Lecture notes on "The Role of Rivers to Mankind"

INSTITUTE

OF HYDRO-ENGINEERING,

GDANSK

Conference on "Selected Problems from the Theory ofSimulation

ofHydrodynamic

Phenomena"

jablonna,june

1969

lecture notes on

"THE ROLE OF RIVERS TO MANKIND"

hy

INSTITUTE

OF HYDRO-ENGINEERING,

GDANSK

Confer

ence on "Selected Probl

e

ms f

r

om the Theory ofSimulation

of Hydrodynamic Phenomena"

Jablonna,June

1969

lecture notes on

"THE ROLE OF RIVERS TO MANKIND"

b

y

H

.

A. Berdenis

v

an Berlekom *

)

*)Assistant Managing-Director, Netherlands Engineering Consultants NEDECO the Hague, the Netherlands

These lecture notes are part of a series of three:

I. "The Role of Rivers to Mankind

v

,

by H.A. Berdenis van Ber

l

e

k

o

m, N

e

th

e

rl

ands Eng

i

neer

in

g

Consultants NEDECO,

t

he H

agu

e

, th

e

N

e

th

er

l

ands.

Ir.

"Solving River Proble

m

s

by Hydraulic

and Mathematical Model

s

"

by Dr. M. de Vries, Delft Hydra

uli

cs Laboratory, Delft

t

he

Netherlands.

IIr. uPhenomena Related to Turbu

l

en

t

F

l

ow in Water Con

t

rol Structures"

by J.E. Prins, Delf

t

Hydra

ulic

s Labora

t

ory, Delft, the Netherlands.

The lecture notes cover separa

t

e themes, having in common as main

objective: a scientific approach

t

o r

i

ve

r

-engineering problems in order

to arrive at effective river con

tr

o

l

.

.. ,,-' _{,~~,' '. h}

.: :...

".:

r,..,:

the other two books ofsthe symposium, sand

transport has been denoted by

T,

and

CHAPTER I THE BASIC PROBLEM

CHAPTER 11 THE PHENOMENON "RIVER"

1. The river's functions

₅

15

2. The longitudinal profile

3.

Independent and dependent variables

in the river valley

4. Water movement

5.

Sediment movement

6.

Bed formation in a straight river

18 20

37

52

CHAPTER 111

SOME SPECIAL FEATURES

1. Cross-sections of a river in a bend

2. Features of cross-sections in a long

narrowed section

61

3.

Rivers under natural cenditions

4. Confluences

68

73

78

CHAPTER IV

IMPROVEMENT SCHEMES

1. Multipurpose character

2. Improvement in stages

3.

The first stage: information

4. The second stage: temporary improvements

5.

Bed regulation

6.

Normalization

7.

Canalization by weirs

8. Discharge Regulation

83

83

85

86

94

104

105

117

BASIC PIlOBLEHS

When describing a country, when consuiting a map, and when looking at t~e earth from an aeroplane, it is always the pattern of rivers, their courses and their appearance that most strike the onlooker, and that determine the character of the land aud the impression it leaves to tbc visitor. And if the visitor's attention

is already at t.r-act.ed to such an extent ,)ythe rive rs, by the way they interseet the land an d incise the hills, hov much more will rivers affect the life of tbe people that live there, depending on

or perhaps even depending from the existence, the whims and the caprices of the watercourses. They rely on its continued supply of the so essential water without which life is doomed to remain poor and humbie, and at the same time on the possibility to dispose of their waste for which the river serves as excellent remover and purifyer. The water bring thernalso food, proteiri of high quality which so ofteri is lacl{in~ to the serious detriment of a healthy

development of the hurnan body and spirit. Logical that unde~ such corditions ~an, several thousands of years ago, cultivated a believe that the ruystic Gods of the rivers had to be kept good friends, and needed their toll to maintain thc people's life artery. Remarkable to see that nowadays, even in the highly developed countries whose people are fu lly aware of the Lmmense capa cities of mankind, re

-mainders of this old believe still exist: fear for the unknown response of the Great River to human interference that it may not like is something which you can meet regulaily, and is sometimes extremely difficult to eli6inate.

But back to the past centuries, when populations were on the move, searching fryr places to settie and start societies. Old civilizations all occurred along rivers, showing ups and downs in the social and cultural expressions remarkably well-phased with the ups and downs of river behaviour, And even nowadays , one can notice that a great many capitals of the world are situated along

Tt is not oulv the supp ly of wate r that,wa s the reason for

settiement. Of course, a large number of aspects jointly led towards the preferenee of people to live near rivers. There are f.i. the easy transport facilities offered by the river; primitive peoples living in the jungle of the tropics often still have their river as the sole route of communication; provided by Nature free of char~e

.and cheap to be used.

There is the existence of a reasonably flat land at the sides

of the river, the so-called floo~ plain, that has fertile soils because of the nutricients brought there by the regular floods of the river, inundating the valley. The area lends itself very ve lI for agrieulture, cattie grazing etc.: the thick humus layer and the very small terrain slopes guarantee a natural conservation of the soil. On the other hand, the floods still oceur regularly, so that the flood plain eannot be fully possessed by Man: it is'part of the river and as sueh untouehable.

With the progress of civilization, the rLver-ai n,p opulations

beeome more edueated, inereasing their produetion and diversifying

their interests. They discover and exploit oth~r uses of the river: it proves to be a souree for irrigation water, for energy generation, for building materials like sand or clay for brick manufacture,

for cooling their industrial plants, and finally even for recrea-t.Lon , :With sueh e. we aLt.h of gifts whieh Nature endowes the people,

it can only be expected that Man enjoys his good life and riehess

and develops hls society. Sueh development takes plàee in two dimensions; viz., in an increase of population and in an increase of demands of each individual member 'of this population. As a consequenee, more and more use is.~ade of the Great Resource of Life, the river~ Water is extraeted and consumed, which for a great part.then evaporates and does not return back to the river; the floodplain, more and more turned into agricultural land, is used also to erect cattie stables, to build small factories and even to establish whole townships which suffer danrage through floods, s0 that people demand the floodplain as their full and inalienable property and want to ehase the river awayj

the drainage basin hitherto covered with thick vegetation is deforestated so that it can no longer keep the rainwater stored, and neither resist erosion; factories start to discharge their ever increasing waste products into the river thence gradually

turning it into a sewer rather than a healthy supply of clean

water and fisb.

