chapter 2 roof downspout controls
TRANSCRIPT
Volume 3.bookVolume 3 3- 6 Chapter 2
Chapter 2 Roof Downspout Controls
2.1 Description This section presents the criteria for design and implementation of roof downspout controls. Roof downspout controls are simple pre-engineered designs for infiltrating and/or dispersing runoff from roof areas for the purposes of increasing opportunities for groundwater recharge and reduction of runoff volumes from new development or redevelopment. This chapter may also be applicable for onsite management of other impervious surfaces, as allowed by Environmental Services.
This chapter provides guidance for roof downspout controls for meeting the intent of Minimum Requirement #5, Onsite Stormwater Management. It may be possible to use this guidance for meeting the intent of Minimum Requirement #7 as well. Environmental Services may require additional soils or site information for larger projects when using the guidance in this chapter.
Roof downspout controls are used in conjunction with, and in addition to, any additional flow control facilities that may be necessary to mitigate stormwater impacts from the overall development. Implementation of roof downspout controls may reduce the total effective impervious area and result in less runoff from these surfaces. Flow credits for implementing infiltration and dispersion for controls are available as follows:
• If all the roof runoff is infiltrated according to the requirements of this section, the roof area may be discounted from the total project area used for determining flow control thresholds and sizing stormwater facilities.
• If roof runoff is dispersed according to the requirements of this section on lots greater than 22,000 square feet and the vegetative flow path1 is 50 feet or longer through undisturbed native landscape or lawn/landscape area that meets BMP L613, the roof area may be modeled as grassed surface.
Additional information on flow credits is available in Volume 6, Chapter 2.
NOTE: Flow credits only apply to flow control thresholds. Flow credits do not apply to water quality thresholds.
2.2 Selection of Roof Downspout Controls Per Minimum Requirement #5, Onsite Stormwater Management, all projects shall employ roof downspout controls to infiltrate, disperse and retain stormwater onsite to the maximum extent practicable. The applicant is required to control downspouts in the following order of preference:
• Rain gardens (Section 2.4)
• Downspout infiltration systems (Section 2.6)
• Downspout dispersion systems (Section 2.7) - only allowed if it can be demonstrated that infiltration is not feasible.
• Collect and convey to the City system (Section 2.8) - only allowed if it can be demonstrated that infiltration and dispersion are not feasible.
1.Vegetative flow path is measured from the downspout or dispersion system discharge point to the downstream property line, stream, wetland, or other impervious surface.
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Drywells are generally prohibited within the City of Tacoma. Contact Environmental Services if a drywell is proposed.
2.3 Roof Downspout Infiltration Setbacks and Site Controls Setback requirements are generally required by the City, uniform building code requirements, the Tacoma-Pierce County Health Department, or other state regulation. Where a conflict between setbacks occurs, the City shall require compliance with the most stringent of the setback requirements from the various codes/regulations. The following are the minimum setbacks required per this manual.
• At least 100 feet from drinking water wells and springs used for public water supplies. Infiltration facilities upgradient of drinking water wells and within 1, 5 and 10-year time of travel zones must comply with the Health Department requirements (Washington Wellhead Protection Program, DOH, Publication #331-018).
• At least 10 feet from any building structure and at least 5 feet from any other structure or property line unless approved in writing by Environmental Services.
• Rain gardens designed for infiltration shall not be built on slopes steeper than 20%. A geotechnical analysis and report shall be required on slopes over 15% or if located within 200 feet of the top of steep slopes (40% or greater) or a landslide hazard area. More stringent setbacks may be required based upon Tacoma Municipal Code.
• At least 10 feet from septic tank and septic drainfields. Additional setbacks from DOH Publication #333-117, “Onsite Sewage Systems”, Chapter 246-272A WAC may apply. Shall not be located upstream of residential septic systems unless topography or a hydrology analysis clearly indicates that subsurface flows will not impact the drainfield.
• Environmental Services may require additional setbacks or analysis for infiltration facilities proposed to be sited within the influence of known contaminated sites or abandoned landfills.
2.4 Procedure for Evaluating Feasibility of Downspout Infiltration
Downspout infiltration is considered feasible if a rain garden or infiltration trench designed to meet the minimum design criteria specified in Section 2.5 or 2.6 can fit within the proposed project site and meet the setback site constraint requirements described in Section 2.3.
To determine if downspout infiltration is feasible, a site-specific soils report may be required. To determine if a soils report is required, evaluate if the minimum rain garden size from Table 3 - 4 or the minimum trench length from Table 3 - 5 can fit on the project site while maintaining the design criteria of Section 2.5.2 or Section 2.6.1.1. If the rain garden or infiltration trench can fit on the site and maintain the design criteria, a site-specific soils report is required.
For single lots the soils report must, at a minimum:
• Be prepared by a professional soils scientist certified by the Soil Science Society of America (or an equivalent national program), a locally licensed onsite sewage designer, other suitably-trained professional engineer, geologist, hydrogeologist, or engineering geologist registered in the State of Washington, or persons working under the supervision of one of the soils professionals listed here;
• Contain at least one soils log a minimum of 4 feet deep (from proposed grade) taken at the location of the proposed infiltration system;
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• Identify the SCS series of the soil and the USDA textural class of the soil horizon through the depth of the log;
• Note any evidence of high groundwater level, such as mottling; and
• Note the depth to groundwater.
The design of the rain garden or infiltration trench shall be based upon the soil located at the proposed bottom of the rain garden or infiltration facility. If multiple soils logs are conducted, the least infiltrative soil found at the proposed bottom of the infiltration facility shall be used for design.
NOTE: On sites where soils are insufficient for infiltration based on the soils report and available land to construct facilities and meet setback requirements, a downspot dispersion system per Section 2.7 may be feasible in lieu of infiltration.
2.5 Roof Downspout Rain Gardens
2.5.1 Purpose and Definition Bioretention areas are shallow stormwater retention facilities designed to mimic forested systems by controlling stormwater through detention, infiltration, and evapotranspiration.
2.5.2 Design Criteria 2.5.2.1 Groundwater Separation A minimum of 18 inches of separation is required between the lowest elevation of the bioretention soil or any underlying gravel infiltration layer and the seasonal high groundwater elevation or other impermeable layer.
