Grading and drainage guidelines

It is the responsibility of the property owner, their representative and designers to design and install proper site drainage.

 

For projects with additions up to 500 square feet on the first floor and not located in sharon heights:

  • All net new impervious area stormwater run-off shall be retained/detained on-site. Net new impervious area refers to surfaces that were previously pervious (e.g., grass, soil) and have been changed to impervious (e.g., concrete, asphalt).
  • To alleviate stress on the storm system, the owner is encouraged to retain/detain as much run-off as feasible on-site. This is in addition to the retention/detention requirement of net new impervious area run-off.
  • Grading and drainage plans are not required.

Engineering will require and review grading and drainage plans for the following projects:

  • All residential projects with brand new construction.
  • Projects with additions greater than 500 square feet on the first floor.
  • All new development projects or additions within the Sharon Heights neighborhood or projects located on sloped lots with unstable or unsafe soil conditions.

Requirements for grading and drainage plans:

  1. Grading and drainage plans must be submitted with Building Permit applications.
  2. The drainage should be designed such that the post-development site run-off to the public storm drain does not exceed the pre-development flow to the storm drain. See the impervious area worksheet.
  3. All new construction projects with a net increase in impervious area must submit a hydrology report.
  4. All additions/remodels with a net increase in impervious area of greater than 500 square feet must submit hydrology calculations.
  5. The Architect or Engineer shall coordinate with the project Arborist and Landscape Designer to design the storm drain system to minimize adverse impacts on on-site heritage trees and heritage trees in the public right-of-way. No grading and/or trenching shall be done within the dripline of any heritage tree located within the project area without first obtaining the approval of a certified arborist.
  6. Use Construction Best Management Practices (BMPs).

Design criteria:

  • If fill is to be added adjacent to the property lines, grades on neighboring properties will need to be obtained to document any potential impacts on these properties. Site grading shall not impede existing drainage from adjacent properties.
  • Design the drainage for sheet flow to lawn or pervious landscaped areas of the site, in lieu of area drains and pipe collection systems, wherever possible, without creating ponding and erosion.
  • Show where the roof downspouts are located. These downspouts should direct to approved splash blocks (minimum 2 feet long) that deflect the water away from the building. Show (with arrows) how the water is proposed to move away from the splash blocks.
  • If a basement is proposed for the project, a drainage plan for a separate subgrade drainage system must be included in this plan. The subgrade drainage system shall not be connected to surface storm drain piping to prevent backflow from the surface to the sub-grade. Drainage for basement light wells must be designed to prevent any backflow of surface drainage to the light well, and light wells should not be connected to a sub-drain system (to prevent backflow to the foundation). Subgrade drainage shall be treated as run-off and shall meet the retention/detention requirements.
  • Sites located lower than street grade shall make additional design considerations. Drainage for sites below street grade must be designed to prevent any backflow from the public storm drain system to connected on-site drainage systems.
  • Grades in the first 10 feet adjacent to the structure must drop 6 inches minimum (5% for pervious surfaces), and slope at 2% minimum for impervious surfaces as required by CRC §R401.3.
  • Erosion control plan - The applicant needs to submit an erosion control plan regardless of the season. An erosion control plan (ECP) is typically required year-round, not just during the wet season. While the risk of erosion is higher during periods of heavy rainfall, effective erosion control is essential throughout the entire construction process. A stabilized construction entrance is required per City Detail CG-16.
  • All new projects shall include a Construction BMP Plan Sheet as part of the plan submittal.
  • For projects located within the designated FEMA flood zone area, see FEMA Plan Review Checklist for additional design criteria.
  • On Jan. 18, 2022, the State Water Board’s emergency regulations became law and will remain in place for one year unless the State Water Board acts to end, modify, or readopt it. The following measure is a prohibited action that affects all water users statewide: Using potable water for street cleaning or construction site preparation purposes unless no other method is available (e.g., mixing concrete) or as needed to protect the health and safety of the public.

