The Detention Pond That Moved
A surveyor prepares an as-built topographic survey for a 12-lot residential subdivision. The approved grading plan shows a detention pond with 2:1 side slopes, a 24-inch outlet at invert elevation 812.5 ft, and an emergency spillway crest at 814.5 ft. The as-built survey shows the pond 18 ft north of plan, the outlet invert at 811.9 ft, and the spillway crest at 814.1 ft. The pond is also 0.8 ft deeper than plan. Lot 7's rear corner monument is 1.3 ft from its plat location, and the lot's finished grade is 0.4 ft above plan. The developer asks the surveyor to show the pond 'as designed' so the city will sign off, and to omit the lot corner discrepancy because it will trigger a replat. The city's review engineer will rely on the surveyor's seal.
1. How should the surveyor respond to the developer's requests?
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The surveyor must refuse to show the pond as designed when it was not built that way, and must not omit the Lot 7 discrepancy. An as-built survey is a factual record; the seal certifies the surveyor's work, not the developer's compliance. Falsifying the pond elevations or hiding a corner offset would mislead the city and could affect drainage, freeboard, and lot boundaries. The surveyor should show actual elevations and locations, label design versus as-built, and note the deviations. The surveyor can offer to prepare a separate comparison table or exhibit for the engineer. If the developer insists on a false as-built, the surveyor should withdraw rather than seal it.
2. What specific design and performance concerns should the surveyor flag to the engineer?
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The outlet invert is 0.6 ft lower than design, which changes drawdown and may reduce usable storage. The spillway crest is 0.4 ft lower, reducing emergency freeboard. The pond is 18 ft north, which may affect side slopes, access, easements, or property setbacks. The pond is 0.8 ft deeper, which may partly offset the lower outlet by adding volume, but that must be computed, not assumed. Lot 7's finished grade is 0.4 ft high, which can redirect drainage toward the neighbor or create positive drainage problems. The 1.3-ft corner offset may affect lot area, setbacks, or encroachments. The surveyor should report measurements and flag the issues; the engineer should determine whether the as-built system still meets the approved performance standards.
3. What should the surveyor do about the Lot 7 corner discrepancy before sealing the as-built?
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The surveyor should investigate the Lot 7 corner before treating it as a simple as-built offset. Check the subdivision plat, the original monument record, the found monument's condition, and the adjoining corners. If the monument is original and undisturbed, it controls over the plat dimension, and the discrepancy may indicate a plat error or a boundary problem. If it is disturbed, reestablish it from original evidence and show the basis. The as-built should show the measured monument position, the record position, and the resulting offset. If the offset creates an encroachment or a lot-area problem, the surveyor should recommend review by counsel and possibly a replat or easement. Omitting it would defeat the purpose of the as-built.