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Afdeling der Weg- en Waterbouwkunde

Vakgroep

Algemene en Verkeerswaterbouwkunde

PROF. IR. L. VAN BENDEGOM

REG U LAT ION S T R U C T URE S

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In the last sections a number of structures, applied in the regulation or training of the lowwaterbed of a river were mentioned. The majority of these structures consists of bankrevetments, groynes or spurdikes, and longitudinal dikes. They support the lowwaterchannel over a certain

length or in isolated points. These structures will have the main interest. Another group of structures only has the limited function of forming a certain resistance against flow, and are not situated at the border of the channel. An example is the enclosure-dam of a riverbranch. A combination of the two functions is found in the separationdam between two river-channels. In case of a groyne the head of this structure has the function of channelsupport, while the dike connecting this isolated support with the bank, has the function of resistant body against flow behind the support.

The group of bottom protection structures, discussed in section 6.2 being revetments just like the protective layers covering groynes etc. , will not be discussed separately.

From these structures the following properties will be successively discussed:

1, type and shape;

2, dimensions and levels;

3, construction and execution of core and revetment.

1. Type and shape

As has been discussed before, the fixation or constriction of a channel can be achieved by structures supporting the channel over a larger length (bankrevetments and longitudinal dikes), or by isolated fi~ed points (groynes, spurdikes). Between these extremes there exists the type of the rampart, a rounded fixed point, applied sometimes in case the direction of the current has a certain freedom in approaching the fixed Doint

(fig. 6.3-19).

Apart from this distinction in types there is the distinction between open and closed structures. Although the application of open structures has become an exception in river-engineering, a few remarks have to be made.

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I

Fig. 6.4-1

Pile clumbs acting as spurdike.

Open structures have been applied for the gradual closing of branches or as channel supports (fig. 6.4-1). They consist of rows of timber piles or pile-clumbs, giving a certain resistance against the flow of water through the structure at lower levels. Consequently they absorb a certain part of the energy of this water, resulting in sedimentation between the groynes (fig. 6.4-2).

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-,.

-,.

Fig. 6.4-2

Sedimentation in case of the application of open groynes.

At first sight the application of open structures seems attractive. The situation is not changed abruptly, and sedimentation, erosion and local scouring have a retarded and less pronounced character. However, in practice there are important drawba.cks. In case of open structures bottomrevetments have to be applied in addition, not only in front of the head of the spurdike where the current is accelerated, but often also between and downstrearn the piles where the current is strongly turbulent. In case of rivers, transporting in certain periods of the year floating vegetation, trees or ice, d9mage is often done and the costs of maintenance will be high. Therefore closed structures are normally applied. In case of closed structures these act as weirs with overflow when the stageoof the river is higher than the crest of the structure.

The choise between the different types of channel supports is mainly a question of costs, and depends on the situation of the bank with

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respect to the projected boundary of the channel. In fig.6.4~~

this is explained with the aid of a simple exarnple of channelcorrection.

Fig. 6.4-3

Types of channel fixation structures.

In this exarnple the irregular channel-alignrnent has to be changed into one large curve. The channelboundary chosen at the upstrearn side partly coincides with the existing bank. Logically here a bankfixation is preferabIe regarding costs and execution. In fig. 6_~4-4 the general shape of the cross-section of such a bankrevetrnent is given. Details of th is shape and the construct ion of the revetrnent will be discussed later on. Onderscrrijding in dagen ptr jaar 345-~60 180 cagen 20

à

40 dagen Fig. 6.4-4. Cross-section of a bankrevetrnent.

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In fig. 6.4- 3 downstream of this section the boundary of the future channel is situated landwards of the existing bank. If there is a dredger at hand the safest solution is to remove this part of the bank artificially. Another solution might be to leave this section unprotected for the time being till the river has eroded this part of the bank. There is some risk in doing so; moreover the execution has to be completed in a later period. Downstream of the above mentioned section the alignment of the future left boundary is situated in the existing lowwaterbed. As long as the distance

is small the best solution is the construction of a longitudinal dike, the cr-oss-jsectLon of which is shown in fig. 6.4-5

345-360 dg/jr 180

"

"

20-40

" "

""'7 ~~~ :::-./.~ ~

---

. Fig. 6.4-5

Cross-section of a longitudinal dike.

r

As long as the distance between dike and bank is small, the area between is filled with bedmaterial up to a height somewhat above mean waterlevel, offering the possibility of vegetation growing.xtn this case the inner bankrevetment of the longitudinal dike is only provisional and can be made cheaper.

If the distance between dike and bank becomes very large, such a fill would become very expensive. In that case it will be less expensive to construct at certain distances connecting-dikes between longitudinal dike and bank in order to prevent strong currents at high stages behind the longitudinal dike, followed byerosion of the bottom and the bank. If the distance between the projected alignment and the bank is moderate a cheaper solution might be the construction of isolated supports, that is of groynes or spurdikes (fig. 6.4-6). Now an important problem

x)

E.g. up to 340 days/year, allowing grass to grow regarding oxygen demand.

