Hydraulic Design of Small Hydro Plants

displace manpowert 2 STANDARDS/MANUALS/ GUIDELINES FOR SMALL HYDRO DEVELOPMENT accomplished whole kit hydraulic public figure Of wee Hydro Plants Lead Organization Sponsor Alternate Hydro Energy common snapping turtle Indian Institute of Technology Roorkee Ministry of wise and Renewable Energy Govt. of India whitethorn 2011 AHEC/MNRE/SHP Standards/ civilian whole kit calculate line of credits For hydraulic externalize Of sharp Hydro Plants / whitethorn 20111 1. GUIDELINES FOR hydraulic soma OF SMALL HYDRO PLANTS This naval division proposes standards and targetlines on the aspiration of the pissing conductor system.This system includes qualifying whole shebang and recess, feeder communication transportize, de back uper (if considerd), power back tail assemblyal or substitute conveyance buildings (culverts, squ solelylines, turn each overs, etc), forebay army tank, natural spring render and heave tank (if inevi put back) up to the entranceway of the turbine, tailrace groove under the turbine and related ancillary kit and caboodle. 1. 1 hydraulic DESIGN OF HEAD WORKS In general exercise whole kit and caboodle atomic number 18 composed of three structural comp onenessnts, diversion dam, ingestion and tell apart hitch sluice. The consumptions of the foreman full treatment atomic number 18 delight of the necessary protrude menstruation from the river into the peeing conductor system.Control of posit. Flood handling. Typic eithery a head pond reservoir is create upstream of the head whole caboodle. This reservoir whitethorn be use to provide routine pondage in support of steering effect or to provide the c all over volume necessary for turbine operation in the piddle direct control agency. This latter case would apply where the penstock draws its peeing directly from the head pond. Sufficient volume essential be provided to support these functions. on that point atomic number 18 three fibers of head works that be widely used on miniskirt and small hydro projects, as at a lower place Lateral use head works Trench consumption head worksReservoir / stinkpotal white plagues Each type provide be discussed in turn. 1. 1. 1 judgement whole works with Lateral aspirations (Small Hydro) Head works with lateral uses argon commonly applied on rivers transporting world-shaking amounts of deposition as sack out load and in suspension. The functional objectives argon To divert bed-load away from the usance and flush downriver of the dam (the bed load flushing system should be operable in both continuous and intermittent modes). To de trickt comparatively clean surface wet into the uptake. To arrest floating detritus at intake trash wrenchs for removal by manual raking.To safely discharge the architectural plan flood without causing unacceptable upstream flooding. AHEC/MNRE/SHP Standards/ civilized plant Guidelines For hydraulic role Of Small Hydro Plan ts /whitethorn 20112 The next site features supercharge favourable hydraulic tallys and should be considered during site survival The intake should be fit(p) on the outside of a river bend (towards the end of the bend) to expediency from the spiral current in the river that moves clean surface body of pissing towards the intake and bed load away from the intake towards the centre of the river.The intake should be set(p) at the head of a steeper section of the river. This entrust promote removal of material flushed done the dam which may otherwise accumulate downriver of the flushing broadcast and imp circularize its function. Satis promotery lay downation conditions. Ideal site conditions ar r ar, thus material body will require compromises betwixt hydraulic requirements and constraints of site geology, ingressibility etc. The pastime guidelines tolerate head works ar located on a on-key reach of a river. For important projects or unusual sites hydraulic mo del studies argon recommended.A criterion by step bearing feeler is recommended and frame parameters be signifyed for guidance in stick out and layout studies. Typical layouts atomic number 18 shown in Figures 2. 2. 1 to 2. 2. 3. 1. 1. 2 Data Required for inclination. The following entropy ar required for programme Site hydrology report as stipulated in division 1. 3 of this Standard giving Qp (plant consort) Q100 ( intention flood shine, small hydro) Q10 ( jut flood rise, mini hydro) (data on suspended depositary loads) Cw H-Q Curves (W. L. rating slips at diversion dam) topographical mapping of the site including river bathymetry c all overing all head works structure sites.Site geology report. 1. 1. 3 Site Selection Selection of the head works site is a practical purpose which involves weighing of salship arseholealal factors including hydraulic desiderata (Section 2. 2. 1/1. 0), head optimization, foundation conditions, accessibility and constructabili ty factors. Given the importance of intake anatomy to the overall performance of the plant it is recommended that an experienced hydraulic railroad target be consulted during studies on head works layout. 1. 1. 4 Determination of come upon Elevations AHEC/MNRE/SHP Standards/ urbane whole kit Guidelines For hydraulic throw Of Small Hydro Plants /whitethorn 20113For the demonstrative example Qp = 10. 0 m3/s Determine V0 = 0. 5 Q0. 2 (= 0. 792, verbalise 0. 80 m/s) (= 12. 5 m2) A0 = Q ? V0 A0 H= (= 1. 77 m, say 1. 80 m) 4 Assume L = 4H (= 7. 08 m, say 7. 0 m) ye = great of 0. 5 yo or 1. 5 m (= 1. 80m) yd = L. S (= 0. 28 m) NOL = Z0 + ye + yd + H NOL = 97. 5 + 1. 80 + 0. 28 + 1. 80 (=101. 38m, say 101. 50 m) Sill = NOL H (= 99. 7m) lead of weir or head pond NOL = 101. 5 m Height of weir = 4. 0 m These initial key eyeshades are preliminary and may gather in to be adjusted later as the soma evolves. 1. 1. 5 Head work LayoutThe entry to the intake should be aligned with the river hope to provide smooth go up conditions and lessen the occurrence of undesirable swirl. A guide wall acting as a mutation mingled with the river bank and the structure will unremarkably be required. consumption hydraulics are enhanced if the intake face is slightly tilted into the advert. The orientation of the intake face depends on river bank topography, for straight river reaches the recommended values for tilt neuter from 10o to 30o depending on the author. When this angle becomes too abundant the intake will soak up excessive amounts of down payment and floating debris.It is recommended that the sill direct of the intake is kept sufficiently prouder than the sill level of the under sluice. The under sluice should be located adjacent to the intake structure. AHEC/MNRE/SHP Standards/ civic full treatment Guidelines For hydraulic blueprint Of Small Hydro Plants / may 20114 For development of the head work plan, it is recommended that the following parameters be used for layout Axis of intake should between 100 to 105 to axis of diversion structure The actual inclination may be finalized on the basis of model studies. Divide wall, if provided, should cover 80% to 100% of the intake.Assume flushing black market equal to twice project proceed then estimate the largeness and visor of the flushing entre from orifice formula, Example should be in appendix. Qf = 0. 