Sprinkler & Drip Irrigation I Applicability, Advantages & Disadvantages

Sprinkler Irrigation

It is a method of irrigation in which water is spread uniformly over the land in the form of natural rainfall that comes from the nozzle of a sprinkler. Water is applied at the rate less than infiltration rate of soil so as to avoid any surface ponding and runoff. Also, the rate of application of water must match the rate of usage of water by the plants else the water will percolate deep below the root zones and will be lost to the plants. Surface ditches and prior land preparation is not necessary for sprinkler irritation system. It consists of a pipe network having main pipe, distribution pipe and laterals with the sprinklers placed at regular intervals along the lateral line. A sprinkler irrigation system can be of following different types:

1. Permanent type
2. Semi permanent type
3. Portable type

If the distribution pipe and laterals are fixed in the land i.e. they are often buried then it is called permanent type sprinkler irrigation system. If the distribution pipes are fixed but laterals are movable from place to place then it is termed as semi permanent system. If the entire network can be moved from place to place then it is called portable type sprinkler irrigation system.

Advantages of Sprinkler Irrigation System

• Suitable to all types of soil except heavy clay.
• Suitable for both row and scattered cropping patters.
• Possibility of using soluble fertilizers and chemicals by mixing in water.
• Accurate and easy to measure amount of water distributed.
• No soil erosion as surface runoff can be completely eliminated.

Disadvantages of Sprinkler Irrigation System

• Sediment free water is necessary otherwise frequent clogging occurs.
• Efficiency of water distribution decreases under heavy winds.
• It is not suitable for paddy crops.
• Due to circular spraying of water, there is chance of over irrigation and no irrigation at some area.

Drip Irrigation

It is a method of applying water directly to the plants through a number of low flow rate outlets called emitters or drippers, generally placed at short interval along a small tubing. One of the main characteristics of this method is point irrigation as compared to area irrigation in sprinkler irrigation system. This method of irrigation is suitable only for row crops such as tomatoes.

Advantages of Drip Irrigation System

• Deep percolation and evaporation losses can be minimized because only small portion of ground (especially root zone of plants) is supplied with water.
• Weed growth can be controlled effectively.
• Surface runoff and soil erosion can be minimized.
• Fertilizers can be used with high efficiency.
• Field levelling is not necessary.

Disadvantages of Drip Irrigation System

• Sediment free water is necessary otherwise frequent clogging occurs.
• It is not suitable for scattered cropping patterns.
• Sun heat decreases the useful life of tubes used for drip irrigation.
• Waste of water, time, and harvest if not installed properly.

Suitability of Sprinkler and Drip Irrigation System for Hilly Areas

The sprinkler and drip irrigation methods are suitable for remote hilly area of Nepal because of following reasons:

• The necessary head for the operation of sprinkler and drip irrigation is easily available in hilly area due to its steep topography.
• Sediment free water is necessary for effective operation of sprinkler and drip irrigation system which can be obtained from spring sources in hilly areas.
• These methods can be adopted for both row and scattered cropping patterns. Drip irrigation method is applicable only for row crops where as sprinkler is suitable for both row and scatter crops.
• Drip and sprinkler irrigation system can be established with a simple pipe network in hilly areas where construction of headworks and canal structures is not feasible. 
• Components of sprinkler and drip irrigation system can be transported easily to remote hilly areas.

