Napa County Municipal Code Ch. 13.48 Drainfields

A. The minimum effective sidewall area shall depend upon the stabilized percolation rate as set forth in the following Table 13.48.010.

Table 13.48.010

B. The administrative authority reserves the right to establish minimum effective sidewall area requirements different than those of Table 13.48.010 when, in the opinion of the administrative authority, special flow conditions exist.

C. A minimum of three hundred square feet of effective sidewall area shall be provided for each system. The required sidewall area shall be based on the results of an approved site evaluation or percolation test performed by qualified persons certified by the administrative authority.

D. The required effective sidewall area for the drainfield shall be computed exclusive of any hardpan, rock, clay, glazed areas or other impervious formations.

(Ord. 1109 § 1 (part), 1996: prior code § 5590)

A. All disposal fields shall be constructed such as to meet the following requirements:

Minimum number of leaching lines ..... 2

Maximum length of each line ..... 100 feet

Minimum length of each line ..... 50 feet

Minimum width of trench ..... 18 inches

Minimum depth of trench ..... 30 inches

Minimum spacing between lines:

Six feet or two times trench depth, whichever is greater

Minimum depth of earth cover over lines ..... 12 inches

Minimum filter material under perforated pipe ..... 12 inches

Minimum filter material over perforated pipe ..... 2 inches

B. Trench bottom and all perforated piping are to be level.

C. The administrative authority may alter the above specifications when satisfied that such alteration is necessary for the functioning of the proposed system, or will benefit the function of future systems.

(Ord. 1109 § 1 (part), 1996: prior code § 5592)

A. Drainfield area slope shall be as specified in Section 13.32.040.

B. The administrative authority may require the statement of a civil engineer specializing in soils mechanics where the stability of the soil may be affected by the excavation or the application of the sewage effluent thereto.

(Ord. 1109 § 1 (part), 1996: prior code § 5594)

The spacing of the leaching lines shall be a factor of the ground slope and the planned trench depth, as set forth in the following Table 13.48.040:

Table 13.48.040 Relationship Between Slope, Trench Depth, Rock Depth to Trench Spacing

(Ord. 1109 § 1 (part), 1996: prior code § 5595)

A. Serial Distribution. This method is used on sloping ground. The leach lines shall be serially connected in such a way that the first line fills completely before effluent is conveyed through a crossover to the next leach line(s). Such connection shall be made with solid pipe of the same diameter as the perforated pipe. The invert of the crossover in each trench shall be at an elevation at least two inches higher than the invert of the perforated pipe in that trench such that the trench is filled completely before crossing over to the next trench. The crossover pipe shall be placed on undisturbed earth on a minimum grade of one-quarter inch per foot. The installation may be subject to the prior approval of the administrative authority.

B. Equal Distribution. This method is used on level or near-level ground. The leach lines shall be connected in such a way that effluent is distributed uniformly to the entire system, and each trench receives an equal volume of water. Such connection shall be made with solid pipe of the same diameter as the perforated pipe. The crossover pipe shall be placed on undisturbed earth. All trenches and piping shall be installed according to the Equal Distribution Guidelines Handout. The installation may be subject to the prior approval of the administrative authority.

C. When approved by the administrative authority, a combination of the above may be used.

D. Each unit of trench shall be not less than fifty feet nor more than one hundred feet in length, and shall be separated from the following or adjacent trench unit by a length of undisturbed soil equivalent to twice the total depth of trench. All trenches shall be excavated with a tool which does not pack the side walls, and excavation shall not be accomplished when the soil is wet. All of the effective sidewall area shall be thoroughly scarified and the trench bottom cleaned after scarifying and before placing the rock.

(Ord. 1109 § 1 (part), 1996: prior code § 5593)

A. Perforated pipe shall be supported by grade stakes or equivalent on at least five-foot centers.

B. Crushed stone, gravel, or similar filter materials acceptable to the administrative authority and having adequate voids, varying in size from three-quarters to one and one-half inches in size, shall be placed in the trench to the depth and grade required in this section. Perforated piping shall be completely encased in filter material. Untreated building paper, filter fabric, or similar material shall then be placed over the filter bed to prevent closure of voids with earth backfill.

C. The connecting pipe between a septic tank and a leaching line, or between leaching lines, shall be laid with approved watertight joints on natural ground.

D. Depths of gravel under the perforated pipe in the leaching trench may be not less than twelve nor more than forty-eight inches, as determined by the administrative authority.

(Ord. 1121 § 3, 1997; Ord. 1109 § 1 (part), 1996: prior code § 5591)