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.