Layout & Components of Typical ROR Type Hydropower Plant

Fig: Schematic diagram of typical ROR type hydropower plant

Components of typical ROR type hydropower plant

The general layout and components of hydropower plants may differ based on the types of hydroelectric project. A general layout of typical ROR type hydropower plants constructed in Nepal is shown in figure above in schematic  form. The name of each components is listed below:

1. Headworks Components

The diversion headwork is constructed in a hydro power plant so as to divert necessary amount of sediment free water into a waterway. A diversion headwork may consists of following components:

• Weir / Spillway

• Undersluice / Scouring sluice

• Divide wall

• Flood wall

• Side Intake

• Bed Sluice

• Gravel Trap

• Gravel Trap Side Spillway

• Approach Culvert / Approach Canal

• Settling Basin

• Head Pond

2. Headrace Waterway

It is a long watercourse or passage that conveys water from headworks component to the a surge chamber or a forebay. The flow in the waterway may be either open channel or pipe flow. The structure in waterway my be one of the following types:

• Headrace Tunnel
• Headrace Canal
• Headrace Pipe

3. Surge Tank / Surge Chamber

The chamber provided in between headrace pressure conduit and steeply sloping penstock pipes with a purpose to minimize the effect of water hammer is called surge tank. If the the waterway in headrace is of open channel type instead of pressurized system, forebay is installed in place of surge tank.

4. Penstock

It is a steel pipe that supplies water from surge chamber to the turbines placed inside powerhouse. Penstock pipe operates under very high water pressure.

5. Anchor Block and Saddle Support

These are the structures that provide supports to the penstock pipe. Anchor blocks are provided at the bends where as saddle supports are provided in between the anchor blocks at regular spacing.

6. Powerhouse

It is a building which consists of different hydromechanical and electromechanical equipment in which hydraulic energy is converted into mechanical energy by turbines and the generators convert mechanical energy into electrical energy.

7. Tailrace Culvert / Tailrace Canal

Tailrace is a water channel constructed in the downstream of hydroelectric powerhouse in order to discharge water back to the river or other water bodies safely.

Types of Irrigation | System of Irrigation

Types of irrigation can be broadly divided into following categories:
a) Surface Irrigation

b) Subsurface Irrigation

c) Micro Irrigation

a) Surface Irrigation

It is further divided into two parts:

i) Flow Irrigation

ii) Lift Irrigation

I) Flow Irrigation

When irrigation water is supplied to the agricultural land by flow of water solely under gravity is called flow irrigation. Flow irrigation is further divided into two parts as:

1) Inundation Irrigation

2) Perennial Irrigation

1) Inundation Irrigation

When water is diverted by excavating a canal at the bank of inundation river (i.e. the river which overflows in rainy season and nearly dries up in summer and winter) without regulator at the head of canal is called inundation irrigation. The bed level of canal is fixed in such a way that water flows through the canal when water level rises above the canal bed and stops automatically when it falls below the canal bed level. As there is no head regulator, over irrigation may occur resulting in damage of crops.

2) Perennial Irrigation

If water is diverted into the canal by constructing headworks at a perennial river (i.e. the river which flows throughout the years in its full capacity) is called perennial irrigation. It consists of regulator at the head of canal to control the flow into the canal. It is of following types:

• Direct Irrigation

In this system, headworks consist of a weir or a barrage across the river in order to raise water level upstream of the weir or barrage so that water can be diverted into the canal.

• Storage Irrigation

In this method, a dam is constructed across the river so as to form a storage reservoir at upstream of the dam and flow of water into the canal is controlled by a head regulator. The storage reservoir may be utilized for other purposes as well in addition to irrigation such as water supply, hydroelectricity, fishery, etc.

II) Lift Irrigation

If water is lifted from surface or subsurface sources by means of manpower, mechanical power or electrical power and directly supplied to the agricultural land then it is called lift irrigation. For example, water from a well may be lifted manually in bucket and supplied to the field via same bucket, a water boring may be installed inside the field that operates by electrical power, etc. It is mostly suitable for irrigating small isolated area. The method of lift irrigation has following advantages:

• Water is supplied to the field as per requirements and there is no possibility of over irrigation.
• Water is supplied directly to the field and there is no conveyance loss as in canals.
• There is no loss of valuable land because construction of distribution canals inside agricultural land is not necessary.
• Installation and maintenance cost is low.

