Interim Hurricane Harvey Report

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EXCERPTS FROM INTERIM HURRICANE HARVEY REPORT

The following material contains excerpts from Harris County Flood Control District meteorologist Jeff Lindner’s interim "Immediate Flood Report #2 - Hurricane Harvey 2017," dated October 10, 2017:

GENERAL FLOODING STATEMENT

The tropical wave that would eventually develop into Hurricane Harvey moved off the west coast of Africa on August 11th and entered the Gulf of Mexico on the afternoon of the 22nd and was upgraded again to tropical depression Harvey on the morning of the 23rd. Over the next 48 hours, Harvey would undergo a period of rapid intensification from a tropical depression to a category 4 hurricane and make landfall along the Texas coast near Port Aransas around 10:00 p.m. on August 25th. A strong rainband developed over Fort Bend and Brazoria Counties during the evening hours of the 26th and spread into Harris County and slowed while training from south to north. Flash flooding developed rapidly between 8:00 p.m. and 11:00 p.m. as tremendous rainfall rates occurred across much of Harris County. Additional rain bands continued to develop into the morning hours of the 27th producing additional excessive rainfall amounts.

As the center of Harvey slowly moved east-southeast and back offshore, heavy rainfall continued to spread across Harris County through much of the 29th and the 30th exacerbating the ongoing widespread and devastating flooding.

Early damage estimates suggest hurricane Harvey’s landfall in Texas and flooding across the Houston area will likely become one of the most costliest disasters in US history – potentially surpassing that of Hurricane Katrina’s $160.0 billion damages in 2005. Current damage estimates for Harvey range from 80-190 billion dollars.

RAINFALL

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)
1-Hour
Maximum 6.8” 1,500 (0.0667%)
Weighted Range 4-5” 50-500 (2.0% - 0.2%)
24-Hour
Maximum 28.6” 5,000 (0.02%)
Weighted Range 16-20” 200-1,000 (0.5%-0.1%)
2-Day
Maximum 35.2” 12,000 (0.08%)
Weighted Range 23-30” 1,500-5,000 (0.067%-0.02%)
4-Day
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.

RAINFALL HISTORICAL CONTEXT

There are three ways to examine a rainfall event to determine its historic nature and comparison to other previous events. This includes duration, amount, and spatial coverage of rainfall. Texas A&M State Climatologist Dr. Nielson Gammon examined all of the largest rainfall events ever recorded in United States history and compared against Hurricane Harvey for durations of 48, 72, and 120 hrs and in spatial coverage of 1,000, 2,000, 5,000, 10,000, 20,000, and 50,000 square miles. Harvey exceeded the previous record in all of the 18 different combinations except one. The most astounding statistic is that for the 120 hr duration over 10,000 square miles, Harvey exceeded the previous record from June 1899 by 13.33 inches or 62%. The rainfall amounts and spatial coverage of those amounts have never been experienced across the United States since the late 1800’s.

Additionally, the average 33.7 inches of rainfall from Harvey across Harris County exceeds the worst storm event ever recorded for a similar square mile area as Harris County in the state of Louisiana in August of 1940 by 3.9 inches.

The following table shows the 120 hr rainfall average for Harvey compared against the previous record for various coverage areas:

Area (Sq Mi) Event Average Rainfall (in) Over Previous Record

1000 Harvey 45.71 40%

Louisiana 1940 32.64

2000 Harvey 43.69 47%

Louisiana 1940 29.80

5000 Harvey 39.72 5%

TX June 1899 25.60

10000 Harvey 34.72 62%

TX June 1899 21.39

20000 Harvey 28.22 51%

Beulah 1967 18.70

50000 Harvey 19.05 19%

Beulah 1967 16.00

For Harris County, the 33.7 inches averaged over 1,777 square miles was 68% of the annual rainfall of 49.77 inches at Bush IAH in a 4 day period. Over a 50,000 square mile area, Harvey dropped upwards of 16.6 trillion gallons of water which could supply the entire US water needs for 280 days and fill Lake Conroe 116 times.

