WARNING… This blog may contain no reference to Drennan Acolytes or my Beloved Sheena.
Ok I know this subject won’t be suitable for every reader but you may find it interesting, so get yourself a beer or a brew and read on.
Are we all sitting comfortably?…. good … then I shall begin.
In todays lesson I was going to talk about fishing rivers when in flood but maybe I will leave that for next weeks blog because first I thought we should cover the subject of Liquid Flow Dynamics (that’s half my readership gone already) and how we can use a basic knowledge of this fascinating subject to help us catch more fish.
‘What the heck are you talking about and are you drunk?’ I hear you ask, stick with me here because what you are going to see is how the flow we observe when we stand on the riverbank is not the same as what is happening below the surface.
Introduction over, lets get the technical stuff out of the way. River flow is driven by something called The Hydrostatic Pressure Gradient and this on its own would cause all the water travelling along a river to maintain a constant velocity regardless of depth etc. However there are other factors at play. Firstly wind speed and direction. Now I’m guessing that if you get to your favourite swim and there is a strong wind blowing upstream it may look like the river has stopped flowing or even reversed flow as the thin surface layer is pushed upstream by the wind. Are we fooled by this observation? No, we can see what is happening and know that underneath the water is flowing as normal. So lets look at what’s happening below the surface. For the sake of simplicity, lets imagine a nice long bit of river say 10-15mtrs wide and a fairly constant depth of around 6ft (that’s almost 2 meters kids) A cross section of this river, it would look something like this.
Now what is going to come into play is something called The Viscosity Effect. Simply put the fixed boundary’s (banks and river bed) are offering resistance to water flowing over them slowing it down. This effect happens here (shown in Dark Blue).
This water that has been slowed dramatically by having to ‘Rub’ along the rivers ‘boundary’ has the same slowing effect on the water next to it, albeit to a lesser amount, and so on upwards and inwards until the Viscosity effect becomes all but irrelevant.
One more thing to add. Lets presume that today is totally calm no wind at all. The Viscosity Effect is also present at the surface as the very top layer of water is, very slightly, slowed by contact with the still air.
So finally we have this.
So as we see the maximum flow will always be in the centre of the river/the deepest part, just below the surface.
I do hope you have got this far, in this strange blog, as its about to become very relevant to your river fishing.
Lets now look at a side on view of the same river, flowing right to left as we look.
Now lets bait up with some groundbait/mashed bread etc.
You gauge the flow asses the depth, make a quick calculation and splosh, splosh there we go baited up and all that’s left is an accurate cast to where (you think) your feed has landed.
But, and here is the point of this whole blog, you have missed the spot, often by quite a distance in deeper water. Taking into account what we learnt earlier the flow will affect the trajectory of the groundbait less as it falls through the water. So what is actually happening in real life is this.
In Deeper water the effect is even more noticable.
If we now compare expected results with actual result we can clearly see the difference.
One other thing to bare in mind is that water depth and topography also affect the distribution of flow
The next picture shows examples of the effect of topography on flow rates.
We can see in the irregular channel example that the depth also has a baring on where the maximum flow will be, something to bare in mind.
I do hope this makes sense to you. I have tried to explain things as simply as possible and managed to get this far without the need to talk about Constant Velocity Gradients (Shear), and the Mean Free Path of Molecules, which is quite handy as I have no idea what they are.
My preferred method of accurately baiting a swim, especially in winter when less loose feed/groundbait is required is this, the humble Bait Dropper. I use it with liquidised bread, Chopped Worm, dead Maggots, Corn etc etc It gets the loose feed down to the bottom very accurately indeed.
Hand feeding of groundbait or mashed bread is fantastic for feeding clear runs but I believe that the humble Bait Dropper comes into its own in tight snaggy swims. For example imagine an overgrown far bank with plenty of overhanging cover, very Chubby. Now if you were to hand feed into a gap your feed would land downstream, probably underneath the overhang, right where you are unable to cast. But if you use a bait dropper you can place your loose feed very accurately indeed, right in the gap, and then cast your hook bait right on it.
So In summery:
The strongest flow on a strait section will be just below the surface in the center/Deepest part of the river.
The flow will deminish the closer you get to the banks or bottom.
Your groundbait/bread on the bottom will have little flow to wash it far down stream.
In the past you will proberbly have being fishing beyond your feed.
“Oh Is that the bell already? Ok class next week we will be discussing how quantitive modelling approaches should be used to influence your choice of float….. Bye all.”
Only joking, I will discuss how I approach flooded rivers.
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