The document discusses various low impact development (LID) stormwater management techniques including eco-roofs, downspout disconnection, cisterns, rain gardens, bioretention planters, and detention basins. It provides details on how each technique works and its benefits such as reducing runoff volumes and improving water quality, as well as potential limitations. For example, eco-roofs filter runoff through growing media but have challenges with weight and leaks, while rain gardens naturally infiltrate water but require appropriate soil conditions.

1. The Urban Watershed
and Low Impact Design
Materials courtesy of the SFPUC
Urban Permaculture
Institute

3. W h a t i s L o w I m p a c t D e s i g n ?
 LID is a stormwater management approach that aims to re-create and
mimic these pre-development hydrologic processes by increasing
retention, detention, infiltration, and treatment of stormwater runoff at
its source.
 LID is a distinct management strategy that emphasizes on-site source
control and multi-functional design, rather than conventional pipes
and gutters.
 Whereas BMPs are the individual, discrete water quality controls, LID
is a comprehensive, watershed- or catchment-based approach.
 These decentralized, smallscale stormwater controls allow greater
adaptability to changing environmental and economic conditions than
centralized systems.

5. Eco Roofs
 Green roofs, or eco-roofs, are roofs that are entirely or partially
covered with vegetation and soils.
 Eco-roofs have been popular in Europe for decades and have grown
in popularity in the U.S. Recently as they provide multiple
environmental benefits.
 Eco-roofs improve water quality by filtering contaminants as the
runoff flows through the growing medium or through direct plant
uptake.
 Studies have shown reduced concentrations of suspended solids,
copper, zinc, and PAHs (polycyclic aromatic hydrocarbons) from eco-
roof runoff.

7. D e s i g n D e t a i l s
 An intensive eco-roof may consist of shrubs and small trees planted
in deep soil (more than 6 inches) arranged with walking paths and
seating areas and often provide access for people.
 In contrast, an extensive eco-roof includes shallow layers (less than 6
inches) of low-growing vegetation and is more appropriate for roofs
with structural limitations.
 Both categories of eco-roofs include engineered soils as a growing
medium, subsurface drainage piping, and a waterproof membrane to
protect the roof structure.

8. Benefits
 Provides insulation and can lower cooling costs for the building
 Extends the life of the roof – a green roof can last twice as long as a
conventional roof, saving replacement costs and materials
 Provides noise reduction
 Reduces the urban heat island effect
 Lowers the temperature of stormwater runoff, which maintains cool
stream and lake temperatures for fish and other aquatic life
 Creates habitat and increases biodiversity in the city
 Provides aesthetic and recreational amenities

9. Limitations
 Poor design or installation can lead to potential leakage and/or roof failure
 Limited to roof slopes less than 20 degrees (40 percent or a 5 in 12 pitch)
 Requires additional structural support to bear the added weight
 Potentially increased seismic hazards with increased roof weight
 Long payback time for installation costs based on energy savings
 May attract unwanted wildlife
 Inadequate drainage can result in mosquito breeding
 Irrigation may be necessary to establish plants and maintain them during
extended dry periods
 Vegetation requires maintenance and can look overgrown or weedy,
seasonally it can appear dead

10. Downspout Disconnect
 Downspout disconnection, also called roof drain diversion, involves
diverting rooftop drainage directly into infiltration, detention, or
storage facilities instead of into the sewer.
 Rainwater can be harvested from most types of rooftops.
 In areas where site conditions allow infiltration, roof drainage can be
conveyed to drainless bioretention planters, dry wells, or can be
simply dispersed onto a rain garden, lawn, or landscaped area
 On sites that are not amenable to infi ltration, roof drains can be
routed into cisterns which are available in a range of materials, sizes,
and models.

12. Benefits
 Reduces runoff volume and attenuates peak flows
 May decrease water usage through lowered irrigation requirements
 Low installation costs
 Low maintenance requirements
 Large variety of implementation locations and scales

13. Limitations
 Pre-fi ltration (such as a first-flush diverter) is required if water is to be
stored
 Added complexity for buildings with internally plumbed stormwater drains
 Secondary system is required to deal with water after it leaves the
downspout, such as a cistern or a rain garden

14. Retrofit where appropriate

15. Cisterns
 Cisterns are a traditional technology employed in arid climates to
capture and store rainwater.
 Cisterns reduce the stormwater volume by capturing rainwater for
non-potable uses, such as irrigation or fl ushing toilets.
 Suitable for a single house or an entire neighborhood, cisterns range
in size and may be placed above ground or underground.

16. C a s e S t u d y : C a m b r i a , C
A
Cambia Elementary School captures and
stores runoff water from the entire school site
in a cistern located underneath athletic fields
and uses the stored water to irrigate the fields
year round. All of the stormwater on the 12
acre campus is captured and stored in large
pipes that are located under 130,000 square
feet of new athletic fields. Up to 2 million
gallons of water can be stored.

