Sustainable Landscape Planning and Design

Almost all PPs have a similar general structure [5, 8–11] (see Fig. 1):

Top layer (surface layer, cover or permeable paver units): This is the top layer seen by humans and the pavement type is defined according to this layer.

Bedding layer (bedding course): This is the layer which fill the voids of interlocking concrete and stone paver and is laid under the pave material. Usually sand is preferred.

Gravel base (base reservoir, base course, small stone granular base): In the majority of permeable pavements, a gravel support layer is needed to increase strength and load- bearing capacity. This layer also allows the stormwater to be removed quickly.

Sub-base (sub-base reservoir, rock sub-base, large stone granular base): This is the actual layer where the water is stored, which contains a larger aggregate than the gravel base.

Underdrains: These are drainage lines consisting of small-diameter plastic pipes and are located just below or near the sub-base to collect the water and convey it to the sewer network. They are frequently used in clay containing soils and high water tables.

Geotextile: This is laid under the sub-base to separate the subgrade from the aggregate so that the soil does not fill the gravel pores.

Subgrade (sub-base, in situ soil, underlying soil): This is the bottom natural ground and is compressed at different levels depending on the purpose of the pavement.

Fig. 1. General structure of PPs

2.1. Open-jointed paver units

Paving blocks are concrete, brick or stone units and aggregate or turf in the joints enables these to be porous [1] (see Fig. 2). They are used on driveways, walkways and low-use parking lots. They can be found in many different colors, forms, sizes and thicknesses. Thicker pavers are suitable for heavier use. It is visually impressive [12]. However, paver blocks are more expensive, deformation can occur in the lower layers and require coating to prevent swelling [1]. Retaining is required to fix the edges. It is not suitable for areas with more than a 5% slope. Occasionally, joint material needs to be renewed and weeded [12].

Fig 2. Types of open–jointed paver units

2.2. Pervious concrete

Pervious concrete is a mixture of cement, coarse aggregate, water and admixtures. It does not contain sand, and has a void content between 15–30% [15], see Fig. 3 [16].

Fig. 3. Pervious concrete

2.3. Pervious asphalt

Pervious asphalt is a mixture of fine and coarse aggregate and bituminous asphalt binder [1, 12]. It contains void between 15–20%. Depending on user load, the thickness ranges from 7.5 to 18cm. A base course is required to increase strength and storage capacity [10]. Pervious asphalt, parking lots, walkways and roads are suitable. [12]. Asphalt is a common and cheap paving material. In addition, pervious asphalt is permeable [1]. Pervious asphalt is paved quickly and easily. The cost is similar to conventional asphalt. With it, uninterrupted and flexible surfaces can be obtained. Its maintenance requirement is low and it should be cleaned with low pressure water or mechanical sweepers [12]. In some applications the asphalt binder may cause clogging. Or, when the binder is too liquid, it may flow down the pores and release the aggregate [1]. Recycling is limited and the surface is rougher [12].

2.4. Unbound aggregate

Aggregate types are gravel, crushed stone, crushed brick, decomposed granite etc, see Fig. 4. A single-size aggregate mass contains void between 30–40% and has high permeability. Aggregate can consist of rounded or angular granules and can be in dense-graded or open-graded form. The granule size affects the porosity, stability and walking comfort [1].

Fig. 4. Unbound aggregate samples

2.5. Turf

Turf areas cope easily with low pedestrian traffic. Roots maintain the permeability of the soil. In such areas, it is necessary to use a seed mixture suitable for being trodden on, as well as climate and soil. Maintenance work such as mowing, irrigation, fertilization and weeding is required as for all green areas.

If the soil is clay, heels and wheels damage the surface in wet weather. Such surfaces can be reinforced for heavier traffic using geocells, grids or blocks [1].

2.6. Soft pervious pavements

Fig. 5. Bark mulch

3. Functions and Benefits

Storm water management: A significant part of the rainfall in urban areas ends up on impermeable surfaces [3]. Impervious surfaces cause runoff during even small storms, because they are impermeable and they have limited storage [21]. Impermeable pavements can cause flooding and erosion of stormwater drainage systems. PPs reduce the amount of runoff by leaking some of the rainfall to the ground thanks to the voids. At this point, groundwater is charged and the need for a storm water drainage system is removed or the size of the necessary system is minimized [1–3, 5, 6, 10]. Compared to conventional drainage systems, storm water retention and infiltration is a more sustainable and cheaper process for urban areas [10].

Pollutant control and improving water quality: Pollutants such as heavy metals, total phosphorous, total nitrogen, oils, hydrocarbons, nutrients, bacteria, and organic matters are accumulated on impermeable surfaces and transported by runoff to reach water resources directly without being cleaned [3, 8, 21]. PPs are the technologies that can be installed to control and retain these pollutants [8]. They reduce the contamination of water resources by reducing the amount of runoff, filtering pollutants during underground infiltration, and helping to biodegrade hydrocarbon pollutants [2, 3, 5, 21]. They also charge aquifers and groundwater [2].

Fig. 6. PPs for urban trees

Reducing urban heat island effect: Approximately 90% of the increase in temperature in cities is due to building surfaces that absorb and store solar heat. These surfaces cause the temperature to increase even after sunrise by re-radiating the heat they have stored [1]. This is called the urban heat island effect, and impermeable pavements enhance it [1, 5, 22]. PPs, also referred to by the EPA as cool pavements, have been accepted by the United States Environmental Protection Agency (EPA) as a strategy to reduce the urban heat island effect [23]. The increase in temperature causes more energy to be used for cooling and more carbon dioxide emissions from more energy-producing plants [1, 7, 24]. High temperatures accelerate chemical reactions that convert emissions into ozone and cause air pollution. At the same time, runoff from the hot surfaces increases the temperature of the nearby waters, reduces the oxygen holding capacity and affects vitality [1] and sensitive biological communities [6] in the waters. High temperatures also bring about problems related to human comfort and health [1, 21]. The voided structure of PPs cools the surface with natural evapotranspiration. It reduces the urban heat island effect by allowing long-lived shade trees to grow [1].

Safety: PPs are more successful than impermeable pavements in terms of safety of pedestrians and vehicles. Water and oil that accumulate on the pavements leak down from the pores and surface accumulation is prevented. This reduces the risk of slipping. For example, the coefficient of friction of the impermeable asphalt drops to one in four when it is wet, whereas the permeable asphalt keeps its friction value in dry weather conditions [1].

Aesthetic and naturalness: Open-jointed pavements made from natural stones, unbound aggregate, turf and soft pervious pavements provide a natural image and a satisfactory effect in pedestrian areas. PPs soften the character of paved areas and make them compatible with residential communities. On one hand they restore the perceptual link between the environment and society; on the other hand they improve natural functions. They also maintain the quality of the environment and human life as a whole [1].

4. Fulfilling ASLA and LEED sustainable design criteria

The Sustainable Sites Initiative, constituted by American Society Landscape Architects (ASLA), has developed a rating system for sustainable land design and development [26]. PPs can contribute to some of the credits in this system, see Table 1.

Table 1. ASLA’s sustainability credits which are fulfilled by PPs