A Summary of Middleton, WI, Stormwater Practices on Streamflow and Water Quality
Date & Time
Tuesday, September 14, 2021, 2:00 PM - 2:30 PM

In the 1960s, Middleton was concerned about negative effects of urbanization, on channel erosion and sediment and phosphorus loads in Pheasant Branch that drains into Lake Mendota. To address concerns, Middleton Water Resource Commission was formed in 1970 to protect and enhance the water resources of Middleton. The first action was development of a stormwater runoff control ordinance to prevent developments from increasing flood peaks due to increased impervious area. The ordinance was modified several times to address phosphorus and sediment loads and loss of groundwater recharge. A streamflow and water quality monitoring program with USGS started in 1974 on the main branch and in 2012 on South and North Forks. The monitoring program provided Middleton with 45 years of data for practice evaluation Data analysis showed flood peak discharge increased 22% for the 2-year flood and 84%for the 100-year flood with an increase of 45% for the average annual runoff. The increases are due to increases in annual precipitation and urbanization in the South Fork Pheasant Branch watershed. The stormwater-management practices have been successful in decreasing the sediment and phosphorus in Pheasant Branch, in spite of increased annual runoff and flood peaks. The largest decrease in sediment and phosphorus loads occurred after 2001, when a large detention pond (Confluence Pond) began operation. Since 2001, annual sediment loads decreased from 2,300 to 1,000 tons per year, a decrease of 56 %. Annual phosphorus loads decreased from 12,200 pounds to 6,300 pounds per year a decrease of 48 %. To understand trapping efficiency of Confluence Pond, and streamflow and water quality loading from the two branches, gaging stations were established on the South and North Forks. The inflow and outflow data from the three-gaging station, 2012-2019, that Confluence Pond trapped 63 % of the sediment load and 28 % of the phosphorus load. The total sediment load trapped was 6,480 tons and the total phosphorus load trapped was 22,200 pounds. The average annual flood peaks on the urban South Fork are 132 cfs/sq mi compared to 64 cfs/ sq mi. on the rural North Fork. The gaging station, downstream of the Confluence Pond, had an average annual peak discharge of 48 cfs/sq mi . indicating effectiveness of Confluence Pond in reducing flood peaks. The monitoring program illustrates the value of showing the positive improvement in water quality characteristics in spite of the increases in streamflow due to urbanization and climate change.

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