1900hurricane's WunderBlog

Last Updated: 6:52 PM GMT on January 21, 2010 — Last Comment: 7:34 PM GMT on January 14, 2010

	Texas Hydrology Blog #1: San Jacinto River Basin

Posted by: 1900hurricane, 3:14 AM GMT on June 18, 2008

Despite being interested in meteorology for as long as I can remember, my interests in hydrology have been only recently formed. I believe that my interest in the subject first developed last year during the Great Summer Flood Event of 2007. Early in the morning on June 27th, a "Rain Bomb" exploded over the Central Texas town of Marble Falls, dumping over 18 inches of rain in only 6 hours over the surrounding area, causing record flooding. The flooding then moved downstream via the Colorado River into Lake Travis, where a chunk of my family's property was threatened by the rising waters. While I was monitoring the flood levels, I became interested in how the flooding actually happened, many of the flood events that had happened prior to this one, and about everything else about hydrology. Recently, I've been calculating some of the historical streamflows of past Texas Floods, and some of the numbers are staggaring. Therefore, I've decided to put together a series of blogs showing these crazy numbers. The first in the blog series is on the San Jacinto River Basin, the river basin that I actually live in.

Above is an image of the San Jacinto River's watershed, which is all of the land area which drains into the San Jacinto River. The watershed is divided into 4 different areas. The 1st area, 12040101, is the area which drains into the West Fork of the Upper San Jacinto River. The second area, 12040102, is the area which is drained by Spring and Cypress Creek. It is also the part of the watershed where I live. My house is located less than a mile from Cypress Creek, although it has never flooded. The 3rd portion of the watershed, 12040103, is drained by the East Fork of the Upper San Jacinto River. The remaining portion of the watershed, 12040104, is mostly drained by Buffalo Bayou, which is then drained by the Lower San Jacinto River. The 1st 3 portions of the watershed more or less converge into Lake Houston, which then drain into the 4th portion of the watershed via the Lower San Jacinto River.

The San Jacinto watershed is not very large and only drains one full county and parts of 7 others. However, the flood events that have occured in the watershed have often been extreme and the discharge of the San Jacinto River has on more than one occasion rivaled the streamflow of the Mississipi River. Major Flood events in the watershed occured in 1929, 1940, 1973, 1983, 1989, 1994, 1998 (multiple times), and most recently in 2001. Several of these events will be mentioned in detail in the text below.

The most recent extrodinary flood event took place in early June of 2001 due to the deluge produced by Tropical Storm Allison. Many of the local streams inside the wathershed set record crests. Record crests were set on Greens Bayou, Halls Bayou, Brays Bayou, Hunting Bayou, Cypress Creek, White Oak Bayou, Sims Bayou, and Willow Creek with almost every other waterway (West and East Forks of the Upper San Jacinto River, Caney Creek, Spring Creek, Buffalo Bayou, the Lower San Jacinto River, and probably more) going into major flood stage, especally those in Harris County. However, the most extreme flooding occured on Greens Bayou on the east side of Houston. Greens Bayou is one of the larger tributaries of Buffalo Bayou, but still only has ~200 square miles of dranage area. Still, just below the buyou's confluence with Halls Bayou at Ley Road, the raging waters were measured to be flowing at almost 70000 cfs (cubic feet per second), which would result in a basin yield of 382.97 cfs/square mile! That would be an impressive reading even for a river with a dranage area 10x larger. The next highest crest (amazingly produced by another tropical storm named Allison, this time in 1989) doesn't even have half as high of a streamflow. This was caused by Allison's multi-day rain event producing almost 40 inches of rain, with over half of that being in one 12 hour period, directly over Greens Bayou. Due to Allison's intense flooding over an urban area, the cost of the event exceeds $6 billion in damages, making Tropical Storm Allison the most costly tropical storm in history and one of the most expensive flood events as well.

Example of some of the flooding caused by Tropical Storm Allison.

The magnatude of Allison's rains were unprecidented for anywhere in the San Jacinto watershed, but they were more localized than many of the other extrodinary flood events in the watershed's history, and the amount of rainfall between two points dropped off sharply. The epicenter of the heavy rains was centered in 12040104, the lowest part of the watershed, meaning that there was nothing extreme coming from upstream, thereby aiding in the dranage of the deluge. However, in October of 1994, there was a extrodinary rain event that did not drop as much rain as Allison, but distributed heavy rain almost evenly across the entire San Jacinto Watershed.

