Drainage Design Manual - the City of San Luis Obispo

Detention for the 10-year design storm may be permitted with specific approval of the CityEngineer or County Public Works Director. The design criteria for a 10-year storm would be thesame as the requirements of a 100-year storm, as noted above, with the exception of the depth ofwater stored. The depth of water detained cannot exceed .150 mm (0.5 ft) at any location in theparking lot under the 10-year design storm.9.1.4 Regional Facilities vs. On-Site DetentionIf storage facilities are planned for an individual site, rather than as a component of an overallregional plan, the storage is referred to as On-site Detention Facility. Such on-site facilities aredesigned to control short, intense storms that produce the greatest peak flows. The facilitiestypically are small in scale and can be used when regional detention is not available or if on-sitestorage is necessary to reduce peak discharge for downstream pipes, culverts, ditches, or streams.Facilities designed as a component of a Master Drainage Plan are classified as a RegionalDetention Facility. Generally, a Drainage Master Plan that incorporates such regional storagefacilities can produce more economical and effective mitigation of increase runoff than ispossible with numerous small detention basins. Regional facilities typically are larger than onsite,privately owned basins, and are designed to control systematically runoff from the totalwatershed. Detention facilities identified within these regional drainage studies must meet allapplicable drainage performance requirements. Any variation from these standards requiresprior approval from the City or County.Coordinated Regional Detention Facilities that take into account the entire watershed area arepreferred over individual stormwater facilities. When a regional drainage study has beenconducted and regional basins are designed, the regional facility will always take precedenceover local basin design. When necessary, on-site detention can be provided on a temporary basisuntil a Regional Detention Facility is built, and removed subsequent to completion of theRegional Detention Facility.9.1.5 LocationWhen choosing a location for a basin, the following factors should be taken into consideration:• Topography: Check for obvious low and high points in the area. The greater theexcavation, the higher the costs. Also, look for natural neckings of the watershed if adam is necessary.• Location in the Watershed: Locate the basin far enough down in the reaches of thewatershed to most effectively reduce the peak flow by intercepting the greatestnumber of tributaries to the main drainage channel. However, a basin located too lowin the watershed may require upstream channel improvements. The timing of outflowhydrographs should be examined closely for basins low in the watershed.• A basin located too high in the watershed allows all downstream lands to drainuncontrolled, which will increase the total flow to an undesirable level. This maySLO Creek Drainage Design Manual 101 February 2003