Sonoma County Municipal Code Art. 14 Standards

This article provides ministerial standards for the proper conduct of construction grading and drainage. All construction grading and drainage subject to the provisions of this chapter shall comply with the requirements of this article, regardless of whether a permit is required by this chapter.

(Ord. No. 6338, § II(Exh. B), 3-16-2021)

Construction grading shall be designed and constructed in compliance with the following requirements.

A. General.

1. Areas of construction grading. Construction grading shall be limited to the grading area identified and delineated on the approved plans and specifications.

2. Final contours. Contours, elevations, and shapes of finished surfaces shall be blended with adjacent natural terrain to achieve a consistent grade and natural appearance. Borders of cut slopes and fills shall be rounded off to a minimum radius of five feet (5') to blend with the natural terrain.

3. MS4 permit. Where construction grading is within an MS4 permit boundary, the construction grading shall comply with the applicable standards and provisions of the MS4 permit.

B. Cuts.

1. Slope. The slope of cut surfaces shall be no greater than two feet (2') horizontal to one foot (1') vertical [fifty percent (50%)], unless a soils report justifies a greater slope.

2. Terracing and drainage. Terracing and drainage of cuts shall be provided as required by Subsection D.

C. Fills.

1. Fill location. Fill shall not be placed on existing slopes greater than two feet (2') horizontal to one foot (1') vertical [fifty percent (50%)].

2. Surface preparation. Ground surfaces shall be prepared to receive fill by removing vegetation, topsoil, and other unsuitable materials, and scarifying the ground to provide a bond with the fill material.

3. Benching. Benching into sound bedrock or other competent material, as determined by a civil engineer, shall be required where existing grade is at a slope greater than five feet (5') horizontal to one foot (1') vertical [twenty percent (20%)] and the depth of the fill exceeds five feet (5'). Benching shall be provided in accordance with Figure 11-1, subject also to the following requirements:

a. A key at least ten feet (10') in width and two feet (2') in depth shall be installed.

b. The key shall be installed at least one foot (1') into sound bedrock or other competent material.

c. The area beyond the toe of the fill shall be sloped for sheet overflow or a non-erosive drain shall be provided.

d. Cuts for benching and keys shall be accepted by a soils engineer as a suitable foundation for fill prior to the placement of the fill material.

[Figure: Figure 11-1 Benching Details]

Figure 11-1 Benching Details

4. Fill material. Fill material shall not include organic, frozen, or other deleterious materials. No rock or similar irreducible material greater than six inches (6") in any dimension shall be included in fills, except where a soils engineer devises a method of placement of larger rock, continuously inspects its placement, and approves fill stability, subject also to the following requirements:

a. Potential rock disposal areas shall be shown on the plans and specifications.

b. Rocks shall be placed so as to assure filling of all voids with well-graded soil.

5. Fill placement. Fills shall be constructed in lifts not exceeding eight inches (8") in depth. Completed fills shall be stable, well-integrated, and bonded to adjacent materials and the materials on which they rest. Fills shall be competent to support anticipated loads and be stable at the design slopes shown on the approved plans and specifications.

6. Compaction. Fills shall be compacted to a minimum of ninety percent (90%) of maximum dry density, as determined by ASTM D1557, Modified Proctor, or as specified by a soils engineer.

7. Slope. The slope of fill surfaces shall be no greater than two feet (2') horizontal to one foot (1') vertical [fifty percent (50%)], unless a soils report justifies a greater slope.

8. Limitation on fill in flood-prone urban area. No fill shall be placed in the flood- prone urban area, unless an analysis of pre-development and proposed post- development site topography demonstrates for no less than the design discharge specified in Section 11.14.040.B that natural and existing off-site drainage patterns will be maintained to the extent feasible and stormwater runoff will be carried to disposal location(s) that have capacity to accommodate the stormwater runoff without worsening existing drainage conditions on the site and adjacent properties.

9. No net fill in special flood hazard areas. No fill shall be placed in any special flood hazard area, unless prior to permit issuance an analysis comparing the pre- development and proposed post-development site topography demonstrates that no reduction in flood storage capacity in the special flood hazard area will result from the fill placement and related improvements, and prior to the permit being finaled an analysis comparing the pre-development and as-built topography confirms that no reduction in flood storage capacity in the special flood hazard area has resulted from the fill placement and related improvements.

10. Terracing and drainage. Terracing and drainage of fills shall be provided as required by Subsection D.

D. Terracing and drainage of cuts and fills. Terracing and drainage of cuts and fills shall be designed and constructed to ensure the integrity of the cuts and fills. Unless otherwise justified by a soils report, terracing and drainage of cuts and fills with surface slopes greater than three feet (3') horizontal to one foot (1') vertical [thirty-three percent (33%)]) shall comply with the following requirements. Additional requirements applicable to the provision of drainage facilities and systems are established by Section 11.14.040.B.

