Chapter 4, Problem 4.2P

Rain is falling on a poorly designed 150-m" flat roof of a contemporary home. The design flaw requires the water level on the roof to reach approximately 5 cm above the plane of the roof before the water flows into drains.

1. Taking the accumulated rain on the roof as the system, can the process before the water level reaches 5 cm be considered batch, semi-batch, or continuous? Is it transient or steady-state? Once the water starts to flow into the drains, how do your answers change?
2. Before the w ater starts draining, which of the terms in the general balance equation (Equation 4.2-1) can be omitted? Why? How do your responses change after water starts draining?
3. Water begins flowing from the roof after 30 minutes of rainfall. What will the rate of draining (L/s) be if rain continues to fall at the same rate?
4. The home owner is considering installation of a “green" roof that is partially or completely covered with vegetation. If this is done, how would your answers to Parts (a) and (b) change? List at least two concerns and two environmental benefits of the new roof.