# Presentation: 353489

## Paradigm Shifts for Stormwater Management

**Paradigm Shifts for Stormwater Management**

- Jenny Molloy
- U.S. Environmental Protection Agency
- Office of Water

## Slide 2

## Relationship between Impervious Cover and Runoff Coefficient

- Schueler, 1987

## Land Development and Population Growth in the US, 1982-2002

**It’s how and where we are growing that are driving our significantly increasing rate of land consumption, not domestic population growth.**

## Development patterns

- 80% of residential development occurs on urban fringe or beyond
- 94% of that development on 1 acre or more

- USDA Economic Research Service

## Consequences of Development to Urban Streams

- Consequences of Development to Urban Streams

## Blakeslee Creek

- CN=47

- CN=58

- CN=67

**Blakeslee Creek**

## Blakeslee Creek

- CN=47

- CN=67

- CN=67

**Blakeslee Creek**

## 70% increase in peak flow.

- Pre-development

- Post-development

- 170% increase in runoff volume.
- Former instantaneous peak flow now lasts ~4 hours.

- Blakeslee Creek

**Notes:**
70% increase in peak flow, 167% increase in runoff volume, former instantaneous peak flow now lasts ~4 hours

## Era of the Big Basin

-    Stormwater management designs that manage only discharge rates often exacerbate                                         the problem.
-    Natural systems                                  respond to runoff                       volumes, frequencies and durations as well.

## Increased rates and volumes of storm water discharges lead to stream widening and down-cutting, or incision.

*Copyright 2001, Center for Watershed Protection*

## This stream is incised several feet in elevation because of the stormwater flow.  The forested wetland in the floodplain is now hydraulically disconnected from the stream that sustained it.  This also means that the flood storage capacity of the floodplain has been lost.

## Channel Enlargement as a Function of Total Basin Imperviousness

- From MacRae et al, 1999

- Total Basin Imperviousness, %

- Enlargement Ratio

**Notes:**
The channel enlargement ratio is defined as the post-development channel cross-sectional area of the channel at bankfull divided by the pre-disturbance cross-sectional area of the channel at bankfull.

##      Pre-Dev      

-      Pre-Dev
-      Post-Dev

- adapted from Center for Watershed Protection

## Slide 15

## Geomorphic Effects of              Stormwater Runoff

- Degradation of habitat structure
- Fragmentation of riparian tree canopy
- Disconnection of stream and floodplain
- Loss of pool-riffle structure
- Decreased substrate quality
- Decreased channel                      stability
- Reduced fish passage
- Embeddedness

**Notes:**
The six criteria comprising the unified approach include: reduce runoff, maintain groundwater recharge, meet pollutant removal goals, reduce channel erosion, provide overbank flood control, and pass extreme floods. It is important to note that often multiple criteria can be met with one treatment practice. 

## Stormwater Pollutants

## Slide 18

## Slide 19

## Slide 20

## Typical Pollutant Loadings from Runoff by Urban Land Use (lbs/acre-yr)

| Land Use | TSS | Total P | TKN | NH3-N | NO2+NO3-N | BOD | COD | Pb | Zn | Cu |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Commercial | 1000 | 1.5 | 6.7 | 1.9 | 3.1 | 62 | 420 | 2.7 | 2.1 | 0.4 |
| Parking Lot | 400 | 0.7 | 5.1 | 2 | 2.9 | 47 | 270 | 0.8 | 0.8 | .04 |
| High Density Residential | 420 | 1 | 4.2 | 0.8 | 2 | 27 | 170 | 0.8 | 0.7 | .03 |
| Med Density Residential | 190 | 0.5 | 2.5 | 0.5 | 1.4 | 13 | 72 | 0.2 | 0.2 | .14 |
| Low Density Residential | 10 | 0.04 | .03 | 0.02 | 0.1 | NA | NA | .01 | .04 | .01 |
| Freeway | 880 | 0.9 | 7.9 | 1.5 | 4.2 | NA | NA | 4.5 | 2.1 | .37 |
| Industrial | 860 | 1.3 | 3.8 | 0.2 | 1.3 | NA | NA | 2.4 | 7.3 | 0.5 |
| Park | 3 | 0.03 | 1.5 | NA | 0.3 | NA | 2 | 0 | NA | NA |
| Construction | 6000 | 80 | NA | NA | NA | NA | NA | NA | NA | NA |

- NURP Study: Homer et al, 1994

## Urban Areas

## Increased Loads & Concentrations of Stormwater Pollutants can Cause:

- Impairments to aquatic life
- Full- and partial-body contact recreational activities threats/restrictions
- Shell-fish bed closures
- Fish consumption advisories
- Drinking water threats/restrictions
- Sediment accumulation: lake aging, increased need for sediment removal

**Notes:**
The six criteria comprising the unified approach include: reduce runoff, maintain groundwater recharge, meet pollutant removal goals, reduce channel erosion, provide overbank flood control, and pass extreme floods. It is important to note that often multiple criteria can be met with one treatment practice. 

