Thursday, November 9, 2017

River Bottom Friction: Manning's n

This will be a fairly quick one.  The condition of the river bottom affects the speed of the river.  Surprised?

Smooth river beds make for smooth, faster flows; rugged or weedy bottoms create turbulence and slow things down.  Here's a fairly lengthy list of sample values (gleaned from, but to be honest, you could probably get away with just using 0.035 as a default for most rivers.  Follow that link if you want to work out flow rates for things like sewers...there's a good range of materials there.

Earth channel - clean 0.022
Earth channel - gravelly 0.025
Earth channel - weedy 0.03
Earth channel - stony, cobbles 0.035
Floodplains - pasture, farmland 0.035
Floodplains - light brush 0.05
Floodplains - heavy brush 0.075
Floodplains - trees 0.15
Natural streams - clean and straight 0.03
Natural streams - major rivers 0.035
Natural streams - sluggish with deep pools 0.04
Natural channels, very poor condition 0.06

The last two factors we need to figure out are width and depth, and that is going to be a fairly lengthy, math-heavy post.  But first, next time, I'll go over some different types of rivers and make at least a general link between width and depth, before taking that plunge (I couldn't resist).

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