4: Groundwater Flashcards
What is an aquifer?
Permeable unit which holds and transmits significant amounts of water
What is an aquitard?
Low permeability unit
What is an aquiclude?
Essentially impermeable unit
What is hydraulic head (saturated)?
The sum of the pressure & elevation head
P/pg + Z
(ignore velocity head as it is negligible)
In which direction does water flow?
From high to low hydraulic head
What is Darcy’s law in terms of:
- total discharge?
- specific discharge?
- confined flow?
- unconfined flow?
Q = -Ks.A.dh/dx q = -Ks.dh/dx Q/m = -Ks.b.dh/dx Q/m = -Ks.h.dh/dx
What is specific discharge?
q = Q/A [m/s]
What is the horizontal head gradient?
dh/dx
What is the vertical head gradient?
dh/dz
What is a potentiometric surface?
h(x): the imaginary hydraulic line that a reservoir of fluid will ‘equalise’ out to
What does steady state mean?
dS/dt = 0
i. e. I(t) = O(t)
also: dh/dt = 0
What is the groundwater equation for confined flow (horizontal)?
S.∂h/∂t = T(∂^2h/∂x^2 + ∂^2h/∂y^2)
What is storativity (equation, units, definition)?
S = bSs [-]
Volume of water released per unit decline in hydraulic head per unit area
What is transmissivity (equation, units, definition)?
T = bKs [m2/s]
Rate at which groundwater flows horizontally through an aquifer
How do you derive the potentiometric surface h(x) from the confined flow groundwater equation (1D steady state with no source/sink)?
S.∂h/∂t = T(∂^2h/∂x^2 + ∂^2h/∂y^2) S.∂h/∂t = 0, ∂^2h/∂y^2 = 0 d^2h/dx^2 = 0 dh/dx = c1 h = c1x + c2
What is the groundwater equation for unconfined flow (horizontal)?
Sy.∂h/∂t = K(∂/∂x(h.∂h/∂x) + ∂/∂y(h.∂h/∂y))
How do you derive the potentiometric surface h2(x) from the unconfined flow groundwater equation (1D steady state with no source/sink)?
Sy.∂h/∂t = K(∂/∂x(h.∂h/∂x) + ∂/∂y(h.∂h/∂y)) Sy.∂h/∂t =0, ∂/∂y(h.∂h/∂y) = 0 d/dx(h.dh/dx) = 0 h.dh/dx = c1 ∫h dh = ∫c1 dx h^2 = c1x + c2
What is specific yield (units & definition)?
Sy [-]
Volume of water released per unit horizontal area of aquifer per unit decrease in water table
What is specific storage (units & definition)?
Ss [m^-1]
Volume of water released per unit volume per unit decline in hydraulic head
