GoHilliard Intelligence Brief: Big Darby Oversight & Stormwater Compliance

Regulatory Jurisdiction: Big Darby Accord Watershed Master Plan & Advisory Panel (BDAAP)

Municipal Authority: City of Hilliard Planning & Zoning Commission / City Engineering Division

Cooperating Entities: Ohio EPA (General NPDES Permit for the Big Darby Watershed) & Franklin Soil and Water Conservation District (FSWCD)

Focus Sector: Flex Industrial, Logistics, and Commercial Infill along the Western Growth Boundary (Alton Darby, Roberts, and Hayden Run Corridors)

Executive Overview

The Big Darby Creek watershed—spanning 555 square miles across Central Ohio, including approximately 6.7 square miles inside the City of Hilliard—is one of the Midwest’s most biologically diverse and stringently protected river systems. Designated as both a State and National Scenic River and classified as an Exceptional Warmwater Habitat by Ohio EPA, the watershed hosts over 100 fish species and 44 native freshwater mussel species, nearly 40 of which are endangered or rare.

As industrial developers push westward toward the rural-urban boundary along the Roberts Road and Alton Darby corridors, the conversion of permeable agricultural acreage into large flex warehouses, logistics yards, and commercial footprints presents an acute environmental hazard: impervious surfaces generate rapid, warm stormwater runoff loaded with hydrocarbons, heavy metals, and sediments that directly threaten fragile aquatic habitats.

To safeguard the watershed without halting regional commercial growth, commercial and flex industrial developments along Hilliard’s western perimeter must clear technical engineering hurdles under the Big Darby Accord Watershed Master Plan and Hilliard’s Chapter 1114/1117 design standards.

The Engineering Gauntlet: Western Perimeter Stormwater Standards

Flex warehouse and commercial sites in Hilliard’s western overlay face stringent post-construction stormwater retention requirements:

+-----------------------------------------------------------------------------------------+
|                    BIG DARBY COMPLIANCE MATRIX: FLEX WAREHOUSE STANDARDS                |
+--------------------------+-----------------------+------------------+-------------------+
| Parameter                | Standard Commercial   | Big Darby Accord | Technical Goal    |
+--------------------------+-----------------------+------------------+-------------------+
| Groundwater Recharge     | Pre- vs. Post-Rate    | 100% Pre-Dev     | Preserves base    |
| (Hydrologic Balance)     | Attenuation Only      | Baseline Volume  | flow in headwater |
|                          |                       | Infiltration     | tributaries.      |
| Water Quality Volume     | Standard 0.75" WQv    | 100% Runoff from | Captures the "first|
| (WQv Capture Depth)      | Detention Drawdown    | 2-Year, 24-Hour  | flush" sediment & |
|                          |                       | Storm Event      | nutrient load.    |
| Riparian Setbacks &      | Standard 25'–50'      | Tiered 100'–300' | Filters thermal   |
| Conservation Buffers     | Drainage Easement     | Undisturbed      | runoff before     |
|                          |                       | Vegetated Buffer | stream entry.     |
| Hydrocarbon & Chemical   | Optional / Standard   | Required Oil/Grit| Prevents petroleum|
| Pre-Treatment            | Sump Catch Basins     | Separators on All| and diesel spills |
|                          |                       | Truck Dock Aprons| from aquifer entry|
+--------------------------+-----------------------+------------------+-------------------+

1. Mandatory Groundwater Recharge Neutrality

In standard suburban commercial design, developers merely detain stormwater to match pre-development peak release rates ($Q_{peak}$). Within the Big Darby overlay, developers must maintain volumetric groundwater recharge neutrality.

• Warehouse footprints act as rain shields. Because flex distribution facilities require expansive rooflines and wide concrete truck aprons, rain cannot soak into the aquifer.

• Engineers must demonstrate via site-specific hydrogeologic modeling that post-development infiltration volumes equal or exceed pre-development agricultural baselines. This requires specialized subsurface infiltration galleries, unlined bio-retention cells, and permeable overflow basins positioned outside riparian flood zones.

2. Hydrocarbon Interception at Loading Docks

Flex warehouses, light assembly facilities, and logistics centers handle Class 7 and Class 8 heavy transport trucks daily. In the western transition zone, commercial applications trigger strict pre-treatment rules:

• Runoff from truck docks, exterior staging lots, and trailer parking cannot discharge directly into retention basins.

• Sites must install mechanical oil-grit separators (coalescing plate hydro-dynamic separators) engineered to capture petroleum hydrocarbons, ethylene glycol, and heavy metals prior to stormwater release.

• For retail/commercial fuel nodes (as adjudicated in cases like the PZ-26-1 Circle K docket at Roberts & Alton Darby), storage tanks and stormwater retention structures require complete physical separation and dedicated shut-off valves to contain catastrophic spills.

3. Thermal Mitigation & Stream Buffering

Because Big Darby Creek’s endangered mussel species rely on cool, stable water temperatures, the rapid discharge of summer stormwater baked on dark warehouse roofs or expansive asphalt lots can cause fatal thermal shock.

• Ponds must employ serpentine flow paths, extended detention underdrains, and bottom-withdrawal structures that discharge cooler water slowly.

• Discharge points must pass through wide, naturalized riparian buffers planted with deep-rooted native prairie vegetation to filter residual nitrates and phosphorus before reaching headwater ditches.

Policy Headwinds: The Big Darby Accord Amendment

The regulatory backdrop is complicated by ongoing discussions surrounding updates to the original 2006 Big Darby Accord.

• The “Town Center” vs. Logistics Pressure: While the original Accord established density transfer frameworks and targeted intense growth into specific, sewer-serviced “Town Center” zones, market pressures have shifted demand toward flex industrial parks, tech campuses, and advanced manufacturing along rail and highway interfaces.

• Inter-Jurisdictional Alignment: While the City of Hilliard has preserved over 680 acres of open space in the watershed, differing development philosophies among the nine Accord partners (including Columbus, Brown Township, Prairie Township, and Norwich Township) require constant vigilance. Columbus’s shifting positions on utility extensions and proposed Accord text updates make local municipal zoning enforcement the front line of environmental protection.

What you should be watching

1. Scrutinize Stormwater Pollution Prevention Plans (SWPPPs): When preliminary development plans for flex industrial or commercial parks land on the P&Z agenda, look past the building elevations. Focus on the civil utility plan to confirm whether underground infiltration beds are sized for the 100-year storm and verify that post-construction recharge calculations show a net surplus.

2. Inspect Spill Prevention Control & Countermeasure (SPCC) Bindings: Ensure that conditional-use approvals for logistics, automotive, or industrial outdoor storage (IOS) along Roberts and Alton Darby Creek Roads include legally binding covenants requiring routine maintenance, certified annual cleanouts of oil-grit separators, and city inspection access.

3. Guard the Greenbelt Continuity: Hold applicants to the mandatory 50% open space conservation minimums. Resist developer requests to convert required open-space conservation buffers into dry stormwater retention detention basins that displace native vegetation.