City of Bellingham Municipal Code

15.42.060 - APPROVAL STANDARDS

A. New Development Requirements: All new development shall provide protection for the quality of the waters of the State of Washington by the application of Best Management Practices and/or Source Controls.

All new development that requires either a building permit or has a land disturbance area greater than 500 sf shall at a minimum comply with Minimum Requirement #2 within BMC Section 15.42.060 F, all other applicable federal, state, and local ordinances, codes and regulations for stormwater management, and shall obtain a stormwater permit from the City of Bellingham prior to commencement of activity. In addition, New Development that exceeds certain other thresholds, as further identified herein, shall be required to comply with additional Minimum Requirements as follows.

1. The following new development shall comply with Minimum Requirements #1 through #5 within BMC Section 15.42.060 F:

a. New Single Family homes or duplexes, or

b. Developments that create or add 2,000 sf, or greater, of new, replaced, or new plus replaced impervious surface area, or

c. Land disturbing activity of 5,000 sf or greater, or

d. A subdivision, or the lots created from a subdivision, that contains two or fewer lots and is not likely to exceed the land disturbance and/or impervious thresholds from future property development, or

e. Developments within the Lake Whatcom Watershed that create or add, 120 sf or greater, of new, replaced or new plus replaced impervious surface area, or

f. Land disturbing activity within the Lake Whatcom Watershed of 500 sf or greater, or

g. Other development that is determined by the Public Works Director to contain a significant risk for the degradation of stormwater.

a. Creation or addition of 5,000 sf, or more, of new, replaced or new plus replaced impervious surface area; or

b. Convert ¾ acres, or more, of native vegetation to lawn or landscaped areas; or

c. Convert 2.5 acres, or more, of native vegetation to pasture; or

d. One acre or more of land disturbing activity; or

e. A subdivision containing two or more lots that is likely to exceed the land disturbance and/or impervious thresholds from future property development; or

f. Through a combination of impervious surface and land altering activities the development will result in a 0.1 cfs increase in flow from the predeveloped condition for the 100 year storm event as identified with a qualified continuous flow duration model.

g. Other development that is determined by the Public Works Director to contain a significant risk for the degradation of stormwater.

3. Lake Whatcom New Development: In addition to the requirements for new development under 1 and 2 above, all new development that drains to Basin One of Lake Whatcom that is subject to the Lake Whatcom Regulatory Chapter (LWRC) 16.80 and that creates or replaces more than 300 sf of impervious or partially pervious surface shall provide for stormwater management as follows:

a. An engineered stormwater site plan shall be provided, for review and approval, which determines site specific controls to limit stormwater runoff and phosphorus transport from the total site to levels associated with a predeveloped forested condition. The plan shall at a minimum provide soils reports, groundwater studies, hydrologic analyses, appropriate BMC's and BMP maintenance plans to demonstrate the ability to meet this code. Additionally all stormwater site plans shall maintain or create a minimum of 30% "Natural Forested Condition" upon the site per BMC 16.80.

Any BMP or combination of BMP's will be considered that would lead to successful management of flow and phosphorus transport. Developed flow shall meet the forested flow duration standards of this code and the DOE Manual. Phosphorus transport through surface flow or interflow shall be limited to an average discharge of no more than 0.15 lbs/acre/yr. Possible BMP's include but are not limited to:

i. Full Infiltration of up to the 100 year storm (with analysis of soil suitability).

ii. Full Infiltration of up to the 2 year storm (with analysis of soil suitability) and overflow connection to an adequately sized piped conveyance system that drains directly to lake Whatcom. Any open channel elements must be found capable of conveying the 100 year storm without erosion potential.

iii. On site water storage with reuse within a building (subject to State Water Law).

iv. Forest Retention or Creation (with long term maintenance and retention agreements).

v. Filtration of up to the 10 year storm (allowance dependent on long term phosphorus removal ability).

vi. Forest Retention or Creation (that portion of any site that meets the "natural forested conditions" contained within BMC 16.80 shall be considered to have met the predeveloped forested condition of this code and the DOE Manual).

vii. Soil Remediation (with analysis of soil suitability and infiltration capability.