1. Terraces. Terraces shall be established on cut and fill slopes to control surface drainage and debris. Suitable access shall be provided to permit proper cleaning and maintenance of terraces.

a. For cut and fill slopes up to thirty feet (30') in vertical height, terraces need not be provided.

b. For cut and fill slopes greater than thirty feet (30') and up to sixty feet (60') in vertical height, one (1) terrace at least six feet (6') in width shall be established at mid-height.

c. For cut and fill slopes greater than sixty feet (60') and up to one hundred twenty feet (120') in vertical height, terraces at least six feet (6') in width shall be established at not more than 30-foot intervals or one (1) terrace at least twelve feet (12') in width shall be established at mid-height.

d. For cut and fill slopes greater than one hundred twenty feet (120') in vertical height, terrace widths and spacing shall be designed by a civil engineer.

2. Terrace drainage.

a. Swales or ditches. Swales or ditches shall be provided on terraces, and shall:

(1) Have a minimum gradient of twenty feet (20') horizontal to one foot vertical (1') [five percent (5%)];

(2) Be paved with reinforced concrete not less than three inches (3") in thickness, or with other materials suitable to the application; and

(3) Have a minimum depth of one foot (1') and a minimum width of five feet (5').

b. Limitation on single run of swale or ditch. A single run of swale or ditch shall not collect runoff from a tributary area exceeding thirteen thousand five hundred (13,500) square feet (projected) without discharging into a down drain.

3. Subsurface drainage. Cut and fill slopes shall be provided with subsurface drainage as necessary for stability.

4. Building pad drainage. Building pads shall have a drainage gradient of fifty feet (50') horizontal to one foot (1') vertical [two percent (2%)] toward approved drainage facilities.

5. Surface interceptor drains. Surface interceptor drains shall be installed along the top of all cut and fill slopes receiving drainage from a tributary width greater than forty feet (40'), measured horizontally upslope. The drains shall be paved with reinforced concrete not less than three inches (3") in thickness, or with other materials suitable to the application. The drains shall be designed in compliance with the Flood Management Design Manual, or superseding document, for the 100-year design discharge. This surface interceptor drains requirement shall not apply to outslope constructed roadways designed to prevent the concentration of stormwater runoff, provided that the stability of the cut and fill slope is maintained.

E. Setbacks. Cut and fill slopes shall be set back from property lines in compliance with the following requirements. Setback dimensions shall be measured perpendicular to the property line and shall be as shown in Figure 11-2.

1. Top of slope. The setback at the top of a slope shall be not be less than that shown in Figure 11-2, or than is required to accommodate any required surface interceptor drains, whichever is greater.

2. Toe of slope. The setback at the toe of a slope shall not be less than that shown in Figure 11-2.

[Figure: Figure 11-2 Cut and Fill Slope Setback Requirements]

Figure 11-2 Cut and Fill Slope Setback Requirements

F. Protection of footings, buildings, and structures. Footings that may be affected by any excavation shall be underpinned or otherwise protected against settlement and shall be protected against lateral movement. Fills or other surcharge loads shall not be placed adjacent to any building or structure unless the building or structure is capable of withstanding the additional loads caused by the fill or surcharge. The rights of adjacent affected property owners shall be as set forth in Civil Code section 832.

(Ord. No. 6338, § II(Exh. B), 3-16-2021)

A. Dams and reservoirs subject to county jurisdiction. Dams and reservoirs subject to the county's jurisdiction shall be designed and constructed in compliance with the approved plans and specifications and the following requirements:

1. Embankment design. Embankments shall be designed and constructed in compliance with the following requirements:

a. Compaction. Embankments shall be compacted to a minimum of ninety percent (90%) of maximum dry density, as determined by ASTM D 1557, Modified Proctor.

b. Embankment crest. The embankment crest shall have a minimum width of ten feet (10'), a minimum freeboard of two feet (2') and slope toward the dam or reservoir at minimum of fifty feet (50') horizontal to one foot (1') vertical [two percent (2%)].

2. Spillways. Dams and reservoirs six feet (6') or more in height from existing grade shall have a spillway as an emergency outlet and shall be designed and constructed in compliance with the following requirements:

a. Spillway design flood. The spillway design flood shall be in compliance with the Flood Management Design Manual, or superseding document, for the 100-year design discharge.

b. Minimum freeboard. The minimum freeboard (vertical distance from spillway crest to dam or reservoir crest) shall be two feet (2').

c. Minimum residual freeboard. The minimum residual freeboard (vertical distance from maximum reservoir stage for spillway design flood to dam or reservoir crest) shall be one foot (1').

d. Downstream chute. The downstream chute of the spillway must not be over-topped for the spillway design flood.

e. Location. Unless otherwise recommended by a soils engineer, the spillway shall not be located over the dam or reservoir embankment.

f. Spillway material. Unless otherwise recommended by a soils engineer, the spillway shall be paved with reinforced concrete not less than six inches (6") in thickness.