17. Benefits
 Reduces runoff volume and attenuates peak flows
 May decrease water usage if retained for irrigation purposes or toilet
flushing
 Low installation costs
 Low maintenance requirements (for above ground cisterns)
 Low space requirements (for underground cisterns)
 Good for sites where infiltration is not an option

18. Limitations
 Poor design, sizing, and siting can lead to potential leakage and/or
failure
 Storage capacity is limited
 Provides no water quality improvements
 Lower aesthetic appeal (for above ground cisterns)
 Water reuse options limited to non-potable uses
 Requires infrastructure (pumps or valves) to use the stored water
 Inadequate maintenance can result in mosquito breeding and/or algae
production

20. Rain Gardens
 Rain gardens are stormwater facilities integrated into depressed
landscape areas.
 They are designed to capture and infiltrate stormwater runoff.
 Rain gardens include water-tolerant plants in permeable soils with
high organic contents that absorb stormwater and transpire it back
into the atmosphere.
 Rain gardens are a subset of bioretention planters except that they
do not typically include engineered soils or an under-drain
connection.
 Plant species can be selected to stack functions and provide yields.

22. Benefits
 Reduces runoff volume and attenuates peak flows
 Improves water quality
 Improves air quality
 Improves urban hydrology and facilitates groundwater recharge
 Low installation costs, low maintenance requirements, low space
requirements
 Creates habitat and increases biodiversity in the city
 Provides aesthetic amenity
 Easily customizable

23. Limitations
 Depth to bedrock must be over 10 feet for infiltration based systems
 Limited to slopes less than 5 percent, slopes greater than 5 percent require
check dams
 Seasonal fluctuation in water quality benefits based on the plants’
ability to filter pollutants
 Vegetation requires maintenance and can look overgrown or weedy,
seasonally it may appear dead
 Site conditions must be conducive to partial or full infiltration and the
growing of vegetationor an underdrain is needed
 10 foot minimum separation from groundwater is required to allow for
infiltration,unless the Regional Water Quality Control Board approves
otherwise
 Non-underdrained systems must have minimum soil infiltration rates

24. Typical Rain Garden

25. Bioretention Planters
 Bioretention is the use of plants, engineered soils, and a rock sub-
base to slow, store, and remove pollutants from stormwater runoff.
 Bioretention planters improve stormwater quality, reduce overall
volumes, and delay and reduce stormwater runoff peak flows.
 Bioretention planters can vary in size from small, vegetated swales to
multi-acre parks; however, there are limits to the size of the drainage
area that can be handled.
 System designs can be adapted to a variety of physical conditions
including parking lots, roadway median strips and right-of-ways,
parks, residential yards, and other landscaped areas and can also be
included in the retrofits of existing sites.

27. Benefits
 Reduces runoff volume and attenuates peak flows
 Improves water quality
 Improves air quality
 Improves urban hydrology and facilitates groundwater recharge
 Lowers the temperature of stormwater runoff, which maintains cool
stream temperatures for fish and other aquatic life
 Creates habitat and increases biodiversity in the city
 Provides aesthetic amenity
 Reduces the heat island effect

28. Limitations
 Depth to bedrock must be over 10 feet for infiltration based systems
 Limited to slopes less than 5 percent, slopes greater than 5 percent require
check dams
 Seasonal fluctuation in water quality benefits based on the plants’
ability to filter pollutants
 Vegetation requires maintenance and can look overgrown or weedy,
seasonally it may appear dead
 Site conditions must be conducive to partial or full infiltration and the
growing of vegetationor an underdrain is needed
 10 foot minimum separation from groundwater is required to allow for
infiltration,unless the Regional Water Quality Control Board approves
otherwise
 Non-underdrained systems must have minimum soil infiltration rates

30. Street-side bioretention planter
based on Portland’s Green Streets

31. Detention Basins
 Detention basins are temporary holding areas for stormwater that
store peak flows and slowly release them, lessening the demand on
treatment facilities during storm events and preventing flooding.
 Generally, detention basins are designed to fi ll and empty within 24
to 48 hours of a storm event and therefore could reduce peak flows
and combined sewer overflows.
 If designed with vegetation, basins can also create habitat and clean
the air whereas underground basins do not.
 Surface detention basins require relatively flat slopes.

32. Four Basic Types of Detention
Basins
Traditional dry detention basins simply store water and gradually
release it into the system. Dry detention basins do not provide water
quality benefits, as they only detain stormwater for a short period of
time. Maintenance requirements are limited to periodic removal of
sediment and maintenance of vegetation. Dry detention basins are
good solutions for areas with poorly draining soils, high liquefaction
rates during earthquakes, or a high groundwater table, which limit
infiltration.

33. Four Basic Types of Detention
Basins
Extended dry detention basins are designed to hold the first flush of
stormwater for a minimum of 24 hours. Extended dry detention basins
have a greater water quality benefit than traditional detention basins
because the extended hold time allows the sediment particles to settle to
the bottom of the pond. Collected sediments must be periodically
removed from the basin to avoid re-suspension.

34. Four Basic Types of Detention
Basins
Underground detention basins are well suited to dense urban locations
where land costs make surface options unfeasible. Underground
detention basins work best if partnered with an ‘upstream’ BMP that
provides water quality benefi ts, like bioretention planters, if water is not
returned to the combined sewer overflow. Underground detention basins
need to be on a slight slope
to facilitate drainage but should not be placed on steep slopes because
of the threat of erosion. They can be placed under a roadway, parking lot,
or open space and are easy to incorporate into other right-of-way
retrofits.

35. Four Basic Types of Detention
Basins
Multi-purpose detention basins are detention basins that have been
paired with additional uses such as large play areas, dog parks, athletic
fields or other public spaces. Generally detention basins are only filled
with water during storm events and can act as open spaces during dry
weather.