Sediment deposition in a rigid monsoon drain

Transcription

1 Intl. J. River Basin Management Vol. 6, No. 1 (2008), pp IAHR, INBO & IAHS Sediment deposition in a rigid monsoon drain AMINUDDIN AB. GHANI, Professor and Deputy Director, River Engineering and Urban Drainage Research Centre (REDAC), Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, Nibong Tebal, Penang, Malaysia. redac02@eng.usm.my NOR AZAZI ZAKARIA, Professor and Director, REDAC, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, Nibong Tebal, Penang, Malaysia MAHATHIR KASSIM, Postgraduate Student, REDAC, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, Nibong Tebal, Penang, Malaysia ABSTRACT Field data collections to study the physical sediment characteristics and trends of sediment deposition were carried out at Raja River monsoon drain made up of concrete channels for the period of 2000 and Assessments of the existing incipient motion equations developed from experimental works were made using the measured field data. The results show that equations by Novak & Nalluri (1975), El-Zaemey (1991) and Ab. Ghani et al. (1999) are able to predict satisfactorily the sediment deposition in rigid channel. Keywords: Incipient motion; sediment deposits; rigid channel; storm drains. 1 Introduction Rapid development in major towns in Malaysia results in the construction of new drainage system mainly open monsoon or storm drains to cater the increase in surface runoff. Sediment depositions in these storm drains have been found to be a major cause of flash flood due to the loss in the hydraulic capacity of the drains. A constant minimum velocity of 0.9 m/s is recommended by the Department of Irrigation and Drainage (DID), Malaysia to minimize sedimentation problems. Recent studies (Ab. Ghani et al. 2000; Kassim et al. 2004; Kassim, 2005) in several major cities in Malaysia confirm the presence of loose deposited beds of non-cohesive sediments in rigid open storm drains with average sediment sizes between 0.35 mm and 2.40 mm. Ashley et al. (2004) found out that these ranges of sediment sizes in Malaysia are similar to European sediments which were collected from combined sewers. Kassim (2005) carried out field data collection along Raja River drainage system to identify the trend of sediment depositions in open storm drain. This paper will highlight the result of this new data collection programme and the assessments of existing incipient motion equations for rigid boundary channels (Delleur, 2001; Ashley et al. 2004). 2 Existing incipient motion criteria for rigid boundary channel The self-cleansing approach in terms of either minimum velocity (V c ) or shear stress (τ c ) is usually used to minimize sediment problems in urban drainage systems. The application of this approach is meant to avoid any deposition at any time or at least over a long period of time no deposits will build up (i.e. high flows flush the deposits). A number of experimental works have been carried out for the past 30 years especially in Europe (Delleur, 2001; Ashley et al. 2005) on sediment transport in urban drainage systems. Field data measurements of sediment deposits in combined sewers were also made to study the physical characteristics of sewer sediments and the trend of depositions (Bachoc, 1992; Blaszczyk & Ashley, 1996). The present study attempts to study the nature of sediment deposition in open storm drains. Experimental works at University of Newcastle upon Tyne, UK (Novak & Nalluri, 1975; Ojo, 1978; Novak & Nalluri, 1984; El-Zaemey, 1991; Nalluri et al. 1994; Nalluri & Ab. Ghani, 1996) and recent works at Universiti Sains Malaysia (Salem, 1998; Ab. Ghani et al. 1999; Ashley et al. 2005) suggest new methods to improve the constant velocity and shear stress criteria by taking into account several factors affecting sediment transport and incipient motion namely sediment characteristics (size, Received on August 10, Accepted on July 21,

