An Alternative Temporal Rainfall Distribution for Hydrologic Analysis and Design

Transcription

1 An Alternative Temporal Rainfall Distribution for Hydrologic Analysis and Design Joseph P. Wilson, PE, PH Wilson Hydro, LLC PhD Candidate Missouri University of Science Technology Rolla, Missouri

2 Sorry Folks, Missouri is CLOSED due to high water MoDOT Road Conditions

3 I think we can all agree that this Is not the same as this

4 Session Topics Why is the temporal distribution of rainfall important? Rainfall temporal distributions currently used in hydrologic modeling. Some of the issues with currently used rainfall distributions. An updated temporal distribution.

5 Design Storms Most jurisdictions in the United States use the design storm approach to hydrologic modeling for design and analysis. A design storm is a way of representing storm rainfall with respect to depth, duration, temporal distribution, frequency and spatial distribution. Based on the assumption that, under average conditions, a rainfall of a given frequency will produce runoff of the same frequency.

6 Rainfall Temporal Distributions The selected design storm for hydrologic modeling has a significant impact on the computed results. Rainfall Temporal Distributions are generally categorized as: Simple geometrical shapes. Nested intensity. Statistical analysis of a gage network. Simulation based on stochastic rainfall models Based on an observed storm.

7 Temporal Distributions for Discussion NRCS Type I, Ia, II, and III Huff s Quartile Atlas 14 Distributions New Frequency Modified Pilgrim-Cordery

8 NRCS 24-Hour Distributions Nested Intensity. Developed by SCS using rural rain gages, expanded on work by Hershfield. NRCS proposing to discontinue use.

9 Huff s Quartile Distributions Derived from 49 gage network in Illinois Events > year period of record, Revisited in 1990 to add 5 urban gages and Results included in Bulletin 71. Median Curves Point Rainfall

10 Atlas 14 Distributions Published with Atlas 14 Region 8, 2013 Based on precipitation depth > 2-year recurrence interval Distribution by duration 6-hour curves not based on true quartiles Median Curves 24-Hour Duration

11 They are Somewhat Similar

12 Frequency Modified Pilgrim- Cordery Distribution Based on the method developed by Pilgrim and Cordery, PC Ranking method based on arbitrary number of observed rainfall events. Annual exceedance series used. Depths non-dimensionalized. Original method average period used for each rank. FMPC uses most frequent rank. Average depth per rank assigned.

13 Frequency Modified Pilgrim- Cordery Method This analysis uses the NOAA hourly rainfall for 48 stations in Missouri defined as Hydrologic Region 4 in Atlas 14, Volume 8. Period of record ranges from years. The annual exceedance series was used. Durations of 6-, 12-, and 24-hours.

14 48 Stations Studied

15 Data is Messy!

16 Data Processing Period of record >30 years. Missing data does not exceed 5%. Accumulated totals excluded. Consecutive totals summed for consecutive 6-, 12-, and 24-hour periods. Precipitation must occur in at least 80% of the time periods. Overlapping time periods eliminated.

17 Data Processing, cont d The WILRAIN program was developed to process over 27 million lines of rainfall data. A separate MODPC program was developed to derive the Frequency Modified Pilgrim- Cordery Distribution and compute the RMSE. The programs can be applied to data from any NOAA hourly or 15-minute rainfall station or can be customized for other data formats.

18 FMPC Distribution - 48 Stations

19 Computed Error

20 St. Louis Lambert International Cumulative Mass Diagram, Hourly Data

21 St. Louis Regional Analysis

22 St. Louis Regional Analysis St. Louis County Flood Study, MSD, FEMA

23 St. Louis FMPC compared to Huff s 2 nd Quartile

24 St. Louis FMPC compared to Huff s 3 rd Quartile

25 St. Louis FMPC compared to NRCS Type II

26 Summary Frequency Modified Pilgrim-Cordery Distribution has a smaller RMSE than Huff s or the NRCS Type II, and more closely represents observed data. The method can be applied to any NOAA station recording hourly or 15-minute data. Can be used at a single station or as a regional analysis. It is important to use the best available data.

27 Recommendations Select the distribution that best fits your data. Prefer a statistical based distribution. Rainfall distributions in order of preference: 1. Frequency Modified Pilgrim-Cordery. 2. Pilgrim-Cordery. 3. Huff s quartile/atlas As a last resort: Nested intensity from local data.

28 Questions? Osage River Tuscumbia, MO 1977 Phone