OR

b. If the site has retained or created 75% or more of forested/native vegetation on a site per Lake Whatcom Management Code 16.80 and the site will not exceed 2,000 sf of impervious surface or up to 20% of the total site, whichever is the lesser. And the site will also not exceed 1,000 sf of partially pervious surface as defined within BMC 16.80 or up to 10% of the total site, whichever is the lesser. And the total of impervious surface and partially pervious surfaces will not together exceed 25% of the total site area nor exceed 2,500 sf.

If these conditions can be met, stormwater mitigation for the site shall require compliance to the maximum extent practicable with On Site Water Management BMPs as provided for in the Manual.

OR

c. If the site redevelopment creates or replaces more than 120 sf and less than 300 sf of impervious surface or partially pervious surfaces and the addition of said surfaces does not cause the property to be in noncompliance with the limits on such surfaces through BMC 16.80, the site shall provide mitigation for stormwater and phosphorus transport at the rate of 10 sf of mitigation for every one sf of creation or replacement of these surfaces.

Impervious surfaces developed under this method shall at a minimum be mitigated to the standard provided for by an On Site Water Management Plan as provided in the Manual.

Mitigation of partially pervious surfaces shall at a minimum be by way of lawn removal.

OR

d. Construction of phosphorus limiting or flow limiting facilities only on a property may be permitted outright with a no fee stormwater permit upon review of the facilities for compliance with Lake Whatcom Management standards.

a. The new, replaced, or total of new plus replaced impervious surfaces is 2,000 sf or more, or 5,000 square feet or more of land disturbing activities; or

b. A subdivision, or the lots created from a subdivision, that contain two or fewer lots and is not likely to exceed the land disturbance and/or impervious thresholds from future property development; or

c. Any redevelopment within the Lake Whatcom Watershed that creates or adds, 120 sf or greater, of new, replaced or new plus replaced impervious surface area; or

d. Any land disturbing activity within the Lake Whatcom Watershed of 500 sf or greater; or

e. Other development that is determined by the Public Works Director to contain a significant risk for the degradation of stormwater.

2. The following redevelopment shall comply with Minimum Requirements #1 through #10 within BMC Section 15.42.060 F for the new impervious surfaces and converted pervious areas:

a. Creation or addition of 5,000 sf, or more, of new, replaced or new plus replaced impervious surface area; or

b. Converts ¾ acres, or more, of native vegetation to lawn or landscaped areas; or

c. Converts 2.5 acres, or more, of native vegetation to pasture; or

d. One acre or more of land disturbing activity; or

e. A subdivision containing two or more lots that is likely to exceed the land disturbance and/or impervious thresholds from future property development; or

f. Through a combination of new impervious surface and land altering activities, the development results in a 0.1 cfs increase in flow from the predeveloped condition for the 100 year storm event as identified with a qualified continuous flow duration model.

3. Lake Whatcom Redevelopment: In addition to the requirements for redevelopment under 1 and 2 above, all redevelopment that drains to Basin One of Lake Whatcom, and that is subject to the Lake Whatcom Regulatory Chapter (LWRC) 16.80, and that creates or replaces more than 300 sf of impervious or partially pervious surface, shall provide for stormwater management as follows:

a. An engineered stormwater site plan shall be provided, for review and approval, which determines site specific controls to limit stormwater runoff and phosphorus transport from the total site to levels associated with a predeveloped forested condition. The plan shall at a minimum provide soils reports, groundwater studies, hydrologic analyses, appropriate BMP's and BMP maintenance plans to demonstrate the ability to meet this code. Additionally all stormwater site plans shall maintain or create a minimum of 30% "Natural Forested Condition" upon the site per BMC 16.80.

Any BMP or combination of BMP's will be considered that would lead to successful management of flow and phosphorus transport. Developed flow shall meet the forested flow duration standards of this code and the DOE Manual. phosphorus transport through surface flow or interflow shall be limited to an average discharge of not more than 0.15 lbs/acre/yr. Possible BMP's include but are not limited to:

i. Full Infiltration of up to the 100 year storm (with analysis of soil suitability).

ii. Full Infiltration of up to the 2 year storm (with analysis of soil suitability) and overflow connection to an adequately sized piped conveyance system that drains directly to lake Whatcom. Any open channel elements must be found capable of conveying the 100 year storm without erosion potential.

iii. On site water storage with reuse within a building (subject to State Water Law).

iv. Forest Retention or Creation (with long term maintenance and retention agreements).

v. Filtration of up to the 10 year storm (allowance dependent on long term phosphorus removal ability).

vi. Forest Retention or Creation (that portion of any site that meets the "natural forested conditions" contained within BMC 16.80 shall be considered to have met the predeveloped forested condition of this code and the DOE Manual).

vii. Soil Remediation (with soils analysis).