To all this, the river responds, not because its Hystic God becomes angry, but because

.

the river's delicate equIlibrium is disturbed. Mildly flowing rivers become torrents, carrying-large masses of sediment, and depositing these at places where Man does

not want them; flash floods threaten the riverain towns and cause scour of the banks; sandbanks start moving fast and deep ehannels_

in the river become shallow, hampering the navigation and endangering intake structure for irrigation; and a prolongation of the period

of low flow in the river occurs, equally detrimental as th~ floods,

inasmuch as agricultural production suffers from lack of water,

navigation becomes impossible a~d waste products of the people and

_industries is no longer removed properly, jeopardizing even a

minimum in hygiene. As long as the people behave themselves, taking

cognizance of the Natural laws that rule phenomena, the rive~ is

-a frierid. But if the balance is interfered w i t.h unwisely, the consequences become manifest; they inevitably turn the river ioto an enemy, which we have to eombat for our survival, and in many cases it will be a battle for ever.

Of course, it is not always the fault of th~ people that rivers are bad. Some rivers have never been good rivers, have always shown high flash floods, alternated-with extremely low discharges, and have always had very mobile beds and tapidly eroding banks. Sueh rivers were, in the first instanee, considered to be unsuitable for establishing settlements, and remained unused. But here, too, the increasing population necessitates an expansion of the inhabitation,

-so that in the course of time also these rivers started to become

subjects of human interest, and those peopl~ that populated their valleys, not content with what they find, require alterations.

Even with the relatively good rivers, we become discontent.

Economic development nnd progress of our civilization, pose hi7he~ and higher r-eqn i remerrts to what the river pr ovides, -Je warrt Lncreased channel depths to cope with our ever increaning s~ip sizes; we want

to construct bridrres Bcross the river which necessitates a control of the course, a barnessin~ into a fixerl frane of Romething which

essentially is mobile nnd dynamic; we want a gllaranteedminimum

amount of water to safeguard the corrtinu otesan d proper removal of

our wastes; aod yet, we want the water to be pure and clear for our domestic, industrial or agricultural uses. Many of these desires

are incompatable, and only a careful and integral anaJysis of the physical potentialities can yield a result in which all aspects are

coordinated and comprises reached between the various demands.

Golving the problems is partly an economie and political matter,

insofar as ft concerns the evaluation of the demands and the

assessment of the priorities. Dut it is clear that, in arriving at

the final solution, particularly the river engineer has to ~an the central controlroom. It is the task aud.target of TIiverEngineering

as a profession to provide all the tools :Zorarriving at an optixrun

utilization of the p otent iaI resources, optizum in the sense of

promoting the benificial characteristics of the river and eliminating

or at least checking the adverse qualities.

To strive for this aim, there must be a scientific understanding

of the complex pattern of natural forces that exert their influence;

CHAPTER 11

THE PENOMENON

"RIVER"

11.1 The Biver'. funetion••

In principle the basic natural function of a river is to

co11eet water direct1y or indirectly or~ginating from preeipitation

and to transport this water from areas wi th a higher elevatio-n

towards p1aees with a lower elevation, mostly the sea. Therefore

what is basical1y typica1 of a river is the continuous Gr

nGn-continuGus presence of water which f10ws under the àction Gf

gravi ty.

Aa the flowing water has the capability to transport partiele.

Griginating frGm the erGsion of the aaath crust, all natural rivera

will carry a greater or 1esser amount Gf erGded p,,

~

tieles (aedime~t)

in their f1GW Gf water. Part Gf the erGsion prGduet. is sG11uble and

wil1 therefore be earried by the flow in the form Gf a IGlution, which

will nGrmally not play a ro1e of significanee in

.

the preeees ef

river behaviGur. The remaining SGlids, hGwever, fulfill a majGr

funetion in this respect. RGllin:g and jumping ever the riverbed

intermittently Gr een't

Lnunua Ly

suspended in the flGw, tem~orarily or

perm&nently depGsited, exehanged against Gther partieles and generally

subject to a prGeess Gf abration, the partieles play a predGminant

role in

·

the formatiGn of the riverbed and in the appearanee and

eharacteristies Gf a river.

Thes. twO'

,

f.notions

_'

Gf

th. river, the transportatiO'nO'fwater and

the transportation of sediment, together fODm the instruetiO'nO'fNature

I

tG which our rivers has tG eomply. Both fune~iO'nsare related to a

certain (dynamic) equilibiu~ in the physieal proeesses of O'urearth,

the first funetion to what is

.

called the hydrO'logiealeycle whieh

describea the perennia1 eireular course O'fthe water interehanging

between

.

the atmosphere and the earth, and the second funetion to the

sG-ca1led erosiGn cyele which deals with the geologieal phenomena O'f

mountain formation and mountain destruction. For a gGGd understanding

_.

-.;

Gf the river, bO'thcycles will be ana1ysed in some .ore detail

here-below.

~

(1). !7~r!l~,!c!1_cle!e~

In principle, the mGst simp1e form Gf hydrGlógieal eyele is a

straightfGrward sequenee Gf evaporatiGn Gf water from the sea, the

transpGrt of the damp air masses to the catehment area Gf the river,

tbe rainfall and tbe run-off bac

k

to the sea. I

f

tbis were a

permanent pbenomenon, it would be eas

y

to eompu

t

e tbe quant

i

ties

involved, sinee ra

i

nfall run-off and evaporat

i

on would all be equal.

