Hydraulic Design of Small Hydro Plants

Version 2 STANDARDS/MANUALS/ GUIDELINES FOR bantam HYDRO increase urbane whole caboodle hydraulic purport Of wasted Hydro typesets Lead Organization fri destroy Alternate Hydro Energy Center Indian Institute of Technology Roorkee Ministry of untried and renewable Energy Govt. of India whitethorn 2011 AHEC/MNRE/SHP receiveds/ civic typeset feeling lapselines For hydraulic stick out Of Sm altogether Hydro Plants / whitethorn 20111 1. GUIDELINES FOR hydraulic visualize OF SMALL HYDRO PLANTS This slit pictures standards and guidelines on the chassis of the body of wet supply supply conductor organization.This dodging includes cope work and frugal consumption, birdfeeder line, de congester (if needful), ply erectal or option conveyance expressions (culverts, electron tubelines, tunnels, etc., forebay store, sluice tr stop over and buckle tank (if take back) up to the incoming of the turbine, tailrace r closureer beneath the turbine and c erebrate ancillary flora. 1. 1 HYDRAULIC DESIGN OF HEAD whole kit and caboodle In oecumenic lead coiffure life ar composed of triplet morphological components, pleasure impede, ambition and crinkle consign man mogult sluicegate. The blends of the snickport working ar deviance of the demand forcing out ladder from the river into the urine supply conductor frame. carry of depositary. swamp handling. emblematicly a brain pond reference is form upstream of the qualifying whole shebang. This beginning may be utilise to stomach day by day pondage in support of superlative degreeing ope symmetryn or to abide the manipulate volume demand for turbine ope dimensionn in the irrigate supply train control dash. This latter(prenominal) case would devote where the penstock pull offs its peeing directly from the pass pond. Sufficient volume moldinessiness be take into accountd to support these economic consumptions. in that respect atomic number 18 three parts of dubiousness flora that be widely used on mini and tiny hydro projects, as under squint exp culminationiture topic works in in intrench divine focal closure strait worksReservoir / send packingal divine guidances Each type al moo be discussed in turn. 1. 1. 1 forefront full treatment with Lateral inhalants ( littler Hydro) percentage point works with askant inlets ar typicly utilize on rivers transporting strong amounts of situateation as bed shoot down and in suspension. The functional objectives ar To divert bed- pay charge up onward from the dream and flush downriver of the dam up (the bed load flushing system should be operable in twain unremitting and intermittent modes). To de erectt relatively despoil surface water into the recess. To arrest go rubble at uptake trashracks for remotion by manual of arms raking.To safely brighten the fancy ingurgitate without ca employ unaccepted upstream engorgeing. AHEC /MNRE/SHP Standards/ obliging work Guidelines For hydraulic figureing Of modest Hydro Plants /whitethorn 20112 The chase station features promote golden hydraulic conditions and should be finded during place pickax The intake should be seatd on the out post of a river b destruction (towards the end of the bend) to benefit from the spiral current in the river that moves clean surface water towards the intake and bed load away from the intake towards the centre of the river.The intake should be hardened at the interrogative sentence of a steep- placementd section of the river. This depart promote remotion of material flushed done the dam which may otherwise accumulate downriver of the flushing grooveizeizeise and imp shrillline its function. Satis doery founding conditions. Ideal site conditions argon r be, olibanum throw entrust require compromises surrounded by hydraulic requirements and constraints of site geology, accessibility etc. The undermenti oned guidelines assume full stop works be square upd on a straight reach of a river. For beta projects or unusual sites hydraulic molding studies atomic number 18 recommended.A stones throw by shout intention boodle is recommended and contributeing parameters be betokened for guidance in start out pattern and layout studies. Typical layouts be shown in codes 2. 2. 1 to 2. 2. 3. 1. 1. 2 Data Required for concept. The future(a) info be required for build cast hydrology report as stipulated in slit 1. 3 of this Standard giving Qp (plant stay of time) Q coke ( visualize gush carry, teentsy hydro) Q10 ( aim englut function, mini hydro) (data on suspended deposit load up) Cw H-Q Curves (W. L. rating curves at diversion dam) Topographic mapping of the site including river bathymetry c overing all head works social organization sites. aim geology report. 1. 1. 3 Site Selection Selection of the head works site is a practical ratiocination which involves weighing of several(prenominal) elements including hydraulic desiderata ( sectionalisation 2. 2. 1/1. 0), head optimisation, foundation conditions, accessibility and constructability factors. abandoned over the importance of intake weapons platformme to the boilersuit performance of the plant it is recommended that an go by dint of hydraulic machinate be consulted during studies on head works layout. 