Episodic change analysis of the annual flood peak time series for a flood insurance study

Decreasing annual maximum flood peaks on the rivers and streams in the Wisconsin driftless area have been reported in recent studies. Various explanations have been offered, generally suggesting different episodes or change points separating the early periods of higher peak flows from the more recent lower peak flows. The present research used two statistical tests to detect a change point in annual flood peaks at Freeport on the Pecatonica River for the period 1914-2008. Both tests indicated that the most significant change occurred in 1954. Next, to find an explanation for the decreasing peaks, this research carried out a seasonal analysis of flood timing. The decrease in winter flood peaks was partly explained by the decrease in snow depth and the increase in winter temperature, providing less favorable conditions for winter flooding. In turn, the decrease in winter peak flows made once smaller summer peak flows more dominant in recent years, causing the shift in flood timing. Similar analysis showed a significant degree of resemblance between the Pecatonica River and several streams in its vicinity.