Group 1 Definition of Runoff Runoff can be defined as the portion of the precipitation that makes it‟s way towards rivers or oceans etc, as surface or subsurface flow. Portion which is not absorbed by the deep strata. Runoff occurs only when the rate of precipitation exceeds the rate at which water may infiltrate into the soil. Portions of Runoff 1. Surface runoff 2. Groundwater flow 3. Direct precipitation over the river stream. Runoff Process
W h e n r a i n f a llll o c c u r s :
A part of rainfall/precipitation is intercepted by vegetation. Some part is stored in depressions on the ground surface known as depression storage (S d ), which later infilterates or evaporates. Some part of rainfall is absorbed by the soil, the amount of which depends upon the soil moisture condition at the time of percolation.
Now if the rain con tinues further;
The water starts infiltrating/percolating to the water table and if the rate of rainfall or the rate at which the water is reaching the ground exceeds the infiltration rate (f), resulting the surface detention (D). This water flow overland and joins the rivers, lakes, streams, oceans, etc, and is known as surface runoff . Surface Runoff 1) The surface runoff is important for maximum flow. 2) Where as the ground water flow is important for minimum flow. 3) Direct Precipitation over the river or stream is negligible. So for peak flow we are generally concerned with surface runoff and therefore we can say runoff as surface runoff. Channel Runoff Streamflow , or channel or channel runoff , is the flow Streamflow, of water of water in in streams, rivers, and other channels, other channels, and is a major element of the water cycle. runoff of water from the It is one component of the runoff of land to waterbodies, the other component being surface runoff. Water flowing in channels comes from surface runoff from adjacent hillslopes, from groundwater flow groundwater flow out of the ground, and from water discharged from pipes. The discharge of water flowing in a channel is measured using stream gauges or can be estimated by the Manning equation. The record of flow over time is called ahydrograph. Flooding occurs when the volume of water exceeds the capacity of the channel.
Factors Affecting Runoff 1. Precipitation characteristics, It is the most important factor for runoff Runoff depends on the type of the storm and it‟s duration, which causes precipitation. Runoff depends on the intensity of rainfall. More the rainfall, more will be runoff. If the rainfall intensity is very less and it rains as light showers then much of the water will be lost in infiltration & evaporation resulting less runoff. If precipitation is in case of snow then less runoff. 2. Shape and size of the catchment, Runoff depends upon size, shape and location of the catchment. Generally more rainfall on smaller area resulting in greater runoff. Less runoff in larger catchments because due to uniform rainfall over the entire area, thus only few tributaries of the stream feed water to a main stream during a particular storm. Types of catchment Fan shaped catchment All the tributaries are approximately of the same size. Gives greater runoff because the peak flood from the tributaries is likely to reach the main stream approximately at the same time. Fern leaf catchment The tributaries are generally of different lengths and meet the main stream at the regular intervals. In such a narrow catchments the peak flood intensity is reduced since discharges are likely to be distributed over a long period of time. 3. Topography, Runoff depends on surface smoothness and slope. If slope is steep, flow will be quick and less evaporation and absorption, resulting greater runoff. If the catchment is in mountainous area and on the windward side of the mountain, then more rain fall resulting more runoff. 4. Geological characteristics, It is one of the important factor. It includes the type of surface soil, subsoil, type of rock and their permeability characteristics. If soil and subsoil is porous, seepage will be more resulting reduction of the peak flood. If the surface is rocky, then absorption will be nil resulting more runoff. If rocks have fissures, are porous in nature, have lava tunnels water will be lost resulting less runoff.
5.
Meteorological characteristics, Runoff may also be affected by temperature, wind and humidity. If temperature is low and ground is saturated then runoff will be greater. If temperature is high and greater wind velocity give rise to greater evaporation loss and resulting in less runoff. Character of the catchment surface, Runoff depends upon the surface conditions like drained, undrained, natural or cultivated. If the surface has no natural drainage then absorption loss will be more. If more area of a catchment is cultivated resulting less runoff. Vegetal cover reduces the runoff in smaller storms. No vegetal cover reduction in bigger storm. Storage characteristics. The artificial storage such as dams, weirs, etc and natural storage such as lakes, ponds, etc tend to reduce the peak flow. They also give rise to greater evaporation losses.
6.
7.
Similarly the flow into the filled depression is essentially balanced by over land flow and infiltration. Thus at this stage over land flow mainly contribute to stream flow. Sub surface flow also contributes to stream flow. After the end of rainfall When rain and overland flow ceases (stops), the stream flow consists of only base flow and channel storage. Evaporation takes place quite from soil moisture. Transpiration also takes place from vegetative cover. Water from depression storages also continues to infiltrate. Also the gravity water still not drained up to the water table continues its downward journey to join water table.
4.
