Hydraulic Design of Small Hydro Plants

Version 2 STANDARDS/MANUALS/ GUIDELINES FOR SMALL HYDRO DEVELOPMENT Civil whole kit and boodle hydraulic concept Of Sm whole Hydro brings Lead Organization Sponsor Alternate Hydro null Center Indian Institute of Technology Roorkee Ministry of New and Renewable Energy Govt. of India whitethorn 2011 AHEC/MNRE/SHP Standards/ Civil whole kit Guidelines For hydraulic finis Of low Hydro Plants / may 20111 1. GUIDELINES FOR hydraulic DESIGN OF SMALL HYDRO PLANTS This surgical incision proffers stocks and happenlines on the conventioning of the peeing conductor system of rules.This system includes layer working and usance, feeder epithelial duct, desilter (if overtopd), power ignoreal or alternative conveyance structures (culverts, organ pipelines, tunnels, etc), forebay tank, sluice and surge tank (if essential) up to the main course of the turbine, tailrace kindleal downstairs the turbine and related ancillary working. 1. 1 HYDRAULIC DESIGN OF chief WORKS In g eneral spot whole kit and caboodle ar composed of three geomorphological servings, deflexion dam, intake and seam load sluice. The functions of the top works atomic number 18 Diversion of the required project hang up from the river into the piss conductor system.Control of repository. alluvium handling. Typically a dubiousness pond reservoir is formed upriver of the intellect works. This reservoir whitethorn be used to provide daily pondage in support of peaking operation or to provide the control passel necessary for turbine operation in the water level control path. This latter topic would apply where the penstock overtakes its water directly from the head pond. Sufficient volume mustiness be provided to support these functions. There are three qualitys of head works that are widely used on mini and small hydro projects, as below Lateral intake head works Trench intake head worksReservoir / canal intakes Each type volition be discussed in turn. 1. 1. 1 ex t abolish Works with Lateral Intakes ( venial Hydro) level works with lateral intakes are typically applied on rivers transporting significant amounts of depositary as bed load and in suspension. The functional objectives are To divert bed-load away from the intake and flush downstream of the dam (the bed load flushing system should be operable in both(prenominal) continuous and intermittent modes). To decant relatively clean surface water into the intake. To arrest floating debris at intake trashracks for removal by manual(a) raking.To guardly put down the fig flood without causing unacceptable upriver flooding. AHEC/MNRE/SHP Standards/ Civil Works Guidelines For hydraulic founding Of picayune Hydro Plants /whitethorn 20112 The next site features promote favourable hydraulic conditions and should be studyed during site endurance The intake should be located on the outside of a river bend (towards the end of the bend) to benefit from the spiral current in the river tha t moves clean surface water towards the intake and bed load away from the intake towards the centre of the river.The intake should be located at the head of a steeper section of the river. This will promote removal of material flushed through the dam which may other accumulate downstream of the flushing contrast and impair its function. Satisfactory foundation conditions. Ideal site conditions are rare, thus tendency will require compromises surrounded by hydraulic requirements and constraints of site geology, accessibility etc. The followers guidelines assume head works are located on a straight reach of a river. For classic projects or unusual sites hydraulic model studies are recommended.A step by step practice approach is recommended and objective parameters are suggested for guidance in goal and layout studies. Typical layouts are shown in Figures 2. 2. 1 to 2. 2. 3. 1. 1. 2 Data Required for design. The adjacent data are required for design Site hydrology report as s tipulated in Section 1. 3 of this Standard giving Qp ( adjust run) Q100 (design flood conflate, small hydro) Q10 (design flood flow, mini hydro) (data on suspended sediment loads) Cw H-Q Curves (W. L. rating curves at diversion dam) Topographic mapping of the site including river bathymetry covering all head works structure sites.Site geology report. 1. 1. 3 Site Selection Selection of the head works site is a practical decision which involves weighing of several factors including hydraulic desiderata (Section 2. 2. 1/1. 0), head optimization, foundation conditions, accessibility and constructability factors. Given the importance of intake design to the overall performance of the plant it is recommended that an experienced hydraulic engineer be consulted during studies on head works layout. 1. 1. 4 Determination of Key Elevations AHEC/MNRE/SHP Standards/ Civil Works Guidelines For hydraulic externalize Of Small Hydro Plants / may 20113For the illustrative example Qp = 10. 0 m3/s mould V0 = 0. 5 Q0. 2 (= 0. 792, say 0. 80 m/s) (= 12. 5 m2) A0 = Q ? V0 A0 H= (= 1. 77 m, say 1. 80 m) 4 strickle 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 Height of weir = 4. 0 m These initial key elevations are preliminary and may have to be adjusted later as the design evolves. 1. 1. 5 dot Works LayoutThe entry to the intake should be aligned with the river rim to provide smooth approach conditions and minimize the occurrence of undesirable swirl. A guide wall acting as a transformation between the river bank and the structure will usually be required. Intake hydraulics are raise if the intake face is slightly tilted into the flow. The orientation of the intake face depends on river bank topography, for straight river reaches the recommended values for tilt var y from 10o to 30o depending on the author. When this fish becomes too large the intake will attract excessive amounts of sediment and floating debris.It is recommended that the sill level of the intake is kept sufficiently risqueer than the sill level of the under sluice. The under sluice should be located adjacent to the intake structure. AHEC/MNRE/SHP Standards/ Civil Works Guidelines For hydraulic number 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 bloc of diversion structure The actual object may be finalized on the basis of model studies. Divide wall, if provided, should cover 80% to 100% of the intake. win flushing flow equal to twice project flow then approximate the breadth and bill of the flushing gate from orifice locution, Example should be in appendix. Qf = 0. 