The Role of Gravity In Site Planning?
When creating a site plan design, a land planner or civil engineer will typically be focused on how the grade elevations will allow storm water runoff and sewer lines to flow with gravity to existing infrastructure. Without gravity, constructing new sewer and storm water infrastructure can be very costly. Other utilities, such as water or electricity or natural gas, do not require gravity to be cost efficient in construction.
With respect to storm water runoff, a land planner will be looking at how water currently runs off the undeveloped property and where this runoff eventually ends up. With this is mind, the site plan design will work towards lot elevations and street grades to allow the storm water to get to the same place post-development. Or, the designer might look at existing storm drain infrastructure and look for other options for the runoff to go.
In terms of sewer design, the land planner will be looking at where the new sewer lines can connect to the existing sewer mains in the most cost-effective manner. Gravity sewer lines are the most cost effective, avoiding sewer lift stations or deep sewer lines that can add significant costs to a development. In some cases, a portion of the property may be too low and not allow gravity to “flow up” to the existing main. The options can include the lift station or possibly importing dirt to raise the low area, but that can also be an expensive option.
In some cases, you might hear a land planner state the site is “too flat”. This means that the storm water and sewer do not have any grade to allow gravity to take effect. These flat sites typically need some extra dirt to raise one part of the site and provide for gravity. Good designers will understand these issues, and sometimes the solutions take some extensive effort between the land planner and developer to find cost effective solutions.
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