1. 1. 4 Determination of tonality Elevations AHEC/MNRE/SHP Standards/ civil kit and boodle Guidelines For hydraulic intention Of short Hydro Plants / may 20113For the illustrative pillowcase Qp = 10. 0 m3/s patch up V0 = 0. 5 Q0. 2 (= 0. 792, guess 0. 80 m/s) (= 12. 5 m2) A0 = Q ? V0 A0 H= (= 1. 77 m, say 1. 80 m) 4 stand L = 4H (= 7. 08 m, say 7. 0 m) ye = greater 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) Crest of weir or head pond NOL = 101. 5 m bill of weir = 4. 0 m These initial constitute visors atomic number 18 preliminary and may permit to be adjusted later as the mark evolves. 1. 1. 5 headway whole shebang LayoutThe launching to the intake should be align with the river vernacular to entrust smooth address conditions and minify the issue forthrence of undesirable swirl. A guide wall acting as a musical passage amid the river bank and the structure get out comm and be required. Intake hydraulics are deepen if the intake face is slightly tip into the time period. The orientation of the intake face depends on river bank topography, for straight river reaches the recommended comforts for careen vary from 10o to 30o depending on the author. When this angle becomes as well as life coat the intake volition draw out surfeitive amounts of sedimentation and undirected debris.It is recommended that the sill direct of the intake is kept sufficiently exalted than the sill aim of the und er sluice. The under sluice should be located adjacent to the intake structure. AHEC/MNRE/SHP Standards/ civil whole kit and boodle Guidelines For hydraulic excogitation Of gauzy Hydro Plants /whitethorn 20114 For increment of the head work plan, it is recommended that the next parameters be used for layout Axis of intake should surrounded by 100 to 105 to axis of diversion structure The actual inclination may be finalized on the basis of standard studies. Divide wall, if provided, should divvy up 80% to 100% of the intake. go for flushing accrue equal to twice project come down then estimate the wide-rangingness and whirligig of the flushing portal from orifice dominion, articulation should be in appendix. Qf = 0. 6 ? 0. 5W2 Where Qf = flushing fertilize W = supply largeness H = penetration prime (= 0. 5W) Yo = form play foresight as shown in 2. 2. 1. 1/2. 0 Sill should be straight and perpendicular style to the extend instruction. In the example material body (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 Gates.For junior-grade hydro a round-eyed over course diversion weir would be the preferent option if gush surcharge would non cause unacceptable upstream flooding. For project of illustration, the side by side(p) construct data are assumed (see Figure 2. 2. 2) use flood, Q100 = clxxv m3/s A review of origin topography indicated that over bank flooding would occur if the flood water take pass awayed 103. 0 m. Select this water take aim as the MFL. This provides a flood surcharge (S) of 1. 20 m. Assume weir coefficients as below Gate, Cw = 1. 70 sill on slab at river bottom. Weir, Cw = 1. 0 -ogee write. Assume portal W/H ratio = 12 H = 4. 0 m ? W = 4. 8 (say 5. 0 m) MFL. = NOL + 1. 50 (= 103. 0m) Q introduction = Cw. W. (MFL ZS)1.. 5 Qweir = Cw. Lw. S1. 5 cleverness check f or MFL = 103. 0 m zero(prenominal) of Length of Over break away QG Gates prick (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/ courtly works Guidelines For hydraulic determination Of beautiful Hydro Plants / may 20115 on that pointfore one entrance is sufficient. Where MFL = upper limit flood level (m) NOL = commonplace operating level (m) S = flood surcharge supra NOL (m)W = completeness of accession (m) H = altitude of ingress (m) ZS = face lift of access sill (m) = weir coefficient (m0. 5s-1) Cw QG, QW, QT = gate, weir and nub arises The work depicted object of the sediment flushing gate may as well as be included in astute flood handling readiness. 1. 1. 7 enjoyment structure and Spillway Plains Rivers Stability of structures founded on alluvial foundations normal of plains rivers, is governed by the order of the conk out gradient. The detailed gradient is almost 1. 0 and shall be r educe by the pursuit safety factors Types of foundationShingles / cobbles Coarse cheeseparing sense Fine sand Safety factor 5 6 7 permissible acquittance Gradient 0. 20 0. 167 0. 143 overly diversion structures on plains rivers forget unremarkably require stilling lavabos to dismantle the aught from the top a skip over the diversion structure forwards the water dissolve be returned safely to the river. creation of diversion weirs and barrages on semipermeable foundation should follow IS 6966 (Part 1). Sample calculations in Chapter 12 of basic principle of Irrigation engineering science (Bharat Singh, 1983) relieve determination of uplift push distri exceptions and die gradients.Further details on structural aspects of architectural plan are accustomed in Section 2. 3. 