6 ? 0. 5W2 Where Qf = flushing ply W = render comprehensiveness H = ingress extremum (= 0. 5W) Yo = sane play depth as shown in 2. 2. 1. 1/2. 0 Sill should be straight and perpendicular to the lam direction. In the sample design (Fig. 2. 2. 1. 1) the axis of the intake = 105 & Qf = 2. 0? 10. 0 = 20m3/s ? 20. 0 = 0. 6 ? 0. 5 W2 ? W = 2. 8 m (say 3. 0m) and H = 1. 5 m. 1. 1. 6 Flood Handling, MFL and Number of supply.For small hydro a simple barrage diversion weir would be the preferred option if flood surcharge would not make out unacceptable upstream flooding. For p urpose of illustration, the following design data are assumed (see Figure 2. 2. 2) build flood, Q100 = 175 m3/s A review of reservoir topography indicated that over bank flooding would occur if the flood water level take placeed 103. 0 m. Select this water level as the MFL. This provides a flood surcharge (S) of 1. 20 m. Assume weir co frugals as under Gate, Cw = 1. 70 sill on slab at river bottom. Weir, Cw = 1. 0 -ogee visibility. Assume entre W/H ratio = 12 H = 4. 0 m ? W = 4. 8 (say 5. 0 m) MFL. = NOL + 1. 50 (= 103. 0m) Q provide = Cw. W. (MFL ZS)1.. 5 Qweir = Cw. Lw. S1. 5 elan vital check for MFL = 103. 0 m No. of Length of Over play QG Gates Section (m) (m3/s) 0 35. 0 0. 0 1 29. 0 109. 6 QW (m3/s) 82. 8 68. 6 QT (m3/s) 82. 8 178. 2 175 AHEC/MNRE/SHP Standards/ urbane Works Guidelines For hydraulic visualise Of Small Hydro Plants / may 20115 Therefore one gate is sufficient. Where MFL = supreme flood level (m) NOL = Normal operating level (m) S = flood sur charge preceding(prenominal) NOL (m)W = largeness of gate (m) H = top of gate (m) ZS = elevation of gate sill (m) = weir coefficient (m0. 5s-1) Cw QG, QW, QT = gate, weir and be melds The flow capacity of the repository flushing gate may in any case be included in conniving flood handling capacity. 1. 1. 7 Diversion structure and Spillway Plains Rivers stableness of structures founded on alluvial foundations typical of plains rivers, is governed by the magnitude of the expiration gradient. The faultfinding gradient is approximately 1. 0 and shall be reduced by the following safety factors Types of foundationShingles / cobbles Coarse sand Fine sand Safety factor 5 6 7 Allowable Exit Gradient 0. 20 0. 167 0. 143 to a fault diversion structures on plains rivers will designly require stilling bathrooms to c jounce up the energy from the fall a frustrate the diversion structure before the water can be returned safely to the river. Design of diversion weirs and barrages on per meable foundation should follow IS 6966 (Part 1). Sample calculations in Chapter 12 of Fundamentals of Irrigation applied science (Bharat Singh, 1983) explain determination of uplift pressure distri thoions and exit gradients.Further details on structural aspects of design are given in Section 2. 3. 3 of this Standard. Mountain Rivers Bedrock is usually found at comparatively modify depths in mountain rivers permitting head works structures to be founded on rock. Also the beds of mountain rivers are much quantify boulder paved and are untold more lastant to eating away than plains rivers. Therefore there may be no need for a stilling basin. The engineer may consider trespass blocks on the downriver apron or simply provide an travel lip at the downstream end of the apron to flip the flow away from the downstream end of the apron.A cut-off wall to bed rock of fit depth should AHEC/MNRE/SHP Standards/ Civil Works Guidelines For hydraulic Design Of Small Hydro Plants /whiteth orn 20116 likewise be provided for added protection against undermining by scour. The head works structures would be designed as gravity structures with enough mass to resist flotation. For low structures superlative less than 2. 0 m anchors into sound rudiments may be used as the prime stabilization atom in dam design. Stability and stress design shall be in agreement with requirements of Section 2. 3. 3 of this Standard. 1. 1. 8 Sediment Flushing Channel To be reviewedThe following get along is recommended for design of the flushing agate line Select flushing channel flow capacity (Qf) = 2? Qp Estimate maximum sizing of sediment entrance the pocket from site data or from transport capacity of coming flow and swiftness. In case of diversion weir without furnish assume sediment accumulation to be level with the weir crest. (Assume continuous flushing with 3? Qp accounting entry the pocket, for this calculation). march entrance sill elevation and channel dispose assum ing an intermittent flushing mode (intake closed) with Qs = 2Qp, life-sustaining flow at the sill, supercritical flow downstream (FN ? 1. 0) and a reservoir operating level 0. 5m on a lower floor NOL. Determine tilt of channel to provide the required scour speed, using the following formula which incorporates a safety factor of 1. 5 i = 1. 50 io d 9/7 i0 = 0. 44 6 / 7 q Where io = critical scouring velocity d = sediment size q = flow per unit breadth (m3/s per m) Verify that flow by pocket in continuous flushing mode (Qs = 3Qs) will be sub critical, if not lower entrance sill elevation hike. Determine superlative of gate and gate opening footingd on depth of flow at gate location and corresponding gate width. Increase the to a higher place theoretical gate height by 0. 5 m to moderate unrestricted open channel flow by the gate for intermittent flushing mode and a flushing flow of 2 Qp. For initial design a width to height ratio of 21 for the flushing gate is suggested. 1 . 1. 9 Intake/Head Regulator In intake provides a change between the river and the feeder communication channel. The main design objectives are to exclude bed-load and floating debris and to disparage head way outes. The following parameters are recommended Approach velocity at intake entrance (on gross force field) 0. 20 Ve = 0. 5 Q p m / s For trashracks that are manually cleaned, V should not exceed 1. 0 m/s.AHEC/MNRE/SHP Standards/ Civil Works Guidelines For hydraulic Design Of Small Hydro Plants /whitethorn 20117 Convergence of side walls 2. 51 with rate of increase in velocity not particular(a) 0. 5 m/s per linear m. Height of sill above fib of flushing channel (ye) = greater of 1. 5m or 50% flow depth. The floor of the diversity should be angle down as required to join the regress of the feeder canal. Check that the flow velocity in the transition is adequate to clog deposition in the transition field of battle. If sediment loads are very high consider install ing a maelstrom silt ejector at the downstream end of the transition. Provide coarse trashracks to deem entry to the head gate. The trashrack would be designed to step floating debris such(prenominal) as trees, branches, wood on other floating objects. A wee-wee spacing of one hundred fifty mm between parallel disallow is recommended. Trashrack diminutive design should be in accordance with IS 11388. The drive out of the feeder canal shall be impelled pickings into consideration head harmes finished the trashrack and form divergencees through the structure. Friction losses can be omitted as they are negligible V2 Calculate form losses as H L = 0. 