2 24 Aminuddin Ab. Ghani et al. sediment concentration) and drain characteristics (size, slope and roughness). Table 1 shows a list of available incipient motion criteria from experimental works (Kassim, 2005) used to predict sediment deposition along the Raja River channel. The required parameters for the applications of these equations are the average sediment size (d50 ), the sediment specific gravity (Ss = 2.65), and the flow depth (yo ). The hydraulic radius, R is determined from the flow depth and cross section of the drain, and the dimensionless sediment size, Dgr (= d50 (g(ss 1)/ν2 )1/3 ; ν the kinematic viscosity of water) is determined from the average sediment size. The water density (ρ) is assumed as 1000 kg/m3 and the gravitational constant (g) is taken as 10 m2 /s. The width of deposition (B) is measured across the section. 3 Study area The Raja drainage catchment is part of the Alor Setar City Drainage scheme, Phase I (Figure 1). The scheme comprises Table 1 Existing incipient motion criteria from experimental works (Kassim, 2005). Researcher Equation Cross section Equation No. Novak & Nalluri (1975) Novak & Nalluri (1975) Novak & Nalluri (1975) Ojo (1978) Ojo (1978) Ojo (1978) Novak & Nalluri (1984) El-Zaemey (1991) El-Zaemey (1991) Ab.Ghani et al. (1999) Ab.Ghani et al. (1999) 0.24 Vc = 0.17(Ss 1)1/2 d50 Vc /(gd50 )1/2 = 0.61(Ss 1)1/2 [d50 /R] τc = 0.128(Ss 1)d Vc = 0.184(Ss 1)1/2 d50 Vc = 0.65(gd50 )1/2 (Ss 1)1.2 [d50 /R]-0.28 τc /ρgd50 (Ss 1) = 0.062(U /νd50 ) 0.54 Vc /[gd50 (Ss 1)]1/2 = 0.5[d50 /R] 0.4 Vc /[gd50 (Ss 1)]1/2 = 0.75[d50 /R] 0.34 Vc /[gd50 (Ss 1)]1/2 = 0.80[d50 /R]0.325 [y0 /B]0.04 Vc /[gd50 (Ss 1)]1/2 = 1.07[d50 /R] 0.23 τc /[ρgd50 (Ss 1)]1/2 = 0.17D 0.57 gr ; Circular ; Circular Deposited Circular Bed Deposited Circular Bed STUDY AREA SR7 SR8 SR9 SR6 SR5 SR4 SR3 SR10 SR2 SR1 Sg.Raja Pumping Station Figure 1 Study area Raja River urban drainage system for Alor Setar City.

3 Sediment deposition in a rigid monsoon drain 25 three catchment basins (Raja, Langgar & Putera) with a total catchment area of 300 ha and was proposed as part of a flood alleviation programme for the region. The Raja basin is the largest in the area (233 ha) and is also the most prone to problems of flooding. Flows from the Raja basin are drained by gravity to a pump station before being discharged via pumping mains to the River Kedah waterway. The main lengths of the drainage system take the form of trapezoidal/rectangular, open channel sections. The sections are lined in concrete, vary in width from 4 to 16 m and have been designed to a minimum velocity criterion of 0.9 m/s for the purpose of sediment self-cleansing. The study area has two typical monsoons; namely, the northeast monsoon and southwest monsoon. The northeast monsoon usually occurs from November to February. The southwest monsoon usually reaches the west coast of Peninsular Malaysia from the Indian Ocean and prevails over Peninsular Malaysia from May to August. In the transition period between the above two monsoons, from September to November, the western wind prevails and causes the heaviest rainfall in the study area in a year. Thus, the study area tends to have two rainy seasons in a year: one from April to May, and another from September to November. The annual rainfall depth in the study area is about 2,000 to 3,000 mm, while the temperature is about 27 Celsius on average. 4 Field data collection and results A sediment data collection programme (Kassim, 2005) was carried out in 2000 and 2001 whereby measurement of sediment deposition were made at ten stations along Alor Derga and Alor Siam forming the upstream branches Raja River and the main reach of the Raja River (Figure 1). The first measurements were carried out before the 2000 rainy season while the second measurements were conducted after 2001 rainy seasons. Thus a comparison can be made on the sediment characteristics and deposition for conditions before and after rainy seasons. For each station (Figure 2), the thickness of sediment deposits and flow depths along the channel was measured over a 20-metre distance at an interval of 2 m spacing (Figure 3). The accuracy of the sediment thickness survey is ±5 mm. Two persons at both banks of the drain were required to hold the survey staff vertical via a rope throughout the measurement. A plate was attached to the survey staff at the bottom to ensure that the staff remained on the top of the sediment deposition. For each interval, three measurements of sediment thickness were made. Samples of sediment (Figure 4) were also collected by grab during the measurement of sediment profiles. Figure 5 shows an example of the slope of sediment deposit (S b ) as compared to the as-built slope (S o ). Similar trends of sediment deposition were found at all stations. All the measurements were made at low discharge to make sure no sediment transport occurred during the field work. This would allow the highest depth of deposition to be measured. Tables 2 to 4 give the summary of the sediment deposit and hydraulic characteristics during the field works carried out in 2000 and The ranges of data for the field work are 0.18 < V(m/s) <0.25, 0.45 < d 50 (mm) <1.80, S s = 2.50, 0.24 < y o (m) <1.23, and 0.36 < y s (m) <1.25 where V is the flow velocity, and y s the main channel sediment deposit thickness. The sediment size distributions for both periods of measurements are given in Figure 6 indicating that they are all non-cohesive. The flow velocity was computed from Manning equation based on the measured flow depth and main channel sediment slope. Taking into account the presence of the sediment deposits, a value of for the coefficient of Manning (n) was assumed for all stations in computing the flow velocity. It can be seen from Tables 2 and 3 that coarser sediments are found in the bed after rainy season (Table 3) compared to before Figure 2 Sediment deposit thickness survey at station SR1 (10th June 2000).

