Groundwater Hydraulics - Arjumand Zaidi

Groundwater Hydraulics - Arjumand Zaidi

GROUNDWATER HYDROLOGY HYDROLOGY AND WATER RESOURCES RG744 December 18, 2015 Institute of Space Technology Institute of Space Technology

GROUNDWATER (GW) Groundwater: A component of Hydrologic Cycle Comprises more than 97% of all freshwater on the earth (not considering water trapped in glaciers and icecaps) Globally more than one-half of the worlds population depends on groundwater Occurring in the saturated zone of a soil profile

Revisit: Some Important terms Zone of aeration: below the soil surface pores contain both air and water. Water stored here is called 'soil moisture' or 'vadose water' Zone of saturation: pores of the soil or rock saturated with water through gravity drainage The top of zone of aeration is called the water

table Water stored in zone of aeration is groundwater BASEFLOW Groundwater Contribution to Stream Gaining Stream

Losing Stream A stream that is gaining during low-flow periods can temporarily become a losing stream during flood stage Baseflow recession Q = Qoe-at

Q = flow at some time t after the recession started Qo= flow at the start of the recession a = recession constant for the basin (1/T) t = the time since the recession began Find the recession constant for the basin Groundwater Flow

Groundwater flow: through the rock and soil layers of the earth until it discharges as a spring, or as seepage into a pond, lake, stream, river or ocean Similar to drainage basin for surface water, in groundwater hydrology the concept of groundwater basin is used It is surrounded by groundwater divides

GROUNDWATER (GW) Moves very slowly Typical average horizontal velocity = 100 meters per year Typical average vertical velocity = one meter per year (K. R. Rushton)

Sometimes difficult to extract Problem associated with GW are: contamination and deep pumping may become uneconomical GROUNDWATER HYDROLOGY Groundwater hydrology is important in the field of surface water flood hydrology Soil properties and rate of infiltration affect the

proportions of rainfall as surface runoff and groundwater losses Thus soil properties hydrologic designs affect surface

water WATER MOVEMENT IN SOIL When soil pores filled with water - gravity dominates When field capacity is exceeded water starts flowing down

GW TERMS Aquifer Water bearing porous soil or rock strata that yields significant amount of water to wells Aquiclude Water bearing soil or rock strata that are effectively

impermeable which can absorb water but can not transmit significant amounts such as clay, shales, slates, etc. Aquitard Rocks that are poorly permeable (silt and mudstone) GW TERMS

Aquifuge A geologic formation with no interconnected pores and hence can neither absorb nor transmit water Exemples are basalts, granites, etc. Water Table Undulating plane below the ground surface at which GW water pressure is equal to atmospheric pressure (also

dividing line between saturated and unsaturated zone) CONFINED AND UNCONFINED AQUIFERS Aquifers that contain water and is in direct contact with the atmosphere through porous material are called unconfined aquifers A confined aquifer is separated from atmosphere by an impermeable layer or aquiclude An unconfined aquifer can become a confined aquifer at some

distance from the recharge area Confined aquifers, also called artesian aquifers, contain water under pressure Water pressure (P), or pressure potential, is a function of the height of the water column at a point (hp), the density of water (), and the ), and the force of gravity (g) P = g hp = hp

CONFINED AND UNCONFINED AQUIFERS The potentiometric (or piezometric), surface of an artesian aquifer describes the imaginary level of hydraulic head to which water will rise in wells drilled into the confined aquifer The direction of groundwater flow, or flow lines, can be determined by constructing lines perpendicular to the water table contours from higher to lower elevation contours The potentiometric surface declines because of friction losses

between points When the land surface falls below the potentiometric surface, water will flow from the well without pumping (artesian or flowing well) ARTESIAN WELL SPRINGS

Where GWT intersects the topography or ground surface, springs are formed AQUIFER CHARACTERISTICS The amount of water stored or released from a water bearing strata depends on porosity, the size of pore spaces, and the continuity of pores Therefore, mapping the GW Flow is not Easy!