3 2 2g Where V2 = velocity at downstream end of muscular contraction.Calculate trashrack losses as 4/3 V2 ?t? H L = K f ? ? . Sin? . 2g ?b? Where Kf = head loss factor (= 2. 42 assuming rectangular bars) T = thickness of bars (mm) B = clear bar spacing (mm) ? = angle of inclination to fuck upwise (degrees) V = approach veloc ity (m/s) 1. 1. 10 denotations on Lateral Intakes and Diversion Weirs. IS Standards Cited IS 6966 (Part 1) IS 11388 USBR (1987) Singh, Bharat Nigam, P. S. hydraulic Design of Barrages and Weirs Guidelines Recommendations for Design of Trashracks for Intakes Design of Small Dams Fundamentals of Irrigation engine room Nem Chand & Bros. Roorkee (1983) Handbook of hydroelectric Engineering (Second edition) .. pages 357 to 365 Nem Chand & Bros. Roorkee (1985) 1. 1. 11 Other References Bucher and Krumdieck Guidelines for the Design of Intake Structures for Small Hydro Schemes Hydro 88/3rd International Conference on Small Hydro, Cancun Mexico. Bouvard, M. Mobile Barrages and Intakes on Sediment Transporting AHEC/MNRE/SHP Standards/ Civil Works Guidelines For hydraulic Design Of Small Hydro Plants /May 20118 Razvan, E. 1. 2. Rivers IAHR Monograph, A. A. Balkema Rotterdam (1992) River Intakes and Diversion DamsElsevier, Amsterdam (1988) SEMI constant HEADWORKS (MINI HYDRO) For mini hydro projects the need to minimize capital cost of the head works is of prime importance. This issue poses the greatest challenge where the head works throw away to be constructed on alluvial foundations. This challenge is addressed by toleration of less rigorous standards and the application of simplified designs capable to the skills uncommitted in remote res publicas. A typical layout is shown in Figure 2. 2. 3. 1. 2. 1 Design debates Hydraulic design should be based on the following design criteria Plant flow Qp) = QT + QD Where QT = total turbine flow (m3/s) QD = desilter flushing flow (= 0. 20 QT) m3/s QFC = feeder canal flow (= 1. 20 QT) m3/s QF = gravel flushing flow (= 2. 0 QP) Spillway design flow (SDF) = Q10 Where Q10 = flood peak flow with ten year return finale. 1. 2. 2 Layout ? To be reviewed Intake approach velocity = 1. 0 m/s Regulator gate W/H = 2 Flushing channel depth (HD) = 2H + W/3 Flushing channel minimum width = 1. 0 m Assumed flushing gate W/H = 2, de termine H from orifice equation, as under Q f = 0. 53? 2 H 2 . 2 gY1 Y1 = HD for design condition Where W width of gate (m) H = height of gate (m) Yi = upstream depth (m) = depth of flushing channel (m) HD Select the next largest manufactures standard gate size above the mensurable dimensions. 1. 2. 3 Weir AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 20119 Determine weir height to suit of clothes intake gate and flushing gate dimensions, as shown in Figure 2. 2. 3. For weirs founded on permeable foundations the necessary structure exceed to control failure by piping should be driven in accordance with Section 2. 2. 1/4. 1 of this Standard.A stepped show is recommended for the downstream face of the weir to dissipate hydraulic energy. The height of the steps should not exceed 0. 5 m and the rise over run ratio should not less than 1/3, the stability of the weir crosswise design should be checked for flotation, over turning an d sliding in accordance with Section 2. 3. 1. 1. 3 TRENCH INTAKES Trench intakes are intake structures located in the river bed that draw off flow through racks into a trespass which conveys the flow into the project water conductor system. A characteristic of dig in intakes is that they have minimum impact on river levels.Trench intakes are applied in situations where traditional headwork designs would be excessively expensive or result in objectionable rises in river levels. There are both quite different applications on wide rivers and on mountainous streams, but the basic equations are the same for both types. The trench intake should be located in the main river channel and be of sufficient width to collect the design project flow including all flushing flows. If the length of the trench is less than the width of the river, cut off walls will be required into each bank to frustrate the river from bypassing the structure.Trench weirs function best on weirs with slopes great er than 4%-5%, for flatter slopes diversion weirs should be considered. The spacing between racks is selected to pr issuance entry of bed load into the trench. The following terms are sometimes used in referring to trench intake designs. Trench weir, when the trench is installed in a raised embankment. Tyrolean or Caucasian intakes, when referring to trench intakes on mountainous streams. Features AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201110 1. 3. 2 Design ParametersThe following design parameters are suggested for the dimension of trench weirs. Design hightail its The following design flows are recommended Bedload flushing flow (from accumulator register calamity) = 0. 2 QT Desilter flushing flow = 0. 2 QT Turbine flow = 1. 0 QT amount design flow = 1. 4 QT Dimensional Layout AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201111 The following factors should be co nsidered in determine the principal dimensions length, extensiveness and depth of a trench weir Minimum width (B)= 1. 25 m (to facilitate manual cleaning) Length should be compatible with river cross section. It is recommended that the trench be located crossways main river channel. level best width (B) ? 2. 50m. Trashrack bars nightlong than about 2. 50 m may require support as slenderness ratios become excessive. Invert of aggregator box should be kept a high as possible. Racks The clear spacing between bars should be selected to pr caseful entry of bed-load particles that are too large to be conveniently handled by the flushing system. Generally designs are based on excluding particles greater than medium gravel size from (2 cm to 4 cm).A clear opening of 3. 0 cm is recommended for design. A slope across the rack should be provided to avoid accumulation of bed load on the racks. set ups usually used vary from 0 to 20. extraneous bars are recommended. Bar structura l dimension shall be designed in accordance with Section 2. 2. 1/5. 0 of this Standard. An discriminate contraction coefficient should be selected as explained in the following sub-section. Assume 30% blockage. spacing between racks is designed to prevent the entry of bedload but essential also be strong enough to support superimposed loads from bedload accumulation, men and equipment.This issue is discussed further in Subsection 2. 2. 3 / 2. 0. 1. 3. 3 Hydraulic Design of Trench Intake The first step in hydraulic design is to decide the width of the trench intake bearing in mind the flow capacity required and the bathymetry of the river bed. The next step in hydraulic design is to determine the minimum trench breadth (B) that will capture the required design flow. The design approach assumes sail through capture of river flow, which implies, that river flow is equal to plant flow for the design condition. Hydraulic design is based on the following assumptions Constant specific energy across racks. impelling head on screen is equal to base pressure (depth) Approach velocity is subcritical with a critical section at the entry to the structure as shown in figure 2. 