S u m m a r y o f R a in f a l l -R u n o f f P r o c e s s
Runoff Cycle It is a part of hydrological cycle The part between the precipitation from the atmosphere over land areas and it‟s subsequent discharge through streams channels. Conditions of Runoff Cycle 1. End of dry period At the end of dry period and just beginning of heavy rainfall all the surface and channel storage gets depleted (dried) expect from lakes, reservoirs and ponds resulted from the previous rains. The only source of stream flow is the ground water flow entering the river channel. 2. Shortly after beginning of rainfall Shortly after beginning of rainfall and before interception, depression storage have been satisfied. The stream flow intercepted by vegetation and buildings can‟t contribute to runoff. This intercepted water is eventually returned to the atmosphere through evaporation. At this stage, a part of precipitation falls directly on the stream which gives an immediate increment to stream flow. 3. Near the end of isolated heavy rainfall After many hours of heavy rainfall virtually all depression storage and interception requirements gets filled up. The soil moisture deficiency is also satisfied to a considerable extent. Infiltration rate is near the minimum.
When a rain starts falling, it is first of all intercepted by buildings, trees, and other objects, which prevent it from reaching the ground. This quantity is known as rainfall interception . Since this quantity is generally very small, it is not of much importance for intense rains; but many a times, large portions of lighter rains are disposed of in this manner. The difference between the total rainfall and that which is intercepted is called g r o u n d r a i n f al l . when the rainfall rate exceeds the interception rate, water starts reaching the ground and infiltration into the sub soil starts. The maximum rate at which the soil in a given condition can absorb water is known as its infiltration capacity . The excess rainwater gets collected into the innumerable small and large depressions existing in the basin, filling them to their overflow levels. This quantity is known as . depression storage All this storage is either evaporated or used by vegetation, or later infiltrates into the soil. None of it appears as surface runoff. If after the depression storage is filled, the rain intensity ( p) continues to exceed the infiltration capacity of the soil (f ), the difference appears as rainfall excess , which initially accumulates on the ground as surface detention (D), and then flows as o v e r l a n d f l o w on the basin surface before entering a stream channel. The water that reaches the stream channel of a basin in this manner is called surface runoff (SRO) or d i r e c t runo ff (DRO) . The surface runoff can, therefore, occur only from those storms, which can contribute to excess rainfall, and are simply not dissipated in fulfilling the interception, depression storage, and infiltration needs of the basin. Hence, Excess Rainfall = Rainfall – Interception - Depressio n storage – Infiltration
The sum total of interception and depression storage for a basin is usually called the initial loss or b a s i n l o s s or initial basin recharge . Excess rainfall can therefore be represented as: Excess r ainfall = Rainfall – Initial Basin los s – Infiltration
The sum total of initial basin loss and infiltration, is called potential infiltration . Since for intense rains, the initial loss is very small as compared to excess rain, it is usually ignored in hydrological analysis of such rainfalls; or is considered to be included in the infiltration itself. The excess rainfall is, thus, represented as:
Application of method Where USED? Design of storm water drains. especially for Small water control projects urban areas where the percentage of impervious area is quite high. Where should be AVOIDED? This method of computing runoff should be avoided for rural areas and for the analysis of major storms. 2.
USE OF INFILTRATION CAPACITY CURVE The infiltration capacity (IC) curve is a plot of the infiltration capacity against time. If the IC curve is super imposed on the rainfall hyetograph, the resultant amount will represent nothing but the runoff. Natural rains of varying intensities, sometimes below and some time above the prevailing infiltration capacity, results in a distortion of a capacity time curve. It is generally assumed that the infiltration capacity at any time is determined by the mass infiltration, which has occurred up to that time.
3.
USE OF INFILTRATION INDICES The infiltration capacity curve already determined on test plot cannot be applied to large basins or heterogeneous areas. At any instant, in a large area, the IC as well as the rainfall rate will vary from point to point. Moreover subsurface flow or interflow will also be substantial. Since this water flow is part of infiltration, it will not normally be included in the runoff computed by IC curve determined on a test plot. Hence runoff volumes are generally computed by using infiltration indices. W-index and Φ-index are the two indices which are commonly used.
W-index is the average infiltration rate or the infiltration capacity averaged over the whole storm period, and is given by
Excess Rainfall = Rainfall – Potential Infiltration
The rain that falls in the beginning of a storm before the depression storage is completely filled is called the initial , rain and the rain that falls near the end of the storm at a rate less than the infiltration capacity is called the residual rain . The intervening period is the net supply interval. The infiltration occurring after the net supply interval is called the residual infiltration . R u n o f f and s u r f a c e r u n o f f are two different terms and should not be confused. R u n o f f o r D i s c h a r g e o r t h e S tr e am f l o w includes all the water flowing in the stream channel at any given section. While S u r f ac e R u n o f f o r D i r ec t R u n o f f includes only the water that reaches the stream channel without first percolating down to the water table. Yield of a drainage basin is same as runoff, with the only difference that it is expressed over long periods 3 (M.m /year), while 3 3 R u n o f f is expressed for short periods (m /sec or m /hr). Runoff =Surface runoff + Ground water inflow (i.e. Base flow).
