The Low Corner That Drains Nowhere
A 1.4-acre commercial redevelopment drains to a single 15-inch storm sewer inlet at the low southwest corner of the proposed parking lot. The inlet rim is at elevation 313.0 ft; the pipe invert is 312.4 ft. The approved drainage report gives a 25-year hydraulic grade line (HGL) of 313.2 ft and a 100-year HGL of 313.7 ft. The building finished floor is 313.9 ft, 35 ft from the inlet. The lot is designed at a 2.0% minimum slope, but the contractor reports standing water after a 1.2-inch rain and proposes raising the low corner to 313.5 ft “to make it drain.” A 12-inch landscape berm and a 6-ft privacy fence occupy the only overland relief path to the street. The owner wants the lot open for a holiday event in three weeks. The contractor has submitted a no-cost change order to raise the grades and has already moved 40 cubic yards of asphalt base.
1. Evaluate whether the proposed grade change solves the drainage problem. Identify the controlling elevations and the failure mode you would expect during a 100-year event, and state what you would direct.
Read the worked answer
No. The controlling elevation is the 100-year HGL of 313.7 ft, not the inlet rim. Raising the rim to 313.5 ft leaves only 0.2 ft of freeboard; the inlet would be submerged and the parking lot would store water. The building finished floor at 313.9 ft is only 0.4 ft above the proposed rim, so a blocked inlet, debris, or wind-driven surge could put water against the building. Standing water after 1.2 in is more likely a construction defect-silt in the pipe, a crushed inlet, or grades flatter than shown-than a need for fill. Before directing any change, require a survey of the inlet and pipe, a CCTV inspection, and an as-built check of the 2% slope. Then evaluate 100-year ponding depth and volume and require a continuous overland relief path that does not rely on the berm or fence. A no-cost change order is not evidence that the change is safe; it may hide the flooding while increasing risk at the building.
2. The contractor’s change order is “no cost” and the owner wants to proceed. What documentation and approvals should be in place before any grade change is authorized, and how do you handle the schedule pressure?
Read the worked answer
A grade change affecting storm drainage is a design change, not a field adjustment. Require a signed and sealed revised grading and drainage plan, updated hydraulic calculations for the 25- and 100-year events, and a construction sequence showing how the inlet and relief path will be protected. Give the owner a written explanation that the “no cost” change does not address the HGL and may not solve the ponding. Issue approval through a formal change order or supplemental instruction, never a verbal direction. Check whether the change triggers floodplain, easement, or site-plan amendment review. Document the contractor’s request, the standing-water observation, and the owner’s direction in a field report. If the owner insists on opening for the event, recommend temporary barriers, signage, and a monitored pumping plan rather than implying the lot is safe.
3. During construction, a silt fence and a topsoil stockpile have been placed across the overland relief path. What immediate action would you take, and what permanent design fix should be shown on the plans?
Read the worked answer
Immediately direct the contractor to remove the stockpile and silt fence from the relief path, clear the inlet and pipe of sediment, and restore a continuous positive overflow route. Confirm the action in writing and notify the owner of the temporary risk. The permanent fix is a defined emergency overflow-a curb cut, concrete apron, or vegetated swale sized for the 100-year flow-shown on the grading plan with flow arrows and spot elevations. If the overflow crosses a property line, secure an easement or drainage right. The berm and fence should be relocated or interrupted so they cannot dam the overflow. Add maintenance notes requiring inspection before each rainy season and after major storms. The overflow must work even if the inlet is completely blocked.