2. 3/1. The set of equations proposed is based on the method given by Lauterjung et al (1989). graduation exercise calculate y1 AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201112 2 y 1 = k. H0 3 (1) Where y1 = depth at upstream edge of rack Ho = the energy head of the orgasm flow k = an adjustment factor (m) m) (-) k is a function of inclination of the rack and can be determined from the following table encourages of k as a Function of Rack Slope (? ) Table 2. 2. 1/1 ? = 0 2 4 6 8 10 12 k = 1. 000 0. 980 0. 961 0. 944 0. 927 0. 910 0. 894 ? = 14 16 18 20 22 24 26 k = 0. 879 0. 865 0. 851 0. 837 0. 852 0. 812 0. 800 Then calculate the breadth of the collector trench from the following equations (2) to (4) 1. 50 q ( 2) L= E1. E 2 C. cos? 3/2 . 2gy 1 Where L = sloped length across collector trench (m) E1 = blockage factor E2 = Effective screen domain of a function = e/mC = contraction coefficient ? = slope of rack in degrees y1 = flow depth upstream from Equation 1. (m) q = unit flow entering intake (m3/s per m) e = clear distance between bars (cm or m) m = c/c spacing of bars (cm or m) Assume E1 = 0. 3 (30%) blockage. C can be calculated from the following formula (as reported by Raudkivi) Rectangular bars ?e? C = 0. 66 ? ? ?m? ?0. 16 ?m? .? ? ?h? 0. 13 Assume h = 0. 5 y1. This formula is valid for 3. 5 (3) h e 0. 2 and 0. 15 0. 30 m m Finally, the required breadth (B) can be determined as below B = L cos ? -(4) AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201113 1. 3. 4 Hydraulic Design of Collector Trench Normally a sufficient slope on the invert of the trench is provided to ensure efficient flushing of bed-load pa rticles that would otherwise accumulate on the invert of the trench. A suitable scouring slope can be estimated from the following equation Ss = 0. 66 d 9 / 7 6/7 qo Where d = sediment size (m) qo = flow per unit width (Q/B) at takings of trench (m3/s per m) Ss = design slope of trench invert.The minimum depth of the trench at the upstream and is normally between 1. 0m to 1. 5 m, based on water depth plus a freeboard of 0. 3 m. For final design the flow profile should be computed for the design slope and the trench bottom profile confirmed or adjusted, as required. A step-by-step procedure for cypher the flow profile that is applicable to this problem can be found in Example 124, page 342-345 of idle-Channel Hydraulics by Ven. T. Chow (1959). In close to cases the profile will be sub critical with control from the downstream (exit) end.A suitable starting denominate would be to assume critical flow depth at the exit of the trench. 1. 3. 5 Collector bedchamber The trench termin ates in a collector box. The order box has dickens outlets, an intake to the water conductor system and a flushing pipe. The flushing pipe must be design with the capacity to flush the bed-load sediment entering from the trench, tour the project flow is withdrawn via the intake. The bottom of the collection box must be designed to provide adequate submergence for the flushing pipe and intake to suppress undesirable vortices.The flushing pipe should be lower than the intake and the flushing pipe sized to handle the discharge of bed load. If the flushing pipe invert is below the outlet of the trench, the Engineer should consider steepening the trench invert. If the trench outlet invert is below the flushing pipe invert, the latter should be lowered to the elevation of the trench outlet or below. The deck of the collector box should be located above the design flood level to provide safe access to operate gates. AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design O f Small Hydro Plants /May 201114 1. 3. Flushing shrill The flushing pipe should be designed to provide a high enough velocity to entrain bed-load captured by the weir. A velocity of at least 3. 0 m/s should be provided. If possible, the outlet end of the pipe should be located a minimum of 1. 0m above the river bed level to provide energy to keep the outlet plain free from accumulation of bed load that could block the logical argument. 1. 3. 7 References on Trench weirs CBIP, (cc1) manual on Planning and Design of Small Hydroelectric Scheme Lauterjung et al (1989) Planning of Intake Structures Freidrich Vieweg and Sohn, Braunswchweig GermanyIAHR (1993) Hydraulic Structures Design Manual Sedimentation Exclusion and Removal of Sediment from Diverted Water. By Arved J. Raudkivi publishing house Taylor & Francis, rising York. Chow (1959) Open- Channel Hydraulics Publisher McGraw-Hill Book Company, New York. 1. 4 RESERVOIR, CANAL AND PENSTOCK INTAKES The designs of reservoir, cana l and penstock intakes are all based on the same principles. However, there are significant variations depending on whether an intake is at the forebay reservoir of a run-of-river plant or at storage reservoir with large draw down or is for a power tunnel, etc.Examples of a variety of layouts can be fond in IS 9761 Hydropower Intakes Criteria for Hydraulic Design or Guidelines for Design of Intakes for Hydropower Plants (ASCE, 1995). The features common to all designs are shown in the following sketch AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201115 The objectives of good design are To prevent entry of floating debris. To avoid formation of aerate entraining vortices. To minimize hydraulic losses. 1. 4. 1 Control of floating debrisTo prevent the entry of debris a trashrack is placed at the entry to the intake. For small hydro plants the trashrack overall size is determined based on an approach velocity of 0. 75 m/s to 1. 0m /s to facilitate manual raking. Trashracks may be designed in panels that can be lowered into place in grooves provided in the intake walls or permanently attacked to anchors in the intake face. The trashracks should to sloped at 14 from the vertical (4V1H) to facilitate raking. The spacing between bars is determined as a function of the spacing between turbine runner blades.IS 11388 Recommendations for Design of Trashracks for Intakes should be consulted for cultivation about spacing between trashracks bars, structural design and thrill problems. Also, see Section 2. 2. 1/5 of this Standard. 1. 4. 2 Control of Vortices jump of all the direction of approach velocity should be axial with value the intake if at all possible. If flow approaches at a significant angle (greater than 45o) AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201116 from axial these will be significant risk of vortex problems.In such a situation an experienced hydraulic engineer should be consulted and for important projects hydraulic model studies may be required. For normal approach flow the submergence can be determined from the following formulae S = 0. 725VD0. 