A Baseline Climatology of Precipitation Rates that Induce Flash Flooding in an Urban Environment Suzanne M. Fortin National Weather Service Pleasant Hill/Kansas City October 19, 2005

Objectives To perform a quantitative assessment of short duration, extreme rain rates and historical precipitation return frequencies in the Kansas City metropolitan area and surrounding region Downscale U.S.-scale precipitation frequency estimates utilizing HPD and Alert system Build runoff return frequencies for the urban area Integrate precipitation and runoff frequencies to build excess runoff climatology TP40

Why? Kansas City has a history of extreme flooding and flash flooding. Several news-worthy extreme flash flooding events in the past 50 years, with four extreme events occurring in the past 30 years. Intense/extreme precipitation events have increased across CONUS (Knight, 1988) Operational forecaster rule of thumb for flash flooding onset is outdated. “Recent extreme” rain events and data excluded Current technology – WSR-88D and Flash Flood Monitoring and Prediction program – often detect rain rates 3-4-5”/hour Bullet 1 1947 Holt, MO 1951 floods, 1961 TS Carla floods, 1993 floods Bullet 2 Sept. 11-12, 1977, Oct. 4, 1998, May 19, 2004 and Aug. 27, 2004. Bullet 3 Knight et al. paper addressing increasing frequency of flash flooding in CONUS Bullet 4 1”/hour

Caveats Will not address meteorological conditions associated with extreme rain events in Missouri and eastern Kansas (Junker, 1993; Moore, 1995 & various; Glass, 1998; Glass et al., 1995; Maddox et al., 1979). Will not address WSR-88D rain rate climatology Extensive library of flash flood/heavy rainfall events in the vicinity. Hydrologic issues a very complex in the urban environment – not an expert on storm water management.

Historical rain rates Comparison of US and local extreme rain rates derived from HPD and Alert micronet. Alert system mirrors US records in the critical flash flood period of <6 hours

Urban Flash Flood Hydrology By definition (NWS), flash flood producing rains have durations of six hours or less Onset of flooding in urban areas much greater due to impervious surfaces, disruption of natural water balance, increased flood peaks (Young, H.Y, 2000; Sauer et al., 1983) Four of the most significant flash flood episodes in the past 25 years in the urban core of Kansas City have occurred within 1 to 3 hours of the onset of rain, producing twice the quantity of rain indicated in NWS Hydro-35 (NOAA, 1977) NWSI 10-922 Data from Sauer et al. suggest that at 50% impervious cover, peak flow from a 100-year event can be twice the peak of a rural watershed 24-hour, 90% storm event based on rainfall spectrum developed from local rainfall records (excludes pp < 0.1 “); 100 year peak runoff expensive and limits development Important for forecasters to be familiar with level of storm water mgmt in urban areas, as storm water design dictate when runoff will become uncontrolled.

Urban Flash Flood Hydrology Storm water treatment and design typically utilize 10 year, 24-hour, 90% rainfall event for cost mitigation (SWRC, 1998) Analysis of KC storm water design found system will accommodate 1.37”/hour (Young & McEnroe, 2002) Runoff return rates at other durations and frequencies difficult to obtain/access Design rather than geomorphology dictate level of excess – a difficult concept for forecasters to grasp More aggressive flood control projects in parts of city – Brush Creek aft. 1977, Blue R. aft 10/4/1998 More difficult part of project to ascertain – need cooperation/assistance with/from city engineers; Jacob Creek/Turnpike FF resulted from design limitations

Comparison of Rainfall Frequencies Hydro35 based on ~25 years of data (1972), length of record of alert gages 15-20 years Hydro35 –Gumbel PDF; OP – Generalized Extreme Value PDF Courtesy of OPSWM

Revised 60-minute Rainfall Frequency Curve for Kansas City Utilized QC’d hourly precipitation data from 20 HPD sites Developed frequency distribution of rainfall categories ≥1.0 (NOAA Hydro35, Young & McEnroe, 2002) Utilized Hydro35 regression equations to develop revised distribution Station records 30-50 years, include 4 major flash flood events Frequency distribution chosen based on Hydro35 documentation – Hydro35 dataset a subset of current HPD 100-year value based on 99% confidence interval of KC HPD PDF

Revised KC Precipitation Frequency Downscaled Precipitation Frequency utilizing HPD and Alert; Utilized coefficients derived from Hydro35

Conclusions Downscaling of rainfall frequencies indicate rates greater than values from Hydro35 Developing excess runoff return frequencies not as tangible, but have something to build upon Integrate rainfall/excess runoff frequency to establish urban FFTI Based on rainfall rate frequencies, KC urban core will need to continue to enhance storm water design to mitigate flooding Important to take advantage of recent, dense data networks

Future Research Recalibrate coefficients for return frequencies Expand analysis to develop frequency curves based on 15-, 30-, 90-, 120-minute data (ASOS, Alert, DCP) Reprocess data utilizing PDF employed by Young & McEnroe, 2002 and Angel & Huff, 1992 Work with local storm water management groups to establish excess runoff frequencies