Drainage Design Manual - the City of San Luis Obispo

9.5 Basin DesignDetention and retention basins constructed as a component of a stormwater management systemmust satisfy the following requirements. Exceptions to these requirements may be granted bythe City or County Public Works Director.• The basin must be designed to aesthetically harmonize with its surroundings.• The length-to-width ratio of the basin must be at least 2:1; a ratio of 5:1 is preferred.The basin inlet and outlet must be located as far apart as possible.• Interior earthen side slopes shall be no steeper than 3H:1V; exterior side slopes shallbe no steeper than 2.5 H:IV. Steeper slopes shall be confirmed by a qualifiedgeotechnical engineer and approved by the City or County.• Basin walls may be retaining walls, provided that the design is prepared and certifiedby a qualified engineer. A fence must be placed along the top of the wall.• The basin flow must slope towards the low-flow channel with a minimum slope of1%, measured perpendicular to the low-flow channel. The slopes must be designedas close to the minimum as possible to facilitate sedimentation. Because sedimenttends to accumulate around the lowest outlet, the invert elevation of any outlet shallbe located 150 mm (0.5 ft) above the basin floor to minimize clogging. Care must betaken to eliminate accumulation of stagnant water within the pond.• All finished earthen slopes must be re-vegetated as soon as possible.• Maintenance of all storage facilities must be addressed in the design and constructionpursuant to Section 3.10. For basins greater than 5,000 m 3 (4 ac-ft) storage; vehicularaccess for maintenance of the pond and outlet works, removal of sediment, andremoval of floating objects during all weather conditions must be provided. Anaccess road must be provided to the basin floor of all detention faculties. This roadmust have a minimum width of 3.7 m (12 ft) and a maximum grade of 20%. Turnaroundsat the control structure and the bottom of the basin must have a 12-m (40-ft)minimum outside turning radius. A maintenance plan must be developed andprovided along with the design documents.• Any storm water concentrated due to the grading of the access strip(s) shall beconveyed to a point of adequate discharge in a manner acceptable to the City orCounty. The facilities handling the concentrated storm water shall be considered asstorm drains and shall be so designed. An outlet structure is required if the pipedischarges into a channel or the detention basin. Erosion protection shall be installedat the lower terminus of the pipe.• If a basin is to double as a park during the dry months, it must have at least three (3)public points of access.SLO Creek Drainage Design Manual 105 February 2003