6 ? 0. 5W2 Where Qf = flushing flow W = gate width H = gate heig ht (= 0. 5W) Yo = normal flow depth as shown in 2. 2. 1. 1/2. 0 Sill should be straight and perpendicular to the flow 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 provide.For small hydro a simple overflow diversion weir would be the pet option if flood purloin would non cause unacceptable upstream flooding. For purpose of illustration, the following design data are fictional ( turn back Figure 2. 2. 2) innovation flood, Q100 = 175 m3/s A review of reservoir topography indicated that over bank flooding would occur if the flood water level exceeded 103. 0 m. Select this water level 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 compose. Assume gate W/H ratio = 12 H = 4. 0 m ? W = 4. 8 (sa y 5. 0 m) MFL. = NOL + 1. 50 (= 103. 0m) Qgate = Cw. W. (MFL ZS)1.. 5 Qweir = Cw. Lw. S1. 5 Capacity check for MFL = 103. 0 m No. of Length of Overflow 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/ Civil Works Guidelines For Hydraulic function Of Small Hydro Plants /May 20115 Therefore one gate is sufficient. Where MFL = Maximum flood level (m) NOL = Normal operating level (m) S = flood surcharge supra NOL (m)W = width of gate (m) H = height of gate (m) ZS = elevation of gate sill (m) = weir coefficient (m0. 5s-1) Cw QG, QW, QT = gate, weir and total flows The flow capacity of the sediment flushing gate may alike be included in calculating flood handling capacity. 1. 1. 7 Diversion structure and spillage Plains Rivers Stability of structures founded on alluvial foundations typical of plains rivers, is governed by the magnitude of the exit gradient. The slender gradient is approximately 1. 0 and shall be reduced by the following safety factors Types of foundationShingles / cobbles Coarse backbone Fine sand Safety factor 5 6 7 Allowable Exit Gradient 0. 20 0. 167 0. 143 Also diversion structures on plains rivers will normally require stilling basins to dissipate the vitality from the fall a torment the diversion structure before the water can be returned safely to the river. Design of diversion weirs and barrages on permeable foundation should follow IS 6966 (Part 1). Sample calculations in Chapter 12 of Fundamentals of Irrigation Engineering (Bharat Singh, 1983) explain determination of uplift force distri andions and exit gradients.Further details on structural aspects of design are habituated in Section 2. 3. 3 of this Standard. Mountain Rivers Bedrock is usually found at relatively shallow depths in multitude rivers permitting head works structures to be founded on rock. Also the beds of mountain rivers are practically boulder paved and are much more resistant to e ating away than plains rivers. Therefore there may be no need for a stilling basin. The engineer may consider refer blocks on the downstream apron or simply provide an angled 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 courtingable depth should AHEC/MNRE/SHP Standards/ Civil Works Guidelines For Hydraulic Design Of Small Hydro Plants /May 20116 to a fault be provided for added protection against undermining by scour. The head works structures would be intentional as gravity structures with enough mass to resist flotation. For low structures height slight than 2. 0 m anchors into bottom fundamental principle may be used as the prime stabilization element in dam design. Stability and stress design shall be in accordance with requirements of Section 2. 3. 3 of this Standard. 1. 1. 8 Sediment Flushing Channel To be reviewedThe following approach is recommended for design of the flushing channel S elect flushing channel flow capacity (Qf) = 2? Qp Estimate maximum size of sediment entering the pocket from site data or from transport capacity of approaching flow and velocity. In case of diversion weir without gates assume sediment accumulation to be level with the weir crest. (Assume continuous flushing with 3? Qp entering the pocket, for this calculation). Establish entrance sill elevation and channel vend assuming an intermittent flushing mode (intake closed) with Qs = 2Qp, critical flow at the sill, supercritical flow downstream (FN ? 1. 0) and a reservoir operating level 0. 5m below NOL. tick slope of channel to provide the required scouring velocity, 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 width (m3/s per m) Verify that flow through pocket in continuous flushing mode (Qs = 3Qs) will be sub critical, if not lower entrance sill elev ation further. Determine height of gate and gate opening establish on depth of flow at gate location and alike(p) gate width. Increase the in a higher place theoretical gate height by 0. 5 m to ensure unrestricted open channel flow through 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 transition between the river and the feeder canal. The main design objectives are to exclude bed-load and floating debris and to minimize head losings. The following parameters are recommended Approach velocity at intake entrance (on gross sphere of influence) 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 /May 20117 Convergence of side walls 2. 51 with rate of profit in velocity not exceeding 0. 5 m/s per linear m. Height of sill above floor of flushing channel (ye) = greater of 1. 5m or 50% flow depth. The floor of the transition should be sloped down as required to join the countermand of the feeder canal. bust that the flow velocity in the transition is adequate to observe deposition in the transition area. If sediment loads are very high consider installing a vortex silt ejector at the downstream end of the transition. Provide coarse trashracks to guard entry to the head gate. The trashrack would be designed to step floating debris such as trees, branches, wood on other floating objects. A cleared spacing of 150 mm between bars is recommended. Trashrack detailed design should be in accordance with IS 11388. The backtrack of the feeder canal shall be obstinate taking into friendliness head passinges through the trashrack and form losses 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 condensation.Calculate trashrack losses as 4/3 V2 ?t? H L = K f ? ? . Sin? . 