Most MS4 programs run on multiple inspection cadences at once. Construction sites get inspected during active disturbance. Post-construction BMPs are typically annual. Outfall screening follows quarterly cycles. Municipal facilities are usually annual. Mixing those cadences in a spreadsheet — or worse, in someone’s head — is how audit findings happen.
This post covers the cadences in plain terms, what your permit usually expects, and why “we inspect on the cadence” isn’t enough. You need to be able to show you inspected on the cadence.
Why cadence matters specifically
Auditors don’t grade your inspections subjectively. They check that inspections happened, when they were supposed to. If your construction permit says inspections during active disturbance and an inspection date is missing for a 60-day window of recorded ground disturbance, that’s a finding regardless of how many other inspections you did.
The cadence isn’t a soft target. It’s a record requirement.
Construction site inspections
Typical cadence: During active disturbance, at intervals defined by your permit — often weekly or after qualifying rainfall events.
What “active disturbance” means in practice: From the time grading begins until the site reaches stabilization (commonly defined as 70% perennial vegetative cover or equivalent erosion control). The permit defines when this period starts and ends; you need a record of each.
What auditors check:
- Is there a record of the start of disturbance for each site?
- Are inspection dates spaced according to the permit’s frequency requirement?
- Are post-rainfall inspections logged with the precipitation event date?
- Does the inspection record include findings, photos, and follow-up actions?
Common failure: Sites that go inactive (paused construction) but stay enrolled. The cadence requirement may pause too — but only if you have documentation of the pause. Without that, the audit reads as missed inspections.
Post-construction BMP inspections
Typical cadence: Annual for structural BMPs, with additional inspections after major storm events for some BMP types.
What auditors check:
- BMP inventory is current and matches development records
- Each BMP in the inventory has an inspection within the past year
- Maintenance triggers (sediment accumulation thresholds, vegetation conditions) have documented responses
- Privately-owned BMPs have current ownership and access records
Common failure: A new development was approved three years ago, the post-construction BMP was installed two years ago, but the BMP didn’t get added to your inventory until last year. Two years of inspections are now missing for that BMP from the audit’s perspective.
Outfall screening (dry-weather)
Typical cadence: Quarterly dry-weather screening on a rotating schedule, with all outfalls screened at some defined interval (often every five years for the full network).
What auditors check:
- Outfall map matches the screening log
- Each outfall has a recent screening within the program’s stated cycle
- Indicator panels (color, odor, floatables, deposits) are documented per screening
- Outfalls flagged with indicators have follow-up records
Common failure: New outfalls discovered during field work don’t get added to the map, so they don’t show up in the screening rotation. Auditors compare the field map (the drainage system) with the inspection log; the gap between them is the finding.
Municipal facility inspections
Typical cadence: Annual for high-priority facilities — maintenance yards, fleet operations, salt storage — with documentation of operations and maintenance practices.
What auditors check:
- High-priority facility list is current
- Each facility has an annual inspection
- Pollution prevention practices are documented (fleet washing, material storage, fueling)
- Spill response events are logged with cause, response, and corrective action
Common failure: The facility list was generated at the start of the permit cycle and never updated. Facilities added later (new annex buildings, satellite operations) don’t appear, and so don’t get inspected.
Re-inspection cadence
When an inspection finds a deficiency, the re-inspection cadence kicks in. Permit conditions usually define how quickly re-inspection must happen and how the issue must be resolved.
What auditors check:
- Each deficient finding has a re-inspection date
- Re-inspections happened within the required window
- Closure status is documented per finding
- Repeated deficiencies have escalation records (NOV, compliance order)
This is where cadence-tracking becomes specifically useful. Manual systems tend to forget about deficient findings the moment the inspector leaves the site.
How cadence drift becomes audit findings
Programs don’t usually fail on cadence because of negligence. They fail because nobody owns the calendar:
- The construction inspector knows when they last visited each site. The coordinator doesn’t, until they ask.
- The BMP inventory lives in a spreadsheet that gets updated when someone remembers.
- The outfall map and the screening log are two separate documents.
- Re-inspections fall off because the original deficiency record isn’t surfaced again.
The fix isn’t more discipline — it’s a system that surfaces what’s overdue and due-soon, by program, on every dashboard a coordinator opens. When cadence is something the system tracks rather than something a person tracks, drift gets flagged before it becomes a finding.
How software fits
NPDESTracker tracks cadence per site and per program, surfacing overdue and due-soon items on the dashboards staff already use. Re-inspection schedules are auto-created from deficient findings. Outfall screening rotation runs against the live outfall map, not a snapshot. The full inspection workflow is on the inspections page, and the cadence-aware readiness rollup is part of reporting.