Pressure Flushing of Cohesive Sedimentin Large Dam Reservoirs

Samad Emamgholizadeh1 and Manoochehr Fathi-Moghdam2

Abstract: Pressure flushing of deposited sediments behind large dam walls causes development of a funnel-shaped crater due to the inducedvortex and considerable shear flow around the outlets. Laboratory experiments were conducted to investigate interaction of flow and sedimentparameters on the development and final size of the scour cone. The volume of the flushed sediment increases with increase of discharge, anddecreases with increase of sediment bulk density and the water level above the sediment. While the water depth over the sediments is the mostsignificant parameter in the collapse of sediments above the intake and the initial development of the cone, sensitivity analysis indicated bulkdensity of cohesive sediment to be the most effective parameter to determine the final size of the scour cone. Using experimental results, twoequations were developed to estimate scour cone volume and length with readily defined and measurable parameters of flow and sediment.The field measurements confirm the capability of the equations for large-scale application and flushing practice. DOI: 10.1061/(ASCE)HE.1943-5584.0000859. © 2014 American Society of Civil Engineers.

Author keywords: Dam reservoir; Pressure flushing; Scour cone; Cohesive sediment.

Introduction

A considerable volume of reservoirs is lost annually around theworld due to the lack of effective watershed measures and the grow-ing trend of sedimentation in dam reservoirs. Effective actions suchas hydraulic flushing to sustain the useful storage capacity can beof great help to the productivity of dams and powerhouses. In aridand semi-arid zones of the world with finer eroded soils, riverstransport more suspended material into dam reservoirs. Conse-quently, the deposition of finer particles near dam walls reducesthe dam efficacy more rapidly (Jothiprakash and Garg 2009; Gargand Jothiprakash 2010). In some developing countries where water-shed management measures are not carried out effectively, reservoirstorage is being lost at a much greater rate (Fathi-Moghadam et al.2011). An overall estimation by International Commission onLarge Dams (ICOLD) reveals that 0.5 to 0.75% of the total storagevolume of reservoirs is lost each year as a result of sedimentation.This amount is generally higher in the Asian nations compared tothe world average (Liu et al. 2002). Therefore, sustaining the stor-age of existing reservoirs has become an important issue as build-ing new reservoirs is rather difficult due to strict environmentalregulations, high cost of construction, and lack of suitable damsites. Hence, techniques for reservoir desilting have receivedincreasing attention recently (Lai and Shen 1996). For control ofsedimentation in addition to the use of soil control measures toreduce sediment inflow from the watershed into the reservoir,the following approaches have been adopted: (1) increase passing

sediment through reservoir during high flows with heavy sedimentconcentrations, (2) disable high flows with heavy sediment concen-tration from entering the reservoir by using bypass techniques,(3) flush sediment from reservoir via density currents, and (4) re-move reservoir sediment by mechanical means such as dredgingand siphoning (Shen 1999). Sedimentation problems and manage-ment techniques vary widely from one site to another, and by study-ing specific sites one can appreciate the complexity of sedimentproblems and the manner in which they can be addressed (Morrisand Fan 1997; Wu et al. 2007). However, estimation of sedimen-tation and time variation of reservoir trap efficiency is an importantissue for flushing (Julien 1996; Lee et al. 2006; Ulke et al. 2009;Senthil Kumar et al. 2012; Mizumura 2012).

In pressure flushing, sediment deposits in the vicinity of thesluice-gate opening can be scoured within a short period of time.Once the water flowing through the opening is clear, the coneformation is fairly stable. Fig. 1 shows the schematic of pressureflushing and development of the scour cone in this study.

Erosion of Cohesive Sediment

Erosion of cohesive soils can occur in two forms of surface erosion(fluvial or particle erosion) and mass erosion (Morris and Fan1997). In surface erosion, individual particles or small aggregatesare removed from the soil mass by hydrodynamic forces such asdrag and lift (Millar and Quick 1998). The ability of a cohesivesoil to resist surface erosion is known as erosional strength (Zreiket al. 1998). Mass erosion is determined by the soil’s un-drainedstrength, or yield strength (Millar and Quick 1998). Mass erosionoccurs at higher values of shear stress and is characterized by theremoval of large clumps of sediment from the streambed (Morrisand Fan 1997).

The erodibility of submerged cohesive sediment depends onthree groups of factors (Grabowski et al. 2011): physical factors(mainly including particle size and degree of compaction whichcan be represented by bulk density in practice), electrochemicalfactors (mainly including clay mineralogy and total salinity whichaffect physical properties like LL-liquid limit, PL-plastic limit, andPI-plastic index), and biological factors (including sediment

1Assistant Professor, Dept. of Water Engineering, Shahrood Univ. ofTechnology, Shahrud 36155-316, Iran. E-mail: s_gholizadeh517@yahoo.com

2Professor, Dept. of Hydraulic Structures, Shahid Chamran Univ.,Ahwaz 6135743135, Iran (corresponding author). E-mail: mfathi@scu.ac.ir

Note. This manuscript was submitted on March 21, 2012; approved onJune 1, 2013; published online on March 14, 2014. Discussion periodopen until September 1, 2014; separate discussions must be submittedfor individual papers. This paper is part of the Journal of HydrologicEngineering, Vol. 19, No. 4, April 1, 2014. © ASCE, ISSN 1084-0699/2014/4-674-681/$25.00.

