Experimental Investigation of Sediment Trap Efficiency in Reservoirs

Transcription

1 ENGINEER - Vol. XLVII, No. 0, pp. [1-8], 014 The Intitution of Engineer, Sri Lanka Experimental Invetigation of Sediment Trap Efficiency in Reervoir N.M.T.K. Revel, L.P.G.R. Ranairi, R.M.C.R.K. Rathnayake and K.P.P. Pathirana Abtract: Reervoir edimentation ha become one of the major problem facing water reource development project in many countrie around the world. However, only a limited number of tudie ha been reported in thi field, particularly addreing the trap efficiency of reervoir. In addition, whatever available tudie in thi area alo conider few parameter governing the reervoir edimentation. A a reult, the available knowledge on trap efficiency i not very well defined. Brune curve [4] ha been widely ued for etimating trap efficiency of reervoir at preent, but it ha everal limitation, a it conider only the reervoir capacity and inflow ratio for etimating trap efficiency. The objective of thi tudy i to formulate an improved methodology in etimating reervoir edimentation through laboratory experiment. A mall-caled laboratory model wa et-up in repreenting a reervoir and a erie of tet were conducted by varying inflow rate, inflow ediment concentration, reervoir capacity and outflow rate. The experimental reult were compared with the available theorie and found that they are not very much in agreement with many of the exiting theorie which are motly baed on limited number of parameter. A comprehenive data analyi wa performed uing dimenional analyi to develop an improved relationhip to etimate reervoir edimentation incorporating many parameter governing the problem. However, the applicability of the propoed method i till limited only to the reervoir with continuou pilling condition. In addition, only one type of ediment gradation (d 50) wa ued in the experimental run and thu, the effect of ediment ize i not well repreented in thi method. However, the relationhip developed in thi tudy could be further improved by conducting more experimental run by varying few other parameter which are not conidered in the preent tudy. Keyword: Reervoir edimentation, Trap efficiency, Releae efficiency, Sediment inflow rate 1. Introduction Reervoir edimentation i a evere problem in managing water reource development project in many countrie around the world, a it reduce the original capacity ignificantly which affect the irrigation, hydropower and drinking water upply, flood control and recreational activitie. Rantambe reervoir in Sri Lanka i one of the reervoir which i everely affected by edimentation. Due to lack of reervoir management practice uch a periodical ediment fluhing, reervoir ediment routing and improving catchment protection to reduce oil eroion, the edimentation of reervoir ha now becoming a major threat to many countrie around the world. Though reervoir edimentation i becoming a major threat to engineer who are reponible in managing water reource project at preent, a limited number of tudie ha been reported in the field of reervoir edimentation. Brune [4] and Churchill [5] have propoed empirical method to evaluate trap efficiency of reervoir however, the applicability of thee method i highly limited a their formulation conider only few parameter which affect the reervoir edimentation. Thi paper preent an experimental tudy carried out in formulating an improved methodology to etimate reervoir edimentation.. Literature Review Very few tudie have been reported in literature on reervoir edimentation and mot of which are baed on analytical method. In general, the amount of ediment accumulated in a reervoir i expreed a a percentage of Eng. N.M.T.K. Revel, B.Sc. Eng. (Hon), Department of Civil Engineering, Univerity of Peradeniya. Eng. L.P.G.R. Ranairi, B.Sc. Eng. (Hon), Department of Civil Engineering, Univerity of Peradeniya. Eng. R.M.C.R.K. Rathnayake, B.Sc. Eng. (Hon), Department of Civil Engineering, Univerity of Peradeniya. Eng.(Prof.) K.P.P. Pathirana, B.Sc. Eng. (Hon.) (Peradeniya), M.Eng., Ph.D., C.Eng., FIE(Sri Lanka), MICE (London), Profeor of Civil Engineering, Department of Civil Engineering, Univerity of Peradeniya. 1 ENGINEER

