Title page for ETD etd-01182011-210957

Type of Document Master's Thesis
Author Mason, Jessica Irene
URN etd-01182011-210957
Title Analysis of Major Hydrologic Events in Ascension Parish, LA
Degree Master of Science in Civil Engineering (M.S.C.E.)
Department Civil & Environmental Engineering
Advisory Committee
Advisor Name Title
Willson, Clinton Committee Chair
Keim, Barry Committee Member
Tsai, Frank Committee Member
  • Frequency Analysis
  • hydrologic model
  • hydraulic model
  • TP-40 method
  • SRCC method
Date of Defense 2010-04-13
Availability unrestricted
Ascension Parish is located in Southern Louisiana and is characterized by low slope and low elevation. This combination allows for much hydrologic storage during a flood event. Because Ascension Parish is one of the most rapidly growing areas in the United States, it is necessary to accurately predict the hydrologic and hydraulic properties of flow and accurately model them. Several processes within a hydrologic model greatly influence the output flow hydrographs so selecting correct methods and parameters is important. Transform methods describe how excess precipitation is transformed into runoff. There are several methods that can be used as a transform method, including the SCS method, Snyder method and Clark Unit Hydrograph method. Each method uses a separate set of equations and processes to determine runoff. By estimating method parameters and using these in a hydrologic model, the effects of varying transform method can be quantified. Results showed significant differences amongst the three methods. The Clark Unit Hydrograph method, which accounts for storage within the watershed, resulted in the most accurate peak flows. Accuracy of these outflow hydrographs was tested through an unsteady hydraulic model for Grand Goudine Bayou. Parameters within the hydraulic model were also tested for accuracy by varying factors, channel Manning’s n value and baseflow, until the simulated stage resembled the actual observed stage. A Manning’s n value of 0.090 and a baseflow of 20 cfs resulted in the most accurate model.

Another important hydrologic design parameter is the rainfall depth determined for the area given a particular return period. There are several different methods and distributions which predict rainfall depths, including the TP-40 method, which utilizes the Gumbel distribution, and the SRCC method. For most areas, the SRCC method offers greater rainfall depth estimates of the higher return periods, (i.e. the 50- and 100- year storms); however, for Ascension Parish, the SRCC method yielded lower rainfall depth estimates for these return periods. When compared after hydraulic modeling, the differences between the SRCC method and TP-40 method were relatively insignificant.

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