Key Points of the Sponge City Standards
Release Time:
2016-04-15
Source:
Key Points of the Sponge City Standards
1. Permeable Pavement
Pervious brick paving, pervious cement concrete paving, and pervious asphalt concrete paving, as well as grass-paved pavers and pebble or crushed-stone paving in landscape design, are also classified as permeable pavements.
(1) When permeable pavement poses a significant potential risk to the strength and stability of the road subgrade, semi-permeable pavement may be used.
(2) When the soil’s permeability is limited, drainage pipes or drainage boards shall be installed within the permeable subbase of permeable pavement.
(3) When permeable pavement is installed on the roof slab of a basement, the thickness of the soil cover above the slab shall not be less than 600 mm, and a drainage layer shall be provided.
2. Sunken Green Space
The depression depth refers to the average depth by which a sunken green space lies below the surrounding paved surface or roadway. Sunken green spaces with a depression depth of less than 100 mm are excluded from the calculation (except for infiltration facilities with limited depression depth due to local soil permeability and other site-specific conditions). For water-surface features such as detention ponds and rainwater wetlands, the term refers to the storage-and-regulation depth.
Permeable pavement ratio = Permeable pavement area / Total impervious surface area;
Green roof ratio = Green roof area / Total building roof area.
(1) The depression depth of sunken green spaces shall be determined based on the flood-tolerance of vegetation and the soil’s infiltration capacity, typically ranging from 100 to 200 mm.
(2) Inlet sumps should generally be provided within sunken green spaces (e.g., stormwater inlets) to ensure overflow discharge of runoff during heavy rainfall; the elevation of the inlet sill is typically 50–100 mm above the green space surface.
3. Bioretention Facilities
(1) In catchment areas with severe pollution, vegetated swales, vegetative buffer strips, or sedimentation ponds should be employed to pre-treat runoff rainwater, removing coarse particulate pollutants and reducing flow velocity. Additionally, measures such as stormwater diversion and salt flushing should be implemented to prevent high-concentration pollutants—such as deicing agents and petroleum products—from harming vegetation.
(2) Roof runoff can be conveyed via downspouts into bioretention facilities, while pavement runoff can enter through curb inlets; the dimensions and number of curb inlets shall be determined by calculation based on factors such as the longitudinal grade of the roadway.
(3) When bioretention facilities are installed in roadside green belts, if the longitudinal grade of the road exceeds 1%, water-retaining weirs or berms shall be provided to reduce flow velocity and enhance rainwater infiltration; the portion of the facility adjacent to the road subgrade shall be equipped with impermeable lining to prevent adverse impacts on the stability of the road subgrade.
(4) Overflow facilities shall be provided within bioretention facilities, which may include overflow risers, grated overflow wells, or stormwater inlets. The top of the overflow facility shall generally be 100 mm below the catchment surface.
(5) Bioretention facilities should be distributed in a decentralized manner and should not be excessively large; the ratio of bioretention facility area to catchment area is generally 5%–10%.
(6) Permeable geotextiles shall be installed on the outer surfaces and at the base of the structural layers of complex bioretention facilities to prevent infiltration of the surrounding native soil. If assessment indicates that infiltration could pose a collapse risk to adjacent buildings or structures, or if the effluent from the facility’s base is intended for collection, storage, and reuse, an impermeable geomembrane may be installed at the base and around the perimeter of the bioretention facility.
(7) The depth of the storage layer in a bioretention facility shall be determined based on the flood-tolerance of the vegetation and the soil’s infiltration characteristics, typically ranging from 200 to 300 mm, with an additional 100 mm of freeboard provided. The type and thickness of the soil replacement layer shall meet the effluent water-quality requirements and also comply with the technical specifications for plant planting and landscape maintenance and management. To prevent loss of the soil replacement material, a permeable geotextile barrier is generally installed at the base of the replacement layer; alternatively, a sand layer (fine or coarse sand) with a minimum thickness of 100 mm may be used instead. The gravel layer serves to facilitate drainage and is typically 250–300 mm thick; a perforated drainage pipe with a diameter of 100–150 mm may be embedded at its base. The gravel must be thoroughly washed and have a particle size no smaller than the aperture of the perforations in the drainage pipe. To enhance the stormwater retention and regulation capacity of the bioretention facility, a gravel detention layer of specified thickness may be added beneath the perforated drainage pipe.
4. Infiltration ponds (depressions, primarily for infiltration and evapotranspiration, with no stormwater diversion)
(1) Prior to infiltration ponds, preliminary treatment facilities such as grit chambers and detention ponds shall be installed to remove coarse particulate pollutants and reduce flow velocity; in cities with snowfall, measures such as first-flush diversion and salt drainage shall be implemented to prevent deicing agents from harming vegetation.
(2) The slope gradient of the infiltration pond’s side slopes (vertical:horizontal) generally does not exceed 1:3, and the depth from the pond bottom to the overflow water level is generally no less than 0.6 m.
(3) The bottom structure of a infiltration pond typically consists of a 200–300 mm layer of planting soil, a permeable geotextile, and a 300–500 mm layer of filter media.
