Why Permeable Pavement? With the increase in urbanization Harris County, space that was once being occupied by soil is being replaced by impermeable surfaces. Water that is falling in these impermeable surfaces becomes polluted and is carried away as storm water. By implementing permeable surfaces, polluted water would be able to infiltrate into the ground the same way that it would with soil. This technology would not only help with water quality, but it would also help alleviate some flooding.
"Let Nature Take Its Course" Once rain water hits the ground it begins to infiltrate into the soil. The soil is then able to remove polutants from the water naturally. The rate of water infiltration (measured in inches/hour) is determined by the type of soil in the area. Sandy soils allow water to infiltrate at much faster rates than clay soils. As water seeps into the ground it saturates the soil and this process allows for aquifers to be recharged. Once the soil is completely saturated, the water remains at the surface and becomes runoff.
Cons of Permeable Pavement Permeable pavement also has its disadvantages. One of the cons of permeable pavement is that it is not meant for high-traffic areas with heavy vehicles [23]. Another disadvantage is the maintenance that it requires. Water that carries heavy loads of debris must be diverted in order to keep the pavement as clean as possible. The pavement has to be vacuumed yearly to get rid of sediments that might prevent the infiltration of water [24]. One of the biggest disadvantages is the price compared to impermeable concrete. The installation cost of impermeable pavement in Harris County would be higher than that of other cities because most of the soil in Harris County is clay.
Soil Conservation Service.General Soil Map, Harris County, Texas [25]
As seen in the map above, most of the soil in Harris County is of poor permeability. If the infiltration rate of the soil where the permeable pavement will be installed is slow, then additional steps must be taken to ensure the permeable pavement works effectively.
Potential Storage Capacity According to studies done by the American Concrete Institute, permeable concrete has a porosity of 15 to 25 percent. [36] Uniformed 3/4 in. gravel has a porosity of about 40 percent. [37] If we were to have a four-inch-thick slab of permeable concrete and a six-in-layer of gravel beneath it, we would be able to calculate how many inches of rain could be captured while that water infiltrates through the third layer of sandy soil. Shown below is the schematic of these layers.
Using the simple formula [38]: Porosity x Thickness of layer (in) = Inches of rain stored. Each amount of rain stored per layer can be added to obtain the total amount of inches stored. If we use a porosity of 20 percent for our permable pavement and a porosity of 40 percent for our gravel, the total amount of rain stored would be: (0.20 x 4 in.) + (0.40 x 6 in) = 3.2 in. of rain stored in the first two layers.
We can then calculate the amount of gallons stored by using the formula [39]: Surface Area of Permeable Concrete x Inches of Rain x 0.6 = Total Gallons stored If we were to have paved a total area of 3,000 sqft with permeable concrete, then the total amount of gallons stored would be: 3,000 sqft x 3.2 in. x 0.6 = 5,760 gallons stored
Cost Overview
Data was obtained from University of Maryland Fact Sheet. [53] Prices were adjusted with inflation. [54]
Permeable - allowing liquids or gases to pass through it. Infiltration Rate - the velocity or speed at which water enters into the soil. It is usually measured by the depth (in) of the water layer that can enter the soil in one hour (in/hr). Porosity - the quality of being porous, or full of tiny holes. Liquids go right through things that have porosity.