5 S D V = submergence to the roof of the gate section (m) = diameter of penstock and height of gate (m) = velocity at gate for design flow. (m/s) Where A recent paper by Raghavan and Ramachandran discusses the merits of assorted formulae for determining submergence (S). 1. 4. 3 Minimization of Head lossesHead losses are minimized by providing a streamlined transition between the entry section and gate section. Minimum losses will be produced when a streamlined bellmouth intake is used. For a bellmouth intake the transition section is formed with quadrants of ellipses as shown in the following sketch. The bellmouth type intake is preferred when ever the additional costs are economically justified. For little, mainly mini hydropower stations, simpler designs are often optimal as the cost of manifestation of curved cover surfaces may not be offset by the value of reduction in head losses.Details on the geometry of both types are given Bellmouth Intake Geometry Geometries for typical run-of-river intakes are shown below A gate width to height of 0. 785 (D) 1. 00 (H) with H = D is recommended. This permits some reduction in the cost of gates without a significant sacrifice in hydraulic efficiency. There is a second transition between the gate and penstock, rectangular to circular. For a gate having H = D and W= 0. 785D the flow velocity at the gate will be equal to the velocity in the penstock so no further flow acceleration is produced in this section. A length for this transition of 1. x D should be satisfactory. AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201117 The head loss co-efficient for this arrangement in Ki =0. 10 Details for layout of bell mouth transitions connecting to a sloping penstock are given in IS9761. Simplified layout (Mini-Hydro) For smaller/mini hydro projects intake design can be simplified by forming the transition in plane surfaces as shown below The head loss for this design (Ki) = 0. 19V2/2g. AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201118 . 4. 4. AIR VENT An air vent should be placed downstream of the head gate to facilitate air exchange between atmosphere and the penstock for the following conditions Penstock filling when air will be expelled from the penstock as water enters. Penstock draining when air will enter the penstock to occupy the space previously change by water. The air vent (pipe) must have an adequate cross section theatre to effectively handle these exchanges of air. The following design rules are recommended Air vent area should the greater of the following values Where (m3/s) AV = 0. 0 Ap or QT AV = 25. 0 (m2) AV = cross-section area of air vent pipe AP = cross-section ar ea of penstock (m2) QP = turbine rated flow ( ? QT of more than one turbine on the penstock) The air vent should exhaust to a safe location unoccupied by power company employees on the general public. 1. 4. 5 PENSTOCK FILLING A penstock should be filled slowly to avoid excessive and dangerous blowback. The recommended give is to control filling rate via the head gate. The AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201119 ead gate should not be opened more than 50 mm until the penstock is completely full. (This is sometime referred to as cracking the gate. ) 1. 4. 6 REFERENCES ON PENSTOCK INTAKES 1. 4. 7 Indian Standard Cited. IS 9761 Hydropower Intakes Criteria for Hydraulic Design OTHER REFERENCES Guidelines for Design of Intakes for Hydroelectric Plants ASCE, New York (1995) Validating the Design of an Intake Structure By Narasimham Raghavan and M. K. Ramachandran, HRW September 2007. laypersons Guidebook European Smal l Hydro Association Brussels, Belgium (June 1998)Available on the internet. Vortices at Intakes By J. L. Gordon Water major power & Dam Construction April 1970 1. 5. TRASHRACKS AND SAFETY RACKS 1. 5. 1 Trashracks Trashracks at penstock intakes for small hydro plants should be sloped at 4 V 1H to facilitate manual raking and the approach velocity to the trashracks limited to 1. 0 m/s or less. Use of rectangular bars is normal practice for SHPs. Support beams should be alignment with the flow direction to minimize hydraulic losses. Detailed trashrack design should be done in accordance with IS 11388. 1. 5. 2Safety Racks Safety racks are required at tunnel and upside-down siphon off entries to prevent animals or people who may have fallen into the canal from being pulled into these submerged water ways. A clear spacing of 200 mm between bars is recommended. Other aspects of design should be in accordance with IS 11388. 1. 5. 3 References on Trashracks IS11388 Recommendations for D esign of Trashracks for Intakes. ASCE (1995) Guidelines for Design of Intakes for Hydroelectric Plants. AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201120 DRAWINGSAHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201121 AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201122 2. HYDRAULIC DESIGN OF WATERWAYS The waterways or water conduction system is the system of canals, aqueducts, tunnels, modify siphons and pipelines connecting the head works with the forebay tank. This Section provides guidelines and norms for the hydraulic design of these structures. 2. 1 2. 1. 1 CANALS Canals for small hydro plants are typically constructed in masonry or built concrete.Several typical cross section designs are shown below AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201123 Lined canals in earth, if required, should be designed in accordance with Indian Standard IS 10430. A further division of canal types is based on function Feeder canal to connect the head regulator (intake) to the desilter Power canal to connect the desilter to the Forebay tank. 2. 1. 2 Feeder Canals 2. 1. 2. 1 Feeder canal hydraulic design shall be based on the following criteria = Turbine flow (QT) + Desilter flushing flow (QF).Design flow (Qd) AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201124 2. 1. 2. 2 Scouring velocity A sufficiently high velocity must be provided to prevent deposition of sediment within the canal. This (scouring) velocity can be determined from the following formulae d 9/7 S C = 0. 66 6 / 7 n = 0. 015 q 1 1 ? VS = . R 2 / 3 . S C/ 2 n Where Sc = Scouring slope d = Target sediment size (m) q = range per unit width (Q/W) (m/s/m) R = hydraulic gas constant (m) Vs = scouring velocity (m/s) n = Mannings roughness coefficient 2. 1. 2. 3 optimisationThe optimum cross section dimensions, slope and velocity should be determined by economic analytic thinking so as to minimize the total life time costs of capital, O&M and head losses (as capitalized value). The economic parameters for this analysis should be chosen in point of theatrical role with the appropriate regional, state or central power authorities these parameters include price reduction rate (i) Escalation rate(e) Plant load factor Service life in years (n) Annual O+M for canal (% of capital cost) Value of energy losses (Rs/kWh). Also see Section 1. 7 of this Standard. The selected design would be based on the highest of Vs or Voptimum. . 1. 2. 