2.5.2.2 Flow Entrance/Presetting • Flow velocity entering the facility shall be less than 1 ft/sec for the 100-year, 24-hour
storm event. If 1 ft/sec cannot be obtained, Environmental Services may approve the discharge with the addition of a designed flow dispersion or energy dissipation device.
• Use one of the following types of flow entrances (other alternatives may be considered on a case-by-case basis):
Dispersed, low velocity flow across a grade or landscape area. A minimum 2-inch grade change between the edge of the contributing area and flow entrance is required.
Pipe flow entrance. The inlet pipe invert elevation shall be higher than the overflow elevation. Place rock or other erosion protection material at the facility entrance to dissipate energy and/or provide flow dispersion.
• Environmental Services may require flow dispersion for rain gardens depending upon type of inlet design and size of the facility.
• Do not place plants directly in the entrance flow path as they can restrict or concentrate flows.
• Install flow diversion and erosion control measures to protect the bioretention area from sedimentation until the upstream area is stabilized.
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• Based upon project site constraints, Environmental Services may require a presettling facility.
2.5.2.3 Cell Ponding Area • The ponding depth shall be 12 inches.
• The surface pool drawdown time shall be less than 24 hours. If the rain garden is sized per Table 3 - 4, this requirement is met.
• Bioretention sizing is based on bottom area. Once bottom area is determined, top area shall be based upon side slopes and total depth of facility.
• The minimum freeboard measured from the maximum ponding water surface elevation to the top of the facility shall be 2” for drainage areas less than 1,000 square feet and 6” for drainage areas 1,000 square feet or greater.
• If berming is used to achieve the minimum top elevation, maximum slope on berm shall be 3H:1V, and minimum top width of design berm shall be 1 foot. Soil for berming shall be imported bioretention soil or amended native soil compacted to a minimum of 90% dry density.
2.5.2.4 Overflow • Unless designed for full infiltration of the entire runoff volume, bioretention systems must
include an overflow. Rain gardens sized using Table 3 - 4 are designed for full infiltration and are not required to have an overflow.
• An emergency overflow pathway shall be provided for all facilities to ensure that all potential overflows are directed into the downstream conveyance system or the public right of way.
2.5.2.5 Bioretention Soil Mix The bioretention soil mix (BSM) shall:
• Have a 60% aggregate component as represented in Table 3 - 2 below and a 40% compost component as represented in Table 3 - 3 below.
Table 3 - 2: Bioretention Soil Mix Aggregate Component Gradation
Aggregate Component (60% by Volume)
Sieve Size Percent Passing
3/8” 100
#4 95-100
#10 75-90
#40 25-40
#100 4-10
#200 2-5
• The compost component shall be stable, mature, and derived from organic waste materials including yard debris, wood wastes or other organic matter. Compost must meet the Washington State compost regulations in WAC 173-350, which is available at http://www.ecy.wa.gov/programs/swfa/compost and meet the the following:
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Have a compost component meeting the size requirements in Table 3 - 3
Table 3 - 3: Bioretention Soil Mix Compost Component Gradation
Compost Component
Have a manufactured inert material (plastic, concrete, ceramics, metal, etc.) less than 1.0% by weight
Have a organic matter content between 45 and 65 percent dry weight bases as determined by TMECC 05.07A, “Loss-On-Ignition Organic Matter Method”.
Have a soluble salt content less than 6.0 mmhos/cm tested in accordance with TMECC 04.10-A, “1.5 Slurry Method, Mass Basis”.
Have a maturity greater than 80% in accordance with TMECC 05.05-A, “Germination and Vigor”.
Have a stability at 7 or below in accordance with TMECC 05-08.B, “Carbon Dioxide Evolution Rate”.
Have a compost product that contains a minimum of 65% and up to 100% by volume recycled plant waste as defined in WAC 173-350-100 as “Type 1 Feedstocks”. A maximum of 35% by volume of other approved organic waste as defined in WAC 173- 350-100 as “Type III”, including post-consumer food waste but not including biosolids, may be substituted for recycled plant waste.
Have a carbon to nitrogen ratio of less than 25:1 as determined using TMECC 04.01 “Total Carbon” and TMECC 04.02D “Total Kjeldhal Nitrogen”. A ratio up to 35:1 can be used if all plants are native species.
The Washington State Department of Ecology keeps a list of approved composting facilities in Washington at:http://www.ecy.wa.gov/programs/swfa/compost/
• Minimum depth of bioretention soil mix must be 18 inches.
• Contact Environmental Services for assistance in verifying bioretention soil mix suitability.
2.5.2.6 Planting • Plants must be tolerant of summer drought, ponding fluctuations, and saturated soil
conditions.
• Consider rooting depth when choosing plants. Roots must not damage underground infrastructure.
• Consider adjacent plant communities and avoid potential invasive species.
• Consider aesthetics, rain gardens should blend into surrounding landscapes.
• “Low Impact Development: Technical Guidance Manual for Puget Sound” is a good tool for selecting proper bioretention plants.
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• Submit a planting plan showing the type, location and size of each plant.
• Irrigation may be required until plants are fully established and in the summer months.
2.5.2.7 Mulch Layer Bioretention facilities should be designed with a mulch layer. Properly selected mulch material reduces weed establishment, regulates soil temperatures and moisture, and adds organic matter to the soil.
• Mulch should be free of weed seeds, soil, roots, and other material that is not trunk or branch wood and bark. Mulch shall not include grass clippings, mineral aggregate or pure bark.
• Mulch shall be:
Compost in the bottom of the facilities, depth 3 inches
Wood chip mulch composed of shredded or chipped hardwood or softwood on side slopes, depth 4 inches.
• A dense groundcover can be used as an alternative to mulch although mulch shall be required until the dense groundcover is established.
2.5.3 Sizing Table 3 - 4 shall be used for sizing facilities managing roof runoff to meet Minimum Requirement #5, Onsite Stormwater Management only. See Volume 6, BMP L630 for sizing facilities for projects that must comply with Minimum Requirement #6, Runoff Treatment and/or Minimum Requirement #7, Flow Control.
Table 3 - 4 gives the square footage of the bottom of the rain garden per 1000 square feet of roof area.
