Sediment exchange between the main channel and the groyne fields of a river

Sediment exchange between the main

channel and the groyne fields of a river

Design of a mobile-bed experiment

Sediment exchange between the main

channel and the groyne fields of a river

Design of a mobile-bed experiment

Mohamed F.M. Yossef

Delft University of Technology

Faculty of Civil Engineering and Geosciences

Section Of Hydraulic Engineering

Delft cluster project no. 03.03.04

F O R W A R D

This report describes and presents the design of mobile-bed laboratory experiments that aim to study the sediment exchange between the m a i n channel and the groyne fields of a river. The experiments are to be conducted i n the Fluid Mechanics Laboratory of Delft University of Technology, D e l f t , the Netherlands. The experiments are part of the author's P h . D . study. This P h . D . study is carried out w i t h i n the framework of D I O C Water, theme 1.3 (Intermediate-scale morphological developments i n rivers due to human interventions) and D e l f t Cluster, theme 3 (Coasts and Rivers).

T A B L E O F C O N T E N T S

Forward ii Table of Contents iii

1. Introduction 1 2. The Need for Experiments 2

2.1. W h a t to model? 3 2.1.1. Flow pattern 3 2.1.2. Sediment transport 3 2.2. A i m of the experiments 4 3. Experimental Setup 5 3.1. Dimensions 5 3.2. Tests programme 7 3.3. Sediment transport 8 3.4. Hydraulic aspects 9 3.5. Measurement procedures 9

4. Time and Cost 11 4.1. Timetable 11 4.2. Cost estimate 11 References 12 Appendix I 14 Appendix I I 23 Appendix I I I 24 i i i

1. I N T R O D U C T I O N

W i t h i n the framework of the research project "Ruimte voor Rijntakken" (in English: "Space for the Rhine Branches") several measures have been devised to achieve a de-crease of the water levels at peak discharges, by means of enlarging river space. A wide range of fourteen measures is mentioned i n detail i n the 'Landscape Planning River Rhine' (LPR) report, see Figure 1. One of those measures is lowering of the existing groynes.

inside the dike |

13 < ^ r ^ l 2

winter bed

flood plain summer bed flood plain

1 = narrowing 2 = lowering of groynes 3 = dredging 4 = redumping of sediment 5 = permanent layer G = natural bank 7 4 * flood level low water level

7 = removing summer embankment 14 = dike raising 8 = secondary channel

9 = lowering of flood plain (excavation of clay/sand) 10 = nature development

11 = removing of high-water free areas 12 = dike reinforcement

13 = dike repositioning

Figure 1 River engineering measures as proposed in (LPR) report

However, i f the groynes are lowered the balance of hydrodynamic forces acting on the groyne fields w i l l change, and there w i l l be a large-scale morphological impact. This may involve t i l t i n g of the entire river; similar to the effect of the normalisation works i n the first half of the previous century. I t may also necessitate dredging or other maintenance measures, so as to ensure a sufficient flood conveyance capacity, sufficient navigable depth, stability of structures, etc.

To carry on w i t h such a plan, a thorough understanding of the effect of groynes on the morphology of the river is necessary. The sediment exchange between the groyne-fields and the main channel needs to be more comprehensible.

According to ten Brinke et al. (1999), erosion of sand is thought to result f r o m currents and ship waves at moderate river stages. Deposition of sand probably takes place mainly at times of high discharge, when the groynes are completely submerged. D u r i n g these flood events, sand is transported f r o m the main channel into the groyne fields and f u r t h e r landward onto the natural levees.

Lately, Lauchlan (2001) conducted experiments to investigate the sediment transport over steep slopes as the case of the upstream face of a groyne. The results of the experi-ments showed that the capacity to model sediment transport for this specific case is very

l i m i t e d and i t highlights the importance and the need for experimental work when studying the sediment transport for the case of complex geometry and near structures.

2. T H E N E E D F O R E X P E R I M E N T S

Ten Brinke et al. (2001) estimated the sediment balance for the Waal River d u r i n g the last three decades and particularly during the high water period of 1995, see Tables 1&2. From Table 2, we can deduce that during high water conditions, the groyne fields sedi-mentation plays a m a j o r role i n the t o t a l sediment balance of the river. Yet, on the long r u n as we see f r o m Table 1, there is no net erosion or deposition f r o m / t o the groyne fields section. Accordingly, we can conclude t h a t the amount of sediment t h a t is depos-ited during flood conditions is released d u r i n g normal t o low flow conditions, which is also confirmed by ten Brinke et al. (1999). T h i s result emphasises the role t h a t groyne fields play i n the t o t a l sediment balance of the river.

Table 1 Sediment balance for the Waal River during the last three decades after ten Brinke et al (2001) Sediment source/sink Volume x 1000 1970 - 1990 (m3/year) 1990 - 2000

Input sediment (US) +682 +507

Output sediment (DS) -761 -578

Bed degradation +489 +264

Groyne fields erosion/deposition — —

Floodplain deposition -53 -53

Dredging -357 -140

Table 2 Sediment balance for the Waal River during the high water of 1995 after ten Brinke et al (2001)

Sediment source/sink Volume x 1000 (m3)

Input sediment (US) +250 Output sediment (DS) -270 Bed degradation +573 Groyne fields erosion/deposition -373

Floodplain deposition -180

Dredging 0

Currently, there is a gap i n the knowledge concerning the sediment exchange process between the river's m a i n channel and its groyne fields. W i t h our present modelling capabilities, we are unable to include this role i n our large-scale morphological predic-tions. I n fact, even on the small-scale level the groyne fields sedimentation/erosion is not reproduced adequately.

To include the contribution of the groyne fields i n the sediment balance of the river we need t o f u r t h e r understand the sediment exchange process between the m a i n channel region and the groyne fields region.

W i t h the implementation of horizontal large eddy simulation (HLES) i n D e l f t 3 D , i t is now possible to simulate the flow near groynes and inside the groyne fields i n a satisfac-tory way see for example Jagers & van Schijndel (2000). I n addition, the morphological pattern in the main channel is better reproduced, see Yossef & Klaassen (2002). However when i t comes t o the morphology of the groyne fields, the results are still poor.

2 . 1 . W H A T T O M O D E L ?

