PAES 603:2016 Open Channels Design PDF

PHILIPPINE AGRICULTURAL ENGINEERING STANDARD PAES 603:2016 Open Channels – Design of Main Canals, Laterals and Farm Ditches Foreword The formulation of this national standard was initiated by the Agricultural Machinery Testing and Evaluation Center (AMTEC) under the projec...

PHILIPPINE AGRICULTURAL ENGINEERING STANDARD PAES 603:2016 Open Channels – Design of Main Canals, Laterals and Farm Ditches Foreword The formulation of this national standard was initiated by the Agricultural Machinery Testing and Evaluation Center (AMTEC) under the project entitled “Enhancement of Nutrient and Water Use Efficiency Through Standardization of Engineering Support Systems for Precision Farming” funded by the Philippine Council for Agriculture, Aquaculture and Forestry and Natural Resources Research and Development - Department of Science and Technology (PCAARRD - DOST). This standard has been technically prepared in accordance with BPS Directives Part 3:2003 – Rules for the Structure and Drafting of International Standards. The word “shall” is used to indicate mandatory requirements to conform to the standard. The word “should” is used to indicate that among several possibilities one is recommended as particularly suitable without mentioning or excluding others. In the preparation of this standard, the following documents/publications were considered: Chow, V.T. 1959. Open Channel Hydraulics. McGraw Hill Book Company, Inc. New York. Food and Agriculture Organization. 1992. Irrigation and Water Management Training Manual No.7: Canals. National Irrigation Administration. 1979. Design Guides and Criteria For Irrigation Canals, O & M Roads, Drainage Channels & Appurtenant Structures. National Irrigation Administration. n.d. Design Manual for Canals and Canal Structures. National Irrigation Administration. n.d. Design Manual on Irrigation Facilities. National Irrigation Administration. 1991. Irrigation Engineering Manual for Diversified Cropping. United States Bureau of Reclamation. 1967. Design Standards No.3: Canals and Related Structures. United States Department of Agriculture. 1977. Technical Release No. 25: Design of Open Channels. A-66 PHILIPPINE AGRICULTURAL ENGINEERING STANDARD PAES 603:2016 Open Channels – Design of Main Canals, Laterals and Farm Ditches 1 Scope This standard provides minimum requirements and criteria for hydraulic evaluation and stable design of open channels specifically for main canals, laterals, sublaterals and farm ditches assuming uniform and steady flow. 2 References The following normative documents contain provisions, which, through reference in this text, constitute provisions of this National Standard: PAES 602:2016 – Determination of Irrigation Water Requirements 3 Symbols and Nomenclature Parameter Symbol Unit area A m2 channel bed slope s - depth d m energy grade line EGL - hydraulic depth D m hydraulic grade line HGL - hydraulic radius R m side slope z - top width T m wetted perimeter P m 4 Definitions 4.1 area cross-sectional area of the flow which is measured perpendicular to the direction of flow (see Figure 1) 4.2 channel bed slope inclination or elevation drop per unit length of the channel bottom (see Figure 2) 4.3 depth depth of water in the channel cross-section (see Figure 1) A-67  PAES 603:2016 4.4 energy grade line specific energy line grade line of the water surface profile plus the velocity head in open channels (see Figure 2) 4.5 freeboard vertical distance from the top of the channel to the water surface at the design condition (see Figure 1) 4.6 hydraulic depth ratio of flow area to the wetted top width (see Figure 4b) A D= T Where: D= hydraulic depth, m A= cross-sectional area of flow, m2 T= wetted top width, m 4.7 hydraulic grade line hydraulic gradient profile of the free water surface (see Figure 2) 4.8 hydraulic radius cross-sectional area of flow divided by the wetted perimeter A R= P Where: R= hydraulic radius, m A= cross-sectional area of flow, m2 P= wetted perimeter, m 4.9 lined channel lined canal canals with impermeable material (usually concrete) for channel stabilization and/or reduced seepage (see Figure 1) A-68  PAES 603:2016 Figure 1 – Cross-Section of a Lined Channel 4.10 normal depth constant flow depth along a longitudinal section of a channel under a uniform flow condition (see Figure 2) 4.11 open channel flow water flow that is conveyed in such a manner that top surface is exposed to the atmosphere such as flow in canals, ditches, drainage channels, culverts, and pipes under partially full flow conditions 4.12 slope of the hydraulic grade line slope of the free water surface 4.13 slope of the energy grade line slope of the water surface profile plus the velocity head in open channels 4.14 side slope ratio of the horizontal to vertical dimension of the channel wall (see Figure 1) 4.15 top width width of the channel cross-section at the free surface (see Figure 1) 4.16 uniform flow occurs when flow has a constant water area, depth, discharge, and average velocity through a reach of channel A-69  PAES 603:2016 Figure 2 – Profile of a Longitudinal Section of an Open Channel with Uniform Flow 4.17 unlined channels unlined canal canals that are cut through the soil, in which the soil excavated from the bed are used to form the embankment Figure 3 – Cross-Section of an Unlined Canal 4.18 wetted perimeter portion of the perimeter of the canal that is in contact with the flowing water A-70  PAES 603:2016 5 Design Requirements and Procedure The following parameters shall be determined in order to design an open channel: 1. Design Discharge or Flow Capacity 2. Flow Velocity 3. Roughness Coefficient 4. Canal Cross Section 5. Bottom Slope 6. Freeboard 5.1 Design Discharge 5.1.1 The design discharge for the main canal shall be equal to the diversion water requirement. Determination of diversion water requirement is shown in PAES 602:2016 – Determination of Irrigation Water Requirements. 5.1.2 The design discharge for the main farm ditch shall be equal to the farm water requirement. Determination of farm water requirement is shown in PAES 602:2016 – Determination of Irrigation Water Requirements. 5.2 Flow Velocity 5.2.1 For unlined channels, the design flow velocity of the canal shall not exceed the maximum permissible velocity in order to avoid destructive erosion to the channel and progressively destroy fertile lands. The maximum permissible velocity depends on the resistance to erosion of the banks of the canal. The values for different materials are given in Table 1. 5.2.2 For lined channels, the minimum permissible velocity shall be considered. However, the flow velocity shall be in no case too low to produce sand deposits or allow weed growth inside the canal. The permissible minimum velocity can be estimated as 0.6 m/s for plain water and 0.9 m/s for water with sediments. Table 1 – Maximum Velocities for Different Types of Materials Type of Material Maximum Permissible Velocity, m/s rock 2.00 decomposed, disintegrated rock 1.50 gravel with silt and sand 1.20 clay 0.90 clay loam 0.80 loam 0.70 sandy loam and silty loam 0.60 sandy soil 0.45 SOURCE: Irrigation Engineering Manual for Diversified Cropping, 1991 A-71  PAES 603:2016 5.3 Roughness Coefficient 5.3.1 Values for roughness coefficient of different materials forming the channel are shown in Table 2. 5.3.2 The value for roughness coefficient shall not be based on the degree of the original finish but on the surface that will exist after a few years of operation. 5.3.3 For the design of farm ditches, the value for roughness coefficient of 0.03 may be adopted since such canal is not well maintained, section is not uniform and there isusually grass or weed growth (Design Manual On Irrigation Facilities, n.d.). Table 2 – Mean Roughness Coefficients for Different Strata and Condition of the Channel Material and Condition Mean Roughness Coefficient Natural Channels Streams on Plain 1. straight, no weeds, neither cracks nor depths are present 0.0300 2. same as item 1 but weeds and stones are present 0.0350 3. no weeds but sinuous and with some depths and fords 0.0400 4. same as item 3 but weeds and stones are present 0.0450 5. same as item 3 but little changes in slope and cross-section, 0.0480 low water level 6. same as item 4 but more stones 0.0500 7. weeds and deep crevices are seen along slow stream flow 0.0700 sections 