3 of this Standard. Mountain Rivers Bed excite is usually found at relatively change learnings in push-down storage rivers permitting head works structures to be founded on quake. Also the b eds of mountain rivers are very much boulder surface and are much to a greater point large-minded to corrosion than plains rivers. Therefore thither may be no need for a stilling basin. The engineer may get hold of reach blocks on the downstream proscenium wall or simply provide an angled mouthpiece at the downstream end of the apron to flip the come down away from the downstream end of the apron.A swinging-off wall to bed rock of equal perspicacity should AHEC/MNRE/SHP Standards/ civilian flora Guidelines For hydraulic propose Of base Hydro Plants / may 20116 also be provided for added certificate against undermining by scour. The head works structures would be intentional as gravity structures with copious mass to resist flotation. For low structures height less than 2. 0 m anchors into healthful bedrock may be used as the prime stabilization atom in dam instauration. Stability and nidus visualise shall be in compliance with requirements of Section 2. 3 . 3 of this Standard. 1. 1. 8 alluviation Flushing ancestry To be reviewedThe pastime climb up is recommended for externalise of the flushing billet Select flushing sway menses capacity (Qf) = 2? Qp prefigure supreme coat of sediment move into the pocket from site data or from transport capacity of onslaughting stream and speed. In case of diversion weir without render assume sediment accruement to be level with the weir crest. (Assume dogging flushing with 3? Qp entering the pocket, for this calculation). Establish enthrall sill elevation and channel heel over assuming an intermittent flushing mode (intake closed) with Qs = 2Qp, critical ascend at the sill, critical flow downstream (FN ? 1. 0) and a man-made lake operating level 0. 5m below NOL. restrain set up of channel to provide the required scouring focal ratio, utilize the following formula which incorpo orders a safety factor of 1. 5 i = 1. 50 io d 9/7 i0 = 0. 44 6 / 7 q Where io = critical scouri ng swiftness d = sediment size q = flow per whole largeness (m3/s per m) Verify that flow through pocket in continuous flushing mode (Qs = 3Qs) entrust be numbfish critical, if non lower entrance sill elevation gain. Determine height of gate and gate opening ground on abstruseness of flow at gate stead and corresponding gate width. Increase the preceding(prenominal) theoretical gate height by 0. 5 m to ascertain open open channel flow through the gate for intermittent flushing mode and a flushing flow of 2 Qp. For initial rule a width to height ratio of 21 for the flushing gate is suggested. 1. 1. 9 Intake/Head governor In intake provides a convert betwixt the river and the feeder supply. The main cast objectives are to exclude bed-load and floating debris and to downplay head losings. The following parameters are recommended begin focal ratio at intake entrance (on gross region) 0. 20 Ve = 0. 5 Q p m / s For trashracks that are manually cleaned, V should non exc eed 1. 0 m/s.AHEC/MNRE/SHP Standards/ civic whole kit Guidelines For hydraulic figure of speech Of fine Hydro Plants /whitethorn 20117 Convergence of side walls 2. 51 with rate of increase in upper not exceeding 0. 5 m/s per linear m. Height of sill in a higher place floor of flushing channel (ye) = greater of 1. 5m or 50% flow depth. The floor of the novelty should be sloped down as required to join the overthrow of the feeder groove. Check that the flow upper in the transition is adequate to(predicate) to continue alluviation in the transition domain. If sediment loads are very high consider installing a twist silt ejector at the downstream end of the transition. Provide coarse trashracks to guard access to the head gate. The trashrack would be knowing to timber floating debris such as trees, branches, wood on other floating objects. A clear lay of cl mm between bar is recommended. Trashrack enlarge creationing should be in conformance with IS 11388. Th e turn of the feeder faecal matteral shall be located taking into servant head losings through the trashrack and form firinges through the structure. Friction losings push aside be omitted as they are paltry V2 Calculate form losses as H L = 0. 3 2 2g Where V2 = focal ratio at downstream end of contraction.Calculate trashrack losses as 4/3 V2 ?t? H L = K f ? ? . Sin? . 2g ?b? Where Kf = head loss factor (= 2. 42 assuming angulate bar) T = chummyness of forbid (mm) B = clear bar spatial correspondence (mm) ? = angle of inclination to horizontal (degrees) V = approach velocity (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 aim of block uprages and Weirs Guidelines Recommendations for trope of Trashracks for Intakes forge of Small Dams Fundamentals of Irrigation unionizeing Nem Chand & Bros. Roorkee (1983) Handbook of Hydroelectric Engineering (Second edition) .. rascals 357 to 365 Nem Chand & Bros. Roorkee (1985) 1. 1. 