In June 1940 the United States Army Corps of Engineers released its Flood Protection Plan for Houston (Buffalo Bayou). I was able to find the original “Definite Project Report” and the associated drawings at the Rice University library.  This post provides some highlights from the documents.

I’ve posted the plan overview before, but its worth reposting here for context.  The green items are levees which form flood damage reduction reservoirs; the blue items are canals or channels; and the yellow text provides identification labels.

The Design Storm

The Corps reviewed a number of different “storms of record” and decided to use a design storm created by combining two of the worst-case scenarios they had seen up to that point.  They derived the design storm by taking the total depths of rain observed from June 27 to July 1, 1899 at Hearne, Texas and combining it with the rainfall intensities (inches per hour) observed in September 9-10, 1921 at Taylor, Texas.  They then placed this hypothetical worst-case storm directly over the planned reservoirs and facilities.

The Hearne storm produced 31.4 inches of rain in 3 days and the average depth of rain over 1,000 square miles was 25.8 inches.  The Taylor storm produced intensities ranging from 4.4 to 0.44 inch per hour.

The combination of these two events is depicted in the graphic below, which shows the hypothetical cumulative rainfall in inches vs. time for the design storm.

The design storm has the total rainfall amount (the horizontal end of the curve in the upper right) of the Hearne event coupled with the crazy steep slope, indicating high rainfall intensity (from 36 to 42 hours) of the Taylor event.  Yikes.

The Design Storm Runoff

To size reservoirs and channels the Corps needed to convert the rainfall depth to actual runoff rates and volumes. This is critical to determining how much of the rain falling from the sky actually runs off and either flows harmlessly around homes and businesses or inundates them.

The Corps conservatively assumed that 90% of the rainfall from the design storm described above would runoff.  To estimate the runoff flowing to each element of the overall plan, the design storm was then moved to be directly above each element.  Runoff timing, total volume, and flow rates (depicted in curves called hydrographs) were estimated using Franklin P. Snyder’s Synthetic Unit Graphs, published in the Transactions of the American Geophysical Union (1938).

Runoff hydrographs for each of the key definite plan elements are presented below.  Note that “second-feet” is equivalent to the flowrate expressed in “cubic feet per second.”  There are about 7.48 gallons in each cubic foot, so you can multiply by 7.48 to get gallons per second and then by 60 to get gallons per minute.  Also, an “acre-foot” is a volume measurement. It is 1 acre of area covered by 1 foot of water or about 325,829 gallons.

Definite Plan Elements

The Corps devised the definite plan to accommodate the design storm and the predicted runoff described above.  The definite plan included the following elements:

• White Oak Reservoir
• Brickhouse Gully Bypass Channel
• North Canal
• Cypress Creek Levee
• Addicks Reservoir
• Barker Reservoir
• Rectification of Buffalo Bayou Above South Canal
• South Canal
• Improvement of Buffalo Bayou Through City

A few details of each of these plan elements are provided below.

White Oak Reservoir

• Peak Design Inflow:                30,800 cubic feet per second
• Total Design Inflow:                103,900 acre-feet
• Levee Length:                         4.7 miles
• Levee Max. Height:                 35 feet
• Levee Max. Elevation:            90 feet above mean sea level
• Storage:                                  24,400 acre-feet
• Max. Pool Elevation:               85 feet above mean sea level

Brickhouse Gully Bypass Channel

• Channel Length:                     1.9 miles
• Lining:                                     Grass / Rip Rap in Selected Locations
• Top Width:                               From 70 to 100 feet
• Bottom Width:                         6 feet
• Side Slopes:                            2 to 1
• Depth:                                      From 10 to 25 feet
• Conveyance:                           1,500 cubic feet per second

North Canal

• Channel Length:                     20 miles
• Lining:                                     Concrete Paving
• Top Width:                               From 140 to 200 feet
• Bottom Width:                         25 feet
• Side Slopes:                            2 to 1
• Depth:                                     From 28 to 38 feet
• Conveyance:                           22,000 cubic feet per second

Cypress Creek Levee

• Levee Length:                         14.9 miles
• Levee Max. Height:                 12 feet
• Levee Max. Elevation:             180 feet above mean sea level
• Top Width:                               10 feet
• Max. Bottom Width:                 82 feet
• Side Slopes:                            3 to 1

Addicks & Barker Reservoir System (Combined)

• Max. Combined Outflow:15,000 cubic feet per second (based on navigation in the Houston Ship Channel)
• Release Conduits: Four 8 feet diameter (Addicks); Two gated “spare” conduits of equal size (Addicks); Five 8 feet diameter (Barker); Two gated “spare” conduits of equal size (Barker)

 Addicks Reservoir

• Drainage Area:                         134 square miles
• Peak Design Inflow:                  50,500 cubic feet per second
• Levee Length:                          10.4 miles
• Levee Max. Height:                  43 feet
• Levee Max. Elevation:             115 feet above mean sea level
• Max. Pool Elevation:                108.3 feet above mean sea level
• Design Storage:                       134,000 acre-feet

Barker Reservoir

• Drainage Area:                        152.8 square miles
• Peak Design Inflow:                 40,300 cubic feet per second
• Levee Length:                          13.8 miles
• Levee Max. Height:                  37 feet
• Levee Max. Elevation:             109 feet above mean sea level
• Max. Pool Elevation:                101.7 feet above mean sea level
• Design Storage:                       135,800 acre-feet

Rectification of South Mayde and Buffalo Bayou Above South Canal

• Channel Length:                     7.2  miles
• Lining:                                     Grass / Rip Rap in Selected Locations
• Top Width:                              From 140 to 200 feet
• Bottom Width:                       15 feet
• Side Slopes:                            3 to 1
• Depth:                                     From 5 to 22 feet
• Conveyance:                           15,000 cubic feet per second

South Canal

• Channel Length:                      39.2 miles
• Lining:                                     Concrete Paving
• Top Width:                               From 120 to 270 feet
• Bottom Width:                          From 30 to 163 feet
• Side Slopes:                            2 to 1
• Depth:                                      From 16 to 42 feet
• Max. Conveyance:                  15,000 cubic feet per second (above Brays Bayou)
• Max. Conveyance:                  28,800 cubic feet per second (below Middle Bayou)

Total Cost

The total cost of all plan elements, plus utility work and other associated details was $13,243,700 (in 1940 dollars).

If you are interested in reading more from the 1940 Definite Plan, I’ve made a scan of it available here.