3. Outlets. Reservoirs shall include a low level outlet for emptying or lowering the reservoir in case of emergency and for inspection and maintenance of the dam or reservoir and appurtenances. Outlet conduits shall have an upstream control device (gate or valve), with a trash rack in front of control device, capable of controlling the discharge through all ranges of flow. Outlet conduits shall be designed for internal pressure equal to the full reservoir head and for superimposed embankment loads, acting separately. A low level outlet need not be provided where the reservoir floor is below grade and a pump capable of emptying or lowering the reservoir in case of emergency or for inspection and maintenance is installed.

B. Dams and reservoirs subject to state jurisdiction. Dams and reservoirs subject to the state's jurisdiction shall be designed and constructed in compliance with the requirements of the California Division of Safety of Dams and this chapter.

(Ord. No. 6338, § II(Exh. B), 3-16-2021)

A. Drainage patterns and runoff. Construction grading and drainage shall be designed and constructed to maintain natural and existing drainage patterns. Post-development stormwater runoff shall not exceed pre-development stormwater runoff using the calculation methodologies in the Storm Water Low Impact Development Technical Design Manual, or superseding document, or equivalent calculation methodologies.

B. Design and construction of drainage facilities and systems.

1. Except as provided in Subsection B.2, drainage facilities and systems shall be designed and constructed in compliance with the Flood Management Design Manual, or superseding document, for no less than the 10-year design discharge.

2. Stream crossings shall be designed and constructed in compliance with the Flood Management Design Manual, or superseding document, for no less than the 100-year design discharge and include a minimum clearance from soffit to design water surface of one foot (1').

3. Drainage facilities required for cuts and fills are also subject to Section 11.14.020.D.

C. Disposal of stormwater runoff. Drainage facilities and systems shall convey stormwater runoff to disposal locations that maximize infiltration and minimize erosion, and shall dissipate the energy or diffuse the flow prior to releasing the stormwater runoff to any setback area or off the site.

D. Prevention of soil loss. Drainage facilities and systems shall prevent or minimize soil loss through the use of storm drain culverts (pipes), storm drain inlets and outlets, storm drain outfalls, energy dissipators, flow dispersion, check dams, rolling dips, critical dips, proper location and sizing of culverts, revegetation of exposed or disturbed slopes, minimizing cross drains through road outsloping, minimizing the use of artificial slopes, and other best management practices referenced or detailed in the department's best management practices for construction grading and drainage.

(Ord. No. 6338, § II(Exh. B), 3-16-2021)

Where human remains or archaeological resources are discovered during construction grading and drainage, all work shall be halted in the vicinity of the find, the director shall be notified, and the following shall occur before work may be resumed:

A. Human remains. If human remains or suspected human remains are discovered, the permittee shall notify the county coroner and comply with all state law requirements, including Health and Safety Code section 7050.5 and Public Resources Code section 5097.98, to ensure proper disposition of the human remains or suspected human remains, including those identified to be Native American remains.

B. Archaeological resources. If archaeological resources or suspected archaeological resources are discovered, the director shall notify the State Historic Preservation Officer and the Northwest Information Center at Sonoma State University, and the permittee shall retain a qualified archeologist to evaluate the find to ensure proper disposition of the archaeological resources or suspected archaeological resources. All costs associated with the evaluation and mitigation of the find shall be the responsibility of the permittee. The director shall provide notice of the find to any tribes that have been identified as having cultural ties and affiliation with the geographic area in which the archaeological resources or suspected archaeological resources were discovered, if the tribe or tribes have requested notice and provided a contact person and current address to which the notice is to be sent. The director may consult with and solicit comments from notified tribes to aid in the evaluation, protection, and proper disposition of the archaeological resources or suspected archaeological resources. The need for confidentiality of information concerning the archaeological resources or suspected archaeological resources shall be recognized by all parties. For the purposes of this section, archaeological resources include historic or prehistoric ruins, burial grounds, pottery, arrowheads, midden, or culturally modified soil deposits. Artifacts associated with prehistoric ruins include humanly modified stone, shell, bone, or other cultural materials such as charcoal, ash, and burned rock indicative of food procurement or processing activities. Prehistoric domestic features include hearths, fire pits, or floor depressions; mortuary features are typically represented by human skeletal remains.

(Ord. No. 6338, § II(Exh. B), 3-16-2021)

Construction grading and drainage within, adjacent to, or involving the alteration of watercourses shall comply with the provisions of Article II (Water Clarity) of Chapter 23 of this code, any necessary state and federal permits, approvals, or authorizations, and the following requirements.

