How do impervious surfaces affect land development?

When we think of impervious surfaces in land development, our minds probably go straight to paved roads. That’s a good start, but we also have to take into account other non-porous surfaces. Examples would be improvements like sidewalks, cul-de-sacs, roundabouts, parking lots and homesite pads. Plus, how interactions take place with natural conditions like topography or immovable features such as clay layers or rock formations.

Why are impervious surfaces such a big deal?

The fast an all encompassing answer is the effect on natural drainage. If impervious surfaces like asphalt roads are used in the project, it will produce changes in how surface water will runoff and also impact the natural absorption of water by the soil. When water absorption points are changed, recharge of water into underground aquifers can change too. It seems we have both surface and subsurface considerations to think about.

These simple equations show the impact of impervious surfaces:

If we have a 26’ wide asphalt road, the amount of impervious surface created per 100’ is 2,600 sq. ft.

26’ x 100 = 2,600 sq. ft.

If our road system is exactly 1 mile long at 26’ wide, we are creating 137,280 sq. ft. of impervious surface per linear mile.

2,600’ x 52.8 sections of 100’ = 137,280

That’s a lot of potential impact and you’re not the only one thinking about it. Public Works and state Ecology Departments are going to be interested too and they can be pretty picky about how our impervious surfaces will change native conditions.

This raises some fairly immediate concerns during the design phases. Anyone that’s ever paid for asphalt is going to be thinking about how to use less of it, albeit for different reasons than the county and state who would like less of it for environmental reasons. More on that later…

Drainage systems

We are not going to get far in the project review without a detailed plan for handling the drainage issues that impervious surfaces cause. That means a quality storm water drainage plan and a look at how the onsite improvements are going to affect offsite hydrology. Yes, rerouting of water within the project can have area and even regional impacts.

This link provides a more in-depth look at drainage considerations:

Why Is Drainage Needed In Land Development: https://www.landdevelopmentrealities.com/home/2020/8/3/v09ipvf9g9osk15z75xgdv5wskbiit-fktfy-jb8nf-dbfhh-c2xey-67bef-jyhwg-zbf58

Engineered plans for impervious surface mitigation must consider a variety of factors, to include:

Routing surface runoff, channel and point drainage.
Collection points.
Detention ponds.
Retention ponds.
Filtration of water.
Topography of the land.

Topography:

Cliffs, gullies and slope conditions are three examples of how natural topographic conditions affect the project. An in-depth look at topography can be found here:

Topography In Land Development: Topography in Land Development — Land Development Realities

Beyond that, topography is also a key factor in where roads are placed…and hence, where massive impervious surfaces wind up. There are two engineering theories out there:

Design the roads first, then plan the homesites around them.
Choice #2 is the opposite - plan the homesites, then design the road.

I’ve done it both ways but topographic features place a heavy impact on road placement and the location of the drainage systems either way. I figure the best way to deal with topography is having the engineering team factor it in conceptually before you wind up owning the property in the first place.

Alternative surfacing:

When I look at the material and labor costs of asphalt, plus the burden it puts on surface runoff, I always wind up dreaming about other ways…and that’s about all I do… dream about it. Gravel and dirt roads just don’t cut it in my opinion and with sub grade compaction and layering they don’t wind up being very water absorbing anyway. Look how many water channels you see on older gravel and dirt roads for proof.

Bituminous Surface Treatment (BST) can be generally less expensive than asphalt, but not that much in my experience. I think a better promise for the future is porous asphalt-like materials, but I have no experience or expertise with using them.

For those that do, see this article for details on asphalt alternatives from Remi Network: https://www.reminetwork.com/articles/the-green-alternative-to-traditional-asphalt/

Contact me at: ldr@landdevelopmentrealities.com