In practiee, bowever, all k

i

nd

s

o

f

var

i

a

ti

ons ex

i

st,

i

n time

as weIl as

i

n space.

The clim

a

t

o

l

o

gical con

d

itions in t

he en

vi

ronment

eause a

s

ometimes q

u

ite re2u

l

ar oe

c

uran

c

e

of

pe

rio

ds

wi

th mueh raiD

alternated by drought

s, w

hieh

i

s a

v

ar

i

a

ti

on

in ti

me

. A

lso tbe raiDs

may be limited

i

n expanse cover

i

ng only a small

p

ort

i

on of tbe

catcb-ment area of the r

i

ver leav

i

ng

t

he

rest liab

l

e to e

vaporation, thue

ereating shortcuts in

t

he cyc

l

e.

Apart from eaus ing a var

i

abie r

un-off in t

he

ri

ver,

i

t

i

s elear

that such variations

i

n the movemen

t

o

f w

ater have one more very

imp.rtant consequence, viz., the

ne

cess

i

t

y t

o

i

ntroduce storage or

aeeummulation

in the cons

i

derations.

A

f ter a prolonged dry season,

the soil is dried ou

t

and the greater par

t

o

f th

e pores between the

aetual soil partieles is empty. Fresh ra

i

ns w

i

ll then have the

tendeney to tirstly fill these s

t

orage areas before actual run-off

commences, so that under s

u

e

h

c

on

d

i

t

i

o

n

s

t

here

is

no longer a simple

and unique relation between

r

a

i

nfa

ll

a

nd

ru

n

-of

f

. The eomputation

of rUDn-off from rainfall f

i

gures becomes

hi

ghly complex and it

requires tbe full attention of an entire

l

y

s

eparate profes8ion

nowadaY8, the hydrology, to make a reaso

n

ab

l

e approach toward. a

.olution.

The bydrological phenomena are sc

h

emat

i

zed

in

a simpl

i

f

i

ed

sketch, given in Fig.

1

, whereas analyt

i

ea

lly

they can be expressed

by the general rule

R-P-E-A

where

a

- run-off,

p

_{- precipitat}

_i

_on,

E

- evaporat

i

o

n,

a

n

d

A ..acc

um

u

l

a

ti

o

n,

whieb form

u

la

i

s

nothing

e

lse than

t

he m

a

them

a

tic

a

l

exp

r

es

~i

ori of

tbe

l

a

w

o

f eontinuity for the hydrolo"iral cycl

e

. It w

o

ul

d

l

ead

us

too far awa

y f

rom

ou

r

riv

e

r hydr

a

ul

i~·',I"

lIea

lher

e a

t l

e

ngth w

i tb

the various magn

i

t

u

de

s in this form

ulfi

.

n

a

the

r would w

e

r

ev

i

e

w tb

e

LIJ I..) z ~ _~ 0 0: ~ ;:: ::;) ~ « 111 .... _{Z lil} LIJ _....I <{ LIJ

LIJ <:>LIJ _{Ö Ol::} 111_... X

U _{> 111 «}

rt z x_« ...

0 :::E z :::E :::E

ar: 0 ....I ₁₁₁

::;) ;:: 0

~ 0 0 0 Z

111 « Ol:: _ar: Ol::

0 Z 111 _«

....

_... u. Ol:: u. u. _z « z UI ....I _{Cl 0: « « u} Ö LIJ z· 111 Z Z 111 UI 0 I/) _> 0 _z 0 0 0: I..) ;:: _;:: ~ I/) UI 0 « 0: _> UI Z z « 0: « LIJ _ik: ::J: 0 0 0: 0- ar: 0: _{> LIJ} 0-0 0 0 0 ik: ::J: Z Z IL IL IL IL :::E 0- _:::E Cl Cl ~ ~ ~ ~ _z 0 _Z 0

Ol:: Ol:: LIl LIl LIl LIl ₀ Ol::

0 Ol:: U. u.

II

1

z z 0 z 0 z ;:: 0 _;:: 0 ;:: ~ :! -e :! 0: Ol:: _0: Ol:: 0 0 Ü IL Ü IL LIl ~ LIl ~ Ol:: Ol:: IL LIl IL LIl

GROUNO WATER STORAGE

F'ig. 1

A separate analysis, moreo

v

e

r, is in most c

ases not

i

nteresting

for river engineers;

t

he accu

r

ac

y with whi

c

h

h

y

drologis

ts

are able

to translate rainfall f

i

gure

s into disch

a

rg

e f

ig

ures

i

s not great

as yet. That is to sa

y

, for averag

e

a

n

n

u

a

l

cond

i

t

i

ons, they may

be able to arr

i

ve a

t

rea

s

onab

ly good

r

esul

t

s

,

but f

or

i

nstantaneous

eonditions they w

i

ll need a co

nsi

de

ra

b

l

e

tim

e to collect physieal

information on the caracterist

ics of t

he entire drainage basin

ineluding particularly the eapae

ity t

o

s

tore water (in the three

forms : groundwater,

surf

a

ce w

ate

r and ie

e and snow). It is mueh

easier to eollect d

i

scharge

f

igures d

i

rectly by taking measurements

.

in the river. On

ly

i

f

da

t

a

on

d

iseh

a

rg

e

s (

or

w

aterlevels) in the

river relate to a

s

hort

p

e

rio

d

of rec

or

ds, t

o

o

short to be used

for statist

i

cal anal

y

sis

, m

a

y

eompleme

nt

ar

y i

nformation from rain

.

-figures be useful. For t

h

e

r

e

st

, pract

ic

a

l c

onsiderations do not

provide the arguments for deta

i

led

hy

drolog

i

cal studies and

investigations.

