$(20$ pts$)5.$ For a $2$ hour, $10$ mm$/$h intensity rainfall, the hydrograph given in Table $4$ is observed. The area of watershed is $500$ km$^2$

Table $4.$ Propeller readings

Time $($hours$)$: $0|2|4|6|8|10|12|14|16|18|20|22|24$

Flow rate $($m$^3/$s$)$: $14|20|24|36|51|44|34|30|24|20|16|14|14$

a$)$ Draw the hydrograph

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