OR

b. If the total site has retained or will create 75% or more of forested/native vegetation on the site, and will contain less than 2,000 sf of impervious surface, and will not exceed 20% impervious surface on the site, and will contain less than 1,000 sf of partially pervious surface, and will not exceed 10% partially pervious surface, and the total of impervious surface and partial pervious surface will not exceed 25% of the total site area nor will it exceed 2,500 sf of those combined surfaces.

And if the impervious surfaces developed under this method are mitigated to the standard provided for by an On Site Water Management Plan as provided for in 15.42 in the Manual.

Then, the total site development shall be considered to comply fully with stormwater standards.

OR

c. If the site redevelopment creates or replaces more than 120 sf and less than 300 sf of impervious surface or partially pervious surfaces and the addition of said surfaces does not cause the property to be in noncompliance with the limits on such surfaces through BMC 16.80, the site shall provide mitigation for stormwater and phosphorus transport at the rate of 10 sf of mitigation for every one sf of creation or replacement of these surfaces.

Impervious surfaces developed under this method shall at a minimum be mitigated to the standard provided for by an On Site Water Management Plan as provided in the Manual.

Mitigation of partially pervious surfaces shall at a minimum be by way of lawn removal.

OR

d. Construction of phosphorus limiting or flow limiting facilities only on a property may be permitted outright with a no fee stormwater permit upon review of the facilities for compliance with Lake Whatcom Management standards.

1. For road-related projects, except for projects that drain to Basin One of Lake Whatcom, runoff from the new, replaced and/or new plus replaced impervious surfaces and converted pervious surfaces (including pavement, shoulders, curbs, and sidewalks) shall meet all of the Minimum Requirements listed in BMC Section 15.42.060 F, subject to the modification to Minimum Requirement #7 below. The project limits shall be defined by the length of the project and the width of the right–of-way.

2. For the replaced surfaces for road-related projects, Minimum Requirement #7 is modified as follows:

Replaced surfaces may be allowed to be mitigated differently than new surfaces. When required to comply with the forested standard of Minimum Requirement #7 in BMC Section 15.42.060 F only 50% of the replaced surfaces must be mitigated to comply with the forested standard. The remaining 50% of the replaced surfaces may either be considered in the forested condition or if desirous, may be considered in the condition existing as of September 1, 1995 as may be determined using aerial photography or other means acceptable to the City.

3. Other types of projects, except for projects that drain to Basin One of Lake Whatcom, shall comply with all of the Minimum Requirements detailed in BMC 15.42.060 F for the new and replaced impervious surfaces if the total of new plus replaced impervious surfaces is 5,000 sf or more, and/or the valuation of proposed improvements – including interior improvements – exceeds 50% of the assessed value of the existing site improvements. With the exception that, for those projects that meet the definition of redevelopment, only 50% of replaced surfaces shall be required to comply with the forested standard of Minimum Requirement #7 in BMC Section 15.42.060 F. The remaining 50% of the replaced surfaces may either be considered in the forested condition or if desirous, may be considered in the condition existing as of September 1, 1995 as may be determined using aerial photography or other means acceptable to the City.

4. Underground utility projects that replace the ground surface with in-kind material or materials with similar runoff characteristics are not subject to redevelopment requirements except construction site erosion control.

1. Minimum Requirement #1 - Preparation of Stormwater Site Plans (“SSP”): All projects meeting the thresholds in BMC Sections 15.42.060(1) OR 15.42.060(2) shall prepare a Stormwater Site Plan (“SSP”) for the City’s review. SSP’s shall be prepared in accordance with the current editions of the “Ecology Manual”, the City of Bellingham “Development Guidelines and Improvement Standards” and this BMC. This SSP may be incorporated with building, grading or clearing plan sets as applicable. Those projects that are subject to Minimum Requirements #1 through #10 shall include an engineering report that addresses all Elements and Minimum Requirements of the project’s stormwater management along with an analysis that supports the SSP and the Construction Stormwater Pollution Prevention Plan (“Construction SWPPP”). Project facilities that are required by state law to be designed by a professional engineer must also be certified by the engineer of record that facilities have been constructed to design specifications. This shall be accomplished by providing a certified as built of the facility/ies.