W h e n studying the sediment exchange between the main channel and the groyne fields some parameters are of importance and need t o be considered. For example the flow pattern i n the groyne filed region, the secondary flow contribution, the suspended sedi-ment distribution and the role of sedisedi-ment diffusion. I n the following sections we present some of these parameters.

2.1.1. Flow pattern

A well-reproduced flow field is essential to further study the sediment exchange process. I t is now possible to reproduce the flow pattern using D e l f t 3 D for the emerged case i n a satisfactory way, see for example Jagers & van Schijndel (2000) and Yossef & Klaassen (2002). Yet, for the submerged condition the resulting flow pattern is still unsatisfactory. The emerged case i n the absence of navigation induced water motion is of a minor importance for the total sediment balance of the river. On the other hand the submerged case is significant as i t explains the filling process of a groyne field w i t h sediment.

T h r e e - d i m e n s i o n a l i t y

Peng et al. (1997) studied the case of submerged groynes. They compared three-dimensional numerical results w i t h experimental results and f o u n d t h a t the flow pattern i n the case of submerged groynes shows strong three-dimensional features behind groynes. The recirculation size at the back of the groyne is reduced gradually as the top of the groyne is approached. Consequently, the reattachment length decreases f r o m b o t t o m to top planes. The location of recirculation centre also varies i n Z-direction. I t moves f r o m the t i p of the groyne (near the bed), towards the bank (close to the top surface plane). I n the upstream face of the groyne, the flow shows an upward motion because of the blockage effect of the groyne.

S e c o n d a r y flow c o n t r i b u t i o n

Another feature for the flow pattern i n the case of submerged groynes, is the secondary flow structure. I t occurs simply because of the disturbance t h a t groynes present. The secondary flow (in YZ-plane) has the same direction in both regions on top of the groyne and behind the groyne i n the recirculation region. The flow direction is towards the mid-channel near the water surface and towards the mid-channel banks near the bed, see for example Peng et al. (1997) and Krebs et al. (1999).

2.1.2. S e d i m e n t t r a n s p o r t

The sediment concentration i n the main channel is relatively high when compared t o that inside the groyne fields, which implies a large contribution of sediment diffusion

f r o m the main channel to the groyne fields region. However, the results of Delft-3D shows that the concentration drops rather fast i n the transverse direction while entering the groyne fields, and nearly no sediment could enter the groyne fields region.

For example; one of the i m p o r t a n t parameters i n a 2D depth-averaged morphological computation is the sediment horizontal diffusion coefficient (e„). The value of which determines to a certain extent the shape of the transverse concentration profile, conse-quently, determining the deposition p a t t e r n along the normal line and f u r t h e r i n the groyne fields section. Increasing the horizontal diffusion coefficient (e ) d i d n ' t improve the results significantly. W h e n f u r t h e r increasing (e^), a different unrealistic deposi-tion/erosion pattern occurs, i n which the local scour holes near the tips of the groynes disappear and a deposition ridge develops through the normal line see Figure 2.

0, ? ?

Slrrl Ittm; 2001/OS/2S OacOCD Erd T*™ 20O1 /07/2S 5ï-tt7

• r-s i • <-Ci6 FJt-CH I •—If II II 1- p PJ<-üfl

I I J

— I

i SI

J— iL JU—_ —=4L Ii— gS? n<oj a<o.' • « a • 03_7 • « 0 4 WO.* • <o.f • rt.fi • <dj • < m • >0J -I — j

—| -

—r -I -I-I -I-I if ^ = 1 1 1 1 1

Figure 2 deposition/erosion pattern for a channel with groynes unsubmerged condition Left panel: exy = 0.001 m2/s, right panel: exy — 1.0 m2/s

To correctly choose a diffusion coefficient the transverse concentration profile needs to be known beforehand. There is a gap i n our knowledge concerning this point and such a profile is still t o be investigated.

Moreover, the difference i n the results i n Figure 2 is not only because of the high sensi-t i v i sensi-t y of sensi-the model sensi-to sensi-the diffusion coefficiensensi-t, busensi-t also i sensi-t is due sensi-to sensi-the formulasensi-tion of sensi-the sediment transport model. The role of the horizontal large eddies (high vorticity and high turbulence intensity) on the sediment transport process is not included i n the model formulation, and need t o be f u r t h e r investigated.

2 . 2 . A I M O F T H E E X P E R I M E N T S

To gain insight into the mechanism governing the sediment exchange process between the main channel and the groyne fields mainly during high water conditions i.e. when the groynes are submerged. W i t h the gained knowledge i t is hoped to:

- I f possible, devise a relation between the discharge stage represented i n the submer-gence level and the erosion/deposition pattern i n the groyne fields.

- Investigate the validity of the current sediment transport models i n mixing zones, i.e. i n areas of high turbulence intensity and high vorticity.

The f o r m a t i o n of scour holes and groyne flames could be as well reported, but i t is not a primary objective of the proposed experiments.

3. E X P E R I M E N T A L S E T U P

The basic consideration of setting up the experiment is based on the experience gained f r o m previous experiments concerning the flow pattern for a river w i t h groynes by U i j t t e w a a l et al. (2002), and sediment transport over steep slopes by Lauchlan (2001).

3 . 1 . D I M E N S I O N S

The dimensions of the experiment are based on the River Waal dimensions (Table 3) as a 'prototype', and indeed the space available i n the laboratory (L = 20 m , B = 2.0 m ) . W h e n choosing the dimensions of such experiments, some dimensionless relations are of importance t o guarantee a well representation of the flow characteristics. For example: - Groynes' spacing to length (S/L) ratio governs the flow p a t t e r n i n the horizontal

plane.

- Groynes' spacing to height (S/hg) ratio governs the flow pattern i n the vertical plane.

- The groyne side slopes, govern the strength of the down flow component and the vortex formation.

