Alleys Go Green with PicP

Summer 2012

Consider design carefully when paving green alleys

By David Smith


Alleys Go Green with PicP

Green alleys, using permeable interlocking concrete pavement (PICP), require careful consideration during the design stages. Start with the end in mind, specifically calculating the amount of water that needs to be managed and where it is draining.

The first steps of the project are calculating runoff volumes from adjacent pavements and buildings, as well as timing how quickly water moves from roofs to the alley and into storm sewers.

In downtowns and highly urbanized areas, alley sewers receive roof runoff from downspouts and adjacent pavements. Green alleys provide a means to slow this flow, and in some cases, infiltrate a portion of the water into the soil subgrade. Slowing the flow can be done with berms and/or constricted outflow pipes that pool into catch basins before draining into the larger sewer system.

slow the flow

Berms are often required to slow flows when the subgrade slope exceeds 3 percent. Berms can consist of concrete, soil left in place, or lateral trenches dug across the width of the alley. The simplest berms consist of an open-graded base wrapped with two layers of geotextile, sitting atop the soil subgrade. Regardless of the material, the height of each berm depends on how much water needs to be detained and infiltrated (if soils are cooperative). Modeling flow is characterized by small ponds flowing one into the next. The outflow from one to the next can be via sheet flow, a knotch, or through a pipe (or pipes) penetrating the berms and aligning with the slope (i.e., oriented longitudinally).

Roof downspouts should drain water into the PICP base instead of its surface if there is an abundance of adjacent impervious surfaces. This reduces the risk of downspouts draining across impervious pavement and mobilizing sediment into the PICP openings. In addition, downspouts joining PICP base will flow longer in winter. In suburban settings, water from downspouts can run across vegetated areas, allowing for infiltration. Estimating the amount of runoff from the vegetated areas should assume the worst-case scenario for saturated soil infiltration rates.

modeling extremes

Depending on the geographic region, there could be extreme storms and rainfall depths that cannot be drained by the alley. When designing a green alley in these areas, the conditions that create flooding need to be modeled (i.e., the worst-case flooding scenario, including the depth and duration of standing water). If surface or subsurface bypass pipes or overland passages drain water from extreme rainfalls, all the better. If not, then the alley should be designed to minimize adjacent property damage in the event of deluge and flooding.

In downtowns and highly urbanized areas, alleys run next to building foundations. The foundations or sides of the alley will require covering by an impermeable liner to prevent water from infiltrating building foundations and basements, especially if the alley is designed as a detention/infiltration facility. This can be accomplished with restricted outflow pipes or perforated pipes along the soil subgrade that hold water until it deepens and then flows out of elevated drainpipes. In other instances where soil infiltration is negligible, there may be perforated drain pipes at the bottom of the alley that remove water almost immediately, which might obviate the need for liners against foundations.

Alleys often have utility poles that should not be surrounded with open-graded base. The utility poles should be moved or the open-graded base should be directed around them. They should be treated as structures; therefore, impermeable liners should be placed several feet away from their perimeter. Another option may be encasing the underground portion of the poles in concrete. Consultation with an engineer from the company that owns the poles is essential. The depth of the poles and distance of undisturbed soil around each should be discussed.

Alleys may have some distance between the back of the buildings and the alley surface. This space can be lawn, a separate garage or outbuilding, or gravel or dirt parking spaces. To reduce the risk of sedimentation and clogging, consider paving these surfaces with permeables as well. PICP alleys require curbs, which occasionally can be sloped to block or divert water and sediment flows from adjacent parking spaces away from the permeable surface.

Alley maintenance is almost non-existent in many cities because surface cleaning, snow removal and deicing are not budgeted. All permeable pavements for green alleys will require regular surface vacuuming. Because some sewer authorities have installed green alleys, they have assumed surface cleaning, especially if the city doesn’t maintain alleys. Surface cleaning is essential, because incidental litter from trash, leaves and grass clippings can accumulate on alley surfaces. For whomever maintains the alleys within a city, green alleys should be included in computerized pavement or sewer management systems.

transforming character

Underground utilities generally are not an issue, as long as they consist of waterproof piping and are deep enough to survive compaction of stone base materials over them without damage. Keep in mind that some utility lines are encased in open-graded stone, and that stone can be a pathway for water if adjacent to a permeable pavement base. In such cases, the flow of water onto the stone around a pipe exposed at the side of a permeable pavement excavation will need to be stopped with low-strength concrete fill.

When alleys go green with PICP, they can be paved with light colored paving units, which can enhance night lighting and help reduce urban temperatures. Alleys in older cities are often made of stones or bricks, and PICP can take on the appearance of either surface, thereby preserving and reinforcing the paving tradition and design context of these older cities.

The transformation from an unsightly alley in a backyard to a clean, well-kept space may increase private property values and earn money for the city in the form of higher property taxes. And for cities with combined sewers, green alleys are another effective tool in the green roads/green infrastructure toolbox that can reduce combined sewer overflows with stormwater detention and infiltration.

Combined sewers or not, green alleys can increase a neighborhood’s character and provide much-needed public spaces in dense urban areas.