Lift irrigation has following disadvantages:

• It may not be feasible for irrigating large areas.
• Pumping from deep wells may not feasible in the areas where water tables decreases below the suction head.

b) Subsurface Irrigation

In this method, water is supplied to the plants by maintaining an artificial water table at some depth below the ground surface. Moisture then moves upward till the root zones by capillary action.

c) Micro Irrigation

The sprinkler and drip irrigation system falls under this category. In sprinkler irrigation, water is sprayed from the nozzle of sprinkler uniformly over the field. In case of drip irrigation, water is supplied to row crops through a small tube and released from the drippers provided at regular spacing along the tube.

Head Loss in Pipe Flow I Major and Minor losses in Pipe

Concept of Head Loss in Pipe Flow

When a fluid under pressure flows through a pipe, the pipe offers resistance to the flow due to which total head (or energy) of flowing fluid gets reduced by certain amount which is called head loss $(h_L)$ in pipe flow. The head loss in fluid flow may occur due to several reasons which are mainly divided into two parts as:

1. Major Head Loss
2. Minor Head Loss

The head loss in fluid flow due to friction is called major head loss. It is also called friction loss $(h_f)$. The head loss in fluid flow caused by various factors such as bend in pipe, change in pipe diameter, presence of valves and fittings, etc are termed as minor head loss $(h_m)$. It is called so because its value is small in relation to friction loss for a large length of pipe. However, in case of short pipe, minor loss may become greater than the friction loss. The total head loss in the pipe is sum of major loss and minor losses.

Major Loss / Friction Loss in Pipe Flow

The friction loss in a pipe of length $(l)$ and diameter (d) is generally calculated using Darcy-Weibach Equation as below:

$$h_f=\frac{flv^2}{2gd}$$

where,

f = friction factor

l = length of pipe

v = velocity of flow in the pipe

g = acceleration due to gravity

   = $9.81 m/s^2$

d = diameter of pipe

The friction factor is evaluated depending upon the regime of flow (laminar or turbulent) and the type of surface boundary (smooth or rough). The friction factor for pipe flow is determined using Moody diagram for which Reynold's Number, pipe roughness, and diameter of pipe is required to be known. Moody diagram can be used for laminar as well as turbulent flow regime. Alternatively,  for laminar flow, friction factor can be formulated as below:

$$f=\frac{64}{Re}$$

Here, the Renyold's Number (Re) can be calculated as below:

$$Re=\frac{v\rho d}{\mu}$$

$$Re=\frac{vd}{\nu}$$

where, kinematic viscosity $(\nu)$ for water is approximately equal to $10^{-6}m^2/s$ and is formulated as below:

$$\nu=\frac{\mu}{\rho}$$

In case of turbulent flow, friction factor is determined based on whether the pipe is hydrodynamically smooth or rough. For turbulent flow in hydrodynamically smooth pipe, friction factor is given as:

$$f=\frac{0.3164}{Re^{1/4}}\space for \space 4*10^3<Re<4*10^5$$

The friction factor for turbulent flow in commercial pipes can be calculated using Colebrook-White Equation as below:

$$\frac{1}{\sqrt{f}}=-2log\left(\frac{e}{3.70d}+\frac{2.51}{Re\sqrt{f}}\right)$$

Here, the roughness height (e) for different pipe material is given below:

Minor Loss / Local Loss in Pipe Flow

When the velocity of a flowing liquid changes suddenly either in magnitude or in direction, there is a large scale turbulence generated due to the formation of eddies in which the energy possessed by the flowing liquid is utilized which is ultimately dissipated as a heat and hence loss of energy occurs. This loss of energy of flowing fluid due to sudden change in velocity is known as minor loss. Minor losses in pipe flow can be formulated in different situations as below:

1. Head loss due to sudden enlargement

$$h_m=\frac{(v_1-v_2)^2}{2g}$$

$$=k\frac{v_1^2}{2g}$$

where,

$$k=\left(1-\frac{A_1}{A_2}\right)^2$$

2. Head loss due to sudden contraction

$$h_m=k\frac{v_2^2}{2g}$$

where,

$$k=\left(\frac{1}{C_c}-1\right)^2$$

$C_c$ is coefficient of contraction. The value of $C_c$ or k is not constant but depends on the ratio $(\frac{A_2}{A_1})$. The value of 'k' is generally taken as 0.50.

