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Seasonal Precipitation Frequency and Distribution Analysis

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The Three Rivers Wet Weather (3RWW) Program and the Allegheny County Sanitary Authority (ALCOSAN) have been working in a collaborative effort to evaluate regional sanitary sewer overflow controls and assisting individual communities in evaluating and planning for local sanitary sewer overflow controls. Extensive flow monitoring conducted by ALCOSAN and 3RWW has shown that hydrologic capture coefficients (percent of precipitation that becomes sewer flow) in separate sewer systems vary monthly/seasonally. It is also widely known that precipitation patterns vary seasonally. Therefore, to identify cost-effective SSO controls, it is important to align the hydrologic conditions with the appropriate precipitation pattern in evaluating wet weather controls. For example, aligning large/intense summer precipitation patterns with larger winter capture conditions would result in over sized wet weather controls.

The use of design storms in lieu of continuous modeling is impacted by the choice of a design season. The usual design storm frequency analysis conducted by NWS and NOAA is dominated by summer storms as it uses the entire yearly data for determining the precipitation-frequency relationships and does not distinguish between seasons. Therefore, the primary goal of this analysis was to identify the seasonal precipitation patterns and associated temporal distribution for the ALCOSAN service area.

The following three steps were followed to identify the seasonal precipitation patterns and associated temporal distributions:

Identify seasonal precipitation periods;

Develop seasonal precipitation Intensity-Duration-Frequency (IDF) curves; and,

Develop seasonal temporal precipitation distributions.

Precipitation data from the Pittsburgh International Airport (PIA) gauge recorded between 1948 and 2009 (62 years) was used for this analysis. The Generalized Extreme Value (GEV) Probability Distribution was used to determine the precipitation-frequency characteristics for each month/period. Results from this statistical analysis were adjusted to account for partial-durations versus annual durations and for n-minute precipitation versus clock-hour precipitation consistent with NOAA Atlas 14. The temporal distribution of Pittsburgh precipitation was accomplished by analyzing the temporal distribution of the 20 largest 24 hour duration storms for each season between 1948 and 2009. The precipitation peaks were “aligned” (i.e. each peak precipitation was defined as occurring in the 22nd hour of the 44 hours storm). These storm intensities were then averaged throughout the entire storm duration and plotted to illustrate the temporal distribution. Results were compared with the SCS Type II storm distribution.

The following summarizes the key results and findings of the analyses.

The months of June, July and August exhibited similar characteristics (Summer Period), the months of October through April (Winter Period) exhibited similar characteristics but different than the Summer Period, and the months of May and September (Transition Period) exhibited similar characteristics and different from the Summer and Winter Periods.

The precipitation-frequency curves for the three seasonal periods (Summer, Winter and Transition) were developed.

The temporal distributions of these storms within each season have a pattern similar to the SCS type II precipitation distribution.

Using the IDF curves the hourly precipitation increments were determined and rear-ranged following the Alternating Block method used for the SCS type II distribution. Using this method the 10-Year, 24-Hour storm hyetographs were developed for the Summer, Winter and Transition periods.

A comparison of the 10 Year 24 Hour hyetographs based on the SCS Type II versus Summer Season from this analysis shows a close comparison.

Results show a significant difference in the magnitude and the temporal distribution between Summer and Winter Season events.

Keywords: Generalized Extreme Value Probability Distribution; Intensity-Duration-Frequency Curve; SSO; Seasonal Precipitation; Temporal Precipitation Distribution

Document Type: Research Article


Publication date: 2011-01-01

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