Storm Sewer Surcharge in an Infill Development
A 1970s subdivision is being redeveloped as an infill townhome project (9.8 acres, 72% impervious). The existing storm sewer along the frontage is 600 ft of 24-inch RCP (n = 0.013) laid at 0.004 ft/ft between two manholes. The upstream manhole rim is at elevation 100.0; the pipe invert is 92.0. Existing peak flow reaching the pipe is 12 cfs; the project adds 10 cfs. The drainage report concludes the pipe "has adequate capacity at full flow" and reports a design HGL of 99.4 at the upstream manhole. The city requires the 10-year flow to pass without surcharge above the crown, and the 100-year HGL to remain at least 1.0 ft below the manhole rim. Several existing lots have basement slabs at elevation 98.5 with gravity sanitary laterals to the street.
1. Test the report's capacity conclusion. Does the pipe meet the city's 10-year criterion, and is the reported 99.4 HGL a defensible basis for approval?
Read the worked answer
Full-flow capacity (Manning, US customary):
- D = 2.0 ft, A = π(1.0)² = 3.14 ft², R = D/4 = 0.50 ft, R^(2/3) = 0.630
- S^(1/2) = 0.004^0.5 = 0.0632
- Q = (1.486/0.013)(3.14)(0.630)(0.0632) ≈ 14.3 cfs
Design flow is 12 + 10 = 22 cfs, about 54% above full-flow capacity. The 10-year criterion (no surcharge above the crown) is therefore not met, and the statement “adequate at full flow” confuses “flows full” with “has capacity.”
The 99.4 HGL is not a defense. A surcharged pipe can be modeled to produce almost any HGL depending on the assumed downstream boundary, n, and whether the excess flow is forced through. The reported value also does not say which event it represents (10-year or 100-year), and a 10-year surcharge already violates the stated standard. Approving on the HGL number alone would waive the capacity criterion without saying so.
Recommendation: reject the capacity conclusion and require a model run of both the 10-year and 100-year events with stated downstream boundary conditions, plus a routing check of any proposed attenuation.
2. Assume the 100-year HGL really is 99.4. What is the residual risk, and what conditions should the reviewer place on approval?
Read the worked answer
Residual risk is basement flooding, not rim overflow. The upstream manhole rim (100.0) is only 0.6 ft above the modeled HGL, and the basement slabs are at 98.5 - 0.9 ft below the modeled HGL. In a surcharged system, that means:
- Storm water is above the basement floor slab at the design event; any floor drain, slab penetration, or cracked lateral becomes an entry point.
- The gravity sanitary laterals at those lots are shallow and share the same trench corridor. A surcharged storm HGL near 99.4 implies the sanitary sewer will also be under head, creating a public-health exposure that a stormwater-only analysis will not catch.
- HGL values for surcharged pipes carry meaningful uncertainty (n, junction losses, downstream boundary). A 0.6-ft margin to the rim is not a margin.
Conditions to attach: (1) compute and show the 100-year HGL explicitly; (2) require backflow prevention on sanitary laterals serving slabs below the 100-year HGL; (3) provide an overland relief path so that if the HGL is exceeded, flow goes to the street rather than into structures; (4) document that no habitable space is below the 100-year HGL.
3. The applicant proposes three options: on-site detention, a relief sewer downstream, or a drainage easement with backflow devices. Which is most defensible here and why?
Read the worked answer
On-site detention sized to the pre-development peak is the cleanest fix, with backflow devices as a companion, not an alternative.
- If post-development peak is attenuated back to 12 cfs, the flow is below the pipe's 14.3 cfs full-flow capacity and the gravity, non-surcharged condition the city standard contemplates is restored. The problem is solved at its source.
- The required storage is modest: shaving 10 cfs off a short-duration urban peak on 9.8 acres typically requires a small basin (order of 0.1–0.3 ac-ft), which an infill site can often accommodate in a below-grade vault.
- Relief sewer treats the symptom, is expensive at 600 ft of new pipe, and may simply move the surcharge downstream unless the downstream system is verified to have capacity.
- Easement plus backflow devices does not meet the city's stated 10-year no-surcharge criterion; it only manages the consequence. It is appropriate for the existing homes whose slabs are already below the HGL, not as the applicant's mitigation.
Best package: detention to the pre-development peak for the applicant's flow, plus a requirement that the existing low slabs receive backflow protection because the pre-project 12 cfs condition may already surcharge downstream.