Meanwhile, hydrology can

n

ot

b

e om

i

t

t

ed entirely beeause it

explains mueh of the river, and part

i

e

ul

arly its so-ealled hydrograph,

whieh is a graph giving a eontinuous

pi

c

t

ure of either the water

discharge (for instanee expresse

d i

n

m3

/

s

ec) or the waterheight

(in m), plotted alon

g

the ver

tie

a

l

ax

is

a

g

a

i

n

s

t a hor

i

zontal time

axis. Mostly a

f

ul] year

i

s take

n

, and if possible not the

artifi-oi.i

aaD-made ealendar year but ra

t

her the more natura 1

hydrologi-cal year, whieh starts at the end of the dry season, when the

aeeumulation of water in the catchment area is lowest.

The shape of the hydrograph grea

t

l

y

depends on the hydrologieal

eonditions in the eatehment area.

If the supply of the run-of

f fr

om the catchment area varies

over tbe year, periods with h

i

gh

w

ater

l

evels and discharges alternate

with low waterlevels and discharge periods. Thê yearly eyele in its

8implest form has one elear "wet season and one dry" season. Rivers

in tropical env

i

ronmen

ts

ver

y

o

ft

e

n h

a

ve su

eh

e

lear

l

y defined and

simple eyeles

,

wh

i

e

h

a

re in m

a

ny cases also

re

m

ar

k

ably reg

ul

ar

in

their timin

g

. In

m

odera

te clim

a

tes

,

how

ever,

th

e c

y

cles ofte~

The above-~entioned supply of run-off incorporates all three

factors P',E and A from the hvdr olo-rica I formu La, 50 that aLs o the

hydrograph is a full and intef}ralreflection of all conditions

in the river basin, And j_Î it was shown heretofore that cornputation

of run-off from the prime ele~ents, P, E and A is still not a

pr actical proposition, the reverse is certainly worthwhiLe to un

der-take. Starting from the hydrograph(the R or rather the

Q

which is

the symbol for discharges), it can be endeavoured- to analyse the

influence of each of the elements: rain, evaporation and accumulation;

and of these particularly the latter, the accumulation, proves to be

of great importance. Storage conditions in the river basin cause

a slower inflow of the rainwater into the river. Gently sloping

terrain, weIl covered with forest and other vegetation, will retain

rainwater to a considerable degree and wi Ll release temporary

stored water long after the rains have stopped. The hydrograph for

such an area will therefore have a much smoother appearance than

~he precipitation pattern would suggest, while considerable

retar-dation will be manifest. The more downstream the more these effects

are observed. À river with lar.r:e:flo0dplains and many tributaries have relatively large storage capacities whieh tend to further

)

modify the hydrograph, "erasing" irregularities and promoting

r-et.ar dat ion , This eiear distinetion between upper rivers and Lowe r

rivers is illustrated in Fig. 2.

Of eonrse~ the storage eonditions, although important, are not

the sole reason for this differenee in hydrographs. Naturally, rain

and evaporation play their role too. hainfall normally oeeurs over

a fairly limited area as showers or rainstorm. Small streams,

there-fore tend to show large daily variations. In large catchment areas,

~here the major rivers collect many streams, only some of these

receive rainfall at aoy particular moment, and this is another reason

for the smooth hydror;raph of 3. Lowe r river.

Considerin~ hydro~raphs in t~is way already gives some indi

aa-tion of the river's charaeteristic. But conceptions like "irregularity"

and "sruoothening out" are qualitative ,expressions, with which we

should not be content. A proper analysis of the phenomina

- 10 -9. 97 96 95 9... 93 92 z c ~ .7 lil .. ~ ... 5 c -E .... ~ 26 25

THA PLA (nan) ..j ~ _94 lil ..j 90 c lil 'lil

.

_._.-

.

_._._._

.

_._._.-._._._._._

.

_._

-

-

-iil

2... > lil 23 ~ ~ 22

I

24

NAKORNSAWAN

'chao phya)

.

-,

.

-

---:-

/

_

.

.

_

.

_._

.

_._

-

---_._

.

_

.

_

.

_

-

.:..__

.

_

.

_._._

.

_

.

-BANGSAI

(ehao

phya)

NOYEW8ER DECEM8ER

.4964 4962

Fig. 2 1)pièaIhydrographs

One sueb number is obvious, and follows immediately·from the bydrograpb, viz., the ratio

Q

max./Q min. It is most interesting

to notiee that this ratio highly differs for different rivers, ran,ging between as low as 2} for the Congo and over 250 for purely tropieal monsoon ~iv~rs like the Behue in Nigeri~. That the Congo has sueh a low figure is mainly eaused by the faet that its drainage basin is partly in the southern hemisphere, with a rainy season

in November and Mareh and partly in the Northern hemisphere with a rainy season in May - September.

Anotber

int.re.ting

quantitat

i

Te

tisure by it•• lt i. tho'

aT.rag.

di.obarle,

.spr •••• d per unit area ot drainale ba.in.

It app.ar. tbat tbi. fi,ure Tari •• oon.id.rably

10•• tban the

pr.Tioua

one, appar.ntly,

tb. aTerage aDDual conditiena

for ..ny

riT.ra ar. in tb•• a•• ord.r of magnitud.,

wb.r.a •• oa.onal

Taria-tien. aay be T.ry large inde.d. Table

3

b.low ..y ••rT. a. an

illu.tration

in tbi. r••p.ct. fieur •• baTe be.n giTon for a nuaber

of lar,. riT.r.

in ••T.ral part. of tb. world.

