Hydrogeology in engineering geology

 Hydrogeology

• It is the branch of geology that deals with the study of occurrence, movement and quality of water beneath the ground surface.

• Groundwater/ subsurface water: The water that exists in the pore spaces and fractures in rocks and sediments beneath the earth surface.

• Sources of groundwater maybe precipitation, water bodies like stream, lakes etc. or irrigation.

         

Fig: Hydrological cycle

River channel Morphology

River: A natural path defined by the flow of water is known as river. Rivers mostly originates from high hills or mountains. Small rivers in mountains or hills are called streams, rivulets or tributaries. When the river flows, the major geological action or the function carried by the river are erosion, transportation and deposition.

Geological action of river

• Erosion: The process of breaking down or wearing of rock due to geomorphic agents like moving water, glaciers melt and blowing wind is known as erosion. Erosion involves hydraulic action, abrasion and attrition.

• Transportation: The amount of sediment transported by stream is called load. Load may be of 3 types: 

1. Dissolved load: Stream receiving underground water generally have dissolved load than water from surface run off, load in dissolved state is called dissolved load. Example: Calcium , sulphate, chloride, Mg, K.

2. Suspended load: Load in suspension or in the form of  floating bodies. Example: Fine particles of silt and clay.

3. Bed load: Load apart from dissolved and suspension which are coarser in size. The bed load move slower than velocity of stream water, they move discontinuously by rolling or sliding. Example: Sand, gravel, pebble.

• Deposition: The loose materials transported downstream are deposited when the velocity of flowing water is reduced.

Types or Pattern of River Channels

 

Straight River	Meandering River	Braided River
The river that follows straight path. Originated at high or steep topography. Gradient is high. Velocity is high ( Greatest in middle away from banks and bed of the channel . High erosion ( Mainly scouring than side cutting) and transportation capacity but low depositional capacity. Mostly dominated at high hills or mountains.	Latin word for Wandering The river that follows zigzag path. Originated at moderate topography Moderate gradient and low velocity Dominated by flood plain Erosion and deposition occurs simultaneously Channel shifted Creates flood plain and oxbow lakes	Bifurcated into several branches and may convert into single Originates at very low relief Low gradient and low velocity Depositional rate is very high Channel is shifted Dominated with channel bars.

Features developed by river

• Point-bar and Ox-bow lake( Fig 5): It is a feature developed in meandering river. In meandering river, the meanders grow by eroding it’s outer bank and depositing sediments at the inner bank. Such moon shaped sediments deposited in the inner bank is called Point-bar. During this process, the sharpness of river bends increases and the neck of meander becomes narrow and narrow. Finally, a stage comes when due to intense erosion, the two banks meet each other. Then during high floods, the river may follow a straight path leaving behind a curved stagnant water body called an oxbow lake. 

Fig: Point-bar and Ox-bow lake

• Alluvial fan(Fig 6): An alluvial fan is a fan or cone-shaped deposit of sediment crossed and built up by the streams. The material constituting a fan includes coarse boulders and pebbles at its head to finer material down it’s slope.

• Flood plain( fig 7): These are areas of low and relatively flat land bordering the channel on one or both the sides, at bank level. These areas are readily submerged under water during flooding, when the river water overtops the banks of the channel and rises above the channel at low water.

• Deltas(fig 8): When a river enters a lake or sea it’s velocity is checked rapidly and the process of deposition is accelerated. The coarser and heavier material is laid down first and the finer and lighter material is carried further out. Thus, the load brought by the river gets deposited at its mouth, which give rise to delta. These deposits are triangular in outline and resemble the Greek letter Δ (delta).

Fig 7: Flood plain

Fig 8: Delta

  

                                Occurrence of subsurface water

• Based on mode of occurrence, subsurface water is classified into two zones. They are:

(a) Zone of aeration/ Vadose zone: Water unsaturated zone consisting of air and water, water can’t be extracted.

(b) Zone of saturation: Groundwater extraction zone, fully saturated.

Water table separates zone of aeration and zone of saturation.

Fig: Different Zones of subsurface water

 

Factors controlling occurrence and movement of Groundwater

• Climate

• Topography

• Porosity

• Permeability

• Hydraulic Gradient

• Hydraulic Conductivity

• Porosity: Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%.The larger the pore space or the greater their number, the higher the porosity and the larger the water holding capacity. It is denoted by ρ.