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comes to the fore, the distance of these SUDDorts. _--:: MIN 345-360 dg/jr _--:::;_-,--_ HoIJfi 180 -~ __ . l.M( 20-40

r'0.

h•tf .-G"'~/,4..,

" "

" "

: : ·Fig.6.4-6

Plane view and longitudinal section of a groyne.

In literature very limited information is given about this problem, and in case figures about groyne distances are given, these are not related to the stability of the channel but to the interests of navigation. In the following an impression is given about the factors governing this problem.

In section 6.3 .. it was made clear that the distance between successive groynes would have to be so small that no meandering of the whole channel in between the supports could occur. This would limit the distance till about five or six times the width of the channel.

However, there are reasons to take this distance much smaller. The

presence of stable strong eddies in the groynefields between successive groynes prevents the river flowlines from entering in the groynefield; in consequence it prevents also the development of strong accelerations and deep scouring in front of the head of the groynes.

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I

or bali bearings, who give support to the river channel. The larger this support, the smaller the attack upon the groynehead. Eddies which

penetrate further into the river decrease the attack upon the groynehead.

This depends on the centrifugal force of the eddy, so on

v

2jgor. Astrong

velocity and a small radius are favourable. Astrong velocity asks for

a favourable energy transfer between channel and eddy. This transfer is achieved by friction and by water exchange in helicoidal flow. This asks

for not too strong a vortex, so not too strong a velocity gradient

between channel and eddy; and for astrong curvature. A minimum curvature in case of one single eddy is obtained with circular flow; in consequence with a groynefield Hith a length more than twice the width offers no possibility for a single, stabie eddy. Another problem is the total surface of the eddy. If this is large, compared to the channel, not enough energysupply is possible, the velocity of the eddy will be small,

and so will be the support. In fig. 6.4-7 some examples are given.

I

Fig. 6.4-7

Eddy formation between groynes.

I

As a general rule it can be said th'at the length between groynes should

be limited till one or one and a half times the width of the channel; on

the other hand this length should not exceed twice the width of the groynefield.

Following another argumentation the waterlevel in an eddy will be nearly horizontal. The energyhead at the upstream groynehead will be almost

zero, at the downstream head somewhat larger due to the loading. At that

point the energy in the river must still be higher. This leads to the

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formula: in which:

I

I

=

energyslope river L

=

distance between groynes v

=

velocity in the river. Now is: v

=

ClhI or L « 1

Assuming C

=

45 m2/s, would give L « 100 h.

The problem of the distance of groynes in rivers constrmcted for

navigational purposes is of a different nature. In that case the eddies,

penetrating into the channel in order to to achieve energy, cause

accelaration and retardation of the current in the channel, hindering

navigation by this. This is the reason that in such a case it is advised

to limit the distance between groynes till about 0.6-0.8 times the width

of the river (fig. 6,4-8).

Fig. 6.4-8

Constricted river with groynes.

I

In all considerations about groyne distances it must be taken into

account that a larger distance results 'ina stronger current-contraction

and a deeper scouring in front of the head of the groyne, so in a more

expensive construction. On the other hand a distance which is too small,

will result in more groynes, being perhaps more expensive than a

longitudinal dike.

Another probleIT.in case of the application of groynes is the direct ion

of the spurdike. Some people prefer groynes pointing downstream in order

to reduce the current contraction. Others prefer groynes pointing

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simple and cheapest solution is the situating of spurdikes in such a

way that the shortest connection is made between groynehead and bank.

2. Dimensions and levels

The most important dimensions and levels determining the form of

bank-revetments and longitudinal and spurdikes are the height of the foot,

the height of the crest, the height and width of ber~s, and the slopes

of the construction.

Knowledge of the expected equilibrium-depth of the foot of a

bank-revetment, constructed in case of the dredging and fixation of a

short-cut, the closing of branches, or the fixation of an eroding bank

is important. It is less important in case the scouring in front of the

bankrevetment is left to nature (fig. 6.4-5) and the underwater

slope is protected in successive stages. Such a solution is the only

possible one in case of the absence of dredgers. However, this is not

only a risky undertaking because of the possibility of unexpected

slidings, but it requires also the execution of the works in a number

of stages. Therefore it is preferabie to dredge a cut in front of the

bankrevetment down till the calculated equilibrium depth, followed

by the construction of the revetment till that equilibrium depth

(fig. 6.4-4 and 6).- The problem now is the calculation of the

equilibrium bottomdepth in front of the structure. A first impression

of this depth might be obtained by comparing the riversection under

construction with other sections of the same river. However, such

other sections will hardly be found; capacity and radius will be different

and instead of channel stability shifting with erosion and sedimentation is more likely. Therefore, some calculations might add to the insight,