674 / JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / APRIL 2014

J. Hydrol. Eng. 2014.19:674-681.

Dow

nloa

ded

from

asc

elib

rary

.org

by

STA

NFO

RD

UN

IV o

n 03

/14/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

disturbance, feeding and egestion, biogenic structures, burrows,roots). The electrochemical and biological effects are importantfor slow flows in lakes and estuaries. For high shear and turbulentflow in pressure flushing, their effects are assumed minor in thisstudy. On the other hand, for regions with similar hydrological fea-tures and geographic location (e.g., arid and semi-arid zones in thisstudy), most of the electrochemical and biological properties can beassumed similar in practice. This is based on an assumption thatsoil property and watershed cover are consistent for the climatolog-ically similar regions. Result of field surveys for four large damsites (Dez, Abbaspour, Marun, and Karkheh) in the semi-arid zonesof Iran is a view of this hypothesis as deposited sediments behindthe dam walls have relatively similar properties (Samadi Boroujeniet al. 2009). Ranges of variation of these properties were 38 to 43%clay and 57 to 62% silt for texture, 42 to 45% for LL, 21 to 23%for PL, and 21 to 22% for PI. Review of literature also revealssubstantial attention to the wet bulk density as a major sedimentproperty for erodibility. Hwang and Mehta (1989), Winterwerp andvan Kesteren (2004), and Amos et al. (2004) also used wet bulkdensity as a fundamental parameter to develop relationships forestimation of critical shear and bed surface erosion.

The purpose of this study is to simulate pressure flushing anddevelop equations for estimating volume of flushed cohesive sedi-ments and geometry of the scour cone for large dams’ reservoirsin arid and semi-arid zones of the world. Sediment samples weretaken from the Dez Dam reservoir, which represents the finesediments for these types of zones. Effects of discharge, flow depthabove the deposits, and degree of sediment consolidation on thesize of the scour cone are the main concerns in this study.

Methods and Materials

Theory

The equilibrium of scour cone volume and length, which is typi-cally developed in a reservoir after pressure flushing, is mainly de-pendent on the reservoir water depth (Hw) and thickness of deposits(Hs) above the intake, fluid density (ρw), particle density (ρs), in-take diameter (D), water velocity at the intake (u), fluid dynamicviscosity (μ), gravitational acceleration (g), and wet bulk density ofcohesive sediment (ρwb). Wet bulk density is measured by mass ofsediment in unit volume of mixtures.

Physical parameters affecting erodibility include clay contentand clay type, temperature, bulk density (largely a reflection ofthe deposit age) and pore pressure (Winterwerp et al. 1990). Astime passes during sediment consolidation, water drains from thesolid mass and density of sediment particles and thus bulk densityincreases. Studies have been performed before to correlate erosionof cohesive soils with their bulk densities rather than other sedimentproperties like void ratio (Hwang and Mehta 1989; Roberts

et al. 1998). The bulk density is also considered in this study asthe main sediment property because of importance and ease of de-termination in practice.

While development and equilibrium of the scour cone volume Vand length L can be ideally assumed to be dependent from a hydro-dynamic standpoint, they are considered independent in practicedue to difference in sediment conditions and materials. Therefore,they are separately expressed as

V ¼ fðu;Hw;Hs;D; g; ρs; ρwb; ρw;μÞ ð1Þ

L ¼ fðu;Hw;Hs;D; g; ρs; ρwb; ρw;μÞ ð2Þ

Considering theories of orifice flow and sediment transport, thefinal forms of the dimensionless parameters that define the geom-etry of the scour cone are derived as follows:

VD3

¼ f1

�uffiffiffiffiffiffiffiffiffigHw

p ;ρwuDμ

;Hs

Hw;ρwb − ρw

ρs

�ð3Þ

LD

¼ f2

�uffiffiffiffiffiffiffiffiffigHw

p ;ρwuDμ

;Hs

Hw;ρwb − ρw

ρs

�ð4Þ

The first parameter in the right hand side of the equations is thepressure coefficient. According to the U.S. Bureau of Reclamation(USBR) (2001), while the pressure coefficient increases with de-crease of submergence depth (Hw=D, flow depth upstream of anorifice to the height of opening) for Hw=D < 4, it remains essen-tially constant for higher flow depths. Reddy and Pickford (1972)defined this submergence depth limit as a function of the intakeFroude number as Hw=D ¼ 1þ F , where F ¼ u=ðgDÞ0.5. Be-cause maximum F in the experiments was around 4, the latermethod is more conservative, and is selected as limited submer-gence depth for analysis and application of the result in practice.However, since submergence depth is normally higher than theabove limit in practice, the pressure coefficient is assumed to bea constant coefficient in Eqs. (3) and (4).

