Much of the Western United States is mountainous hilly land, and as the population expands, we are 
continually encroaching on those hills.  The sight of land movers cutting into and topping hillsides is 
becoming commonplace.  Many times I have seen large, barren slopes between new buildings.  
Whether it is a large commercial park, a new residential community, or just a single home being 
erected on graded land, it all can have adverse affects on water runoff.
Houses, buildings, parking lots, and roads are all impervious surfaces that increase waste water 
runoff, but many people don’t realize that removing plant matter or topsoil, grading the land surface,
and making steeper slopes reduces the infiltration of water into the ground, and increases runoff volumes.  During rainfall, this accelerates water into drainage systems, ditches, and streams.  The result of all this is an increase in frequency of flooding.
Flooding, and therefore water runoff, should be a major concern for the design industry, not only in relation to building owners and liability issues, but also in regard to the environment.  Graded hillsides without plant life increase storm water runoff, can have problems with soil movement or mud slides, and can lead to increased flooding.  But there are things that can be done.
We can mitigate the effects of grading land.  Hillsides and slopes can be seeded, and shrubs and small plants can be planted to help slow water, allowing it to seep into the soil.  When water infiltration is increased, groundwater is recharged and downstream flooding is reduced.  There are other benefits to this approach.  Planting trees, shrubs, any plant life is “green.”  It’s ecologically friendly, good for the environment.  Plants give off precious oxygen while the roots help stabilize the soil.  Plants also help reduce the heat-island affect.
Besides plant life, there are other options for slopes to help reduce water runoff.  Woven fabric matting works well on steeper grades by slowing the flow of water, and over time, will help plants take hold in the slope.  Coir logs and netting increase water infiltration and reduce erosion.  Made from natural coconut fiber, coir logs collect and accumulate local seeds and, once vegetation is established, biodegrade naturally.  Boulders and barriers can make a difference too.  Adding larger rock or timbers at right angles on slopes should decrease soil movement and slow the water.  Then there are terraces.  Terracing hillsides has been done for centuries, and can be used not only to control water runoff, but also to promote vegetation.
With building owners and the general public looking for environmental and “green” alternatives, designers can’t be one-dimensional, but need to be balanced.  We have to take into consideration not only the existing infrastructure and buildings, but also the landscape and environment. 

Marcus Dodson
editor & publisher
Water Infiltration
Mitigating the Effects of Wastewater Runoff