Methods of computation of runoff 1.
Use of runoff coefficient The volume of runoff can be directly computed approximately by using an equation of the form Q=k.P Where Q= runoff P= precipitation k= constant depending upon imperviousness of the drainage area Truly speaking this equation is not rational because: “runoff not only depends upon the precipitation but also depends upon the recharge of the basin”. but the equation gives more and more reliable results as the imperviousness of the area increases and the value of k tends to approach unity.
W index = F / tr = (P – Q) / tr where F = total infiltration including initial basin recharge P = total precipitation Q = total runoff tr = duration of rainfall in hour Φ-index is defined as, “the average rate of loss such that the volume of rainfall in excess of that rate will be equal to the volume of direct runoff”.
It can be defined the other way round as, “the rate of rainfall above which the rainfall volume equals to the runoff volume”. Φ-index can be represented graphically as shown.
Φ-index and W-index will be equal for a uniform rain, but they may not be equal for a non-uniform rainfall. However for rains which are reasonably uniform or for heavy rains these two indices are found to be nearly equal.
In a usual case of moderate rain of non-uniform intensities the Φ-index will be somewhat higher than W-index. These indices will change with a change in interception, depression storage and initial soil moisture. They will also change with the amount of precipitation. These indices are not the actual infiltration rate but the measure of potential basin recharge. The runoff coefficient k can be determined if the W index is known by using the equation k = (P – W index) / P where, P = rainfall rate
Group 2 Groundwater Hydrology Groundwater Groundwater is the liquid water flowing through shallow aquifers, but technically it can also include soil moisture, permafrost (frozen soil), immobile water in very low permeability bedrock, and deep geothermal or oil formation water. Groundwater is the water below the ground surface occupying the pore spaces in rocks and soils. Groundwater is present everywhere beneath land surface and ocean bottom. Most ground water originates from precipitation and surface water. Groundwater is always in motion. Groundwater is recharged from, and eventually flows to, the surface naturally; natural discharge often occurs at springs and seeps, and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal and industrial use by constructing and operating extraction wells. Typically, groundwater is thought of Groundwater is hypothesized to provide lubrication that can possibly influence the movement of faults. It is likely that much of the Earth's subsurface contains some water, which may be mixed with other fluids in some instances. Nearly 3/4 of the water that comes to our homes goes down the drain. Of all of the earth‟s water that‟s useable by humans, 98% is groundwater.
A faucet that leaks at a rate of one drop per second wastes five gallons of water per day and 2,082 gallons per year. What is poured on the ground today can end up in our drinking water many years later. USES OF GROUNDWATER Irrigation production of mineral water, aqua-culture production of energy -Groundwater has a good heat capacity and can therefore be employed as a source of energy when using heat pumps for either heating or cooling. self-supplied domestic, and public supply livestock purposes HYDROGEOLOGY/GROUNDWATER HYDROLOGY the study of the distribution and movement of groundwater. AQUIFER - A moderately to highly permeable rocks where ground water is stored in, and moves slowly through. -The word aquifer comes from the two Latin words, aqua, or water, and ferre, to bear or carry. Aquifers literally carry water underground. Topography and its Effect on Groundwater Topography of the land surface also determines the general direction of groundwater flow, and it influences groundwater recharge and discharge. A recharge area is where water moves downward from a topographical high area into the zone of saturation. A discharge area is where groundwater moves towards the surface to escape into a spring, lake, wetland, or a stream. Groundwater flow from the recharge area to a discharge area is called a flow system. Even though recharge often occurs at topographic high areas, direct runoff may also occur. During high intensity storms, water falling in steep areas is more likely to become surface or direct runoff, rather than replenish the groundwater. Conversely, flat areas are more likely to absorb the water. In addition to topography, soil type, water levels, and underlying bedrock can also affect recharge. Subsurface Zones Zone of Saturation or Saturated Zone
Subsurface zone in which all the void spaces within the rock or sediment are filled with water. Also known as the phreatic zone. (Pheratic is derived from the Greek phrear , -atos meaning a well). Zone of Aeration or Unsaturated Zone
Subsurface zone in which all void spaces in the rock or sediment are not filled with water. Also known as the vadose zone or zone of aeration. (vadose is derived from the Latin vadosus meaning shallow ).