The October 1994 flood event had a complex orgin. a stationary front was in place over the area and moisture both from the Gulf of Mexico as well as from the Pacific due to the remmanents of Hurricane Rosa was overrunning the boundary, setting the stage for multiple days of heavy rain over a large area. Although the rainfall was evenly distributed at about 16 inches througout the entire watershed, a maximum of 28 inches did occur in southern Motgomery County at Kickapoo Creek: a tiny and remote tributary of Spring Creek. The rain maximum was enough to drive the tiny creek all the way up to an incredible streamflow of 84600 cfs, giving it a basin yield 1480 cfs/square mile, by far the largest I have ever seen. I'm not even sure how this is possible, considering the creek (which I have seen with my own eyes) is situated in a 6 foot deep ditch about 4 feet across and surrounded by flat countryside. The streamflow was an offical measurement however, which means that it did somehow happen. Other incredible measurements include 33 ft and 74100 cfs on the East Fork of the Upper San Jacinto River near New Caney (in area 12040103), 44.05 ft and 78800 cfs on Spring Creek near Spring (in area 12040102), and 32.3 ft and 115000 cfs on the West Fork of the Upper San Jacinto River near Conroe (in area 12040101). All of the above crests and streamflows set new records that still stand today (although the records are limited: more on that later). When all of the water converged in Lake Houston, the result was a new record lake height at 52.79 ft and almost doubling the normal capacity of 136000 acre feet of the lake at 249000 acre feet. Since Lake Houston was not built for flood control, the raging floodwaters had to move on. At Sheldon, the Lower San Jacinto River was measured to have exceeded the 100 year floodlevel by over 4 feet with a gauge reading of 27.09 feet. The streamflow of the river was also measured at an incredible 360000 cfs, one of the highest recorded in Texas and 160% higer than the 100 year streamflow value of 225000 cfs. Although the 1994 flood event was one on an epic scale, the flooding did not occour in urban areas like the flooding of Tropical Storm Allison, and the price tag on this flood event only amounted up to $1.5 billion, mostly from flooding of resedintal areas throughout the watershed and the trashing of stuff at the Port of Houston due to the incredible floodlevel and streamflow on the San Jacinto River. However, even though that streamflow value of 360000 cfs is easily the highest measued in the San Jacinto watershed and one of the highest in Texas, higher river crests have been recorded on the river on more than one occasion during a time before streamflow was measured on the river, which leads me to believe that much higher streamflows have occured. This is where my original resarch begins.

When one looks at the hydrograph of the Lower San Jacinto River near Sheldon, it is easy to note the historic crests off to the side. In these historic crests, the 1994 crest is almost 7 feet higher than the next highest crest. But, the 1994 crest is only the 3rd highest. The top ten highest crests according to the NWS are as follows:

(1) 32.90 ft on 05/01/1929
(2) 31.50 ft on 11/16/1940
(3) 27.09 ft on 10/19/1994
(4) 20.12 ft on 06/15/1973
(5) 20.10 ft on 05/19/1989
(6) 19.61 ft on 11/15/1998
(7) 17.50 ft on 05/23/1983
(8) 16.36 ft on 06/10/2001
(8) 16.36 ft on 10/20/1998
(10) 15.00 ft on 11/07/2002

There have been two recorded higher crests than the one in 1994, and both of them are over 4 feet higher! This leads me to wonder what the streamflow values for the flood events in 1929 and 1940 would be if the streamflow for the 1994 flood event was 360000 cfs.

I could not find much information on both the 1929 and 1940 flood events. About the best information I could find came from the Tropical Storm Allison Public Report. In the report, it describes the events as follows:

April 1929
Enormous gulf storm descends on Houston and Harris County and lasts 14 hours. Many areas of county report rainfall of close to 10 inces. Extensive damage sustained to businesses and residencies in almost all areas of Harris County. All bayous are reported to be out of their banks.

May 1929
As the area is still reeling from the last storm, another major storm hits Harris County. Structural damage, heavy street flooding and widespread crop damage reported. San Jacinto River 30 feet above normal.

November 1940
Heavy rains last for 5 days in Northeast Harris County. 10,000 head of cattle lost.

Based on this, the 1940 flood event appears to be similar to the 1994 flood event. The 1929 flood event appears to be unique because it is composed of multiple events in quick sucession. Anyway, now it is time to estimate streamflow values.

Above is the San Jacinto River hydrograph near Sheldon. To calculate the streamflows, I decided to match up the river stage on the left of the hydrograph with the streamflow on the right and put them into Microsoft Excel. Then I would graph the points and get a trend line and use the equation of the trend line to find any streamflow value I wanted to.

My first attempt to make a graph didn't turn out so well. Even with the best fitting line, the line was too high in the center of the graph and too low at the end. I determined the cause of this to be the changing of the floodplain above 18 feet, which I would divide into two diferent graphs for above and below 18 feet.

After dividing the streamflow graph into two seprate graphs, the trend lines fit extremely well on each seperate graph, allowing reasonable estimates of streamflows to be made. Using the above formulas on the graphs above, I have calculated the streamflow of the 10 highest crests on the river. My calculations are below:

Both the 1929 and the 1940 flood events appear to have had streamflows of over 500000 cfs, more than double the 100 year flow of 225000 cfs. However, It must be noted that using my formula only gave the 1994 flood a flow of ~350000 cfs, about 10000 cfs too low! This could possibly mean that both the 1929 and 1940 floods are underestimated as well. Therefore, it is possible that the streamflow from the 1929 flood could have approached or even exceeded 600000 cfs, making the 1929 flood even more poverful than the Mississippi River above its confluence with the Misourri River during their 1993 and current 2008 floods!!! Apparently, everything's bigger in Texas, including floods...

*Please Note that many of the values in this blog are a result of unverified individual resarch and should be taken with a grain of salt.

View (11) Comments For This Post

Updated: 9:51 PM GMT on June 18, 2008

Permalink | A A A