Stormwater MSLA
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Transcript of Stormwater MSLA
Functional Aestheticsfor Maine
• Depth vs. Edge Condition
• Native Plants in a non-native condition
• Safety considerations
• Treatment options can attract invasives
• Maintenance considerations
• Can you address a sense of place?
Integrating Treatment with Design Considerations
Stormwater strategies:
Treadable landscapeRaingardenReflection pool used for storage
CongregationBet Ha’amSouth Portland
Photograph: Shim Sutcliffe
Working through the design
LL BeanFreeport
Aesthetic alternatives to depth
Aesthetic alternatives to spillways
Natural materials, create transition into surrounding landscape.
This fits perfectly with the client’s greater image, treats stormwater and meets LEED criteria, by integrating the design into the site we can create an amenity out of a necessary function.
Consider construction and survival of plant material.
Graphic obtained: Landscape Architectural Graphic Standards - Leonard J. Hopper, RLA, ASLA
Considerations for damage
Bio-retention areas/ Rain Gardensshould never double as snow storage areas.
Quick Concept Sketch - “Testing Design”
Gravel Wetlands
Considerations:
• Great for retro-fitting old quantity detention ponds• Great for Nitrogen Removal• Generally less impacted by temperature• Soils are not a factor as it is fully lined• Better suited for limited outlet conditions 4-8” depth vs. 30” depth• Cost is generally higher than pond and similiar to underdrain soil filter
Gravel Wetland Design Specifications Page 7 of 10 Stormwater Manual - City of South Portland, Maine www.southportland.org
clogging and will prevent flows out of the filter. The geotextile fabric shall be Mirafi 170n or equivalent.
8” (20 cm) minimum thickness of wetland soil
24” (60 cm) minimum thickness of 3/4” (2 cm) crushed stone (gravel)
3” (8 cm) minimum thickness ofgraded filter (i.e., pea gravel) ifneeded
Low permeability soil or liner if underlying soils are very permeable
Figure 3: Diagram of typical gravel wetland construction.
Gravel Wetland Drain Design Except where local slopes (e.g., coastal areas) prohibit this design, each GW shall have a drain pipe that can completely or partially drain wetland soil layer. The drain pipe shall have an elbow or protected intake within the GW to prevent sediment deposition, and a diameter capable of draining the permanent pool within 24 hours. Access to the drain pipe shall be secured by a lockable structure to prevent vandalism and/or accidental draining of the pond, which could pose a safety hazard due to high drainage velocities.
Safety Features Design The Bypass outlet (emergency spillway, or secondary spillway) is sized to pass designs
flows for the 25‐year 24 hours storm event and/or in accordance with City requirements. This outlet is sized by using conventional routing calculations of the inflow hydrograph through the surface storage provided by the subsurface gravel wetland system.
Proposed graded side slopes to the GW shall not exceed 3:1 (H:V), and shall terminate on the safety bench.
The principal spillway opening shall not permit access by small children, and endwalls above pipe outfalls greater than 48 inches in diameter shall be fenced to prevent a hazard.
“Token” or emergency spillways (those placed above the water elevation of the largest managed storm) may be required if not already provided as part of the conveyance of
Gravel Wetland Design Specifications Page 1 of 10 Stormwater Manual - City of South Portland, Maine www.southportland.org
City of South Portland Stormwater Manual Design Specif icat ions
Gravel Wetland Adapted from RI Stormwater Design Manual and UNH Design Specifications.
Overview
HOW IT WORKS
Gravel wetlands (GW) are horizontal flow retention and filter systems. The GW utilizes temporary storage for solids settling and soil media filtration as the primary mechanisms for pollutant removal. Gravel Wetlands maintain a saturated gravel bed and provide treatment by stormwater movement through the gravel bed and plant/soil treatment processes. Gravel Wetlands are well suited for poorly draining subsoils. These systems are well suited because: 1) there is a limited hydraulic head requirement, and 2) does not require separation from groundwater due to lack of infiltration. This is one of the University of New Hampshire Stormwater Center’s most successful systems for overall pollutant removal. Hydraulic head requirements for gravel wetlands are approximately four inches, whereas underdrained filtration systems may require as much as three feet or more. Because the Gravel Wetland is a horizontal porous media flow system it does require a hydraulic head to drive the water through the system. At a minimum, the driving head is the difference between the vertical distance from the ponded water level above the wetland surface and the invert of the primary outlet. To maintain the system in its saturated condition, it must be situated in low hydraulic conductivity soils or lined below the gravel layer. Because infiltration is not designed to occur, separation from groundwater is not required and the GWs are sited much like stormwater ponds.
Figure 1: Diagram of a gravel wetland. Source: UNH Stormwater Center.
How does it work?
University of New Hampshire Stormwater Center (UNHSC) Gregg Hall ● 35 Colovos Road ● Durham, New Hampshire 03824-3534 ● http://www.unh.edu/erg/cstev
UNHSC
Subsurface Gravel Wetland
Design Specifications
June 2009
Image and Figure from: University of New Hampshire Stormwater Center
• Treatment comes through uptake of plants rather than infiltration
• Underlayer is always wet.• Aesthetics around the basins to create better
sense of natural environment
Gorham Road at Western Ave - South Portland
Thank you.