The old axiom that “water runs downhill” is definitely true. Since modifications to existing property normally tend to interfere with the soil’s ability to absorb rainfall, development generally causes increased runoff from the affected property. As such, rules and laws have been established to protect downstream property owners from excessive increases in runoff flows across their property. For this reason, care must be taken to calculate and mitigate any significant increases in flow leaving the property. Typically, this is done by several methods such as piping the flows directly to a natural conduit capable of conveying the increased runoff without detriment to others or by capturing the excess runoff and slowly releasing it so that it leaves at a rate not exceeding the predevelopment rate.
In addition, care must be taken not to flood streets or buildable property within the proposed subdivision itself.