Capillary Fringe – region above water table where water rises due to capillary forces in the porous medium. Water table and Perched Water table The top of an unconfined aquifer is the water table or Pheratic Surface. The amount of water in storage in an aquifer is reflected in the elevation of its water table. If the rate of recharge is less than the natural discharge rate plus well production, the water table will decline and the aquifer's storage will decrease. A perched aquifer's water table is usually highly sensitive to the amount of seasonal recharge so a perched aquifer typically can go dry in summers or during drought years. Interstices The spaces where groundwater occupies are known as voids, interstices, pores or pore spaces. The number, size, and shape of these openings depend upon the character of the rocks; therefore, the occurrence of ground water in any region is determined by the geology of that region. The interstices or voids in rocks range in size from microscopic openings in clay to huge caverns in limestones. The porosity of a rock aggregate is its property of containing interstices. Porosity is expressed as the percentage of the total volume occupied by the interstices. Types of Interstices w.r.to Size Capillary: sufficiently small, water held by surface tension forces Subcapillary: very small, water held primarily by adhesive forces Supercapillary: larger than capillary forces Types of Interstices w.r.to Connection Communicating Isolated
Original Interstices Created by geologic processes governing the origin of geologic formation. Found in sedimentary and igneous rocks. Secondary Interstices Developed after rock formation. Examples: joints, fractures, solution openings, opening formed by plants and animals. Porosity Ratio of pore volume to total volume of a soil or rock . Open porosity or effective porosity Ratio of accessible pore volume to total volume . Porosity determines the amount of water that a given volume of soil or rock can contain. Diogenesis – The formation of rock; pores fill up with precipitations of mineral and reduce porosity
Rock Porosity The porosity of a sedimentary rock is affected by several factors: Sizes and shapes of the mineral grains. Compactness of their arrangement. Weight of overlying rock or soil. Extent to which the pores become filled with the cement that holds the particles together. The porosity of intrusive and metamorphic rocks generally is low. Mathematically, Porosity (η) is the ratio of openings (voids) to the total volume of a soil. η = (Vt-Vs)/Vt = Vv/Vt Vt= total volume of the soil or rock Vs= volume of solids in the sample Vv= volume of openings (voids) Expressed as percentage. Permeability Capacity of a rock or soil to transmit fluid through pore space and fractures. Rocks/Soil that allow water to flow easily are referred to as permeable. While those that do not allow water to flow easily are referred to as impermeable. In hydrology, permeability describes the capacity of soil to transmit water. The permeability of water-bearing material generally is expressed as a coefficient of permeability. The coefficient of permeability is defined as the number of gallons of water a day at a temperature of 60°F that will be conducted through each mile of the water-bearing bed under investigation, measured at right angles to the direction of flow, for each foot of thickness of the bed and for each foot per mile of hydraulic gradient (Meinzer's coefficient, or meinzer). Coefficients of permeability have a wide range in value. Clay and silt, which are fine grained, may have high porosity, but very low permeability; a coarse-grained sand may have a lower porosity, but a high permeability, owing to the greater ability of the coarse-grained material to transmit water. Relation to hydraulic conductivity: The proportionality constant specifically for the flow of water through a porous media is called the hydraulic conductivity; permeability is a portion of this, and is a property of the porous media only, not the fluid. Intrinsic and absolute permeability states that the permeability value in question is an intensive property (not a spatial average of a heterogeneous block of material), that it is a function of the material structure only (and not of the fluid), and explicitly distinguishes the value from that of relative permeability.
Additional Information Because of tensional forces, some water clings to the walls of the interstices and reduces the size of the interconnected openings that transmit ground water. (openings represent the effective porosity) Specific yield is the ratio of the volume of water that drains from the material. It indicates the volume of water that drains from a porous substance, but does not tell how quickly the water drains out. Hydraulic conductivity (generally called permeability) refers to the relative ease with which a porous substance can transmit a liquid. There is no direct relationship between porosity or specific yield and permeability. This is because the rate at which water passes through a substance is very sensitive to the arrangement of individual particles. Group 3 Specific Yield and Specific Retention Porosity: maximum amount of water that a soil/rock can contain when saturated. SPECIFIC YIELD: Portion of the groundwater draining under influence of gravity. Portion of the GW: retained as a film on soil/rock surfaces and in very small openings: SPECIFIC RETENTION. SPECIFIC YIELD AND SPECIFIC RETENTION
η = Sy+ Sr Sy = Vd / Vt Sr = Vr / Vt Where η = porosity Sy= specific yield Sr = specific retention Vd = volume of water that drains from a total volume of Vt., Vr = volume of water retained in a total volume of Vt Vt = total volume of a soil or rock sample Darcy’s Law Study of percolation of water experimentally Statement: “For laminar flow condition in a saturated soil, the rate of flow is proportional to the hydraulic gradient”. Q=kiA or v = Q/A = k i where Q = rate of flow i = hydraulic gradient k = coefficient of permeability A = total x-sectional areaof soil mass perpendicular to the direction of flow v = velocity
Coefficient of Permeability (K) It is the velocity of flow which will occur through the total x-sectional area of the soil (or aquifer) under a unit hydraulic gradient. K varies in space and time (so does discharge) PERMEABILITY VS. HYDRAULIC CONDUCTIVITY Permeability
The property or capacity of a porous rock, sediment, or soil to transmit fluid. Technically, permeability is an intrinsic property of the porous medium through which fluid moves and is independent of the properties of the fluid.