Frontage improvements:

  • All existing frontage improvements that are damaged, cracked, uplifted, or depressed during the course of construction, or that were damaged prior to construction, shall be removed, replaced, and/or repaired. Replaced and repaired sections shall meet City standards along the entire property frontage. The City will not bear the costs of reconstruction.
  • Pavers and/or stamped/decorative concrete shall not be installed in the public right of way.
  • All frontage improvement work shall be in accordance with the latest version of the City Standard Details.
  • A separate encroachment permit is required for any work in the public right of way.

Landscaping:

  • Provide documentation with the building permit application indicating the amount of irrigated landscaping. Detailed information regarding the Water Efficient Landscape Ordinance can be found on the City’s webpage at: Water Efficient Landscaping Ordinance
  • Submit a Landscape Application Checklist.
  • On March 1, 2022, the City Council adopted the following water conservation measure: Newly constructed homes and buildings must irrigate with drip or microspray only.

Grading and drainage plan guidelines:

The following requirements are based on guidelines listed in the California Residential Code. These are minimum standards, not maximum provisions, which can guarantee adequate drainage under all conditions. Depending on the topography, layout, or soil conditions, more restrictive requirements may be necessary as determined by the reviewing official.

  • The designer, in coordination with the soil engineer, must determine the necessities of each individual site on its own merits and design for problems peculiar to the site. Long-term performance must be considered with enough conservatism in design to take into account the general lack of maintenance received by residential sites.
  • Grading and drainage plans must be signed, dated, and stamped by a registered architect or civil engineer, on the original drawing.
  • For hillside lots (between Alameda De Las Pulgas and Interstate 280), a geotechnical engineer must submit a certified soils report and stamp the grading plan.

Drainage gradients:

The following minimum gradients for drainage are required for the development of private property:

  • Dirt/Grass Swale: 2% (Longitudinal)
  • Slope Away from Structure on Pervious Surface: 5% (Within 10-Feet)
  • Slope Away from Structure on Impervious Surface: 2% (Within 10-Feet)
  • Terrace/Interceptor Drains: 5%

The following are maximum gradients:

  • Graded earth swales: 6%
  • Driveways: 20%

Cut and fill slopes:

  • All cut and fill slopes shall be no steeper than 2:1. For steeper slopes, a soils engineer must submit a soils report and stamp, date, and sign the original drawing of the grading plan.
  • Drainage standards for slopes are established to prevent excessive erosion and subsequent instability. No surface water from buildings or pads should be permitted to flow over the slopes. Drainage from the natural slopes above the graded cut slope should be diverted away by a terrace drain or a “V” ditch.

 

The following is a checklist of items which as a minimum, must be shown on the plot and finished grading plans:

Plan requirements

Cover sheet

  1. The site address
  2. The owner's name, address and phone number
  3. The  names, addresses  and  phone  numbers  of  the  architect, civil  engineer, surveyor or other designer
  4. The volume of cut and fill needed and net new impervious area to be added
  5. Vicinity map with enough detail so the site can be easily found
  6. North arrow, scale and legend
  7. General notes

