Subsurface Movement of Water (Part - 3) - Civil Engineering (CE) PDF Download

Examples of ground water flow 

Although ground water flow is three – dimensional phenomenon, it is easier to analyse flows in two – dimension.  Also, as far as interaction between surface water body and ground water is concerned, it is similar for lakes, river and any such body.  Here we qualitatively discuss the flow of ground water through a few examples which show the relative interaction between the flow and the geological properties of the porous medium. Here, the two – dimensional plane is assumed to be vertical. 

1. Example of a gaining lake and river.

Figure 11 shows an example of a lake perched on a hill that is receiving water from the adjacent hill masses. It also shows a river down in a valley, which is also receiving water. 

FIGURE 11. Example of a lake and a river, both of which are receiving warer from the adjoining soils.

2. Example of a partially losing lake, a disconnected losing lake, and a gaining river.  Figure 12 illustrates this example modifies the situation of example 1 slightly. 

FIGURE 12. An example of two lakes, one of which is gaining water, as well as loosing; one river that is continuously gaining; and another lake perched on a hill, disconnected from the water table, and thus loosing water by infiltration

3. Example of flow through a heterogeneous media, case I.

This case (Figure 13) illustrates the possible flow through a sub-soil material of low hydraulic conductivity sandwiched between materials of relatively higher hydraulic conductivities.

FIGURE 13. Example of sub-soil flowthrough heterogeneous media - Case I

4. Example of flow through a heterogeneous media, case II.

This case (Figure 14) is just opposite to that shown in example 3. Here, the flow is through a sub-soil material of high hydraulic conductivity sandwiched between materials of relatively low hydraulic conductivities. 

FIGURE 14. Example of sub-soil flowthrouhg heterogenous media - Case II

Water table contours and regional flow

For a region, like a watershed, if we plot (in a horizontal plane) contours of equal hydraulic head of the ground water, then we can analyse the movement of ground water in a regional scale.  Figure 15 illustrates the concept, assuming homogeneous porous media in the region for varying degrees of hydraulic conductivity (which is but natural for a real setting).

FIGURE 15. Movement of ground water in a regional scale

Aquifer properties and ground water flow 

Porosity 

Ground water is stored only within the pore spaces of soils or in the joints and fractures of rock which act as a aquifers.  The porosity of an earth material is the percentage of the rock or soil that is void of material.  It is defined mathematically by the equation 

   (2) 

Where n is the porosity, expressed as percentage; v_v is the volume of void space in a unit volume of earth material; and v is the unit volume of earth material, including both voids and solid.

Specific Yield

While porosity is a measure of the water bearing capacity of the formation, all this water cannot be drained by gravity or by pumping from wells, as a portion of the water is held in the void spaces by molecular and surface tension forces.  If gravity exerts a stress on a film of water surrounding a mineral grain (forming the soil), some of the film will pull away and drip downward.  The remaining film will be thinner, with a greater surface tension so that, eventually, the stress of gravity will be exactly balanced by the surface tension (Hygroscopic water is the moisture clinging to the soil particles because of surface tension).  Considering the above phenomena, the Specific Yield (Sy) is the ratio of the volume of water that drains from a saturated soil or rock owing to the attraction of gravity to the total volume of the aquifer.

If two samples are equivalent with regard to porosity, but the average grain size of one is much smaller than the other, the surface area of the finer sample will be larger.  As a result, more water can be held as hygroscopic moisture by the finer grains.

The volume of water retained by molecular and surface tension forces, against the force of gravity, expressed as a percentage of the volume of the saturated sample of the aquifer, is called Specific Retention S_r, and corresponds to what is called the Field Capacity. 

Hence, the following relation holds good: 

n = S_y + S_{r     (3)} 

Specific storage (s_s) Specific storage (ss), also sometimes called the Elastic Storage Coefficient, is the amount of water per unit volume of a saturated formation that is stored or expelled from storage owing to compressibility of the mineral skeleton and the pore water per unit change in potentiometric head.  Specific Storage is given by the expression 

S_s = γ (α + nβ )      (4) 

where γ is the unit weight of water, α is the compressibility of the aquifer skeleton; n is the porosity; β is the compressibility of water.

Specific storage has the dimensions of length^-1

The storativity (S) of a confined aquifer is the product of the specific storage (S^s) and the aquifer thickness (b). 

S = bS_s        (5) 

All of the water released is accounted for by the compressibility of the mineral skeleton and pore water.  The water comes from the entire thickness of the aquifer.

In an unconfined aquifer, the level of saturation rises or falls with changes in the amount of water in storage.  As water level falls, water drains out from the pore spaces.  This storage or release due to the specific yield (S_y) of the aquifer.  For an unconfined aquifer, therefore, the storativity is found by the formula.

S = S_y + hS_s    (6) 

Where h is the thickness of the saturated zone.

Since the value of S_y is several orders of magnitude greater than hS_s for an unconfined aquifer, the storativity is usually taken to be equal to the specific yield. 

Aquifers and confining layers 

 It is natural to find the natural geologic formation of a region with varying degrees of hydraulic conductivities.  The permeable materials have resulted usually due to weathering, fracturing and solution effects from the parent bed rock.  Hence, the physical size of the soil grains or the pre sizes of fractured rock affect the movement of ground water flow to a great degree.  Based on these, certain terms that have been used frequently in studying hydrogeology, are discussed here.

• Aquifer: This is a geologic unit that can store and transmit water at rates fast enough to supply reasonable amount to wells.
• Confining layers: This is a geologic unit having very little hydraulic conductivity.  Confining layers are further subdivided as follows:
  • Aquifuge: an absolutely impermeable layer that will not transmit any water.
  • Aquitard:  A layer of low permeability that can store ground water and also transmit slowly from one aquifer to another.  Also termed as “leaky aquifer’.
  • Aquiclude:  A unit of low permeability, but is located so that it forms an upper or lower boundary to a ground water flow system.

Aquifers which occur below land surface extending up to a depth are known as unconfined.  Some aquifers are located much below the land surface, overlain by a confining layer. Such aquifers are called confined or artesian aquifers. In these aquifers, the water is under pressure and there is no free water surface like the water table of unconfined aquifer.