Rainfall Analysis

Houston, TXTyler Gentry

Tropical Storm Allison Analysis

Tropical Storm Allison Analysis Cont’d

Harris County Rainfall

• A great website for Harris County rainfall analysis over up to 365 days located at http://www.hcoem.org/

Harris County Rainfall Cont’d• Once you select the gauge you want to explore, the site takes you further

and allows you to find detailed information about the location.

Analysis

• In order to make a calculation of water tank capacity requirements for the micro area of study, we will look directly at the Buffalo Bayou.

• The drainage area of the Buffalo Bayou is 103 square miles.

• The population in that drainage area is over 410,000.• The open stream is 116 miles.• During Hurrican Allison, a gauge on the Buffalo Bayou

read 23.54 inches of water in 5 days, 14.45 inches of that were in 2 days.

Analysis

• If the entire area of the Buffalo Bayou’s drainage system received 7.3 inches of rain in a day, that would amount to:– 1 sq. mile = (5280x5280) or 27,878,400 s.f.– 7.3” = .6083333 of 1 foot– This equates to (.6083333x27,878,400/.6083333) 16,959,360

cubic ft. of water per mile.– This is (1 US Gal = .133680556 cubic ft.) or 126,865,350 Gallons

per mile.– The total gallons in the Buffalo Bayou Drainage system through

the day would then be (126,865,350x103) or 13,067,131,059 Gallons in a 24 hour period.

Determining Impervious Land Cover

• A precedent two-step process to estimate the fraction of an urban watershed covered by a hydraulically effective impervious area:

• The first step applies maximum likelihood classification of fine-scale multispectral satellite imagery to derive urban land cover. The second step uses an automated macro in a geographic information system to trace the water flow path from pixels classified as impervious and subclassify them as noneffective or effective. The two steps were verified independently, with verification of the second step using idealized data. The two-step process was then tested with a

small watershed study of model calibration and rooftop connectivity impact on runoff. At the watershed scale the land cover classification differences were approximately 6%, while at the pixel scale matches of 50, 60, and 83% were achieved for the rooftop, asphalt/concrete, and vegetation land covers, respectively. The effective impervious area was estimated to comprise 16% of the watershed surface, which was close to the actual value of 22%. At the pixel scale, the effective impervious area match was less accurate at 48%. Differences in both land cover and effective impervious area classification at all scales are attributed to high land surface heterogeneity, data limitations and errors, and tree canopy covering impervious surfaces. The verification tests and runoff simulations validate the method as a useful means to rapidly estimate with reasonable

accuracy an essential urban hydrologic model parameter. • J. Hydrologic Engrg. Volume 14, Issue 2, pp. 111-120 (February 2009)

The Process

• I am going to use the aforementioned pixel analysis method in order to obtain a relative rate of impervious land cover for use to calculate the rainfall impact on the watershed.

• Step 1. Overlay a satellite image onto a map of the Buffalo Bayou watershed area from the Harris County Flood Control District website: http://www.hcfcd.org/L_buffalobayou.html

• Step 2. Remove the area this is not within this area.• Step 3. Separate Green / Yellow pixels• Step 5. Assess the approximate percentage of

impervious land cover.

Images of Process

Images of Process Cont’d

Analysis Cont’d

• From these analysis images, we can assume that approximately 60% of the land in this area is impervious cover.

• If we assume that at least 60% of the total gallons per day must drain through the bayou, this is still (13,067,131,059 Gallons*60%) 7,840,278,635 gallons per day or 326,678,277 gallons per hour flowing into the buffalo bayou.

• How fast is the bayou flowing into the gulf? Well….

A look at a precedent study…• The inventors of the thruster system have applied for a pattent at

http://www.freepatentsonline.com/7419334.html .• In their research, they state that “In the case of the Great Flood of 2001 in the City of

Houston, the elevation between the flooded area and the Gulf of Mexico was about 24 feet above sea level and the distance from the flooded area to the Gulf Mexico was about 20 miles. So the driving force of the rainwater was a head of about 24 feet. It literally would not do a substantial amount of good to make the waterway significantly deeper because if the waterway were significantly deeper it would potentially be below sea level. To make the waterway progressively wider to increase the volume in a highly urbanized area is a massive investment in the purchase of land and the movement of earth, and the changes to other civil engineering structures such as bridges and roads.”