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be it calculations of a very rough nature. As has been discussed in chapter 4

dent parameters:

Q-S-D-I-b-h, ignoring in first instance the fact that these are only

a river is defined by six indep

en-averages. Other parameters, like roughness, meanders etc. are dependent on the foregoing. From these six normally three are given by nature; in case of a river in equilibrium-situation: Q-S-D, and in case of a river not in equilibrium Q-I-D. So in order to solve the problem three

equations are necessary. Two of them are the watertransport-formula (e.g. Chézy) and the sedimenttransport-formula (e.g. Meyer-Peter and Müller). The problem can be solved if a third formula or parameter is given.

x)

Zie voor een meer uitvoerige behandeling par. 3.2-sedimentmechanica van de handleiding rivieren en rivierwerken (deel 1, 1974).

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To start with the most simple example a long, straight and constricted river is considered. Moreover constant discharges

Q

and S, a homogenious grainsize D, and no moving sandbanks is assumed. In that case the botto~ in the cross-section will be horizontal and the height of that bottom can be calculated with the two above ~entioned formulas. The only difficulty is the roughness factor C of Chézy, which has to be measured in other riversections, assuming an equal value in the section under consideration.

A first complication occurs when the river is not straight, but is curved. In case of a long bend an equilibrium situation is reached wh ere the flowlines of bedload and mean current are parallel to the channelcurvature. In those cross-sections the bot tom is not horizontal but sloped. This slope can be calculated, for instanee with the van Bendegom formula:

I

sin ex

=

óh

=

h· I

ór liD h

r

Theoretically the factor A is about 20, but has to be established more accurately from observations in the river under consideration.

In this case the depth has not a constant value h; by integration the following value is found:

1

=

1

+

(l

h hO r

1 halo

-)

20-.lID

ra

in which the index 0 means the value of the parameter in the outer bend.

In combining this function with the applied formulas for water and sedimentdischarge, the cross-section can be calculated. This computation is.easily done by computer. If calculating by hand a number of values for hO and 10 have to be assumed! Interpolation ~ill giv.e the result.

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The next complication occurs in case the river is not constricted but

only regulated by fixation structures in the outer bends. In that case

the depth of the foot of the structures in these bends can be calculated

in the same way; only an assumption has to be made with regard to the

width of the channel. The growing of vegetation, followed by the silting

of the inner bend will limit this width and has to be established by

observation.

From the foregoing it will be obvious that such calculations will always be of a very global character and need comparison with the

natural situation. In that respect four complications have to be wentioned;

these are:

a, demixing of grainsize distribution in bends;

b, the problem of the "bedforming" discharge

Q;

c, the shifting of bends observed in nature;

d, the influence of isolated supports instead of continuous supports.

ad a. As has been discussed in chapter 4 demixing of the sediment

composition will take place in bends. This is caused by the fact that the

forces working on the grains in the cross-section are of a different

nature (gravity force working on the mass and helicoidal force working

on the surface)~ In the formula related to the bottomslope an increase

in diameter D will decrease the slope, and vice versa. As aresuit the

larger grains will move towards the deeper parts, the smaller grains

towards the high sandbanks. The consequence is that the strong slope in

the outer bend will decrease, the flat slope in the inner bend will

increase. In fig. 6.4-9 this is shown with the earLier-mentioned

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,

900 950

}

"

l

~

o

E 2 c: ~ 3 a. Q) "'0 5 +x ... o. = meetpunten = theoretische ---- = idem 6 -~---+-

I

-- -I I Fig. 6.4-9 straal r in m 1000

..

)( )( 1050 1100 1150

~

-

_

-

.

-

-

'+... -;c-_ - --.-.

-

~

t

-~

---3---l

--

I

_ - ..

-:~~

o 0 .;. ,.~

-

··

·

I

_

f·~

_

+

)

_.

_

~

---I-

----+

i---l

• • ~o +

bodemlijn (De constant) • I'

0-(0:: h) )(

+ I

-

-

-

-

---+

----

+---

x

Slope of a cross-section in a bend of the river Waal, measured in nature, and calculated with demixed and not deximed sediments.

In case of strong demixing the slope becomes nearly straight. In case of a bankfixation project investigations therefore have to be carried out about the demixing process in the river under consideration.

ad b. A second complication is caused by the variation in discharge

Q,

the phenomena of the and as aresultin water level h. In chapter 4.

"breathing" of a meandering river have been discussed. At high stages

the cross-section in bends will erode, in crossings sedimentate. Moreover at high stages the slope of the cross-section in bends will increase (fig. 6.4-10).

Fig. 6.4-10

-~ ..r

j, IV

-

- -

....

-

-Equilibrium of a riverbottom after a long period of high resp. low water.