3. Head loss at the entrance to a pipe

$$h_m=0.5\frac{v^2}{2g}$$

4. Head loss at exit from a pipe

$$h_m=\frac{v^2}{2g}$$

5. Head loss due to bend

$$h_m=k\frac{v^2}{2g}$$

The value of 'k' depends on total angle of bend and relative radius of curvature R/d, where, R is radius of curvature of pipe axis and d is diameter of pipe.

6. Head loss in various pipe fittings

$$h_m=k\frac{v^2}{2g}$$

The value of coefficient 'k' actually depends on the type of pipe fittings.

What is Simulation and Optimization in Water Resource Engineering ?

Simulation 

Simulation is the process of designing a model of a system and conducting experiments with it for understanding the behavior of the system and for evaluating the various strategies for its operation. A simulation model can be physical, analog or mathematical. The vehicle used to solve the mathematical model is commonly a computer. While performing a simulation using computers, the first step is to prepare a simulation model within a specific computer software. The simulation model should exactly replicate the real world problem. Then, known input parameters are fed to the simulation software as boundary conditions. And, the final step includes running the simulation model using the simulation software in which fundamental mathematical equations that govern the given problem are solved using computer resources. Different techniques like finite element method (FEM), finite difference method (FDM), finite volume method (FVM) etc are used to solve the fundamental governing equations. The advantages of computer simulation over a physical modelling are as follows:

1. A physical model is a time consuming and costly affair.
2. Complexities may arise in physical modeling if it is required to evaluate the alternative configurations and sizes of the facility.

Optimization

Optimization is the process of finding a best solution from a number of possible alternatives. The optimization method finds a set of decision variable such that the objective function is optimized. Optimization techniques are used to find optimal or near-optimal solutions to complex problems where exhaustive search is not practical. The objective could be to minimize cost, maximize profit, proper utilization of resources etc.

Applications of Simulation and Optimization Techniques in Water Resources Engineering 

There are various simulations software available free of cost or in paid versions that can be used to simulate the problems in water resources engineering. And analyzing the results of such simulations, optimal solutions can be obtained. For example, simulation of water flow in rivers can be performed in HecRAS in order to understand the flow hydraulics in natural streams, simulation of water hammer effect can be performed in a software named Hammer, three dimensional sediment laden flow simulation can be performed in Flow3D, the rock fall simulation can be performed in a software called RocFall by rocscience.

Anchor Block & Support Pier in Hydropower Plant

Anchor Block / Thrust Block

Anchor block is an encasement of penstock pipe at particular section, designed to restrain the pipe movement in all direction. It is a massive concrete block that anchor down the pipe securely to the ground. It should be stable against various forces acting on it. The shape and size of anchor block is confirmed by stability analysis. An expansion joint in the pipe is placed immediately downstream of the anchor block. Anchor block is required at following locations along the pipe line:

1. At every horizonal and vertical bends. Due to change in direction of flow, huge hydrostatic force acts at pipe bends that tend to move the pipe out of the alignment which is resisted by the anchor block.
2. At immediately upstream of the powerhouse. This minimizes the stresses in turbine housing.
3. In straight section at an interval of 100 to 150 m. This interval may become considerably small for micro hydel plants where the total head and discharge is relatively low.
4. At bifurcation and trifurcation in pipeline system. The flow redistribution at such branching creates imbalances in flow rates, pressure and velocities leading to uneven forces which is resisted by the anchor blocks.