Table 3 - 4: Sizing Table for Rain Gardens
Soil Type Rain Garden Bottom
(square feet)
Medium sands 50
Sandy loam 50
Loam 150
For sites with soils other than given in Table 3 - 4 or sites proposing an underdrain, an engineered design will be required. Refer to Volume 6, BMP L630 for sizing guidance.
2.5.3.1 Flow Credit If roof runoff is infiltrated according to the requirements of this section, the roof area may be deducted from the project area used for determining flow control thresholds and sizing stormwater facilities for flow control only. The deduction cannot be applied to pollutant- generating surfaces that require water quality treatment. If the thresholds for water quality treatment are met, water quality is required. A Stormwater Site Plan and Construction Stormwater Pollution Prevention Plan are still required.
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2.5.4 General Construction Criteria • Do not install or excavate during soil saturation periods.
• Excavation and soil placement should be done from equipment operating adjacent to the facility – no heavy equipment should be operated in the facility if possible.
• If equipment must be operated within the facility, use lightweight, low ground pressure equipment and scarify the base to reduce compaction upon completion.
• Do not use fully excavated bioretention facilities for erosion and sedimentation control during construction;
Consider partial excavation of bioretention cells prior to construction (to within 12 inches above finished bottom grade) for use as temporary stormwater detention, if necessary.
• Clogged soil and silt shall be removed during excavation to finished bottom grade prior to installing bioretention cell profile.
• Scarify sides and bottom to roughen where heavy equipment may have compacted soil.
• Ensure the bioretention facility is protected from erosion and sedimentation until all contributory areas are fully stabilized.
• If sedimentation occurs within the bioretention facility, excavate the area a minimum of 12 inches below final grade to remove sediment.
2.5.5 Maintenance Criteria Per Minimum Requirement #10, an operation and maintenance plan shall be prepared for all stormwater management facilities. See Volume 1, Appendix D, Maintenance Checklist #22 for specific maintenance requirements for rain gardens. Maintenance shall be a basic consideration in design and cost-determination of the stormwater management facility.
Any standing water removed during the maintenance operation must be disposed of in a City approved manner. See the dewatering requirements in Volume 4 of this manual. Pretreatment may be necessary. Solids must be disposed of in accordance with state and local waste regulations.
Facilities shall be constructed such that the facility can be easily inspected by one person. This may require the construction of additional inspection ports or access manholes.
2.6 Downspout Infiltration Systems Downspout infiltration systems are trenches designed for flow control and/or complying with Minimum Requirement #5 and are intended only for use in infiltrating runoff from roof downspout drains. They are not designed to directly infiltrate runoff from pollutant-generating impervious surfaces. Volume 5, Chapter 5 contains a discussion of infiltration trenches for water quality treatment.
2.6.1 Flow Credit for Roof Downspout Infiltration If roof runoff is infiltrated according to the requirements of this section, the roof area may be discounted from the project area used for determining flow control thresholds and sizing flow control facilities. The deduction cannot be applied to pollutant-generating surfaces. If the thresholds for water quality treatment are met, water quality treatment is required. A Stormwater Site Plan and Construction Stormwater Pollution Prevention Plan are still required.
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2.6.1.1 Design Criteria for Infiltration Trenches Figure 3 - 1 shows a typical downspout infiltration trench system, and Figure 3 - 2 presents an alternative infiltration trench system for sites with coarse sand and cobble soils. Applicants may use either of these designs in conjunction with the soils table below or may model the site separately. Modeling must be conducted by a licensed engineer in the State of Washington.
General
1. The following minimum lengths (in linear feet [LF]) per 1,000 square feet of roof area based on soil type may be used for sizing downspout infiltration trenches. For soil types other than those presented in Table 3 - 5, additional geotechnical information and/or engineering analysis may be required by Environmental Services.
Table 3 - 5: Downspout Infiltration Length for USDA Soil Type
USDA Soil Type Trench Length (linear feet)
Coarser 20
Sand 85
Silt Loam 270
2. Maximum length of trench must not exceed 100 feet from the inlet sump.
3. Minimum spacing between trenches shall be 4 feet measured from the edge of trench.
4. Non-woven geotextile fabric shall be placed around the walls, bottom and top of the trench aggregate. A 6-inch minimum layer of sand may be used as a filter media at the bottom of the trench instead of the geotextile. Volume 5, Appendix C contains specifications for geotextile fabric.
5. A minimum of three feet of separation is required from the proposed final grade to the seasonal high groundwater table.
6. A minimum of 1 foot of separation is required from the bottom of the infiltration trench to the seasonal high groundwater table.
7. Infiltration trenches may be placed in fill material if the fill is placed and compacted under the direct supervision of a geotechnical engineer or professional civil engineer with geotechnical expertise, and if the measured infiltration rate is at least 8 inches per hour. Trench length in fill must be 60 linear feet per 1,000 square feet of roof area. Infiltration rates can be tested using the methods described in Section 6.4.3.
8. Trenches may be located under pavement if a small yard drain or catch basin with grate cover is placed at the end of the trench pipe such that overflow would occur out of the catch basin at an elevation at least one foot below that of the pavement, and in a location which can accommodate the overflow without creating a significant adverse impact to downhill properties or drainage systems. This is intended to prevent saturation of the pavement in the event of system failure.
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2.6.1.2 Maintenance Criteria Per Minimum Requirement #10, an operation and maintenance plan shall be prepared for all stormwater management facilities. See Volume 1, Appendix D, Maintenance Checklist #3 for specific maintenance requirements for infiltration trenches. Maintenance shall be a basic consideration in design and cost-determination of the stormwater management facility.
Any standing water removed during the maintenance operation must be disposed of in a City approved manner. See the dewatering requirements in Volume 4 of this manual. Pretreatment may be necessary. Solids must be disposed of in accordance with state and local waste regulations.
Facilities shall be constructed such that the facility can be easily inspected by one person. This may require the construction of additional inspection ports or access manholes.