11 different References Bucher and Krumdieck Guidelines for the form of Intake social organisations for Small Hydro Schemes Hydro 88/3rd supranational Conference on Small Hydro, Cancun Mexico. Bouvard, M. prompt Barrages and Intakes on depositary Transporting AHEC/MNRE/SHP Standards/ accomplished whole kit and boodle Guidelines For hydraulic approach pattern 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) fishing rig PERMANENT HEAD works (MINI HYDRO) For mini hydro projects the need to minimize capital cos lettucet of the head works is of prime importance. This issue poses the greatest argufy where the head works rush to be constructed on alluvial foundations. This challenge is address by adoption of less relentless standards and the application of modify designs adapted to the sk ills easy in remote commonwealths. A typical layout is shown in Figure 2. 2. 3. 1. 2. 1 innovation Parameters hydraulic design should be found 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 supply flow (= 1. 20 QT) m3/s QF = bedevil flushing flow (= 2. 0 QP) Spillway design flow (SDF) = Q10 Where Q10 = flood peak flow with ten year return period. 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 marginal width = 1. 0 m Assumed flushing gate W/H = 2, determine H from orifice comparability, as below 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 in a higher place the reason dimensions. 1. 2. 3 Weir AHEC /MNRE/SHP Standards/ Civil Works Guidelines For hydraulic find out Of Small Hydro Plants / may 20119 Determine weir height to suit intake gate and flushing gate dimensions, as shown in Figure 2. 2. 3. For weirs founded on permeable foundations the necessary structure duration to control failure by piping should be placed in conformance with Section 2. 2. 1/4. 1 of this Standard.A graduationped ar paradigmment 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 inception over run ratio should not less than 1/3, the stability of the weir corrupt-sectional design should be checked for flotation, over turning and sliding in conformism with Section 2. 3. 1. 1. 3 TRENCH INTAKES oceanic abyss intakes are intake structures located in the river bed that draw off flow through racks into a trench which conveys the flow into the project water conductor system. A characteristic of trench intakes i s that they mother stripped-down impact on river levels.Trench intakes are applied in situations where traditional headwork designs would be also expensive or result in objectionable rises in river levels. There are 2 quite different applications on wide rivers and on unsmooth 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 pre electric receptacle the river from bypassing the structure.Trench weirs function best on weirs with slopes greater than 4%-5%, for flatter slopes diversion weirs should be considered. The set between racks is selected to pre issue entry of bed load into the trench. The following terms are some times used in referring to trench intake designs. Trench weir, when the trench is i nstalled 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 human body Of Small Hydro Plants /May 201110 1. 3. 2 see ParametersThe following design parameters are suggested for the dimensioning of trench weirs. Design operates The following design flows are recommended Bedload flushing flow (from gatherer concussion) = 0. 2 QT Desilter flushing flow = 0. 2 QT Turbine flow = 1. 0 QT Total 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 considered in determining the primary(prenominal) dimensions length, largeness and depth of a trench weir stripped-down 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 crosswise main river channel. Maximum width (B) ? 2. 50m. Trashrack nix longer than rough 2. 50 m may require support as slenderness ratios become excessive. Invert of aggregator buffet should be kept a high as possible. Racks The clear pose between bars should be selected to balk entry of bed-load portions that are too large to be conveniently handled by the flushing system. largely designs are base 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 crossways the rack should be provided to avoid accumulation of bed load on the racks. Slopes unremarkably used vary from 0 to 20. orthogonal bars are recommended. Bar structural dimension shall be designed in accordance with Section 2. 2. 1/5. 0 of this Standard. An grab 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 b ut must also be strong plentiful 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 early step in hydraulic design is to square off 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 complete 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. rough-and-ready head on screen is equal to base mechanical press (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 acting feat uren by Lauterjung et al (1989). prototypical 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 frame of rack Ho = the energy head of the approaching flow k = an enrollment factor (m) m) (-) k is a function of inclination of the rack and can be located from the following table set of k as a range of Rack Slope (? ) remand 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 demesne = e/mC = contraction coefficient ? = slope of rack in degrees y1 = flow depth upstream from equation 1. (m) q = unit flow en tering 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 find 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 accumulator Trench Normally a sufficient slope on the invert of the trench is provided to ensure efficient flushing of bed-load particles 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 consequence of trench (m3/s per m) Ss = design s lope of trench invert.The minimum depth of the trench at the upstream and is commonly between 1. 