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7. Trenches may be located under pavement if a small yard drain or catch basin with grate cover is placed at the end of the trench pipe such that overflow would occur out of the catch basin at an elevation at least one foot below that of the pavement, and in a location which can accommodate the overflow without creating a significant adverse impact to downhill properties or drainage systems. This is intended to prevent saturation of the pavement in the event of system failure. Design Criteria Figure 3.1.4 shows a typical downspout infiltration drywell system. for Infiltration These systems are designed as specified below. Drywells General 1. Drywell bottoms must be a minimum of 1 foot above seasonal high ground water level or impermeable soil layers. 2. When located in course sands and cobbles, drywells must contain a volume of gravel equal to or greater than 60 cubic feet per 1000 square feet of impervious surface served. When located in medium sands, drywells must contain at least 90 cubic feet of gravel per 1,000 square feet of impervious surface served. Drywells must be at least 48 inches in diameter (minimum) and deep enough to contain the gravel amounts specified above for the soil type and impervious surface served. 4. Filter fabric (geotextile) must be placed on top of the drain rock and on trench or drywell sides prior to backfilling. 5. Spacing between drywells must be a minimum of 10 feet. Downspout infiltration drywells must not be built on slopes greater than 25% (4:1). Drywells may not be placed on or above a landslide hazard area or on slopes greater than 15% without evaluation by a professional engineer with geotechnical expertise or a licensed geologist, hydrogeologist, or engineering geologist, and with jurisdiction approval. Volume III— Hydrologic Analysis and Flow Control BMPs — December 2014 3-6