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Our problem now is the calculation of the deepest possible scouring

in front of regulation structures in bends. A rough estimate can be made

by calculating the equilibrium at different high levels, taking into

account the duration of floods exceeding these levels.

ad c. Free meandering rivers are shifting their bends byerosion of the

outer bank on sedimentation in the remaining part of the cross-section.

This implies not only sedimenttransport in the direction of the riveraxe

but also a component perpendicular to this direction. The result is a

smaller slope in the cross-section bottom. In chapter 4 this has

been discussed more in detail. By measuring the factor A in the formula

for the cross-section slope this has to be taken into account. In case

there is an overall shifting of the bends with E meters per year We

found a decrease in cross-section slope, formulated by:

sin Cl êh A· hl h

0

rE

)

=

êr

=

flD r 3.10S.Sx

So in case of a scouring bend we can calculate the factor A, notwithstanding

the scouring. Observations about shifting and about the average bedload

however, are in .that .caae ft necesaaty .

ad d. All these calculations are related to the waterdepth in front of

a continuous bankrevetment. In case of isolated bankfixation points like

groynes the equilibrium depth will be larger. In case the distance between

the groynes is small the extra depth will be limited to 5 or 10%; but

in case of large distance this extra percentage will grow. However, this

optimation problem is not solved yet.

From the foregoing it can be concluded that an exact calculation of the

height of the foot of channelfixation structures after the establishment

of an equilibrium situation of the cross-section is not possible. Only a

rough calculation is available, and moreover there will be variation due

to waterdischarge and -level fluctuations.

With the calculation of the crest-height of these structures we meet

other uncertainties. In that case a distinction has to be made between

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having the purpose of leading away the current from branches to be

elosed, and structures for channel-eonstrietion.

For all these structures there is a minimum and a maximum height. For economie reasons the erestheight will be kept as low as possible. However, the erest of these structures must be strong and this asks for pitchings, so execution above the waterlevel. In order to give the contractor enough time for execution and maintenance the crestheight cannot be projected far below mean waterlevel. The maximum hèight of the structures is given by the height of the floodplain. The crest must remain below floodnlainlevel in order to prevent during high stages current concentration and erosion behind the root end of the groynes etc. So the possibility of variation ln crestheight normally is not much more than a few meters.

In case of normal channelfixation-points the aim is to erect a helicoidal flow of such a strength that channelstability is assured. If the erest-height of the groyne is relatively low, the helicoidal aetion might be weakened if the surfacewater is flowing over the groyne and out of the channel, resulting in a decrease of waterlevel-slope in the cross-section. This outflow will be neglected if the distance between groynehead and high bank of the floodplain is a nearly constant one. Moreover, the rootend of groynes is always sloped (1:100 till 1:200); this will reduee the outflow in case of long groynes.

In case of a furcation the risk of outflow and instability is increasing; in this case the erestheight must be higher than mean waterlevel. This is

also the case for closure dams in branches to be closed. Theoretically

the height of these dams must be such that even during high discharges no bedloadtransport may start in the closed branch downstream the dam. In practiee the erestheight may be projected at a lower level, beeause there will be sedimentation at lower level.

The problem of the crestheight in case of riverconstriction for

navigation-purposes is different. In this case the whole riverdischarge must be concentrated into the navigation-channel below the waterlevel, which gives restriction to navigation. For instance if the desired navigation-depth is 9 feet, the erestheight of the groynes must be projected at least 9 feet above the bottom in the dominant crossings. In reality this height must be increased in case of large variations in cross-sections at higher levels. These variations involve sedimentation and

scouring during high stages; during the fall of the river the tota1 discharge must be coneentrated in the low waterbed at an early stage

I

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in order to scour the undesired sedimentations. In the next section

this will be discussed further.

The slopes applied in bankrevetments, groynes etc. depend on the

quality of the subsoil and the type and composition of the revet~nt and

will be discussed later on. Normally underwater-slopes are between 1:2~

and 1:3~, while slopes constructed in the dry are somewhat steeper

(1:1! till 1:2!). In some cases the slope at the riverside of the head

of a groyne is constructed less steep (1:5 till 1:10). The aim is to

form a more gradual transition between eddy and main current by which

the vortex will become less pronounced, giving a decrease in scour.

Berms are often projected in order to facilitate the execution, to

give a better stability, or to form a good transition between two

types of revetments. In the figures 6.4-4, 5·and 6 ..J examples of

berms are given. The transition between the revetment constructed under

water and the revetment made above the waterlevel is situated at a level

allowing for construction and maintenance above water during abóut 2-3

months a year.