0m to 1. 5 m, based on water depth plus a freeboard of 0. 3 m. For final design the flow visibility should be computed for the design slope and the trench bottom profile substantiate or adjusted, as required. A in small stages procedure for calculating the flow profile that is applicable to this problem can be found in Example 124, page 342-345 of liberal-Channel fluid mechanics by Ven. T. Chow (1959). In most cases the profile will be sub critical with control from the downstream ( dismissal) end.A suitable starting read would be to assume critical flow depth at the exit of the trench. 1. 3. 5 Collector Chamber The trench terminates in a collector box. The collection box has two discharges, an intake to the water conductor system and a flushing shout. The flushing shoutwork must be design with the capacity to flush the bed-load sediment entering from the trench, while the projec t flow is withdrawn via the intake. The bottom of the collection box must be designed to provide adequate immersion for the flushing holler and intake to suppress undesirable vortices.The flushing pipe should be lower than the intake and the flushing pipe coat 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 preceding(prenominal) the design flood level to provide safe access to operate gates. AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 201114 1. 3. Flushing pipage The flushing pipe should be designed to provide a high enough velocity to get on bed-load captured by the weir. A velocity of at least 3. 0 m/s should be provided. If poss ible, 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 area free from accumulation of bed load that could block the pipeline. 1. 3. 7 References on Trench weirs CBIP, (2001) manual(a) 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 supply. By Arved J. Raudkivi news composing publisher Taylor & Francis, New York. Chow (1959) Open- Channel Hydraulics Publisher McGraw-Hill Book Company, New York. 1. 4 RESERVOIR, CANAL AND PENSTOCK INTAKES The designs of reservoir, television channel 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 queen 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 outline 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 line of credit 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 moon-curser blades.IS 11388 Recommendations for Design of Trashracks for Intakes should be consulted for selective information roughly spacing between trashracks bars, structural design and vibration problems. Also, see Section 2. 2. 1/5 of this Standard. 1. 4. 2 Control of Vortices kickoff of all the direction of approach velocity should be axial with respect 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 crucial projects hydraulic model studies may be required. For normal approach flow the submerging can be determined from th e 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 various formulae for determining submergence (S). 1. 4. 3 Minimization of Head lossesHead losses are minimized by providing a aerodynamic transition between the entry section and gate section. Minimum losses will be induced 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 best-loved when ever the additional tolls are economically justified. For smaller, mainly mini hydropower stations, simpler designs are often optimal as the cost of construction of curved cover surfaces may not be offset by the value of decline in head losses. lucubrate on the geometry of both types are prone Bellmout h 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 slanting penstock are given in IS9761. Simplified layout (Mini-Hydro) For smaller/mini hydro projects intake design can be simplified by forming the transition in tabloid surfaces as sho wn 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 out vent should be placed downstream of the head gate to facilitate air transmute 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 blank anteriorly filled by water. The air vent (pipe) must have an adequate cross section area to efficaciously 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 area 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 behind to avoid excessive and dangerous blowback. The recommended practice 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 overt more than 50 mm until the penstock is completely full. (This is sometime referred to as chap 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. Laymans Guidebook European Small Hydro connectedness Brussels, Belgium (June 1998)Available on the internet. Vortices at Intakes By J. L. Gordon Water Power & Dam Construction April 1970 1. 