The second parameter on the right-hand side of Eqs. (3) and (4)is the intake Reynolds number. Because experiments were in a lim-ited range of turbulent flow (Reynolds number range was 21,978 to175,821), sensitivity of the pressure coefficient and cone develop-ment to Reynolds number was assumed to be negligible. This isalso a good assumption in practical applications of pressure flush-ing as design parameters usually have a limited range of variation inlarge dams projects. The rest of the parameters were used in a non-linear regression to develop relationships for estimating the volumeand length of the flushing cones. Let V� ¼ V=D3, H� ¼ Hs=Hw,L� ¼ L=D, ρ� ¼ ðρwb − ρwÞ=ρs, then Eqs. (3) and (4) can bewritten as

V� ¼ kvH�αvρ�βv ð5Þ

L� ¼ klH�αlρ�βl ð6Þwhere kv, αv, βv and kl, αl, βl are coefficients for scour conevolume (subscript v) and length (subscript l). A part of kv andkl coefficients account for the pressure coefficient. The numberof parameters and form of the equations are selected to define flowand sediment interaction as a nonlinear phenomenon while reduc-ing the propagation of uncertainties for use of the equations forlarge-scale cases and practice. It is worth noting that the scale effectis governed by the intake diameter (D) and the water elevation (Hw)in Eqs. (3) and (4). Because (D) is normally designed based on thewater elevation (Hw) in large dams.

Fig. 1. Experimental setup for pressure flushing and scour cone invicinity of dam intakes

JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / APRIL 2014 / 675

J. Hydrol. Eng. 2014.19:674-681.

Dow

nloa

ded

from

asc

elib

rary

.org

by

STA

NFO

RD

UN

IV o

n 03

/14/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

Experimental Setup

Experiments were carried out at the Hydraulic Laboratory of theChamran University of Ahwaz, Iran. A 4 m long flume with a1.50 × 2.3 m rectangular cross section was used to simulate a res-ervoir and cone formation due to pressure flushing. To observe flowand sediment erosion, sides and front walls of the channel weremade of Plexiglas. Water was circulated in the first upstream meterof the flume to set up a constant head over a 0.42 m sediment layerresting on the flume bed (Fig. 1). The 2-in. pipe and gate valve inthe downstream end were used to simulate discharge through thedam outlets. The outlet discharge (Q) was drained into the firstsection of a 3.5 × 1.0 × 0.8 m settling flume where outflow watercould be measured using a 26° V-notch weir. Water was steadilypumped in the system using a small pump over a relatively largesump, which had a constant water level.

Cohesive sediments of Dez reservoir behind the dam wall wereused for testing. Having borehole information and vertical densitygradient of the deposit, it was intended to prepare five sedimentclasses with wet bulk densities of 1,230 to 1,405 kg=m3 for testing.This range is selected based on the borehole information for depos-its behind the Dez Dam wall. The samples with densities of lessthan 1,200 kg=m3 (suspension and fluid mud sediments accordingto Coastal Engineering Manual 2002) were not strong enough toresist the overflow shear in the small scale model, and the five sedi-ment classes were selected with densities above this limit.

A large quantity of sediment from around 30 m below the low-level outlets (LLO) with bulk density around 1,405 kg=m3 wastaken to the laboratory for preparation of the five sediment classes.The initial sample with bulk density of 1,405 kg=m3 and void ratioof 0.84 [using the ASTMD854 (ASTM International 2010) pro-cedure] was selected as class 1 in Table 1. The sediment classesof 2, 3, 4, and 5 were prepared by adding and mixing some waterto the sediment class 1 and left to still and smooth on the flume bedfor a day before testing. According to the Raudkivi (1998) classi-fication, the prepared samples were in fairly compacted based ontheir void ratio, and in the mud class based on their bulk density andYang and Wang’s (1996) classification. A Malvern Mastersizer2000 was used to analyze particle size and generate the gradationcurve for the cohesive sediment samples (Fig. 2).

The prepared samples were laid and leveled on the flume bedand tested with three flow depths of Hw ¼ 0.52, 0.90, 1.20 m, andfour discharges of 0.001, 0.003, 0.006, and 0.008 m3=s. Consider-ing the laboratory and experimental limitations, the model variableswere kept within proper range of scaling of hydraulic models(around 1=50). The mean flow velocities at the opening of outletpipe were calculated to be 0.51 to 4.08 ms−1. To avoid high shearflow effects on the cone due to lowering of the water level at the endof tests, the flume was discharged through a 3-in. valve upstream ofthe sediment layer (Fig. 1). The water inside the cone was slowlydrained through a small opening of the 2-in. gate valve. Depending

on the flushing cone volume, this procedure may take severalminutes. Using a point gauge of 0.1 mm accuracy, stream-wiseand cross-sectional profiles of the cone were measured beforethe sediment could get dry.

