Trees and Stormwater Management

1
Trees and Stormwater Management

• Promoting forest preservation and restoration at
  development sites through stormwater credits

2
Goals

• 1) Preserve forests and natural vegetation at
  development sites
• 2) Protect and enhance urban and suburban tree
  canopy
• 3) Increase the use of trees in stormwater
  practices

Photo by Jay Wilson
3

• The Chesapeake Bay Program has formally
  recognized the benefits of trees for stormwater
  management
• WE FURTHER RECOGNIZE THAT URBAN TREE CANOPY COVER
  offers stormwater control and water quality
  benefits for municipalities in the Chesapeake Bay
  watershed and can extend many riparian forest
  buffer functions to urban settings. (Chesapeake
  Executive Council, Directive No. 94-1, Riparian
  Forest Buffers)

4
Old News

• The first models for predicting runoff appeared
  in the 1800s and used runoff coefficients to
  account for different land use and land covers.
• The Rational Method (Mulvaney 1851)
• Peak Discharge Runoff Coefficient x Rainfall x
  Basin Area
• Runoff Coefficients
• Forested ground is typically assigned a value of
  near zero.
• Pavement is given values approaching 100 percent.
• But why emphasize trees in particular versus
  other stormwater practices?

5
Meets Multiple CBP Directives

• Promote expansion and connection of forest
  habitat
• Chesapeake Bay 2000
• Minimize forest loss and fragmentation.
• Directive 94-3 Framework for Habitat Restoration
• Increase urban and suburban tree canopy cover
• Directive 94-1 Riparian Forest Buffers
• Promote innovative stormwater BMPs
• Directive No. 98-3 Accelerating Bay Restoration
  Through implementation of Innovative Technologies

6

• Premise Trees provide so many benefits beyond
  stormwater that they should be a priority
  consideration in site design.
• Unlikely to happen unless trees are given credit
  for their work.
• Stormwater is one place where we can give them
  credit.

USDA Forest Service Urban Watershed Forestry
Manual, Part 1
7
Dont trees get some credit already?

• Not in most municipalities
• Not enough to matter to most developers.
• Not enough to equal their true value

Source CWP (borrowed from one of Tom Schulers
presentations)
8
Ways that trees improve water quality

• Three main ways
• Rainfall interception
• Evapotranspiration
• Infiltration
• The overall effect
• Delay peak runoff during storms
• Increase soil infiltration and groundwater
  recharge
• Remove nutrients and other pollutants
• Decrease volume of stormwater that needs to be
  treated, especially from small, frequent storm
  events.
• Thats great, but can you give me some numbers?
• How else can our engineers calculate credits?

9
A Few Examples from the Scientific Literature

• Rainfall Interception
• 10-40 of annual rainfall depending on tree
  species and climate (Zinke 1967)
• Evapotranspiration (ET)
• Eastern Forests 300 900 mm (12-36)
  (Kittredge, 1948).
• Infiltration
• Kays, 1982
• 12.4 in/hr for forest,
• 4.4 in/hr for lawn
• 1.9 in/hr for suburban development (Kays, 1982)

10
Thats fine, but what about MY trees?

• New models have been developed to estimate the
  water quality and quantity function of trees
• STRATUM Quantifies stormwater volume benefit of
  street trees (i.e how much runoff volume was
  avoided via tree canopy interception).
• UFORE Hydro A more advanced model calculates
  both stormwater volume and water quality benefits
  for individual trees and forest patches using
  interception, infiltration, and
  evapotranspiration.
• Designed for ease-of-use
• Simple Windows interfaces (no technical knowledge
  required)
• For municipal foresters, commercial arborists,
  environmental consultants, planners, etc.
• Do we have to run the model for every site?
• UFORE Urban Forestry Effects Model
• STRATUM Street Tree Resource Analysis Tool for
  Urban Forest Managers

11
Use models to generate regional Tree Credit
guidelines

• UFORE Hydro is almost ready.
• STRATUM software is written and data for our
  climate region is coming soon.
• Well be able to fill out charts like these
  (below) to create tree credit guidelines.

Type 1 Trees, Piedmont Climate Region Type 1 Trees, Piedmont Climate Region Type 1 Trees, Piedmont Climate Region
Size (DBH) Water quantity credit Water quality credit
0 3    
3 6    
6 12    
12 18    
18 24    
24 30    
30 36    
36 42    
12
Example Output from STRATUM
STRATUM calculates both economic and
environmental benefits of trees. (Example above
is from the street tree population of North
Vancouver)
13
Other approaches being explored

• Premise We have many manuals and publications
  about LID, but we have seen minimal
  implementation by developers and in local
  ordinances.
• Issue paper Obstacles to LID implementation
• Based on interviews with developers, local
  officials, and public works people.
• Find out the challenges and misconceptions.
• Identify potential solutions.
• Compile some successful case studies.

14
Summary of Efforts

• Use scientific literature and models to provide
  real numbers for the stormwater quality and
  quantity impact of trees.
• Demonstrate how forests and tree canopy help meet
  important goals and directives
• Cleaner air, lower energy use, habitat
  enhancement and protection, forest connectivity,
  carbon sequestration, etc.
• Speak with developers and local government
  officials to find out
• Why more LID practices arent used
• How to lower the barriers to using these
  techniques.
• Use case studies to highlight successful examples.

15
Seeking Workgroups Advice

• How can we use this information to affect change
  (i.e. preserve and enhance tree canopy cover in
  urban areas and at new development sites)?
• Who are the target audiences?
• How do we package this information to be most
  effective?
• Final Products?
• Tree Credit Guidelines Use UFORE Hydro and
  STRATUM models to create regional tree credit
  charts that state/local governments could use.
• Issue Paper Obstacles to Incorporating
  Trees/Natural Areas into SW

GOALS REVISITED 1) Preserve forests and natural
vegetation at new development sites 2) Protect
and enhance urban and suburban tree canopy 3)
Increase the use of trees in stormwater practices