By Sara Heger, Ph.D.
Topography and landscape position are the first things seen when the site is visited. In the broadest sense, topography describes the physical features of the land surface, including relative elevations and the aspect of the surface. Landscape position describes the location of the site relative to the location on a slope.
The characteristics of the topography and landscape position influence the way that water moves both on and within the soil. For example, the site may be at the top (summit) or the bottom (toe) of a slope. This determines the nature of surface water movement. The upslope portions of the landscape have good external drainage as water flows away from them. The lower areas have poor external drainage as water flows into them.
The shape of the slope is another important characteristic that predicts water movement on the site. The slope description describes the land surface along and perpendicular to the slope using terms of planer/linear, convex and concave. This information is useful in determining surface and subsurface drainage patterns. For example, sloping convex areas typically have good surface and subsurface drainage away from the area, while concave sloping areas such as potholes, drainage ways, and foot slopes are more likely to possess wetter soil conditions.
If the proposed wastewater treatment area is located at the bottom of a concave type landscape where water flows to one common area, the design should include precautions if the system could not be relocated to an area of less surface water flow. Some of these precautions may include surface water redirection by means of berms and swales to route the surface water around the onsite wastewater treatment system.
Groundwater also moves toward a common area in concave slopes and can raise the seasonal high groundwater table in these areas relative to the rest of the parcel of land. When excavating in these areas, the installer must be careful in assessing if the observed seasonal high groundwater table agrees with the site and soil evaluation report.
Topography and landscape can affect all activities related to the installation. If a site has drainage issues, the timing of the installation may have to be adjusted on the basis of weather delays. Temporary drainage may be needed for access to the installation area or to protect adjacent property. Equipment selection may also be affected. Staging areas must be carefully chosen to ensure access. Sediment and erosion control measures may be required.