2 inflow ediment quantity which i called a ediment Trap Efficiency (TE). Amount of ediment depoit TE (1) Amount of ediment in inflow Brune [4] ha developed a curve relating TE to capacity (V) to annual average inflow (I) ratio a preented in the Figure 1, extracted from Reervoir Sedimentation Handbook by Morri et.al [7]. Brune curve ha been widely ued to etimate the edimentation of reervoir. Siyam [9] ha alo demontrated that the above equation with value of β = , and would well-define the upper, median and lower Brune [4] curve repectively a hown in Figure 1. Churchill [5] developed a graphical relationhip between Releae Efficiency (RE) and Sedimentation Index (SI) a hown in Figure which i extracted from Reervoir Sedimentation Handbook by Morri et. al [7]. Figure 1 Brune curve [4] Thi curve can alo be preented in the following; V log I TE () where, TE = trap efficiency; V = reervoir capacity; and I = annual average inflow; Brune [4] had conidered only two parameter in hi formulation namely; the reervoir capacity and average annual inflow rate and many other parameter that affect the reervoir edimentation are not repreented in hi method. In addition, Brune [4] had ued only normally ponded reervoir in deriving thi empirical relationhip. However, hi method found to be not accurate enough in etimating edimentation quantity in reervoir with highly variable inflow, a the ediment trapped in the reervoir i very much influenced by the ediment inflow rate. Further, Siyam [9] ha hown that Brune [4] curve i a pecial cae of a more general trap efficiency function given by the following equation; V I TE 100 e... () where, β i a edimentation parameter that reflect the reduction in the reervoir torage capacity due to edimentation. Figure - Churchill curve [5] Sedimentation index can be defined a follow; Detention time S.I... (4) Mean velocity (Reervoir capacity) g S.I. (5) (Reervoirinflow) Reervoirlength Churchill ha alo conidered only a limited number of parameter uch a detention time and mean velocity, omitting other parameter affecting reervoir edimentation. But Trimble et.al [1] concluded that the Churchill [5] method appear to produce more realitic etimate of ediment yield than from the Brune method [4], for reervoir receiving ediment from an uptream reervoir. Borland [] ha alo concluded that the Churchill [5] curve i omewhat uperior to Brune curve [4], in etimating reervoir edimentation. Vertraeten et.al [6] have tated that although the ue of the Churchill curve [5] may give a better prediction of TE compared to Brune curve [4], but it i very difficult to obtain the neceary input data for calculating the edimentation index for Churchill [5] curve. Thi i probably the reaon why Brune (195) approach i ued o extenively a oppoed to Churchill [5]. ENGINEER

3 Brown [] alo ha alo preented a correlation for TE incorporating the Capacity (V) to Waterhed (W) ratio a illutrated in Figure. ha alo tated about the overetimation of both Brune [4] and Churchill [5] curve.. Methodology.1 Experimental Set-up Figure - Brown' curve [] The relationhip can alo be expreed a; 1. (6) TE D V W where, V = reervoir torage capacity expreed in m ; and W = catchment area expreed in km. D i a coefficient whoe value ranging from to 1, with a mean value of 0.1. Here, Brown [] alo conidered about the capacity (V) and waterhed area (W) of the reervoir where more ignificant number of parameter affecting the reervoir edimentation. Harbor et al [8] ha carried out experimental invetigation on reervoir edimentation and derived a curve which i imilar to the Brune curve [4]. Thi relationhip i given in Eq. (7). V I (7) TE V I Toniolo et al [11] ha alo carried out everal experiment on reervoir edimentation, but their reult appear to be very preliminary tage and have failed to produce any concluive reult. Bahar et al [1] have carried out bathymetric urvey on Roeire reervoir which i located in Sudan, taking the baeline a the deign torage capacity of the reervoir at the different level in 1966 and compared with both Brune [4] and Churchill [5]. There he concluded that both Brune [4] and Churchill [5] method overetimate the trap efficiency. Lewi et al [10] The experimental et-up conit of an overhead tank which ha dimenion of 1. m 1. m 1 m, to feed ediment laden water into the reervoir, a ediment agitator to keep ediment in upenion mode inide the overhead tank and a eparate tank of 0.5 m high, 1 m wide and m long, to repreent a reervoir. The ediment agitator conit of an electric motor, vertical haft with horizontal wing and a tand to fix the motor. Figure 4 how a chematic diagram of the experimental et-up. Sediment Agitator Overhead Tank Valve Inflow Experimental Reervoir Spillway Figure 4 - Schematic Diagram of Experimental Set-up The rectangular tank wa modified by placing four timber plate at four comer of the tank in order to avoid water tagnation at thoe corner. The reervoir bed wa made linearly loping toward the dam uing compacted oil to repreent the model much cloer to a real reervoir. The dam wa made out of timber which can be moved back and forth to change the reervoir capacity (V). Actual photograph of the experimental et-up i preented in Figure 5. The following important feature were available in the experimental et-up; - Poibility of controlling the inflow rate of ediment laden water to the reervoir by uing a gate value, - Ability to change the reervoir capacity by adjuting the downtream boundary (dam) of the model reervoir. - Poibility of changing the pillway length by fixing different ize of pillway ENGINEER