2. Minimum Requirement #2 - Construction Stormwater Pollution Prevention Plan (“Construction SWPPP”):

a. All new development and redevelopment shall comply with Construction SWPPP Elements #1 through #12 as described in this section below.

b. Projects outside the Lake Whatcom Watershed in which the new, replaced, or new plus replaced impervious surfaces total 2,000 square feet or more, or disturb 5,000 square feet or more of land must prepare a Construction SWPPP as part of the Stormwater Site Plan [see BMC 15.42.060(6)(A)]. Each of the twelve elements must be considered and included in a Construction SWPPP unless site conditions render the element unnecessary and the exemption from that element is clearly justified in the narrative of the SWPPP. The SWPPP shall include, at a minimum, the narrative, the Stormwater Site Plan and copies of Best Management Practice detail sheets that will be utilized as a part of the SWPPP.

c. Projects outside the Lake Whatcom Watershed that add or replace less than 2,000 square feet of impervious surface and disturb less than 5,000 square feet of land are not required to prepare a Construction SWPPP. They must, however, consider all of the twelve Elements of Construction Stormwater Pollution Prevention detailed below and propose controls for all Elements that pertain to the project site within the Stormwater Site Plan.

d. Those projects, that are within the Lake Whatcom Watershed, that create or add, 200 square feet or greater, of new, replaced or new plus replaced impervious surface area or that disturb more than 500 square feet of land shall provide a Construction SWPP Plan and a Stormwater Site Plan as described above.

e. Elements of Construction Stormwater Pollution Prevention:

i. Element 1: Mark Clearing Limits: Prior to beginning land disturbing activities, including clearing and grading, all clearing limits, sensitive areas and their buffers, and trees that are to be preserved within the construction area should be clearly marked, both in the field and on the plans, to prevent damage and offsite impacts. Plastic, metal, or stake wire fence may be used to mark the clearing limits.

ii. Element 2: Establish Construction Access:

(a) Construction vehicle access and exit shall be limited to one route if possible.

(b) Access points shall be stabilized with quarry spall or crushed rock to minimize the tracking of sediment onto public roads.

(c) Wheel wash or tire baths should be located on-site, if applicable.

(d) Public roads shall at a minimum be cleaned thoroughly at the end of each day. Sediment shall be removed from roads by shoveling or pickup sweeping and shall be transported to a controlled sediment disposal area. Street washing will be allowed only after sediment is removed in this manner.

(e) Street wash wastewater shall be controlled by pumping back on-site, or otherwise be prevented from discharging into systems tributary to state surface waters.

iii. Element 3: Control Flow Rates:

(a) Properties and waterways downstream from development sites shall be protected from erosion due to increases in the volume, velocity, and peak flow rate of stormwater runoff from the project site. Properties subject to Minimum Requirement # 5 and/or #7 shall implement controls as early in the development as is practicable to mitigate for flow rates.

(b) Downstream analysis is necessary if changes in flows could impair or alter conveyance systems, stream banks, bed sediment or aquatic habitat. See the Ecology Manual for offsite analysis guidance.

(c) Where necessary to comply with Minimum Requirement #7, Stormwater retention/detention facilities shall be constructed as one of the first steps in grading. Detention facilities shall be functional prior to construction of site improvements (e.g. impervious surfaces).

(d.) If permanent infiltration ponds are used for flow control during construction, these facilities should be protected from siltation during the construction phase.

iv. Element 4: Install Sediment Controls

(a) The duff layer, native topsoil, and natural vegetation shall be retained in an undisturbed state to the maximum extent practicable.

(b) Prior to leaving a construction site, or prior to discharge to an infiltration facility, stormwater runoff from disturbed areas shall pass through a sediment pond or other appropriate sediment removal BMP. Runoff from fully stabilized areas may be discharged without a sediment removal BMP, but must meet the flow control performance standard of Element 3(i) above. Full stabilization means concrete or asphalt paving; quarry spalls used as ditch lining; or the use of rolled erosion products, a bonded fiber matrix product, or vegetative cover in a manner that will fully prevent soil erosion. Sediment ponds, vegetated buffer strips, sediment barriers or filters, dikes, and other BMPs intended to trap sediment on-site shall be constructed as one of the first steps in grading. These BMPs shall be functional before other land disturbing activities take place.

