The rational method for peak flow, detention and retention pond sizing, storm drain pipe sizing, and the grading patterns that give a site positive drainage.
Every acre of pervious ground converted to roof and pavement pushes site hydrology the same direction — more runoff volume, a higher peak flow, and a shorter time of concentration — which is why stormwater quantity control is a required, not optional, part of civil site design. This module builds the rational method (Q=CiA) from its assumptions and limitations, works through how time of concentration and IDF curves set design rainfall intensity, and covers how detention and retention ponds are actually sized using storage-indication routing rather than a peak-flow shortcut.
By the end of this module you should be able to explain why an overestimated time of concentration is the most common way a rational-method design ends up quietly undersized, and why storm drain pipe slope is bounded by minimum and maximum velocity limits even when Manning's-equation capacity looks adequate on paper.