Field Studies

Dez Dam was constructed in 1963 over the Dez River located200 km north of the city of Ahwaz, Iran. The area is categorizedas a semi-arid zone. The reservoir is 60 km long and only fine sedi-ments (mainly carried by short period density currents) can reachthe dam wall. Results of borehole (using piston-type core sampler)testing performed about 100 m upstream of the dam wall reveal asimilar texture of 40.3% clay and 59.7% silt for all layers of thedeposits (Fathi-Moghadam et al. 2011; Samadi Boroujeni et al.2009). The deposited sediments near the dam wall of the Dez res-ervoir have always been submerged and water layer over the sedi-ments has always been more than 50 m during the operating periodof 45 years. The density gradient of deposits behind the dam wallalong with position of normal water level (NWL), upper boundarylevel for the sediment layer (SSL), and low-level dam outlets (LLO)for pressure flushing are shown in Fig. 3. The depth of boreholesampling in the sediment layer was 62 m. Because the top layersamples were being disturbed during sampling, it was decidedto prepare the testing sample classes in the laboratory with boreholesamples based on their bulk density (Table 1) and field information(Fig. 3). The sample densities are selected within fairly compactedzones that rest about 19 m above the low-level outlets (90 m belowwater level).

Results and Discussion

The experiments were conducted with five classes of cohesive sedi-ments having bulk densities of 1,405, 1,360, 1,315, 1,280, and1,230 kg=cm3 in Table 1. The sediments started to discharge fromthe outlet approximately half an hour after the gate valve wasopened. The sediment discharge increased over time while flushingdensity decreased. The flushing cone reached stable conditionwhen the drained water was clear. The field observations of pres-sure flushing for the Dez Dam reservoir showed that a few days arerequired before clean water could be passed through. However,total time was less than 4 h for most of the tests in this study.Furthermore, the drainage of sediment and development of a stablecone for the samples with lower densities took less time, and thevolume of the cone was larger than that for the denser samples.Both types of erosions were observed in the experiments though the

Table 1. Characteristics of the Testing Samples

Sedimentclasses

Voidratio

Raudkivi’sclassification (1998)

Wet bulkdensity(kg=m3)

Yang andWang’s

classification(1996)

1 0.84 Fairly compacted 1,405 Mud2 0.89 Fairly compacted 1,360 Mud3 0.95 Fairly compacted 1,315 Mud4 0.98 Fairly compacted 1,280 Mud5 1.02 Loose-fairly compacted 1,230 Mud

10

5

0

100

50

0 0.01 0.1 1 10 100 1000

Particle Diameter (10-3mm)

Fig. 2. The gradation curve for the tested cohesive sediment samples

676 / JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / APRIL 2014

J. Hydrol. Eng. 2014.19:674-681.

Dow

nloa

ded

from

asc

elib

rary

.org

by

STA

NFO

RD

UN

IV o

n 03

/14/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

mass erosion was more effective. Fig. 4 shows a three-dimensionalview of the developed flushing cone for the experiments withsediment class 3, outflow discharge 0.003 m3=s and water depthof 0.90 m. In the following, effects of single parameters of flow andsediment on the cone development are discussed.

Discharge Effect

Discharge of water through low-level outlets to flush sedimentscreates a free vortex flow with a pressure gradient towards thecenter. This causes a pulling effect on sediment particles on topof the shear flow effect. Figs. 5 and 6 demonstrate a considerableeffect of flushing discharge on the scour cone volume and length.They also reveal the greater effect of discharge for less dense sedi-ment (class 5) in Fig. 6 than the denser one in Fig. 5. These figuresreveal the inverse effect of water depth on the flushed sediments.

The relative effect of discharge on the flushing sediment interms of percentage is shown in Fig. 7 for the sediment class 1.This figure demonstrates a relative increase of the cone volumeand length with discharges of 0.001, 0.003, and 0.006 to the conedevelopment with discharge of 0.008 m3=s. The figure reveals thegreater effect of discharge on the early stages of cone development.Similar trends were obtained for other classes of sediments.

Water Depth Effect

A decrease in water depth increases the vertical gradient of velocityand bed shear stress, thus producing a higher flushing rate andlarger cone. The development of scour cones with three flow depthsof 0.52, 0.90, and 1.20 m over the sediment classes 1 and 5 areillustrated in Figs. 8 and 9 for variable discharges. A considerableincrease of the cone size with decrease of flow depth was recordedfor both sediment classes, particularly for the class 5. This is due tothe rapid increase in the transverse velocity of vortex flow and sheargradient under lower flow depths. A similar result for effect ofwater depth is reported by White (2001). For the range of flowdepths and sediment classes in this study, the curves reveal an aver-age reduction of 10% in water depth causes the flushing cone vol-ume and length to be increased by about 7 and 5%, respectively.

Bulk Density Effect

Water content, bulk density, and porosity are important propertiesof cohesive sediments and are measures of the proportion of solid toliquid states in the sediment (Winterwerp et al. 1990, Avnimelechet al. 2001; Grabowski et al. 2011). In this study, sediment samplesare submerged in water and have similar texture, thus the bulk den-sity is the most important parameter to characterize the cohesivesediment samples. The importance of bulk density to cohesive sedi-ment erodibility is well supported in the literature. Bulk density isnegatively correlated with erodibility. In other words, dense sedi-ment beds have lower erosion rates (up to 100 times), and highererosion thresholds (up to 5 to 8 times) than the less dense beds(Jepsen et al. 1997; Lick and McNeil 2001; Bale et al. 2006,2007). Results of this study also reveal the considerable effectof the bulk density of sediments on development of the scour conevolume and length as shown in Fig. 10. These curves reveal thehigh sensitivity of the flushed sediment to the bulk density, asan average of 1% decrease in bulk density caused the scour conevolume and length to increase by 12 and 7%, respectively. Thisshows the sensitivity of eroded cohesive sediment to bulk densityis more than 10 times that of water level (Hw).