Site plan

  1. Fully dimensioned property lines and boundaries
  2. Existing and proposed easements, streets with center lines, sewer, and storm drain and access easements
  3. Location, diameter and drip line of all existing trees both on the property and within the public right of way
  4. Detailed plans of all drainage devices, walls, cribbing or other protective devices to be constructed as part of the proposed work
  5. All cut and fill slopes with continuous "daylight" lines
  6. Location of any buildings, structures, driveways, drainage ditches or element of the project such as pool, patio, tennis court, etc., on or within 15 feet of the property where the work is to be performed
  7. Existing and proposed elevations of building pad and finished floor
  8. Existing and proposed elevations of ground at property lines, relevant locations and spot elevations showing site grading and drainage paths
  9. Existing and proposed elevations of flowline at street gutter or edge of pavement along property frontage to a point 50 feet beyond the property lines
  10. Existing and proposed frontage improvements including curb, gutter, valley gutter, sidewalk and/or parking strips to be replaced
  11. Location and height of all retaining walls (note: retaining walls with a height exceeding 4 feet from the bottom of footing require a special permit per Section 301 of the UBC
  12. Top and toe of all cut and fill slopes
  13. Existing and proposed impervious areas, with a tabulation of each type of surface (e.g., patio, roof, landscaping, pool, driveway) and its area in square feet. Clearly show these areas on the plans
  14. Adequate drainage notes and specifications. Stormwater run-off shall be collected and conveyed to an on-site stormwater treatment/retention/detention facility. Grass swales shall be provided to drain side yards to front or rear yards. Provide design consideration for safe overflow discharge of a 100 year storm event
  15. Details for storm drainage devices and stormwater treatment measures
  16. Provisions for protecting adjacent properties
  17. For hillside lots, erosion control and/or slope protection
  18. Cleanouts at each bend in the underground drain pipe, including the bend at the downspout
  19. Tree protection plan for all trees to be retained and a tree removal notes for trees to be removed. (See Heritage Tree Ordinance)

Driveways

  1. Show driveway location, width and slope
  2. Approach must conform to City Standard Details

Sidewalk, curb and curb and gutter requirements

  1. Show existing curb, gutter, driveways and ADA ramps
  2. Broken or damaged sidewalk, curb and gutter must be replaced
  3. New sidewalk, curb and gutter must conform to City Standard Details
  4. Any construction work within the public right of way requires a permit from the Engineering Division

Utilities

  1. Show appropriate City Details for new water service and/or storm drains if proposed
  2. Proposed locations of sanitary sewer and storm drain system cleanouts
  3. A separate encroachment permit is required for any work within the public right of way. The applicant/contractor shall obtain the permit from the Engineering Division before start of any work within the public right of way or public easement areas. The applicant shall obtain permits from utility companies before applying for city encroachment permit.
  4. Show any easements affecting the property.
  5. Undergrounding all new utility services is encouraged.

Written hydrology report

Introduction - The introduction should include but is not limited to the following:

  1. State the nature of the project
  2. State the existing on-site and off-site conditions. State the existing total site composite run- off coefficient, total lot impervious area, and the total lot run-off rate Q10-year and Q100-year.
  3. State the proposed on-site and off-site conditions. State the proposed total site composite run-off coefficient, total lot impervious area, and the total lot run-off rate Q10-year and Q100-year.
  4. Identify the drainage basin (San Francisquito Creek, Atherton Channel or San Francisco Bay). See attached map showing the drainage basins. (Attachment A).

Body - The body of the report can be written in any format but must include the following as a minimum requirement:

  1. Describe in detail the existing on-site and off-site conditions. Specify the location and size of the existing off-site storm drain system receiving the project site run-off.
  2. Analyze whether upstream drainage patterns will be altered by the proposed project and any possible off-site impacts resulting from such alterations. Analyze how downstream sites are affected by existing and proposed new on-site runoff. Describe, if any, drainage facilities proposed to protect affected upstream and downstream sites.
  3. Summarize the hydrology calculation method used in this report
  4. Itemize any important information, assumptions or findings

Exhibits

  1. Include a site plan delineating watershed partitions for the proposed condition. On the same plan, provide the following:
    1. Use flow arrows to show directions of run-off
    2. Show the paths used for calculating time of concentration for each watershed
    3. Show the average run-off coefficient for each watershed
    4. Show the area in acres and ft2 for each watershed
    5. Show the proposed storm drain system and label the size of each pipe
  2. Complete and attach impervious area worksheet
  3. Include a site plan detailing impervious areas for the existing conditions
  4. Include a site plan detailing impervious areas for the proposed conditions

 

 

Back to top