Bioswale Freeboard and Utility Conflict
A 0.9-acre parking lot drains to a 300-foot bioswale along its downhill edge. The 10-year design storm intensity is 3.2 inches per hour, and the swale is sized with a 1.0% longitudinal slope and check dams every 50 feet. At the outlet, the swale bottom is at elevation 103.0, the existing 12-inch storm drain invert is at 102.4, and seasonal high water is at 101.8. The proposed ponding depth behind the check dams is 12 inches, with 6 inches of freeboard to the adjacent pavement edge at elevation 104.2. The contractor's shop drawing shows a 6-inch orifice outlet and an overflow weir set 4 inches above the ponding design elevation. The project is in a combined-sewer area, and the utility company has an 8-inch water main 3 feet from the swale centerline.
1. Evaluate whether the proposed swale geometry provides adequate freeboard and overflow.
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The freeboard claim does not check out, and the overflow is set too high.
- Swale bottom 103.0 + 12 in ponding = 104.0 water surface.
- Pavement edge 104.2 gives only 0.2 ft (2.4 in) of freeboard, not 6 in.
- The overflow weir at 4 in above the ponding design is 104.33, which is above the pavement at 104.2. The pavement would flood before the weir functions.
- Revise the design: reduce ponding to 8–10 in, raise the pavement edge, or lower the overflow weir to at least 6 in below the pavement. Confirm the outlet orifice and tailwater at the 12-in storm drain invert 102.4; the swale bottom is only 0.6 ft above the invert, so the outlet may be submerged.
- The design storm must be routed, not just sized by a rule of thumb.
2. What changes would you require in the construction documents?
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Revise the construction documents to show the actual water surface and overflow route.
- Add a swale profile with check dam elevations, ponding elevations, and the overflow weir elevation.
- Show the emergency overflow path across the pavement and to a safe discharge point; do not rely on a note.
- Detail the check dams, orifice plate, and outlet protection.
- Add a note requiring the contractor to verify the storm drain invert and tailwater before constructing the outlet.
- Coordinate the 8-in water main: show the utility easement and the minimum clearance. If the swale excavation conflicts, require a utility relocation or a redesigned swale section.
- Include a sequence note: install the outlet and overflow before the parking lot is paved.
3. How should the water main conflict be addressed?
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The water main is a conflict, not a coordination footnote.
- A main 3 ft from the swale centerline may be within the excavation side slope, especially with a 12-in ponding depth and check dams. The first step is a utility locate and survey of the actual main elevation.
- If the main is within the excavation zone, options are relocation, a structural crossing, or a revised swale alignment. Do not simply add a note to 'protect in place'.
- Obtain the utility owner's approval and required clearance; in many jurisdictions, excavation within a utility easement requires a permit and inspector.
- Show the resolution on the grading plan and sections. If relocation is required, issue a change order with cost and time before the swale is excavated.