3. Construction and execution of core and revetment

For economie reasons the core of bankrevetments, groynes and longitudinal

dikes will, if possible, be made of the bedmaterial of the river. In

case of strong currents, for instanee near the head of groynes, or in

case of closure dams this is not always pOBsible. Then the core-material

will consist of stone or sunken mattresses or sausages, loaded with

stones, and washed in with sand in order to limit the settling of the

core. In case of a bankrevetment the core is already present, only same

profile dredging or filling will be required. Filling up will never be

done by hydraulic fill, the natural slope becoming toa flat. Dumping

by hand or crane will be necessary. Dredging underwater-slopes has to

be done with the aid of dredgers (bucket or cutter), the application of

draglines giving very poor results on greater depth. In general it can

be stated that the construction of structures, mentioned abave , asks for

floating equipment, and in consequence for navigable rivers. Is the river

not navigable than it might still be possible to supply small floating

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Revetments

In general the purpose of revetments can be threefold: a, to protect the subsoil against erosion;

b, to form a lining, a watertight cover in order to prevent seepage; c, to influence the bottomroughness.

In the case of regulation structures for rivers the protective function is the most important one.

In the following first the requirements for revetments will be discussed with the acting forces and the calculation of dimensions; af ter that a number of examples will be given, having not the pretention to be complete. The requirements are:

1. The subsoil under the revetment (slope or horizontal) must be stabie, and remain stable in case of foreseable scouring at the edge (foot) of the revetment;

2. The revetment as a whole must be stable, so no lifting, sliding etc; 3. Inside the revetment no disturbance of the equilibrium, for instance

between successive layers, may occur;

4. The cover-material of the revetment must have stability with regard to the forces of currents and waves and ice;

5. The subsoil particles direct under the revetment must remain in place, so no creep in case of a watertight revetment, and no sucking through in case of an open revetment;

6. The construction must be durable with regard to mechanical, chemical and organic attack;

7. Execution and maintenance must be feasible.

In the following these items will be discussed somewhat more in detail.

1. Stability subsoil

Calculation of stability is norrnally done with the aid of circular sliding planes, unless the presence of clay or peatlayers gives a motive for

other planes. The determination of the safety-coëfficient is a difficult point. Groundwatertable and groundwaterflow play a dominant role.

Therefore special attention should be given to these factors, in particular ~ and peatlayers; including measurements of groundwatertable and [case orl

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I

riverlevel during falling river, and in case of sudden drop in riverlevel (translation- and shipwaves). In case of an homogenious and pervious subsoil and a normal Q-value the equilibrium-slope normally will have the following values:

revetment under water - 1:3 till 1:4

pitching above water - 1:1~ till 1:2~

grass covered slope - 1:6 till 1:10

2. Stability revetment

A first requirement for revetments is flexability. If it could not absorb the settling of the subsoil, the erosion of the foot, or the irregularities in the dredged underwater-slope, it would be broken, or sliding down due to lack of shear stress with the bottom.

A second requirement is related to this shear stress with the subsoil. Neglecting for the time being groundwaterinfluence this shear stress must be smaller than the critical shear stress. Therefore the critical

shear stress must be as large as possible, meaning a rough transition between revetment and subsoil and no layers in between who decrease this value (decay of straw, etc.).

The shear stress caused by the revetment comes partly from the water flowing over the revetment, and -in case of a slope- partly from the component along the bottom of the weight of the revetment. The shear stress from the current will be very small; in case of a horizontal bottom and a normal frequency distribution of T, extreme values will be of the order of magnitude of 5 p ghI N/m2, while in case of a slope

w

this value will be still smaller, perhaps 4 p ghI. This value can

w

be neglected in comparison with the gravity component along the slope.

50, if no groundwater comes into the picture, theoretically the angle a of the talus must remain smaller than the critical angle of shear stress ~. This would lead to a maximum angle of the slope of 30° till 400 (1:1~). However, this is much too steep. The reason is

the influence of groundwater, which is nor~ally dominating in this problem. Some calculations will make this clear.

A simple example is the impervious slab, made of concrete or asphalt on a horizontal riverbottom (fig. 6.4-11).

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Fig. 6.4-11

Stability of a bottomrevetment.

The horizontal shear stress caused by the current and acting upon the slab is:

K

=

T ~ 5 P ghI N/m2

max

w

The vertical force is:

G

=

(p - p ) gd N/m2

m w

If the critical shear stress between battom and slab is given by the

angle ~ we obtain:

-2

Changing hl into ~, and adjusting for C2 the unfavourable value of

about 1000, and tg ~

=

0,5 gives the result:

d >

In case of a current of 3 mis, and a p

=

2400 (concrete blocks)

material

we come to: d

=

0,065 m (in case the surface of the slab is hydraulically

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I

Now the influence of groundwater.

If the porosity of the revetment is very large in relation to the porosity

of the subsoil, groundwater flowing from the subsoil towards the surface-water can hardly build up an overpressure under the slab; Only in case of

a sudden drop of the waterlevel, some retardation in pressure drop under the poreus slab can occur.

