§ 23.31.040 DESIGN STORM.
   (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.