Steady state seepage is taking place through a soil element at $Q$, $2$ m below the ground surface immediately downstream of the toe of an earthen dam as shown in the sketch. The water level in a piezometer installed at $P$, $500$ mm above $Q$, is at the ground surface. The water level in a piezometer installed at $r$, $500$ mm below $Q$, is $100$ mm above the ground surface. The bulk saturated unit weight of the soil $18 \: kN/m^3$ and the unit weight of water is $9.81 \: kN/m^3$. The vertical effective stress (in kPa) at $Q$ is

1. $14.42$
2. $15.89$
3. $16.38$
4. $18.34$