Degradation Downstream From a Sluice Gate

8/6/2019 Degradation Downstream From a Sluice Gate

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Moayad S. Khaleela

, Khalil I. Othmanb

aDepartment of Irrigation and Drainage, College of Engineering, University of Mosul, Mosul, IraqbSadam Research Centre for Dams and Water

Resources, University of Mosul, Mosul, Iraq


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Abstracty R esults observed from a laboratory study

concerning the degradation of an alluvial channel

owing to the flow of clear water are reported.

y Two sizes of sand, of median diameter 0.47 mmand 0.79 mm and geometric standard deviation

4.65 and 3.54 respectively, are used as a bedmaterial.


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Abstracty The objectives of this study is to :y studying the variation of the surface layer size

with time and distance,y the variation of sediment discharge with time,y the sediment size of the armored layer, and the

time required for stabilization of the channelbed.

y Some useful equations for predicting thesediment size of the armored layer and the totaltime for the degradation are also given.


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In troductio n y G enerally, a river running through a deep layer of

alluvial sediment, or sand, will change its flow

and sediment characteristics after theconstruction of a dam or a hydraulic structure.The clear water released from the structureusually causes degradation of the downstreamriver bed. The finer fraction of the bed material will be removed from the bed surface by sorting,and will be transported downstream.


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In troductio ny The median diameter of the bed material

becomes larger and the sediment discharge

decreases with time. After a period of time, a newbed profile will form; this bed contains all thecoarse particles that the flowing water is not ableto remove.


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In troductio n y B ed degradation may extend over long distances

(up to 300 km) (Simons and Senturk, 1977)

downstream from the hydraulic structure whichcauses it. On the other hand, natural or artificialunerodible obstacles and a control point, such asa diversion dam, can reduce and limit the effect of degradation.


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Objectives of this study 3. To predict the median diameter of the bed surface

layer with time and distance according to the

degradation process and armored conditions.4. To predict the time required to stabilize thedegrading bed.


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Experimental equipment1. R ecirculating flume of rectangular cross-section

of 24 m length, 0.81 m width and 0.70 m depth.

2. instruments and equipment required for tracing water and bed surface profiles, measuring thesediment discharge, collecting sedimentsamples along the flume length, and measuring

water discharge 3. Sluice gate at the upstrem4. Sill at the down stream acts as control point.


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Rec ircu l ti g fl ume of r ec t g u l rc ross-s ec tio


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Procedurey The working reach of the flume was 20 m in

length, and was filled with graded sand, Two

different sizes of sand were used, with mediandiameters of 0.47 mm and 0.79 mm and geometricstandard deviations ( ) of 4.65 and 3.54,respectively.

y

A total of 21 experiments were carried out for thetwo sizes of sand.


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Procedurey The water was gradually allowed to flow in the flume

until it reached a certain value. At the same time, thesediment was fed in by a mechanical feeder at the

upstream end of the test reach. The feeding in of sediment was continued until equilibrium conditions were achieved

y (the establishment of equilibrium flow meant that the sediment discharge at the endof the flume was equal to the feed rate at the upstream end, and there was nonoticeable change in water surface and bed profiles).

y After equilibrium flow was established, centre-linebed levels and water surface levels were measured by six point gauges fixed along the flume. And samplesof the bed surface layer were collected at threelocations along the reach.


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Procedurey After analyzing the bed at equilibrium, feeding in

of sediment was then stopped and the bed was

allowed to degrade.y After that, the water surface and bed elevations

were measured , bed samples were taken at thethree locations, and the sediment transported in

the flume was measured at gradually increasedtime intervals. The run was continued until thesediment transport was small (less than 1% of initial sediment discharge).


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p rocedurey E leven runs with different discharges were carried

out using the sand with D 50 =0.47mm (Sample A)y

Ten runs with different discharges were carriedout using the sand with D 50 =0.79mm (Sample B )

y In the experimental programme the bed and water surface elevations were measured 1074times, 4296 samples were collected from thechannel bed and the sediment discharge wasmeasured 179 times.


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An alysis of data 1. Variation of bed and eroded material:y The size distribution curves for the surface bed

material at the three locations along the channel weredrawn at the equilibrium condition (Fig. 1 for Run A5,as an example) and at the end of each run (end of thedegradation process) (Fig. 2 for Run A5, as an

example). In these figures, the distribution curve of the original bed material is also shown.