4 Freeboard A freeboard remuneration above the steady state design water level is required to contain water safely within the canal in event of power outages or floods. A minimum of 0. 5 m is recommended. 2. 1. 3 Power Canals Power canal design shall be based on the following criteria a) Design flow = total turbine flow (QT) b) Power canal design should be based on optimization of dimensions, slope and velocity, as explained in the previous section. For mini-hydro plants Q 2. 0 m3/s optimal geometric design dimensions for Type 1 (masonry construction) can be estimated by assuming a longitudinal slope of 0. 04 and a Mannings n value of 0. 018. Masonry construction would normally be preferred for canals with widths (W) less than 2. 0 m (flow area = AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201125 2. 0 m2). For larger canals with flow areas greater than 2. 0m2, a Type 3, box culvert design would be preferred based on economic analysis. c) Freeboard A freeboard allowance above the steady state design level is required to contain water safety within the canal in event of power outages. The waterway in most SHPs terminates in a Forebay tank.This tank is normally equipped with an escape weir to discharge surplus water or an esc ape weir is provided near to the forebay tank. For mini-hydro plants a minimum freeboard of 0. 50 m is recommended. The adequacy of the above minimum freeboard should be verified for the following conditions Maximum flow in the power canal co-incident with sudden outage of the plant. Design flow plus margins for leakage losses (+0. 02 to +0. 05 QT) and above rated operation (+ 0. 1QT). Characteristics of head regulator flow control. The freeboard allowance may be reduced to 0. 5 m after taking these factors into consideration. The maximum water level occurring in the forebay tank can be determined from the weir equation governing flow in the escape weir. 2. 1. 4 Rejection Surge Designs which do not incorporate downstream escape weirs would be sphere to the occurrence of a rejection freshet in the canal on sudden turbine shutdown, giving above static water levels at the downstream end, reducing to the static level at the upstream (entry) end of the water way. Methods for evaluat ing water level changes due to a rejection surge are explained in Section 2. 2. 2 / 7. 0 of this Standard. . 2 AQUEDUCTS Aqueducts are typically required where feeder or power canals pass over a gully or side stream valley. If the length of the aqueduct is relatively improvident the same channel dimensions as for the canal can be kept up(p) and there would be no change in hydraulic design. For longer aqueducts design would be based on economic analysis subject to the proviso that flow remains sub critical with NF ? 0. 8 in the flume sections. The following sketch shows the principal dimension of aqueduct entry and exit transitions and flume section. AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design OfSmall Hydro Plants /May 201126 The changes in invert elevation across the entry and exit structures can be calculated by Bernoulis equation as below Entry transition consider cross section (1) and (2) V2 V2 Z 1 + D + 1 = Z 2 + d + 2 + hL 2g 2g and 2 b? V ? hL = 0. 10 ? 1 ? ?. 2 ? B ? 2g Z2 can be determined from the above equations, since all geometric parameters are known. Flume Sections (2) to (3) The slope of the flume section is determined from Mannings equation 2 ? Vn ? ( S ) = ? 2 / 3 ? . A Mannings n = 0. 018 is suggested for concrete channels. ?R ?Some designers increase this slope by 10% to provide a margin of safety on flow capacity of the flume. Exit transition consider cross section (3) and (4) V2 V2 Z 3 + d + 3 = Z 4 + D + 4 + hL 2g 2g AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201127 and 2 b? V ? hL = 0. 20 ? 1 ? ?. 3 ? B ? 2g Z4 can be determined from the above equations, since all geometrical parameters are known. The same basic geometry can be adapted for transition between trapezoidal canals sections and rectangular flume section, using mean flow width (B) = A/D. . 3. INVERTED SYPHONS 2. 3. 1 Inverted siphon offs are used where it is more economical to route th e waterway underneath an obstacle. The inverted syphon is made up of the following components Entry structure siphon barrels Exit structure Entry Structure Hydraulic design of the entry structure is uniform to the design of reservoir, canal and penstock intakes. ensue the guidelines given in Section 2. 2. 2/2. of this Standard. Syphon barrels The syphon barrel dimensions are normally determined by optimization ? V? ? does not tudies, with the proviso that the Froude Number ? N F = ? gd ? ? ? exceed 0. 8. Invert elevations are determined by accounting for head losses from entry to exit of the structure using Bernoulis equation. For reinforced concrete channels a Mannings n value of 0. 018 is recommended. The head loss coefficients for mitre bends can be determined from USACE HDC 228. 2. AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201128 AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Pla nts /May 201129 Exit structure The exit structure is designed as a diverging transition to minimize head losses the design is similar to the outlet transition from flume to canal as discussed in Subsection 2. 2. 2/2 of this Standard. The following sketches show the layout of a typical inverted siphon. AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201130 2. 3. 2 Reference on Aqueducts and Inverted Syphons Hydraulic Structures By C. D. Smith University of Saskatchewan Saskatoon (SK) Canada 2. 4. humble PRESSURE PIPELINESLow pressure pipelines may be employed as an choice to pressurized box culverts, aqueducts or inverted syphons. Concrete, plastic and steel pipes are suitable depending on site conditions and economics. brand pipe is often an attractive alternative in place of concrete aqueducts in the form of pipe bridges, since relatively large diameter pipe possesses significant inherent structural strength. Steel pipe (with stiff ening rings, as necessary), concrete and plastic pipe also have significant resistance against external pressure, if buried, and offer alternatives to inverted syphons of reinforced concrete construction.Generally pressurized flow is preferred. The pipe profile should be chosen so that pressure is positive through out. If there is a high point in the line that could trap air on filling an air bleeder valve should be provided. Otherwise, hydraulic design for low pressure pipelines is similar to the requirements for inverted syphons. The choice of type of design low pressure pipeline land pipeline material), inverted syphon or aqueduct, depends on economic and constructability considerations, in the context of a given SHP. Mannings n determine for selected Pipe Materials Material Welded Steel Polyethylene (HDPE) Poly Vinyl Chloride (PVC)Asbestos Cement externalize iron Ductile iron Precast concrete pipe Mannings n 0. 012 0. 009 0. 009 0. 011 0. 014 0. 015 0. 013(2) Note (1) From Tab le 5. 4 Laymans Guide Book ESHA (2) From Ven T. Chow Open Channel Hydraulics AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201131 2. 