The issue of
impervious surfaces is one that is difficult for some people to grasp. What’s an impervious surface? Simply said it is any surface that sheds
water. With that in mind a lawn is a
pervious surface as rainwater will generally soak into the ground. A driveway is an impervious surface as
drainage occurs and water runs to the sides of this structure surface. So is a
roof. Run off can occur on all surfaces
given enough volume with large storms but the standard that is looked at is surfaces
that create run off all the time are considered impervious.
The reasons
we look at impervious surfaces carries a few concerns. With the lakes we have we want to maintain
water quality. Controlling impervious
surfaces lessens the amount of runoff created by them. The more impervious you have the greater the
runoff and the greater the velocity of that runoff. Velocity of runoff is important as the
greater the velocity the greater potential for erosion. With erosion comes silt, dirt and pollutants
that get carried away to end up in our streams and lakes. Water quality is then affected.
Erosion
carries its own problems. With gravel
soils and roadways heavy rains degrade the roadway surfaces, fill in the
drainage ways and limit our ability to adequately deal with rain events. Soils that are deposited by erosion
eventually need to be removed from drainage ways, storm water ponds, and other
areas. The removal is a costly
effort. Limiting the source of some of
this runoff lessens the problems of erosion by decreasing the volume of runoff,
reducing the velocity, and keeping the pervious surfaces available for
absorption.
Another
issue with impervious surfaces is the need for drainage facilities and storm
sewers. The more impervious surfaces
that we have the more need there is for adequate drainage facilities. It is an easy concept to grasp that if you
have 50% impervious surfaces you have more runoff than 25% impervious
surfaces. Having more runoff means, in
the case of storm sewers, larger pipes are needed at a higher cost. Some of this can be managed with storm sewer ponds
as they collect water and release it as the outflow allows. This seems like a good solution and does
solve the part of the problem. Detention ponds take space that in many
instances can’t be used for other purposes.
They also control sediment which needs to be periodically cleaned out.
If there is
a lack of or inadequate drainage facilities the issues get compounded with
greater impervious coverage. More runoff
and continued inadequate facilities to deal with it. A smaller problem becomes a larger one. Solving the problem in the first place is
costly. Correcting it can be more costly
and managing it over time means constant maintenance.
I hope the
significance of managing impervious surfaces becomes a bit clearer. A property owner may not think their
situation causes a problem. In all
rights a single property may not be that significant but extending that
privilege to all creates the problem so having a standard is important and
should be adhered to.
