Lesson 3 River Geomorphology.pdf

8/10/2019 Lesson 3 River Geomorphology.pdf

1/43

LESSON 3 RIVER GEOMORPHOLOGY


2/43

FLUVIAL SYSTEMS

LESSON 3 River Geomorphology

1. An erosional zone ofrunoff production and

sediment source

2. A transport zoneofwater and sediment

conveyance; and

3. Adeposition zoneofrunoff delivery and

sedimentation


3/43

SEDIMENT EROSION, TRANSPORT

and DEPOSITION by RIVER

Interaction between flow and riverbed

(a) General View

(b) Free body diagram showing shear stress at flow base-river bed interface



4/43



SHEAR STRESS

fo ghS =

where : Sf = slope of theEGL

t

V

gx

V

g

v

x

hSS of

==

1

SHEAR STRESS

For NORMAL FLOW

oo ghS =



5/43


and DEPOSITION by RIVERShields Method (Julien, 1995)

31

2*

)1(

=

v

gGdd s

where: G = specific gravity

of sediment particle

v = kinematic viscosity

of the fluidg = acceleration due to

gravity

sss

o

gdG

u

d )1()(

2

**

=

=

Shields Parameter

where:

= boundary shear

stress

u*

= shear velocity

defined by

o

u =*



6/43





7/43



Particle TypeDiameter

ds (mm)Critical Shear Stress

(N/mm)

Cobble 130 111Gravel 8 5.7

Sand 0.25 0.194



8/43

INSTABILITY OF FLUVIAL SYSTEMS



9/43

RIVER CHANNEL PATTERn



10/43

RIVERBED DEGRADATION



11/43

RIVERBED DEGRADATION



12/43

RIVERBED AGGRADATION



13/43


Channel Aggradation



14/43




15/43

BAR FORMATION IN ALLUVIAL RIVERS

BARS refer to large bed forms on the bed of a river that are

often exposed during low flows

these deposited segment mounds are not static and

often get transported under high flows

they may again appear when the flow subsided but

may not necessarily be at the same location as the

earlier ones



16/43


Bar formation in rivers

(a) Alternate bars form in

straight channels with

deposits alternationfrom right bank to left

bank. Froude number is

high



17/43



(b) Point bars form due to the presence of secondary flowsLESSON 3 River Geomorphology


18/43


Secondary flow in rivers

(a) Streamlines in plan at different levels



19/43


Secondary flow in rivers

(b) Rotation movement of water in river cross-section

(c) Effect of secondary flow: deposition on inner back



20/43



(c) Mid-channel point bar



21/43

RIVER MEANDERING


G


22/43

RIVER MEANDERING


LATERAL MOVEMENT OF RIVERS AND


23/43

LATERAL MOVEMENT OF RIVERS AND

ITS BANK INSTABILITY


VARIABLES AFFECTING


24/43

VARIABLES AFFECTING

RIVER BEHAVIOUR

1. Stream discharge

2. Sediment load

3. Longitudinal slope4. Bank and bed resistance to flow

5. Vegetation

6. Geology including types of sediment

7. Works of dam


SUMMARY


25/43

SUMMARY

SUSPENDED SOLIDS measured by sampling the water, filtering toremove the sediment, drying, and weighing the filtered material


SUSPENDED SEDIMENT LOAD


26/43


SUSPENDED SEDIMENT LOAD measured by sampling the

water, filtering to remove the sediment, drying, and weighingthe filtered material

expressed in parts per million (ppm)

computed by dividing the weight of sediment by the weight

of sediment and water in the sample and multiplying the

quotient by 106.




27/43


US DH-81 SAMPLER

It consists of an adaptor, cap with

an internally molded vent tube and3/16 or 1/4 or 5/6 nozzle, all of

which are autoclavable.




28/43


U.S. DH-48 DEPTHINTEGRATED SUSPENDED

WADING-TYPE SEDIMENT

SAMPLER

This is a lightweight sampler

for collection of suspended

sediment samples where

wading rod sampler

suspension is used




29/43

Where:

Qs= suspended sediment transportQ = streamflow

n = commonly varies from 2 to 3


ns kQQ =




30/43




31/43

BED LOAD


32/43

BED LOAD

TheBLS 30 andBLS 48 pressure difference cable suspe

nded Bed Load samplers are

used in natural streams for c

arrying coarse sediments.


RESERVOIR SEDIMENTATION


33/43


)log(100

%)log(100%)log(

100% 332211 TBWclayTBWsiltTBWsandW +++++=

Where:

W = specific weight (dry) of depositT = age of deposit

B1, B2, B3 = constants having same unit as Wthat relate to the

compaction characteristics of each soil type

W1, W2, W3 = specific weights of sand, silt and clay, respectively

%sand, %silt, %clay is on weight basis




34/43





35/43


Example 1.Estimate the specific weight of (dry) of deposited

sediment that is always submerged. The sediment is 20% sand,

30% silt, and 50% clay by weight. Calculate how the specific

weight of the deposited material varies with time and find the

volume occupied by 500 tons of first-year and tenth-year depositedsediment.




36/43


Example 2.If the specific gravity of sediment particles is 2.65 and

the specific weight (dry) of a cubic foot of deposited sediment is

70 pcf, what is the porosity of the deposited sediment and what

does 1 ft3 of that sediment weigh?




37/43


TRAP EFFICIENCY the percentage of the inflowing sediment

that is retained in the reservoir, which is a function of theratio of reservoir capacity to total inflow.

decreases with age as the reservoir capacity is reduced

by sediment accumulation

SMALL RESERVOIR on a large stream passes most of its

inflow so quickly that the finer sediments do not settle but

are discharged downstream.

LARGE RESERVOIR may retain water for several years and

permit almost complete removal of suspended sediment.




38/43


CONTROL

Common procedure for dealing with sediment problems

designate a portion of the reservoir capacity as SEDIMENT

STORAGE

select a site where the sediment inflow is naturally low

use of soil-conservation methods within the drainage basin

(terraces, strip cropping, contour plowing, check dams,

vegetal cover, stream-bank protection, revetment, etc)provide means of discharging some sediment such as sluice

gates


RESERVOIR CLEARANCE


39/43

RESERVOIR CLEARANCE

Disadvantages from leaving the vegetation in the reservoir are:

1.trees wil l eventually float and create a debris problem at

the dam

2. decay of organic material may create undesirable odor or

taste in water-supply reservoirs

3.tress projecting above the water surface may createundesirable appearance and restrict the use of reservoir

recreation


RESERVOIR SITE SELECTION


40/43


General rules for choice of reservoir sites are:

1. A sui table dam site must exist. The cost of the dam is often

a controlling factor of a site.

2. The cost of real estate for the reservoir (including road, rail

road, cemetery, and dwelling relocation) must not be

excessive.

3. The reservoir si te must have adequate capacity.




41/43


4. A deep reservoir is preferable to shallow one because oflower land costs per unit of capacity, less evaporation loss

and less l ikelihood of weed growth.

5. Tributary areas that are unusually productive of sedimentshould be avoided if possible.

6. The quality of the stored water must be satisfactory for its

intended use.




42/43


7. The reservoir banks and adjacent hillslopes should bestable. Unstable banks wil l contribute large amount of soil

material to the reservoir.

8. The environmental impact of the proposed reservoir mustbe studied and made available to the public to ascertain

the social acceptabili ty of the project.



43/43

LET LEARNING CONTINUE.

Documents

Lesson 3 River Geomorphology.pdf