Rivers & Streams: Sedimentation and Erosion

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River & Stream: Sedimentation and Eroion New dam and reervoir a deigned and planned What happened later contrary to plan Benoit Cuhman-Roiin The larget river on the planet, the Amazon, form from the

At the river conjunction, the muddy, tan-colored water of the Solimõe meet the "black" water of the Negro River.

1 River & Stream: Sedimentation and Eroion New dam and reervoir a deigned and planned What happened later contrary to plan Benoit Cuhman-Roiin The larget river on the planet, the Amazon, form from the confluence of the Solimõe (the upper Amazon River) and the Negro at the Brazilian city of Manau in central Amazona. At the river conjunction, the muddy, tan-colored water of the Solimõe meet the "black" water of the Negro River. The unique mixing zone where the water meet extend downtream through the rainforet for hundred of mile, and attract tourit from all over the world. common.wikimedia.org/wiki/ It i the vat quantity of ediment eroded from the Ande Mountain that give the Solimõe it tan color. By comparion, water in the Negro derive from the low jungle where reduced phyical eroion of rock preclude mud entering the river. In place of ediment, organic matter from the foret floor tain the river the color of black tea. 1

2 Weight of particle Almot all ediment particle regardle of ize have a denity equal to =,650 kg/m. Auming pherical particle of diameter d, we have: Volume: Weight: V d 6 F mg gd 6 From thi, one define the pecific gravity, which i the ratio to the water denity:, ,000 Apparent weight Apparent weight (net weight) = actual weight minu buoyancy force F F Fb ( ) gd 6 ( 1) gd 6 in which i the denity of water.

Settling peed The ettling peed, alo called terminal fall velocity, i the peed acquired by a falling ediment particle when it downward apparent weight i balanced by the upward drag force due to the

3 Settling peed The ettling peed, alo called terminal fall velocity, i the peed acquired by a falling ediment particle when it downward apparent weight i balanced by the upward drag force due to the movement with repect to the water. Drag force i: 1 1 d Fd CDAw CD w 4 The balance of force yield: 1 d F F C w gd 4 6 d D ( 1) 4gd w ( 1) Problem i that C C D varie D with peed and ize! Eroion A particle on top of the bed will be entrained into the flow if - it center of gravity i vertically above the point of contact, - lift and drag force combine into a force capable of pivoting the particle upward at the downtream point of contact. Note: If the ediment i coheive, a coheion force mut alo be overcome.

4 The force onto a particle ituated at the top of the bed i intimately related to the bottom tre b exerted by the water flow onto the bed. Comparing the force caued by thi tre (force = tre x area), we have: b d ( 1) gd 4 6 frictional force b ratio particle weight ( 1) gd Then ignoring the / factor and recalling that the bottom tre b can be expreed in term of the friction velocity u *, we define the dimenionle ratio: u* u* Sh ( 1) gd ( 1) gd Thi i called the Shield tability parameter. The thinking then become a matter of comparing the actual Shield tability parameter value to a critical value for which entrainment begin. Shield tability parameter Reynold number Bed load begin when the Shield parameter Sh exceed the threhold value (gray zone) depending on the particle Reynold number. 4

5 Sh c The Miiippi River Delta Where the water peed low down, upended particle ettle a ediment. All river delta are formed in thi way. 5

(diolved chemical) (particle in upenion) Bed Bed (particle that roll around a the water pae by) Definition: - Bed = ediment particle not in motion, wet but not moving - Bed load = et of particle

6 (diolved chemical) (particle in upenion) Bed Bed (particle that roll around a the water pae by) Definition: - Bed = ediment particle not in motion, wet but not moving - Bed load = et of particle crawling along the bed, dilodged by lift and drag force but unable to tay aloft becaue of their weight many colliion - Supended load = et of particle moving with the tream colliionle - Diolved load = concentration of chemically diolved element from ediment, weightle and therefore at any level. u* Sh ( 1) gd ilt and gravel Note: No longer Reynold number but a meaure of the particle ize Particle witch from crawling on the bed (bed load) to being fully upended when the friction velocity u * exceed their ettling peed w. 6

7 Bedload tranport The amount of material being tranported in the bedload i, per unit width of tream: ma tranported (per unit time and unit width) m q c u crawling peed thickne of bedload layer volumetric flow rate (per unit width) volumetric particle concentration (volume/volume of water) A common choice i: c ( Sh Sh ) d u 4.8 u * c in which cae the bedload tranport i found to be: u * m d u ( 1) gd * Many other formula have been propoed over the year Here: q * * c Sh * Sh c m ( 1) gd 7

Stream meandering Exce centrifugal force of

The effect of the econdary circulation i

8 Stream meandering Exce centrifugal force of fat flow at top and inufficient centrifugal force of low flow near bottom caue a tranvere circulation, called Secondary Circulation. The reulting D flow i a helical flow. The effect of the econdary circulation i caue a pattern of eroion at the outer bank and edimentation at the inner bank. (Scorer, 1997) 8

Sedimentation pattern in a tream meander Evidence of eroion Evidence of

defined a the middle of the Miiippi River at the time of the deciion.

coincide with the middle of the river at a number of location, leaving pocket

9 Sedimentation pattern in a tream meander Evidence of eroion Evidence of edimentation The tate line between the tate of Louiiana and Miiippi wa defined a the middle of the Miiippi River at the time of the deciion. Since then, the Miiippi River ha modified it coure, and the border no longer coincide with the middle of the river at a number of location, leaving pocket of Miiippi State to the Wet of the River, and pocket of Louiiana State to the Eat of the River. 9

Flow near the entrance of a ide channel (uch a an irrigation channel) will lead to unwanted ediment at the entrance of the channel eventually blocking the entrance of the ide channel.

10 Flow near the entrance of a ide channel (uch a an irrigation channel) will lead to unwanted ediment at the entrance of the channel eventually blocking the entrance of the ide channel. Gradual choking of entrance to ide irrigation channel by ediment depoition ha been claimed to have contributed to the demie of the Babylonian civilization. Poible remedie 1. Have the entrance channel on the outide of a bend of the main channel.. Configure the geometry uch that the flow i firt partitioned and then forced to make the turn. 10

FLUID MECHANICS. TUTORIAL No.4 FLOW THROUGH POROUS PASSAGES

FLUID MECHANICS. TUTORIAL No.4 FLOW THROUGH POROUS PASSAGES FLUID MECHANICS TUTORIAL No.4 FLOW THROUGH POROUS PASSAGES In thi tutorial you will continue the work on laminar flow and develop Poieuille' equation to the form known a the Carman - Kozeny equation. Thi