     Mathematically, 

      ρ= Volume of voids (v)/ Total volume (V) * 100

                              Porosity values in different sediments/ rock

 

Rock/ Sediment	Porosity ( %)
Sandstone	5-15
Shale	1-10
Limestone/ Dolomite	5-15
Sand/ Gravel	24-55
Clay	34-60
Silt	34-61

 

• Permeability: 

The capacity of a rock to transmit a fluid. It varies with the fluid’s  viscosity, hydrostatic pressure, the size of openings, and particularly the degree to which the openings are interconnected. High permeability rocks are conglomerate, sandstone, basalt,  limestone etc. Low permeability rocks are shale, unfractured granite, quartzite, gneiss etc.

Grain shape, grain packing, and cementation affect permeability. 

• Hydraulic gradient: 

Water flows from high to low fluid potential or head. Hydraulic head is used to determine the hydraulic gradient.

Hydraulic head = the driving force that moves groundwater. The hydraulic head combines fluid pressure and gradient.

Hydraulic gradient for an unconfined aquifer = approximately the slope of the water table.

It is also defined as head difference between the two points divided by its length.

Hydraulic gradient 

(I)= h1-h2/L

• Hydraulic Conductivity

Movement of groundwater depends on rock and sediment properties and the groundwater’s flow potential. Porosity and permeability are important components of hydraulic conductivity. 

HYDRAULIC CONDUCTIVITY = K (or P)

units = length/time (m/day)

Ability of a particular material to allow water to pass through it.

The definition of hydraulic conductivity (denoted "K" or "P" in hydrology formulas) is the rate at which water moves through material. Internal friction and the various paths water takes are factors affecting hydraulic conductivity. Hydraulic conductivity is generally expressed in meters per day.

Porosity and hydraulic conductivity of selected earth minerals (Keller, 2000)

Groundwater Movement - Darcy’s Law

• Q = KIA Henry Darcy, 1856, studied water flowing through porous material. His equation describes groundwater flow.

• Darcy’s experiment: 

• Water is applied under pressure through end A, flows through the pipe, and discharges at end B.

• Water pressure is measured using piezometer tubes.

• Hydraulic head = dh (change in height between A and B)

• Flow length = dL (distance between the two tubes)

• Hydraulic gradient (I) = dh / dL

• The velocity of groundwater is based on hydraulic conductivity (K), as well as the hydraulic head (I).

• The equation to describe the relations between subsurface materials and the movement of water through them is

                            Q = KIA

Q = Discharge = volumetric flow rate, volume of water flowing through an aquifer per unit time (m3/day)

A = Area through which the groundwater is flowing, cross-sectional area of flow (aquifer width x thickness, in m2)

• Rearrange the equation to Q/A = KI, known as the flux (v), which is an apparent velocity

Aquifers

• An aquifer is a formation that allows water to be accessible at a usable rate. Aquifers are permeable layers such as sand, gravel, and fractured rock. 

• Aquifers can be sand, gravel, and fractured rock. To be an aquifer, the stored water must be accessible at a usable rate. 

• Synonyms: Groundwater reservoir,  water bearing formation.

Types of Confining bed

• An impermeable or less permeable layer that stops or retards the movement of groundwater into and out of it is called confining bed. They are of three types:

• Aquiclude- It is an impermeable layer which doesn’t transmit water at all. Although the formation is capable of absorbing water slowly. Clay soils, shales, and non-fractured, weakly porous igneous and metamorphic rocks are examples of aquiclude.

• Aquifuge- It is a geological formation that can neither absorb nor transmit water. Example: granite.

• Aquitard:  It is a geological formation that is permeable enough to transmit the water in significant quantities for large area and long period. Example: sandy clay

Types of Aquifers

• Confined aquifers have non-permeable layers, above and below the aquifer zone, referred to as aquitards or aquicludes. These layers restrict water movement. Clay soils, shales, and non-fractured, weakly porous igneous and metamorphic rocks are examples of aquitards.

•  Sometimes a lens of non-permeable material will be found within more permeable material. Water percolating through the unsaturated zone will be intercepted by this layer and will accumulate on top of the lens. This water is a perched aquifer.

•  An unconfined aquifer has no confining layers that retard vertical water movement.

Geological Factors for Formation of Different Hydrogeological Conditions

• Coarse sediments like sand, gravels, pebbles may store water within their pores.

• Presence of excessively jointed and fractured rocks may signify presence of water bodies.

• Porous rocks like sandstone, limestone, etc.

• Presence of geological structures like fault, fold.

Aquifer System of Nepal ( Terai, Hills and Mountains)

 

Terai	Hills	Mountain
Loose unconsolidated sediments ( Alluvial deposit comprising of coarse sediments such as sand, gravels, pebbles, cobbles and boulders.	Percolation of water in hilly areas is usually associated with highly fractured and jointed rock mass, porous rock deposits like sandstone, limestone, etc.	Aquifer formed due to primary porosity and secondary porosity. Caves, caverns and solution channels in Limestone and dolomites.