Design of Canals

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These canals occupy less of the agricultural land, but are also is more costly to construct Figure 3.

Fish production in irrigation canals A review

Unlined canals, whilst attractive because of their low construction costs, have a number of disadvantages which may have serious consequences for crop production, and which also have implications for fish production. These problems are examined in section 3. These canals are normally lined with concrete, brick or stone. Other types of lining more recently used include polythene films, bitumous mixtures, soil cement, chemical sealants and impervious earth materials Michael, These have a limited life, however, and are susceptible to damage by livestock and excessive water velocity.

Concrete linings, although costly, have a longer lifespan with minimal repair and maintenance costs. Canal banks - The slope of the banks can be greater in lined canals than in earth canals, but should not exceed a slope of , unless the concrete is hand placed and the canal over 60 cm in depth. A typical cross-section is shown in Figure 3. In certain areas of the world there are extensive aquifers; Bangladesh is a prime example, and much of the irrigated area is supplied by numerous tube wells which tap the underground water resources. These wells each supply only small areas, a few hectares at most.

Water is pumped from the well on demand, and is distributed to the fields by a series of tertiary and quaternary canals. In such systems there is no potential for fish culture in the irrigation canals. There are several advantages in using pressurised underground systems compared to that of open canals. Pipes take less land out of cultivation and do not interfere with farm operations. Equation 5. On recognising the effect of the sediment size on the critical velocity, Kennedy modified Eq.

Here, the velocity U is the critical velocity for all sizes of sediment, whereas U o is the critical velocity for Upper Bari Doab sediment only. This means that the value of m is unity for sediment of the size of Upper Bari Doab sediment. For sediment coarser than Upper Bari Doab sediment, m is greater than 1, while for sediment finer than Upper Bari Doab sediment, m is less than 1.

Kennedy did not try to establish tiny other relationship for the slope of regime channels in terms of either the critical velocity or the depth of flow. Thus, the equations enable one to determine the unknowns B i.

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In Eqs. Such considerations limit the range of slope. However, within this range of slope, one can obtain different combinations of B and h satisfying Eqs. The resulting channel sections can vary from very narrow to very wide. While all these channel sections would be able to carry the given discharge, not all of them would behave satisfactorily.

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    Design and construction

    If this value of mean velocity matches with the value of the critical velocity obtained earlier, the assumed value of h and the computed value of B provide channel dimensions. If the two velocities do not match, assume another value of h and repeat the calculations. For this purpose draw a vertical line through the intersection of the relevant discharge curve and horizontal line representing the desired value of S. The vertical line would intersect several bed width curves. A horizontal line drawn through each of such points of intersection would enable determination of the depth of flow h and critical velocity U 0 for the chosen width B corresponding to the selected point of intersection.

    Assume that the bed slope is equal to 1 in Since the two values of the velocities are matching, the depth of flow can be taken as equal to 2. Lacey stated that the width, depth and slope of a regime channel to carry a given water discharge loaded with a given sediment discharge are all fixed by nature. According to him, the fundamental requirements for a channel to be in regime are as follows:. Incoherent alluvium is the loose granular material which can scour or deposit with the same ease. The material may range from very fine sand to gravel, pebbles and boulders of small size.

    The channels which have lateral restraint because of rigid banks or imposed slope are not considered as regime channels. For example, an artificial channel, excavated with width and longitudinal slope smaller than the required, will tend to widen its width and steepen its slope if the banks and bed are of incoherent alluvium and non-rigid. In case of rigid banks, the width is not widened but the slope becomes steeper. Lacey termed this regime as initial regime. A channel in initial regime is narrower than what it would have been if the banks were not rigid.

    This channel has attained working stability. If the continued flow of water overcomes the resistance to bank erosion so that the channel now has freedom to adjust its perimeter, slope and depth in accordance with the discharge, the channel is likely to attain what Lacey termed as the final regime. The river bed material may not be active at low stages of the river, particularly if the bed is composed of coarse sand and boulders. However, at higher stages, the bed material becomes active, i.

    As such, it is only during the high stages that the river may achieve regime conditions. This fact is utilised in solving problems related to floods in river channels. Based on the analysis of data Lacey finally gave the following regime relations which can be used to obtain channel dimensions, channel slope S and the flow velocity U for given discharge Q and sediment size d in an alluvial channel carrying sediment-laden water:. Equations 5. For wide channels, the hydraulic radius is almost equal to the depth of flow h. Hence, Eq. This helps in estimating the levels of foundations, vertical cutoffs and lengths of launching aprons of a structure constructed across a river.

    The use of these charts involves obtaining the intersection of the relevant fi curve with the relevant discharge curve and then read the values of the bed width B and depth of flow h from the abscissa and ordinate. Figure 5. The regime equations are purely empirical relations lacking any sound theoretical basis. These equations consider only sediment size to be important and do not take into account the sediment load. It should be noted that the regime charts i.

    The layout and alignment of an irrigation canal should be such that it ensures equitable distribution of water with minimum expenses. Watershed is the dividing line between the parts of two catchment areas from where rain water flows into a drain or stream of two adjacent streams and is obtained by joining the points of highest elevation on successive cross-sections taken between the two streams or drains.

