ENCN342 - Hydraulics and Applied Hydrology

Where is GW?

Open spaces underground. Pores, fractures and open caverns. 

Vadose zone?

Also known as the unsaturated zone. Open space underground is filled by air and water.

Saturation?

The fraction of open space filled by water.

Porosity?

All the open space.

Phreatic zone?

All open space is filled by water and no air. Fully saturated. 

Where does the water table begin?

Start of phreatic (saturated zone).

Recharge?

Water entering the GW system. Through soil column to water table.

Hydraulic conductivity?

The ease in which drops of water move between pores.

Aquifer?

High porosity and conductivity. Good source of GW.

Gravels and sands.

Aquitard?

Low porosity and conductivity. Not a source of GW but provide protection for aquifers.

Clays, silts, and peats.

Confined aquifer?

Top of aquifer is the bottom of an aquitard.

Unconfined aquifer?

Top of aquifer is the water table.

Water table is higher than the bottom of the stream?

Water exits into the stream. Creates base flow. Called a gaining stream. 

Water table is lower than the bottom of the stream?

Water leaves stream and enters GW system. Called a losing stream.

Perched aquifer?

Infiltrated water is trapped by a aquitard and is unable to flow. Vulnerable to overuse.

Seepage face?

Aquifer-aquitard contact at a cliff face.

Spring?

GW naturally wells up out of the ground. 

Why do springs occur?

When the natural water level (level of water table) sits above the ground surface.

High water levels can be maintained by rainfalls elsewhere that transmit a pressure force through the aquifer, driving the water upwards.

Artesian?

Water wants to flow out of the ground, possibly prevented by an aquitard.

Well?

A long hole in the ground. Drilled or dug. 

What is the purpose of a well casing?

To prevent water from unwanted aquifers entering the well.

What is the purpose of a screen in a well?

To filter out large particles.

Groundwater head?

The level at which water would rise in a straw that is open to just that aquifer. Measured relative to a datum.

Hydrostatic pressure?

Due to the weight of water.

Piezometric surface?

Contours of constant head.

When is the piezometric surface the same as the water table?

In an unconfined aquifer.

Where is the piezometric surface in a confined aquifer?

The surface is hypothetical. It usually sits above the aquifer.

If it is above the ground surface the aquifer is artesian.

Where does water flow?

Down the piezometric surface.

What does Darcy’s law describe?

Volume flow rate of water.

What are the assumptions used when using Darcy’s law?

• Constant cross sectional area

• Linear hydraulic gradient

Drawdown?

Change in the piezometric surface or head compared to pre pumped state. 

A cone of depression will form around a well during drawdown. 

Storativity?

Volume of water that must be extracted to induce one unit of drawdown evenly across one unit area of aquifer.

Storativity is equal to?

Drainable porosity or specific yield.

What is steady condition groundwater capture?

Total recharge is balanced by total discharge. No change in total water volume.

What happens to ground water capture during drawdown?

Initially dV/dt = -Q. 

Over time a new water table will form. A new equilibrium will be reached eventually.

When can we use the logarithmic Theis solution?

When (r²S)/(4Tt) < 0.05

What is c?

Hydraulic resistance of a confining layer.

Why must we do data corrections on pumping test data?

• Seasonal variations in recharge

• Atmospheric pressure

  • Long term trends

Well yield?

The volume rate that a well can be pumped at over a specified time period that indues a drawdown no larger than h_max. 

h_max represents a limit of the system, if exceed usability or sustainable goals may be compromised. 

How can we model two well pumping near each other?

Superposition. A sum of Theis solutions.

How can we model pumping a well with flow barriers and recharge sources?

Use an image well. This steepens the drawdown curve.

For a flow barrier that is a constant head (stream or lake), use an image well with a negative pumping rate (injecting water). This flattens the drawdown curve.

How do we model a step rate pump test?

Using the super positions of multiple wells. Each well has its own pumping rate and beings at different times. The pumping rate is equal to the change in pumping from the previous step. 

Driven well?

A well constructed by driving a pointed screen end into unconsolidated ground and then attaching lengths of pipe.

Drilled well?

Made by using a rotary drill to excavate a hole. A casing is then placed in the hole to prevent collapse.

What is the primary constraint of screen sizing? And why?

Entrance velocity. 

• To minimise head lose

• To prevent clogging

  • To prevent sand pumping

When should an artificial gravel pack be used?

When d₁₀ < 0.25 mm.

What is the recommended screen length?

The entire thickness of the aquifer.