## Development & Runoff

- Impervious cover = 65%
- Runoff/acre = 40,000 ft3/yr
- Runoff/unit = 5,000 ft3/yr

- Impervious cover = 20%
- Runoff/acre = 19,000 ft3/yr
- Runoff/unit = 19,000 ft3/yr

- Impervious cover = 38%
- Runoff/acre = 25,000 ft3/yr
- Runoff/unit = 6,000 ft3/yr

## . . . and these numbers don’t even take into account the additional impervious cover associated with additional road networks, parking lots, etc.

## But watershed managers are not dealing with 8 houses...

- Accommodating 10,000 units on a 10,000 acre watershed at different densities

- 10,000 houses on 10,000 acres produce
- 187 million ft3 /yr stormwater runoff
- Site: 20% impervious Watershed: 20% impervious

- 10,000 houses on 2,500 acres produce
- 62 million ft3 /yr
- stormwater runoff
- Site: 38% impervious Watershed: 9.5% impervious

- 10,000 houses on 1,250 acres produce
- 49.5 million ft3 /yr
- stormwater runoff
- Site: 65% impervious Watershed: 8.1% impervious

- The lower density scenario creates more run-off and consumes 2/3 more land than the higher density scenario

**1 unit/acre**

**4 units/acre**

**8 units/acre**

## Which is Better for Water Quality on a Watershed Basis?

- Swale

- Pond
- (maybe wetland)

- Buffer

**Sidewalk with infiltration strip**

**Shared drive aisle for **
**clustered housing**

**Short set backs = shorter driveways**

## Which is Better for Water Quality on a Watershed Basis?

- Housing like this....

- ...is, by design, served by
- retail and roads like this

## What needs to change?

**Notes:**
This presentation is an overview of Low Impact Development (LID) and how it can be used to help protect and restore the water quality and biological resources of Puget Sound.  

## Paradigm Shift:  Rain is a Resource, Not a Waste

- Drinking water
- Ground water recharge
- Stream baseflow
- Trees & other plants
- Aesthetic qualities

## Paradigm Shift:  Get away from the curb and gutter, big basin approach

- Shift from the concept of moving stormwater as far away as quickly as possible in large, buried collection and conveyance systems.
- Shift towards the concept of managing stormwater the way mother nature would do it: where it falls; plants & soils.

## Paradigm Shift:  Trifocal Approach to Stormwater Management

## How can we make the built environment function hydrologically like natural systems?

**Notes:**
The challenge is how to make a residential subdivision, or a commercial property, function hydrologically like the wooded area shown on the upper left. This residential subdivision is heavily wooded and landscaped. However, because of the amount of directly connected impervious area (streets, driveways, rooftops and sidewalks), the highly compacted soils and the efficient storm drainage pipe collection systems, there is a huge increase in the volume, frequency and rate of stormwater runoff, as well as a decrease in infiltration.

## Principles of Good Site Design

- Maintain natural flow paths
- Decentralize and micromanage storm water at it’s source
- Use open drainage
- Flatten slopes
- Disperse drainage
- Lengthen flow paths
- Save headwater areas
- Maximize sheet flow
- Harvest and reuse stormwater

**Notes:**
Maintaining the natural runoff rate from a site protects receiving waters, such as streams and wetlands, stream channels, and fish and wildlife habitat. The goal is to maintain the historic, pre-developed volume, rate, frequency and duration of stormwater discharges so that discharges are not excessively high during wet, winter months or excessively low during dry, summer months. A number of techniques are available to achieve this. In this example, runoff is directed to this vegetated swale, which slows down flows and allows for infiltration.  

## Principles of Good Site Design

- Reduce storm pipes, curbs and gutters
- Reduce building footprints
- Preserve sensitive soils
- Reduce road widths
- Minimize grading
- Limit lot disturbance
- Reduce impervious surfaces
- Promote infiltration

**Notes:**
LID design requires the planner and designer to carefully evaluate the physical and ecological characteristics of the site and consider how to minimize development impacts. The goal is to work with the site characteristics to maintain hydrologic functions and processes rather than attempt to mitigate impacts. For example, avoiding the disturbance and grading of vegetated areas can significantly reduce the need for stormwater controls and will help to recharge groundwater. Reducing impervious surfaces by reducing road widths, clustering buildings and using permeable surfaces for parking reduces surface runoff and improves infiltration.