10' MINIMUM
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Figure 3 - 1. Typical Downspout Infiltration Trench
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Figure 3 - 2. Alternative Infiltration Trench System for Coarse Sand and Gravel
Simple 10-foot trench per Trench X-Section above
Maximum 50-foot trench w/notch board per Figure 3-4
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Figure 3 - 3. Typical Downspout Dispersion Trench
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Figure 3 - 4. Standard Dispersion Trench with Notched Grade Board
2.7 Downspout Dispersion Systems Downspout dispersion systems are splash blocks or dispersion facilities that spread roof runoff over vegetated pervious areas. Dispersion attenuates peak flows by slowing entry of the runoff into the conveyance system, allowing for some infiltration, and providing some water quality benefits.
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2.7.1 Application Downspout dispersion may be used on all sites that cannot infiltrate roof runoff and that meet the feasibility and setback requirements provided in the general design criteria below.
2.7.2 Flow Credit for Roof Downspout Dispersion If roof runoff is dispersed according to the requirements of this section, and the vegetative flow
path1 of the roof runoff is 25 feet or greater through undisturbed native landscape or lawn/ landscape area that meets BMP L613, the roof area may be modeled as a grassed surface for determining thresholds and sizing flow control devices. The deduction cannot be applied to pollutant-generating surfaces. If the thresholds for water quality treatment are met, water quality treatment is required.
2.7.3 General Design Criteria • No erosion or flooding of downstream properties shall result.
• Downspout dispersion trenches designed as shown in Figure 3 - 3 or Figure 3 - 4 should be used for all downspout dispersion applications except where splash blocks are allowed.
• Perforated stub-out connections shall not be used.
• For sites with septic systems, the discharge point of all dispersion systems must be downgradient of the drainfield. This requirement may be waived if site topography clearly prohibits flows from intersecting the drainfield.
• For sites with septic systems, the discharge point must be downslope of the primary and reserve drainfield areas. This requirement may be waived if site topography clearly prohibits flows from intersecting the drainfield or where site conditions (soil permeability, distance between systems, etc) indicate that this is unnecessary.
• Place all dispersion systems at least 5 feet from any property line. If necessary, setbacks shall be increased from the minimum 5 feet in order to maintain a 1:1 side slope for future excavation and maintenance.
• Setback dispersion systems from sensitive areas, steep slopes, landslide hazard areas, and erosion hazard areas as goverened by the Tacoma Municipal Code.
• All dispersions systems shall be at least 10 feet from any structure. If necessary, setbacks shall be increased from the minimum 10 feet in order to maintain a 1H:1V side slope for future excavation and maintenance.
• For sites with multiple dispersion systems, a minimum separation of 10 feet is required between flowpaths. Environmental Services may require a larger separation based upon site conditions such as slope, soil type and total contributing area.
• Runoff discharged towards landslide hazard areas must be evaluated by a geotechnical engineer or a licensed geologist, hydrogeologist, or engineering geologist. The discharge point shall not be placed on or above slopes greater than 20% (5H:1V) or above erosion hazard areas without evaluation by a geotechnical engineer or qualified geologist and City approval.
1.Vegetative flow path is measured from the downspout or dispersion system discharge point to the downstream property line, stream, wetland, or other impervious surface.
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Design Criteria for Dispersion Trenches
• Design dispersion trenches as shown in Figure 3 - 3 and Figure 3 - 4.
• A vegetated flowpath of at least 25 feet in length must be maintained between the outlet of the trench and any property line, structure, stream, wetland, or impervious surface. A vegetated flowpath of at least 50 feet in length must be maintained between the outlet of the trench and any slope, 20% or greater. Sensitive area buffers may count towards flowpath lengths if approved by the City of Tacoma.
• Trenches serving up to 700 square feet of roof area may be simple 10-foot-long by 2-foot wide gravel filled trenches as shown in Figure 3 - 3. For roof areas larger than 700 square feet, a dispersion trench with notched grade board may be used as approved by the City. The total length of this design must not exceed 50 feet and must provide at least 10 feet of trench per 700 square feet of roof area.
Design Criteria for Splashblocks
A typical downspout splashblock is shown in Figure 3 - 5. In general, if the ground is sloped away from the foundation and there is adequate vegetation and area for effective dispersion, splashblocks will adequately disperse storm runoff. If the ground is fairly level, if the structure includes a basement, or if foundation drains are proposed, splashblocks with downspout extensions may be a better choice because the discharge point is moved away from the foundation. Downspout extensions can include piping to a splashblock/discharge point a considerable distance from the downspout, as long as the runoff can travel through a well- vegetated area as described below.
• A vegetated flow path of at least 50 feet shall be maintained between the discharge point and any property line, structure, steep slope, stream, wetland, lake, or other impervious surface. Sensitive area buffers may count toward flow path lengths.
• Flows shall not be directed onto sidewalks.
• A maximum of 700 square feet of roof area may drain to each splashblock.
• A splashblock or a pad of crushed rock (2 feet wide by 3 feet long by 6 inches deep) shall be placed at each downspout discharge point.
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Figure 3 - 5. Typical Downspout Splashblock Dispersion
2.8 Collect and Convey Where it can be demonstrated that infiltration and dispersion are not feasible for roof downspout controls, it may be allowable to collect and convey to the City stormwater system. This may be either the curb, if present, or the actual pipe and structure conveyance system.
Conveyance to the curb will only be allowed if a catch basin is located within 350 feet downstream of the discharge point. If a catch basin is not located within 350 feet of the discharge location, a storm main extension shall be required.
Minimum pipe size for conveyance to the curb shall be 3 inches in diameter. Where capacity greater than a 3 inch diameter pipe is required, Environmental Services shall review the proposal and may require a storm main extension.
NOTE: Environmental Services will only approve on a case-by-case basis those facilities that would require more than one through-curb discharge point.
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For total roof areas 2,000 to 5,000 sf, roof runoff may be allowed to be collected and conveyed to either the curb or directly connected to a structure. The runoff shall not be conveyed over driveways, sidewalks or other areas reserved for pedestrian traffic. A detail for the discharge shall be submitted to Environmental Services for review and approval.
For roof areas between 5,001 sf and 9,999 sf, roof runoff may be allowed to be collected and conveyed to the curb or stormwater structure unless Minimum Requirement #7 applies and requires alternative mitigation. Capacity analysis of the road gutter, conveyance piping and catch basin leads will be required to ensure that adequate capacity exists (see Section 9.6). Environmental Services may require more than one through curb outlet for discharge to the curb.