4 mould on the dam. In thi way, outflow from the reervoir could be changed. - Poibility of changing inflow ediment concentration by adding different quantitie of ediment to the overhead tank and mixing it with the agitator fixed to thi tank kg/m to 1.0kg/m. In addition, inflow rate were alo changed in the range of 00 ml/ to 400 ml/ and two pillway length of 00 mm and 00 mm were ued which control the outflow rate from the reervoir. 4. Reult and Dicuion 4.1 Experimental Reult Reervoir Model Figure 5 - Experimental Set-up A reddih andy oil which paed through 1 mm ieve wa ued a ediment for the propoed tet. Figure 6 illutrate the particle ize ditribution of the ediment mixed with the inflow to the reervoir. The mean diameter (d 50) of the oil wa found to be 0.0 mm. Figure 6 - Particle Size Ditribution of the Sediment. Experimental Procedure Overhead Tank Adjut the gate valve connected to the outflow pipe of the overhead tank to releae known flow rate to the reervoir and allowed to pill over the pre-et length of pillway. The experiment wa carried out for a duration of one hour. During the experiment, ix ediment ample were taken from the inflow at different time to get the averaged ediment concentration. After the experiment are over, the ediment depoited on the reervoir bed were collected and quantified. Thi procedure wa repeated for three different reervoir capacitie 97.1 l, l and l, while varying the inflow ediment concentration from A a typical reult, one et of experimental data collected from the experiment carried out for the reervoir capacity 97.1 l i given in Table 1. The reult tabulated here indicate the inflow ediment concentration (C), water inflow rate to the reervoir (Q), pillway length (S) and dry weight of depoited ediment on the reervoir bed (W ). Similar et of reult were collected for the reervoir capacitie of l and l. Table 1 - Experimental Reult for Reervoir Capacity 97.1 l C /(kg/m ) Q /(ml/) S/(mm) W /(g) The variation between trapped ediment weight (W ) and inflow rate (Q) i hown in Figure 7. It i not poible to interpret thi graph a there are few other parameter influencing the edimentation quantity (W ). ENGINEER 4

