§ 13.24.090. Design standards for drainage.

A.

Drainage Standards. Site development shall have no adverse impacts on adjacent and downstream locations. If adverse impacts are identified, off-site erosion, sediment and flood control improvements to the drainage way will be required to eliminate the adverse impacts. Designs for site area drainage and terraces shall be consistent with the low impact development (LID) requirements set by the RWQCB and the following minimum standards:

1.

Design and Construction. Drainage systems and facilities subject to drainage plan review and approval that are to be located in existing or future public rights-of-way are to be designed and constructed as set forth in the latest edition of the public works department's public improvement standards, or as per the project's conditions of approval. Applicants may request an adjustment pursuant to the public improvement standards in order to allow for a design that is more compliant with LID practices. Other systems and facilities subject to drainage plan review and approval are to be designed in accordance with good engineering practices. The design of drainage facilities in new land divisions and other new development subject to minor use permit or conditional use permit approval shall maximize groundwater recharge through on-site or communitywide stormwater infiltration measures. Examples of such measures include constructed wetlands, vegetated swales or filter strips, small percolation ponds, subsurface infiltration basins, infiltration wells, and recharge basins. Where possible, recharge basins shall be designed to be available for recreational use.

2.

Natural Channels and Runoff. Proposed projects are to include design provisions to retain natural drainage patterns and, when required, limit peak runoff to pre-development levels. To the maximum extent feasible, all drainage courses shall be retained in, or enhanced to appear in, a natural condition, without channelization for flood control. On downhill sites, encourage drainage easements on lower properties so that drainage can be released on the street or other appropriate land area below.

3.

Best Management Practices (BMPs). All new development subject to drainage plan review shall use BMPs to address polluted runoff. BMPs shall be consistent with the guidance found in documents such as the LID Handbook. Such measures shall include, but not be limited to: minimizing the use of impervious surfaces (e.g., installing pervious driveways and walkways); directing runoff from roofs and drives to vegetative strips before it leaves the site; and/or managing runoff on the site (e.g., percolation basins); and other low impact design (LID) techniques. The installation of vegetated roadside drainage swales shall be encouraged and, if used, calculated into BMP requirements. The combined set of BMPs shall be designed to treat stormwater runoff up to and including the eighty-fifth-percentile storm event.

4.

Runoff Volume. Runoff conveyance systems shall be capable of carrying the computed runoff volume from a twenty-five-year frequency storm or greater if deemed necessary by the director of community development. This may be reduced to a ten-year storm for small watersheds.

5.

Interceptors. Concrete ditches, bio-swales or other approved methods capable of intercepting surface runoff waters shall be installed along the top of all cut slopes where the tributary drainage area has a slope ten (10) percent or greater and a horizontal projection greater than forty (40) feet. Interceptor drains shall be paved with a minimum of three inches of concrete or gunite and reinforced. They shall have a minimum depth of twelve (12) inches and a minimum paved width of thirty (30) inches measured horizontally across the drain. The slope of drain shall be approved by the director of community development. Energy dissipaters may be required by the director of community development.

6.

Berms. Berms or drainage divides at least one foot high and three feet wide at the base shall be constructed at the top of all fill slopes where runoff would be directed towards the top of fill. Berms, ditches, interceptor drains, or swales shall be constructed at the top of cut and filled slopes for protection against water runoff. Paved interceptor drains shall be installed along the top of all cut slopes where the tributary drainage area above slopes towards the cut and has a drainage path greater than forty (40) feet measured horizontally.

7.

Over Side Drains. Over side drains shall be of concrete or corrugated metal pipe having a diameter required by runoff calculations, but not less than eight inches, and shall be aligned so as to minimize velocity at discharge points. Alternate designs, such as LID methods, approved by the director of community development may be permitted.

8.

Inlets. Inlets shall be constructed of galvanized iron, or approved equivalent, and shall be provided with overflow structures.

9.

Outlets. Outlet structures shall be provided with approved velocity reducers, diversion walls, rip-rap, concrete aprons or similar energy dissipaters where necessary and aligned to minimize downstream erosion and reasonably maximize recharge at discharge points, and shall be approved by the director of community development.

10.

Culverts. Culvert size and materials shall be determined by the civil engineer in accordance with standard design criteria and as approved by the building official. Minimum diameter shall be eighteen (18) inches.

11.

Terraces. Terraces at least six feet in width shall be established at not more than thirty-foot vertical intervals on all cut or fill slopes to control surface drainage and debris except that where only one terrace is required, it shall be at mid-height. For cut or fill slopes greater than sixty (60) feet and up to one hundred twenty (120) feet in vertical height, one terrace at approximately mid-height shall be twelve (12) feet in width. Terrace widths and spacing for cut and fill slopes greater than one hundred twenty (120) feet in height shall be designed by the civil engineer and approved by the director of community development. Suitable access shall be provided to permit proper cleaning and maintenance. Swales or ditches on terraces shall have a minimum gradient of five percent and must be paved with reinforced concrete not less than three inches in thickness or an approved equal paving. They shall have a minimum depth at the deepest point of one foot and a minimum paved width of five feet. A single run of swale or ditch shall not collect runoff from a tributary area exceeding thirteen thousand (13,000) square feet (projected) without discharging into a down drain.