5. TUNNELS 2. 5. 1 Tunnels often provide an appropriate solution for water conveyance in mountainous areas. Tunnels for SHP are generally of ii types. seamless tunnels Concrete lined tunnels On SHP tunnels are usually used as part of the water ways system and not subject to high pressures. . 5. 2 Unlined tunnels Unlined water tunnels can be used in areas of favourable geology where the following criteria are satisfied a) stone mass is adequately water tight. Rock surfaces are sound and not vulnerable to erosion (or erodible zones b) are fitly protected. The static water pressure does not exceed the magnitude of the diminished field c) rock stress. Controlled perimeter blasting is recommended in order to minimize over break and produce sound rock surfaces. Additionally, this construction approach tends to produce relatively uniform surfaces and minimizes the hydraulic roughness of the completed tunnel surfaces.Design velocities of 1. 5 to 2. 0 m/s on the mean AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201132 cross section area give optimal cross section design. It is normal practice to provide a 100mm thick reinforced concrete paving material over leveled and compacted tunnel muck in the invent of the tunnel. IS 4880 Part 3 provides additional guidance on the hydraulic design of tunnels and on the survival of the fittest of appropriate Mannings n values. 2. 5. 3 Lined Tunnels Where geological are unfavourable it is often necessary to provide concrete linings for support of rock surfaces.IS4880 Parts 1-7 give comprehensive guidelines on the design of lined tunnels. 2. 5. 4 High Pressure Tunnels Design of high pressure tunnels is not cover in this standard. For high pressure design, if required, the designer should consu lt an experienced geotechnical engineer or engineering geologist. For the purpose of this standard, high pressure design is be as tunnels subject to water pressures in excess of 10m relative to the crest of the tunnels. 2. 5. 5 Reference on Tunnels IS Standards IS 4880 Code of Practice for the Design of Tunnels imparting Water. Other References Norwegian Hydropower Tunnelling (Third volume of collected papers) Norwegian Tunneling guild Trondheim, Norway. www. tunnel. no Notably Development of Unlined Pressure Shafts and Tunnels in Norway, by Einar Broch. 2. 6. CULVERTS AND CROSS-DRAINAGE WORKS Small hydro projects constructed in hilly areas usually include a lengthy power canal routed along a hillside contour. Lateral inflows from streams and gullies intercepted by SHP canals often transport large sediments loads which must be prevented from entering the canal. The first line of defense is the canal upstream ditch which intercepts local anesthetic lateral runoff.The flow in the se chains must be sporadically discharged or the drain capacity will be exceeded. Flow from these drains is usually evacuated via culverts passing underneath the canal. These culverts would normally be located where gullies or streams cross the canal alignment. The capacity of canal ditches should be decided taking into consideration the sightly distance between culverts. In the rare cases when distance between culverts is excessive, consideration should be given to diverting AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201133 itch flows across the canal in flumes or half round pipes to discharge over the downhill side of the canal at suitable locations. Culverts are usually required where the canal route crosses gullies or streams. Culverts at these points provide for flow separation between lateral inflows and canal inflows and often present the most economical solution for crossing small but steep valley locations. It is recom mended that culverts design be based on the following hydrological criteria. For mini hydro projects, 1 in 10 year flood (Q10) For small hydro projects, 1 in 25 year flood (Q25)Where it is practical to extract the necessary basin parameters, the procedures given in Section 1. 4 should be applied. Otherwise design flows should be estimated from field measurements of cross section area and longitudinal slope at representative cross section of the gully or side stream. A survivable design approach is further recommended with canal walls strengthened to allow local over topping without damage to the canal integrity when floods exceed the design flood values. Detailed hydraulic design should be based on information from reliable texts or design guidelines such as Design of Small Bridges and Culverts Goverdhanlal 2. 7 2. 7. 1 Engineering and Design drainage and Erosion Control. Engineering Manual EM 1110-3-136 U. S. Army army corps of Engineers (1984) www. usace. army. mil/publica tions/eng-manuals Manufacturers guides, notably American Concrete Pipe Association www. concrete-pipe. org Corrugated Steel Pipe Institute www. cspi. ca Power Canal Surges Power canals that are not provided with escape weirs near their downstream end will be subject to canal surges on quick load rejections or load additions.The rejection surge will typically cause the downstream water level to rise above static level and may control the design of canal freeboard. For load additions there is a risk that the level will fall to critical at the downstream end and restrict the rate at which load can be taken on by the unit. The following formulae taken from IS 7916 1992 can be used to estimate the magnitude of canal surges. AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201134 Maximum surge height in a power channel due to load rejection may be calculated from the empirical formulae given belowFor abrupt cloture hmax = K 2 + 2 Kh For gradual closure within the period required for the first wave to travel twice the length of the channel K hmax = + V . h / g 2 Where hmax = maximum surge wave height, K = V2/2g = velocity head, V = mean velocity of flow, and area of cross sec tion h = effective depth = top width Maximum water level resulting from a rejection surge at the downstream of a canal Maximum W. L. = Yo + hmax Minimum water level resulting from by a start up surge at the downstream end of a canal Minimum W. L. = YS hmax Where Yo YS = steady state downstream water level static downstream water level. The maximum water level profile can be approximated by a straight line joining the maximum downstream water level to the reservoir level. 2. 7. 2 Canal Surges on Complex Waterways For waterway systems comprising several different water conductor types, the above equations are not applicable. In such cases a more detailed type of analysis will be required. The U. S. interior(a) Weather Service FLDWAV computer program can be used to solved for the transient flow conditions in such cases (Helwig, 2002). 2. 7. 3 References IS Standards citedIS 7916 1992 Open Channel Code of Practice. Other References Application of FLDWAV(Floodwave) Computer dumbfound to Solve for Power Canal Rejection Wave for Simple and Complex Cases. P. C. Helwig Canadian Society for Civil Engineering Proceedings, Annual Conference Montreal, Canada (2002). AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201135 3. HYDRAULIC DESIGN OF DESILTERS 3. 