The other extreme is a watertight revetment. Nowa drop in waterlevel will not be followed directly by a drop in groundwater-pressure. The overpressure is the same as a decrease in specific density ~. We can write: d .Q..d

v

2 ~ ~l 2g in which: ~ - I groundwater

z

= ~

-d

We can also write:

d >

+

z

ó. In our- case: d > ~ 32

+

0,30 1,4 20 1,4 d > 0,065 + 0,215

=

0,28 m

The influence of groundwater dominates.

The problem becomes more complicated in case of a watertight revetment upon a slope. In case of an asphalt revetment a number of complications would have to be taken onto account: the plasticity of the material, the possibility of creep between revetment and subsoil, the waterpressure in the hardly porous asphalt, the possible absorption of stress, fatigue of material, etc. For these complications reference is made to literature; here a simplified and not quite correct formula is given, assuming:

1, every part of the revetment must be in equilibrium, so no stress, cre ep etc.;

2, the revetment is watertight only in the underside of the slab (plastic sheet). This gives the formula:

I

d >

z

(fig. 6.4-12)

ó.·cosa (1 - ~)

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Fig. '6~4-12

Equilibrium of a watertight revetment uron a slope.

In case there is a current over the revetment the thickness of the

slab has to be increased a little bit.

3. Internal equilibrium

There are a few examples where the equilibrium of a revetment was

disturbed by actions inside ·the construction. An example is the revetment

consisting of a number of filterlayers. If the critical shear stress

between two layers would be smaller than the angle of the slope, e.g. in

case of a rounded rivergravel, sliding might occur. Another sliding

inside might occur if the porosity in upward direction is decreasing.

In this case an overpressure might be building up inside.

In case of asphalt revetments the problem again is very complicated. If

the quantity of bitumen would be a few percent too large, the skeleton

formed by sand, filler etc. would loose its stabie structure and start

flowing down the slope.

In the past asphalt revetments were sometimes composed of a number of

layers, the bottomlayer sometimes having a larger porosity than the

toplayer. So overpressure could build up directly under the toplayer,

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4. Stability of covermaterial

Many revetments, in majority pervious but sometimes also impervious ones, have a coverlayer of dumped stone or of basaltpitchings, concrete blocks, etc. Their function is to proteet the underlying structure against the forces from currents, waves or against other possible damage; or to provide the necessary load upon the revetment.

A

first requirement for this coverlayer is stability. For this calculation we apply the same formules as in case of bed-Ioad in an alluvial river. In this formula:

hl

~D < ~cr~t.

.

we gave ~ the value of 0.05 in case of a hydraulic rough bottom, without ripples and with a normal frequency-distribution for the shear stress T. In that case there is already some movement of sediment. In case of arevetment this movement must be reduced further, especially in case of a slope. Here we have an optimation problem, the one extreme

'being a dimension of stone preventing every movement even under extreme shear stress, resulting in hardly any maintenance; the other extreme small stones with much maintenance. Therefore the figure for ~. is

cr~t. chosen more or less lower than 0.05, for instance 0.03 or 0.04. In case of a normal turbulence and T-distribution this gives aresuit being in accordance with measurements. However, revetments are often constructed in areas around structures or downstream outlets, etc. where turbulence is much larger resulting in a requirement for larger stones. If in the formula:

hl

~D or d >

hl ~~

the term hl is changed into ~v2 , and instead of y2 the Bernoulli-term y2/2g is given, the formula changes into:

d >

A

y2

~ 2g

in which

A

= ~~~

In normal circumstances a safe result will be found with C2

=

1000 and

~ =

~.03 resulting in A

=

0.7. In case of heavy turbulence measurements have shown this factor becoming about two times 0.7

=

i.4.

(24)

So

A

=

0.7 1.4

small turbulence + heavy turbulence + rounded stone square stone more movement allowed + no movement alloweq,

In case the velocity distribution in the vertical is abnormal, the factor A has to be adopted; after flowing over a concrete slab the bottomcurrent will be larger than normal; the same when the flow is

contracted over acrest.

If groundwater is flowing out of the bottom, the term ~ must be decreased as has been mentioned before.

If the stone is situated on a slope the gravity-component along the surface must be taken into account, resulting in:

d > A !:J.

v

2 2g 1 2 ' sin a sin

z ~

When ordering stone from a.quarry normally not the diameter but the weight of the required stone is given in pounds or kilogram-force. In this case we are also accustomed to work wi th the old "specific weight". Quarries cannot deliver stone in one diameter orweight;a certain

weight distribution has to be accepted, and is also preferred in order to limit the pore volume.

In connection with this pore volume the question arises of the required depth of the stonelayer. In order to have a good protection of the revetment this depth must be at least 1~ till 2 times dSO' If the

execution is inaccurate -for instance dumping in deep water- this figure will have to be enlarged.

I

Instead of dumped stone sometimes basaltpitchings, concrete blocks, etc. are applied as coverlayer for the revetmept (fig. 6.4-13)

(25)

Fig. 6.4-13

Concrete block pavement upon gravel filter.