Hydraulic Conductivity
The volume of water that will move in unit time under a unit hydraulic gradient through a unit area of a porous medium, which is at right angles to the direction of flow. Unlike permeability, hydraulic conductivity is a function of not only the porous medium through which the fluid moves, but of the fluid itself. Commonly, in the fields of ground water hydrology and hydrogeology, the two terms are used interchangeably.
Applicability of Darcy’s law Darcy law is valid only for laminar flow which can exist in small pore dimensions of fine grained soils. Applicability depends on: Particle size Velocity of flow and Hydraulic gradient According to Trencher et al, flow is laminar through sands, if R e ≤ 1 where ρ = mass density d = diameter or particle size v = velocity μ = dynamic viscosity Natural groundwater flow is within validation limits. Flow through rock aquifers, unconsolidated aquifers with steep hydraulic gradients, soils containing larger diameter solution openings is beyond validation limits, hence darcy‟s law is not applicable for such formations. Three important pairs of terms in flow systems Homogeneity/heterogeneity Isotropy/anisotropy Steady state/transient Homogeneity/heterogeneity Homogeneous - K does not vary in a given hydro-geologic unit Heterogeneous - K depends on position - Different Types of Heterogeneity Layered, e.g., sedimentary rocks Discontinuous,e.g.,fractures/faults, unconformities Trending, e.g., deltas, alluvial fans
Heterogeneity A - Aquifer wedge (trending) B - Sedimentary layers of different K (layered) C - Sandy to shaly sediments (trending) Isotropy/anisotropy A – Isotropic K same in all directions B – Anisotropic K depends on direction Steady-state vs. Transient Flow Refers to magnitude and direction of flow - Steady-state means no change in flow with time - Transient means flow changing with time Transmissivity (or Coefficient of Transmissibility) Rate of flow of water through a vertical strip of aquifer of unit width and extending the full saturation height under unit hydraulic gradient at a temperature o of 60 F. How much and easily water will be transmitted horizontally through an aquifer Full saturated thickness of aquifer = dh/dl = 1 T=bK 2 T (m /s), b (m), K (m/s) 2 T > 0.015 m /s for good aquifers Transmissivity is developed for 1 dimension - more complicated for 2 and 3 dimensions Storativity (or Storage Coefficient) Volume of water that a confined aquifer will absorb or expel from storage, per unit surface area, per unit change in head Typical range of values for storativity are 0.00005 < S < 0.005 Storativity can calculate amount of water lost from storage Vw = S A Δh It is valuable for estimating water loss during drought Storativity of an unconfined aquifer corresponds to its specific yield when water level is lowered by one meter Group 4 Geologic Formations Aquifer
A saturated, permeable geologic unit that can transmit significant quantities of water under ordinary hydraulic gradients. Contains and transmits
Aquitard
A geologic unit that may be permeable enough to transmit groundwater in quantities significant to regional groundwater flow, but not permeable enough to allow producing groundwater wells. Contains and transmits insignificant (e.g. sandy clay, leaky confining layer)
Aquiclude
A saturated geologic unit that is incapable of transmitting significant quantities of water under
ordinary hydraulic gradients.Contains but doesn‟t transmit (e.g. confining layer, clay) Aquifuge
An absolutely impermeable unit that will not transmit groundwater.Neither contains nor transmits (e.g. Massive compact rock without any fracture)
What is an Aquifer? Body of saturated soil/rock through which water can easily move. Aquifers must be both permeable and porous. Include rock types such as sandstone, conglomerate, fractured limestone and unconsolidated sand and gravel. Fractured volcanic rocks also make good aquifers. In order for a well to be productive, it must be drilled into an aquifer. Rocks such as granite and schist are generally poor aquifers because they have a very low porosity. However, if these rocks are highly fractured, they make good aquifers . Amount of water in storage in the aquifer can vary from season to season and year to year. Depending on the permeability, ground water velocity may vary up to 50 feet per year or 50 inches per century. Every aquifer has a recharge zone or zones and a discharge zone or zones. Recharge of an aquifer is due to rain, snowmelt, river, reservoir leakage or irrigation. Discharge from an aquifer occurs through springs near the stream and in wetlands at low altitude, and also from wells and high-altitude springs. Wet Land: a large area covered with very wet soft land, and is usually near a lake, river or sea, that tends to flood and is always wet. Types of Aquifers Unconfined Aquifer – water is in contact with atmospheric pressure – drill and well hit the water table. Confined- Water is under pressure due to the weight of the up gradient water and the confinement of the water between “impermeable” layers. Water flows to surface under artesian pressure in an Artesian Well. Perched- It is an aquifer on small scale formed when a lens or localized patch of impervious stratum occurs inside an unconfined aquifer. An UNCONFINED aquifer has access to the surface and water can percolate freely from rainfall into the aquifer. (Also called Water Table Aquifer ) A CONFINED aquifer is bound by low permeability beds above and below and can only get water from surface exposures. (Also called Artesian Aquifer, because a number of such aquifers were found in Artois a former province of north France )