Table ., Comparative basic data of some rivers

No. Name Drainage Annual water Avera.ge₂ Q Water diecharge Q max m3 / sec

-area transport per ka

Q ain km2 109 m3 _105,m3 High Low 1. Amazon 7 050 000 3 000 1t.3 200 000

-

Abt. It 2. Nile 2 860'000 85 0.3

-

-3. Yangtze Kiang 1 830 000 700 3.H 80 000 5 200 16 lt. Congo 3 700 000 1 ItOO 3.8 65 000 27 000 2.5 5. Miuouri 1 370 no')

-

-

25 500

-6. Bwang Bo 771 000 20.0 2.6 25 000 21t5 100 7. Mekong 795 000 ItOO 5.0 60 000 1 700 35 8. Niger 1 890 000 180 1.0 30 000 1 200 25 9. Miuiuippi 3 222 000 600 1.8 76 500 3 500 22 10. Volga t 500 000 250 1.7

-

-11. St.Lawrence

-

300

-

-

-12. ParaDa (Plata) 3 000 000 600 2.0

-

-13. lndu8 1t52 000 200 1t.4 20 000 1t90 33 lIt. Brahmaputra 938 000 380 4.0

-

1t25 15. Danube 1 16? 000 200 1.7 10 000

-16. Zambezi 1 300 000 500 3.8

-

-17. Ganges 905 000

-

-

60 000 1 71t0 34 18. Dnieper

-

50

-

-

-19. lr~.awadd)' 1t15 000 520 1.2 61t000 1 310 1t8 20. Bhine 162 000 80 5 12 000 500 20\

.(2) lir2.si0!!.

!,y~l!..

Erosion

is

the na

t

ural proces

s of

demol

i

t

i

on of the surface

of the earth ann the rem

o

val

o

f

th

e eros

i

on products b

y

wind and

water. The primary (or exoge

net

ic

)

s

l

ope of the lann

i

s changed

byerosion

at the hi~her lev

p.ls

a

nrld

epos

i

tion a

t

the lower levels

to the endogenetic slope as s

hown

i

n Fi

g.

'

1.

,

EXOGENETIC

SLOPE

·

"

/'

/

~'

"~'

EROSION

Î'