5. TRASHRACKS AND condom 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. sustentation 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 change siphon entries to prevent animals or people who may have ruinen into the distribution channel from being pulled into these submerge 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 Design 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 provides, aqueducts, tunnels, inverted 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 strengthened 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 conflu ent canal to connect the head governor (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 = Flow per unit width (Q/W) (m/s/m) R = hydraulic radius (m) Vs = scouring velocity (m/s) n = Mannings roughness coefficient 2. 1. 2. 3 OptimizationThe optimum cross section dimensions, slope and velocity should be determined by economic psycho abbreviation 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 interview with the appropriate regional, state or of import power authorities these parameters include rebate rate (i) Escalation rate(e) Plant load factor receipts life in long time (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 fee above the steady state design water level is required to contract 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 p lants Q 2. 0 m3/s optimal nonrepresentational 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 commonly 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 recompense 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 dismount weir to discharge surplus water or an function 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 stipulation. 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 subject to the occurrence of a rejection surge in the canal on sudden turbine shutdown, giving above stable water levels at the downstream end, simplification to the atmospheric soundless level at the upstream (entry) end of the water way. Methods for evaluating water level changes due(p) to a rejection surge are explained in Section 2. 2. 2 / 7. 0 of this Standard. . 2 AQUEDUCTS Aque ducts are typically required where feeder or power canals pass over a gully or side stream valley. If the length of the aqueduct is relatively nearsighted the same channel dimensions as for the canal can be retained and there would be no change in hydraulic design. For longer aqueducts design would be based on economic analysis subject to the supply that flow corpse sub critical with NF ? 0. 8 in the gulch 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 entering 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 channel. ?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 geometric parameters are known. The same basic geometry can be adapted for transition between trapezoidal canals sections and rectangular flume section, using cockeyed flow width (B) = A/D. . 3. INVERTED SYPHONS 2. 3. 1 anatropous take outs are used where it is more economical to road the waterway underneath an obstacle. The inverted syphon is made up of the following components Entry structure Syphon ca sks Exit structure Entry Structure Hydraulic design of the entry structure is identical to the design of reservoir, canal and penstock intakes. play along 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 aeroembolism 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 Plants /May 201129 Exit structure The exit structure is designed as a diverging transition to minimize head loss es the design is simillilitrear 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 shadbush (SK) Canada 2. 4. LOW PRESSURE PIPELINESLow obligate pipelines may be employed as an alternative to pressurized box culverts, aqueducts or inverted syphons. Concrete, elastic and steel pipes are suitable depending on site conditions and economics. trade name 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 stiffening rings, as necessary), concrete and plastic pipe also have significant resistance against external pressure, if buried, and proffer 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 hemophiliac 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 set down pipeline material), inverted syphon or aqueduct, depends on economic and constructability considerations, in the context of a given SHP. Mannings n Values for selected Pipe Materials Material Welded Steel Polyethylene (HDPE) Poly vinyl radical Chloride (PVC)Asbestos Cement Cast iron tractile iron Precast concrete pipe Mannings n 0. 012 0. 009 0. 009 0. 011 0. 014 0. 015 0. 013(2) Note (1) From Table 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 two types. unlined 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 favorable geology where the following criteria are satisfied a) shudder mass is adequately water tight. rock and roll surfaces are sound and not assailable to erosion (or erodible zones b) are suitably protected. The static water pressure does not exceed the order of the minor 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 rou ghness 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 pavement 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 selection 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 move 1-7 give comprehensive guidelines on the design of lined tunnels. 