Table 3 Dimensions of groynes in the River Waal, after Schans (1998)

Parameter Min. Max. Mean Stdev. Median Mode

Groynes spacing (S) (m) 50 420 198.2 37.7 200 200 Groynes length (L) (m) 0 175 67.9 28.6 65 50 Orientation of a groyne (deg) -30 10 -8.0 8.7 -5 0

Spacing—length r a t i o (S/L)

The (S/L) ratio determines the number and shape of the horizontal eddies t h a t f o r m along the normal line and inside the groyne fields, see Uijttewaal (1999). A n aspect ratio close to unity gives rise to a single eddy. A larger aspect ratio i.e. f r o m 2 to 4, gives room for two eddies: a large one called p r i m a r y eddy i n the downstream part of the groyne-field, and a smaller secondary eddy emerging near the upstream groyne. A n extremely long groyne-field leads t o the penetration of the main flow into the groyne-field. I n the River Waal the (S/L) ratio is around 3. This means that the dimensions of the experi-ment should be i n the range of (S/L = 2 to 4). Considering the available space, a value of S/L = 2.85 is found t o be convenient.

Spacing—height r a t i o (S/hg)

As (S/L) ratio controls the flow pattern i n the horizontal plane, the (S/hg) ratio governs

the flow p a t t e r n i n the vertical (XZ) plane. Detaching flow over the upstream groyne may reattach the groyne-field bed and the bed shear stress recovers its large value (that is usually reduced because of the groynes), i f the groynes are spaced far apart. Too close groynes w i l l prevent the flow reattachment to the bed maintaining the bed shear stress at low value, see Peng et al. (1997). Further, Okabe (1988) noted some increase i n the drag coefficient associated w i t h the groynes w i t h the increased (S/h ) ratio.

Information about the extension of the separation region i n the lee-side of a groyne could be deduced f r o m the case of a back-facing step. For turbulent flow w i t h high Reynolds number, the separation region extend (7±0.5) times the step height, see for example K i m (1978), and Jovic (1996).

I n the River Waal S/hg = 28, i.e. the flow reattachment to the bed takes place; this

should be guaranteed i n the experiment as well. A m i n i m u m value of (S/hg) should be

(7.5) and preferably a value of (28) t o fully represent the natural situation. However, the space available doesn't allow such a large (S/hg) ratio. Three values of S/h w i l l be

utilised which are 16, 20, and 26.5 for three different test cases.

G r o y n e ' s side slopes

I n a flume test i t is much more handy to use a vertical groyne rather than a sloped face one. However, Lauchlan (2001) reported i n her experiment a significant difference be-tween the sediment transport over a vertical weir and over a sloped weir, a situation that is to some extent similar to the transport over a groyne. I n nature groynes are generally sloped (3:1, H : V ) . Thus, a sloped-faced groyne w i t h (3:1) is more representative of reality.

S u m m a r y

I n the light of the above-mentioned arguments, the dimensions of the proposed experi-ments could be summarised i n Table 4, and Figure 2.

Table 4 Summary of the dimensions for the proposed experiments Test Unit Parameter G l G2 G3 Unit Groyne height (hg) 0.125 0.100 0.075 m Groyne Length (Lg) 0.65 0.65 0.65 m Groynes spacing (S) 2.00 2.00 2.00 m Spacing to length ratio (S/Lg) 3.08 3.08 3.08

—

Spacing to height ratio (S/hg) 16 20 26.67

—

Dimensions in meters -—ZD >,—• | ! 1 1 1 1 1 1 Fkow d i r e c t i o n

L

A A A A

^ _

Figure 3 Experimental flume with proposed set-up

3 . 2 . T E S T S P R O G R A M M E

I t is proposed to undertake a mobile-bed experimental investigation simulating the case of a river w i t h groynes. I t is envisioned that b o t h suspended-load and bed-load sediment movement w i l l be simulated.

P i l o t test

I t is proposed t o conduct a pilot experiment prior to the main experimental programme i n order to:

- ... identify the morphodynamic process t h a t take place because of the presence of groynes b o t h i n the main channel and i n the groyne fields sections.

- ... quantify the morphological time-scale of the different process, and accordingly define the time-scale of the experiment.

- ... get acquainted w i t h the different instrumentation and operation procedures of the flume.

Table 5 summary of pilot test cases

Pilot test cases P I P2

Geometry H/h9 0.125 1.0 0.125 2.0 H (m) 0.125 0.25 umc (m/s) Q (m3/s) 0.277 0.320 Hydraulic umc (m/s) Q (m3/s) 0.058 0.134 conditions i (cm/20 m) 0.99 0.50 Re 11250 23970 Fr 0.27 0.20 Suspended sediment u,/ws Z Suspension 1.34 1.87

upto water depth 1.34 1.87

upto water depth -g Qs o E & H T-H (ms/s) (Kg/hr) 3.11x10-° 29.66 3.47xl0"6 39.51 3 Q s (ms/s) 0.34xl0'6 0.54xl0"6

& van Rijn (Kg/hr) 3.20 5.18

= _{E & H} Cbed (ra/h)

time (hrs) 0.30 65.88

0.46 43.36 S van Rijn Cbed (m/hr)

time (hrs) 0.03 617.9

0.06 334.4

I t is envisioned t h a t these processes are: 1- i n the main channel

- local scour near the t i p of the groynes - groyne flames along the normal line

- establishment of equilibrium longitudinal profile 2- i n the groyne fields

- minor deposition of sediment into the groyne fields d u r i n g the emerged flow condition

- m aj or deposition of sediment into the groyne fields during the submerged flow condition

From those processes, i t has been possible to reproduce the part concerning the m a i n channel using a numerical model. Yet, the morphological process inside the groyne fields was not possible to reproduce. So, as a first step i t is to be confirmed as assumed before further experiments.

I t is proposed to r u n two cases i n the pilot test to include b o t h the emerged and the submerged conditions. These cases w i l l be chosen f r o m the dimensions given i n (Table 4 - G l ) , a summary of the pilot test cases is given i n Table 5.

M a i n tests p r o g r a m m e

Based on the pilot test results and after confirming the m a i n assumptions, the experi-ments time-scale w i l l be estimated. I t is proposed to r u n at least three different flow conditions for each test. I f i t is considered necessary, (and i f i t is feasible) the number of test cases might be increased. This is to be decided i n a later stage. The following parameters w i l l be measured during each experiment:

- Bed level changes - Flow velocity

- Suspended sediment concentration profiles

As the effect of submergence is a primary goal for the experiments, the flow depth is preferred to be the variable parameter. Accordingly, the ratio (H/hg) as the dimensionless

f o r m of defining the relative submergence w i l l be varied. The three proposed submer-gence ratios are 1.4, 1.7, and 2.0 consequently. Comparison of the sediment transport and the bed development at the different submergence conditions w i l l provide informa-t i o n on informa-the effecinforma-t of submergence on informa-the morphological behaviour of b o informa-t h informa-the m a i n channel and the groynes-sections.