CHANNEL FLOODING (Kingwood area information only)

Disastrous flooding occurred on many of the watersheds in Harris County except Sims Bayou, portions of White Oak Bayou and Horsepen Creek. Historical records held by previous massive floods in October 1994, Tropical Storm Allison, and April 2016 (Tax Day) were exceeded by Harvey at many locations.

San Jacinto River

Catastrophic record flooding occurred along the entire San Jacinto River system including the West Fork, East Fork, mainstem below Lake Houston, and major tributaries along the river including Gum Gully. Massive flooding occurred throughout Humble, Kingwood, Huffman, Crosby, Highlands, and portions of Sheldon. Extreme flows on the lower portion of the San Jacinto River around Banana Bend completely lifted houses off their elevated pilings and resulted in severe damage to roadway access into that subdivision. The previous record flood levels of October 1994 were exceeded at all locations along each section of the river. Along the West Fork of the San Jacinto River water levels surpassed October 1994 by 3.0-4.0 ft, and as much as 5.0 ft along the East Fork of the San Jacinto River. Mainstem river flooding below Lake Houston exceeded the previous record in October 1994 by 1.0-3.0 ft and at the I-10 crossing water levels exceeded Hurricane Ike’s storm surge by 4.0 ft. Water levels along the West Fork of the San Jacinto River averaged above the .2% (500-yr), along the East Fork of the San Jacinto River were 5.0 ft above the .2% (500-yr) level and along the mainstem of the river below Lake Houston averaged between the 1% (100-yr) and .2% (500-yr) annual exceedance probabilities. Several locations along the river system experienced water levels into the second floor of homes or the first floor of elevated structures requiring extensive water rescue efforts. Additionally, large amounts of debris and heavy sedimentation upwards of 4.0-6.0 ft in some locations have been noted especially along the West Fork of the San Jacinto River.

Lake Houston

A record pool elevation of 53.1 ft was recorded at the Lake Houston Spillway surpassing the previous record of 52.3 ft in October 1994. An estimated discharge of 425,000 cfs or 5.0 times the average flow of Niagara Falls occurred at the peak flow over the Lake Houston spillway. This amount of flow would fill the Houston Astrodome in less than two minutes.

Houston Ship Channel

Incredible amounts of rainfall run-off were discharged into the Houston Ship Channel from many of the bayous and creeks draining Harris County. The NOAA tide gage at Manchester (610 E Loop) recorded a peak water surface elevation of 12.01 ft on August 29th. Hurricane Ike’s storm surge flooding at the same tide gage was 12.30 ft suggesting Harvey’s rainfall run-off was only .29 of a foot lower than Ike’s storm surge.

Lake Conroe

A new record pool elevation of 206.20 ft was recorded for Lake Conroe surpassing the previous record pool of 205.60 ft in October 1994. A peak release rate of 79,141 cfs was passed through the Lake Conroe flood gates into the West Fork of the San Jacinto River in accordance with emergency procedures for an extreme event to protect the integrity of the dam structure. While Lake Conroe released 79,141 cfs, three other uncontrolled watersheds: Spring Creek, Cypress Creek, and Lake Creek contributed a total of 168,200 cfs into the West Fork of the San Jacinto River. It is estimated that 200,500 cfs flowed through the West Fork of the San Jacinto River at Humble (US 59) of which 39% was water from Lake Conroe. Of the total estimated inflow of 444,200 cfs into Lake Houston 17.8% was from Lake Conroe. The table below shows the peak discharge rates into Lake Houston from the major watersheds that drain into the lake.

Watershed Peak Discharge (cfs)

E Fork of San Jacinto River 119,000

Peach Creek 31,300

Caney Creek 20,900

Cypress Creek 28,100

Spring Creek 82,100

W. Fork of San Jacinto River (Porter) 130,000

Luce Bayou 32,800

Total 444,200