For average site drainage design, what should the frequency or return period generally be?

For average site drainage design, a return period of 10 years is commonly utilized because it strikes an effective balance between the likelihood of heavy rainfall events and the need for reasonable engineering costs. This time frame provides a practical estimate that allows designers to account for typical storm events without excessively oversizing drainage systems, which can lead to increased construction and maintenance costs.

Using a 10-year return period means the system is designed to handle rainfall events that have a 10% chance of being equaled or exceeded in any given year. This frequency is frequently accepted in the industry, particularly for urban stormwater management systems. It ensures that the drainage infrastructure can effectively manage most normal precipitation scenarios while being mindful of economic implications.

Choosing a shorter return period, such as 5 years, might result in a system that is under-designed for larger storms, leading to flooding issues. Conversely, selecting a longer return period, like 20 or 30 years, could overestimate the capacity requirements and increase installation costs without significantly enhancing performance during typical storm events. Thus, a 10-year return period aligns well with both practical design considerations and the expected serviceability of drainage systems.