A.   The drainage study and conceptual plan shall be submitted under the following concepts:
   1.   Drainage Patterns: Drainage flows of all frequencies should enter and depart from the property to be developed in substantially the same manner as predevelopment conditions.
   2.   Street Relationships/Emergency Access: All lots, structures, etc., within a development shall be accessible by at least one route during a 100-year storm with a maximum street water depth of one foot (1'). In all cases, the flow of water shall be contained within rights of way.
   3.   Retention Facility: In order to reduce the storm runoff as much as feasible, retention facilities shall be sized for local (on site) stormwater using the following procedures:
      a.   The design frequency shall be the 25-year storm, using the 24-hour rainfall depth "D" as determined from the national weather services for Idaho. For the Jerome area the 24-hour rainfall on a 25-year storm frequency is 1.6 inches.
      b.   Volume requirements for the retention facility are calculated by the following method:
(12) = (1.6) * (A), where
Volume required in acre feet
24 hour rainfall depth in inches on a 25-year storm frequency
Tributary area
   4.   Side Slopes: Where possible, the retention facility shall have four to one (4:1) side slopes.
   5.   Parking In Floodplains; Commercial: Parking within the 100-year floodplain shall be allowed provided that there are no unattended vehicles and that there is no substantial obstruction to the free flow of water. If the depth of water exceeds one foot (1') during the base flood, the parking area shall be signed to show the parking area is subject to periodic flooding. (Ord. 994 §2, 2006)