Saddle Support / Support Pier

Support piers are short columns that are placed between anchor blocks along straight sections of exposed penstock pipe. Supports piers or saddle supports are provided at uniform spacing along the pipeline. These structures prevent the pipe from sagging and becoming overstressed. However, support piers allow pipe movement parallel to pipe alignment that occurs due to thermal expansion and construction. 

Mandatory Rule of Thumb in Building Design

In building design for Nepal, there are several mandatory rule of thumb that architects and designers typically follow. While these rules may or may not be legally binding, they are based on experience and practical knowledge to ensure safe and functional buildings. The main objective of these Mandatory Rules of Thumb (MRT) is to provide ready-to-use dimensions and details for various structural and non-structural elements for ordinary residential buildings commonly built in Nepal. Their purpose is to replace the non-engineered construction presently adopted with pre-engineered construction so as to achieve the minimum seismic safety requirements. There are major three codes that explain the thumb rule for building design in Nepal. They are NBC 201, NBC 202 and NBC 205. NBC 201 is for RCC building with masonry infill. NBC 202 is for load bearing masonry building and NBC 205 talks about MRT for RCC building without masonry infill. Few important rule of thumbs for building design as per these codes are pointed below:

For Load Bearing Masonry Building

• Brick masonry with cement mortar can be used upto three storey building.
• Brick masonry with mud mortar can be used upto two storey.
• Stone masonry with cement mortar can be used upto two storey.
• The concrete to be used in footings, columns, beams and slabs, etc., shall have a minimum crushing strength of 15 kN/m² at 28 days for a 150 mm cube.
• Cement-sand  mortar for masonry bond shall be of 1:6 and 1:4 for one-brick and half-brick thick walls, respectively.
• All plasters shall have a cement-sand mix not leaner than 1:6 on outside or inside faces.
• In order to achieve the full strength of masonry, the usual bonds specified for masonry shall be followed so that the vertical joints are broken properly from course to course.
• Openings should be as small and as centrally located as practicable.
• A building shall not be constructed if the proposed site is : water-logged, a rock-falling area, a landslide-prone area, a subsidence and/or fill area, and, a river bed or swamp area.

For RCC Building

• The span of beam shall not exceed 4.5m
• Each slab panels must be lesser than 13.5 square meter.
• The size of cantilever projection shall not exceed 1m.
• The length to width as well as height to width ratio of building must not exceed 3.
• The maximum height of structure is 11m or 3 storey whichever is less from the level of lateral restraint. However, an additional storey of smaller plan area(not exceeding 25% of typical floor area) shall be permitted.
• The length of wings on the structure shall restricted such that they are lesser than 25% of the length of rectangular part in either direction.
• No walls except a parapet wall shall be built on a cantilever slab. Such walls shall be constructed only if the cantilever slab is formed with beams.
• The foundation shall be at uniform level.
• Buildings shall not have soft storey.

Factors Affecting Selection of Turbine

Hydropower projects utilize various types of turbines depending on the site conditions and project requirements. The major types of turbines installed in the powerhouse of a hydropower project are Pelton turbine, Francis turbine, Propeller and Kaplan turbine, Deriaz turbine etc. Among them, Francis and Pelton turbine are the most widely used turbines in hydropower production because they have been extensively studied, developed and optimized over the years. The selection of a turbine for a hydropower project is a crucial decision that depends on various factors which are explained below:

1. Head and Discharge

As a general rule, Francis turbine is mostly suitable for high discharge and low head applications where as Pelton turbine used in case of high head and low discharge condition.

2. Part Load Operation

Efficiency of turbine is maximum when it is running at design load condition. In case of part load operation, Pelton turbine proves to be more efficient than Francis turbine.

3. Rotational Speed

As the turbine and generators are directly coupled, the rotational speed of turbine is same as synchronous speed of generator and is given as:

$$N=\frac{120f}{N_p}$$

where,

N = Rotational speed, rpm

$N_p$=Number of poles

f = Electrical frequency, Hz (50Hz for Nepal)

As the rotational speed (N) increases, number of pole ($N_p$) required is less. This means size of generator is reduced which ultimately reduces the cost of construction of powerhouse.