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At the three locatio n s alo ng the cha nn el, the bed material becomes coarser tha n theori gin al material. This is due to erosio n of fin e p articles from the bed surface by the

flowi ng water. The curve for the tra n sp orted material co n firms this: a clear reductio n in grai n size is observed for this material


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At the e n d of the de gradatio n p rocess (Fi g. 2). Further i n creases i n the grai n size of bedsurface layer were observed, whereas that of the tra n sp orted material co n tin ued to

decrease with time. From the above dia grams, o n e ca n also n otice that the grai n size of the bed surface layer decreases i n the dow n stream directio n .


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An alysis of datay It is found from experimental measurements (see

Othman (1992)) that the slopes of the channel bed and

water surface are reduced, as is the velocity of thefollowing water.


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An alysis of data 2 . Variation of surface layer median diameter with

time and distancey

To clarify the change in surface layer size with timeand distance, the values of D50 at four time intervals were plotted against the distance for all theexperiments, as shown in Fig. 3 (for Run A6 as an

example). Also, the variations of D50 for the threelocations along the channel were plotted against timeas shown in Fig. 4 (for Run B5 as an example).


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In gen eral, the size of the surface layer i n creased with time at the same locatio n in the

cha nn el (Fig. 3), a n d as dista n ce i n crease the media n diameter decrease.


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The i n crease i n D 50 had a maximum value at the dista n ce X = 0.0, the n it decreased as

the value of X i n creased. The mai n cha ng es i n the surface layer size took p lace a short

time after the de gradatio n p rocess was started (Fi g. 4). After that, the i n crease of D 50con tin ued with time, but at a much slower rate tha n in the be ginn ing .


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Equatio n for determi n atio n the surface layermedia n diameter at various dista n ces

W here;y q is discharge per unit width

y qs is sediment discharge per unit widthy X is distance measured along the channel from a fixed

point (upstream)y X 0 is effective channel length (under degradation

condition)y D50i0 is median diameter of the surface layer at the

equilibrium time at the upstream end of the channel.


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An alysis of data 3 . Variation of sediment discharge with timey The rate of sediment transport was plotted against

time for various runs from Set A and Set B as shown inFig. 5 and Fig. 6, respectively. These figures indicatethat the sediment discharge is decreasing with time,because at the beginning of degradation the fine

particles will be eroded first and the particle size of thebed layer becomes coarser as time goes on, so the flow will no longer be able to carry the same amount of sediment.


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The slo p e of the curves i n Fig. 5 seems to be ge n tle at the be ginn ing , a n d the n starts to i n crease shar p ly un til the e nd of the ex p erime n t


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whereas i n Set B, Fi g. 6, the slo p e of the curves i n creases i n a more gradual waytha n in Set A. This is due to the u n iformity of the p article size distributio n of the

material used i n Set B, where the geometric sta ndard deviatio n of material i n Set A was 4.65 a n d in set B was 3.45.


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An alysis of data 4 . Prediction of median diameter of armored layer

y The following equation was obtained by regression analysis (correlationcoefficient 0.97):

y D 50a 1 is the median diameter for the armored layer at the first locationy D 50i 1 is the median diameter of the surface bed material at the first location at

equilibrium timey

C is the critical shear stress calculated from a modified Meyer- P eter andMuller equation

y0 is the average shear stress

y1 is the geometric standard deviation for the bed material at the first location

at equilibrium time.


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An alysis of data 5. Estimation of time period required for channel

stabilizationy

Any alluvial stream exposed to degradation will bestabilized after a fixed period by forming an armoredlayer or through reduction in the stream slope, or as aresult of both factors.

y

The important factors that affect the stabilization of any alluvial stream are the amount of f low , streamlength and slope, and size of bed material and its variation with the distance .


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An alysis of datay Depending on previous factors, and from the data observed through the

experiments, an equation for the time required for stabilization of the channelbed (T ac) was obtained (correlation coefficient 0.99):

y Si is the bed slope at the equilibrium timey SD50i is the variation in median diameter with distance at equilibrium

time


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C on clusio n y The size of bed material increases as the degradation

continues. The increase varies inversely with the

downstream flow direction. The percentage of increase of bed material size at the end of thedegradation varies between 340% and 1070% of theoriginal size of Sample A and between 262% and 742%of the original size of Sample B.

y The median diameter of the armor coat is in the rangeof D80 - D97 of the original bed material for both sandsizes.


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C on clusio ny These equations are based on experimental data of the

study, and in spite of the high correlation coefficienttheir applicability should be tested using otherexperimental and field data.

y The rate of sediment transport through thedegradation decreases with time, owing to the increaseof bed material size and the reduction of bed slope. It

was found that the rate of sediment transport (bedload) fell below 24% of the initial rate after a periodequal to 30% of the time required for the degradationprocess.


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Degradation Downstream From a Sluice Gate