DeKalb officials receive Snap Finger Basin update; consultants recommend targeted inspections, some pond decommissioning and a bioretention pilot

DeKalb County commissioners heard an update July 14 on a detailed phase of the county’s stormwater master plan focused on the Snap Finger Creek watershed, where consultants from Arcadis said they have inspected critical culverts and detention ponds and will use the findings to build a prioritized capital improvements plan.

Arcadis senior project manager Rich Bruel and colleague Anwar Ahmed told the Public Works and Infrastructure Committee that the firm had mapped about 11,000 stormwater assets in the Snap Finger watershed and selected roughly 1,100 of the highest-consequence elements for closer inspection. The team said it has completed a little more than half of the initial inspections, using pole-camera and drone-like devices equipped with LiDAR and thermal cameras to get condition data from inside culverts and long pipelines.

The consultants described the inspection approach — including “pole camera” checks for deep pipes and a caged drone that collects high-definition video and a LiDAR-derived 3-D profile — and said the field work informs condition ratings, safety assessments, and an updated risk assessment that will feed a capital improvements plan. “We took that 11,000 and went down to 1,100 of the ones that are going to be the most impactful if they fail,” Bruel said.

Why it matters: The county’s strategic master plan previously cataloged about 77,000 stormwater assets at a high level. The Snap Finger work is the “next level down” officials said — moving from inventory into condition assessment, hydrologic and hydraulic modeling, and project prioritization to reduce the risk of road closures, property flooding, and other failures.

Inspection findings and implications

- Asset classes and inspections: Arcadis described five asset classes (class 1: corrugated metal pipes beneath single-access roads; class 2: pipes under collector/arterial roads; class 3: creek-conveyance structures; class 4: pipes in residential right-of-way; class 5: off-right-of-way assets). The firm said about 10% of mapped assets in the basin were flagged as “very high consequence of failure” and therefore targeted for inspection.

- Condition examples: In video and stills shown to commissioners, the team highlighted culverts with missing invert sections and severely damaged segments that could not be fully evaluated with pole cameras, demonstrating the value of the aerial/drone inspection tool. Bruel said some inspected class-1 assets had already shown a roughly 1-in-7 rate of being inoperable or in very poor condition among the inspected subset.

- Modeling and sizing decisions: Consultants emphasized that sizing or replacing culverts without hydrologic modeling can shift problems downstream. They said model runs showed some culverts, for example on Cherry Glen Trail and Cedar Park Drive, are being overtopped frequently, and those findings will be validated with field checks and neighborhood feedback before any design or construction.

Detention ponds: decommissioning and water-quality retrofits

Arcadis reported a desktop review of roughly 218 county-owned or -maintained detention ponds within the watershed. The review identified: about 64 ponds (≈30%) as candidates for decommissioning because they provide little detention benefit; nine ponds that could be repurposed with water-quality retrofits; and nine pond pairs where consolidating two ponds into one would likely improve performance and reduce maintenance burden.

As an example, consultants said the Royal Town Park townhome development contains two county-maintained ponds (labelled Pond A and Pond B in the presentation). Arcadis’ hydrologic modeling indicated Pond B provides little detention benefit and discharges directly to the creek, and Pond A captures a small contributing area. Arcadis recommended decommissioning Pond B and converting Pond A to a bioretention (biofiltration) basin — a retrofit that would treat runoff and “polish” water before it reaches the creek.

Commissioners and staff questions focused on resident notification, property ownership and maintenance arrangements for ponds (many ponds sit on private property but are maintained by the county via prior agreements), pilot opportunities, and costs. Commissioner Terry and others asked for the inspection video and the LiDAR screen shots; Arcadis said they would provide screen captures and video to commissioners.

Next steps and staff direction

Arcadis and county staff said next steps include: - Completing the Snap Finger inspections and updating the digital asset inventory and condition ratings; - Performing hydrologic/hydraulic analyses to confirm whether culverts should be upsized or repaired, then incorporating findings into a prioritized capital improvements plan (CIP) for the watershed; and - Further studying pond decommissioning and water-quality retrofit approaches, and using the Royal Town Park pond recommendations as a pilot to develop a template for community engagement and technical analysis.

Countywide master-planning framework

Anwar Ahmed presented how the Snap Finger study will inform a countywide approach. Arcadis showed a prioritized list of 24 watersheds and grouped remaining basins into three study packages (Peachtree Creek north area; South River eastern tributaries and Yellow River; and South River western and southern tributaries). Arcadis estimated each phase of the watershed master plan would cost on the order of $1 million.

Staff cautioned commissioners that many ponds are on private property and that any decommissioning or conversion will require engagement with homeowners associations and affected property owners. Director Lemke noted maintenance liabilities and emphasized communication plans will be needed where county maintenance agreements exist.

What the board directed

The committee accepted the update and directed staff and consultants to finish the Snap Finger inspections, validate model results with on-the-ground checks and neighborhood input before recommending construction, develop the CIP prioritization using the inspection results, and prepare a public-engagement approach and a cost estimate for pond decommissioning and pilot retrofits. The Royal Town Park pond pair was identified as a candidate pilot for a decommissioning/bioretention conversion template.

Ending note

Consultants and staff said countywide rollout of this next-phase master planning will be done in prioritized packages rather than all basins at once; funding, scheduling and intergovernmental coordination (because watershed boundaries cross municipal lines) will guide sequencing. Commissioners asked to receive the inspection video and associated LiDAR screenshots for review.