3 . 3 . S E D I M E N T T R A N S P O R T

A fine-grained u n i f o r m l y graded sediment* w i t h a diameter of D50 = 0.164 m m , w i l l be

used i n these experiments. The i n i t i a l thickness of the sand layer is chosen at 12.5 cm, (see appendix I ; I I I ) . The flow velocities w i l l be set to ensure t h a t b o t h bed-load and suspended-load is possible. A n i n i t i a l estimate of the t o t a l sediment transport rate has been calculated using the method of van R i j n (1984a; 1984b) and Engelund & Hansen (1967), Table 6. Details of sediment transport calculations are given i n A p p e n d i x I . D u r i n g the pilot test the sediment transport rate w i l l be refined.

* Same sediment that was used by Lauchlan (2001), yet to be confirmed prior to the start of the experi-ments.

Table 6 Initial estimate of the total sediment transport rate'

H/hg H E&H van Rijn

Test (cm) (cm) Qs (m3/s) (Kg/hr) Qs (m3/s) (Kg/hr)

G l i r 12.5 3.04E-06 29.03 3.37E-07 3.22

Gla

12.5 1.4 17.5 3.52E-06 33.54 4.34E-07 4.14 Gib 12.5 1.7 21.3 3.80E-06 36.29 4.92E-07 4.69

Glc 2.0 25.0 4.05E-06 38.67 5.41E-07 5.16

G2i r 10.0- 2.75E-06 26.22 2.77E-07 2.64

G2a

10.0 1.4 14.0 3.20E-06 30.51 3.69E-07 3.52 G2b 10.0 1.7 17.0 3.47E-06 33.14 4.25E-07 4.06

G2c 2.0 20.0 3.71E-06 35.42 4.74E-07 4.52

G3i r 7.5" 2.39E-06 22.81 2.04E-07 1.95 G3a

7.5 1.4 10.5 2.81E-06 26.82 2.90E-07 2.76 G3b 7.5 1.7 12.8 3.07E-06 29.29 3.43E-07 3.27

G3c 2.0 15.0 3.29E-06 31.43 3.89E-07 3.71

Calculated for flow velocity, umc = 0.3 m/s.; Emerged groynes

3 . 4 . H Y D R A U L I C A S P E C T S

I n all test cases the flow condition is chosen to ensure a sub-critical flow condition and a f u l l y developed turbulent flow, see Appendix I I . The proposed hydraulic conditions for the different tests are given i n Table 7.

Table 7 Hydraulic conditions for the different test cases*

Hg H/hg H Q Re Fr Test (cm) (cm) (m/s) (m3/s) G i l r 12.5" 0.25 0.058 11 250 0.27 Gla 1.4 17.5 0.30 0.088 15 626 0.23 Gib 1.7 21.3 0.30 0.111 19 737 0.21 Glc 2.0 25.0 0.30 0.134 23 970 0.20 G2I 1" 10.0" 0.25 0.044 7 895 Ö27 G2a 1.4 14.0 0.30 0.067 11 924 0.24 G2b " 1.7 17.0 0.30 0.085 15 089 0.23 G2c 2.0 20.0 0.30 0.103 18 352 0.22

H

G3I

r

7.5" 0.25 0.031 5 523 0.29 G3a 1.4 10.5 0.30 0.047 8 385 0.26 G3b ' 1.7 12.8 0.30 0.060 10 639 0.25 G3c 2.0 15.0 0.30 0.073 12 967 0.24 Calculated for velocity t h r o u g h groyne fields section, ugf = 0.3 x umc

3 . 5 . M E A S U R E M E N T P R O C E D U R E S

For the pilot test cases see section 3.2. For the main tests, i t is proposed to start the experiments f r o m an emerged situation, and r u n i t to reach an equilibrium low water bed level, then the submerged condition may start. I n this way the low discharge morphologi-cal pattern w i l l be developed. The bed changes due to high water w i l l then have a clear reference t o start f r o m and the changes w i l l be known to take place only because of the high water.

The suspended sediment concentration measurements shall start after the relatively rapid i n i t i a l bed changes. Velocity measurements w i l l be carried out simultaneously w i t h the concentration measurements.

The measurement sections w i l l be located i n a groyne field far downstream, for example the groyne field located between 12.0 and 14.0 m f r o m the entrance of the flume. The measurements w i l l cover bed changes, sediment concentration profiles, velocity and water level. The details of the measurement locations and procedures are given i n Appendix I I I .

4. T I M E A N D C O S T

4 . 1 . T I M E T A B L E

The following timetable is proposed for the experimental programme:

Activity Weeks Activity 1 2 3 4 5 6 7 8 9 i 10 l 1 i i 12 ! 13 1 1 14 15 | 16 Setup _{m m} 1 ' _WMË i 1 1 l l i + J o P I 1 1 1 i 1 i P2 1 1 1 1 l ( ! G l m cS Pj G2 _i

_{B I}

G3 j i i i _i Analysis Reporting ! 1 l 1 1 • • • • • Activity Weeks Activity 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Setup _{P I} o P I P2 G l cc3 Ö G2 i fa G3 Analysis i Reporting

•

i !

This timetable relies on the availability of suitable student assistance, preferably MSc students.

4 . 2 . C O S T E S T I M A T E

I n accordance w i t h the timetable given i n the previous section the cost of the experi-ments is given i n the following table:

Fixed coast Unit cost/week Total

ö ö CD CD .10 so.. Ö Ö O O O O Activity Weeks personnel 13 Laboratory structure 13 materials

—

Researchers Yossef experimenter 20 32 supervision 4 Total 11

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Asian and Pacific Regional Division, IAHR, K y o t o , Japan, 309-316.

Peng, J., Kawahara, Y . , and Tamai, N . (1997). "Numerical analysis of three-dimensional turbulent flows around submerged groynes." 27th IAHR congress,, San Francisco,

USA.

