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Identify the NRCS series of the soil and the USDA textural class of the <br />soil horizon through the depth of the log, and note any evidence of high <br />ground water level, such as mottling. <br />4. Downspout infiltration is considered feasible on lots or sites that meet <br />all of the following: <br />• 3 feet or more of permeable soil from the proposed final grade to the <br />seasonal high ground water table. <br />At least 1-foot of clearance from the expected bottom elevation of <br />the infiltration trench or dry well to the seasonal high ground water <br />table. <br />• The downspout full infiltration system can be designed to meet the <br />minimum design criteria specified below. <br />Design Criteria Figure 3.1.2 shows a typical downspout infiltration trench system, and <br />for Infiltration Figure 3.1.3 presents an alternative infiltration trench system for sites with <br />Trenches coarse sand and cobble soils. These systems are designed as specified <br />below. <br />General <br />The following minimum lengths (linear feet) per 1,000 square feet of <br />roof area based on soil type may be used for sizing downspout <br />infiltration trenches. <br />Coarse sands and cobbles <br />20 LF <br />Medium sand <br />30 LF <br />Fine sand, loamy sand <br />75 LF <br />Sandy loam <br />125 LF <br />Loam <br />190 LF <br />2. Maximum length of trench shall not exceed 100 feet from the inlet <br />sump. <br />3. Minimum spacing between trench centerlines shall be 6 feet. <br />4. Filter fabric shall be placed over the drain rock as shown on Figure <br />3.11 prior to backfilling. <br />Infiltration trenches may be placed in fill material if the fill is placed <br />and compacted under the direct supervision of a geotechnical engineer <br />or professional civil engineer with geotechnical expertise, and if the <br />measured infiltration rate is at least 8 inches per hour. Trench length in <br />fill must be 60 linear feet per 1,000 square feet of roof area. Infiltration <br />rates can be tested using the methods described in Section 3.3. <br />6. Infiltration trenches should not be built on slopes steeper than 25% <br />(4:1). A geotechnical analysis and report may be required on slopes <br />over 15 percent or if located within 200 feet of the top of slope steeper <br />than 40%, or in a landslide hazard area. <br />Volume III — Hydrologic Analysis and Flow Control BMPs — December 2014 <br />3-5 <br />