(4) The drainage time of infiltration ponds shall not exceed 24 hours. Infiltration ponds shall be equipped with overflow facilities and connected to the urban stormwater drainage system and the excess stormwater runoff discharge system. Safety protection measures and warning signs shall be installed around the perimeter of the infiltration pond.
5. Detention Pond (for stormwater regulation and storage, with a regulating volume for reused stormwater)
(1) Intake and overflow outlets shall be equipped with energy-dissipating structures such as riprap and energy dissipating weirs to prevent scouring and erosion caused by water flow.
(2) The detention pond serves as a pretreatment facility for wet ponds, functioning to settle out large-particle pollutants in runoff. The pond bottom is typically constructed of concrete or riprap to facilitate desilting. Detention ponds shall be equipped with desilting access channels and protective structures; the revetment should preferably be an ecological soft revetment, with a slope (vertical:horizontal) generally ranging from 1:2 to 1:8. The volume of the sedimentation zone in the detention pond shall be determined based on the desilting interval and the suspended-solids pollutant load of the incoming stormwater runoff.
(3) The main pond generally comprises a permanent storage volume below the normal water level and a detention volume; the depth of the permanent storage volume is typically 0.8–2.5 m. The detention volume is usually determined in accordance with the “control volume per unit area” specified in the relevant planning for the site. Wet ponds designed to attenuate peak flows also include an adjustment volume, which should be drained within 24–48 hours. A planted aquatic vegetation zone (rainwater wetland) is recommended between the main pond and the pre-treatment pond, and the bank of the main pond should be an ecological soft revetment, with a slope ratio (vertical:horizontal) not exceeding 1:6.
(4) Overflow outlets shall include overflow risers and spillways, and their drainage capacity shall be determined in accordance with the drainage capacity of the downstream stormwater pipe network or the overdesign stormwater runoff discharge system.
(5) Wet ponds shall be equipped with safety barriers, warning signs, and other safety protection and warning measures.
6. Rainwater Wetland
(1) Intake and overflow outlets shall be equipped with energy-dissipating structures such as riprap and energy dissipating weirs to prevent scouring and erosion caused by water flow.
(2) Rainwater wetlands shall be equipped with a detention pond to pre-treat runoff rainwater.
(3) The marsh area comprises shallow and deep marsh zones and serves as the primary purification zone for rainwater wetlands. In the shallow marsh zone, water depth typically ranges from 0 to 0.3 m, while in the deep marsh zone it generally ranges from 0.3 to 0.5 m, depending on the water…
Plant different types of aquatic plants in deep water.
(4) The regulating storage capacity of rainwater wetlands shall be drained within 24 hours.
(5) The effluent basin primarily serves to prevent the resuspension of settled solids and to reduce water temperature; its typical water depth is 0.8–1.2 m, and its volume accounts for approximately 10% of the total basin volume (excluding the regulating volume).
7. Grass Swales
(1) The cross-sectional shape of shallow ditches should preferably be inverted parabolic, triangular, or trapezoidal.
(2) The slope gradient of vegetated swales (vertical:horizontal) should not exceed 1:3, and the longitudinal slope should not exceed 4%. When the longitudinal slope is relatively steep, a stepped vegetated swale should be adopted, or energy-dissipating sills should be provided at intermediate locations.
(3) The maximum flow velocity in vegetated swales shall be less than 0.8 m/s, and the Manning’s roughness coefficient is recommended to be 0.2–0.3.
(4) The vegetation height in conveyance-type vegetated swales should be maintained at 100–200 mm.
8. Seepage Pipes/Channels
(1) Infiltration pipes/channels shall be equipped with pretreatment facilities such as vegetated swales and sedimentation (sand) basins.
(2) The perforation rate of seepage pipes/channels shall be controlled within the range of 1% to 3%, and the porosity of sand-free concrete pipes shall be greater than 20%.
(3) The laying slope of seepage pipes/channels shall meet the requirements for drainage.
(4) The perimeter of the infiltration pipe or channel shall be backfilled with gravel or other porous materials, and the gravel layer shall be wrapped with a permeable geotextile, with an overlap width of not less than 200 mm.
(5) When infiltration pipes/channels are installed beneath the roadway pavement, the cover depth shall not be less than 700 mm.
9. Vegetation Buffer Zones
Vegetative buffer strips are vegetated areas with relatively gentle slopes that, through vegetation interception and soil infiltration, reduce the velocity of surface runoff and remove a portion of the pollutants carried by the runoff. The slope of vegetative buffer strips typically ranges from 2% to 6%, and their width should not be less than 2 meters.
10. Initial Rainwater Overflow Facility
Common initial runoff diversion methods include volume-based diversion and small-pipe diversion (flow-switching method), among others. Diversion configurations encompass automated diversion, infiltration diversion, diversion ponds, and downspout diversion, among others. These methods are suitable for the upstream end of low-impact development facilities, such as downspouts for rooftop stormwater and centralized inlets for overland runoff.