1 BACKGROUND Sediment transported in the flow, especially particles of hard materials such as quartz, can be harmful to turbine components.The severity of damage to equipment is a function of several variables, notably sediment size, sediment hardness, particle shape, sediment concentration and plant head. The control of turbine wear problems due to silt erosion requires a comprehensive design approach in which sediment propertie s, turbine mechanically skillful and hydraulic design, material selection and features to facilitate equipment maintenance are all considered (Naidu, 2004). Accordingly the design parameters for desilter design should be made in consultation with the mechanical designers and turbine manufacturer.Where the risk of damage is judged to be high a subsiding basin (or desilter) should be constructed in the plant waterway to remove particles, greater than a selected target size. 3. 1. 1 Need The first design decision is to determine whether the sediment load in the river of interest is sufficiently high to merit construction of a desilter. There is little guidance available on this topic however, the following limits are suggested by Naidu (2004) Table 2. 2. 3/1. 0 Concentration Suggested Maximum Allowable Sediment versus Plant Head. Parameter Head Maximum allowable sediment concentrationLow and Medium Head Turbines ? one hundred fifty m High Head Turbines 150 m 200 ppm 150 ppm 3. 1. 2 Removal Size There are also sizeable divergences of opinion on the selection of design size for sediment removal. Nozaki (1985) suggests a size range of between 0. 3 mm to 0. 6 mm for plant heads ranging from 100 m to 300 m. Indian practice is to design for a particles size of 0. 20 m regardless of head. Some authors suggest that removal of particles smaller than 0. 20 mm is not practical. The adoption of 0. 20 mm is the design (target) sediment size is recommended for Indian SHP designs.AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201136 3. 1. 3 Types of Desilters There are two basic types of desilters Continuous flushing type Intermittent flushing type Guidelines for design of both types are given in this section. 3. 2. DESIGN CONSIDERATIONS 3. 2. 1 Data Requirements (Small Hydro Plants) It is recommended that a program of suspended sediment ingest be initiated near the intake site from an early stage during site investigations to ensure that sufficient data is available for design.The sampling program should extend through the entire rainy season and should comprise at least two readings daily. On glacier fed rivers where diurnal flow variations may exist, the schedule of sampling should be adjusted to take this phenomenon into account and the scheduled sampling times be adjusted to coincide with the hour of peak daily flow with another sample taken about twelve hours later. While it is often assumed that sediment load is directly related to flow, this is only lawful on the average, in a statistical sense.In fact it is quite likely, that the peak sediment event of a year may be associated with a unique upstream event such as a major landslide into the river. Such events often account for a disproportionately large proportion of the annual sediment flow. Therefore, it would also be desirable to design the sediment measurement program to provide more detailed information about such events, basically to in crease the sampling frequency to one sample per 1 or 2 hours at these times. A louver year long sediment collecting program would be ideal. little than one monsoon season of data is considered unsatisfactory.Some authors suggest that the vertical variation of sediment concentration and variations horizontally across the river be measured. However, on fast stream rivers inherent turbulence should ensure uniform mixing and sampling at one representative point should be sufficient. The data collected in a sediment sampling program should include Mean daily concentration of suspended sediment (average of two readings twelve hours apart) Water temperature Flow (from a related flow gauging program) The following additional information can then be derived from collected samples.AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201137 A sediment rating curve (sediment concentration versus flow where possible) Particle size gradation c urve on combined sample Specific gravity of particles. It is also recommended that a petrographic analysis be carried out to identify the component minerals of the sediment mix. It is likewise recommended that experiments be made on selected ranges of particles sizes to determine settling velocities. A further discussion on the subject of sediment sampling is given in Avery (1989)The characteristics of the sediment on a given river as obtained from a data collection program will assist in selection of appropriate design criteria. 3. 2. 2 Data Requirements (Mini Hydro Plants) On mini hydro projects where resources and time may not be available to undertake a comprehensive sampling program, selection of design parameters will depend to a great extent on engineering judgment, supplemented by observations on site and local information. The following regional formula by Garde and Kothyari (1985) can be used to support engineering decision making. 0. 19 ?P ? 0 Vs = 530. 0 P0. 6. Fe1. . S 0. 25 Dd . 10 .? max ? ?P? Where Vs = mean sediment load in (tonnes/km2/year) s = average slope (m/m) Dd = drainage density, as total length of streams split up by catchment area (km/km2) P = mean annual precipitation (cm) Pmax = average precipitation for wettest month (cm) Fe = ground cover factor, as below 1 Fe = 0. 80 AA + 0. 60 AG + 0. 30 AF + 0. 10 AW ? Ai = arable land area AA = grass land area (all in km2) AG AF = forested area AW = waste land area (bare rock) 3. 2. 3 Design Criteria The principle design criteria are 1. The target size for removal (d) d = 0. 20 mm is recommended 2.Flushing flow QF = 0. 2 QP is recommended 3. Total (design) flow QT = QP + QF = 1. 2 QP. Where QP is plant flow capacity in (m3/s). AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201138 3. 2. 4 Siting The following factors control site selection 1. A site along the water way of appropriate size and relatively level with remark to cross section top ography 2. A site high enough above river level to provide adequate head for flushing. For preliminary layout a reference river level corresponding to the mean annual flood and minimum flushing head of 1. 0 m is recommended. In principle a desilting tank can be located anywhere along the water conductor system, upstream of the penstock intake. Sometimes it is convenient to locate the desilting basin at the downstream end of the waterway system where the desilter can also provide the functions of a forebay tank. However, a location as close to the head works is normally preferred, site topography permitting. 3. 3 Hydraulic Design A desilter is made up of the following elements Inlet section Settling tank Outlet section Flushing system 3. 3. 1