A. Flood carrying capacity. The flood carrying capacity of any altered or relocated portion of a watercourse shall be maintained.

B. Obstruction of watercourses. Watercourses shall not be obstructed unless an alternate drainage facility complying with Section 11.14.040.B is installed.

C. Fills within watercourses. Fills placed within watercourses shall have protection against erosion.

D. Streams in closed conduits. Except for stream crossings, streams shall not be placed in closed conduits. Stream crossings shall be limited to the minimum width necessary to cross the stream.

E. Heavy equipment. Heavy equipment shall not cross or disturb stream channels, unless best management practices referenced or detailed in the department's best management practices for construction grading and drainage are in place.

F. Materials storage. Materials that could contribute to pollution shall not be deposited or stored in or adjacent to a watercourse.

(Ord. No. 6338, § II(Exh. B), 3-16-2021)

Construction grading and drainage shall not remove or disturb trees and other vegetation except in compliance with the department's best management practices for construction grading and drainage and the approved plans and specifications. Construction grading and drainage shall be conducted in compliance with the following requirements.

A. The limits of the development area shall be clearly identified and delineated on the approved plans and specifications, and defined and marked in the field to prevent damage to surrounding trees and other vegetation.

B. Trees and other vegetation within the limits of the development area that are to be retained shall be identified and protected from damage by marking, fencing, or other measures.

(Ord. No. 6338, § II(Exh. B), 3-16-2021)

Construction grading and drainage shall replant disturbed surfaces in compliance with the approved plans and specifications and the following requirements.

A. Topsoil. Topsoil removed in preparation for construction grading and drainage shall be stored on or near the site and protected to prevent soil loss while the work is underway. Topsoil shall not be stored on top of root systems of trees intended to be preserved. Topsoil shall be restored to disturbed surfaces prior to revegetation.

B. Methods of revegetation. Mulching, seeding, planting of groundcover, shrubs, or trees, or other suitable stabilization measures shall be used to protect disturbed surfaces to minimize soil loss, and to maximize slope stability. Use of drought-tolerant, fire resistant native plant species is encouraged; use of invasive plant species is prohibited.

C. Timing of revegetation. Revegetation shall be installed as soon as practical after vegetation removal, but in all cases prior to final inspection.

(Ord. No. 6338, § II(Exh. B), 3-16-2021)

Construction grading shall be set back fifty feet (50') from the high water mark of lakes, ponds, and reservoirs, unless a greater setback is required by the general plan, local coastal program, or zoning code. The setback requirements in this section shall not apply to construction grading for construction drainage; trails; public projects; resource conservation, restoration, or enhancement projects; or lake, pond, or reservoir maintenance.

(Ord. No. 6338, § II(Exh. B), 3-16-2021)

Construction grading shall be set back twenty-five feet (25') from the top of the higher bank of streams, unless a greater setback is required by the general plan, local coastal program, or zoning code. The setback requirements in this section shall not apply to construction grading for stream crossings and approaches; dams and reservoirs; construction drainage; trails; public projects; resource conservation, restoration, or enhancement projects; or stream bank restoration or stabilization.

(Ord. No. 6338, § II(Exh. B), 3-16-2021)

Construction grading shall be set back from wetlands in compliance with the requirements in Table 11-3, unless:

A. A greater setback is required by the general plan, local coastal program, or zoning code; or

B. All necessary state and federal permits, approvals, or authorizations to fill the wetlands have been obtained.

Table 11-3 Wetland Setback Requirements

(Ord. No. 6338, § II(Exh. B), 3-16-2021)

A. During the work. Construction grading and drainage shall prevent or control soil and other pollutant discharges during the work through the use of best management practices referenced or detailed in the department's best management practices for construction grading and drainage.

B. During qualifying rain events. Construction grading and drainage shall prevent or control soil and other pollutant discharges during qualifying rain events by implementing stormwater best management practices referenced or detailed in the department's best management practices for construction grading and drainage at least forty-eight (48) hours prior to any qualifying rain event.

C. Post-development. Construction grading and drainage shall be designed to limit post-development soil and other pollutant discharges to pre-development levels in compliance with the department's best management practices for construction grading and drainage.

(Ord. No. 6338, § II(Exh. B), 3-16-2021)

Construction grading and drainage shall only be permitted during the rainy season when on-site soil conditions permit the work to be performed in compliance with this article and the department's best management practices for construction grading and drainage, and sufficient materials are available on-site to implement the stormwater best management practices required by Section 11.14.120.B, if necessary. The area of erodible land exposed at any one (1) time shall not exceed one (1) acre or twenty percent (20%) of the site, whichever is greater, and the time of exposure shall be minimized.

(Ord. No. 6338, § II(Exh. B), 3-16-2021)