ENDOGENETIC

~

DEPOSITION

~

,

1

~~~

SLOPE

~

--:,. --

EROSION BASIS

E

.

G

.

SEA LEVEL

...

DELTA

BUILDING

FIG.4

SL:OPE CHANGES

DUE TO

EROSION

Fournier has estimated

t

ha

t

ero

si

on

i

s taking place at the

average rate of

I.

cm per century over the w

h

ole area of tbe world

not covered

by

water.

The cau~es of erosio

n

are we

ll

-known. The

b

are primary rock

-Alternating high and low temperatures eause eraeks ta for~ in the

bare roek, water eau thcn enter and freezin~ oeeurs thus further

brealrinz the r ocr, roots of plants can enter and attactc by dissolved ehemieals is aeeelerated. The ~toded rocks form a talus or alluvial eone around the bare roek and this debris is then removed by running

water and carried into torrents whenee it is transported to the lakes, valleys, and rivers and, eventually, to the sea.

An important feature is that the transport of material does not take plaee direetly to the sea (exeept for a proportion of the

finest partieles, silt and elay), but by a series of stages of scour and deposition. Material carried by torrents is deposited at the eonfluenee with the river. The river then earries the material downstream and it may be many times deposited and pieked up again on the way. ~iamematerial wilI he move d-a long the bed of

the river itself, so~e will be ~eposited in the flood plain at high

.,water diseharges. As the river meanders fresh material is deposited on the inside of the benrlsand material depositerl at a previous time is removed by attaelc on the banles at the outside of the bends,

It may take a grain .many hundreds, or even thousanrls, of years to .,

travel from the point at which it leaves the parent roek untill it reaches the sea (This eontrary to a waterparticle whieh travels with an average veloeity of 1 m/sec or 85 km/ day). By ploughing

through the valley the river is eontinuously selecting its transport

material~ droppin~ the larger grains and picking up the smaller sizes. ITorizontal seleetio~ takes plaee by meandering and vertieal selection is carisedby irreg~larities in the bed, sinee at different stages the river has differing .powers to transport material and therefo~e scours to differing depths. As a eonsequenee, the grain

size of the bed material of rivers deereases in the downstream direetion.

One part of the erosion products is deposited directly below the mountains to form the hills and the process of surface weathering together with vegetable growth, which becomes vegetabIe.

mould, yields humus.

Another part of the erosion produets is deposited in the

sea forming layers of sand, silt, etc. With great thicknesses of

deposits and by high pressures and temperatures and by cementation

new sedimentary rocks are formed. By geological forces the new rocks

may be lifted up and another cycle of erosion is started. The whole

cycle may take millions of years and during the erosion process the

valleys go through stages of differential development from youth,

through maturity to old age.

!o~t~ is the period of formation of torrents. Water runs

down the steep slopes of the mountains and is concentrated into the

existing folds. Due to the steep slope there is a high velocity and

consequently large energy, large transport capacity and large erosive

•

power. Such torrents will scour the bedrock and form a channel, but

they are also fed from the side slopes with water and erosion products.

Gradually the length of the torrent incTeases as it cuts backwards

fnto the fills; the slope decreases, transport capacity decreases and

the rate of scouring decreases until an equilibrium slope is reached.

Then the walls of the valley are attacked (Fig

5).

OrL.:;inalprofile Flattor slop0 ~ivinG 0quilibrium Original val Lcy \ Suc ce s s i.vo Ly Lar gc r vaLLcys

-

-

.

-

.

--=---

-~

-

'

/.,

-

-

-

--~7·---

-~

._

-_

.

~

-

-

-

-

_/

Scction A.A

-

;

~

-

'-

l

-

r

r

-~ Valley with ~quilibriu~ hcd slopt) Valley with uquilibriun b0d and ai.do 810po::;

As the val~ey walls recede the river has space in which to

meander. The slope of the valley sides is also reduced byerosion until equilibrium is reached and vegetation begins to take root.

In reality, of course, the picture is much more compli-cated since there is not just one torrent but a whole network of torrents, every side slope having its own secondary torrents causing erosion and tending to produce the equilibrium situatiop. The result of this process is the typical fully developed drainage pattern called a !a~u!e_r!v!r_v~llel.

Finally, this develops into a !e~ile_ valley formation (or peneplain) where the whole landscape tends to become aplain, with such a gentie

slope that the capacity of the flow to transport sedimentparticles has been reduced to zero. In most cases, this will take millions of years, and often Nature does not leave the time to actually see

such final equilibrium situation because of new tee tonic movement which causes a fresh start of the erosion process.

Just like in the hydrological cycle the most important

aspect for river engineers is the actual water discharge of the river, the part of the erosion cycle which interests us most is the transport-ation of sediment in the river. In this connection, one often speaks about the collecting part, the transporting part and the depositing part of the river, although scientifically, it is more accurate to use the terms upper river, middle course and lower river. Please note that this division is different from the distinction in young rivers, mature river and old rivers. The latter is a series in time, the former a series in distance along the river axis.

11.2. The longitudinal profile.

In considering the river from its source in the headwaters towards its erosion basis, the sea, it is noticed that there are a certain given vertical difference and horizontal distance to be covered. Is has been outlined in the previous paragraph, that in the course of time the vertical difference decreases byerosion of mountains, and the horizontal distance increases by backward incision and by delta building. But apart of that, rivers never take a straight

course between source and mouth. Within its (sometimes wide) valley,

the alignment of which greatly depends on orographic boundary

conditions (ridges, and plains, geologic faults), the river as such

flowing freely through its self-built bed, may take all forms between

extremely winding (meandering) or practically straight (reach type).

Thus with a given elevation and distance to be covered between

source and river mouth the average longitudinal slope of a river

is still undetermined.

Moreover, nowhere along the course will the slope be

equal to the average slope. Actual slopes deviate considerably from

the mean value, this deviation occurring in two days:

( i) local slopes vary from place to place and even from time to time;

as aresuit it is extremely difficult to know always exactly

the values of such slopes, the observations essentially being

averages over some length of river section.

(ii) along the full course from source to sea, the river shows a

gradually decreasing slope, in the headwaters steep and in its

lowest section gentie. Considerable 'drops over short distances

(rapids and waterfalis) occur in the mountainous areas, where

the river is still forming its valley by incision, so that the

bed mainly consists of rock and its slope is greatly dictated

by the original mountain slopes.

On leaving the mountain areas, the river takes its course

through regions where the erosion products from the headwater are

part-ly deposited. The slopes decrease considerably and are less subject

to great variations. The river now flows more and more through à bed

built from its own sediments (alluvial river) and creates its own

slope, which is no longer dictated by the boundary conditions. But

here, too, it continues to decrease in downstream direction, through

the middle course into the lower course where the river flows through

low lying areas, mainly riverain deposits in what was originally part

of t.he- sea, There,slopes attain very low values, sometimes even

negative where under the influence from the sea, the tide penetrates.

It is clear that the riverbed increases its width from

head-waters to the sea, not only because of the decreasing slope but also

And before finally entering into the sea, the width generally increases very considerably where tidal flows (flood and ebb.) are superimposed upon the river's own discharge (upland discharge). The place will be reached where tidal motion becomes more important than the upland discharge. This part of the river, into which

saline water from the sea penetrates, is named the estuary. The reason for'this continuous decrease in slope all