Side Slope of the Cone Flushing

The side slope of the flushing cone depends on discharge, flowdepth above the sediment, as well as physical properties of sedi-ments and degree of consolidation. The water level is more effec-tive for the lower flow depths than higher ones. The observations

Bulk Density (Kg/m3)

Fig. 3. Low level outlet and density gradient of deposits behind the Dez dam wall

020

4060

80 100

Length (cm)

Width (cm) 0

2550

75

-25-50

-750

20

40

Hei

ght(

cm)

Inflow

Outflow

Fig. 4. A three dimensional (3D) view of sediment layer and the de-veloped flushing cone around outlet

JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / APRIL 2014 / 677

J. Hydrol. Eng. 2014.19:674-681.

Dow

nloa

ded

from

asc

elib

rary

.org

by

STA

NFO

RD

UN

IV o

n 03

/14/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

and measurements show that the developed scour cone around out-let are not a perfect cone due to the vicinity of the created vortexflow with dam wall. The side slopes at the right and left sides ofthe cone are different. In addition, longitudinal wall slope wasabout 6% larger than average cross flow wall slope. The side slopeof the scour cones for five classes of the bulk density in Table 1were measured as 55.4, 46.7, 43.2, 37.8, and 32.1° with an averageof 43° for Q ¼ 0.003 m3 s−1 and HW ¼ 0.90 m. Field measure-ments of the Dez reservoir showed an average angle of 21° forthe cone wall slope. The field measurement of the reservoirs inChina (Kongazhue, Bikou, Qington Gorge, Fen He, and Yan

Gou Gorge reservoirs) showed the scour cone side slopes to be4 to 17° in practice (Fang and Cao 1996). However, this is not anextraordinary result for cohesive sediment studies with large inher-ent uncertainty as differences in two orders of magnitude is re-ported for some properties of cohesive sediments in Pejrup andMikkelsen (2010), and Fathi-Moghadam et al. (2011). It should benoted that besides the sediment type and degree of compaction,thickness of the sediment layer above the outlet is also a governingparameter on the wall slope, as the thicker sediment layers createlarger slopes. This is due to the created vortex flow, which is morepowerful in deeper cones.

0

20

40

60

1050

Q (10-3 m3/s)

V(

10-3

m3 )

Hw=0.52 m

Hw=0.90 m

Hw=1.20 m

(a)

0.1

0.2

0.3

0.4

0.5

1050

Q (10-3 m3/s)

L (

m)

Hw=0.52 m

Hw=0.90 m

Hw=1.20 m

(b)

Fig. 6. Effect of discharge on the cone volume and length for cohesive sediment class 5

0

10

20

1050Q (10-3 m3/s)

V( 1

0-3 m

3 )

Hw=0.52 m

Hw=0.90 m

Hw=1.20 m

(a)

0.05

0.15

0.25

0.35

1050

Q (10-3 m3/s)

L (

m)

Hw=0.52 m

Hw=0.90 m

Hw=1.20 m

(b)

Fig. 5. Effect of discharge on the cone volume and length for cohesive sediment class 1

0

20

40

60

80

100

Increase of discharge (%)

Incr

ease

of f

lush

ing

cone

Vol

ume

(%)

Hw=0.52 m

Hw=0.90cm

Hw=1.20 m0

20

40

60

80

100

0 20 40 60 80 100 0 20 40 60 80 100

Increase of discharge (%)

Incr

ease

of f

lush

ing

cone

leng

th (%

)

Hw=0.52 m

Hw=0.90cm

Hw=1.20 m

(a) (b)

Fig. 7. Relative increase of the flushing cone volume and length with increase of discharge for the sediment class 1

678 / JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / APRIL 2014

J. Hydrol. Eng. 2014.19:674-681.

Dow

nloa

ded

from

asc

elib

rary

.org

by

STA

NFO

RD

UN

IV o

n 03

/14/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

Estimation of Flushing Cone Volume and Length

The most effective parameters for development of the flushing coneare presented in Eqs. (5) and (6). Using Statistical Package for theSocial Science (SPSS) for the experimental result of this study, anonlinear correlation of the dimensionless parameters for estima-tion of flushing cone volume and length are as follows:

VD3

¼ 0.99

�Hs

Hw

�0.59

�ρwb − ρw

ρs

�−2.85ð7Þ

LD

¼ 0.33

�Hs

Hw

�0.40

�ρwb − ρw

ρs

�−1.44ð8Þ

Eqs. (7) and (8) can be used to estimate the flushing conevolume and length in a pressure flushing strategy for removal ofcohesive sediment. The application of the equations is limited tothe freshly layered sediment above the dam intakes (densities of1,200 to 1,500 kg=m3), and where water level above the sedimentis more than F þ 1. The advantages of the equations are simplicityand need of measurable parameters. As no method is provided toestimate the volume of flushed sediment, the equations haveconsiderable contribution for practice of flushing and design ofthe large dams intakes (including diameter and level of intakesand outlets as well as the selection of proper intake valve).