Group 5 Tube Wells Discharge from a tube well can be several times more than that from an open well. The reasons are: 1. It can be sunk to several hundred feet, and hence, tap water from several aquifers. 2. Due to Graded filter, velocity towards tube well >> critical velocity of flow 3. Due to very large x-sectional area, because of radial flow from several aquifers. Main Parts of a Tube well 1) Housing Pipe 2) Blind Pipes 3) Screens or Strainers 4) Bail Pipe or Sand Trap 5) Gravel Pack, and 6) Pumping Equipment Brief Description of Some Parts of Tube well As the name suggests tube well consists of a pipe (tube) inserted in the ground. The portion of the pipe passing through aquifer is provided with slots (openings). The pipe with slots is called a screen or strainer . The lengths of the pipe, which pass through impervious layers and do not have slots, is called blind pipes. These pipes are surrounded by graded filter , which prevents entry of sand particles with water into the pipe. sand trap is a short length of blind pipe, provided at the bottom of the tube well to collect very fine sand particles, which have entered the tube well pipe through strainer. The well is generally plugged at the bottom by cement concrete. 1) Housing Pipe usually a mild steel pipe. lengths varies with the type of pump used. for centrifugal pump suction pipe with a length of 30 ft. and diameter of 6-8 in. is suitable for pumping 2 cfs. In case of turbine pumps the length of housing pipes range between 100 ft. to 200 ft 2) Blind Pipe A tube well may penetrate several layers of both aquifer and impervious layers. A pipe called blind pipe which does not contain any openings, and is provided through impervious layers 3) Screen or Strainer These are perforated pipes around which the fine wire mesh is wrapped to arrest the sand particles. Serves as the intake section: sucks water from an aquifer. A well designed screen is that which allows sand free water in ample quantity with minimum loss of head to enter into the well. Types of Strainers Cook strainer a hollow brass tube manufactured by rolling.
The slots are cut on the pipe in the form of a truncated hollow pyramid . The slots are narrow outside and wide towards inside. Fine wire mesh is wrapped around the strainer pipe to cover the slots. The size of the slots varies from 0.15 to 0.40 mm. Tej strainer a commonly used strainer. manufactured from brass sheet by bending it to the shape of a circular pipe. consists of a welded joint . The tube is wrapped with fine wire mesh. Brownlie strainer manufactured from steel pipe having star shaped projections. The perforations are made on the circular body. The strainer pipe is first encased within a wire net. Then fine wire mesh is wrapped around the net. The net and the mesh are generally made of copper . Ashford strainer made of perforated steel tube. a thick copper wire is wound round the pipe on which fine wire mesh is wrapped. thus a gap is maintained between the tube and the mesh. Properly Designed Well Strainer should be continuous and Openings/slots uninterrupted. Slots to provide maximum percentage of open area consistent with adequate strength of the screen. Slots should widen inwardly. Single material of construction to avoid galvanic corrosion. 4) Bail Pipe or Sand Trap small length of plain pipe used at the bottom end of the screen. allows sand which has entered the screen in spite of all the precaution to prevent its entry. Sand is settled down in the pipe without interfering with the performance of the screen. Sand trap is usually 4 to 6 ft. in length. usually kept a little above the bottom of the bore because it would give way under the weight of the whole pipe length. In fact, the tube well pipe does not rest on the ground at the bottom but is held in position by skin friction of the bore walls. 5) Gravel Pack In sandy and unconsolidated formations of aquifer, fine particles of soil seep into the well pipe during pumping. To prevent it, a graded filter called gravel pack, is provided between the screen and the bore hole walls. 6) Pumping Equipment a) Constant Displacement Pumps Piston pumps
b)
Rotary pumps Constant displacement pumps deliver same volume of water against any head within which they can operate. The use of constant displacement pumps is confined to places where discharge is low . In tube wells these pumps are rarely used. Variable Displacement Pumps Centrifugal pumps Volute Type Diffusion Type Mixed Flow Type Bore Hole Type Pump Turbine Pumps Submersible Pumps
i) Centrifugal pum ps
most important class of pumps. very extensively used as compared to other classes of pumps. maximum suction head under which the pump can practically work effectively is about 20 ft. to 25 ft . The pump is driven by power from an external source, which rotate its vanes. This gives a centrifugal head to the water in the pump chamber. The water leaves the vanes at the outer circumference with a high velocity and pressure. The high pressure of the water is utilized in overcoming the delivery head of the pump.
ii) Bo re Hole Type Pum p T u r b i n e Pu m p s
Installed where ground water is very deep installation of a conventional centrifugal pump is difficult and expensive very compact design The top 50 ft. to 100 ft. of the bore hole and casing pipe is generally kept larger than the remaining bore, so as to accommodate the pump in the casing itself. size of the housing pipe, impellers and bowl assembly is usually small . hence, discharge as well as the lift is usually small . The pump is driven by a vertical shaft type electric motor, coupled directly and placed to the top of the line shaft at ground level. Such pumps are generally called line shaft pumps.