8. densely weeded section with many crevices and bushes 0.1000 Streams in mountainous area, no vegetation in channel, usually steep with trees and bushes along banks submerged at high water level: 1. boulder and gravel bed 0.0400 2. big boulder bed 0.0500 Excavated or Dredged Canal Earth, Straight and Uniform: 1. clean (immediately after completion) 0.0225 2. clean (after weathering) 0.0250 3. gravel, no weed and uniform section 0.0250 4. short grass, but little weeds 0.0270 Earth, Curved and Irregular Section: 1. no vegetation 0.0250 2. some weeds/grass 0.0300 3. with dense water weeds 0.0350 4. earth bottom and rubble sides 0.0300 5. earth bottom and weedy sides 0.0350 6. cobblestone bottom and clean sides 0.0400 Dragline Excavation or Dredged: 1. no vegetation 0.0280 A-72  PAES 603:2016 2. some bushes on the banks 0.0500 Rock Excavation 1. smooth and uniform 0.0350 2. irregular 0.0400 Lined Canal Rigid 1. concrete 0.0130 2. grouted riprap 0.0300 3. stone masonry 0.0320 4. soil cement 0.0220 5. asphalt 0.0160 Gravel Riprap 1. 1-inch D50 0.0330 2. 2-inch D50 0.0410 Rock Riprap 1. 6-inch D50 0.0690 2. 12-inch D50 0.0780 SOURCE: Irrigation Engineering Manual for Diversified Cropping, 1991 5.4 Canal Cross Section The recommended shapes for irrigation canals are trapezoidal or rectangular (for lined channels or rock formation) due to their stability and higher resistance to scouring thus, the guidelines in this standard are limited to such shapes. Sample cross-section is shown in Figure 3. Cross-sections of other canal shapes are shown in Annex A. 5.4.1 Side Slope 5.4.1.1 For lined channels, the recommended side slope is 1.5:1. 5.4.1.2 For unlined channels, side slopes shall depend on soil characteristics and should be flat enough to avoid the possibility of caving in after saturation. Steeper slopes can be permitted in cutting reaches in rock or hard strata but not for embankment reaches. Recommended values are shown in Table 3. Table 3 – Stable Side Slopes for Unlined Channels Location/Strata Stable Side Slope (Horizontal:Vertical) Cutting Reaches 1. hard rock ¼:1 to ½:1 2. decomposed rock, soft rock ¼:1 to 1:1 3. granular soils, sand, gravel and silt 1:1 to 1.5:1 4. highly plastic, expansive clays, loams 1.5:1 to 2:1 5. noncohesive and loose sand 2:1 Embankment Reaches 1. with granular material, sandy, silty soils 1.5:1 2. with highly plastic, expansive clays, loams 2:1 3. noncohesive and loose sandy soils 2:1 to 3:1 SOURCE: Design Manual on Irrigation Facilities, n.d. A-73  PAES 603:2016 5.4.2 Channel Bed Width and Depth Ratio (b/d ratio) 5.4.2.1 For lined and unlined channels, several b/d ratio can be selected in order to satisfy a given design discharge. 5.4.2.2 The bed width and channel depth can be computed by trial and error using the formula: 5⁄ Qn A 3 1⁄ = 2⁄ 3 S 2 P 3 where: Q = discharge, m /s n = roughness coefficient A = cross-sectional area, m2 S = bed slope P = wetted perimeter, m 5.4.2.3 The bed width and channel depth can also be determined by using Table 4 which shows the recommended b/d ratio for a corresponding design discharge. Table 4 – Recommended b/d ratio Range of b/d Ratio Channel Discharge, m3/s unlined channel lined channel 0.1 1.00 - 2.50 0.5 1.25 – 3.00 1 1.50 – 3.50 1.00 2 2.00 – 4.00 5 2.50 – 4.50 10 3.00 – 5.50 15 3.50 – 6.00 20 3.75 – 6.50 1.25 25 4.00 – 7.00 50 4.50 – 9.00 1.50 75 5.00 – 10.50 2.00 100 5.50 -11.50 more than 100 - 2.50 SOURCE: Design Manual on Irrigation Facilities, n.d. 5.5 Bottom Slope The bottom slope or the slope of the channel bed shall be determined from the topographic surveys. 5.6 Freeboard 5.6.1 The flow capacity in the cross sectional area including the freeboard shall be about 1.25 to 1.35 times the design discharge. 5.6.2 For unlined and lined channels, the freeboard shall be calculated based on the formula listed below. The largest value shall be selected for the freeboard. A-74  PAES 603:2016 5.6.2.1 For depths of flow of 0.3 m up to 2 m, Fb = 0.4d where : Fb = freeboard, m d = depth of flow during crop maintenance, m 5.6.2.2 For depths of flow of more than 2 m, Fb = 0.25d + 0.3m where : Fb = freeboard, m d = depth of flow during crop maintenance, m 5.6.2.3 Accounting for velocity head, Fb = 0.05d + hV + (0.05 m to 0.15 m) where : Fb = freeboard, m d = depth at design discharge, m hv = v2/2g, velocity head, m A sample computation is shown in Annex B. 6 Canal Route Selection 6.1 The canal route shall be selected such that the design discharge and design depth can be maintained. 