For roof areas 10,000 sf and greater, please refer to Minimum Requirement #7.
Chapter 2 Roof Downspout Controls
2.1 Description This section presents the criteria for design and implementation of roof downspout controls. Roof downspout controls are simple pre-engineered designs for infiltrating and/or dispersing runoff from roof areas for the purposes of increasing opportunities for groundwater recharge and reduction of runoff volumes from new development or redevelopment. This chapter may also be applicable for onsite management of other impervious surfaces, as allowed by Environmental Services.
This chapter provides guidance for roof downspout controls for meeting the intent of Minimum Requirement #5, Onsite Stormwater Management. It may be possible to use this guidance for meeting the intent of Minimum Requirement #7 as well. Environmental Services may require additional soils or site information for larger projects when using the guidance in this chapter.
Roof downspout controls are used in conjunction with, and in addition to, any additional flow control facilities that may be necessary to mitigate stormwater impacts from the overall development. Implementation of roof downspout controls may reduce the total effective impervious area and result in less runoff from these surfaces. Flow credits for implementing infiltration and dispersion for controls are available as follows:
• If all the roof runoff is infiltrated according to the requirements of this section, the roof area may be discounted from the total project area used for determining flow control thresholds and sizing stormwater facilities.
• If roof runoff is dispersed according to the requirements of this section on lots greater than 22,000 square feet and the vegetative flow path1 is 50 feet or longer through undisturbed native landscape or lawn/landscape area that meets BMP L613, the roof area may be modeled as grassed surface.
Additional information on flow credits is available in Volume 6, Chapter 2.
NOTE: Flow credits only apply to flow control thresholds. Flow credits do not apply to water quality thresholds.
2.2 Selection of Roof Downspout Controls Per Minimum Requirement #5, Onsite Stormwater Management, all projects shall employ roof downspout controls to infiltrate, disperse and retain stormwater onsite to the maximum extent practicable. The applicant is required to control downspouts in the following order of preference:
• Rain gardens (Section 2.4)
• Downspout infiltration systems (Section 2.6)
• Downspout dispersion systems (Section 2.7) - only allowed if it can be demonstrated that infiltration is not feasible.
• Collect and convey to the City system (Section 2.8) - only allowed if it can be demonstrated that infiltration and dispersion are not feasible.
1.Vegetative flow path is measured from the downspout or dispersion system discharge point to the downstream property line, stream, wetland, or other impervious surface.
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Drywells are generally prohibited within the City of Tacoma. Contact Environmental Services if a drywell is proposed.
2.3 Roof Downspout Infiltration Setbacks and Site Controls Setback requirements are generally required by the City, uniform building code requirements, the Tacoma-Pierce County Health Department, or other state regulation. Where a conflict between setbacks occurs, the City shall require compliance with the most stringent of the setback requirements from the various codes/regulations. The following are the minimum setbacks required per this manual.
• At least 100 feet from drinking water wells and springs used for public water supplies. Infiltration facilities upgradient of drinking water wells and within 1, 5 and 10-year time of travel zones must comply with the Health Department requirements (Washington Wellhead Protection Program, DOH, Publication #331-018).
• At least 10 feet from any building structure and at least 5 feet from any other structure or property line unless approved in writing by Environmental Services.
• Rain gardens designed for infiltration shall not be built on slopes steeper than 20%. A geotechnical analysis and report shall be required on slopes over 15% or if located within 200 feet of the top of steep slopes (40% or greater) or a landslide hazard area. More stringent setbacks may be required based upon Tacoma Municipal Code.
• At least 10 feet from septic tank and septic drainfields. Additional setbacks from DOH Publication #333-117, “Onsite Sewage Systems”, Chapter 246-272A WAC may apply. Shall not be located upstream of residential septic systems unless topography or a hydrology analysis clearly indicates that subsurface flows will not impact the drainfield.
• Environmental Services may require additional setbacks or analysis for infiltration facilities proposed to be sited within the influence of known contaminated sites or abandoned landfills.
2.4 Procedure for Evaluating Feasibility of Downspout Infiltration
Downspout infiltration is considered feasible if a rain garden or infiltration trench designed to meet the minimum design criteria specified in Section 2.5 or 2.6 can fit within the proposed project site and meet the setback site constraint requirements described in Section 2.3.
To determine if downspout infiltration is feasible, a site-specific soils report may be required. To determine if a soils report is required, evaluate if the minimum rain garden size from Table 3 - 4 or the minimum trench length from Table 3 - 5 can fit on the project site while maintaining the design criteria of Section 2.5.2 or Section 2.6.1.1. If the rain garden or infiltration trench can fit on the site and maintain the design criteria, a site-specific soils report is required.
For single lots the soils report must, at a minimum:
• Be prepared by a professional soils scientist certified by the Soil Science Society of America (or an equivalent national program), a locally licensed onsite sewage designer, other suitably-trained professional engineer, geologist, hydrogeologist, or engineering geologist registered in the State of Washington, or persons working under the supervision of one of the soils professionals listed here;
• Contain at least one soils log a minimum of 4 feet deep (from proposed grade) taken at the location of the proposed infiltration system;
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• Identify the SCS series of the soil and the USDA textural class of the soil horizon through the depth of the log;
• Note any evidence of high groundwater level, such as mottling; and
• Note the depth to groundwater.
The design of the rain garden or infiltration trench shall be based upon the soil located at the proposed bottom of the rain garden or infiltration facility. If multiple soils logs are conducted, the least infiltrative soil found at the proposed bottom of the infiltration facility shall be used for design.
NOTE: On sites where soils are insufficient for infiltration based on the soils report and available land to construct facilities and meet setback requirements, a downspot dispersion system per Section 2.7 may be feasible in lieu of infiltration.
2.5 Roof Downspout Rain Gardens
2.5.1 Purpose and Definition Bioretention areas are shallow stormwater retention facilities designed to mimic forested systems by controlling stormwater through detention, infiltration, and evapotranspiration.
2.5.2 Design Criteria 2.5.2.1 Groundwater Separation A minimum of 18 inches of separation is required between the lowest elevation of the bioretention soil or any underlying gravel infiltration layer and the seasonal high groundwater elevation or other impermeable layer.