(c) Earthen structures such as dams, dikes, and diversions shall be seeded and mulched according to the timing indicated in Element 5 below.

v. Element 5: Stabilize Soils

(a) All exposed and unworked soils shall be stabilized by application of effective BMPs that protect the soil from the erosive forces of raindrop impact and flowing water, and wind erosion.

(b) From October 1 through April 30 of each year, no soils shall remain exposed and unworked for more than 2 days. From May 1 to September 30 of each year, no soils shall remain exposed and unworked for more than 7 days. This condition applies to all soils on site, whether at final grade or not.

(c) Applicable practices include, but are not limited to, temporary and permanent seeding, sodding, mulching, plastic covering, soil application of polyacrylamide (PAM), early application of gravel base on areas to be paved, and dust control.

(d) Soil stabilization measures selected should be appropriate for the time of year, site conditions, estimated duration of use, and potential water quality impacts that stabilization agents may have on downstream waters or ground water.

(e) Soil stockpiles must be stabilized and protected with sediment trapping measures.

(f) Work on linear construction sites and activities, including right-of-way and easement clearing, roadway development, pipelines, and trenching for utilities, shall not exceed the capability of the individual contractor for his portion of the project to install the bedding materials, roadbeds, structures, pipelines, and/or utilities, and to re-stabilize the disturbed soils, meeting the timing conditions listed above.

(g) In addition, at the discretion of the Public Works Director those sites unable to maintain the quality of their stormwater discharge may be required to provide soil stabilization to all exposed soil areas regardless of the working status of the area. Upon written notification, the property owner shall provide full stabilization of all exposed soil areas within 24 hours.

vi. Element 6: Protect Slopes

a. Cut and fill slopes shall be designed and constructed in a manner that will minimize erosion.

b. Consider soil type and its potential for erosion.

c. Reduce slope runoff velocities by reducing the continuous length of slope with terracing and diversions, reduce slope steepness, and roughen slope surface.

d. Divert upslope drainage and run-on waters from off-site with interceptors at top of slope. Off-site stormwater should be handled separately from stormwater generated on the site. Diversion of off-site stormwater around the site may be a viable option. Diverted flows shall be redirected to the natural drainage location at or before the property boundary.

e. Contain down slope collected flows in pipes, slope drains, or protected channels.

f. Provide drainage to remove ground water intersecting the slope surface of exposed soil areas.

g. Excavated material shall be placed on the uphill side of trenches, consistent with safety and space considerations.

h. Check dams shall be placed at regular intervals within trenches that are cut down a slope.

i. Stabilize soils on slopes, as specified in Element #5.

vii. Element 7: Protect Drain Inlets

(a) All storm drain inlets made operable during construction shall be protected so that stormwater runoff shall not enter the conveyance system without first being filtered or treated to remove sediment.

(b) All approach roads shall be kept clean, and all sediment and street wash water shall not be allowed to enter storm drains without prior and adequate treatment unless treatment is provided before the storm drain discharges to waters of the State.

viii. Element 8: Stabilize Channels and Outlets

(a) All temporary on-site conveyance channels shall be designed, constructed and stabilized to prevent erosion from the expected velocity of flow from a 2 year, 24-hour frequency storm for the developed condition.

(b) Stabilization, including armoring material, adequate to prevent erosion of outlets, adjacent stream banks, slopes and downstream reaches shall be provided at the outlets of all conveyance systems.

ix. Element 9: Control Pollutants

(a) All pollutants, including waste materials and demolition debris, that occur on-site during construction shall be handled and disposed of in a manner that does not cause contamination of stormwater.

(b) Cover, containment, and protection from vandalism shall be provided for all chemicals, liquid products, petroleum products, and non-inert wastes present on the site (see Chapter 173-304 WAC, as currently enacted or hereafter modified, for the definition of inert waste, which is incorporated herein by this reference).

(c) Maintenance and repair of heavy equipment and vehicles involving oil changes, hydraulic system drain down, solvent and de-greasing cleaning operations, fuel tank drain down and removal, and other activities which may result in discharge or spillage of pollutants to the ground or into stormwater runoff must be conducted using spill prevention measures, such as drip pans. Contaminated surfaces shall be cleaned immediately following any discharge or spill incident. Emergency repairs may be performed on-site using temporary plastic placed beneath and, if raining, over the vehicle.

