Detention pond grading documentation is the final checkpoint between land development grading and stormwater permit closeout. The as-built record proves that the pond was constructed to the design volume and outlet configuration that the civil engineer modeled — and that the stormwater management system will function as designed. Without it, the permit stays open and occupancy permits do not issue.
What documentation is required for detention pond grading closeout?
Detention pond grading closeout documentation requires five components: (1) a pond bathymetric survey on a 10-25 ft grid showing bottom and side slope elevations, used to calculate as-built storage volume at each design stage elevation; (2) outlet structure as-built record showing the measured invert elevation of each outlet pipe, weir crest elevation, and orifice sizes; (3) embankment compaction records for the pond berm — lift-by-lift nuclear gauge or sand cone results demonstrating the berm was compacted to the specified density; (4) side slope gradient verification confirming the constructed slopes match the design (typically 3:1 or 4:1) and are within the limits required by the geotechnical report; and (5) an as-built storage volume calculation comparing the surveyed volume to the design volume at the critical stage elevations. Most municipalities require this package to be signed and sealed by a licensed civil engineer or land surveyor.
The bathymetric survey is the foundation of the detention pond as-built record. It captures the actual bottom and side slope elevations of the constructed pond, which are used to compute the as-built storage volume using the prismatoid or contour area method.
Bathymetric survey procedure:
The outlet structure controls how water leaves the pond and at what rate. The as-built elevations of the outlet structure components must match the design to ensure the pond releases water at the correct rate for each design storm.
| Structure Component | Data to Record | Tolerance |
|---|---|---|
| Low-flow orifice | Center elevation, orifice diameter, material | ±0.02 ft |
| Riser weir crest | Elevation at each side and midpoint; weir length | ±0.02 ft |
| Barrel outlet pipe invert | Invert at inlet and outlet ends; pipe size and material | ±0.02 ft |
| Emergency spillway invert | Lowest elevation of spillway crest; width | ±0.05 ft |
| Anti-seep collar location | Distance from outlet end; collar dimensions | Document actual location |
| Energy dissipator | Type, size, invert elevation, downstream condition | Confirm matches design |
The pond berm is an earthen dam — it must be compacted to specification to provide the structural stability required to retain storm water. Embankment compaction records are required by most stormwater programs and by the geotechnical engineer as a condition of their report.
Required elements for embankment compaction documentation:
Some jurisdictions require a licensed geotechnical engineer to certify the embankment compaction records. Confirm the certification requirement with the stormwater program before starting the work.
Sitemark captures pond bathymetric surveys, outlet structure elevations, and embankment compaction records in a single package — ready for the civil engineer seal and permit submission. Start free.
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