    Such an alignment will. Since the main canal takes off its water from a river which is at the lowest point in the cross-section , the main canal necessarily crosses some streams before it mounts the watershed. In the first four types i. AOSM, OFRB or OF, the discharge, though dependent on the water level in the parent channel, is independent of the water level in the watercourse, so long as the Minimum Modular Head mmh required for its working is available. In the fifth type of outlet i.

    In case of free fall, the difference in the canal water surface and centre of pipe at exit would control the discharge. All outlets within ft upstream of fall structures would be designed as Open Flumes OF. AOSM type of outlet is a long throated flume with a roof block, capable of vertical adjustment, introduced into the upper end of the parallel throat.

    Design of Canals

    So long as the standing wave is steady and remains clear of the exit of the orifice, the discharge co-efficient does not alter. The roof block is so shaped that the jet is made to fill the exit of the orifice and jet contraction is suppressed. Cast iron or pre-cast RCC roof block would be used. Roof block would have its upstream end at distance of three 3 inches down the throat from the crest and its bottom at a height of y varies above the crest.

    Discharge of this type of outlet is computed by the formula:. Setting of the outlet would be equal to 0. At this setting, its sediment drawl capacity would be is This outlet is simply a smooth weir with throat constructed sufficiently long to ensure that the controlling section remains within the parallel throat at all discharges up to maximum. Values of C are as under: For Bt 0. Setting of the outlet would be kept equal to 0. In order to check the over-withdrawal by the outlet, a roof block would always be fixed in the gullet.

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    The roof block would have its upstream end at a distance of H down the throat from the crest and its bottom at a height of 0. As width of outlet is limited to minimum of 0. In such cases, the crest level of open flume outlet would be the same as that of bed in the parent channel. H, the height of full supply level in the channel above crest level of the OF outlet would kept as 1. Zero of the gauge would be fixed at crest level.

    Hydraulic Canals: Design, Construction, Regulation and Maintenance

    Gauge reading at. Crests of all outlets at tail would be kept at the same level. In drowned condition, it works non-modular. Use of this type of outlet is avoided, as far as possible. Its use would be justified in case of lift outlets when the working head available or the discharge of the outlet is so small that a semi-modular outlet cannot be designed.

    Cross drainage structures are constructed to negotiate an aligned channel over, below or at the same level of stream crossing that channel. These structures can be classified under the following three broad categories:. Small drains may be taken under the canal through culverts but for streams crossings it is economical to flume the canal over the stream. Such structure called super-passage is appropriate where the nullah invert level is between canal bed level and full supply level FSL or above FSL.

    The canal section geometry would be kept unchanged to minimize head losses. The structure would consist of: trough to carry the nullah water, supported on piers or piles foundation; downstream bridge to maintain continuity of the canal road supported; foot bridge on trough walls; stilling basin for energy dissipation; upstream and downstream flared out transitions; upstream guide banks to guide the flow through the crossing without inducting excessive scour; upstream and downstream stone aprons; stone pitching for slope protection; upstream and downstream cutoffs; downstream PCC blocks over inverted filter, and reinforced concrete canal lining underneath the trough.

    Guide banks would provide a free board of 3. The structure would comprise: multiple barrels carrying nullah water under canal section; upstream head wall; downstream transitions; downstream cistern floor; upstream and downstream stone aprons; stone pitching for slope protection; and cutoffs. Many irrigation structures incorporate a culvert or a bridge in order to facilitate vehicular access within the scheme.

    Where a road crosses a channel at some distance from any other structure, it will be necessary to provide an independent culvert or bridge. Road crossings usually comprise either pipe or box culverts to convey channel water under the road. The culvert acts as a constriction and creates a backwater effect to the approach flow, causing a pondage of water above the culvert entrance. The flow within the barrel itself may have a free surface with subcritical or supercritical conditions depending on the length, roughness, gradient, and upstream and downstream water levels of the culvert.

    If the upstream head H is sufficiently large the flow within the culvert may or. The pipe typically has a fall of 0. For culverts under patrol roads within an irrigation scheme, the minimum depth of cover to the pipe should be about 0. If this is impracticable, then consideration should be given to:. Transitions are required at both the inlet and outlet of the culvert barrel. An accelerating water velocity usually occurs at the inlet, and a decelerating velocity at the outlet.


    Transitions reduce head losses and prevent channel erosion by making the velocity changes less abrupt. Pipe collars may be required to reduce seepage flows along the outside of the pipe. This should be checked using Lanes seepage path length method, particularly for any culvert where the channel water surface is significantly higher than the potential point of relief of the percolating water.

    Pipe collars may also be necessary to discourage rodents from burrowing along the pipe. Channel erosion protection is required in earthen channels, and usually takes the form of dry stone pitching placed upstream and downstream of the transitions. In addition, for large flows, cutoffs may be provided to the transition structures extending below scour depth. They can be constructed from reinforced concrete, or comprise concrete floor and roof slabs with masonry walls. The minimum height of the culvert should be about 3 ft. Box culverts are normally designed for free flow, with the water surface below soffit level.

    Head loss across the culvert can be calculated by summing entry and exit losses, plus friction loss in the culvert which may be determined using Mannings equation.