• Further in the background they continue, “Now imagine that every one thousand feet along the Houston Ship Channel from downtown Houston to the start of the bay system into the Gulf of Mexico we put a water jet thruster package into the water. It is a distance of about 20 miles. That would be about 5 thruster packages per mile or about 100 thruster packages. Now assume in the normal flood situation, the waters are being carried away from downtown Houston, down the Houston Ship Channel, at approximately 3 miles per hour. The speed is a balance between the energy provided by the water and the frictional forces resisting it.”

Precedent Cont’d• They continue to explain their mathematic calculations: “If the estimate of

water at flood stage in Buffalo Bayou from Houston is a minimum of 36 feet deep, 300 feet wide at the surface, 228 feet wide at the bottom and the water flowing at the rate of 3 miles per hour, that would mean that a total of 1,003,622 cubic feet of water would be flowing, or 62,626,037,760 lbs. of water would be flowing. If the bayou slopes 24 in 20 miles, it drops 1.2 ′feet per mile or 3.6 feet in one hour, or 0.000682 feet per minute. The energy derived is 0.000682 feet times 62,626,037,760 lbs. or 42,699,571 feet-pounds per minute. This divided by 33,000 gives 1,294 horsepower.

• If the power required is a function of the square of the velocity, and the system method is only 50% efficient, then 1294*4/0.5=10352 horsepower. If we divide the 10352 horsepower by the 100 thruster stations, we get that each of the thruster stations would require a minimum of 103.5 horsepower.”

In relation to the vacuum system

• The major difference between the thruster system and the vacuum system is that the vacuum system does not have to fight against the water pushing from the gulf.

• A typical 10 HP Gould’s water pump Centrifugal Pump, @ 60 hz with will pump 345 Gallons per Minute with Water Flow @ 70 Feet.

• In comparison, for the thrusters to move 1,003,622 cubic feet of water, it would require 10,352 HP.

• For this system to remove the same 1,003,622 cubic feet of water, it would only require 560 HP.

Flow Rate / Calculations Cont’d

• Once the bayou floods, and becomes closer to level with the gulf, the flow rate slows to a crawl.

• We can assume that the system requires us to be capable of removing 326,678,277 gallons per hour in a rainfall @ .304” / hour to prevent any flooding.

• If the tanks along the bayou cooperatively hold two days worth of rainfall at this rate, the bayou should be able to flow at a rate allowing to remain flowing.

• This would be that the flood areas on the Buffalo Bayou would need to hold a total of 15,680,557,271 total gallons over the two days.

• As seen in the above diagram, the flood areas along the Buffalo bayou are divided into 7 approximately equivalent areas for implementation flood control systems.• Each area’s tanks would need to be capable of containing 2,240,079,610 gallons.

Evaluation of existing design

• The existing design contains 17 modular tanks that are very close to equal in the amount of potential volume capacity.

• If we average these, we would be required to contain 131,769,389 gallons per container to relieve the city of a 2 day (500 year) flashflood rainfall.

Evaluation of Existing Design, Cont’d

• Currently each triangular tank measures approximately 260’ wide, 300’ tall, 130’ deep.

• Because they are triangular in shape, these areas contains 5,070,000 cubic feet of water or 37,926,391 gallons.

• We need to hold 131,769,389 gallons , but we can only hold 37,926,391 gallons at this point. This means we need 3.5 times as many of these systems in order to prevent a two day 500 year flood rain storm.

• This tells us that we may need 24 sets instead of 7 systems along the bayou, this would suffice for all of the buffalo bayou drainage area.

• However, without implementing these systems at each of the bayou drainage areas, the system would not work.

Resources

• http://www.srh.noaa.gov/hgx/projects/allison01/textproducts/060901pns.txt

• http://www.hcoem.org/HCRainfall.aspx• http://www.hcfcd.org/L_buffalobayou.html• J. Hydrologic Engrg. Volume 14, Issue 2, pp. 111-

120 (February 2009) Feb. 2009• http://scitation.aip.org/getabs/servlet/GetabsSer

vlet?prog=normal&id=JHYEFF000014000002000111000001&idtype=cvips&gifs=yes

• http://www.freepatentsonline.com/7419334.html

• http://www.grainger.com/Grainger/items/1N485?cm_mmc=Google%20Base-_-Pumps-_-Centrifugal-_-1N485