Submersible Pumps
problems posed to a line shaft pump: transmission of power by a long shaft. The verticality of the pipe deep pumps to overcome these difficulties, submersible pumps is developed in which the motor is coupled directly to the impellers and the whole set is submerged in water. The pump is a vertical centrifugal pump with radial or mixed flow impellers. All metallic bearings are water lubricated and protected against ingress of sand.
advantages both the pump and the motor needs no maintenance because all the bearings are water lubricated.
Types of Tube well Depending upon type of intake section and type of flow from aquifer to the well, tube wells can be classified into: Strainer type tube well Cavity type tube well Slotted type tube well • • •
1.
Strainer well Most common and widely used Commonly „ tube well’ refers to “ strainer well” Slots on main pipe are bigger in size than mesh openings. Area of slots = area of mesh openings, to maintain velocity of flow . Little space is provided between perforated pipe and wire mesh to maintain area of flow . General mesh size = D60 to D70 of surrounding soil. It can draw water from an unconfined or/and confined aquifer(s). Flow is radial 2. Cavity well Water is drawn from bottom of the well where a cavity is formed. Essential conditions: in confined aquifer of good specific yield. strong impervious material above aquifer, e.g. clay. Working: At first pumping, initially sand particles come out and form a cavity. On further pumping cavity is enlarged, and critical condition of flow (vc) is achieved. When this equilibrium is formed, clean water stars coming out. Velocity < critical velocity Flow is spherical. 3. Slotted well It is a well which consists of slots on the tube penetrating to a confined aquifer. Selected when: 1) suitable aquifer is not available up to 100 m depth, i.e. strainer well cannot be used. 2) strong roof is not available, so a cavity well cannot be opted. Size of slots = 25 mm x 3 mm @ 10-12 mm spacing To stop silt entry, well shrouding is done with gravel and coarse sand. Tube well Construction Tube well construction involves three distinct operations, including: (a) Drilling Cable Tool Percussion Method Hydraulic Rotary Straight Rotary Reverse Rotary
(b) Installation of tube well components Screen/Strainer, Blind Pipe, and Sand Trap/Bail Plug Housing pipe Gravel Pack/Well Shrouding (c) Development of tube well Over pumping and exerting sudden suction jerk Mechanical surging Blowing compressed air in the hole Back washing a) Drilling merits and demerits of each method depend upon the formation in which drilling is to be done. several types of drilling rigs to drill in different formations. Cable Tool Percussion Method In this method the drilling in the well hole is carried out by hammering action of the drilling bit, which is attached to the lower end of a string cable tools. The drilling bit attached to a string of cable tool is alternately lifted and dropped in the, hole to break the formation. Rig is truck-mounted and is equipped with supportive parts/accessories. Straight Rotary This method is used for drilling large diameter holes in soft and unconsolidated material . Equipment used includes derik, a hoist, a power operated revolving table that rotates the bit, a pump for forming fluid through the drill bit and a power unit. Reverse Rotary reverse rotary system is opposite to the straight rotary system, i.e. instead of pumping of water in the hole, it is sucked from the hole. same equipment as used in the straight rotary system. b) Installation of tube well components (i) Installation of Screen, Blind Pipe, and B ail Plug On the basis of the well logs of the strata prepared during drilling, bail pipe, well screens and blind pipes of appropriate lengths are welded and are hung inside the hole from the top. To keep the component of the tube well exactly in the centre, centralizers are provided. Centralizers are iron strips welded to the blind pipe to keep the parts exactly in the centre of the bored hole. (ii) H o u s i n g P i p e On top of the bail pipe, blind pipe and well screen the housing pipe of the required diameter and length is fixed. (iii) Well Shrouding / Gravel Pack Next step is to provide gravel pack around the well screen. A suitable grading of the filter is designed keeping in view the size of sand particles and the size of screen slots.
The filter material is poured in small quantity at one time, in the space around screen and the casing pipe. In case of tube well for irrigation purposes, gravel pack can be provided up to the ground surface. After providing the gravel pack the casing pipe is pulled up. c) Development of Tube Well i . O v er p u m p i n g a n d e x e r t i n g s u d d e n s u c t i o n j e r k s
In this method pumping of water is done at higher variable rates. By over pumping fine material surrounding, the welI is agitated and carried out of the well. ii. Mechanical surgin g
In this method a surge plunger is moved up and down briskly in the well pipe and the screen. In this way, fines which have entered the well are agitated, and then they are pumped out. iii. Blowing Compressed Air
In this method compressed air is continuously injected, which develops air lift pump system in the well and carries sand particles with water and are pumped out. After injection of the compressed air is completed, mechanical surging is also done to fully develop the well. iv. Back Washing
In this method water is forced in the well in the reverse direction by means of compressed air pressure. All the sand and clay particles, which have got stuck around the strainer, are agitated and are removed. Design of a Tube well for Irrigation It includes: D e s ig n o f P u m p H o u s i n g c a s i n g
i. Diameter ii. Length Design of Screen
i. Diameter of Screen ii. Length of Screen Types of Pump
i. Pump ii. Motor Spacing of Tube wells a.
