Impervious Surface Removal

Quietly, while no one was looking, DDOT has been taking something right out from under you! But, there’s no need to thank us – we’re just trying to make our city a more Sustainable DC. John Thomas, our Chief Forester, explains the clear benefits of a project with a slightly abstruce name: Impervious Surface Removal.

The Urban Forestry Administration (UFA) applied for and received American Reinvestment and Recovery Act (ARRA) Grants to fund three coordinated projects:  Impervious Surface Reduction, Green Median Renovation, and Tree Canopy Renovation.  These projects were all funded by ARRA via the Environmental Protection Agency (EPA), through the Clean Water State Revolving Fund (CWSRF), and were administered by the District Department of the Environment (DDOE).

Each of these ARRA projects at UFA was designed to reduce stormwater runoff and increase the urban tree canopy. The original intent was to focus on the Combined Sewer Overflow (CSO) since the combined sanitary and stormwater sewers frequently have overflows during heavy rains that discharge untreated sewage into the Potomac River and ultimately the Chesapeake Bay.  Over time however, projects were identified in areas outside of the CSO and the project was expanded to include work in these areas as well.

DDOT already plants thousands of new trees each year, but another way the District of Columbia can reach its 40 percent tree canopy goal is to break into areas with impervious surface and make them available for trees.  By increasing the permeable area available for urban tree canopy establishment, UFA is extending beyond a basic tree planting strategy.  Now UFA is advancing urban tree canopy into areas where it has not heretofore been possible.

Impervious Surface Removal: The Impervious Surface Removal Project has focused on increasing the green space within the public space of DDOT roadways.  This was accomplished through a combination of practices including  tree box expansion, tree box creation, continuous strip creation, and large area greening.  By removing impervious surfaces, UFA has increased the soil area for root growth to 36″ in most cases, increased the interception of stormwater runoff and obtained increased environmental benefits by planting larger canopy tree species in the tree boxes and continuous planting strips.

Project Square ft $ Spent Cost/sq ft
Impervious Surface Reduction 80,303 $ 1,223,414.34 $15.23
Green Median Renovation 44,203 $ 636,386.37 $14.40
Total 124,505 $ 1,859,800.71 $14.94
Funding $ Spent Percent
ARRA funded $ 1,850,000.00 99.5%
Local funded $ 9,800.71 .5%
Location Square ft $ Spent  
Projects located in CSO 107,176 (86%) $ 1,614,000.45 (87%)
Projects located in MS4 17,329 (14%) $ 245,800.26 (13%)

To date, UFA has removed more than 3 acres of impervious surfaces, converting these areas into new green space. Here are some examples of where this transformation has taken place.

Tree Box Expansions:  Expanding tree boxes results in greatly expanding the rooting zone for street trees and will provide for increased stormwater retention and allow for a larger, healthier, longer lived tree to develop, ultimately increasing the Urban Tree Canopy and providing many other environmental benefits.

Old Morgan School Way, NW - Before

Old Morgan School Way, NW - After

Tree Box Creation: The addition of 6 new tree boxes down the length of a block, like Champlain Street, NW shown below, results in a significant greening of the streetscape.  This will ultimately provide many more environmental, aesthetic and economic benefits than a block without trees.  The addition of trees also reduces the urban heat island effect.


Champlain Street, NW - Before

Champlain Street, NW - After

Continuous Planting Strip Creation: The creation of a greenway along District streets can significantly reduce the amount of impervious surface within the streetscape.  This continuous planting strip located on P Street, NW removed 7,200 sq feet of impervious surface and created numerous planting locations.  This area now  has the potential -depending on rainfall intensity – to intercept the majority of the rainfall that falls within the sidewalk dimension.  In additon, UFA was able to remove 36″ of old soil and replace it with new top soil with organic matter, therefore increasing the soil volume.

P Street, NW – Before
P Street, NW – After

Large Area Greening:Where opportunities within public space exist to green large expanses of impervious surface there can be a significant change in the aesthetics and environmental impact of the built environment.   When contiguous areas of over 10,000 square feet of impervious surface are greened, the benefits through reduced stormwater runoff, reductions to the urban heat island effect, and increases to the urban tree canopy, are immediately apparent.

P and North Capitol Streets, NE – Before
P and North Capitol Streets, NE – After

Green Median Renovation: Existing medians that are finished with a hardscape such as brick, concrete or pavers can be renovated by removing this impermeable surface, and replacing with a turf or mulched surface and planting street trees.  These changes to the streetscape will result in reductions to the amount of stormwater entering the sewer system and ultimately the District’s waterways.  The increase in planting locations will result in an expansion in the Urban Tree Canopy.  This project delivers substantial reductions in the blighted look that can result when there are multiple travel lanes which reduces the potiential for tree cover.

Bladensburg Road, NE - Before

Bladensburg Road, NE - After

Calvert Street, NW - Before

Calvert Street, NW - After

Tree Canopy Renovation: The goal of the Tree Canopy Renovation project is to improve the condition and coverage of the tree canopy so that additional rain fall is intercepted and does not enter the stormwater system.  This goal is being met by removing trees that are dead or dying and planting new trees in the now open tree boxes.  Replacing dead and dying trees which have a limited canopy with a newly planted tree with a vigorous canopy will provide for net canopy increases.  This will dramatically improve the canopy coverage for the CSO area resulting in increases in rainfall intercepted and decreases in stormwater runoff.

Tree Canopy Renovation by Ward
1 2 3 4 5 6 7 8 Citywide Total
Trees Removed 457 722 83 126 199 538 26 58 0 2209
Stumps Removed 58 67 8 2 19 20 0 1 0 175
Trees Planted 309 616 30 378 458 485 31 140 137 2590

These are just a handful of the projects completed over the past 2 years. Here’s a link to a complete list. Some of these changes are small – just a few feet at a time – and some cover much more ground, but combined they make the District a more beautiful and environmentally friendly place to live, work and play.

In addition, as intended, the recovery money used to fund these projects, put people to work. UFA was able to to hire five full time employees to manage the three projects outlined above.  They have been employed for almost 18 months now with benefits and have been able to gain valuable experience in urban forestry, customer service skills and engineering.  It will provide a solid foundation for their careers.  The ARRA funds also allowed the contractors to keep a tree removal and a concrete removal crew on through the length of the project, including during the winter months  when construction work is otherwise limited since pouring concrete requires warm weather while concrete removal is not affected by the temperature.

John Thomas
Associate Director for Urban Forestry

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One Response to Impervious Surface Removal

  1. Dana Dunmire says:

    This is superb–thanks very much to Urban Forestry and DDOE. My dream is that this becomes routine for DDOT and other departments of transportation–can we do this as a matter of course and not only when there is ARRA funding, say when sidewalks and pavement are being replaced?

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