5 Figure 7 - Variation between Q and W The variation of trapped ediment weight (W ) with the inflow ediment concentration (C) i illutrated in Figure 8. Figure 9 - Comparion of experimental reult with the Brune curve [4] Figure 10 how the comparion of experimental reult with the Churchill curve [5]. In thi cae alo, the data point are cattered within a narrow region but, give a fare agreement with thi curve than that with the Brune curve. But it can be oberved that more than 50% of the reult are below the Churchill curve. Figure 8 - Variation between C againt W It could be een that there i a lightly better correlation between thee two parameter, though the data point are omewhat cattered that could be due to experimental error. Thi figure ugget that the inflow ediment concentration (C) i one of the mot influencing parameter on the amount of ediment trapped in the reervoir. 4. Comparion Experimental Reult with Brune and Churchill Curve Figure 9 illutrate the comparion of experimental data with the Brune curve [4]. Thi graph clearly how that the experimental data are not very much in agreement with the Brune curve [4]. Trap Efficiencie (TE) etimated from the experimental reult are motly located below the Brune curve [4], indicating that TE are omewhat underetimated when compare to Brune curve. Though the experimental data are very much cattered, it can be een that there i an increaing trend of TE with V/I ratio, a imilar to the Brune curve [4]. It could alo be noted that there can be more than one value of TE for a given V/I ratio, indicating that there can be other parameter affecting TE. Figure 10 - Comparion of the experimental reult with Churchill curve [5] It i important to highlight here that few other reearcher have alo reported about the overetimation of TE by the Brune [4] and Churchill [5] method baed on their tudie (Bahar et al [1] and Lewi et al [10]). A neither Brune [4] nor Churchill [5] method ha given a reaonable relationhip for the experimental data obtained in thi tudy, a freh data analyi ha been adopted uing Dimenional Analyi. 4. Dimenional Analyi The dry weight of ediment depoited (W ) in reervoir i affected by the following parameter, where, W f {,d,q,s,l,t,v,g,,,,c }... (8) 50 5 ENGINEER

6 = Dynamic vicoity of water [ML -1 T -1 ] d 50 = Mean diameter of ediment particle [L] Q = Inflow to the reervoir [L T -1 ] S = Weir length (Spillway length) [L] C = Sediment concentration [ML - ] t = Time [T] V = Reervoir volume [ L ] g = Gravitational acceleration [LT - ] = Bed roughne [L] L = Reervoir length [L] = Denity of water [ML - ] = Denity of ediment [ML - ] The capacity of the model reervoir (V) ued for the preent tudy i dependent only on the reervoir length (L). Therefore, the parameter L i removed from the above lit keeping the volume term. ε i removed a the reervoir bed in the experimental etup ha been a mooth urface. The term ρ i alo replaced by adding a * term * which i given by, g. Then, W f {,d,q,s,t,v,,,,c }... (9) 50 Hence, the non-dimenional term could be formulated a given below; * It ha already been noted that the inflow ediment concentration ha a dominant effect on trapped ediment amount over the other model parameter. Figure 11 how the relationhip between non-dimenional term repreenting inflow ediment concentration (C) and the trapped ediment weight (W ). Figure 11 - Variation of non-dimenional term repreenting W and C Further combining the term, inflow ediment concentration (C) and water inflow (Q) give the ediment inflow rate (I ) and thi term ha hown a good correlation with W where it propoe an increaing trend a illutrated in Figure 1. gw 1 1 g d 50 g S gv,, f * g t C Qg,,, 1 5 g,... (10) A experimental duration (t) and ediment denity (ρ ) were not varied during thi tudy, the correponding dimenionle term are conidered to be contant, and they do not affect the amount of depoited ediment in the reervoir for the preent tudy. Hence, the 1 1 term; g d * 50, and g t are g 1 removed from the analyi. 4.4 Improved correlation among model parameter The non-dimenional term repreenting the trapped ediment weight (W ) wa plotted againt the other non-dimenional term repreenting other model parameter which were obtained from the dimenional analyi. Figure 1 - Variation of non-dimenional term repreenting W and CQ Figure 1 how the effect of inflow ediment rate (I ) on the trapped ediment weight (W ) in the reervoir. Combining all the model parameter, a new relationhip wa derived a illutrated in Figure 1, where the amount of trapped ediment in the reervoir can be etimated uing all the parameter conidered in thi tudy. Thi new formulae incorporate the reervoir capacity (V), ediment inflow rate (I ), and pillway length (S) which i directly correlated to outflow from the reervoir. Thi relationhip indicate that the trapped ediment weight (W ) will reach a maximum value with increaing the term (I.V/S 1/ ). ENGINEER 6