2.5.2.2 Flow Entrance/Presetting • Flow velocity entering the facility shall be less than 1 ft/sec for the 100-year, 24-hour
storm event. If 1 ft/sec cannot be obtained, Environmental Services may approve the discharge with the addition of a designed flow dispersion or energy dissipation device.
• Use one of the following types of flow entrances (other alternatives may be considered on a case-by-case basis):
Dispersed, low velocity flow across a grade or landscape area. A minimum 2-inch grade change between the edge of the contributing area and flow entrance is required.
Pipe flow entrance. The inlet pipe invert elevation shall be higher than the overflow elevation. Place rock or other erosion protection material at the facility entrance to dissipate energy and/or provide flow dispersion.
• Environmental Services may require flow dispersion for rain gardens depending upon type of inlet design and size of the facility.
• Do not place plants directly in the entrance flow path as they can restrict or concentrate flows.
• Install flow diversion and erosion control measures to protect the bioretention area from sedimentation until the upstream area is stabilized.
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• Based upon project site constraints, Environmental Services may require a presettling facility.
2.5.2.3 Cell Ponding Area • The ponding depth shall be 12 inches.
• The surface pool drawdown time shall be less than 24 hours. If the rain garden is sized per Table 3 - 4, this requirement is met.
• Bioretention sizing is based on bottom area. Once bottom area is determined, top area shall be based upon side slopes and total depth of facility.
• The minimum freeboard measured from the maximum ponding water surface elevation to the top of the facility shall be 2” for drainage areas less than 1,000 square feet and 6” for drainage areas 1,000 square feet or greater.
• If berming is used to achieve the minimum top elevation, maximum slope on berm shall be 3H:1V, and minimum top width of design berm shall be 1 foot. Soil for berming shall be imported bioretention soil or amended native soil compacted to a minimum of 90% dry density.
2.5.2.4 Overflow • Unless designed for full infiltration of the entire runoff volume, bioretention systems must
include an overflow. Rain gardens sized using Table 3 - 4 are designed for full infiltration and are not required to have an overflow.
• An emergency overflow pathway shall be provided for all facilities to ensure that all potential overflows are directed into the downstream conveyance system or the public right of way.
2.5.2.5 Bioretention Soil Mix The bioretention soil mix (BSM) shall:
• Have a 60% aggregate component as represented in Table 3 - 2 below and a 40% compost component as represented in Table 3 - 3 below.
Table 3 - 2: Bioretention Soil Mix Aggregate Component Gradation
Aggregate Component (60% by Volume)
Sieve Size Percent Passing
3/8” 100
#4 95-100
#10 75-90
#40 25-40
#100 4-10
#200 2-5
• The compost component shall be stable, mature, and derived from organic waste materials including yard debris, wood wastes or other organic matter. Compost must meet the Washington State compost regulations in WAC 173-350, which is available at http://www.ecy.wa.gov/programs/swfa/compost and meet the the following:
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Have a compost component meeting the size requirements in Table 3 - 3
Table 3 - 3: Bioretention Soil Mix Compost Component Gradation
Compost Component
Have a manufactured inert material (plastic, concrete, ceramics, metal, etc.) less than 1.0% by weight
Have a organic matter content between 45 and 65 percent dry weight bases as determined by TMECC 05.07A, “Loss-On-Ignition Organic Matter Method”.
Have a soluble salt content less than 6.0 mmhos/cm tested in accordance with TMECC 04.10-A, “1.5 Slurry Method, Mass Basis”.
Have a maturity greater than 80% in accordance with TMECC 05.05-A, “Germination and Vigor”.
Have a stability at 7 or below in accordance with TMECC 05-08.B, “Carbon Dioxide Evolution Rate”.
Have a compost product that contains a minimum of 65% and up to 100% by volume recycled plant waste as defined in WAC 173-350-100 as “Type 1 Feedstocks”. A maximum of 35% by volume of other approved organic waste as defined in WAC 173- 350-100 as “Type III”, including post-consumer food waste but not including biosolids, may be substituted for recycled plant waste.
Have a carbon to nitrogen ratio of less than 25:1 as determined using TMECC 04.01 “Total Carbon” and TMECC 04.02D “Total Kjeldhal Nitrogen”. A ratio up to 35:1 can be used if all plants are native species.
The Washington State Department of Ecology keeps a list of approved composting facilities in Washington at:http://www.ecy.wa.gov/programs/swfa/compost/
• Minimum depth of bioretention soil mix must be 18 inches.
• Contact Environmental Services for assistance in verifying bioretention soil mix suitability.
2.5.2.6 Planting • Plants must be tolerant of summer drought, ponding fluctuations, and saturated soil
conditions.
• Consider rooting depth when choosing plants. Roots must not damage underground infrastructure.
• Consider adjacent plant communities and avoid potential invasive species.
• Consider aesthetics, rain gardens should blend into surrounding landscapes.
• “Low Impact Development: Technical Guidance Manual for Puget Sound” is a good tool for selecting proper bioretention plants.
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• Submit a planting plan showing the type, location and size of each plant.
• Irrigation may be required until plants are fully established and in the summer months.
2.5.2.7 Mulch Layer Bioretention facilities should be designed with a mulch layer. Properly selected mulch material reduces weed establishment, regulates soil temperatures and moisture, and adds organic matter to the soil.
• Mulch should be free of weed seeds, soil, roots, and other material that is not trunk or branch wood and bark. Mulch shall not include grass clippings, mineral aggregate or pure bark.
• Mulch shall be:
Compost in the bottom of the facilities, depth 3 inches
Wood chip mulch composed of shredded or chipped hardwood or softwood on side slopes, depth 4 inches.
• A dense groundcover can be used as an alternative to mulch although mulch shall be required until the dense groundcover is established.
2.5.3 Sizing Table 3 - 4 shall be used for sizing facilities managing roof runoff to meet Minimum Requirement #5, Onsite Stormwater Management only. See Volume 6, BMP L630 for sizing facilities for projects that must comply with Minimum Requirement #6, Runoff Treatment and/or Minimum Requirement #7, Flow Control.
Table 3 - 4 gives the square footage of the bottom of the rain garden per 1000 square feet of roof area.
