I changed the title of the thread to something more appropriate

Guy Hoelzer  wrote in
news:c0ucho$2ers$1{at}darwin.ediacara.org: 

> in article c0pmn5$uei$1{at}darwin.ediacara.org, William Morse at
> wdmorse{at}twcny.rr.com wrote on 2/15/04 10:07 PM:

(snip)
 
> I agree with all that you say about hydrology, but your exception for
> time of concentration supports the claims I made above.
 
> Regarding semantics, there is a difference between a static
> contribution to something, which results in an increase, and a
> "pressure" exerted on a dynamical system to move in a direction of
> increase.  I think there is value in having a term meaning the latter,
> while I am not sure there is much value in the equilibrial view of
> maximization. 

Despite my flippant remark about physicists (my brother teaches physics), I 
do recognize the value, both mathematically and in analyzing non-
equilibrium systems, of having a function that always has a positive slope 
vs. the typical definition of maximize. The problem is that even - or 
especially - mathematically,  maximize has a generally accepted meaning. 
Perhaps a new term is justified for a "pressure to increase". I
can suggest 
meliorize or majorize, but they both sound kind of weak. Anybody care to 
suggest a Greek derivation?

>> Second we note that, as an overall process in a watershed, erosion
>> moves soil from areas of steeper gradient to areas of lesser
>> gradient. (Actually I am including deposition with erosion, but in my
>> mind - and I think in Tim Tyler's discussion of the subject - the two
>> are linked). This process decreases the overall gradient in the
>> watershed, which increases the time of concentration, which decreases
>> the rate of flow. 
 
> Again, I agree.  The act of dissipating a gradient reduces the
> strength of the gradient and the potential it represents for
> self-organization.  Weaker gradients generally support weaker flows. 
> I am not sure why you think your good points contradict the position I
> took. 

To repeat, because decreasing the gradient increases the time of 
concentration which _decreases_ the rate of flow. Erosion in this case 
equals lower flow rate.

 
>> Now we get to the tricky subject of meanders, which I don't begin to
>> understand, but which I see as critical to the argument. Unless I am
>> mistaken, the net effect of meanders is to decrease the average
>> velocity in a stream, which will,albeit only slightly, decrease the
>> rate of flow. Meanders may be a good example of a dissipative system,
>> and I would be happy to see an explanation of how they represent
>> useful work (a la Lotka) or self organization (a la Kauffman), but I
>> remain unconvinced that they increase the rate of flow.
 
> I never said that meanders increase the rate of flow.  I said that
> erosion does.  Note also that erosion implies that sediments are
> flowing as well as water, but we should probably stay clear of that
> complication.  Where a meander exists, sediments get deposited in such
> a way as to frustrate the flow, resulting in a localized pond or pool
> of slow moving water.  As you correctly described above, erosion
> generates alternative channels, and the one with the highest flow rate
> will tend to draw flows away from the alternatives, thus maximizing
> the net flow rate (reducing the frustration of flow).  The net effect
> of deposition and erosion processes in streams is to create meanders
> that facilitate flows at first, then eventually frustrate flow.  In
> this way, meanders are born and die in a continual churning of the 
> landscape and changing of stream morphology.  At every step along the
> way I would argue that erosion is maximizing the rate of water flow
> through the system (from beginning to end of the gradient).

You and I have both been ignoring sediment flow.Tim Tyler thought it was 
important to the argument, and he may be right. But I will try to ignore it 
a little longer.

I still disagree that erosion always increases  the rate of water flow. 
[Note that this part of my discussion is again limited to "mature" 
landscapes. Erosion starting from sheet flow of water over a freshly 
regraded landscape will decrease time of concentration (due to faster flow 
because of a  higher hydraulic radius) and therefore increase the rate of 
flow. So in very non-equilibrium conditions, erosion does maximise flow 
rate.] There are numerous demonstrations that flow that starts in a 
straight channel section will soon create meanders.Yet it is the straight 
channel that has the higher flow capacity, and to the extent that flow 
capacity affects time of concentration, it is the straight channel that 
increases the flow rate. I have lived in two communities - Houston, Texas 
and Corning, New York - that had Army Corps of Engineers projects in the 
1950's to straighten and line stream channels: Buffalo Bayou in Houston and 
Chemung River in Corning. Both communities subsequently experienced 
devastating floods, caused in part by the decrease in time of concentration 
due to the stream channel straightening. 

Now an interesting thing about the changes in stream morphology caused by 
erosion - including meanders, riffles, braiding, etc. (and I am starting to 
get well away from my actual knowledge of hydraulics and hydrology in 
relation to geomorphology) is that they seem to result in flows that are 
helical (meanders) or exhibit horizontal (eddies) or vertical (souse holes) 
rotations. 
These types of patterns are also frequently seen in other examples of self-
organization, such as tornadoes, Benard cells, and DNA.  I don't know 
whether this is because we call any rotational pattern
"self-organization",  
whether there is no relation, or whether there is some relation between 
rotation and self-organization.

Yours,

Bill Morse
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