Author Topic: River Current Effects on Round-Trip Journey Times  (Read 2264 times)

FloridaWindSUP

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River Current Effects on Round-Trip Journey Times
« on: June 25, 2016, 06:14:28 PM »
A while ago I was thinking about how river current slows down the racers in our local race series on the slow-flowing Imperial River.

It's obviously that the current slows you down when you're paddling against it, and speeds you up when you're paddling with it. It's the same principle as walking the right way on an airport moving sidewalk vs. the walking the wrong way on an airport moving sidewalk; going with the flow gets you to your destination faster, going against the flow gets you there slower. What's less obvious is how the flow affects the time of a round-trip journey, where you go with the flow one way but have to go against the flow the other way. Does the time saved going with the flow make up for the extra time it takes when going against the flow?

It turns out the answer is, "No." You always lose more time going against the flow than you save going with the flow. There's not much difference if the current is very slow relative to the speed of the paddler, but the stronger the current is, and the slower the paddler is, the more the paddler's overall time is reduced relative to her time on a course with equal distance but no current. If it's hard to wrap you mind around that, maybe the math will convince you.

The formula for the total time it takes to complete a round-trip course up and down a flowing river is: t = d/(v+c) +d/(v-c)

d = 1 way distance of course
v = racer speed relative to the water
c = river current speed
t = time to complete course

I put the formula to work in an spreadsheet embedded in this blog post.

http://jimbodouglass.blogspot.com/2016/06/river-current-effects-on-paddling-speed.html

It let's you tweak racer speed, race distance, and current speed to see how much time you lose in a race due to the current in different situations.
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burchas

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Re: River Current Effects on Round-Trip Journey Times
« Reply #1 on: June 26, 2016, 05:54:26 AM »
Nice Job! I Put in the numbers and it definitely reflects what I see in the field.
One caveat with the formula though, while working perfectly in a vacuum, it does not
take under considerations the eddies created due to obstacles and of course the wind.

A smart racer knows every inch of the course and chooses the right line in order to "hide"
from the current or avoid turbulent patch that will cause slow-downs. And if the wind is
in the teens, a skilled racer will know how to squeeze every bump to generate more speed.

These two elements will throw a big wrench into this formula.

It would be helpful if you could add the wind effect to your formula
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PonoBill

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Re: River Current Effects on Round-Trip Journey Times
« Reply #2 on: June 26, 2016, 07:01:47 AM »
The tricky part is determining when wind and bumps overcomes eddies as an advantage. If you plot the current in some throretical river it's a U shaped curve, the highest speed being at the center and lowest at the edges. In a real river the bottom profile, the presence of rock outcroppings and underwater structure, and how the river meanders effects the vater velocity. Generally the fastest way to go upcurrent is in the eddy line, but the most fun is out in the middle, where the biggest bumps are. We deal with this every day in the Columbia. I tend to opt for slow and fun over faster--unless there's someone around I have a chance of beating. I consider downwind runs where I hide in the eddy line to be wasted to sooth my ego.

Interesting formula, it's always a surprise how the order of things can have a bigger effect on outcome than expected. Going upstream before downstream is probably very different from downstream then upstream.  I've been playing around with saving/spending scenarios for the retirement book I'm writing and the accumulate/spend cycle has similarities. Compounding effects and a dwindling pool of resources make that a complicated calculation.
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Ichabod Spoonbill

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Re: River Current Effects on Round-Trip Journey Times
« Reply #3 on: June 26, 2016, 07:23:27 AM »
This is the story of my life. I also have to take into account tides, which change the direction of the river. And, of course, the upstream direction is slower than the downstream. There's also a slack tide period when the river basically stops. Add to the complexity that the direction changes tend to happen between an hour or two after the tide changes, and along the river's edge before the center.
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PonoBill

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Re: River Current Effects on Round-Trip Journey Times
« Reply #4 on: June 26, 2016, 08:04:04 AM »
I used to paddle a lot on the Willamette River, which is tidal all the way to Willamette Falls. Water flow rate vs. tides vs. wind makes it a tricky river for long distance paddles. With the tide change I often found myself going uphill in both directions--bad planning. Or rather planning for the wrong thing--my long distance paddles generally included a stop for lunch at a riverside restaurant. Lunch doesn't pay attention to tides.