(d) Wheel wash, or tire bath wastewater, shall be discharged to a separate on-site treatment system or to the sanitary sewer.

(e) Application of agricultural chemicals, including fertilizers and pesticides, shall be conducted in a manner and at application rates that will not result in loss of chemical to stormwater runoff. Manufacturers’ recommendations shall be followed for application rates and procedures.

(f) Management of pH-modifying sources shall prevent contamination of runoff and stormwater collected on the site. These sources include, but are not limited to, bulk cement, cement kiln dust, fly ash, new concrete washing and curing waters, waste streams generated from concrete grinding and sawing, exposed aggregate processes, and concrete pumping and mixer washout waters.

x. Element 10: Control De-Watering

(a) All foundation, vault, and trench de-watering water, which has similar characteristics to stormwater runoff at the site, shall be discharged into a controlled conveyance system, prior to discharge to a sediment trap or sediment pond. Channels must be stabilized, as specified in Element #8.

(b) Clean, non-turbid de-watering water, such as well-point ground water, can be discharged to systems tributary to state surface waters, as specified in Element #8, provided the de-watering flow does not cause erosion or flooding of the receiving waters. These clean waters should not be routed through sediment ponds with stormwater.

(c) Highly turbid or otherwise contaminated dewatering water, such as from construction equipment operation, clamshell digging, concrete tremie pour, or work inside a cofferdam, shall be handled separately from stormwater at the site.

(d) Other disposal options, depending on site constraints, may include, by way of example: 1) infiltration, 2) transport off-site in vehicle, such as a vacuum flush truck, for legal disposal in a manner that does not pollute state waters, 3) on-site treatment using chemical treatment or other suitable treatment technologies.

xi. Element 11: Maintain BMPs

(a) All temporary and permanent erosion and sediment control BMPs shall be maintained and repaired as needed to assure continued performance of their intended function. All maintenance and repair shall be conducted in accordance with BMPs.

(b) Sediment control BMPs shall be inspected weekly or after a runoff-producing storm event during the dry season and daily during the wet season. All projects that disturb an area greater than one acre shall have a certified erosion control lead available to the site. This erosion control lead shall be responsible to provide overview of ongoing day to day erosion control requirements. The erosion control lead shall (within 24 hours) report to the City and Department of Ecology any site discharges that exceed state water quality standards that have or are likely to have entered waters of the State.

(c) All temporary erosion and sediment control BMPs shall be removed within 30 days after final site stabilization is achieved or after the temporary BMPs are no longer needed. Trapped sediment shall be removed or stabilized on site. Disturbed soil areas resulting from removal of BMPs or vegetation shall be permanently stabilized.

xii. Element 12: Manage the Project

(a) Phasing of Construction - Development projects shall be phased where feasible in order to prevent, to the maximum extent practicable, the transport of sediment from the development site during construction. Revegetation of exposed areas and maintenance of that vegetation shall be an integral part of the clearing activities for any phase.

(b) When establishing these permitted clearing and grading areas, consideration should be given to minimizing removal of existing trees and minimizing disturbance/compaction of native soils except as needed for building purposes. Permitted clearing and grading areas and any other areas required to preserve critical or sensitive areas, buffers, native growth protection easements, or tree retention areas, shall be delineated on the site plans and the development site.

(c) Coordination with Utilities and Other Contractors - The primary project proponent shall evaluate, with input from utilities and other contractors, the stormwater management requirements for the entire project, including the utilities, when preparing the Construction SWPPP.

(d) Inspection and Monitoring - All BMPs shall be inspected, maintained, and repaired as needed to assure continued performance of their intended function.

(e) For any project disturbing more than one acre, a Certified Professional in Erosion and Sediment Control shall be identified in the Construction SWPPP and shall be on-site or on-call at all times. Certification may be through the Washington State Department of Transportation/Associated General Contractors (WSDOT/AGC) Construction Site Erosion and Sediment Control Certification Program or any equivalent local or national certification and/or training program, in the City’s discretion.

(f) Whenever inspection and/or monitoring reveals that the BMPs identified in the Construction SWPPP are inadequate, due to the actual discharge of or potential to discharge a significant amount of any pollutant, the SWPPP shall be modified, as appropriate, in a timely manner.