2g ?b? Where Kf = head loss factor (= 2. 42 assuming angulate bars) T = thickness of bars (mm) B = clear bar spacing (mm) ? = angle of inclination to horizontal (degrees) V = approach velocity (m/s) 1. 1. 10 References 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 Engineering 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 transnational Conference on Small Hydro, Cancun Mexico. Bouvard, M. Mobile Barrages and Intakes on Sedime nt 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, capital of The Netherlands (1988) SEMI PERMANENT 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 have to be constructed on alluvial foundations. This challenge is addressed by adoption of less rigorous standards and the application of simplified designs adapted to the skills available in remote areas. A typical layout is shown in Figure 2. 2. 3. 1. 2. 1 Design Parameters 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 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 minimum width = 1. 0 m Assumed flushing gate W/H = 2, determine H from orifice equation, 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 above the cypher 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 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 yell should be determined in accordance with Section 2. 2. 1/4. 1 of this Standard.A stepped arra ngement 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 bollocks up-section design should be canvass for flotation, over turning and 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 intrench which conveys the flow into the project water conductor system. A characteristic of trench 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 two 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 suffic ient 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 pr fact the river from bypassing the structure.Trench weirs function best on weirs with slopes greater than 4%-5%, for blandish slopes diversion weirs should be considered. The spacing between racks is selected to prevent entry of bed load into the trench. The following terms are some clock 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 dimensioning of trench weirs. Design Flows The following design flows are recommended Bedload flushing flow (from collect or box) = 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 principal dimensions length, breadth 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 across main river channel. Maximum width (B) ? 2. 50m. Trashrack bars longer than about 2. 50 m may require support as slenderness ratios become excessive. Invert of collector box should be kept a high as possible. Racks The clear spacing between bars should be selected to prevent 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. Slopes normally used vary from 0 to 20. Rectangular bars are recommended. Bar structural dimension shall be designed in accordance with Section 2. 2. 1/5. 0 of this Standard. An grant 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 must in like manner 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 requ ired 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. Effective 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 inclined by Lauterjung et al (1989). First 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 approaching flow k = an adjustment factor (m) m) (-) k is a function of inclination of the rack and can be determined from the following table Values 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 area = 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 Hydra ulic 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 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 outlet 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 indite should be computed for the design slope and the trench bottom profile confirmed or adjusted, as required. A step-by-step procedure for calculating the flow profile that is applicable to this problem can be found in Example 124, page 342-345 of Open-Channel Hydraulics by Ven. T. Chow (1959). In most cases the profile will be sub critical with control from the downstream (exit) end.A suitable starting point 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 accrual box has two 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, while the project flow is withdrawn via the intake. The bottom of the collection box must be designed to provide adequate ducking 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 dec k 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 Of Small Hydro Plants /May 201114 1. 3. Flushing thermionic tube 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(prenominal) 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 area free from accumulation of bed load that could block the pipeline. 1. 3. 7 References on Trench weirs CBIP, (2001) 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 Publisher Tay lor & 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, canal and penstock intakes are all based on the same principles. However, there are significant interlingual renditions depending on whether an intake is at the forebay reservoir of a run-of-river plant or at entrepot 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 air 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 information about 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 First 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 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 various 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 fol lowing sketch. The bellmouth type intake is preferred when ever the additional costs 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 lessening 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 slant 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 transmit An air vent should be placed downstream of the head gate to facilitate air exchange between atmosphere and the penstock for the following conditions sluice filling when air will be expelled from the penstock as water enters. Penstock draining when air will enter the penstock to occupy the outer space previously filled by water. The air vent (pipe) must have an adequate cross section area 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 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 late 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 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. seculars Guidebook European Small Hydro Association 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 gumshoe 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. expatiate trashrack design should be done in accordance with IS 11388. 