Raudkivi, A . J. (1998). Loose boundary hydraulics, Balkema, Rotterdam.

van R i j n , L . C. (1984a). "Sediment transport, part i : Bed load transport." Journal of Hydraulic Engineering, ASCE, 110(10), pp. 1431-1456.

van R i j n , L . C. (1984b). "Sediment transport, part i i : Suspended load transport." Jour-nal of Hydraulic Engineering, ASCE, 110(11), pp. 1613-1641.

van R i j n , L . C. (1984c). "Sediment transport, part i i i : Bed forms and alluvial rough-ness." Journal of Hydraulic Engineering, ASCE, 110(12), pp. 1733-1754.

van R i j n , L . C. (1987). "Mathematical modelling of morphological processes i n the case of suspended sediment transport," Ph.D. Thesis, D e l f t University of Technology, D e l f t .

van R i j n , L . C. (1993). Principles of sediment transport in rivers, estuaries and coastal seas, A q u a Publications, Amsterdam.

Schans, H . (1998). "Representativitet van l a ï b v a k m e t i n g e n u i t 1996 en 1997 ten opzichte van de hele waal." ICG 98/15, Universieit Utrecht, Fysische Geografie, Utrecht. Suzuki, K . , Michiue, M . , and Hinokidani, O. (1987). "Local bed f o r m around a series of

spur dikes i n alluvial channels." XXII IAHR Congress, Lausanne, Belgium, 316-321. Uijttewaal, W . S. J. (1999). "Groyne field velocity patterns determined w i t h particle

tracking velocimetry." 28th IAHR congress, Graz, Austria.

Uijttewaal, W . S. J., Berg, M . H . , and van der Wal, M . (2002). "Experiments on physical scale models for submerged and non-submerged groynes of various types." Interna-tional Conference on Fluvial Hydraulics, Louvain-la-Neuve, Belgium.

Yalin, M . S. (1992). River mechanics, Pergamon Press L t d , O x f o r d .

Yossef, M . F. M . , and Klaassen, G. J. (2002). "Reproduction of groynes-induced river bed morphology using les i n a 2-d morphological model." River Flow 2002 - Pro-ceedings of the International Conference on Fluvial Hydraulics, Louvain-la-Neuve, Belgium.

A P P E N D I X I

S E D I M E N T T R A N S P O R T C A L C U L A T I O N S General

I n this appendix we present the design details of the experiment concerning the sediment transport calculations. A l l calculations are based on an assumed sediment size* of:

D10 = 0.112 m m

D50 = 0.164 m m

Dgo = 0.238 m m

I n all the following calculations, the particle fall velocity (ws) is calculated according to

Ahrens (2000) for the sediment characteristic diameter (D50), for: D50 = 0.164 m m

ws = 0.019 m / s , see Figure 4.

1*10 5 W 0 ~4 W O 3 0.01 0.1

Characheristic Diameter (m)

Figure 4 Particle fall velocity according to Ahrens (2000)

Bed Load

For bed-load transport to take place, the bed shear stress must be raised above the critical shear stress of particle i n i t i a t i o n of motion as given by Shields (1936) and modi-fied by Miller et al. (1977); see for example van R i j n (1993) and R a u d k i v i (1998). The critical depth-averaged velocity reads:

ücr = C - ^ A L \ 0 ( I I . 1)

w i t h :

C = Chezy roughness coefficient ^ = (Ps ~ Pw)/Pw

6a. = critical Shields parameter

These values were used by Lauchlan (2001), and they are to be confirmed prior to the start of the experiments.

For live bed situation the roughness coefficient is a f u n c t i o n of the f o r m roughness as well as the grain size. For the current experiment, m i n i ripples are likely to f o r m . A rough 'first' estimate for the bed forms dimensions is given by Yalin (1985), see van R i j n (1993): Ripples height: ZL Ripples length: Ar 50 t o 200 Dt 500 to 1000 D 50 50 {11.2) (II.3)

Based on the analysis of ripple data given by a number of researchers, van R i j n (1993) proposed the following relationship for f o r m roughness (Ks ) related to ripples:

K»=2QlrAr

A,.

(II.4)

where (7,.) is the ripples presence factor (%= 1.0, for ripples alone).

Utilising equations 1 to 4, the critical depth averaged velocity for i n i t i a t i o n of m o t i o n could be summarised i n Table L I .

Table 1.1 Prediction of critical depth-averaged velocity for initiation of motion Flow depth Critical depth averaged velocity*

H(m) _{«er,W (m/S)}

0.25 0.15

0.20 0.14

0.15 0.14

0.10 0.12

Based on form roughness (K = 1.6 cm)

Suspended Load

I n this experiment i t is required as well t o have the sediment i n suspension. Several researchers proposed criteria for the onset of suspension; these criteria could be summa-rised i n Table 1.2.

Table 1.2 Criteria for suspended load motion

Criteria Reference Condition* Remarks

# 1 Bagnold (1966)

* . > !

As given by van R i j n (1984b)

Deduced from experimental investigations # 2 van Rijn (1984b) U* 4 ~w~s>~D* For D* U* 4 ~w~s>~D* ^ > 0 . 4 For D* > 10 # 3 Raudkivi (1990) after — > 0 . 5 Inception of suspension # 4 Chanson (1999) — > 1 . 2

™. Dominant suspended load

# 5 Jullien (1990) after ^ > 0 . 2 w, ^ > 2 . 5 Inception of suspension # 6 Chanson (1999) ^ > 0 . 2 w, ^ > 2 . 5

Dominant suspended load Englund (1965)

# 7 after

van Rijn (1984b) w

-— > 0 . 2 5

Based on crude stability analysis Inception of suspension

* W i t h u* = shear velocity; ws = fall velocity, and D* = particle parameter and takes the

following definition:

A 5

- C r i t e r i o n # 5 defines the lower l i m i t for the inception of suspension and gives the lowest allowable depth-average velocity. I n fact, i t yields as well as Criteria # 3 & # 7 velocities lower than the critical velocity for i n i t i a t i o n of motion, compare Table I . l w i t h Table 1.3.

- C r i t e r i o n # 2 may define an intermediate stage at which locally turbulent bursts w i t h sediment particles are l i f t e d into suspension.

- C r i t e r i o n # 4 defines a stage at which there is dominant suspension. The choice of the flow velocity will be made to satisfy this criterion.

- C r i t e r i o n # 5 yields relatively high critical velocities and w i l l not be considered.

A summary for the results of the suspension analysis based on the above-mentioned criteria applied for different hydraulic conditions is presented i n Table 1.3.