4 26 Aminuddin Ab. Ghani et al. SR7 Cross section Cross section x x x x x x x x x x x x x x x x x x x x x x B Q x x x x x 2m x x x x x x Study Reach L= 20m Sediment deposit sampling X survey points Figure 3 Measurement points at a station (SR 7). Figure 4 Sediment deposit sampling at station SR6 (20th June 2000).

5 Sediment deposition in a rigid monsoon drain 27 Distance (m) Elevation (m) Flow Direction S b = S o = sediment as-built (center) (a) Profile of Sediment Deposition Channel Width (m) Elevation (m) Sediment (b) Thickness of Sediment Deposition Figure 5 Sediment deposition at station SR1 (10th June 2000). Table 2 Sediment deposit and hydraulic characteristics for 2000 data. Station Y o (m) A (m 2 ) P (m) R (m) V (m/s) d 50 (mm) Sediment thickness (m) Left bank Main channel Right bank SR SR SR SR SR SR SR SR SR SR rainy season. This indicates that bed erosion has occurred during high flow in the rainy season and carried along the finer sediments. Table 4 also shows that milder sediment slopes are present before rainy season ( ) as compared to after rainy season ( ). All sediment bed slopes differ than the as-built slopes indicating that attempts should be made to compute possible trend of sediment deposition. Hence the assessment of the existing incipient motion equations are greatly required since almost all the available equations were derived from experimental works.

6 28 Aminuddin Ab. Ghani et al. Table 3 Sediment deposit and hydraulic characteristics for 2001 data. Station Y o (m) A (m 2 ) P (m) R (m) V (m/s) d 50 (mm) Sediment thickness (m) Left bank Main channel Right bank SR SR SR SR SR SR SR SR SR SR Table 4 Comparisons between measured sediment slopes and as-built slopes. Station SR1 SR2 SR3 SR4 SR5 SR6 SR7 SR8 SR9 SR10 Sediment deposit slope, S b (2000) Sediment deposit slope, S b (2001) As-built slope S o Assessments of incipient motion equations Percentage Passing (%) Percentage Passing (%) Size (mm) SR1 SR2 SR3 SR4 SR5 SR6 SR7 SR8 SR9 SR10 (a) Size (mm) Tables 5 and 6 give the results of comparison between the calculated sediment deposit slopes and the observed ones for the ten stations during measurement period in 2000 and For the velocity critical equations (Equations 1, 2, 4, 5 and 7 10), the computed slopes were obtained by the use of Darcy-Weisbach equation. Whereas for the case of critical stress equations (Equations 3, 6 and 11), the computed slopes were obtained from the definition of shear stress (τ c = ρgrs b ). The criterion used to select the best equation is the discrepancy ratio namely the ratio of calculated slopes using the equations listed in Table 1 over the observed slopes. If the discrepancy ratio is between 0.5 and 2.0, the equation is deemed to be accepted for use (Yang 1996). The summary of the assessment of the existing incipient motion criteria is given in Table 7. The results show that the critical velocity criterion given by Novak and Nalluri (Equation 2) seems to give the best prediction of the sediment deposit slopes followed by El-Zaemey (Equation 8) and Ab. Ghani et al (Equations 10 and 11). The highest percentage of discrepancy value in the accepted range of 0.5 and 2.0 is 55% as given by Equation 2 (Figures 7 and 8). The results suggest the different conditions at field compared to the controlled condition in the laboratory. Revision of the existing equations may be needed to improve the simulation of deposition in actual storm drains. SR1 SR2 SR3 SR4 SR5 SR6 SR7 SR8 SR9 SR10 6 Conclusions Figure 6 (b) 2001 Sediment distribution curves for all stations, Raja River. A detailed study on sediment deposition trend was carried out in 2000 and 2001 whereby measurements of sediment deposition

7 Sediment deposition in a rigid monsoon drain 29 Table 5 Discrepancy ratio for 2000 data. Station Equation SR SR SR SR SR SR SR SR SR SR Table 6 Discrepancy ratio for 2001 data. Station Equation SR SR SR SR SR SR SR SR SR SR Table 7 Summary of discrepancy analysis for existing incipient motion equations (2000 and 2001 data). Equation No. No. of data within Percentage (%) discrepancy ratio Figure 7 Assessment of equation 2 for 2000 data. were made at ten stations along Raja River drainage system in Alor Setar. An assessment of existing incipient motion criteria for rigid boundary channel developed from laboratory works shows that the equation developed by Novak and Nalluri (Equation 2) seems to give the best prediction of the sediment deposit slopes followed by El-Zaemey (Equation 8) and Ab. Ghani et al. (Equations 10 and 11).

