The Hollis Creek Cleanup
1In April, after two days of heavy rain, Hollis Creek flooded part of the town of Wexford. When the water went down, Amara Fitzgerald's seventh-grade watershed club spent a Saturday pulling trash and branches out of the creek park. Dr. Yusuf Brennan, a hydrologist who runs the county's stream gauges, met the club there to talk about how floods are forecast.
2A stream gauge measures the creek's water level, called its stage, every 15 minutes. At the park, the creek spills over its banks when the stage reaches 4 m, which the county calls flood stage. At 4.5 m, water starts to reach the first homes. The club found a line of leaves and plastic stuck to the park fence 1.2 m above the ground. The ground at the fence sits at a stage of 3.9 m, so the line marks the highest the April flood reached.
3Dr. Brennan showed the club the highest stage the creek reached in each of the last 40 years, including this year. He also showed a graph of seven earlier storms, comparing the rain that fell in 24 hours with the creek's peak stage at the park. His forecast app shows how much rain is expected over the next 24 hours. "No one can say which year the next flood will come," he said, "but the records show how often floods happen and how much rain it takes."
4The town council is choosing one flood project. It has two criteria. First, in a flood as large as April's, the project must keep the creek below 4.5 m at the park. Second, it must not send more water downstream to the village of Millford. Engineers used computer models to test five choices (Table 1).