4. Specific Speed

The specific speed relation can be written as:

$$N_s=\frac{N\sqrt{P_{HP}}}{H^{5/4}}$$

$\because N_s \propto N$

The turbine with higher specific speed is expected to have high rotational speed. Eg. Kaplan turbine

$\because N_s \propto \frac{1}{H^{5/4}}$

The turbine with low specific speed will have higher head. Eg. Pelton turbine

5. Efficiency

The turbine with highest efficiency under various working condition shall be selected.

6. Maintenance Cost

The maintenance cost of reaction turbine is more than that of impulse turbine.

7. Transport Consideration

In case of larger units, it may be difficult to transport assembled large sized runner to the powerhouse sites.

8. Disposition of Shaft

From previous experience, it is recommended that horizontal shaft arrangement is best suitable for large size impulse turbine eg. Pelton turbine where as vertical shaft arrangement is most suitable for large sized reaction turbine eg. Francis trubine.

9. Cavitation Characteristics

Cavitation characteristics affects the installation of reaction turbine.

10. Water Quality

Water quality is more crucial for reactive turbine than reaction turbine.

Types of Energy Dissipators in Hydraulic Structure

Energy dissipater is a structure provided behind an overflow section, for example  a spillway, in order to dissipate the excess kinetic energy of water at downstream of the spillway. A spillway provided in the dam site always consists of an energy dissipating structure at the the toe of the dam. It kills the excess energy of surplus water and thus prevents damages to the dam and any other appurtenant structures in the downstream. Energy dissipation of water passing over the crest of spillway may be achieved by one of the following methods:

1. Hydraulic Jump Type Stilling Basin
2. Roller Bucket
3. Deflector Bucket / Flip Bucket / Ski Jump Bucket / Trajectory Bucket

1. Hydraulic Jump Type Stilling Basin

An stilling basin is a structure provided at the toe of spillway in order to dissipate the energy of excess water coming from spillway by formation of hydraulic jump within the confines of the basin. The flow passing at critical depth over the crest of spillway becomes super critical at dam toe which when meets the normal flow at subcritical depth in the downstream side, a hydraulic jump is formed during the transition from super critical to subcritical flow. The stilling basin should be so designed that the hydraulic jump is formed within the length of the basin so as to not harm the channel behind the length of stilling basin. In order to achieve this, the post jump depth obtained from sequent depth relation should be exactly equal to tail water depth. A stilling basin consists of a concrete apron and some auxiliary structures such as end sill, chute blocks, baffle blocks, etc.

2. Roller Bucket

Roller bucket is used to dissipate the energy in situation when the tail water depth is much more than the post jump depth. When high velocity sheet of water slides down the spillway, it gets arrested by the tail water. As a result, excess energy is dissipated due to formation of submerged hydraulic jump. A roller may be either a solid roller bucket or a slotted roller bucket the latter being the improved version of the former. The bucket type energy dissipater has a relatively short length as compared to hydraulic jump type stilling basin. The major design parameters for a roller bucket are the radius of bucket (R) and lip angle $(\phi)$. The radius varies from 15 to 25 m and lip angle varies from 20° to 40°.

3. Deflector Bucket / Flip Bucket / Ski Jump Bucket / Trajectory Bucket

Deflector bucket is used to dissipate the energy in situation when the tail water depth is insufficient for the formation of hydraulic jump. i.e. tail water depth is much less than post jump depth. It is in construction very similar to roller bucket but the hydraulic action is entirely different. The trajectory bucket deflects the high velocity jet into the air and is made to strike the river bed at a considerable distance from the structure. This type of energy dissipater is suitable for the situation where foundation rock is of good quality and can withstand the erosive action of striking jet. The energy dissipation is achieved due to combined action of air resistance, viscous effect and turbulence due to impact on the river bed.