along the river'8 course is a morphological one and will be dealt

with later on in this chapter. One of the consequences of this phenomenon is that the flow velocity

in

the river decreases in downstream direction, which in its turn is the cause of the process of sorting the sediment along the riverbed. Pebbles, stones and gravel are mainly found in the upper rivers; middle courses have predominently sand, at first coarse but decreasing in size to fine, whereas in estuaries the greater part of the material often is silt and clay. Since silt behaves diffe!ently from sand and is subject to other mechanisms of motion, estuaries must be dealt with by an entirely separate hydraulic approach.

In the following, we will limit ourselves to middle and

lower river courses, thus to alluvial rivers where the valley con-sists of the river's own sediment and where all characteristies are a result of the river's own aetion.

In such an alluvial river, the shape of the cross-section is generally as i~dicated in Fig.

6,

consisting of a low-water bed with sandbanks, natural levees at both sides, and an extensive flood plain which is inundated during the high-water periode

NATURAL

LEVEES

BANKS

H.W.

.

~

..

FLOOD

PLAIN

LOW WATER BED

IFlOOD

PLAIN

I

18

-The lowest point is part of the so-called "talweg", which is the line along the river-length connecting the deepest points in the successive cross-sections. In most cases, cross-sections are drawn with distorted scales, that is, vertically much larger than

horizontally, because the river's width is very much more than the depth.

11.3 Independent and dependent variables in the river valley.

The general cross-section as given above, is of course a

qualitative indication. Mutual relations between the sizes and other characteristics of each of the elements, highly depend on the river's

"demand" for capacity to discharge its flow and its sediment charge.

The morphological processes that govern the moulding of the bed

will be described later in this chapter (see 11.6); here some general and introductory remarks may be made.

It has been said above that in an alluvial river, the

valley is to a large extent created by the river itself, through deposition of sediment at places where this fits in the river's bed pattern. If part of the valley would not comply to the various

natural forces, the place would immediately be subject to scour or deposition, thus tending towards full and unconditional obeyance of Nature's Laws. All this, of course, on the proviso that the bed and banks wholly consist of erodible material, that is gravel, sand etc.

Clay, the material predominant in the deltaic region, is mostly

too far consolidated to be readily erodible and of~en causes

(temporary) disturbances in the progress of the development of the

bed. For, the riverbed is not something statie; on the contrary, it

is continually on the move and the equilibrium that exists is therefore

a "dynamic" one,'although the changes take place according to a highly

organised system, in time and space.

" If it is true that the valley is created by the river

itself, there must necessarily be an uni-directional relation between

the two functional tasks of the river, its flow

Q

and its sediment

In ~act-thpre is a third facLor on thc independent side of the problem~

namely4he sediment sizc - although perhaps to alesser degree bepause

we have seen that pvcn this size is subject to the river'a influence;

But it depends on the type a nd composi tion of the material originaLly

supplied in thc headwaters, to what extcnt thc abration procese has

been effeetuated, so that, u Lt.Lmat.eLy , the gr-ain size (sYmbol d) may

be .consIder-ed as a given independent quantity.

Prom these tbree, Q, Tand d, all the rest depends ..Dependent va rLabIe-a

are, for instanee, the width of the channel as weIl as of tbe flood~

plain, t.he roughness of the bed offering resistance to tbe flow"

tbe depth, and even the slope. Our knowledge of morphology bas beek

developed to sueh a stage that, for these depend~nt_variables we are

in a position to produee a fair indieation of their values if

Q

,

Tand d

are given. Other dependent variables, for wbieh tbe quantitative

'. ,

functions and eorrelations have not yet been soIved , refer to phenomena

as meandering, island formation and branching bende ncy , et

c

.

,

In the pto~es8ion of river hydraulics, a lot is still fallow nnd open t~

.

further ·scientific study - in fact, fluvial morphology is a very

young branch of the profession and is onlY just stàrting its l~ng and

dltf icul t way towards ful! comprehension of all aapect.s,

Long and difficult, because in many instances sucb Lnve stigation_s

have

to-

draw upon actual rivers for tbeir data, and such data are

still very scarce; If measurements have been taken in a räver , such

measurements often do not cover all required_ .aapeets , and one missing

link renders correlation calculations useless.

For a part, labora~ory tests are applied, and indeed much

information and insight ha s been obtaine~, but a word of ,..arning is

appropriate bere as weIl. Labo~~tories work witb flumes tbat in most

cases are tilted so as to give it a certain slope. Independent vèria

-bles in tbat eaee are

Q

,

d nnd toe slope i, whereas T is reduced to

an dependent magnitude. It is not

f

ully

under$tood what precisely are

the consequences of tbis reversal of funetions, but lt seeJllSat

lee.st wise to regard tbe re suIt.sobtained in such flumes wibh

11.4 Water movement.

Movement of water in the rivercourse can be subjected

to the highly sophisticated theories that have been derived for flumes in laboratories, and that include detailed knowledge on velocity

distribution in verticals, etc. For river engineering, however, the required accuracy is limited; there are so many aspects that lower precis~on, that it is absolutely without significance to endeavour a very high accuracy in discharge information.

Discharges cannot be measured directly; they are

com-puted from velocities and cross-sectional area, and since both depend on the water level, we will start the subject with discussions on level. Levels form one of the most essential aspects of the river, both for the general understanding and for directly practical purposes as flood protection. Almost every construction in the river has some relation to the waterlevel, whether it concerns revetments, bridges, groynes or inlets. Efficient planning is only possible when frequen-ces of occurrence of certain stages, high and low, are known.

The compilation of frequency curves is only possible

with the help of a long period of records - the longer the period of reliable records the better. It is not always necessary to have continuous records at the same site provided that it is possible to establish a connection between the records available and those at an existing gauge elswhere. In fact, knowledge on the relation between the levels of different gauges is not only important for checking

or compléting records but aIso for e.g. prediction •.An early establish-ment of gauges should therefore be undertaken whether or not major projects are envisaged. It is sometimes possible to obtain local

information about high water levels from inhabitants or by observation of marks on tr-ees , buildings, roads, etc.

When compiling records all earlier information shóuld be

expressed in levels at the latest gauge, taking into account the trans-verse and longitudinal gradients of the river. Where actual level ob-servations cover only a short period, sometimes use can be made of rainfall records that usually cover a much longer period; but, as

is extremely difficult to establish.

The number of gauges required depends on the desired

accuracy and on changes in the gradient of the river and, possibly,

its tributaries. Approximate spacings for some rivers are listed

below: River Rhine (lower) Rhine (upper) Mississippi Niger Spacing 10 km

30

km

50

km 100 km

The

l~~~ii~~

of gauges should be chosen with regard to

points of special importance but free from disturbing influences.

Important positions are immediately upstream and downstream of weirs,

locks, bridges, inlets, etc., along dangerous seetions of the river

and downstream of eonfluences. It is not suitable to have gauges

just upstream of eonfluences beeause the level in each tributary river

affects the other. Generally, they should be sited in sueh a way

that it is possible to relate any cross-section of the river to an

existing gauge. A bench mark should be established in the

vieinity for periodieally (lor 2 times a year) eheeking the zero.

It is usually not neeessary for the zero of each gauge

on a river to be based on the same datum but it is frequently

