Catch It, Carry It, Slow It
Down — and Pass Inspection

The system starts at the surface with catch basins, curb inlets, and yard drains, runs through buried pipe, passes through junction boxes where lines meet, and discharges through a headwall or flared end section — a concrete collar that stops the pipe end eroding out — into a ditch, a stream, or a pond. The pond is the part most owners misunderstand. A detention pond is not there to hold water permanently; it is normally dry, fills during a storm, and bleeds down through a restricted outlet over hours so your peak discharge never exceeds what the site produced before you built on it. A retention pond keeps a permanent pool and treats water quality as well. Storm is entirely separate from sanitary, always.

Any new impervious area — building footprint, parking, drive aisles, loading. More pavement means more runoff arriving faster, so the site needs both conveyance and peak-flow control. In South Carolina, disturb an acre or more and you are into the construction general permit and a SWPPP; disturb more than two acres and your plan needs agency approval and certification by a registered engineer, landscape architect, or Tier B surveyor; disturb ten acres or more draining to one outlet and a sediment basin is required during construction. North Carolina and Georgia have their own erosion and sedimentation programs with their own thresholds and their own certifications — the one-acre federal trigger is the only part that travels unchanged, so we settle which program applies before design starts. Beyond new work: redevelopment that adds pavement, and existing failures — chronic ponding, undermined curb, or a detention pond that no longer draws down.

Pipe is the smallest surprise here. Twelve to twenty-four inch HDPE dual-wall at normal depth is the workhorse; reinforced concrete pipe runs materially more per foot and gets specified where loads, burial depth, or an agency spec demand it. Structures are the line that grows on you — every catch basin and curb inlet is a precast box, a frame, and a traffic-rated top, and the count comes off the inlet capacity math, not off the pipe layout. Headwalls and flared end sections are the small line that keeps an outfall from eroding out from under itself. What actually decides the budget is where you put the storage: surface detention is by a wide margin the least costly storage per cubic foot you can build, while underground chamber systems run multiples of it, which is why they only pencil when the land they free up is worth more than the difference. On schedule: permitting is three to eight weeks, storm pipe and structures three to eight weeks overlapping with mass grading, and pond work another two to five. One seasonal warning — you cannot terminate permit coverage until vegetation is established, so you do not want to be seeding in late July.

Rainfall along the Blue Ridge escarpment is genuinely severe and it arrives fast. At Seneca the 25-year, 24-hour storm is 6.74 inches and the 100-year is 8.81, with a 10-year one-hour burst of 2.31 inches — and the numbers run in that neighborhood the whole way along the foothills, from northeast Georgia through the Greenville area and up into western North Carolina. A single summer cell will exceed the capture rate of an undersized inlet immediately, which is why grate count and grate type matter as much as pipe diameter. Red clay does not infiltrate — these are hydrologic group C and D soils, so runoff coefficients are high before you pave anything, and infiltration-based practices frequently do not work here without engineered media. Then there is the rulebook, which is where plans actually get rejected. South Carolina requires controlling the 2- and 10-year events, but Greenville County — right in our back yard — makes the 25-year the critical storm across most of the county, with the 50-year in the Gilder Creek watershed and the 100-year inside Mauldin. North Carolina and Georgia run their own stormwater programs with their own design storms, and their counties layer local overlays on top the same way. Designing to a state floor and assuming it clears local review is a rejected plan waiting to happen in any of the three.