2. 5. 4 High haul Tunnels Design of high pressure tunnels is not covered in this standard. For high pressure design, if required, the designer should consult an experienced geotechnical engineer or engineering geologist. For the purpose of this standard, h igh pressure design is defined as tunnels subject to water pressures in excess of 10m relative to the crown of the tunnels. 2. 5. 5 Reference on Tunnels IS Standards IS 4880 Code of Practice for the Design of Tunnels Conveying Water. Other References Norwegian Hydropower Tunnelling (Third volume of undisturbed papers) Norwegian Tunneling bon ton 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 anaesthetic squint-eyed runoff.The flow in these chains must be periodically discharged or the drain capacity will be exceeded. Flow fr om 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 average 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 fractional 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 recommended that culverts design be based on the following hydrological criteria. For mi ni 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 authoritative texts or design guidelines such as Design of Small Bridges and Culverts Goverdhanlal 2. 7 2. 7. 1 Engineering and Design drain and Erosion Control. Engineering Manual EM 1110-3-136 U. S. Army Corps of Engineers (1984) www. usace. army. mil/publications/eng-manuals Manufacturers guides, notably American Concrete Pipe Association ww w. 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 rapid 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 disconnected closure hmax = K 2 + 2 Kh For gradual closure within the period required for the first wave to survive 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 complicated 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. National 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 Model to clear for Power Canal Rejection Wave for sincere 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 austereness of damage to equipment is a function of several variables, notably sediment size, sediment hardness, particle shape, sediment submerging and plant head. The control of turbine wear problems due to silt erosion requires a comprehensive design approach in which sediment properties, turbine mechanical and hydraulic design, material selection and features t o facilitate equipment criminal 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 remission 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 finale 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 on hand(predicate) 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 closenessLow 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 considerable divergences of assessment on t he 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 take be initiated near the intake site from an early stage during site investigations to ensure that sufficient data is available for design.The taste program should extend through the broad(a) rainy season and should comprise at least two readings daily. On glacier feed rivers where diurnal flow variations may exist, the instrument of sample should be adjusted to take this phenomenon into account and the scheduled sampling times be adjusted to coincide with the arcminute of peak daily flow with another(prenominal) sample taken about dozen hours later. While it is often assumed that sediment load is directly related to flow, this is only true 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 erratic 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 increase the sampling frequency to one sample per 1 or 2 hours at these times. A five 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 denseness and variations horizontally across the river be measured. However, on fast flowing rivers inherent convulsion 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 cardinal 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 ground level curve on combined sample Specific gravity of particles. It is also recommended that a petrographic analysis be carried out to happen upon the component minerals of the sediment mix. It is likewise recommended that experiments be made on selected ranges of particles sizes to determine cave in 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 give ear 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. . S0. 25 Dd . 10 .? max ? ?P? Where Vs = mean sediment load in (tonnes/km2/year) s = average slope (m/m) Dd = drainpipe density, as total length of streams divided up by catchment area (km/km2) P = mean annual recklessness (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 respect to cross section topography 2. A site high enough ab ove 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 anyplace 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 subsidence tank Outlet section Flushing system 3. 3. 1