(g) Maintenance of the Construction SWPPP - The Construction SWPPP shall be retained on-site. The Construction SWPPP shall be modified whenever there is a significant change in the design, construction, operation, or maintenance of any BMP.

3. Minimum Requirement #3 - Source Control of Pollution: All known, available and reasonable source control BMPs shall be applied to all projects. Source control BMPs shall be selected, designed, and maintained according to the Ecology Manual. Source Controls that are applicable to a project shall be either indicated on the Stormwater Site Plan and/or contained within a stormwater engineering report when such report is required.

4. Minimum Requirement #4 - Preservation of Natural Drainage Systems and Outfalls: Natural drainage patterns shall be maintained, and discharges from the project site shall occur at the natural location, to the maximum extent practicable. The manner by which runoff is discharged from the project site must not cause a significant adverse impact to downstream receiving waters and down gradient properties. Additional information on how to comply with this requirement may be found in the Ecology Manual.

5. Minimum Requirement #5 - On-site Stormwater Management: Projects shall employ On-site Stormwater Management BMPs to infiltrate, disperse, and retain stormwater runoff onsite to the maximum extent feasible without causing flooding or erosion impacts. On-site Stormwater Management BMPs shall be designed and provided in accordance with the Ecology Manual.

6. Minimum Requirement #6 - Runoff Treatment:

a. All projects subject to this minimum requirement shall utilize On-site Stormwater BMPs for the treatment of runoff. Additionally, when the following design thresholds are met or exceeded within a threshold discharge area an engineered water quality facility shall be provided. All runoff treatment facilities and BMPs shall be designed, sized and provided for in accordance with the “Ecology Manual”.

b. Water Quality Design Thresholds

i. Projects in which the total of new and/or replaced effective, pollution-generating impervious surface (PGIS) is 5,000 sf or more in a threshold discharge area of the project, or

ii. Projects in which the total of new and/or replaced pollution-generating pervious surfaces (PGPS) is 3/4 of an acre or more in a threshold discharge area, and from which there is a surface discharge in a natural or man-made conveyance system from the site.

c. Additional Requirements. Direct discharge of untreated stormwater from pollution-generating impervious surfaces to ground water is prohibited, except for the discharge achieved by infiltration or dispersion of runoff from residential sites through use of On-site Stormwater Management BMPs. Projects within Basin One of the Lake Whatcom Watershed shall meet these standards for water quality in addition to those contained in other portions of this code.

a. Applicability: Projects must provide flow control to reduce the impacts of stormwater runoff from impervious surfaces and land cover conversions. All projects subject to this minimum requirement shall utilize On-site Stormwater BMPs for flow control. Additionally, when the following design thresholds are met or exceeded an engineered water quantity facility shall be provided. All water quantity facilities and flow control BMPs shall be designed and provided for in accordance with the Ecology Manual. The thresholds and requirements below apply to projects that discharge stormwater directly or indirectly into a fresh water. Those projects that meet flow control exemption criteria of the Ecology Manual are eligible to apply for modification to these requirements. Exception: The exemption of flow standards for Lake Whatcom shall only be allowed with the written approval of the Public Works Department. The basis of that approval shall be a finding that no appreciable risk of water quality degradation will result from the exemption.

b. Water Quantity Design Thresholds: The following require construction of engineered flow control facilities and/or land use management BMPs to satisfy this chapter and the Ecology Manual:

i. Projects in which the total of new, replaced or new plus replaced effective impervious surfaces are 10,000 sf or more in a threshold discharge area; or

ii. Projects that convert ¾ acres or more of native vegetation to lawn or landscape, or convert 2.5 acres or more of native vegetation to pasture in a threshold discharge area, and from which there is a surface discharge in a natural or man-made conveyance system from the site; or

iii. Projects that, through a combination of new, replaced or new plus replaced effective impervious surfaces and converted pervious surfaces, cause a 0.1 cubic feet per second increase in the 100-year flow frequency from a threshold discharge area as estimated using the Western Washington Hydrology Model or other approved model; or

iv. That portion of any development project in which the above thresholds are not exceeded in a threshold discharge area shall apply Onsite Stormwater Management BMPs in accordance with Minimum Requirement #5.

v. Projects within Basin One of the Lake Whatcom Watershed.

