(A) Storm frequency generally. The storm frequency to be used in drainage system design will be the storm frequency applicable for the design point under consideration. The following guidelines give a general rule for determining applicable design storm frequency.
(B) Frequency determined by Engineer. The actual storm frequency used will be specified at the discretion of the Engineer.
(1) Ten-year storm. The drainage system for the ten-year storm is to be designed to minimize inconvenience, protect against minor damage and reduce maintenance costs. Improvements to be designed for the ten-year storm shall generally include on-site and non-sump conditions and local drainage facilities for residential, commercial, office and industrial development. This will almost always include all on-site closed conduit design and minor channel sections.
(2) Twenty-five-year storm. Twenty-five- year storm is required for sump conditions in the public right-of-way.
(3) One hundred-year storm. The drainage system for the 100-year storm is to be designed to protect against loss of life or substantial property damage. Improvements requiring 100-year design capacity are channels, detention ponds and retention basins. Major channels and closed conduit systems shall also meet the requirements for the 100-year storm as specified in these standards.