A building addition can occupy a small part of a site while its drainage design depends on grades across a much larger area. A survey limited to the new walls may show where the addition sits but omit the low point, paved route or existing drain that determines what happens to runoff. The civil designer should identify those questions before the surveyor sets the field limits.
Include the surrounding grades
Consider a hypothetical addition beside a parking area. The pavement continues past the addition toward a street catch basin. The relevant survey area includes more than a narrow line to that basin: adjacent grades, curbs, swales, walls and low spots can affect the direction water takes and the options the engineer can evaluate. A low point outside the addition’s footprint may matter even when it lies away from the apparent route to the basin. Omitting it can leave the engineer without part of the terrain needed to evaluate drainage.
Locate accessible surface structures and measure the existing grades the design will use. A mapped basin, however, is only a possible outlet. Its visible position does not establish the pipe invert, ownership, capacity or permission to connect. Identify which elevations require further measurement and which questions need utility records, engineering analysis or confirmation from the utility owner.
Include off-site conditions when the project requires them
For covered Bellevue land-use submittals, the City’s survey requirements call for adjacent street information and an off-site upstream and downstream structure for sanitary and storm systems. Its stormwater site-plan guidance also calls for existing and proposed contours on and off site and for specified drainage elevations. Other jurisdictions and project types have different requirements; the engineer must establish what the actual design and submittal need.
Mark the addition, surrounding grades, candidate outlet and any unresolved off-site information on the survey request. Terrane’s guide to starting survey and engineering together explains how to settle those responsibilities before fieldwork.