Table 1.3 Prediction of critical depth-averaged velocity for suspension flow depth H (m) = 0.25 0.2 0.15 0.1

Critical depth averaged velocity* Criteria u,crtSm(m/s) ucr<sus (m/s) # 1 0.019 0.25 0.24 0.22 0.20 # 2 0.018 0.24 0.23 0.21 0.20 # 3 0.009 0.12 0.12 0.11 0.10 #4 0.023 0.30 0.28 0.27 0.24 # 5 0.004 0.05 0.05 0.04 0.04 # 6 0.047 0.61 0.59 0.55 0.51 # 7 0.05 0.06 0.06 0.06 0.05

* Based on form rouj jhness (Ksv v= 1.6 cm)

To ensure t h a t suspension occurs and over a large part of the flow depth, a range of velocities for different flow depth was checked against the criteria given by van R i j n (1993), the results are summarised i n Table 1.4.

Table 1.4 Suspended sediment distribution

Flow depth Depth averaged velocity Suspension number* Suspended sediment distribution

(m) (m/s) Suspension is upto: 0.35 1.76 water depth 0.25 0.30 2.05 mid depth 0.25 2.46 mid depth 0.35 1.68 water depth 0.2 0.30 1.96 water depth 0.25 2.35 mid depth 0.35 1.58 water depth 0.15 0.30 1.85 water depth 0.25 2.22 mid depth 0.30 1.69 water depth 0.1 0.25 2.02 mid depth 0.20 2.53 mid depth

* W i t h : K is von K a r m e n constant ( k = 0.4), (3 is a factor > 1 , ((3 = 1 was used), and Z takes the following f o r m :

(3 K U*

Suspend sediment adaptation length

According to van R i j n (1987) the adjustment length (La) is defined as the length after

which the suspended sediment transport entering a channel differs less than 5% f r o m the equilibrium transport. For a value of (w/u, =0.5), we f i n d t h a t equilibrium is reached after (Ljh = 50) i.e. (La = 10 m ) . Consequently, the measurement section should start

after 10 m f r o m the channel entrance.

Bed forms

The type of the bed-forms that are likely t o occur under the given hydraulic conditions was checked using the criteria given by van R i j n (1984c), i n all cases ' M i n i Ripples' were found to take place.

The dimensions of the bed forms were calculated using van R i j n (1984c), Y a l i n (1992) and R a u d k i v i (1998). The results are summarised i n Table 1.5, where we can see a significant difference i n the ripples length (A,.) between van R i j n and the other two methods, the resulting (Ar) f r o m van R i j n (1984c), is an order of magnitude higher. The

basic difference between van R i j n and the other two methods is that; i n b o t h Yalin (1992) and R a u d k i v i (1998), the ripples geometry doesn't depend on the flow depth whereas i n van R i j n (1984c), the ripples dimensions is a f u n c t i o n of the flow depth. For comparison w i t h the experiments conducted by Lauchlan (2001), the observed ripples dimensions compared best w i t h the results obtained f r o m R a u d k i v i (1998).

Table 1.5 Ripples dimensions

Average ripples dimensions

Method r (cm) r (cm) _r

van Rijn (1984c) 1.58 111.83 0.02 Yalin (1992) 0.40 15.12 0.03 Raudkivi (1998) 1.53 13.01 0.12 A,. = height, A,. = length, and Sr = steepness.

Sediment Transport Rate

To estimate the sediment transport rates for the different hydraulic conditions the formula of van R i j n (1984a; 1984b) and Engelund & Hansen (1967) were compared. Assuming that the flow velocity i n the groyne fields section is approximately 0.3 times* the flow velocity i n the main channel, we can split the sediment transport capacity for the two different sections. The t o t a l transport capacity results f r o m summing up the two parts.

Time of the Experiment

The time required for the bed to reach an equilibrium profile is determined by calculat-ing the propagation speed of the bed-forms. A n equilibrium condition is reached i n the time i t takes one bed f o r m to travel the flume length (20 m ) . The bed forms speed w i l l be calculated adopting the bed f o r m height that was deduced f r o m R a u d k i v i (1998) ( / lt

= 1.5 cm) and using the expression given by Raudkivi (1998) and van R i j n (1993) for bed forms velocity which reads:

*

Based on experimental data for submerged groynes as conducted by Yossef (not published)

w i t h ; p = porosity

Using only the bed load transport capacity for the main channel section, the results are summarised i n Table 1.7. As the E & H formula computes a t o t a l load, the bed load is computed using the ratio between the bed load to the total load estimated f r o m van R i j n formulae. The times presented i n this table are i n the conservative side as they are computed only f r o m the bed load part.

Table 1.6 Estimated total sediment transport load E & H Test qs,mc qs,g f Qs Qs (m2/s) (mVs) (m3/s) (Kg/hi-) van Rijn Qsi mc Qs, gf Qs Qs (m2/s) (m2/s) (m3/s) (Kg/hi")

G l i 2.3E-06 5.5E-09 2.9E-06 27.96 G l a 2.6E-06 6.3E-09 3.4E-06 32.30 G i b 2.8E-06 6.8E-09 3.7E-06 34.95 Glc 3.0E-06 7.3E-09 3.9E-06 37.24

2.5E-07 6.7E-09 3.3E-07 3.10 3.2E-07 8.6E-09 4.2E-07 3.99 3.6E-07 9.7E-09 4.7E-07 4.52 4.0E-07 1.1E-08 5.2E-07 4.98 G 2 i / P l 2.0E-06 4.9E-09 2.6E-06 25.25

G2a 2.4E-06 5.8E-09 3.1E-06 29.39 G2b/P2 2.6E-06 6.2E-09 3.3E-06 31.91 G2c 2.7E-06 6.7E-09 3.6E-06 34.11

2.0E-07 5.5E-09 2.7E-07 2.55 2.7E-07 7.3E-09 3.6E-07 3.40 3.1E-07 8.4E-09 4.1E-07 3.91 3.5E-07 9.4E-09 4.6E-07 4.36 G3i 1.8E-06 4.3E-09 2.3E-06 21.97

G3a 2.1E-06 5.1E-09 2.7E-06 25.83 G3b 2.3E-06 5.5E-09 3.0E-06 28.21 G3c 2.4E-06 5.9E-09 3.2E-06 30.27

