Middleton staff say Madison projects and updated rainfall estimates could raise peak flows; group agrees to proceed with remap using updated 100‑year rainfall

City of Middleton staff and their consultant briefed members on August 1 on how planned and recent drainage projects in the City of Madison, together with more recent rainfall estimates, could change flood risk in Middleton the South Fork/Pheasant Branch watershed.

The presenter, a project consultant working under Middleton's remapping contract, summarized City of Madison projects including the Swallowtail Park work and the Pheasant Branch Enhancement Project. He said Madison has designed those projects and obtained funding and permits, and that, in model runs, they increase peak discharge for smaller storms while having little or no effect on the largest floods. The presenter said that in a sensitivity test substituting a higher 100‑year rainfall estimate the model showed roughly a 28 percent increase in peak flow at the city boundary and a rise of more than a foot at South Pond in an extreme simulation.

"If there is more rain, you know, there's more flooding downstream," the presenter said during the briefing, summarizing the practical takeaway from the sensitivity work. He also noted the long‑standing caveat that "all models are wrong; some are useful," to explain why the team was focused on limited, targeted changes rather than a wholesale redo of Middleton's model.

Why it matters: Middleton is performing a FEMA‑compatible floodplain remapping effort for the South Fork watershed. The choice of rainfall inputs and model calibration affects flood insurance mapping, land‑use planning and the size of mapped floodplains that property owners and the city will use for regulation and outreach.

What the briefing found

- Rainfall inputs: City of Madison used NOAA Atlas 14 depths. The presenter described an alternative analysis by UW–Madison civil engineering Professor Dan Wright that uses more recent records and estimates about one additional inch for the 100‑year, which increases modeled peak flows when substituted into Madison's model.

- Event sensitivity: The Madison projects (culvert upsizing and added storage) increase the rate at which smaller storms discharge downstream; model runs show the largest storms remain dominated by regional storage and have smaller proportional change than frequent events.

- Calibration differences: The consultant said Middleton's watershed model is calibrated across multiple historical events and matches three USGS gauges, while Madison's model is tuned to larger‑event data. Those calibration differences help explain why substituting higher rainfall inputs produces different results in each model.

Board direction and next steps

After discussion, members signaled consensus to continue the floodplain remapping process using the higher 100‑year rainfall assumption (the Dan Wright–adjusted value) and to pursue only modest, targeted updates to Middleton's model rather than a full rework to force agreement with Madison's outputs. The group also asked staff and consultants to keep coordinating with Madison, to explore potential intergovernmental agreements or memoranda of understanding to address joint projects or shared costs (for example, potential cost‑sharing for gauge operation), and to bring an overview to Middleton's city council to confirm the remapping direction and to start public outreach on map changes.

What wasn't decided

The advisory group did not adopt any formal intergovernmental agreement at the meeting. Members asked staff to pursue further technical coordination with Madison, to identify potential collaborative structural or green‑infrastructure projects (culvert replacements, selected pond enlargements) and to return cost estimates for those options.

Next procedural step

Consultants will finalize the modest model updates and prepare a high‑level presentation for Middleton city council explaining the mapping options, the rationale for the chosen rainfall input and the tradeoffs of a full FEMA remapping versus local, advisory mapping. The group also recommended staying in regular contact with Madison while continuing the remapping contract.