A look into Seqwater’s upgrade of Leslie Harrison Dam

So we’re at Leslie Harrison Dam here to
talk about the Dam Improvement Program and the upgrade that’s coming for the dam.
We own 26 dams around South East Queensland and in 2012 to 2013, we undertook a
program where we had external people come in and review those dams. Out of that we identified that there are a whole range of dams that need upgrading over the next 20 years and we’ve established a program of work
called the Dam Improvement Program where we’re going to go through and upgrade those
projects over time. So Leslie Harrison Dam has been marked for an upgrade. One of the key things that is going to happen as part of that upgrade is works to
strengthen the main embankment of the dam. And also works to strengthen the spillway. We’re going to use post tensioned anchors or large anchors to tie down the crest of
the spillway. We’re going to do works in the concrete chute to make that more resilient for flood events, particularly large flood events. When we’re talking about
post tensioned ground anchors, we’re talking about large steel cables with
bundled strands of cables together, and we’ve drilled down into the foundation, we’ve secured the bottom of those cables and we pull them tight and then lock it off
so it replies a load. So it pulls down on the concrete crest and holds it down
into the foundation. We’re here at the main embankment wall for Leslie Harrison Dam. The upgrade works on the embankment itself will be the main part of the upgrade works that we’re completing. Looking across we can see
that you’ve got an earth fill embankment and you’ve got a rock fill protection on
the downstream side, so what the upgrade will involve is we’re going to strip off
that rock fill, we’re going to take an excavation down to a rock foundation.
We’re going to build that lower part of the embankment back up again. That will
be done using rock fill. Once we bring that up, we will then strip off the downstream face where all the grass is. We will then
put a specially designed sand filter and then what we do is we put another layer
of material over the top of that sand filter to hold it in place. Now that sand filter is specially designed to be able to cope with any seepage of water that
comes through the embankment and drain it away safely and make sure
there’s no threat to the dam from that and the berm and digging out the
foundation material allows us to strengthen the dam and to be able to
withstand earthquake loading. When the dam was designed, it was
not common for an earthquake to be considered as a load case but in recent times we’ve had a number of earthquakes within Queensland, and certainly around the world and their focus on earthquake as a load case has sort of changed over time. The other thing it does is it strengthens and increases the factor of safety or the
stability of the embankment. It makes it safer for large flood events or really
extreme flood events as well. Leslie Harrison Dam was originally constructed in the late 60s and was upgraded again in 1984 to include four vertical lift gates – large steel gates that get lifted up to allow floodwaters to pass. The steel gates were put in to increase the water supply volume – so how much water is available for the Capalaba community in the Redlands area until eventually they created the Stradbroke Island water supply scheme. So at the moment, Redlands area gets its water from three places. It gets it from Leslie Harrison Dam through the Capalaba Water Treatment Plant. It gets it from North Stradbroke Island, and it gets it through the Eastern Pipeline Interconnector so we can bring water in from the Brisbane region into the Redlands City area. One of the key issues the community has been concerned about is flood mitigation. When a dam is constructed, whether that dam has gates or not, it provides flood mitigation by slowing down floodwaters coming through a creek. As the waters come into the dam, the dam stores them temporarily and it slows down the flow and reduces the peak flow that goes into the river downstream. That provides a benefit to the community
by taking longer for the floodwaters to get to the community, and by reducing the
height of the flooding downstream. Both a spillway dam (a dam that doesn’t
have gates) and a gated structure both provide more benefit than if there were no dam here at all. But in terms of the gates providing flood mitigation, it’s not the gates that provide flood mitigation it’s
the actual storage volume – the temporary storage that’s behind the dam that provides mitigation by slowing down the water, storing it temporarily and allowing
it to be released over a longer period of time allows the mitigation benefit. So
whether we have the gates on or off, both of those configurations – the way the
dam is shaped, and the storage behind the dam provides mitigation for the
downstream community. There’s a similar level of benefit, it doesn’t
matter whether we take the gates off or whether we leave them on. The flood mitigation benefits for the community remain the same. The decision has been made to leave the spillway gates off. The reason behind that decision is, if we’re going to put those spillway gates back on, the upgrade for the dam would cost
incrementally another 18 million dollars on top of what’s already proposed for
that upgrade. Because we have good water security for the Redlands City area, we
don’t need the additional water supply and justifying the additional 18 million
dollars worth of expenditure does not make sense at this point in time. But for
the Leslie Harrison Dam upgrade, we’re in the process of planning that upgrade and finishing detail design, the drawing specifications and all those things
we’ll need to build it. We’ll take that out to tender at the end of this year or
the beginning of next year, and then we’re hoping to be on site and have
construction start in the middle of next year.

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