1.5E-07 4.0E-09 2.0E-07 1.88 2.1E-07 5.7E-09 2.8E-07 2.67 2.5E-07 6.8E-09 3.3E-07 3.15 2.8E-07 7.7E-09 3.8E-07 3.58

Table 1.7 Estimated equilibrium time for the longitudinal profile

E & H (1967) _{van R i j n (1984a )}

Test Qs, bed Ubed Time to Qs, bed Ubed Time to

Test (mVs) (m/d) equilibrium (m2/s) (m/d) equilibrium (days) (days) G l i 7.7E-07 7.39 2.71 8.4E-08 0.81 24.72 G l a 9.6E-07 9.19 2.18 1.2E-07 1.12 17.83 G i b 1.1E-06 10.31 1.94 1.4E-07 1.32 15.19 Glc 1.2E-06 11.28 1.77 1.5E-07 1.49 13.44 G 2 i / P l 6.5E-07 6.26 3.19 6.5E-08 0.62 32.11

G2a 8.3E-07 7.98 2.51 9.5E-08 0.91 21.97

G2b/P2 9.4E-07 9.03 2.21 1.1E-07 1.09 18.29

G2c 1.0E-06 9.95 2.01 1.3E-07 1.25 15.94

G3i 5.1E-07 4.89 4.09 4.3E-08 0.41 48.53

G3a 6.8E-07 6.50 3.08 6.9E-08 0.66 30.21

G3b 7.8E-07 7.49 2.67 8.6E-08 0.83 24.20

G3c 8.7E-07 8.35 2.40 1.0E-07 0.97 20.54

Based on only bed load, and porosity p = 0.4

Local scour

To estimate the equilibrium local scour depth ( ys e) near the t i p of the groynes, a

com-parison between the different local scour formulae is given, see Figure 5. The expression of A h m e d (1953) as given by Hoffmans & Verheij (1997) reads:

ys,e +

K =

k a • ka (l-m) (TI.6) where: ys,e h0 m K\ KA

equilibrium scour depth below i n i t i a l depth i n i t i a l water depth

b/B, b and B are the w i d t h of the groyne and channel respectively.

2.1U1/3 (=1.0rrï1/s. sz/3)

2Kp Ks Ka KM

= correction factor for the influence of channel bend, (inner = 0.85, outer = 1.1-1.4, and for straight reach: Kp = 1.0)

Ks = for the shape of structure, (vertical wall = 1.0, 1:1 sloped =0.85)

Ka = for the angle of attack, (30° to 150° = 0.80 - 1 . 1 0 )

KM = for the influence of porosity (0.2 porosity = 1 . 0 , 0.5 porosity = 0 . 9 - 0 . 6 )

The expression given by G i l l (1972) reads:

1 ys,e +

K = K

a a 11 U - m J 1 l - m ) (%)-(%„) for clear-water for live-bed, rc > > T\

(n.7)

W i t h (n) as the sediment transport formula exponent and (a)is a correction factor that takes the following f o r m :

\0.25

a = 8.375 A o

h 'o )

The expression of L i u et al. (1961) as given by Hoffmans & Verheij (1997) reads:

v

m

=

K L

K

0 l

F3 (II.8)

W i t h (KL) as a correction factor (KL = 2.15 for blunt groynes)

Hoffmans & Verheij (1997) compared a large number of scour predictors w i t h experimen-t a l daexperimen-ta. They proposed a formula, which reads:

Vs,e

=

K

1

[l-m + K

R • b • t a n h

1 6 J (II.9)

w i t h (KB) as a correction factor (KB =1.5 for groynes w i t h sloped face)

We must realise that the equations given by Gill and Hoffmans kVerheij are independent f r o m the flow velocity and they b o t h assume that the velocity is higher than the critical velocity. The equation of A h m e d gives a rather low value of (ysJ) when compared w i t h

the other equations and i t may serve as a lower l i m i t for scour hole depth.

0 . 0

0 0 . 0 5 0.1 0 . 1 5 0 . 2 0 . 2 5 0 . 3

F l o w d e p t h ( m )

A h m e d ( 1 9 5 3 ) H o f f m a n s & V e r h e i j ( 1 9 9 5 ) L i u et a l . ( 1 9 6 1 ) ^ G i l l ( 1 9 7 2 ;

Figure 5 Comparison between the estimated scour depth from different formulae, velocity u = 0.3 m/s

Local scour depth for a series of groynes

W h e n series of groynes are used, the scour depth varies w i t h the groyne location. The scour depth around the first groyne is similar to t h a t of a single groyne. Suzuki et al. (1987), showed through laboratory experiments, t h a t the local scour depth around a groyne located far downstream (ys>DS) i n a series of groynes is a f u n c t i o n of the groyne

spacing to length (S/Lg) ratio, and i t could be expressed roughly i n the following f o r m :

Following the dimensions given i n section 3.1, the local scour reduction factor could be calculated {ys>DJ y$J = 0.34), w i t h ysl the scour depth around the first groyne which is

similar to the scour depth near a single groyne and could be estimated using any of the above mentioned formulae.

For the proposed experiment the m a x i m u m local scour could be estimated to be ( ys l =

0.47 m ) , and for the downstream groynes i t w i l l be reduced to 35% of t h a t value, i.e. (ysDs = 0.17 m ) . However, i t is only possible to provide 10 cm of sand, as the flume depth

is 40 cm and the m a x i m u m flow depth is 25 cm thus leaving only a 5-cm clearance. Therefore, i t is proposed to provide a protection layer to the groynes-tips t o ensure t h a t the scour hole doesn't reach the flume b o t t o m . This protection layer could be provided t h r o u g h coarser sediment.

Time scale of scour

From dimensional analysis and the results of many experiments, Hoffmans &; Verheij (1997) found that the characteristic time (ts) at which (ys = h0) :

= 0.07 • — + 0.14 for 2 < — < 10 (11.10)

ys,i

_ i f A

2

-A

1

-

7

s

(<*u

Q

-u

c

r

in which:

coefficient (K = 330 hours m2-3.s4-3, then 4 is expressed i n hours)

(A - pm)/pw

coefficient depends on turbulence intensity mean, and critical velocities (m/s)

The a-factor was estimated by van der Wal (1991) - as given by Hoffmans & Verheij (1997) to range f r o m 2 to 9 according to the geometry of the groyne. W i t h this range of (a) the estimated time falls i n a very wide range. I t is recommended to estimate the scour time f r o m the pilot test series.

