Level Loop Misclosure at a Detention Pond Outfall
You are laying out a 4.6-acre detention pond in Bibb County. The outfall is an existing 18-inch RCP discharging to a creek, and the design requires the pond's 100-year water surface of 812.00 ft to be at least 0.50 ft below the crown of the pipe invert so the pond drains by gravity. The nearest published NAVD88 mark is brass disk 'Creek 2' at 810.214 ft, 0.9 mi from the site. Your first closed level loop of eight setups returns a misclosure of +0.052 ft. The firm's specification for this class of work is 0.05 ft × √M, where M is the loop length in miles. A second, independent loop over the same route closes at −0.018 ft. From the second run, after adjustment, you have: backsight on 'Creek 2' = 4.372 ft; foresight to a rod held on the pipe invert at the outlet = 3.964 ft; the adjusted correction at the outlet setup is +0.016 ft.
1. Evaluate the first loop against the firm specification and state your next action.
Read the worked answer
Allowable misclosure = 0.05 × √0.9 = 0.05 × 0.9487 = 0.047 ft.
The observed misclosure is 0.052 ft, which exceeds 0.047 ft. The loop fails and the elevations from it cannot be used, regardless of how small the excess looks.
Before re-running, look for the cause, because a misclosure that is only slightly out of tolerance is often systematic rather than random: unbalanced backsight and foresight distances that expose a collimation error, a rod not plumbed on the long shots, heat shimmer over pavement, a settling instrument, a turned or disturbed mark, or a benchmark elevation taken from the wrong datum. Re-observe with balanced sight distances, shorter shots, and a second published mark as a check.
The second loop closed at 0.018 ft, well inside 0.047 ft, so it is acceptable and can be adjusted. Record both loops, the rejection, and the corrective action in the field book. The rejected loop is not erased; it is the evidence that you checked your work.
2. Compute the invert elevation and the crown elevation of the outfall, and evaluate them against the design criterion.
Read the worked answer
Height of instrument = 810.214 + 4.372 = 814.586 ft.
Unadjusted invert = 814.586 − 3.964 = 810.622 ft.
Adjusted invert = 810.622 + 0.016 = 810.638 ft, say 810.64 ft.
Crown = invert + pipe diameter = 810.638 + 1.50 = 812.14 ft.
The criterion requires the 100-year water surface (812.00 ft) to sit at least 0.50 ft below the crown, so the crown must be at or above 812.50 ft, which means the invert must be at or above 811.00 ft.
The outfall is 811.00 − 810.638 = 0.36 ft too low. The crown is only 0.14 ft above the design water surface instead of the required 0.50 ft, so at the 100-year stage the pipe is effectively submerged and the gravity outfall the design relies on will not perform. That is a design and permitting problem, not a layout problem: the pipe is where the creek bank says it is.
3. What do you report, to whom, and what do you do about the pond layout? Show the reasoning behind each action.
Read the worked answer
Report to the design engineer first, in writing, with the benchmark, loop data, misclosure, adjustment, and the computed invert and crown. The engineer owns the hydraulic conclusion; you supply the measured fact and the arithmetic. Copy the owner, since the owner is paying for a pond that may need a design change (raising the outfall, adding a relief structure, revising the water surface, or accepting pumped discharge).
Do not proceed with final grade staking to the plan elevations as though nothing were wrong. If the pond has already been staked, flag it and re-stake only after the engineer issues a revised design. Setting grade to a design that cannot drain creates a defect you would be certifying.
Document the two loops, the rejected run, the datum used, and the date and personnel. If asked later to certify the as-built pond, you certify only measured elevations against the design, never 'meets design intent' as a conclusion on the engineer's behalf. If the owner pressures you to smooth the number, the correct response is that the elevation is the elevation and the discrepancy is the reason the check exists.