The advantage of such a construction is the smaller attack by currents and waves; the disadvantage the necessaty to construct such a layer in a dry pit. Maintenance and reparation is sometimes possible under water with the aid of frogmen etc., but is expensive.

The depth of such a pitching-Iayer is not given by the tractive force

of the current; in case of a smooth surface the factor A will be small as has been explained in the foregoing. Now the diameter and the weight of the blocks depend mainly on the rapid changes in pressure, vibrations, caused by turbulent flow variations. About this phenomenon little is

known and it will be wise in case of strong currents not to apply the

formula for d with the small factor A, even in case of a good porosity.

5. Stability of soil particles under the revetment

This factor plays sometimes a role in case of impervious revetments; it is always of importance in case of pervious revetments.

If, in case of an impervious revetment, e.g. an asnhalt slab, pressure

variation can lift the revetment locally, sediment particles will start

moving gradually down the slope due to this vibration. The end will be

the cracking of the revetment in the lower part of the slope. Although

this phenomenon will especially occur in areas with strong wave action

and seldom in upper rivers, it will be wise to make the weight of the revetment so heavy that local lifting becomes impossible. The calculation

(26)

revetment and subsoil. In case the variation in pressure is a slow

process the depth of the layer must be larger than

d > z

but in case of impact and vibrations this deDth must be much larger.

In case of a pervious revetment the problem of the filter becomes important. A pervious revetment always has two principal functions: 1, the filterfunction, allowing the passage of groundwater without

much resistance, but preventing the sucking through of underlaying

particles;

2, the protective function, resisting the attacks of current and

wave-action.

I

The last function has been discussed in the foregoing. The filterfunction

has been studied in laboratory, resulting in a number of regulations concerning the composition of the filterlayer in relation to the under -lying layer. Important for the prevention of sucking through of particles

is the quotient:

dsO filterlayer

dSO underlayer

But also important is the prevention of blocking the filterpores by

underlaying fine particles, given by the quotient:

d1S filterlayer d15 underlayer

The permissable quotient depends on form, gradation and packing of the filterlayer, and may vary from 5 tillSO (5 - round, badly packed,

homogenious grains, 50 - square, carefully packed, heterogenious grains). If the subsoil to be protected consists of fine sediment (e.g. d

=

100 ~m) and the attacking current asks for coarse stone (e.g. 0.20 m) it will not be possible to suffice with one filterlayer. A number of layers will be necessary, each layer being the underlayer for the next filterlayer.

In fig. 6.4-14 an example is given of successive filterlayers between subsoil and protective coverlayer.

(27)

"

l ~ ~ h>(1 ~M ~

,

~ ~ ';0 ~ ~ !)t> ,0 10 \

r-,

"'-\

~

1\

'S.

"

~ ~ r-~ ~ ~ '"

:.-r\..

"

-,

'",

.

10-I 10-2 10•..s I"-iO Fig. 6.4-14.

Desired sieve-curves of successive filterlayers.

I

A nroblem is the depth of each of the layers. This depth must be such

that in each pore the Hidth is sufficiently restricted. This is a theoretical probability problem. In nractice the following figues can

be given for this minimum depth:

coarse sand - 0.10 m - 0.20 m

1~ till 2 times d.

gravel coverstone

These figures have to be enlarged in case of inaccuracy in execution.

In the foregoing the filters are made of non-cohesive material like sand

and gravel. However, many other filters have been applied all over the world. Well-known examples are filters made of vegetation, like mattresses,

sausages, etc. Modern examples are cohesive filters made of polyethene of

polypropene fabrics (fig.6.4-15).

Fig. 6.4-15

Mattress of Nicolon propylene fabric covered by nrotective reed, to

(28)

The main problems encountered in these applications are of ten related to execution and durability.

6. Durability of materials and construction

Apart from the mechanical forces working upon the revetment like currents, waves and groundwaterflow there are other mechanical, chemical and organic forces responsabie for possible destruction of revetments. In the group of mechanical forces there are frost and ice, ships-propellers and anchors, sport-fishers and youth.

Chemical attack sometimes has an unfavourable influence like hydrocarbons, oxygen, oil.

Organica life sometimes might be the reason for decay, like toredo, algae, weed and vegetation growing even through asphalt.

7. Factors related to execution and maintenance

The possibilities, which the river under consideration offers for the execution and maintenance of the revetments, are of ten decisive in the choice for the type of revetment.

