The basic premise of a design storm’s utility is that a design storm of a given frequency will produce a simulated runoff peak and volume having the same return frequency. Thus, a 100-year design storm should produce a 100-yr runoff and volume. Design storms have used to develop engineering solutions to counter the basin’s flooding responses. Typically, design storms have been developed by statistical analysis of long term point precipitation records with little regard for the runoff consequences (Urbonas, 1979). If developed in this manner, the design storm is likely flawed.

An inherent weakness of the standard design storm is that it lacks true definition of the storm’s aerial coverage in a basin and the temporal distribution of the rainfall producing the runoff (Urbonas, 1979). Typically, these factors are “backed into” by synthetic modeling efforts. Lacking both quantitative storm aerial and temporal distributions from observations, many contemporary design storm approaches have been based on a combination of long term precipitation point records and sophisticated modeling efforts.

However, the SBC design storm has addressed these key weaknesses and is based on a strong local and regional observation base and related basin response modeling. The SBC design storm proposed is based on the integration of four key basin evaluations:

1. statistical South Boulder Creek precipitation record evaluation and analyses to provide the storm magnitude and return frequencies and antecedent moisture characteristics,

2. enhanced radar analysis of regional, historical 100-yr storms to refine the aerial coverage and temporal rainfall of observed Front Range 100-yr thunderstorms,

3. observed topographic impacts on SBC storm placement and development on precipitation distribution within the basin and,

4. paleo-hydrologic basin-specific evidence of past flooding events.

Unlike traditional design storms that rely heavily on creative analyses of precipitation records or historically developed storm types from other parts of the country, the SBC design storm relies on the integration of aerial and temporal characteristics not available until the installation of the NWS WSR-88D Doppler radar network in the early 1990’s. Application of the 1993-2003 WSR-88D data base has addressed the aerial and temporal distributions of storms not available from precipitation records. We recognize that the 10-year WSR-88D record is short in comparison to the many long term precipitation records and that the relationship of radar reflectivity to rainfall is still an imperfect science. Still the advantages of the radar data base outweigh any shortfalls.

Thus, the definition of the SBC design storm is based on observed storm characteristics. These analyses address the inherent weaknesses of most design storms. Most standard design storms rely only on point or network precipitation analyses and statistically generated precipitation characteristics. The SBC design storm has been “observationally customized” for the SBC basin. The basin modeling tasks provide insights into the runoff characteristics generated by the design storm to provide a bridge between the basin design storm and its runoff characteristics.

Instead of a typical, conservative uniform rainfall distribution and design storm application, the SBC design storm is based on the integrated, climatological analyses completed for this study. Based on the SBC storm climatology it evolved into two customized design storms: a 72-hour general storm and a 24-hour thunderstorm. These two design storms reflect the primary meteorological causes of flooding in SBC basin. We refer to these design storms as our “scientifically defensible” Design Storm (SDDS) for South Boulder Creek.