In: Civil Engineering
we can solve this problem by using the kirpich equation
Kirpich equation:
Time of concentration

Where K is kirpich adjustment factor
G=0.0078
L- Longest length of watershed
S= slope
| Ground Cover | Kirpich Adjustment Factor, k | 
| General overland flow and natural grass channels | 2 | 
| Overland flow on bare soil or roadside ditches | 1 | 
| Overland flow on concrete or asphalt surfaces | 0.4 | 
| Flow in concrete channels | 0.2 | 

Time of concentration over grass =1.005 minutes =60.30 seconds
Time of concentration over concrete=0.3185 minutes =19.11 seconds