Combined Sewer System Long Term Control Plan - DC Water

Section 11
Selection of Final LTCP
Selection of Final LTCP
11.1 INTRODUCTION
Based on the comments received, the new TMDL for TSS for the Anacostia River and on additional
CSO Policy considerations, WASA developed additional CSO control alternatives for selection of a
Final LTCP. This section describes the development of the alternatives and the selection of the Final
LTCP.
11.2 BASES FOR EVALUATIONS
11.2.1 Design Storms
The selection of the Draft LTCP was made based on average year conditions as required by the CSO
Policy. Several commenters requested additional evaluations of the quantity of CSO overflow and
the performance of various control plans for design storms such as the 1-year, 2-year, 5-year, 10-
year, 15-year, 25-year and 50-year return frequency storms. Several sources of data were reviewed
to develop design storm characteristics. They are:
USDA Approach – These are synthetic storms developed based on rainfall data collected
throughout the US by the National Weather Service (NWS). Maps of regional rainfall for
various design storms are presented in Technical Paper No. 40 (Hershfield, 1961). This
rainfall is then apportioned in time using a distribution created by the USDA (1986). This
distribution is appropriate for design storms over small areas less than 10 square miles.
Huff Approach – These are also synthetic storms developed based on design storms
presented in NWS Technical Paper No. 40. The approach differs from the USDA method
when apportioning the rainfall over time. The Huff Approach was developed for large
drainage areas, ranging from 50 to 400 square miles (Huff, 1990).
National Airport Data – hourly rainfall data is available at National Airport from 1949 to the
present. This data was used to calculate return frequencies based on actual data.
The procedure for determining design storms is to first select a storm duration. Common durations
are 6-hour, 12-hour and 24-hour storms. Longer storms produce larger rainfall volumes. For each
duration, storms are ranked from largest to smallest based on total rainfall volume. Note that when
using actual rainfall data, intensities may or may not increase with rainfall volume. For example, it is
possible to have a 25-year storm where it rained steadily for the entire 24 hours thereby producing a
lot of rain but having a low intensity. Similarly, a lower return frequency storm such as a 10-year
storm may have rained intensely for a short period of time and then drizzled for the remainder of the
24 hours. These are the types of complexities encountered when dealing with natural phenomena.
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