c. Standard Flow Control Methodology: Stormwater discharges shall match developed discharge durations to predeveloped durations for the range of predeveloped discharge rates from 50% of the 2-year peak flow up to the full 50-year peak flow. The pre-developed condition to be matched shall be a forested land cover. This standard requirement is waived for sites that will reliably infiltrate all the runoff from impervious surfaces and converted pervious surfaces.

d. Alternate Flow Control Methodology: A modified SCS/SBUH Methodology may be used as an alternate to Department of Ecology Western Washington Hydrology Model if adjustments shown below are utilized and the project area is less than one-acre in size. At such time as the City of Bellingham has a calibrated HSPF model available for use, this alternate flow control allowance will be re-evaluated by the Public Works Director for suspension of the allowance.

i. Adjusted target peak flow standard. Limit the peak rate of runoff from individual development sites to 50% of the pre-developed condition 2-year, 24-hour design storm. Limit the peak rate from the 10-year, 24-hour design storm to the pre-developed condition peak rate from the 2-year, 24-hour design storm. Limit the peak rate from the 100-year, 24-hour design storm to the pre-developed condition peak rate from the 10-year, 24-hour design storm.

ii. Restricted variable assumptions.

(a) The flow path length assumed for sheet flow runoff in the pre-developed condition calculations shall be 300 feet.

(b) The Manning’s effective roughness coefficient for pre-developed forested conditions shall be 0.80.

(c) The curve numbers for the pre-developed conditions shall be selected from the Ecology Manual and shall be fair or good forest. The post developed condition shall also be taken from the Ecology Manual.

8. Minimum Requirement #8 - Wetlands Protection:

a. Applicability: The requirements below apply only to projects whose stormwater discharges into a wetland, either directly or indirectly through a conveyance system. These requirements must be met in addition to meeting Minimum Requirement #6, Runoff Treatment.

b. Thresholds: The thresholds identified in Minimum Requirement #6 – Runoff Treatment, and Minimum Requirement #7 - Flow Control shall also be applied for discharges to wetlands.

c. Standard Requirement: Discharges to wetlands shall maintain the hydrologic conditions, hydrophytic vegetation, and substrate characteristics necessary to support existing and designated uses. A wetland can be considered for hydrologic modification and/or stormwater treatment in accordance with guidance documents from the Department of Ecology.

d. Additional Requirements:

i. The standard requirement does not excuse any discharger from the obligation to apply whatever technology is necessary to comply with state water quality standards, Chapter 173-201A WAC, or state ground water standards, Chapter 173-200 WAC. Additional treatment requirements to meet those standards may be required by federal, state, or local government.

ii. Stormwater treatment and flow control facilities shall not be built within a natural vegetated buffer or wetland, except for:

(a) Necessary conveyance systems as approved by the Directors of the City’s Public Works and Planning Departments or their designees; or

(b) As allowed in wetlands approved for hydrologic modification and/or treatment in accordance with guidance from the Department of Ecology; or

(c) Where full dispersion of flow within a buffer has been approved as a Low Impact Development practice.

iii. An adopted and implemented basin plan (Minimum Requirement #9), or a Total Maximum Daily Load (TMDL, also known as a Water Clean-up Plan) may be used to develop requirements for wetlands that are tailored to a specific basin.

9. Minimum Requirement #9 - Basin/Watershed Planning: Projects may be subject to lesser, equivalent or more stringent minimum requirements for erosion control, source control, treatment, and operation and maintenance, and alternative requirements for flow control and wetlands hydrologic control as identified in Basin/Watershed Plans. Basin/Watershed plans shall evaluate and include, as necessary, retrofitting urban stormwater BMPs into existing development and/or redevelopment in order to achieve watershed-wide pollutant reduction and flow control goals that are consistent with requirements of the federal Clean Water Act. Standards developed from basin plans shall not modify any of the above minimum requirements until the basin plan is formally adopted and implemented by the City and other local governments within the basin, and approved or concurred with by the Department of Ecology.

10. Minimum Requirement #10 - Operation and Maintenance: An operation and maintenance manual that is consistent with the provisions within the Ecology Manual shall be provided for all proposed stormwater facilities and BMPs, and the party (or parties) responsible for maintenance and operation shall be identified. At private facilities, a copy of the manual shall be retained onsite or within reasonable access to the site, and shall be transferred with the property to the new owner. For public facilities, a copy of the manual shall be retained in the appropriate department. A log of maintenance activity that indicates what actions were taken shall be kept and be available for inspection by the City or Ecology.