In Metro Atlanta, a detention pond is sized against four separate criteria in the Georgia Stormwater Management Manual, not one. You must treat the water quality volume from the first 1.2 inches of rainfall, provide 24-hour extended detention of the 1-year 24-hour storm for channel protection, hold the post-development 25-year 24-hour peak discharge to the pre-development rate for overbank flood protection, and safely convey the 100-year 24-hour event. A typical single-pond design package for a Metro Atlanta site runs roughly $4,500 to $15,000 in engineering fees depending on drainage area and jurisdiction, with 30 to 90 days of local review before a land disturbance permit is issued.
Four criteria, not one
Most owners think of detention as a single number: hold back the water so the neighbors don’t flood. Georgia does not work that way. The Georgia Stormwater Management Manual sets a unified stormwater sizing framework, and a compliant pond has to satisfy all of it at once. Miss one criterion and the plan comes back with comments.
| Criterion | Storm event | Requirement |
|---|---|---|
| Water quality volume (WQv) | First 1.2 inches of rainfall (85th percentile storm) | Treat the volume and reduce average annual TSS load by 80% |
| Channel protection (CPv) | 1-year, 24-hour | 24-hour extended detention; not required below 2.0 cfs post-development discharge |
| Overbank flood protection (Qp25) | 25-year, 24-hour | Post-development peak discharge must not exceed pre-development rate |
| Extreme flood protection (Qf) | 100-year, 24-hour | Control or safely convey so flooding is not made worse |
The water quality volume is computed as WQv = 1.2 Rv A / 12, in acre-feet, where A is site area in acres and Rv = 0.05 + 0.009(I) with I as percent impervious cover (GSMM Volume 2). That formula is why impervious cover drives your pond size more than anything else on the plan. A 5-acre site at 30% impervious carries a materially smaller WQv than the same 5 acres at 70%, and the difference shows up as usable land.
The 10% rule catches people out
Detention is not just about your outfall. The GSMM requires downstream analysis at the site outlet and at each downstream tributary junction until your site’s drainage area is 10% or less of the total drainage area above that point. The manual’s own example: a control draining 10 acres has a zone of influence that ends where the total drainage area reaches 100 acres or greater. On a tight infill site with a small receiving channel, that analysis can extend several hundred feet off your property and can force a change to the outlet structure.
Where runoff reduction saves you money
Georgia’s framework rewards reducing runoff at the source. Better site design practices that reduce total runoff proportionally reduce WQv, CPv, Qp25, and Qf: all four numbers at once. Groundwater recharge targets scale with hydrologic soil group:
| Hydrologic soil group | Recommended recharge volume |
|---|---|
| A | 0.40 inches of runoff × total impervious area |
| B | 0.25 inches of runoff × total impervious area |
| C | 0.10 inches of runoff × total impervious area |
| D | Not applicable |
If your geotechnical report puts you in an A or B soil, infiltration-based practices are usually worth pricing before you commit to a large surface pond. On C and D soils across much of the north Georgia piedmont, they usually are not, and the honest answer is that you are building a pond.
One caution from the manual: runoff from a hotspot land use should not be infiltrated without effective pretreatment. Fueling areas, equipment yards, and vehicle maintenance sites need the pretreatment train designed in from the start, not added after review comments.
Surface pond vs. underground detention
| Factor | Surface detention pond | Underground detention |
|---|---|---|
| Engineering fee | Lower | Higher: structural and access design |
| Construction cost | Lower per cubic foot | Substantially higher per cubic foot |
| Land consumed | 5–15% of site typical | Effectively zero above grade |
| Maintenance | Mowing, sediment removal, outlet cleaning | Confined-space access, vactor truck, higher long-term cost |
| Best fit | Subdivisions, industrial, larger commercial pads | Constrained infill, high land value per acre, parking-limited retail |
The decision is an economic one, not an engineering one. If the pond eats a lot you could otherwise sell, or removes the parking count that makes a retail deal work, underground detention often pays for itself. Run that math during feasibility, not after the site plan is approved.
Cost and timeline in Metro Atlanta
Engineering fees for detention design are not a standalone line item on most projects. The pond is designed as part of the hydrology study and land disturbance plan set. As a planning range for Metro Atlanta:
| Scope | Typical engineering fee range |
|---|---|
| Single pond, small commercial or residential site, straightforward outfall | $4,500 – $8,000 |
| Multi-pond subdivision or complex outfall requiring extended downstream analysis | $9,000 – $15,000+ |
| Underground detention system design | Add 30–60% over an equivalent surface pond |
Review timelines vary by jurisdiction. Fulton County requires flood management practices per the Georgia Stormwater Management Manual, including detention, for all new development and redevelopment requiring a land disturbance permit (Fulton County Stormwater Management Plan). Practically, expect 30 to 90 days from first submittal to permit across Metro Atlanta jurisdictions, with at least one comment cycle. Georgia’s statutory land-disturbance review clocks are covered in our post on the SB 447 45-day deadline.
What we need from you to price it
- Boundary and topographic survey, ideally with 1-foot contours extending past the property line at the outfall
- Preliminary site plan or building footprint with impervious areas identified
- Geotechnical or soils information, including hydrologic soil group
- The jurisdiction and any prior approvals, variances, or existing stormwater facilities on site
- FEMA panel information if any part of the site is near mapped floodplain
Frequently asked questions
Against the Georgia Stormwater Management Manual’s unified sizing criteria: water quality volume (WQv) from the first 1.2 inches of rainfall, channel protection volume (CPv) as 24-hour extended detention of the 1-year 24-hour storm, overbank flood protection for the 25-year 24-hour storm, and extreme flood protection for the 100-year 24-hour storm.
WQv = 1.2 × Rv × A ÷ 12 in acre-feet, where A is site area in acres and Rv = 0.05 + 0.009 × percent impervious cover.
The GSMM does not require CPv control when post-development discharge is less than 2.0 cubic feet per second.
To the point where your site’s drainage area is 10 percent or less of the total drainage area above the analysis point. A control draining 10 acres has a zone of influence ending where total drainage area reaches 100 acres.
Not always. Runoff reduction and better site design practices reduce WQv, CPv, and peak flows proportionally, and on some sites underground detention or a distributed BMP train costs less than a surface pond.
Related reading
- Stormwater design and hydrology studies
- Stormwater permitting and compliance
- Forsyth County stormwater requirements
- Stormwater fee reduction studies for Atlanta-area properties
Talk to us before the site plan is locked
Detention is the single largest consumer of developable land on most Metro Atlanta sites. The cheapest time to fix it is during concept planning. If you have a site under contract or in due diligence, request a detention feasibility review and we will tell you what the pond costs you in acres before you close.