Dargle river strategically placed boulders.

Zebbedee

Does anyone know the reason for these boulders placed in the river along the People's park?
Definite pattern to them which changes as you go up river towards La Vallee.
If its to aid with bankside erosion prevention I can't see them surviving strong surges over the years even though they're close to a ton weight.
In fact I can imagine tree stumps and branches that are swept down river during storms getting caught up in the boulders resulting in a dam effect and increasing flood risks.

recedite

Zebbedee wrote: »

Does anyone know the reason for these boulders placed in the river along the People's park?
Definite pattern to them which changes as you go up river towards La Vallee.
If its to aid with bankside erosion prevention I can't see them surviving strong surges over the years even though they're close to a ton weight.
In fact I can imagine tree stumps and branches that are swept down river during storms getting caught up in the boulders resulting in a dam effect and increasing flood risks.

Interesting, I haven't seen them yet but maybe they would take some of the power out of the river in a flood?


They could attract the attention of kayakers if some interesting wave patterns appear in their wake, although that would not be the reason for putting them there.
WCC wouldn't want to facilitate too much exotic recreational use along the river, sure haven't the people got the park to walk in.

loobylou

Bumping this, I'd love to know the answer too.

judeboy101

Pics would be nice

Calibos

judeboy101 wrote: »

Pics would be nice

How about HD Drone Footage!

https://www.youtube.com/watch?v=VDwzN_DKbEE

Calibos

loobylou wrote: »

Bumping this, I'd love to know the answer too.

My uneducated guess would be that the boulders are there to keep the river in its current channel at low water levels and stop it meandering and eroding a new channel which at high water levels might direct heavy flow at the opposite banks causing erosion where they don't want it. Those tall concrete monoliths up at the Rivervale apartments look like the are also there to direct waterflow at high water levels to prevent erosion where they don't want it.

recedite

Calibos wrote: »

Those tall concrete monoliths up at the Rivervale apartments look like the are also there to direct waterflow at high water levels to prevent erosion where they don't want it.

They look like a strainer to catch any floating tree before it catches in the bridge further downstream.

Creamy Goodness

Another uneducated guess but could it be to remove energy from the river when it floods?

Zebbedee

Is there nobody on here with some sort of water flow engineering background who can answer?


Perhaps it was the guys on the track machines just having fun on their lunch breaks! In other words 'doodling' with the boulders

Lu Tze

These kind of features are usually to reduce the velocity of the water, to reduce erosion. The video above shows other features which have the same effect, cascades/drop structures and what looked like a plunge pool downstream

Subcomandante Marcos

Some of the structures are to stop the banks eroding and eliminate the potential for landslides, some are to stop the bottom eroding and keeping the central Chanel at a specific depth. the staggered ones in the central channel help stop water from moving in straight lines at speed and moving tonnes of sediment from the bottom in a short period of time, and also look cool.

Zebbedee

Thanks for the replies.
However I can still envisage smallish tree trunks and boughs getting snagged in these rocks during extremely heavy flows.
It doesn't happen often but I have seen large items of this type of detritus getting caught between the arches of the Fran O'Toole bridge (at the main street).
I will watch with interest.

Subcomandante Marcos

Zebbedee wrote: »

Thanks for the replies.
However I can still envisage smallish tree trunks and boughs getting snagged in these rocks during extremely heavy flows.
It doesn't happen often but I have seen large items of this type of detritus getting caught between the arches of the Fran O'Toole bridge (at the main street).
I will watch with interest.

That's not a particularly big issue though, they can be cleared with relative ease when flooding subsides.

Subcomandante Marcos

Oh, another feature of the staggered rocks and upright pillars along the course, as well as the plunge pool, would be that behind them there will be small pockets of slower/calmer water which certain flora and fauna will prefer to hang about in, which leads to greater bio-diversity and increased ecological services.

B0jangles

Sort-of related question; why are there no access points to the riverside anywhere between Fran O'Toole bridge and the end of the People's Park? It's all 15 foot vertical walls on both sides with no ladders or anything - is this standard practice with planned river layouts like this?

I was walking down there a few days ago and it occurred to me that if anyone fell over into the river, there's no way of getting anywhere near them to help, and god help you if you drop something off the boardwalk because you're not getting it back.

Now, I didn't get to look downriver past the bridge, so there may be access points there that I didn't see.

recedite

Good question. The Liffey in Dublin city centre has steel ladders at intervals.
Maybe they reckon that ladders attract kids and other messers, who would end up in the water?


The Dargle is a bit different, ie not quite as tidal and not as deep, but faster flowing in a flood. I think if you fell in, you would either drown or get washed onto a sandbank. Aiming to swim towards a specific ladder or exit point is probably not going to be an option.