The sampling and borehole results of the deposited sedimentsbehind other large dam walls in the southwest of Iran (i.e., Marun,Abbaspour, and Karkheh dams), a semi-arid zone, show that they

0

10

20

30

40

(a)

0.05

0.15

0.25

0.35

0.45

(b)

Q= 0.001 m3 s-1

Q= 0.003 m3 s-1

Q= 0.006 m3 s-1

Q= 0.008 m3 s-1

Q= 0.006 m3 s-1

Q= 0.008 m3 s-1

Q= 0.001 m3 s-1

Q= 0.003 m3 s-1

0.3 0.9 1.5 0.3 0.9 1.5

Hw (m) Hw (m)

L (

m)

V (

10-3

m3)

Fig. 8. Decrease of scour cone volume and length with water depth for sediment class 1

0

20

40

60

(a)

0.15

0.25

0.35

0.45

0.55

(b)

Q= 0.001 m3 s-1

Q= 0.003 m3 s-1

Q= 0.006 m3 s-1

Q= 0.008 m3 s-1

Q= 0.006 m3 s-1

Q= 0.008 m3 s-1

Q= 0.001 m3 s-1

Q= 0.003 m3 s-1

0.3 0.9 1.5 0.3 0.9 1.5Hw (m) Hw (m)

L (

m)

V (

10-3

m3)

Fig. 9. Decrease of scour cone volume and length with water depth, sediment class 5

0

20

40

60

Bulk Density (kg/m3)

Vo

lum

e (1

03 m

3 )

Q=0.001 m3/s

Q=0.003 m3/s

Q=0.006 m3/s

Q=0.008 m3/s

(a)

0.10

0.25

0.40

0.55

1150 1250 1350 1450 1150 1250 1350 1450

Bulk Density (kg/m3)

L (

m)

Q=0.001 m3/s

Q=0.003 m3/s

Q=0.006 m3/s

Q=0.008 m3/s

(b)

Fig. 10. Effect of bulk density on the flushed cone for Hw ¼ 0.90 m

JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / APRIL 2014 / 679

J. Hydrol. Eng. 2014.19:674-681.

Dow

nloa

ded

from

asc

elib

rary

.org

by

STA

NFO

RD

UN

IV o

n 03

/14/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

are in a similar class as those in Table 1 for a large thickness aroundthe normal leveling range of the water intakes (Samadi Boroujeniet al. 2009). In addition, they also show as having similar physicalproperties to other large dams in this area. This allows for assumingsimilar characteristics for large dam deposits in arid and semi-aridzones and supports the wider application of these equations forsuch areas in practice.

Verification of the Models

To validate the modeling results, Eqs. (7) and (8) were used toestimate volume and length of the cone for a real flushing event.In a real pressure flushing of the Dez Dam with low-level outletdiameter of D ¼ 5.0 m and information in Fig. 3 (Hw ¼ 33 m,Hs ¼ 33 m), the volume and length of the flushed cone wereestimated as 100,000 m3 and 70 m, respectively. Using Eqs. (7)and (8) with the same information and an average bulk density1,170 kg=m3, the volume and length of the flushed cone werecalculated as 73 m3 and 54 m, respectively. If the field measuredvolume and length are assumed to be correct, there are 27% and23% errors associated with Eqs. (7) and (8), which is tolerablefor a cohesive sediment study.

Flushing Efficiency

Flushing efficiency of the deposited sediments in dam reservoirsis very dependent on the flushing method. That is, flushing canbe done with partial drawdown of water surface (pressure flushing)or with emptying (free flushing) (Morris and Fan 1997). Qian(1982) defined flushing efficiency (E) as the ratio of volume of theeroded sediment (V) to the volume of water (Vw), as E ¼ V=Vw.Based on several site investigations, Morris and Fan (1997) re-ported flushing efficiency ranges for partial drawdown and empty-ing of reservoirs as 0.00017 to 0.012 and 0.04 to 0.13, respectively.The laboratory results of this study with cohesive sediments showan average value of 0.00343 for efficiency. In other words, 292 m3

of clear water is required to flush 1 m3 of the deposited sediment,which is among the reported range for pressure flushing. Fig. 11shows correlation of water volume versus volume of the flushedsediments in this study.

Conclusions

It is shown that efficiency of pressure flushing as a fast and peri-odical technique to desilt a limited area around the dam intakes

depends mainly on the intake discharge and properties of depositedcohesive sediments. While the pressure effect of water depth is asignificant parameter for initial cone formation in the early desiltingstages, the suction effect due to pressure gradient of the vortex flowis the significant parameter in the final stage of cone development.Results of the conducted sensitivity analysis for the involvedparameters revealed the bulk density (an indicator of grain sizeand degree of compaction) to be most effective parameters in de-velopment of the scour cone. Based on independent dimensionlessparameters, two equations were developed for estimation of thecone volume and length. The advantage of the equations is theircapability to account for the effects of discharge, water and sedi-ment level, and properties of cohesive sediments with readily de-fined and measurable parameters. They also have considerablecontribution to the design of dam outlets and level of intakes. Theapplication of the equations is limited to the freshly layered sedi-ment above the dam intakes (densities of 1,200 to 1,500 kg=m3),and where water level above the sediment is more than F þ 1. Theproposed equations were verified with field and large-scale mea-surements during flushing of the Dez reservoir, and were able toestimate the cone volume and length with acceptable accuracy.Assuming similar characteristics for the deposited cohesive sedi-ments behind the large dam walls in arid and semi-arid zones,the developed equations can be used to estimate flushed sedimentsfor such zones.

