soils, yes shrinking and swelling soils. Those things need to be overcome properly just doing a ‘patch up’ job is really just going to hide the problem and maybe create a worse problem that is going to bit you later.

Anthony Fairfull, you gave a presentation today in regards to some storage losses over different heights and the like. Can you just give us the detail, give a general run down of those problems.

Well, I did a bit of a comparison on a 1000 meg storage assuming that the reservoir has got water in it for 8 months of the year and what that showed is that if you build a reservoir 3 metres high and compared that to a reservoir that is 6 metres high you are saving around 500 megs in evaporation and seepage losses just by reducing your area from 50 hectares to 10.

You are heavily involved in regards in the measurement of actual losses from storage and have been using a calibration there. Can you run through what you have been doing in that regards.

We have got a highly accurate calibration metre. It measures depth and water levels to an accuracy of less than 1mm. What it does is it measures the level in the storage and the wind speed once per second and averages these over a 15 minute interval and stores them every 15 mins and what we do with this is we can use this highly accurate measurement to determine the seepage rate in the reservoir and the evaporation rates for that reservoir for that time of year. We found that there is a very high correlation rate between evapro transpiration rates which are readily available from the Bureau of Meteorology from their silo data and the evaporation rates for your reservoir. So once we have taken these measurements, we can give you a highly accurate seepage rate which won’t change throughout the year and a calibration factor which can be applied to the silo evapro transpiration data and from then on you can easily calculate your evaporation from that storage at any point in time.

You also mentioned in your talk in regard to losses from storages. Can you run through, you had a few criteria’s there and then what are the options for people in regards if they do find they are loosing too much water from their storage?

There are three categories of seepage losses for reservoirs. The first category for a good reservoir is a seepage rate of less than 1mm a day and this equates in volume terms to around 2 ½ megs per hectare of reservoir per year. The seepage rate then goes up to the next category of around the four to five mm per day. Once you get to this point you are reaching the point where it becomes economic to start fixing that seepage or you want to look at not using that reservoir for any long term seepage losses and 5mm a day equates to around 18 megalitres per hectare per year and the final category is the ‘why did I build my reservoir there’ category which is over 10mm a day seepage losses and this equates to around 36megs per hectare losses a year which is fairly significant. And to overcome seepage losses the easiest, by far the easiest way if you have got a suitable source of clay soil available is to line the suspect areas with .6 of a metre of compacted clay. The next cheapest option is to buy some bentonite and incorporate that into the soil. There are some hassles with that because handling incorporating bentonite is fairly difficult to do and the most expensive option but probably the option that guarantees you 100% fixture rate is line it with high density poly ethylene and this costs in the area of $10.00 to $20.00 per square metre and care has to be taken with using a fairly thick 1 – 2 mm thick poly ethylene cause otherwise in during construction or afterwards the plastic will get punctured and you have got a leaking storage again.

Tony Lockery, you have worked extensively with growers up there on the Downs in regards to some of their storages, can you outline some of the work that you have been doing.

Yes we have tried to zero in a bit on some of our bigger losses on farm in terms of irrigation efficiency and they have proven to be that we are loosing a lot of water to evaporation from storages when we have water in them and we are also loosing some to seepage so we are trying to measure how much of that is happening in each dam and where possible try and define where it is happening particularly with seepage and work out ways to treat that so we have trialled a few different things there.

Can you explain what you have been trialling. What your methods are?

We have done four trials with infiltration rings in some floors of some dams. First of all as I said surveys and soil tests to identify the designs we wanted to trial where it was most likely they were leaking and then we put these rings down and we trialled compaction soil pam, poly acrylamide and bentonite as the treatments to reduce seepage. We found the compaction was consistently better up around 70 – 80% reduction in seepage, the poly acrylamide about 50 – 55% and the bentonite around about 40% reduction.

You have also done a little bit of work in regards to some broken storages and what have you found there?

Yes we had a look two years ago when we had a filling event we had something like 28 dams breached on the Downs for a variety of reasons and we have gone into some detail looking at their construction, age that sort of thing to try and put a finger on it. It appears that not many, a small percentage of the failures were construction failures. It was more likely that it was something that a problem that had developed over time. The local irrigators group asked us to try and find someone who could look into the walls and see what was happening and where, a bit of a diagnostic tool. We were able to secure a fellow from Western Australia with ground penetrating radar and we ran that around the top of the walls. It looked down into the walls and we got an idea of the structure, any variation in structure from that.

You were able to use that ground penetrating radar to find areas that weren’t as compacted as others, you did cause there, what were you finding in the cause?

Yes we found fairly consistently where there was a problem that there was an area of soil that was wetting and drying. Mainly drying from root activity in about a zone of about 3 – 3 ½ metres down into the wall. So probably the most common problem is that the compaction and strength of the wall has been lost due to drying out and quite often that has been caused from vegetation. So roots getting down into the wall and drying it out.

You also mentioned that roots were getting down to three metres which surprised a lot of people?

Yes, that is right. Star grass was recommended up until recently as an erosion control on the inside batter of the wall and not thought to be a deep rooted plant we have certainly found evidence of roots and thick roots at that at 3 and 4 metres down into the profile of the wall. In fact areas that have been repaired with it blown out within 18 months the grass has grown back over the batter and roots have penetrated down to that depth so fairly scary so we have outlawed star grass.

Further information: Peter Smith, Chairman of the NW IAA at NSW DPI Tamworth or on (02) 6763 1262.