Flood and Precipitation Frequency of the September 13th 2006, Flash Flood in Railroad Wash through Indirect Discharge E

1. �Flood and Precipitation Frequency of the September 13th 2006, Flash Flood in Railroad Wash through Indirect Discharge Estimates� National Weather Service Tucson, AZ Glenn Lader

2. Overview of Railroad Wash Watershed ~200 square mile watershed that covers all of Southern Greenlee County south of Franklin, AZ and portions of extreme western Hidalgo County, NM Basin is about two-thirds on the Arizona side Majority of contributing headwaters are located in the Peloncillo Mountains in Greenlee County 3 people were killed at Railroad Wash from Flash Flooding that occurred in September 1980.

3.

4. Overview of Event September 12-13 2006 Flooding occurred at tail end of active 2006 monsoon season Two active days of thunderstorm activity over watershed. Radar data indicated greater rainfall on September 12th compared to September 13th when actual flooding occurred. Lack of reliable rainfall data is a significant problem in this case. Watershed is too far away and partially beam blocked, causing a degradation in reliable Doppler radar rainfall totals NWS CO-OP observer just north of watershed in Duncan reported 0.96� on September 12 and 0.40� on September 13 Greenlee County Emergency Management reported flooding of 5-10 residences along Railroad Wash and overtopping of the bridge on Highway 70. NOTE: Survey was done just downstream of bridge on Highway 70.

5. Process of Obtaining Indirect Discharge--Introduction Find straight channel reach, then use approximate distance across channel as the amount of distance between each cross section Determine appropriate cross section locations Find appropriate benchmark that can be used in survey with each cross section

6. Process of Obtaining Indirect Discharge--Finding High-Water Marks Best to go into the field shortly after event Most common usage is debris piles pushed up against vegetation or laid out on the side of the channel Soil differences along edge of channel could also be an additional indicator

7. Process of Obtaining Indirect Discharge�Distance, elevation and slope At every significant break, a distance must be obtained from the high water mark and an elevation must be obtained from the benchmark location Between each cross section, a slope must be surveyed in. In this case the reach was straight only to obtain 4 cross sections. The slope averaged about 0.3 ft/100ft distance. Note channel characteristics for later N number designation

8. Compile all data and run USGS Slope Area Computation Program (SAC) With only four cross sections of data the best software to use to estimate a discharge is the USGS SAC program SAC uses the Slope-Area method from �Measurement of Peak Discharge by the Slope-Area method� (Dalrymple and Benson, 1968) Manning Coefficient of Roughness number N of 0.036 was estimated and used

9. SAC Program Results Slope area calculations from each cross section indicated there was a range in estimated discharge from 5785 cfs to 8996 cfs. Taking an average of the cross sections yields 6916 cfs. This was the value used for discharge in this study.

10. Determining Flood Frequency Return Period using USGS Equations Equations to determine flood frequency in ungaged watersheds have been developed for Arizona (and each state) by the USGS (USGS Fact Sheet 111-98, June 1999) This data is used in the USGS National Flood Frequency program Each state is divided into Hydrologic regions which each have different equations

11. Determining Flood Frequency Return Period using USGS Equations Using the 6916 cfs obtained from the SAC program, and 200 square mile basin yields a recurrence interval of 27 years. Since part of the Railroad Wash watershed is in New Mexico, the same data was plotted for the appropriate USGS New Mexico equations. The return period difference was quite minimal. Even assuming up to a 20% +/- error in discharge using the Slope area method would yield a recurrence interval ranging from 15 years to 40 years.

12. Flood Frequency plot using USGS Equations

13. NOAA Atlas 14 Rainfall Frequency for Railroad Wash watershed

14. Issues Determining Rainfall in Watershed September 12-13th Tucson Doppler radar is located over 100 miles away from Watershed Lowest beam of radar is almost 10,000 feet AGL. This results in radar missing low level precipitation processes. The northern half of the Watershed is beam blocked and thus already underestimating precipitation considerably

15. Issues Determining Rainfall in Watershed September 12-13th Radar derived precipitation amounts were similar across the watershed on September 12th, the day prior to the flood. This rainfall on September 12th in the Watershed was what set the stage to the main flood on the 13th. The watershed was completely saturated, thus nearly all additional rainfall on the 13th would be runoff There was a nearly stationary thunderstorm for 2-3 hours prior to the Railroad Wash Flood on September 13th that was most likely the main contributor.

16. KEMX Radar Loop 2000Z to 2345Z

17. Conclusions from Radar Data review Radar not very useful in determining actual basin average precipitation amounts. Approximate rainfall from radar derived estimate is about 1-1.5 inches from Sept 12 & Sept 13th combined. Taking into account beam blockage issue, watershed average is likely 2-3 inches Based on NOAA Atlas 14, with a watershed average of 2-3 inches in 24 hours, the recurrence interval ranges between 5 years and 40 years. 3 hour radar storm totals were not impressive, however, local unofficial reports indicate 2.5-3 inches occurred near Duncan on September 13th.

18. Comparison to USGS Gages on Gila in Duncan, AZ and Virden, NM

19. Project Conclusions and Discussion Useful Flash Flooding Data can be obtained even well after an event with good high water marks Field survey data input into slope area method obtains peak stream discharge Radar derived rainfall estimates less useful far from radar, stressing the importance of �ground truth� observers WFO�s should be encouraged to engage in flash flood studies through indirect discharge estimates. Discharge data yielding flood frequency can be found which provides an excellent frame of reference for event significance. This is useful to forecasters, emergency managers and flood control districts.

20. Questions????