These possibilities are related to:

a, execution and maintenance above the waterlevel or under water; b, execution and maintenance from the bank or from the waterside; c, execution and maintenance with or without the aid of big mechanical

tools (dredges, cranes, selfunloading barges); d, execution directly or in stages.

ad a. Revetments constructed above the waterlevel are preferabie; the execution is more simple and of ten cheaner, and the constructions are stronger and visible for inspection. However, the contractor must have areasonabie period for execution and maintenance. Therefore, such a solution is only possible in that part of the revetment which is above the water level about 60 days a year. At that waterlevel there is aften the transition between the revetment constructed above water, and that part of the revetment constructed under water.

ad b. In case of a navigable river the sup~ly of materials and equipment is of ten easily achieved by waterway; by land the site is in many cases only attainabl~ at high expenditure. Moreover, in such a case for the underwater revetments a feasible type could be chosen, that is the coherent, floating filter mattress, constructed on a slipway elsewhere,

(29)

transported to the side, and ballasted with cover stone.

ad c. Nowadays an economie execution of regulation structures is hardly

possible without modern floating equipment, like dredgers, and sand -and stone-dumping barges. Cranes and slidin~ gutters operating from the

banks can never replace such equipment satisfactorily and efficiently

without extra costs.

ad d. In regulation projects the risk factor is always great, and the

I

final costs usually appears to be higher than the first calculation.

In selecting the method of execution it seems at first sight attractive to leave a part of sediment removal to nature. But the risks will

increase in that case and often the total costs of execution and maintenance will appear lower by dredging the banks and channels directly into equilibrium situation.

Examples and choice of revetments

In studying a regulation project an investigation has always to be

carried out with regard to the possible presence of natural resistant

banks, like rocks or stiff and hard claylayers. In case of the presence

of such natural channel supports, the alignment of the channel ~ight be projected in such a way that use is made of these supnorts.

In case revetments have to be applied, a first choice has to be made

out of three categories, namely filter revetments, watertight revet~ents

and an intermediate category, watertight from inside but open from

outside~ A number of examnles of these categories is given in

fig. 6.4-16.

If there is no special reason to choose for the second or third category, the filter revetment will always be chosen for underwater pevetment. For

the revetment constructed and maintained above the waterlevel all categories are acceptable unless the groundwater cannot build UD an

overpressure.

A very important factor in the choice of revetment type is the nearby

availability of cheap revetment materials. If there are rocks and quarries

along the river a cheap construction might be the dumping of thick

layers of stone unon the slopes. However, this solution will normally be

(30)

IMPEBMEesLE REVETMENTS

~- GRIISS ON CLIIY

GRIISSONCLIIY IIRMOUREO WIT H PIltHING STONE

T T T T

I

CONCRETE BLOCKS WiTH

I-.&__ L-_...JL-_-''__'' - ASPHIILTIC BITUMEN

~\:;;;;t~t;;;;;~(~;1~

-

C

I

O

.

NCRETE BLOCKSON

'"

!

.

i

PLASTIC·SHEET

STONE GROUTEO WITH

IISPHIILT

- ASPHALTCONCRETE

REmMENIS IMPEBMEABLEFBOM INSlpEI

PERMEABLE FROM OIUSIOE

--CONCRETE BLOCKS , J - FINE GRAINEO GRAVEL / . ~'/.-CLAY • --PlTCHING STONE , -RIP-RAP - BRICK-LAYER -CLAY Fig. 6.4-16 PERMEABhEREVETMENTS - COVER STONE

}_ SUCCESSIVE FILTER LAYERS

. OFSIINO ANC GRAVEL

- COVER STONE

'- DNELAYEROF ("'AVEL - NYLON TISSUE

~-COVER STONE

;;W

LAYEROF BRAlDEO Azolll

-STRIPSONFILTER

MATERlAL

~

r

r

:

--:

';_-CO

,

VERSTONE FILTER MATTRESSOF

"""..."""'''''''''''''...,oQ,....'''"'~

-

FASCINE OR OTHER MATERlIIL

~-COVER STONE

.• . - S.R.O.-SIINO

STONE PACKEDIN EXPANDEDMETliL

COARSEGRAVEL PARrLY WITH PRE PACT MORTAR

~ CONCRETE BLOCKS WITH ~-NYLON TISSUE

I , NYLON TISSUES FILLED UP - WITH PRE PACT - MORTAR

(31)

I

too expensive; the stone must be large in order to resist the currents, and at the same time an acceptable filterworking is necessary. A

better solution might be to dump in layers (fig. 6.4-14).

Above water this is easily possible; in that case however a pitching

is preferred. Underwater this solution asks for gutters and the

execution becomes very complicated.

Instead of non-coherent filters, consisting of sand and gravel layers,

preference is given to coherent filters, mattresses, made of braided

material. In the past this material consisted of vegetation material,

like bushgrass, brushwood and willow, bamboo, lianas, plywood, etc.

This material is still applied, but nowadays a large development is

going on in the construction of filters made of artificial textiles,

like nylon, propylene, etc. However, these textiles are too slack and

have not enough floating capacity. Therefore they are often affixed to

floating and stiff material like straw, reed, etc. This development

of floating matt~esses, consisting of combined materials has not yet

(32)

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