12.

Dispersal Structures. An approved drainage dispersal structure shall be constructed wherever it is necessary to convert channel flow to sheet flow.

13.

Sensitive Habitat and Groundwater Protection. Runoff from roads and development shall not adversely affect sensitive habitat, groundwater resources and downstream areas, and shall be treated to remove floatable trash, heavy metals and chemical pollutants as necessary prior to discharge into surface or groundwater.

14.

Groundwater Recharge Methods. New development shall identify all methods to enhance groundwater recharge.

15.

Impervious Surfaces. New development shall be designed to minimize the amount of impervious surfaces in order to maximize the amount of on-site infiltration.

16.

Rain Gutters. Approved rain gutters shall be provided to receive all roof water and dispose of the water in a groundwater enhancing and non-eroding manner where the director of community development determines it to be necessary because of steepness of slope or presence of erodible materials. Direct connection of rain gutter outlets to impervious surfaces shall be minimized.

17.

Building Site Drainage. All graded building pads shall slope a minimum of five percent for ten (10) feet to an approved drainage device, or as approved by the director of community development. The drainage device shall be an approved system which conducts the water to a street, recharge area or drainage way. The top of footing stems or finish floor, if a concrete slab, shall extend above the top of street curb or inlet to the drainage device by a minimum of six inches plus two percent of the distance from the footing to the drainage device or curb. The director of community development may allow two percent to be used, if, because of terrain or soils, five percent is not reasonably attainable or necessary.

18.

Capacity of Drainage Devices. On graded sites, the director of community development may require that drainage devices calculated to convey runoff from a twenty-five-year frequency storm or greater be installed, if deemed necessary to prevent erosion, to conduct stormwater around buildings or structures and to the nearest recharge area, drainage way, or as approved by the director of community development.

19.

Appearance of Drainage or Recharge Devices. Where drainage devices are highly visible from the street or located in the public viewshed, they shall be shielded from view, if practical. Where visible, drainage devices shall be compatible with the character of the area and the existing topography. Exposed concrete overside drains are prohibited within these situations unless a visual analysis indicates the prohibition to be unnecessary. If they are visible, the size shall be the minimum necessary to handle drainage and ensure ability to maintain all drainage devices which collect from the slopes, and shall convey drainage by means of underground pipes or rock-lined ditches or other approved materials to blend with the natural topography in character, color and design. Transitions from natural drainage courses to developed areas shall be accomplished with comparable landscaping and grading to blend with existing topography. Detention, retention, or recharge basins shall be designed as a visual and/or recreational amenity within a project whenever practical.

20.

Groundwater Recharge. Groundwater recharge elements must be included in the project design to mitigate the impacts on recharge caused by the reduction in the permeability of soil areas on the site, except when any of the following site characteristics exist:

a.

High groundwater in the area limits the effectiveness of recharge efforts or enhancing groundwater recharge would create additional problems related to high groundwater.

b.

The entire site being developed is shown to contain impervious soils that would not benefit from recharge efforts.

c.

There is a known geologic instability that would be negatively impacted by increased groundwater recharge.

d.

It can be demonstrated that no additional runoff will occur from the development.

e.

Federal or state regulations prohibit recharge.

B.

Groundwater Recharge. All areas on the project site that will become impervious or will have their soil permeability impaired (such as compaction of soil under an all weather driveway) must be mitigated to the maximum extent practicable with recharge enhancement elsewhere on the parcel. Off-site mitigation is a secondary alternative.

1.

Areas Subject to Flooding. Buildings or structures are not permitted in an area determined by the director of community development to be subject to flood hazard by inundation, overflow, high velocity flows or erosion, except where the buildings or structures comply with the standards in Section 16.44.050, and provisions are made to eliminate identified hazards to the satisfaction of the public works director. These provisions may include providing adequate drainage facilities, protective walls, suitable fill, raising the floor level of the building or structure, or other means. The building and other structures (including walls and fences) shall be placed on the site so that water or mud flow will not be a hazard to on- or off-site structures or adjacent property. In the application of this standard, the director of community development shall enforce as a minimum the current federal flood plain management regulations as defined in the National Flood Insurance Program authorized by United States Code Title 42, Section 4001-4128 and contained in Title 44 of the Code of Federal Regulations, Part 59 et seq., which are hereby adopted and incorporated into this title by reference as though they were fully set forth here.

2.

Design of Floodproofing Measures. Floodproofing measures required by the director of community development shall be designed by a licensed architect or registered civil engineer.

3.

Subdrains. The director of community development may require the installation of approved subdrains in areas where underground water is anticipated.

4.

Runoff Computations. Runoff computations may be made by the "rational method" except where specific methods for calculating individual residential retention basins have been adopted or with the approval of the director of community development.

5.

Alternate Designs. Alternate designs which provide equivalent safety and are approved by the public works director may be used in lieu of those contained in this section.

6.

Hydromodification Control. If the director of community development has determined that the project could cause off-site erosion or adverse impacts to beneficial uses as a result of an increase in runoff rates and/or duration, the project shall incorporate hydromodification control measures in compliance with low impact development (LID) handbook requirements.

(Ord. No. 660, § 2(Exh. A), 2-25-2014)