8 30 Aminuddin Ab. Ghani et al. Notations Figure 8 Assessment of equation 2 for 2001 data. A Flow cross-sectional area (m 2 ) B Deposition width (m) d 50 Average sediment size (mm) D gr Dimensionless sediment size (= [(S s 1)g/ν 2 ] 1/3 d 50 ) g Gravitational constant n Manning roughness coefficient P Flow wetted perimeter (m) Q Flow discharge (m 3 /s) R Flow hydraulic radius (m) S o As-built Slope of Channel S b Measured Slope of deposition S s Sediment specific gravity (= ρ s /ρ) y s Main channel sediment bed thickness V Average flow velocity V c Critical velocity y o Average flow depth γ s Specific weight of sediment ν Kinematic viscosity of water ρ Density of water ρ s Density of sediment Critical shear stress τ c References 1. Ab. Ghani, A., Zakaria, N.Z., Kassim, M. and Ahmad Nasir, B.(2000). Sediment Size Characteristics of Urban Drains In Malaysian Cities, Journal of Urban Water, 2(4), Ab. Ghani, A., Salem, A.M., Abdullah, R., Yahaya, A.S. and Zakaria, N.A. (1999) Incipient Motion of Sediment Particles Over Deposited Loose Beds in a Channel, 8th International Conference on Urban Storm Drainage, Sydney, 30 August 3 September. 3. Ashley, R.M., Bertrand-Krajewski, J.L., Hvitved- Jacobsen, T. and Verbanck, M. (2004). Solids in Sewers Characteristics, Effects, and Control of Sewer Solids and Associated Pollutants, IWA Publishing, London, 340pp. Scientific and Technical Report No. 14, ISBN Ashley, R.M., Bertrand-Krajewski, J.L. and Hvitved- Jacobsen, T. (2005). Sewer Solids 20 Years of Investigation, Journal of Water Science and Technology, 52(3), Bachoc, A. (1992). Location and General Characteristics of Sediment Deposits into Man-Entry Combined Sewers, Journal of Water Science and Technology, 25(8), Blaszczyk, P. and Ashley, R.M. (1996). Application of New Criteria to Control Sediment Problems in Combined Sewers in Poland, Journal of Water Science and Technology, 33(9), Delleur, J.W. (2001). New Results and Research Needs on Sediment Movement in Urban Drainage, Journal of Water Resources Planning and Management, ASCE, 127(3), El-Zaemey, A.K.S. (1991). Sediment Transport Over Deposited Beds in Sewers. Ph.D. Thesis, Dept Civil Eng., University of Newcastle upon Tyne, England. 9. Kassim, M. (2005). Sediment Deposition in a Rigid Monsoon Drain: A Case Study of Raja River, Alor Setar, MSc Thesis, School of Civil Engineering, Universiti Sains Malaysia. 10. Kassim, M., Ab. Ghani, A., Abdullah, R. and Zakaria, N.A. (2004). Prediction of Sediment Deposition in Raja River Concrete Drainage System: A Case Study, Water and Environmental Management Series: Sewer Networks and Processes Within Urban Water System, International Water Association (IWA), pp Nalluri, C. and Ab. Ghani, A. (1996). Design Options For Self-Cleansing Storm Sewers, Journal of Water Science and Technology, 33(9), Nalluri, C., Ab. Ghani, A. and El-Zaemey, A.K.S. (1994). Sediment Transport Over Deposited Beds in Sewers, Journal of Water Science and Technology, 29(1 2), Novak, P. and Nalluri, C. (1975). Sediment Transport in Smooth Fixed Bed Channels, Hydraulics Division, 101(HY9), Novak, P. and Nalluri, C. (1984). Incipient Motion of Sediment Particles Over Fixed Beds, Hydraulic Research, 22(3), Ojo, S.I.A. (1978). Study of Incipient Motion and Sediment Transport Over Fixed Beds, Ph.D. Thesis, Dept Civil Eng., University of Newcastle upon Tyne, England. 16. Salem, A.M. (1998) Incipient Motion Over Loose Deposited Beds in a Rigid Channel, M.Sc. thesis, School of Civil Engineering, University of Science. 17. Yang, C.T. (1996). Sediment Transport Theory and Practice, McGraw Hill, New York.