Dam Safety¶
Regulatory Summary¶
Regulatory Authority¶
New Jersey dam safety requirements are governed by the Safe Dam Act (N.J.S.A. 58:4-1 et seq.) and administered by the NJDEP Dam Safety Program. Stormwater detention basins that meet the size or hazard thresholds defined in the regulations require a Dam Safety permit in addition to any Stormwater Management (SWM) permit.
When Dam Safety Applies¶
A stormwater detention basin triggers NJDEP Dam Safety review when it meets any of the following criteria:
| Criterion | Threshold |
|---|---|
| Dam height | ≥ 10 feet (measured from downstream toe to crest) |
| Storage capacity | ≥ 10 acre-feet at maximum water surface elevation |
| Hazard classification | High hazard (failure would cause probable loss of life) |
Basins below these thresholds that pose significant downstream hazard may still be classified by the Dam Safety Program on a case-by-case basis.
Hazard Classifications¶
| Class | Definition | Design Flood Standard |
|---|---|---|
| High Hazard (Class I) | Failure would probably cause loss of life | Probable Maximum Flood (PMF) |
| Significant Hazard (Class II) | Failure causes significant property damage; unlikely to cause loss of life | 0.5 × PMF or 500-yr flood |
| Low Hazard (Class III) | Failure causes minimal property damage; no probable loss of life | 100-yr flood |
The hazard classification is determined by the Dam Safety Program based on downstream conditions, not the design intent of the basin.
Engineering Interpretation¶
Key Design Implications¶
Emergency spillway: All regulated dams require a primary spillway (principal outlet) and an emergency spillway capable of passing the design flood without overtopping the embankment.
Embankment freeboard: Minimum freeboard above the design flood water surface elevation is specified by the Dam Safety Program based on hazard class. Typical minimum: 1–3 feet.
Embankment materials and slopes: Compacted earth fill; interior and exterior slopes are prescriptive per the Dam Safety Engineering Standards. Geotechnical analysis required for high-hazard dams.
As-built certification: A licensed professional engineer must certify the as-built condition of the dam and appurtenant structures.
BMP Implications¶
Relationship to Stormwater BMP Permits¶
Detention basins that trigger dam safety are subject to dual permitting:
- NJDEP Division of Land Resource Protection — Stormwater Management (SWM) permit under N.J.A.C. 7:8
- NJDEP Dam Safety Program — Dam Safety permit under the Safe Dam Act
The SWM permit governs water quality and quantity performance (WQV, channel protection, flood control). The Dam Safety permit governs structural integrity and public safety. Both must be obtained before construction.
Design Coordination
Dam Safety requirements may impose embankment geometry and spillway sizing that exceed the minimum stormwater detention design. Coordinate both permit tracks early in the design process to avoid conflicts.
Applicable BMPs¶
Dam safety review is most commonly triggered by the following BMPs when they exceed the size thresholds:
- Extended Detention (Ch. 10 – GI)
- Dry Detention (Ch. 11 – Non-GI)
- Wet Pond (Ch. 10)
- Constructed Wetland (Ch. 10)
Small-scale Ch. 9 BMPs (bioretention, dry wells, green roofs, etc.) do not trigger dam safety.
Cross References¶
- N.J.S.A. 58:4-1 et seq. — Safe Dam Act
- N.J.A.C. 7:20 — Dam Safety Standards
- NJDEP Dam Safety Engineering Standards (current edition)
- N.J.A.C. 7:8 — Stormwater Management Rules (quantity control requirements)