Lets Get This Ball Rolling – Chaffey Dam; Tamworth, NSW!!
Having now completed two weeks here with John Holland in New South Wales (NSW), I thought it was time I provided a brief synopsis of the Chaffey Dam Augmentation and Upgrade, a little on my transition back to the 1980’s and an insight to Tamworth itself.
CHAFFEY DAM
The project centres on the upgrade of an existing earth & rock-fill dam and accompanying spillway in order to increase the storage capacity of the Chaffey Reservoir from 62GL up to 100GL It is a $40 million (AUS) design & build contract (CG21) at the request of NSW State Water Corporation, a project born from the increasingly desperate water shortages experienced in Tamworth region of NSW and surrounding agricultural communities. Three distinct areas of engineering are immediately apparent within the project:
1. The raising of the earth & rock-fill dam.
2. Raising the Morning Glory (Pretty dam funny, I thought so too..)
3. Replacement of a simply supported, concrete road bridge.
1. Raising of the Dam. Unfamiliar with earth & rock-fill dams, I found myself bombarding the site engineer and experienced foreman on the workings, benefits and considerations associated with such a structure. Based on a layer system, the structure has a clay core which is to act as impervious membrane. Surrounding the clay core are several thick layers of coarse aggregate, with each layer increasing in aggregate size until large boulders exist on the external face. The purpose of which is two-fold; firstly the surrounding aggregate/boulders are there for durability/ protection/ stability. Secondly, the aggregate layers act as a wick (details of which I’m yet to fully explore), but effectively it serves to remove moisture away from the clay core, whilst preventing the migration of fines.
2. MGS. What sets the dam apart, particularly amongst the dam enthusiast society, is really the fact that there is a “Morning Glory Spillway” (hours of fun right there..) as opposed to a typical broad crested spillway for example. This is effectively an elevated plug hole within the reservoir that once surpassed by the storage water level, allows water to flow at controllable rates beneath the dam and out into the original watercourse, Peel river. Fortunately, this aspect of the project is what I have been requested to focus on. Due to the uniqueness of the structure, the temporary works design has proven to be quite challenging. The double curvature of the formwork, access, installation and maintaining an operational spillway throughout are just some of the considerations I will be exploring further. (I wont attempt to explain without drawings)
3. Bridge and Road Infra Upgrade. As the storage capacity of the reservoir is to be dramatically increased, clearly so too does the potential water level. A consequence of which is that existing roads/bridges in the surrounding area also need to be raised or protected. 5km upstream of the dam is the first, primary crossing; an arterial route that provides a key link road for several rural settlements. The bridge replacement was deemed necessary as even at the original 62GL capacity the bridge road surface would frequently become submerged. The new bridge is to be approximately 9m higher and constructed in a new location. The final span was completed last week and the brushed concrete deck is currently 50% complete. Interestingly, the bridge aspect of the project was “construct” only, yet issues with unforseen ground conditions (v.hard rock) lead to delays (despite numerous historic ground investigations clearly point this out!)
!NON-PET INFO!
Tamworth is a gorgeous, sleepy little country town with an awesome backdrop of green, misty mountains. Eva loves the red parrots flying about in the mornings but is not so keen on the giant bats (unbelievable, huge horror-movie buggers that literally fill the sky) but the decent local animals more than make up for them (Kangeroos/ Koalas/ Emus and talking birds!, . The people are certainly very warm and welcoming but you do sometimes wish you bought the “Anti-Mutant Cream” (as John would say!). Actually, he’d probably fit in quite well around here; they love a good “Rock-Ape”!
Anyway, enough waffle. First impressions are that I am very fortunate to be involved in a such a unique project with so much variety, in a spectacular location as part of an experienced, welcoming and competent project team.
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Can I assume that your transition back the 1980s comes complete with howling tash?
Hayley’s Dad’s got a tash… Would love one but she says “it ruins the mood” if you know what I mean?…. Hope you’re doing well mate.
Dan, I am loving the idea of an imperious membrane. Something made out of ermin perhaps?
Hi Dan, glad the project is turning out to be as good as it promised. As you bed-in I suggest you keep close tabs on the completion of the bridge whilst the opportunity is there. It doesn’t matter that you are not directly responsible for an aspect of work, you can still analyse, reflect and comment on it. Your role looks promising in terms of both experience and comedy value (morning glories, interesting, are also known as ‘glory holes’ and whilst I’ve not managed to find the authoritative definition I think they are named after the morning glory flower). No doubt John or Richard will elaborate once back from leave.
Hi Dan, sounds brilliant!
Not sure if you have ever been to ladybower reservoir in the peak district but that has a similar glory spillway. What are the plans to raise it up: cofferdam or propping method against existing structure?
Sounds like permeability is important in the design if the dam is made of an aggregate and clay combination. Are you expected to compact in layers, how will you know what to do (layers, amount of compaction, strength requirements) to achieve desired flow rate?
By increasing water storage capacity, as per many of the hydro-electric dam schemes in Scotland back in 60s, how are the associated sustainability concerns (loss of habitats, change of ecology, rehousing or otherwise) being addressed? Will there be a loss of agricultural land, thereby eroding some of the purpose of the project?
All sounds very good and so many aspects to it!
Good intro Dan, thanks. Sorry to say morning glory, despite having all of the right geometry and a bell end to boot are actually named after the flowers. They do however, retain all of the inuendo oportunities you could wish for: If you can keep a straight face your presentation in phase 4 is onto a winner already…
There’s load of good stuff here for TMRs from the hydrology of the flood routing through the structure – fanastic piece of level pool analysis done? The hydraulics of the circular wier, design head (snigger…) and the pipe flow characterisitics also offer strong technical opportunities. presumable increasing the deisgn head will change the risk of vortex formation and potential for cavitation in the flow and therfore shaft scour. Are there any additional protection measures being put in place?
Dan
An excellent start to an excellent project – make the most of it
Kind Regards