K A

a =

Un, K. =

A P P E N D I X I I

H Y D R A U L I C C O N S I D E R A T I O N S

I n this appendix we address the basic hydraulic considerations concerning the experi-ment.

The transverse velocity d i s t r i b u t i o n f r o m the main channel to the groyne fields section was obtained from unpublished data acquired from previous laboratory experiments. The experiments were carried out i n the f l u i d mechanics laboratory (2001). The velocity profile is presented i n Figure 6, from which we can realise that the velocity i n the groyne field region is around 30% of t h a t i n the m a i n channel.

1 2 _{i i} I _i

I

₁ -1

• 9

. 1

L . i

1 *

#

8

+

.a

„....

1

-*

!

I

1 !

• i -0.8 -0.6 -0.4 - 0 2 0 0.2 0.4 0.6 0 8 1 * 0.6 0 4 0 2

Distance from the tip of the groyne (x/L^)

Figure 6 Transverse velocity profile obtained for different submergence ratios

Assuming t h a t the same profile w i l l be present during the experiment, we can then

compute the corresponding Reynolds number (Re) and Froude Number (Fr). I n all test

cases Reynolds number was much larger t h a n 2300 (fully developed turbulent flow) and Froude number was much smaller t h a n 1.0 (sub-critical f l o w ) . We can as well compute the t o t a l discharge corresponding to a given water level and m a i n channel depth-averaged flow velocity, see Table I I I :

Table I I I Total flume discharge for different test conditions

H Discharge (m) (m3/s) 0.075 0.037 0.133 0.066 0.192 0.095 0.250 0.124

Base flow velocity U = 0.3 m/s.

A P P E N D I X I I I

P R A C T I C A L C O N S I D E R A T I O N S

I n this appendix we address some practical considerations concerning the execution of the experiment.

Design and Alignment

1. Groynes shape;

- horizontal crest w i t h a length of 0.65 m

- crest top w i d t h is = 1.0 cm (not sharp-crested) - groyne height = 0.25 m

- side slopes of 1:3 i n all directions

- spacing between the centre line of the groynes = 2.0 m

- i t is recommended t o design a k i n d of cap for the groyne's crest t o ensure the non-submergence during the non-submerged test cases, ( G l i , G 2 i , G3i). The cap height should around 2cm.

Three groyne heights are going t o be tested, b o t h the groyne height and the sand layer thickness are chosen to guarantee t h a t the m a x i m u m water level doesn't exceed the flume wall height of 40 cm. The groyne height is going to be varied through varying the thickness of the sand layer. The proposed sand layer thickness for each test case is as follows:

- test G l : hg = 12.5 cm -> sand layer thickness = 12.5 cm->max. W . L . = 37.5 cm

- test G2: hg = 10.0 cm -> sand layer thickness = 15.0 cm->max. W . L . = 35.0 cm

- test G3: hg = 07.5 cm -> sand layer thickness = 17.5 cm->max. W . L . = 32.5 cm

The groynes should be as well marked at the locations of h , prior t o the installation. A f t e r the completion of each test i t w i l l be required to add a 2.5cm layer of sand to start the new test case.

Sand Supply

2. Inflow sediment distribution

As the ratio between the sediment transport rate i n the groyne fields region to the main channel region i n all test cases is small, the sand could be supplied w i t h a constant

distribution through the m a i n channel w i d t h i.e. through 1.35m-width supply system.

However, the water supply should be through the whole flume w i d t h w i t h the same flow d i s t r i b u t i o n as a far downstream cross-section.

3. Required sand volume

The t o t a l amount of sand t o be used is divided into two parts;

- a base part; that forms the channel bed w i t h a m a x i m u m volume (2x20x0.175 = 7.0 m3)

- per test sand volume; t o t a l for all test cases = 9.91 m3; A d d i n g 25% losses, the t o t a l

volume of required sand = 21 m3

Measurements

4. Measurement locations

A l l measurements should be carried out i n an area far downstream f r o m the inflow boundary to ensure a f u l l y developed flow and suspended sediment profiles. The meas-urements are proposed to be i n parallel and to take place i n the following locations: - Bed level measurements1: between groynes # 6 , and # 7 i n Figure 7.

- Velocity and concentration measurements: between groynes # 7 , and # 8 i n Figure 7

5. Number of bed-profilers and measurement procedures Bed level measurements w i l l be carried out i n two steps;

- 1s t to measure i n the main channel w i t h a t o t a l measuring length of 4.0m

- 2n d to measure i n the groyne fields region w i t h a t o t a l length of around 0.70mu

The active measurements w i d t h is 0.9m and the number of probes depends on their availability e.g. 10@10cm, 7@15cm, etc.

6. Suspended sediment profiles

Concentration profiles w i l l be carried out across 2 transverse sections, Sland S2 avoiding measurements on the groyne side slope. Their locations are:

- S I at 0.50m downstream groyne # 7 - S2 at 0.50m upstream groyne # 8

- Lateral location: distances f r o m the wall i n the transverse direction are: 20, 65, 90, and 170cm.

- Vertical locations: elevations f r o m the bed are: 2, 7, 12, 17, 22, 27, and 32cm. For low water levels the exposed nozzles should be closed.

- Period: 45-minutes period is proposed, i t should be enough to collect enough sand for accurate evaluation of the concentration.

I n t o t a l a m a x i m u m of (2x4x7) 56 measurement points are proposed for each test caseiü.

7. Velocity profiles

Velocity measurements w i l l be conducted for a single transverse profile i n the middle of groyne#7, and groyne#8, at the same lateral distances as the concentration profiles.

8. Water level measurements

The water level measurements aim t o evaluate the slope i n b o t h the main channel and a single groyne field. The water surface fluctuations are not of interest.

- main channel: near the inflow and near outflow boundaries

- groyne field: U.S. of groyne#7, D.S. of groyne#7, and U.S. of groyne#8

I i f possible during the test at time interval of e.g. 1 hour

I I this changes f r o m test to test and w i l l not cover the region very close to the groynes I I I The f i n a l arrangement w i l l be optimised according to the processing method and the

availability of measurement devices