AQUIFER CHARACTERISTICS Porosity: percentage of rock or soil that is void of material Porosity (n)= 100Vv/Vt Where: Vv= volume of void space in a unit volume of rock/ soil

Vt= total volume of earth material including void space AQUIFER CHARACTERISTICS Effective Porosity: ratio of the void space through which water can flow to the total volume If pores are of sufficient size and interconnected to allow water to move freely, the soil or rock is

permeable Aquifer Characteristics It is not possible to measure GW velocities within an aquifer Observation boreholes (piezometers) are constructed to determine the elevation of the water level in piezometer The GW head in an aquifer is the height to which water will rise in a piezometer

GW head gradients can be used to estimate magnitude and direction of GW velocities The amount of water discharged from an aquifer can be approximated with Darcys law Darcy Law: Flow through a Porous Medium Darcy law states: specific discharge in a porous medium is in the direction of

decreasing head and directly proportional to the hydraulic gradient Darcy performed a series of experiments on water flow through columns of sand He packed sand in iron pipes and systematically measured the parameters that he expected to impact the flow Darcy found that the total discharge Q varies in direct proportion

to X-sectional area of the column, hydraulic head difference at each end of the column, and inversely with length of column Darcy Equation Q A (h1-h2)/L Q = KA (h1-h2)/L K = hydraulic conductivity

Darcy Equation Darcy equation can be rewritten as: Q/A = -K (h2-h1)/L V= Q/A = -K (h2-h1)/(l2-l1) This can be written more generally as: q = -K (dh/dl) Where: q = Q/A is the specific discharge

(dh/dl) = Hydraulic gradient Negative sign indicates that positive specific discharge (indicating direction of flow) correspond with a negative hydraulic gradient The Darcy velocity is an average discharge velocity through the entire x-section of the column, the actual flow is limited to the pore channels only The seepage velocity Vs is equal to the Darcy

velocity divided by porosity Vs = Q/nA Actual seepage velocities are therefore much higher (by a factor of 3) than the Darcy velocities Example Darcys Law Determine the discharge of flow through a well sorted gravel aquifer. The change in head is 1 m

over the distance of 1,000 m and the crosssectional area of the aquifer is 500 m2. find Q? Q = K A dh/dL Q = (0.01 cm/sec ) (500 m2) (0.001 m/m) (0.01 m/cm) Q = 0.00005 m3/sec or 4.32 m3/d AQUIFER CHARACTERISTICS Hydraulic Conductivity: velocity of flow through a porous medium resulting from 1 unit

of energy head (m/d) (ability of a porous media to transmit water) Examples of Hydraulic conductivity From Brooks Material Well sorted gravel Well sorted sands, glacial outwash

Silty sands, fine sands Silt, sandy silts Clay Hydraulic Conductivity (cm/sec) 10-2 - 1 10-3 10-2

10-5 10-3 10-6 - 10-4 10-9 - 10-6 AQUIFER CHARACTERISTICS Transmissivity (m2/unit time): amount of water that can flow horizontally through the entire saturated thickness of the aquifer under the

hydraulic gradient of 1m/m Tr = bkv Where: Tr =transmissivity (m2/unit time) b = saturated thickness (m) kv= hydraulic conductivity of the aquifer (m/unit time) AQUIFER CHARACTERISTICS Specific yield: ratio of

the volume of water that can drain freely from the saturated earth material due to the force of gravity to the total volume of the earth material Specific Yield

Storativity of confined aquifer AQUIFER CHARACTERISTICS Specific retention: ratio of the volume of water a rock can retain against gravity drainage to the total volume of rock Increases with decreasing grain size

Porosity = Specific yield + Specific retention Higher porosity not always ensures higher water yield!!! Clay may have a porosity of 50% with specific retention of 48% Other factors that affect water transmission through soil pores