7 ued in the experimental run and thu, the effect of ediment ize i not well repreented in thi method. However, the relationhip developed during thi tudy could be further improved by conducting more experimental run by varying few other parameter which are not conidered in the preent tudy. Figure 1 - Variation of trapped ediment weight (W ) with the model parameter Figure 11 ugget an equation a hown in Equation 11. ρgw μ 10 5 ρ μ 9 g I V 1 S (11) Equation 11 how the relationhip between the non-dimenional term repreenting trapped ediment weight (W ) and the combined nondimenional term of the model parameter. Sediment inflow rate (I ) and reervoir capacity (V) preent a direct proportionality to trapped ediment weight (W ), where pillway length (S) i inverely proportional. 5. Concluion and Recommendation An experimental invetigation wa conducted for invetigating the edimentation of reervoir and particularly, on trap efficiency of reervoir. Baed on the erie of tet reult, an improved correlation wa derived among the parameter governing the reervoir edimentation. Equation 11 preent a relationhip between the non dimenional term repreenting trapped ediment weight (W ) and the other model parameter uch a ediment inflow rate (I ), reervoir capacity (V) and pillway length (S). It can alo be found that the inflow ediment rate (I ) i the mot governing parameter for the edimentation of the reervoir according to the preent tudy. Equation 11 can be ued to etimate the amount of ediment depoited in a reervoir uing other parameter with a reaonable accuracy. However, the applicability of the propoed method i till limited only to the reervoir with continuou pilling condition. In addition, only one type of ediment gradation (d 50) wa A the preent tudy concentrate only on the continuou pilling condition of reervoir, everal experiment hould alo be conducted with non-pilling condition. Further tudie would alo be needed to account the operating rule of reervoir where ediment outflow from the reervoir will be partly retricted. Effect of reervoir hape and it orientation which are alo important factor affecting the amount of ediment trapped in the reervoir, need to be invetigated. Reference 1. Bahar, K. E., and Eltayeb Ahmed, 010. Sediment Accumulation in Roeire Reervoir. Nile Bain Water Science & Engineering Journal, Vol., (Iue ), pp Borland, W. M., 1971: Reervoir Sedimentation. In Shen, H. W., editor, River mechanic. Vol. II, Fort Collin, CO: Colorado State Univerity, pp Brown, C. B., 194: Dicuion of Sedimentation in Reervoir, by J. Witzig. Proceeding of the American Society of Civil Engineer 69, Brune, G. M, 195. Trap Efficiency of Reervoir. American Geophyical Union, 4(), pp Churchill, M. A., 1948: Dicuion of Analye and ue of Reervoir Sedimentation data by L.C. Gottchalk. In Proceeding of the federal interagency edimentation conference, Denver, Colorado, Wahington, DC: US Geological Survey, pp Gert Vertraeten and Jean Poeen, 000. Etimating Trap Efficiency of Small Reervoir and Pond: Method and Implication for the Aement of Sediment Yield,. Progre in Phyical Geography, (4), pp Gregory, L., Morri and Jiahua Fan, Reervoir Sedimentation Handbook. New York: McGraw-Hill Book Co. 8. Harbor, J., Bhaduri, B., Angelaki, L. and Snyder, J. 1997: Sediment bain, uing modified tormwater management bain and ediment bain to reduce water pollution from contruction ite in Ohio. Kent, OH Department of Geology, Kent State Univerity. 9. Siyam, A. M., 000: Reervoir Sedimentation Control. Ph.D. Thei, Univerity of Britol, England. 10. Stephen, E., Lewi Zoe, T., Bainbridge, Petra M. Kuhnert, Bradford S., Sherman, Brent Henderon, Cameron Dougall, Michelle Cooper, and Jon E. Brodie. 01 : Calculating Sediment 7 ENGINEER

8 Trapping Efficiencie for Reervoir in Tropical Setting: A Cae Study from the Burdekin Fall Dam, NE Autralia, Water Reource Reearch, Vol. 49, pp Toniolo, H., and Schultz, J., 005. Experiment on Sediment Trap Efficiency in Reervoir. Lake & Reervoir: Reearch and Management 005, vol 10, pp Trimble, S. W., and Carey, W. P., 1990: A Comparion of the Brune and Churchill Method for Computing Sediment yield Applied to a Reervoir Sytem. USGS Water Supply Paper 40, ENGINEER 8