Pressure rise for gradual and sudden closure

First compare the closure time with the critical time $2L/c$ to decide whether the closure is gradual or sudden.

Critical time

$$t_c = \frac{2L}{c} = \frac{2 \times 2500}{1460} = 3.425\ \text{s}$$

(i) Closure in 25 s

$25\ \text{s} > 3.425\ \text{s}$, so it is gradual closure. The water column decelerates uniformly:

$$\begin{aligned} p &= \frac{\rho L V}{T} = \frac{1000 \times 2500 \times 1.5}{25} = 150\,000\ \text{N/m}^2 = 0.15\ \text{MN/m}^2 \\ H &= \frac{p}{\rho g} = \frac{150000}{9810} = 15.29\ \text{m} \end{aligned}$$

(ii) Closure in 2 s

$2\ \text{s} < 3.425\ \text{s}$, so it is sudden closure. For a rigid pipe:

$$\begin{aligned} p &= V\sqrt{K\rho} = 1.5\sqrt{2.132 \times 10^9 \times 1000} \\ &= 1.5 \times 1.460 \times 10^6 = 2.19 \times 10^6\ \text{N/m}^2 = 2.19\ \text{MN/m}^2 \end{aligned}$$

(Check: $\rho c V = 1000 \times 1460 \times 1.5 = 2.19$ MN/m², same, since $c = \sqrt{K/\rho} = 1460$ m/s.) This equals about 223.3 m of water.

Answer: (i) 0.15 MN/m² (150 kN/m²) for 25 s closure; (ii) 2.19 MN/m² for 2 s closure.