6.2 The supply canal shall follow the highest points of the land to be irrigated by gravity in order to supply the widest area within the range of the available water head. 6.3 If the project area includes elevated areas or depressions, the use of appropriate conveyance structures shall be considered in the route. 6.4 High-banking or deep-cut sections shall be avoided, taking into consideration the available water head and available velocity. 6.5 Soil conditions, interference with houses and public traffic during construction of canals shall be considered in the canal route selection. 6.6 If the irrigation canal is long and its purpose is to irrigate mainly upland fields, establishing a regulating reservoir shall be considered. A regulating reservoir can decrease construction costs due to the reduction of the required canal sections. 6.7 Regional development plans or projects shall be considered in canal route selection. A-75  PAES 603:2016 Annex A Canal Cross-Section (Informative) A.1 Rectangle A.1.1 Area A = by A.1.2 Wetted Perimeter P = b + 2y by A.1.3 Hydraulic Radius R= b+2y A.1.4 Top Width T=b A.1.5 Hydraulic Depth D=y A.2 Trapezoid A.2.1 Area A = (b + zy)𝑦 A.2.2 Wetted Perimeter P = b + 2y√1 + 𝑧 2 (b+zy)𝑦 A.2.3 Hydraulic Radius R= 2 b+2y√1+𝑧 A.2.4 Top Width T = b + 2zy (b+zy)𝑦 A.2.5 Hydraulic Depth D = b+2zy A.3 Triangle A.3.1 Area A = z𝑦 2 A-76  PAES 603:2016 A.3.2 Wetted Perimeter P = 2y√1 + 𝑧 2 zy A.3.3 Hydraulic Radius R= 2√1+𝑧 2 A.3.4 Top Width T = 2zy 𝑦 A.3.5 Hydraulic Depth D=2 A.4 Circle 1 A.4.1 Area A = 8 (θ − sinθ)d0 2 1 A.4.2 Wetted Perimeter P = 2 θd0 1 sinθ A.4.3 Hydraulic Radius R = 4 (1 − ) d0 θ θ A.4.4 Top Width T = (sin 2) d0 = 2√y(d0 − y) θ−sinθ d0 A.4.5 Hydraulic Depth D=[ θ ] sin 8 2 A.5 Parabola 2 A.5.1 Area A = 3 Ty 8y2 A.5.2 Wetted Perimeter P=T+ 3T 2T2 y A.5.3 Hydraulic Radius R = 3T2 +8y2 3 A.5.4 Top Width T = 2 Ay 2 A.5.5 Hydraulic Depth D= y 3 A-77  PAES 603:2016 ANNEX B Sample Computation (Informative) B.1 Given: Flow capacity, Q 20 m3/s Type of Material clay loam Condition clean, earth canal with no vegetation Embankment reach with granular material, sandy, silty soils Bed Slope 0.00025 B.2 Required: Trapezoidal Canal Section for an Unlined Open Channel B.3 From Table 1, the maximum permissible velocity for clay loam, V = 0.8 m/s Q 20 m3 ⁄s A= = = 25 m2 V 0.8 m⁄s B.4 From Table 2, the mean roughness coefficient of the given strata and condition, n = 0.025 B.5 From Table 3, the stable side slope for the condition of embankment reach, the stable side slope ratio is 1.5:1 B.6 From Table 4, the recommended range of b/d ratio is from 3.75 to 6.50 B.7 Assume b/d = 6; b= 11, d=1.83 B.8 From A.2 of Annex A, the canal dimensions are computed as, (NOTE: d=y, b and d are in meters) Area A = (b + zy)y = (11 + (1.5 × 1.83) × 1.83 = 25.21 m2 Wetted Perimeter P = b + 2y√1 + z 2 = 11 + (2 × 1.83√1 + 1.52 ) = 17.6 m (b+zy)y 25.21 Hydraulic Radius R= = = 1.43 m b+2y√1+z2 17.6 B.9 Compute for the actual flow velocity and flow area. B.9.1 The computed flow velocity shall be less than or equal to the maximum permissible velocity. Otherwise, adjust the value of base or depth. B.9.2 The computed flow capacity shall be greater than or equal to but not appreciably different from the design capacity or required flow capacity. Otherwise, adjust the b/d ratio. 1 1 V = n R2⁄3 S 1⁄2 =0.025 × 1.432⁄3 × 0.000251⁄2 = 0.80 m/s Q = AV = 25.21 × 0.80 m = 20.25 m3 ⁄s A-78  PAES 603:2016 B.10 If all conditions are met, compute for the top width and freeboard Top Width T = b + 2zy = 11 + 2 × 1.5 × 1.83 = 16.49 m B.11 From section 5.6.2, the freeboard is computed as From 5.6.2.1 Fb = 0.4d = 0.4 × 1.83 = 0.73 m From 5.6.2.3 Fb = 0.05d + hV + (0.05 m to 0.15 m) Fb = 0.05 × 1.83 + (0.802 ⁄2 × 9.81) + 0.10 = 0.22 m B.12 Take the larger value as freeboard which is 0.73 m. B.13 The design of the required unlined open channel is shown below. A-79