conve-nient that this should be so, beeause then gradients can be directly

eomputed. On the other hand the datum for levels along a river should

preferably not be a horizontal plain, but rather a eurved plain,

parallel to the river's longitudinal profile. This ean in praetiee

most easily be determined by selecting the level that is superceded

during

95%

of the time.

The !r!q~e~et at which readings should be taken depends on

the rate of rise and fall. A large river rises and falls slowly and

one reading per day is probably 3uffieient. In headwaters and

tribu-taries which may be subjeeted to rapid fluctations in level many

readings may be required to be able to draw the hydrograph eorrectly.

Naturally, in tidal waters even more readings 'areneees·sary and here

The most common gauge consists of a

!~!!!,

fixed vertically on

val Is, rock faces, piers or specially driven piles and carrying

reada-bIe 8cales. In inaccessible places or places vhere frequent readings

are required, recording gauges may be used. There is a large variety

of types and brands, operating on different principles such as

mechanical, pneumatic, hydrostatic or electric systems. Whatever

system is utilized great attention must be given to the reliability.

All readings are primarily to be plotted against a

time axis,

yielding a hydrograph of stages (see Chapter I) but much more can

be done with thema Further possiblities, mainly drawing upon

observations during a 'series of many subsequent years, are:

- Envelopes of maximum of minimum records.

- Frequencies of levels occurring on a fixed day.

- Duration percentage riflevels per specific y~a~ or per average year.

Probability of peak levels.

- Level relation between two gauge stations.

An

illustration of each of these types of curves into

which daily gauge readings can be elaborated and processed is given

in Fig.

7.

Particularly the last-mentioned curve (the relation curve)

is useful as it offers the possibility to inform a downstream station

when to expect a certain level.

Time

'

lag and relation of levels can easily be assessed for

specific stages sueh as peaks in the hydrograph that are

recog-nisable on both gauges (Fig.

8).

For intermediate levels the

problem is more complicated. The best method is to plot levels

at A against levels at B. This has been done in Fig.

9

for

Mopti and Dir6 on the

River Niger in Mali, West Africa. If the

level

.

at Mopti is 3.0 m on a certain day then the level at Diré

viII be

3.65

m

45

days later. This is true for both rising and

falling stages. Similar other positions may be cho.en for various

level. and time-lag. and the tvo curves plotte~. Bence, given the

hydrograph at A it i. pO.8ible to con.t~ct

the hydrogràph at B.

Allovance must be made vhere other tributaries enter or rainfall

occurs.

a

b

h h -.-.- YEAR

a

---- YEAR b ENVELOPE OF t t h

c

h

_d

100'0 1 YEAR t A LWAYS

I

EXCEEDED

o

0'0 OF TIME 100 h DURATION f

PROBABILLITY OF PEAK LEVELS DURATION AND FREQUENCY

1961-- -- ---- ----'1962

r--,;UGrSEP-

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Fig.

9

LEVEL RELATION CURVE AND TIME LAq. CURVE FOR :

D!P!h~ are quite,distinet from waterlevels, unfortunately both

depths and levels are expressed by the same symbol h, which probably is the reason for mueh confusion. Depth, however, obviously is the vertical differenee between waterlevel and bed.

Of course, in a certain cross-section, the bed varies,

and therefore the depth, whereas the waterlevel is practieally con-stant (apart from a possible small transversal gradient).

Also depths are of great importance in river engineering. We have seen that we need the depth for computing discharges, but apart from that depth must be known for all kinds of purposes, ranging from navigation to constructing revetments and from channel studies to foundation problems for bridges. Depths are measured by taking soundings, either manually by handlead or eleetronically by eeho-sounders. Of course, measuring the depth should be eombined with some kind of position fixi~g, so that a cross-section or even a full chart can be drawn.

With levels and depths known, the next subjects to be brought under the attention is the flow velocity. It can be

measured by floats, by propeller meters or by resistance meters. Floats form by far the most simple method.Any floating obj~ct

like a banana skin is already a float. However, surface floats have the disadvantage that they are not accurate because wind can cause the surface velocity to bear little relation to the average velocity in the vertical.Staff floats, therefore, are much better (Fig. 10). But the length must be sueh that it does

not touch obstacles on the bottom of thê river, so that it must

~lways be made too short and therefore the velocity it indicates is too high. This problem is partly solved by the chain float

(Fig. 10) but the influence of dragging on the river bed still

LIABLE TO HIT BOTTOM OBSTRUCTION

FIG.10 SPECIAL FLOATS

One disadvantage of floats is that it is necessary to plot a

measured section along the river so as to time their passage.

This is not always possible, particularly not in large rivers.

To overcome this difficulty we can use a ship's log, i.e. a

float on a wire with knots indicating the distanees. The log is

paid out from an anchored ship. Only a very rough value for the

velocity can be obtained. With a long vessel floats may

be

dropped at the upstream end and timed as they pass dOWDstream.

Floats have the advanta~e that they show the path

and pattern of flow. They are therefore sometimes used in addition

to the more sophisticated instruments like the propellor current

meters (Fig

.

•11) , which measures the number of rotations in a

given time; this information is translated into velocity through

calibration. Propellor meters can be fitted with bell-signals,

counters or automatic recording devices. The instruments can be

suspended at any required depth, so that the full vertical can

be measured, taking observations in a aerLes of depths. The chief

disadvantage is that they are delicate and vulnerable to damage,

the most vulnerable part being under water; frequent calibration

and checking is therefore required.

Fig. 11 A propellor current meter

Secondly, the direction of flow is difficult to determine except

with a complicated instrument.

A more robust and reliable instrument, less expensive

and after some practical experience certainly as easy to operate,

is the Pendulum meter or Planeta (Fig. 12), a modern version

of an instrument dating form the 17th century. The principle

on which it works is that water flow exerts a force on the end

of a pendulum causing it to deflect. The angle of deflection

is then a measure of the force and hence of the stream velocity.

The resistanoe body, hung from a thin wire, should not be round

as this would give unstable readings; instead, special shapes

of bodies have been designed to give satisfactory results.

If the ship's compass is used to give the heading at any time,

also the direction of the current can be noted.

Disadvantages of this instrument are:

a correction for depth is necessary for each measuring

point,

due to the slanting position of the wire; weeds

.

accumulate on

the wire influencing the results and must therefore be regularly

removed; and the elaboration of the observations needs some practical

skill, so that exper

i

enced operators are required.

Extent cf swing gives velocity

Angle in plan,

(3,

gives direction of flow.

k

tan

·

a

=

mg

where

F

=

effective area of pendulum body

therefore v

c

v'fi

V

tan

a

v

= (cons~ant) Vtan a

Due to drag on wire

a

1

I

a

2

and a correction must be applied.

I

~

Resistance body on

end of pendulum.

k

End

ot

pointer

Pendulum-pointer system

huhg from reference

frame by a separat

·

e

universal sU6pens10n.

Finally, with all the knowledge now obtained is it

possible to compute the discharge. For a certain flow condition in the river, defined by a certain water level, a suitable cross-section is selected, preferably at right angle to the flow; the cross-section is sounded and velocity and direction of the current

is observed in a number of verticals, and for each vertical in a

number of points. Integration of this information then yields

the

Q.

One way of calculation is to draw lines of equal velocity

(isovels) in the cross-section. The area enclosed within each

isovel is plotted against the value of the isovel and the area

of the diagram gives the total discharge (Fig. 13)

-

-

~~~:

l~LJY

t

Velocity profiles at 6uitablc

Cross-sûction of intervals

river

v

To'tal cross-secti on area

I

~'Area A

, dA

Total disch.::tr5u

Q dA