Acknowledgments

The authors would like to acknowledge the journal editors and thefive anonymous reviewers for their insightful comments and sug-gestions on this paper. The acknowledgement is also extended tothe Shahid Chamran University of Ahwaz, Shahrood University ofTechnology, and the Centre of Excellence on Operation Manage-ment of Irrigation and Drainage Networks for financial support andfacilitation of the experiments.

Notation

The following symbols are used in this paper:A = outlet area;D = diameter of outlet;E = flushing efficiency;F = intake Froude number;g = gravitational acceleration;

Hs = depth of deposited sediment;Hw = water depth above sediment;H� = ratio of Hs=Hw;L = scour cone length;L� = ratio of L=D;Q = outflow discharge;u = intake water velocity;V = scour cone volume;V� = ratio of (V1=3=D);Vw = volume of water;ρ� = density parameter;ρwb = wet bulk density;ρs = particle density;ρw = water density;μ = dynamic viscosity of water;

kv, αv, βv = coefficients for scour cone volumeequation; and

kl, αl, βl = coefficients for scour cone length equation.

V = 7.96 Vw0.448

R2 = 0.51

0

10

20

30

0 2 4 6 8 10

Vw (m3)

V (

10-3

m3 )

Fig. 11. Efficiency of the pressure flushing for the experiments

680 / JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / APRIL 2014

J. Hydrol. Eng. 2014.19:674-681.

Dow

nloa

ded

from

asc

elib

rary

.org

by

STA

NFO

RD

UN

IV o

n 03

/14/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

References

Amos, C. L., et al. (2004). “The stability of tidal flats in Venice Lagoon:The results of in-situ measurements using two benthic, annular flumes.”J. Mar. Syst., 51(1–4), 211–241.

ASTM International. (2010). “Standard Test Methods for Specific Gravityof Soil Solids byWater Pycnometer.” ASTMD854, West Conshohocken,PA.

Avnimelech, Y., Ritvo, G., Meijer, L. E., and Kochba, M. (2001). “Watercontent, organic carbon and dry bulk density in flooded sediments.”Aquacult. Eng., 25(1), 25–33.

Bale, A. J., Stephens, J. A., and Harris, C. B. (2007). “Critical erosion pro-files in macro-tidal estuary sediments: Implications for the stability ofintertidal mud and the slope of mud banks.” Cont. Shelf Res., 27(18),2303–2312.

Bale, A. J., Widdows, J., Harris, C. B., and Stephens, J. A. (2006).“Measurements of the critical erosion threshold of surface sedimentsalong the Tamar Estuary using a miniannular flume.” Cont. ShelfRes., 26(10), 1206–1216.

Coastal Engineering Manual. (2002). “Erosion, transport and deposition ofcohesive sediments.” Chapter III-5, EM 1110-2-1100, U.S. Army Corpsof Engineers, Washington, DC.

Fang, D., and Cao, S. (1996). “An experimental study on scour funnel infront of a sediment flushing outlet of a reservoir.” Proc., 6th FederalInteragency Sedimentation Conf., Las Vegas, NV, I-78–I-84.

Fathi-Moghadam, M., Arman, A., Emamgholizadeh, S., and Alikhani, A.(2011). “Settling properties of cohesive sediments in lakes and reser-voirs.” J. Waterway Port Coastal Ocean Eng., 10.1061/(ASCE)WW.1943-5460.0000078, 204–209.

Garg, V., and Jothiprakash, V. (2010). “Modeling the time variation of res-ervoir trap efficiency.” J. Hydrol. Eng., 10.1061/(ASCE)HE.1943-5584.0000273, 1001–1015.

Grabowski, R. C., Droppo, R. I., and Wharton, G. (2011). “Erodibility ofcohesive sediment: The importance of sediment properties.” Earth Sci.Rev., 105(3–4), 101–120.

Hwang, K. N., and Mehta, A. J. (1989). “Fine sediment erodibility in LakeOkeechobee,” Rep. UFLICOEL-891019, Coastal and OceanographicEngineering Dept., Univ. of Florida, Gainesville, FL.

Jepsen, R., Roberts, J., and Lick, W. (1997). “Effects of bulk density onsediment erosion rates.” Water Air Soil Pollut., 99(1–4), 21–31.

Jothiprakash, V., and Garg, V. (2009). “Reservoir sedimentation estimationusing artificial neural network.” J. Hydrol. Eng., 10.1061/(ASCE)HE.1943-5584.0000075, 1035–1040.

Julien, P. Y. (1996). “Transforms for runoff and sediment transport.”J. Hydrol. Eng., 10.1061/(ASCE)1084-0699(1996)1:3(114), 114–122.

