Delaware's Runoff Reduction Volume and Resource Protection Event Criteria
Delaware's post-construction stormwater regulations under 7 DE Admin. Code 5101 organize around a specific design target: the Runoff Reduction Volume (RPv) tied to the 1-year, 24-hour rainfall event. The RPv is explicitly labeled a "Resource Protection Event" criterion — designed to reduce runoff volume, promote groundwater recharge, mitigate downstream channel erosion, and reduce pollutant loads. For designers used to Maryland's ESD-to-MEP (replicate pre-development woods hydrology) or Virginia's phosphorus-load-based standard, DE's approach is different — more specific in the storm event chosen, and more mechanistic about the volume required.
The RPv — design event and volume
- Definition. Post-development annualized volume of runoff from the 1-year, 24-hour rainfall event (99% probability of occurrence in any given year).
- Labeled a "Resource Protection Event" criterion in the regulations.
- Policy purposes: minimize runoff volume, recharge groundwater, reduce impacts on downstream channels, decrease pollutant loads.
- Treatment volume varies by whether disturbed area was previously forested or non-forested, up to a maximum of 1 inch of runoff difference.
Primary source: dnrec.delaware.gov (Sediment and Stormwater Program). For administration, see our DE Conservation District essay on how Kent, New Castle, and Sussex Conservation Districts plus approved cities (Wilmington, Dover) handle plan review.
Runoff reduction practices
Compliance uses Best Management Practices (BMPs) classified as runoff reduction practices — those that reduce total runoff volume through:
- Canopy interception.
- Surface recharge.
- Evaporation.
- Rainfall harvesting.
- Engineered infiltration.
- Evapotranspiration.
Typical BMPs: bioretention, rain gardens, permeable pavement, green roofs, cisterns for rainwater harvesting, infiltration trenches, grass swales with infiltration, and preservation of natural vegetation.
Impaired waters and alternative standards
Where a receiving waterbody is identified as impaired or requires specific pollutant reduction targets, DNREC may mandate alternative stormwater treatment practices beyond the RPv baseline. This matters for projects discharging to named impaired waters where TMDL-driven requirements can raise the design bar.
Previously forested vs non-forested treatment volume
The treatment volume calculation distinguishes:
- Previously forested land — stricter treatment requirement, reflecting the loss of natural hydrologic function when forest is cleared.
- Previously non-forested land — lower treatment requirement since hydrologic baseline is already degraded.
The 1-inch runoff difference cap provides an upper bound. Practical effect: clearing existing forest for development increases stormwater cost beyond the RPv formula alone — it raises the treatment obligation.
How DE RPv compares to MD ESD-to-MEP
Maryland's ESD-to-MEP (see our MD New vs Redevelopment essay) takes a different philosophical approach:
- Target — replicate "woods in good condition" pre-development hydrology to the Maximum Extent Practicable, not a specific storm-event volume.
- Design storm — 1-year design storm, with specific "PE" rainfall depth based on soil type and proposed imperviousness.
- Flexibility — the "MEP" qualifier allows designer/reviewer negotiation on what's practicable given site constraints. DE's RPv is more prescriptive; compliance is defined by the volume and event.
- Addressing recharge, water quality volume, and channel protection together — all bundled into the ESD-to-MEP target rather than broken into separate criteria.
For a designer moving between DE and MD, the RPv is a more concrete specification; ESD-to-MEP is a more open-ended design goal.
How DE RPv compares to VA VSMP
Virginia's VSMP (see our VA VSMP essay) uses a pollutant-based metric:
- Target — total phosphorus load, not a storm-event volume.
- 2025 update — for plans submitted July 1, 2025 and after, new development cannot exceed 0.26 lb phosphorus/acre/year (down from 0.41).
- Redevelopment — 20% phosphorus load reduction (≥1 acre disturbance) or 10% (<1 acre) below pre-development if no net impervious increase.
- Compliance calculation — Virginia Runoff Reduction Method (VRRM) spreadsheet v4.1.
VA's framework is performance-based: designers can choose how to meet the phosphorus load target as long as the math works. DE and MD lean more on prescribed volume/event concepts.
What DE designers actually deliver
A typical commercial DE project stormwater submittal includes:
- Site characterization — soils, hydrology, existing cover (forested vs non-forested).
- Receiving waterbody identification and impairment check.
- RPv calculation for proposed conditions.
- BMP selection and sizing — typically a mix of runoff reduction practices plus supplemental treatment as needed.
- Site-specific layout demonstrating BMP placement.
- Sediment and erosion control plan for construction phase.
- Post-construction maintenance plan.
- Submission through the appropriate Conservation District or approved city program.
For design professionals unfamiliar with DE's framework, the July 2023 update to the Delaware Erosion and Sediment Control Handbook is essential reading — plans submitted July 1, 2023 or later must use updated standard details.
Coastal Zone and CBPA considerations
Projects near coastal or tidal waters may also engage:
- DE Coastal Zone Act considerations for heavy-industrial-proximate work (see our DE Coastal Zone essay) — mostly not applicable to commercial development in Sussex beach towns despite the name.
- DNREC Wetlands Act for tidal wetlands.
- Subaqueous lands permits.
- Beach Preservation Act for beach/dune-proximate work.
Common misconceptions
- "RPv is only about runoff reduction, not water quality." No — RPv treatment reduces pollutant load as a consequence of volume reduction. Water quality is a first-order purpose.
- "Previously-forested treatment volume is just a paper factor." No — clearing existing forest materially increases stormwater infrastructure cost through the higher treatment requirement.
- "1-year 24-hour event is the same as MD's PE design storm." Similar in spirit, but the calculation specifics and treatment requirements differ. Cross-state designs need recalibration.
- "If I handle RPv, I'm done." Channel protection, flood control, and TMDL-impaired-water requirements can layer on top.
Operational implications for designers
- Site assessment establishes forested vs non-forested cover — materially affects treatment volume.
- Receiving water identification is a step-one task, not an afterthought.
- Runoff reduction BMPs are preferred over traditional pond-dominant approaches.
- Conservation District pre-submission conversations are cheap and high-value — see our DE Conservation District essay.
- July 2023 standard details for plans submitted after that date.
What to do with this
If you're designing a DE commercial project: characterize cover and receiving water first. RPv follows from that.
If you're crossing from MD or VA: don't assume your familiar design storm or pollutant-load approach maps directly. DE's RPv has its own calculation logic.
If you're clearing existing forest: expect higher treatment volume. Preservation often lowers cost.
If you're near impaired receiving waters: DNREC may require alternative practices beyond baseline RPv.
For the broader DE stormwater administration context, see our DE Conservation District essay.
About The Hive
The Hive builds tools and publishes essays for working construction and MEP professionals in the Delaware Valley and Mid-Atlantic. Primary-source-grounded, practitioner-voiced, free to use.