D e s ig n o f P u m p Ho u s i n g c as i n g
i. Diameter Diameter of the pump housing casing should be such that vertical shaft of turbine pump and suction pipe of a centrifugal pump can be easily accommodated in it. Also, in case of small capacity tube well it can serve as the suction pipe of centrifugal pump. The minimum inside diameter of the housing should, therefore be at least 2 inches larger than the outer diameter of the proposed bowl assembly or suction pipe. ii. Length For turbine pump, the length depends on ultimate Pumping Water Level (PWL) and pump setting requirements
So, it is necessary to estimate drawdown in the well. The fluctuation of static water level (SWL) during the year as well as during prolonged periods of time may also be studied to determine the ultimate static water level . The minimum length of housing pipe can be computed by adding ultimate SWL to the expected draw down. Let: SWL (Ultimate) A, ft. Draw down= B, ft Then the minimum length of housing pipe = (A+B) ft. To allow for errors in the estimate of draw down and also for increase in draw down with passage of time due to well deterioration, at least 10 ft. extra length is provided to the housing pipe. Therefore, the final length of housing pipe = (A + B) + 10. A good thumb rule to determine the length of pump housing casing is the lowest expected PWL plus 10ft. or so.
recommended to use centrifugal pumps for smaller capacity tube wells. ii. M o t o r It has been found that an electric driven tube well is much cheaper than the diesel driven tube well so it is recommended that wherever electric power is available, tube wells shall be operated electrically. Comparison Irrigation
1. 2. 3.
Design of Screen
i. Diameter of Screen The screen diameter may be equal to or less than the diameter of pump housing casing in case of vertical turbine pumps. But in case of centrifugal pump, it is generally equal to the housing pipe. ii. Length of Screen Let Q = discharge capacity of tube well in (cfs) q = screen transmitting capacity per ft. length. D = diameter of the screen (ft.) P = percentage open area per ft. length of screen V = entrance velocity (ft./sec) = 0.075 ~ 0.1 ft./sec Then Length of screen, L = Q / q Surface area per ft. length = π D Open area per ft. length = P x π D q = V x Open area cfs q =VxPxπD cfs q = π V P D cfs L = Q / (V x P x π D) c. Types of Pump i. Pum p
If the water table is not lowered beyond 15.0 ft., a centrifugal pump is quite suitable. Because horizontal centrifugal pump works on suction, so it should be placed in a pit close to the sub-surface water level. In case of vertical turbine pumps there is no limitation of suction because they are normally installed in such a manner that in the conditions of lowest expected pumping levels, the first stage impeller is submerged in water. Mounting level of a turbine pump is generally 3.0 ft. above the natural surface level. Since, horizontal centrifugal pump is much cheaper as compared to vertical turbine pump, so it is always
Tube
well
Irrigation
with
Canal
ADVANTAGES OF TUBE WELL IRRIGATION OVER CANAL IRRIGATION
4. b.
of
5. 6.
7.
8. 9. 10.
The well is under the direct control of the owner. Hence wells may be sunk and equipped as required. Isolated area can be irrigated by a well. Wells may be sited to command any desired land. The supply from a well can be maintained fairly constant. Water can be turned off at any moment, taking advantage of rainfall. Since a well is generally centrally located, the loss in transit is very much reduced. The duty of water in well irrigation is generally higher. Volumetric assessment is possible. Well irrigation is helpful in lowering the sub-soil water level and in draining off irrigated land which might become water logged. In canal irrigation, the chances of water logging is much more than in the well irrigation. Unless drought continues. for several years, well irrigation does not fail in drought seasons, while a canal may fail. With the help of well irrigation, more than one crop in a year can be grown. The well water, which is warmer in cold weather, and cooler in hot weather, is more agreeable to crops. The cost of construction of a well is low and irrigation in a locality can be introduced in stages.
DISADVANTAGES OF TUBE WELL IRRIGATION OVER CANAL IRRIGATION
(1) Since water has to be lifted from the well, the working expenses are very high in comparison to the canal irrigation. (2) Due to mechanical defects in the pump or due to interruption in electric supply, water may not be made available to the crop in right time. (3) The well water is clear and free from silt. Clear water does not have manuring value which silt in canal water provides. (4) The tube well strainer is subject to progressive deterioration to mechanical and chemical action. Thus, replacements are necessary after frequent interval of time. (5) The maintenance of mechanical and electrical machinery are also great care and more funds.