Table 3 - 4: Sizing Table for Rain Gardens
Soil Type Rain Garden Bottom
(square feet)
Medium sands 50
Sandy loam 50
Loam 150
For sites with soils other than given in Table 3 - 4 or sites proposing an underdrain, an engineered design will be required. Refer to Volume 6, BMP L630 for sizing guidance.
2.5.3.1 Flow Credit If roof runoff is infiltrated according to the requirements of this section, the roof area may be deducted from the project area used for determining flow control thresholds and sizing stormwater facilities for flow control only. The deduction cannot be applied to pollutant- generating surfaces that require water quality treatment. If the thresholds for water quality treatment are met, water quality is required. A Stormwater Site Plan and Construction Stormwater Pollution Prevention Plan are still required.
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2.5.4 General Construction Criteria • Do not install or excavate during soil saturation periods.
• Excavation and soil placement should be done from equipment operating adjacent to the facility – no heavy equipment should be operated in the facility if possible.
• If equipment must be operated within the facility, use lightweight, low ground pressure equipment and scarify the base to reduce compaction upon completion.
• Do not use fully excavated bioretention facilities for erosion and sedimentation control during construction;
Consider partial excavation of bioretention cells prior to construction (to within 12 inches above finished bottom grade) for use as temporary stormwater detention, if necessary.
• Clogged soil and silt shall be removed during excavation to finished bottom grade prior to installing bioretention cell profile.
• Scarify sides and bottom to roughen where heavy equipment may have compacted soil.
• Ensure the bioretention facility is protected from erosion and sedimentation until all contributory areas are fully stabilized.
• If sedimentation occurs within the bioretention facility, excavate the area a minimum of 12 inches below final grade to remove sediment.
2.5.5 Maintenance Criteria Per Minimum Requirement #10, an operation and maintenance plan shall be prepared for all stormwater management facilities. See Volume 1, Appendix D, Maintenance Checklist #22 for specific maintenance requirements for rain gardens. Maintenance shall be a basic consideration in design and cost-determination of the stormwater management facility.
Any standing water removed during the maintenance operation must be disposed of in a City approved manner. See the dewatering requirements in Volume 4 of this manual. Pretreatment may be necessary. Solids must be disposed of in accordance with state and local waste regulations.
Facilities shall be constructed such that the facility can be easily inspected by one person. This may require the construction of additional inspection ports or access manholes.
2.6 Downspout Infiltration Systems Downspout infiltration systems are trenches designed for flow control and/or complying with Minimum Requirement #5 and are intended only for use in infiltrating runoff from roof downspout drains. They are not designed to directly infiltrate runoff from pollutant-generating impervious surfaces. Volume 5, Chapter 5 contains a discussion of infiltration trenches for water quality treatment.
2.6.1 Flow Credit for Roof Downspout Infiltration If roof runoff is infiltrated according to the requirements of this section, the roof area may be discounted from the project area used for determining flow control thresholds and sizing flow control facilities. The deduction cannot be applied to pollutant-generating surfaces. If the thresholds for water quality treatment are met, water quality treatment is required. A Stormwater Site Plan and Construction Stormwater Pollution Prevention Plan are still required.
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2.6.1.1 Design Criteria for Infiltration Trenches Figure 3 - 1 shows a typical downspout infiltration trench system, and Figure 3 - 2 presents an alternative infiltration trench system for sites with coarse sand and cobble soils. Applicants may use either of these designs in conjunction with the soils table below or may model the site separately. Modeling must be conducted by a licensed engineer in the State of Washington.
General
1. The following minimum lengths (in linear feet [LF]) per 1,000 square feet of roof area based on soil type may be used for sizing downspout infiltration trenches. For soil types other than those presented in Table 3 - 5, additional geotechnical information and/or engineering analysis may be required by Environmental Services.
Table 3 - 5: Downspout Infiltration Length for USDA Soil Type
USDA Soil Type Trench Length (linear feet)
Coarser 20
Sand 85
Silt Loam 270
2. Maximum length of trench must not exceed 100 feet from the inlet sump.
3. Minimum spacing between trenches shall be 4 feet measured from the edge of trench.
4. Non-woven geotextile fabric shall be placed around the walls, bottom and top of the trench aggregate. A 6-inch minimum layer of sand may be used as a filter media at the bottom of the trench instead of the geotextile. Volume 5, Appendix C contains specifications for geotextile fabric.
5. A minimum of three feet of separation is required from the proposed final grade to the seasonal high groundwater table.
6. A minimum of 1 foot of separation is required from the bottom of the infiltration trench to the seasonal high groundwater table.
7. Infiltration trenches may be placed in fill material if the fill is placed and compacted under the direct supervision of a geotechnical engineer or professional civil engineer with geotechnical expertise, and if the measured infiltration rate is at least 8 inches per hour. Trench length in fill must be 60 linear feet per 1,000 square feet of roof area. Infiltration rates can be tested using the methods described in Section 6.4.3.
8. Trenches may be located under pavement if a small yard drain or catch basin with grate cover is placed at the end of the trench pipe such that overflow would occur out of the catch basin at an elevation at least one foot below that of the pavement, and in a location which can accommodate the overflow without creating a significant adverse impact to downhill properties or drainage systems. This is intended to prevent saturation of the pavement in the event of system failure.
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2.6.1.2 Maintenance Criteria Per Minimum Requirement #10, an operation and maintenance plan shall be prepared for all stormwater management facilities. See Volume 1, Appendix D, Maintenance Checklist #3 for specific maintenance requirements for infiltration trenches. Maintenance shall be a basic consideration in design and cost-determination of the stormwater management facility.
Any standing water removed during the maintenance operation must be disposed of in a City approved manner. See the dewatering requirements in Volume 4 of this manual. Pretreatment may be necessary. Solids must be disposed of in accordance with state and local waste regulations.
Facilities shall be constructed such that the facility can be easily inspected by one person. This may require the construction of additional inspection ports or access manholes.
10' MINIMUM
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Figure 3 - 1. Typical Downspout Infiltration Trench
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Figure 3 - 2. Alternative Infiltration Trench System for Coarse Sand and Gravel
Simple 10-foot trench per Trench X-Section above
Maximum 50-foot trench w/notch board per Figure 3-4
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Figure 3 - 3. Typical Downspout Dispersion Trench
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Figure 3 - 4. Standard Dispersion Trench with Notched Grade Board
2.7 Downspout Dispersion Systems Downspout dispersion systems are splash blocks or dispersion facilities that spread roof runoff over vegetated pervious areas. Dispersion attenuates peak flows by slowing entry of the runoff into the conveyance system, allowing for some infiltration, and providing some water quality benefits.
