degradation downstream from a sluice gate
TRANSCRIPT
-
8/6/2019 Degradation Downstream From a Sluice Gate
1/32
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
-
8/6/2019 Degradation Downstream From a Sluice Gate
2/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
3/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
4/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
5/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
6/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
7/32
-
8/6/2019 Degradation Downstream From a Sluice Gate
8/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
9/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
10/32
Rec ircu l ti g fl ume of r ec t g u l rc ross-s ec tio
-
8/6/2019 Degradation Downstream From a Sluice Gate
11/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
12/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
13/32
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).
-
8/6/2019 Degradation Downstream From a Sluice Gate
14/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
15/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
16/32
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
-
8/6/2019 Degradation Downstream From a Sluice Gate
17/32
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 .
-
8/6/2019 Degradation Downstream From a Sluice Gate
18/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
19/32
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).
-
8/6/2019 Degradation Downstream From a Sluice Gate
20/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
21/32
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 .
-
8/6/2019 Degradation Downstream From a Sluice Gate
22/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
23/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
24/32
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
-
8/6/2019 Degradation Downstream From a Sluice Gate
25/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
26/32
-
8/6/2019 Degradation Downstream From a Sluice Gate
27/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
28/32
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 .
-
8/6/2019 Degradation Downstream From a Sluice Gate
29/32
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
-
8/6/2019 Degradation Downstream From a Sluice Gate
30/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
31/32
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.
-
8/6/2019 Degradation Downstream From a Sluice Gate
32/32
Thank you