Construction sites are rarely flat. Changes in elevation across a site can affect how stormwater moves, where runoff concentrates, and which erosion and sediment controls may be appropriate. One term that often comes up when discussing site grading and stormwater management is grade differential.
A grade differential is not a Best Management Practice (BMP) or a stormwater control by itself. Instead, it is a site characteristic that can influence how stormwater behaves and how a project manages runoff.
What Is a Grade Differential?
Simply put, a grade differential is the change in elevation between two locations.
For example, if the elevation at the top of a construction site is 5,020 feet and the elevation at the bottom is 5,000 feet, the grade differential between those two points is 20 feet.
Grade differentials can occur naturally because of existing terrain or be created through construction activities such as excavation, grading, building pads, road construction, and drainage improvements.
The important thing to remember is that grade differential describes the change in elevation between two points. It does not, by itself, describe how steep the area is.
Grade Differential vs. Slope
Grade differential and slope are closely related, but they aren’t the same thing.
Grade differential describes how much the elevation changes between two points. Slope considers that elevation change in relation to the horizontal distance between those points.
For example, if a site drops 20 feet over 100 feet horizontally, the slope is 20%. If the same 20-foot elevation change occurs over 1,000 feet, the grade differential is still 20 feet, but the slope is only 2%.
This distinction is important when evaluating stormwater because the same elevation difference can produce very different runoff conditions depending on the distance and terrain between the two points.
Is Grade Differential a BMP?
No. The grade differential on a site is not a BMP.
A grade differential is simply a characteristic of the site’s terrain. However, grade differentials can help project teams understand why certain BMPs may be necessary.
For example, a significant elevation change may create a slope where runoff gains velocity and has a greater potential to cause erosion. The project team may then use appropriate erosion and sediment controls to manage that runoff.
Why Does Grade Differential Matter for Stormwater?
Water naturally flows downhill, which means changes in elevation play an important role in determining how stormwater moves across a construction site.
As the grade changes, runoff may concentrate in certain areas. Steeper slopes can also increase the potential for runoff to gain velocity, which can contribute to erosion and sediment transport.
Understanding the site’s elevation changes can help project teams identify:
- Where stormwater is likely to flow
- Where runoff may concentrate
- Areas that may be more susceptible to erosion
- Where additional erosion and sediment controls may be appropriate
- Where runoff or velocity-control BMPs could be needed
- How grading changes may affect existing BMPs
A grade differential does not automatically mean that a specific BMP is required. Instead, it is one of many site characteristics that can be considered when developing and maintaining an effective stormwater management strategy.
Final Thoughts
A grade differential is simply the difference in elevation between two points. It is not a BMP, but understanding elevation changes can be important when evaluating how stormwater will move across a construction site.
Grade differentials can influence runoff patterns, erosion potential, and the placement or effectiveness of stormwater controls. As construction changes the site’s terrain, project teams should evaluate whether those changes are affecting existing drainage patterns or BMPs.
When it comes to stormwater compliance, the goal isn’t simply to understand the elevations shown on a plan. It’s to understand how those elevations affect water movement in the field and whether the project’s stormwater controls continue to address actual site conditions.
At Summit Services Group, we help Colorado contractors manage stormwater compliance through inspections, SWMP support, BMP evaluations, and ongoing compliance services.