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2.7.1 Application Downspout dispersion may be used on all sites that cannot infiltrate roof runoff and that meet the feasibility and setback requirements provided in the general design criteria below.
2.7.2 Flow Credit for Roof Downspout Dispersion If roof runoff is dispersed according to the requirements of this section, and the vegetative flow
path1 of the roof runoff is 25 feet or greater through undisturbed native landscape or lawn/ landscape area that meets BMP L613, the roof area may be modeled as a grassed surface for determining thresholds and sizing flow control devices. The deduction cannot be applied to pollutant-generating surfaces. If the thresholds for water quality treatment are met, water quality treatment is required.
2.7.3 General Design Criteria • No erosion or flooding of downstream properties shall result.
• Downspout dispersion trenches designed as shown in Figure 3 - 3 or Figure 3 - 4 should be used for all downspout dispersion applications except where splash blocks are allowed.
• Perforated stub-out connections shall not be used.
• For sites with septic systems, the discharge point of all dispersion systems must be downgradient of the drainfield. This requirement may be waived if site topography clearly prohibits flows from intersecting the drainfield.
• For sites with septic systems, the discharge point must be downslope of the primary and reserve drainfield areas. This requirement may be waived if site topography clearly prohibits flows from intersecting the drainfield or where site conditions (soil permeability, distance between systems, etc) indicate that this is unnecessary.
• Place all dispersion systems at least 5 feet from any property line. If necessary, setbacks shall be increased from the minimum 5 feet in order to maintain a 1:1 side slope for future excavation and maintenance.
• Setback dispersion systems from sensitive areas, steep slopes, landslide hazard areas, and erosion hazard areas as goverened by the Tacoma Municipal Code.
• All dispersions systems shall be at least 10 feet from any structure. If necessary, setbacks shall be increased from the minimum 10 feet in order to maintain a 1H:1V side slope for future excavation and maintenance.
• For sites with multiple dispersion systems, a minimum separation of 10 feet is required between flowpaths. Environmental Services may require a larger separation based upon site conditions such as slope, soil type and total contributing area.
• Runoff discharged towards landslide hazard areas must be evaluated by a geotechnical engineer or a licensed geologist, hydrogeologist, or engineering geologist. The discharge point shall not be placed on or above slopes greater than 20% (5H:1V) or above erosion hazard areas without evaluation by a geotechnical engineer or qualified geologist and City approval.
1.Vegetative flow path is measured from the downspout or dispersion system discharge point to the downstream property line, stream, wetland, or other impervious surface.
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Design Criteria for Dispersion Trenches
• Design dispersion trenches as shown in Figure 3 - 3 and Figure 3 - 4.
• A vegetated flowpath of at least 25 feet in length must be maintained between the outlet of the trench and any property line, structure, stream, wetland, or impervious surface. A vegetated flowpath of at least 50 feet in length must be maintained between the outlet of the trench and any slope, 20% or greater. Sensitive area buffers may count towards flowpath lengths if approved by the City of Tacoma.
• Trenches serving up to 700 square feet of roof area may be simple 10-foot-long by 2-foot wide gravel filled trenches as shown in Figure 3 - 3. For roof areas larger than 700 square feet, a dispersion trench with notched grade board may be used as approved by the City. The total length of this design must not exceed 50 feet and must provide at least 10 feet of trench per 700 square feet of roof area.
Design Criteria for Splashblocks
A typical downspout splashblock is shown in Figure 3 - 5. In general, if the ground is sloped away from the foundation and there is adequate vegetation and area for effective dispersion, splashblocks will adequately disperse storm runoff. If the ground is fairly level, if the structure includes a basement, or if foundation drains are proposed, splashblocks with downspout extensions may be a better choice because the discharge point is moved away from the foundation. Downspout extensions can include piping to a splashblock/discharge point a considerable distance from the downspout, as long as the runoff can travel through a well- vegetated area as described below.
• A vegetated flow path of at least 50 feet shall be maintained between the discharge point and any property line, structure, steep slope, stream, wetland, lake, or other impervious surface. Sensitive area buffers may count toward flow path lengths.
• Flows shall not be directed onto sidewalks.
• A maximum of 700 square feet of roof area may drain to each splashblock.
• A splashblock or a pad of crushed rock (2 feet wide by 3 feet long by 6 inches deep) shall be placed at each downspout discharge point.
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Figure 3 - 5. Typical Downspout Splashblock Dispersion
2.8 Collect and Convey Where it can be demonstrated that infiltration and dispersion are not feasible for roof downspout controls, it may be allowable to collect and convey to the City stormwater system. This may be either the curb, if present, or the actual pipe and structure conveyance system.
Conveyance to the curb will only be allowed if a catch basin is located within 350 feet downstream of the discharge point. If a catch basin is not located within 350 feet of the discharge location, a storm main extension shall be required.
Minimum pipe size for conveyance to the curb shall be 3 inches in diameter. Where capacity greater than a 3 inch diameter pipe is required, Environmental Services shall review the proposal and may require a storm main extension.
NOTE: Environmental Services will only approve on a case-by-case basis those facilities that would require more than one through-curb discharge point.
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For total roof areas 2,000 to 5,000 sf, roof runoff may be allowed to be collected and conveyed to either the curb or directly connected to a structure. The runoff shall not be conveyed over driveways, sidewalks or other areas reserved for pedestrian traffic. A detail for the discharge shall be submitted to Environmental Services for review and approval.
For roof areas between 5,001 sf and 9,999 sf, roof runoff may be allowed to be collected and conveyed to the curb or stormwater structure unless Minimum Requirement #7 applies and requires alternative mitigation. Capacity analysis of the road gutter, conveyance piping and catch basin leads will be required to ensure that adequate capacity exists (see Section 9.6). Environmental Services may require more than one through curb outlet for discharge to the curb.
For roof areas 10,000 sf and greater, please refer to Minimum Requirement #7.