Thursday, September 12, 2013

Why is Imprevious Coverage Important



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.