Peak total rainfall for the various time periods are listed below for HCFCD gages across Harris County.
Time 1-hr 2-hr 3-hr 6-hr 12-hr 24-hr 2-day 4-day
Peak Rainfall (inches) 6.8 11.9 14.8 18.9 20.9 25.6 35.2 47.4
Duration – Rain began Friday morning, August 26th across the county with the first heavy band entering the county Saturday night, August 27th. Heavy rain bands continued to sweep across the entire county through Monday, August 28th. The majority of the rainfall occurred during a 4 day period and breaks in rainfall between bans were short and infrequent.
Total Amounts – Total rainfall amounts ranged from 26 to 47 inches across the county for 4 days. The 2 day amount ranged from 20 to 35 inches and the 1 day from 13 to 25 inches. The lowest totals occurred over the northwest and northeast part of the county with the highest totals focused across the southeast part of the county along I-45 from near the City of South Houston and Pasadena southward to Friendswood and Webster and eastward to I-10 and Baytown. A maximum 2 day rainfall total of 34.5 inches was recorded at Clear Creek and I-45 and 35.2 inches at Berry Bayou and Forest Oaks Blvd. A maximum 4 day rainfall total of 47.4 inches was recorded at Clear Creek and I-45
Exceedance Probability – Rainfall was generally less than a 2% (50-yr) event for the time 15-min to 6-hr time periods for most areas except southeast Harris County and Brays Bayou where 1% (100-yr) to 0.2% (500-yr) and greater rainfall occurred. Rainfall for the 12-hr to 4 day time periods ranged from 1% (100-yr) to 0.2% (500-yr) and greater for all watersheds. The maximum and weighted ranges shown below are county-wide and recurrence intervals greater than 500-yr are approximate. The 47.4 inches of rainfall at I-45 and Clear Creek over a 4 day period is 95% of the Probable Maximum Precipitation (PMP).
Duration Rainfall Amount Return Interval – years (exceedance probability)
Maximum 6.8” 1,500 (0.0667%)
Weighted Range 4-5” 50-500 (2.0% - 0.2%)
Maximum 28.6” 5,000 (0.02%)
Weighted Range 16-20” 200-1,000 (0.5%-0.1%)
Maximum 35.2” 12,000 (0.08%)
Weighted Range 23-30” 1,500-5,000 (0.067%-0.02%)
Maximum 47.4” 50,000 (0.002%)
Weighted Range 30-40” 3,000-20,000 (0.033%- 0.005%)
Two other rainfall factors that influence flood levels is the intensity (inches per hour) variation over time and the distribution in the watershed (area distribution). For Hurricane Harvey, the intensity was moderate to high most of the time. Due to the length of the rainfall event and the numerous bands that developed, the areal distribution in most a watersheds did not vary significantly. The following table compares the extraordinary rainfall associated with Harvey against Tropical Storm Allison in June 2001, the Tax Day Flood of April 2016, and the October 1994 Flood for various time periods. It is interesting that Tropical Storm Allison exceeds Harvey’s rainfall in the 12 and 24-hr periods. In the 2 day period Harvey produced 6.0 inches more than Allison and 8.9 inches more over 4 days.
The following table compares the extraordinary rainfall associated with Harvey against Tropical Storm Allison in June 2001, the Tax Day Flood of April 2016, and the October 1994 Flood for various time periods. It is interesting that Tropical Storm Allison exceeds Harvey’s rainfall in the 12 and 24-hr periods. In the 2 day period Harvey produced 6.0 inches more than Allison and 8.9 inches more over 4 days.
Max Rainfall (inches)
Duration Harvey Allison June 2001 Tax Day April 2016 October 1994
1-hr 6.8 5.7 4.7 3.7
2-hr 11.9 9.9 7.3 4.7
3-hr 14.8 13.5 8.3 5.3
6-hr 18.9 21.2 13.9 7.2
12-hr 20.9 28.3 16.7 12.0
1 day 25.6 28.4 17.4 20.9
2 days 35.2 28.5 17.5 23.1
4 days 47.4 38.5 N/A 28.9
A total of 1 trillion gallons of water fell across Harris County over the 4 day period which would fill the Houston Astrodome 3,200 times and cover Harris County’s 1,777 sq. miles with an average of 33.7 inches of water. This volume of water would also run Niagara Falls for 15 days.