Lai, J. S., and Shen, H. W. (1996). “Flushing sediment through reservoirs.”J. Hydraul. Res., 34(2), 237–255.

Lee, H.-Y., Lin, Y.-T., and Chiu, Y.-J. (2006). “Quantitative estimation ofreservoir sedimentation from three typhoon events.” J. Hydrol. Eng.,10.1061/(ASCE)1084-0699(2006)11:4(362), 362–370.

Lick, W., and McNeil, J. (2001). “Effects of sediment bulk properties onerosion rates.” Sci. Total Environ., 266(1–3), 41–48.

Liu, J., Liu, B., and Ashida, K., (2002). “Reservoir sedimentation manage-ment in Asia.” 5th Int. Conf. on Hydro-science and Engineering,Warsaw Univ. of Technology, Poland.

Millar, R. G., and Quick, M. C. (1998). “Stable width and depth of gravelbed rivers with cohesive banks.” J. Hydraul. Eng., 10.1061/(ASCE)0733-9429(1998)124:10(1005), 1005–1013.

Mizumura, K. (2012). “Prediction of sediment concentration in riversby recursive least-squares and linear minimum variance estimators.”J. Hydrol. Eng., 10.1061/(ASCE)HE.1943-5584.0000528, 851–857.

Morris, G. L., and Fan, J. (1997). Reservoir sedimentation handbook,McGraw-Hill, New York.

Pejrup, M., and Mikkelsen, O. A. (2010). “Factors controlling the field set-tling velocity of cohesive sediment in estuaries.” Estuarine CoastalShelf Sci., 87(2), 177–185.

Qian, N. (1982). “Reservoir sedimentation and slope stability; technical andenvironmental effects.” 14th Int. Congress on Large Dams, Transac-tions, Vol. III, Rio de Janeiro, Brazil, 639–690.

Raudkivi, A. J. (1998). Loose boundary hydraulic, Balkema, Rotterdam,Netherlands, 271–311.

Reddy, Y. R., and Pickford, J. A. (1972). “Vortices at intakes in conven-tional sump.” Int. Water Power Dam Constr., 24(3), 108–109.

Roberts, J., Jepsen, R., Gotthard, D., and Lick, W. (1998). “Effects ofparticle size and bulk density on erosion of quartz particles.”J. Hydraul. Eng., 10.1061/(ASCE)0733-9429(1998)124:12(1261),1261–1267.

Samadi Boroujeni, H., Fathi-Moghadam, M., and Shafaei-Bejestan, M.(2009). “Investigation on bulk density of deposited sediments in DezReservoir.” Trends Appl. Sci. Res., 4(3), 148–157.

Senthil Kumar, A. R., Ojha, C. S. P., Kumar Goyal, M., Singh, R. D., andSwamee, P. K. (2012). “Modelling of suspended sediment concentra-tion at Kasol in India using ANN, fuzzy logic and decision tree algo-rithms.” J. Hydraul. Eng., 10.1061/(ASCE)HE.1943-5584.0000445,394–404.

Shen, H. W. (1999). “Flushing sediment through reservoirs.” J. Hydraul.Res., 37(6), 743–757.

Ulke, A., Tayfur, G., and Ozkul, S. (2009). “Predicting suspended sedimentloads and missing data for Gediz River, Turkey.” J. Hydrol. Eng.,10.1061/(ASCE)HE.1943-5584.0000060, 954–965.

U.S. Dept. of the Interior, Bureau of Reclamation. (2001). Water measure-ment manual, 3rd Ed., ⟨http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/index.htm⟩.

White, W. R. (2001). Evacuation of sediment from reservoirs, ThomasTelford, London.

Winterwerp, J. C., Cornelisse, J. M., and Kuijper, C. (1990). “Parametersto characterize natural muds.” Int. Workshop on Cohesive Sediments,Royal Belgian Institute of Natural Sciences, Brussels, 103–105.

Winterwerp, J. C., and van Kesteren, W. G. M. (2004). Introduction to thephysics of cohesive sediment in the marine environment, 1st Ed.,Elsevier, Amsterdam, Netherlands, 576.

Wu, B., Wang, G., and Xia, J. (2007). “Case study: Delayed sedimentationresponse to inflow and operations at Sanmenxia Dam.” J. Hydraul.Eng., 10.1061/(ASCE)0733-9429(2007)133:5(482), 483–494.

Yang, M., and Wang, G. (1996). “Formulas for incipient motion of mudwith various densities.” Int. J. Sediment Res., 11(1), 34–42.

Zreik, D. A., Krishnappan, B. J., Germaine, J. T., Madsen, O. S., and Ladd,C. C. (1998). “Erosional and mechanical strengths of deposited cohe-sive sediments.” J. Hydraul. Eng., 10.1061/(ASCE)0733-9429(1998)124:11(1076), 1076–1085.

JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / APRIL 2014 / 681

J. Hydrol. Eng. 2014.19:674-681.

Dow

nloa

ded

from

asc

elib

rary

.org

by

STA

NFO

RD

UN

IV o

n 03

/14/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.