## Common Stormwater Site Design Practices

- Keeping Water Out of Pipes
- Bioretention (Rain Gardens, Infiltration Trenches)
- Permeable and Porous Pavements

-   Green Roofs
-   Planter Boxes
-   Soil Amendment
-   Open Swales
-   Rain Barrels

**Notes:**
These are some of the more common integrated management practices used in low impact development. Several research organizations throughout Puget Sound, including the University of Washington and the Washington State Department of Transportation, are gathering information and conducting research on these techniques. 

## Keeping Water Out of Pipes

**Notes:**
Disconnectivity means disconnecting impervious surface areas, and directing stormwater to vegetated areas. This slide shows how the drains from a parking garage or building can be directed to a vegetated area to reduce runoff volume and provide treatment. A conventional system would have just piped the runoff into the downstream system. 

## Bioretention

## Summary of Removal Efficiencies for Conventional Detention and Biofiltration

- Summarized from Davis et al, 2003; Cosgrove et al, 2003; Davis et al, 1998 by Ermilio, 2005

| Parameter | Dry Detention Basins | Biofiltration Basins |
| --- | --- | --- |
| TSS | 70% - 90% | 90% |
| TP | 10% - 60% | 70% - 83% |
| TKN | 20% - 60% | 68% - 80% |
| BOD | 30% - 40% | 60% - 80% |
| Lead | 20% - 60% | 93% - 98% |
| Zinc | 40% - 60% | 93% - 98% |
| TPHC | 60% - 77% | 90% |

## Open Swales

## Grassed Swale Pollutant Removal Efficiencies

- California BMP Handbook, 2003

## Parking Lot Island Infiltration Areas

## Removal Efficiencies of Stormwater Management Practices for Parking Lots

- From R. Winer, 2000

|  | TSS | TP | TN | Metals |
| --- | --- | --- | --- | --- |
| Bioretention Facilities | N/A | 65% | 49% | 95 – 97% |
| Dry Swales | 93% | 83% | 92% | 70 – 86% |
| Surface Sand Filters | 87% | 59% | 32% | 49 – 80% |
| Infiltration Trench | N/A | 100% | 100% | N/A |

## Rain Gardens

## Permeable and Porous Pavements

## Infiltration of Permeable Pavements

- Bean, Hunt & Bidelspach, 2005

## Green Roofs

## Peak Flow Reduction                        of Green Roof Runoff

- Moran, Hunt & Smith,  2005

## Soil Amendment & Structuring

## Planters

## Rainwater Harvesting & Use

- Rainwater Harvesting & Use

## Slide 53

**Notes:**
These practices can be integrated into every part of site design to create a hydrologically functional lot. Trees and native vegetation provide nature’s storage of rainfall. Permeable pavement, grass swales along roads, rain gardens and amended soils store, filter and infiltrate runoff. Adding rain barrels and other cisterns provide additional water storage. 

## Principles of Smart Growth

- Mixed Use, Walkable Neighborhoods, Public Transit – fewer car trips, less pavement: less impervious surface
- Compact Building Designs – smaller footprints:  less impervious surface
- Preserve open space, critical natural areas
- Direct development where there is existing infrastructure; maintain existing infrastructure

## Common Smart Growth Practices

- Infill Development
- Redevelopment
- Tree & Canopy Programs
- Innovative Parking
- Innovative Street Designs
- Transit Oriented Development
- “Fix it First” Infrastructure Policies

## Infill Development

- Takes advantage of areas that are already served by transportation and infrastructure
- Accommodates development that might otherwise occur on greenfield sites

- Can be coupled with site design practices such as green roofs to effectively manage stormwater

## Redevelopment

-   Sites for redevelopment are already typically covered by impervious surfaces.  Redevelopment offers an opportunity to significantly reduce runoff from the existing condition.

- Atlantic Station, Atlanta Georgia
- Former industrial area redeveloped with mixed use and open space.  Pond used not only to help manage stormwater, but is also an amenity.

## Innovative Parking

- Structured parking provides the same amount of parking using less land
- Modifying size & configuration
- Reducing number of spaces/perviousness through:
- Shared parking
- Parking lifts
- Unpaved overflow lots

- Can be coupled with site design practices such as infiltration trenches, pervious pavement or green roofs to effectively manage stormwater

## Street Design

- Connectivity to reduce car trip lengths
- Multiple modes of transportation
- Narrower roads/        less pavement
- Sidewalks to facilitate more walking

## Tree & Canopy Programs

- Trees intercept, and         evapotranspire                 significant amounts                       of water
- Trees filter                          pollutants
- Canopies shade and cool paved surfaces

## Coupling of Development Paradigms like LID and Smart Growth

- Managing stormwater along the continuum from watershed scale to site scale is critical.
- Coupling smart growth                         and green site design                 paradigms is absolutely                 necessary to effectively                     protect water quality.

## Slide 63