$(30$ pts$)3.$ For a $2$ hour, $8m\frac{m}{h}$ intensity rainfall, the hydrograph given in Table is observed. The

area of watershed is $500k{m}^2$

Table: Propeller readings

y: The last digit of your student number $Y=9$

a$)$ Obtain $2$ hour unit $-$ hydrograph for the considered flow section

b$)$ Obtain $4$ hour unit $-$ hydrograph for the considered flow section

c$)$ Obtain the hydrograph of a rainfall with $4$ hour duration and $10m\frac{m}{h}$ intensity