Lessons · Engineering · Runoff from a catchment
How much water arrives from a piece of ground
The rational method estimates peak runoff as a coefficient times rainfall intensity times catchment area. The coefficient says what fraction runs off rather than soaking in: near one for asphalt, far lower for grass. Pave a field and you have not changed the rain, you have changed the coefficient.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is why development floods places downstream that never flooded before. The catchment did not grow and the storm was not new; the ground stopped absorbing.
How to think about it
Take the three quantities the problem gives and keep the units consistent -- intensity in metres per second with area in square metres gives cubic metres per second. Then ask what the coefficient would have been before the work, because the difference is what the drainage must now carry.
Worked example
Q = C x i x A.Coefficient, intensity, area.
C near 0.9 for asphalt, near 0.2 for grass.The surface decides it.
Units must agree before multiplying.Where the arithmetic usually fails.
Development raises C, not the rainfall.Which is the whole flooding story.
Your turn
C = 0.5, intensity 0.00002 m/s, area 10000 m². Write the peak flow in cubic metres per second.
0.5 x 0.00002 x 10000 = m^3/s
Solve one, graded on the server
The trap
Mixing millimetres per hour with square metres. The formula is only as good as the units, and the wrong ones are out by thousands.