The section of the columbia I generally paddle in now is actually a lake with a dam at each end. Since these are extremely large hardoelectic dams the water flow tends to be something close to the general river flow--but it doesn't have to be. The hydro operators match output to demand, and balance flows to anticipate runoff rate. They bank water in the lakes, and thereby vary flow and depth somewhat independent of general runoff and river flow rates. They are also required to divert substantial amounts of water during salmon runs, both for upstream runs of mature fish and downstream runs to divert hatchlings past the giant bass-o-matics. This "fish flush" requirement costs millions of dollars in lost generation, and the BPA was woring hard to get it eliminated, citing no substantial change in fish return rates, when the largest salmon run in recorded history occured and killed their efforts dead in the water (so to speak).

So flow is pretty varied, as is depth. You can even request a flow and depth change for specific recreational events, and the dam operators will do what they can to accomodate. They don't get many requests, it's kind of surprising how much they're willing to fiddle with millions of dollars worth of stored power to accomodate a sailing club or a SUP race.
« Last Edit: June 26, 2016, 08:19:14 AM by PonoBill »
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burchas

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Re: River Current Effects on Round-Trip Journey Times
« Reply #5 on: June 26, 2016, 08:10:55 AM »
This is the story of my life. I also have to take into account tides, which change the direction of the river. And, of course, the upstream direction is slower than the downstream. There's also a slack tide period when the river basically stops. Add to the complexity that the direction changes tend to happen between an hour or two after the tide changes, and along the river's edge before the center.

Well now that you mentioned it, there is also the situation where the tide is going the opposite direction of the current. I'm not sure if this affects you all the way up where you at, but down here near the river mouth where the river is affected both by the LI sound and the open ocean from the apex area this really makes things more interesting as the tidal swells have a major effect on your overall speed, if the conditions allows you ride them.

When all 3 elements are aligned (wind, tide & current) I can see a consistent 8-10mph
on the GPS (the 10 would be a swell set).

Since I only started recording sessions recently with the new VivoActive HR, I don't have such conditions  on record as of yet, but I suspect I soon will.
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burchas

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Re: River Current Effects on Round-Trip Journey Times
« Reply #6 on: June 26, 2016, 08:19:09 AM »
I used to paddle a lot on the Willamette River, which is tidal all the way to Willamette Falls. Water flow rate vs. tides vs. wind makes it a tricky river for long distance paddles. With the tide change I often found myself going uphill in both directions--bad planning. Or rather planning for the wrong thing--my long distance paddles generally included a stop for lunch at a riverside restaurant. Lunch doesn't pay attention to tides.

When I just started and wasn't aware of all the working elements I often found my self paddling in place
at certain times. The only one who enjoyed it was the crowd on the river bank looking at the clown going through the motions but not going anywhere.
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Ichabod Spoonbill

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Re: River Current Effects on Round-Trip Journey Times
« Reply #7 on: June 26, 2016, 09:05:56 AM »
Burchas, the tide affects the Hudson all the way to Albany. You don't really get a current reversal up there, but at incoming tide the river basically stops and fills up. Where I am (Peekskilll) we get a 2–3 foot tide.

I've had the wind and tide aligned where I couldn't move downstream. Things get weird here. Sometimes the mountains create a wind tunnel effect too, where the local wind can be faster than the ambient wind. That's only from certain directions. I used to take advantage of that while windsurfing, with maybe a 300 yard area where I could plane even I couldn't plane anywhere else.
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Re: River Current Effects on Round-Trip Journey Times
« Reply #8 on: June 26, 2016, 09:52:26 AM »
That's how it's always felt to me--that you don't gain as much as you lose.  If it's not true, it still makes me feel better to think that it does.