1. 5. 2Safety Racks Safety racks are required at tunnel and upside-down siphon 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 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 canals, 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 reinforc ed 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 constituent 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 = 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 optimisationThe optimum cross section dimensions, slope and velocity should be determined by economic analysis so as to minimize the total feeling time costs of capital, O&M and head losses (as capitalized value). The economic parameters for this analysis should be chosen in consultation with the becharm regional, state or central power authorities these parameters include Discount 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 allowance above the steady state design water level is required to contain water safely within the canal in event of pow er 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 powe r outages. The waterway in most SHPs terminates in a Forebay tank.This tank is normally equipped with an outflow weir to discharge surplus water or an escape 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 subject to the occurrence of a rejection surge in the canal on sudden turbine shutdown , giving above static water levels at the downstream end, diminution to the static level at the upstream (entry) end of the water way. Methods for evaluating 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 short 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 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 b y 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 geometrical 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 flu me section, using consider flow width (B) = A/D. . 3. INVERTED SYPHONS 2. 3. 1 Inverted syphons are used where it is more economical to route the waterway underneath an obstacle. The inverted syphon is made up of the following components Entry structure Syphon barrels Exit structure Entry Structure Hydraulic design of the entry structure is similar to the design of reservoir, canal and penstock intakes. Follow 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 Plants /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. LOW PRESSURE PIPELINESLow pressure pipelines may be employed as an alternative to pressurized box culverts, aqueducts or inverted syphons. Concrete, plastic and nerve pipes are suitable depending on site conditions and economics. Steel pipe is often an attractive alternative in place of concrete aquedu cts 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 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 Values for selected Pipe Materials Material Welded Steel polythene (HDPE) Poly Vinyl Chloride (PVC)Asbestos Cem ent Cast 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 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 favourable geology where the following criteria are satisfied a) Rock mass is adequately water tight. Rock surfaces are sound and not vulnerable to erosion (or erodible zones b) are suitably protected. The static water pressure does not exceed the magnitude of the minor field c) rock stress. Controlled security deposi t 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 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 excerption 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 split 1-7 give comprehensive guidelines on the design of lined tunnels. 2. 5. 4 High Pressure Tunnels Design of high p ressure 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, high 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 collected papers) Norwegian Tunneling Society 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 lateral runoff.The flow in these 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 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 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 an d 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 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. Eng ineering 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 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 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 empiric formulae given belowFor abrupt closure hmax = K 2 + 2 Kh For gradual closure within the period required for the first wave to incite 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 deta iled 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 Solve for Power Canal Rejection Wave for saucer-eyed 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 denseness 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 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 settling 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 Turbin es ? 150 m High Head Turbines 150 m 200 ppm 150 ppm 3. 1. 2 Removal Size There are also large 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 sampling 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 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 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 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. Less 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 flowing 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 versu s flow where possible) pinch size gradation curve 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 ma king. 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 = drainage density, as total length of streams divided 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 = louse up 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 re latively level with respect to cross section topography 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