Interconnectivity of pores size of the pores Specific yield can be determined in both lab and field Field: Water wells are pumped, and the rate at which

the water level falls in nearby wells is measured Groundwater Development Assessing groundwater potential for GW development requires knowledge of the local geology and aquifers

Surface features ordinarily does not indicate any sign related to the location, depth, and extent of water bearing material or strata Geological maps can be used to help identify potentially productive water

bearing strata by examining the direction and degree of dipping strata, locating faults and fracture zones, and determining the stratigraphy of rocks with different water bearing and hydraulic characteristics As a rule, opportunities for GW development increase as one moves from upland watersheds to lower basins and floodplains

Extensive and high yielding aquifers occur in the most major river valleys and alluvial plains Well A vertical hole dug into the ground

Many types of wells Well Point Lower end of pipe Cone of depression Created by pumping water from well that lowers the water table around the well Drawdown

The difference between original water level and the water level after a period of pumping Discharge rate is measured through flow meter attached to the discharge pipe Interference Locating wells too close together causing more lowering of a water table than spacing them far apart

MANAGEMENT OF GROUNDWATER RESOURCES Means controlled use in accord with some plan Continued extraction of GW may create many problems Use of GW without consideration to its effects is unwise Good management is to minimize the adverse effects of GW use with good knowledge of the probable effect Need to know local area GW conditions (including

quality) and basic research on recharge and movement of GW is required GW can be managed using the concept of safe yield MANAGEMENT OF GROUNDWATER RESOURCES Safe Yield: The rate of water that can be extracted from an aquifer during anytime period that do not produce undesirable effects

(excessive lowering of water table, saltwater intrusion, high pumping cost) Water removed from an aquifer in excess of the safe yield is termed overdraft When water is extracted at a rate that exceeds the recharge of the aquifer, the water table is lowered GW MANAGEMENT

(Sustained withdrawal of GW) Water budget analysis to study quantitative aspects of safe yield I O = SS Where I = inputs to GW (including GW recharge by percolation of rainwater and snowmelt, artificial recharge through wells, and seepage from lakes and streams) O = output from GW (including pumping, seepage to lakes

and streams, springs, etc. S = change in storage GW Recharge Natural Artificial Induced infiltration, spreading, recharge wells

Water moves through aquifer under the influence of gravity, therefore the zone of recharge should be higher than areas of discharge Water table or potentiometric maps Can be shown as contour maps with equal elevation

Groundwater contours Surface water and topography features should be taken into account The datum for water level in wells should be the same as the datum for the surface topography Groundwater contours can never be higher than water surface contours

Typically the depth to groundwater will be greater beneath hills than beneath valleys If a lake is present - the lake surface is flat as is the water table beneath it. Hence the groundwater contours should go around it

The only exception is perched lake having surface elevation above the main water table Groundwater contours form a 'V' pointing upstream when they cross a effluent or gaining stream Groundwater contours bend downstream when they cross a influent or losing stream

The potentiometric surface of a confined aquifer is not influenced by the surface topography and surface water features as there is no direct hydraulic connection between them Potentiometric surface contours can even be above the land surface In areas with shallow water table or potentiometric surface the groundwater contours will be spaced well apart

If the gradient is steep, the groundwater contours will be closer Groundwater will flow in the general direction that the water table or potentiometric surface is sloping Gradient of Potentiometer Surface

Manual contouring is practically always utilized in GW studies (sometimes in conjunction with computer-based methods) Complete reliance on software contouring may lead to erroneous results missing interpretation of Geological boundaries Varying porous media Influence of surface water bodies

Principles of GW flow Triangular Line Interpretation Steps: Graphical method Problems with computer-based method WT Contour maps

Manually drawn Inverse distance to power method using super computer program Recorded WT elevation in feet amsl GW software

MODFLOW GMS A GMS is a comprehensive groundwater modeling environment with GIS based graphical preprocessing tools to automate and

streamline the modeling process. GMS seamlessly interfaces with MODFLOW and several other famous groundwater models, and provides advanced graphical features for viewing and calibrating model results.

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