I didn't know there were tides in the Willamette until PonoBill mentioned it once.  That explained times when there'd be no wind, and I'd expect to be going fast, but instead felt like my board was sticking to the water.

And if current or tides are strong, being a slightly better paddler is everything.   Once I went out in winter with gorgebob and Strand Leper (visiting here and looking for a fun time).  The current was so strong at one spot that they hardly made any progress in one spot, but being faster than me at least they were going forward well enough, while I barely was.  A bit slower and I would have been stuck in place (which I have been before, or even worse--paddling forward and going backwards).  Being able to go only slightly  faster than someone else can mean an infinite difference in making progress, since eventually you'll get where you're going while the slower guy is still right where you started. 

burchas

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Re: River Current Effects on Round-Trip Journey Times
« Reply #9 on: June 26, 2016, 09:55:16 AM »
Burchas, the tide affects the Hudson all the way to Albany. You don't really get a current reversal up there, but at incoming tide the river basically stops and fills up. Where I am (Peekskilll) we get a 2–3 foot tide.

I've had the wind and tide aligned where I couldn't move downstream. Things get weird here. Sometimes the mountains create a wind tunnel effect too, where the local wind can be faster than the ambient wind. That's only from certain directions. I used to take advantage of that while windsurfing, with maybe a 300 yard area where I could plane even I couldn't plane anywhere else.

Gotta love it though. Makes things a lot more interesting and the view up on your end sure is stunning.
The tide here is going almost to 6 foot but the river is a lot more uniform both on the shape and the bottom structure. I can only imagine what happened on your end with all the curves, the uneven bottom and with significant width difference.

Makes you wonder how a formula for this would look like. Are you up for the challenge FloridaWindSUP :D
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Re: River Current Effects on Round-Trip Journey Times
« Reply #10 on: June 26, 2016, 10:46:46 AM »


The tricky part is determining when wind and bumps overcomes eddies as an advantage. If you plot the current in some throretical river it's a U shaped curve, the highest speed being at the center and lowest at the edges. In a real river the bottom profile, the presence of rock outcroppings and underwater structure, and how the river meanders effects the vater velocity. Generally the fastest way to go upcurrent is in the eddy line, but the most fun is out in the middle, where the biggest bumps are. We deal with this every day in the Columbia. I tend to opt for slow and fun over faster--unless there's someone around I have a chance of beating. I consider downwind runs where I hide in the eddy line to be wasted to sooth my ego.


Hah, I run the eddy line more often than I'd like to admit trying to keep up with my faster friends (which is pretty much everybody). There are actually some of my favorite big smooth bumps just past Mitchell by that dip net platform on the inside. The biggest variable for running the eddy line seems to be weeds. Sometimes the eddy line has weed balls that could almost be mistaken for bales of hay floating it 😃
JP

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PonoBill

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Re: River Current Effects on Round-Trip Journey Times
« Reply #11 on: June 26, 2016, 10:50:24 AM »
Lots of variables, and a number of them would be exponential. And even then, just an approximation. Flow though a smooth pipe of specified diameter and length with known differential pressure is just an approximation--a close one, but chaos intervenes.

Yeah, JP, Mitchell is an odd duck. The eddy line disappears near the pipe, and one of the best waves in the river is 50 feet from shore. The other bit of craziness is the Wells Express, where you can escape shallow water drag by finding the edge of the sandbar, but still bet the steep faces that shallows kick up. Do it just right and you can get 100 yard rides. Do it wrong and your board rounds up so briskly you can be going backwards before you can react.

And yeah, the weed balls are the price you pay for an eddy. With my rudder board the price is too high. If I hit a weed ball with that spitfire fin with a gap at the bottom it's all gonna come with me. I've pulled my board out at the event center with what looks like a whole salad bar following me.
« Last Edit: June 26, 2016, 10:52:23 AM by PonoBill »
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

 


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