How many times in the last nine months since Hurricane Harvey have you heard someone mention something about building a “third reservoir?”

If I had a dollar for every time I heard or read about that, I’d have enough money to pay for it. (Well not actually, but you get the joke.)

This post provides some information on all three of the “third” reservoirs that I’ve heard about. Please add some information about any additional “third” reservoirs you’ve heard about in the comment section below.

White Oak Reservoir (1940)

The earliest mention of a third reservoir I can find is from the U.S. Army Corps of Engineers “Definite Plan” of 1940.

This reservoir was known as the “White Oak Reservoir” and was proposed to be located just north of the area of the future intersection of US 290 and IH-610, near the future T.C. Jester and future West 34th. (Well before those roads were built.)  Here’s a map of the general location with our modern road network and current floodplains shown.

Here’s an image of the reservoir from the original, hand-drawn plans prepared by the U.S. Army Corps of Engineers in 1940.

Key statistics about the proposed 1940 White Oak Reservoir are provided below:

• Drainage Area:                                            72 square miles
• Site Area:                                                     4,280 acres
• Maximum Berm Height:                               35 feet
• Storage Volume:                                          24,400 acre-feet
• Peak Water Surface Elevation:                    84 feet above mean sea level
• Discharge Rate:                                           23,500 cubic feet per second

The White Oak Reservoir was designed to reduce the risk of flooding of downstream areas by capturing, holding, and rerouting larger storms up to a specified large, but rare, depth and intensity of rainfall.

If built, the White Oak Reservoir would have captured stormwater runoff generated from a specific amount of rainfall falling on 72 square miles of the upper portion of the White Oak Bayou watershed.  It would have slowly released the captured water through two planned conveyances. One conveyance was the proposed North Canal, which was a 20-mile long ditch about 30 feet deep that could carry 22,000 cubic feet per second of water. The second conveyance was about 2 miles of channelized Brickhouse Gully that was to carry about 1,500 cubic feet per second into Buffalo Bayou (with obstructions removed).  See detailed image from the 1940 plan set below:

The Cypress Creek Overflow

In order to more fully understand the purpose of the other two proposed “third reservoirs,” it is important to know about the Cypress Creek Overflow. As described in the Final Study Report: Cypress Creek Overflow Managment Plan (August 18, 2015), prepared for the Harris County Flood Control District, Harris County, and the Texas Water Development Board by Michael Baker International (Burton Johnson, P.E.), SWA Group, and Mitigation Resources; the Cypress Creek Overflow,

refers to a large overflow of stormwater runoff from the Cypress Creek watershed into the Addicks and Barker Reservoir watersheds during moderate to severe storm events in the upper Cypress Creek watershed, upstream of US 290. When rainfall levels … [exceed 5.8 inches in 24 hours] ... runoff drains into upper Cypress Creek and makes its way downstream. The overflow begins to occur at the point where the creek shifts from a north-south flow direction to an east-west flow direction near the Waller-Harris county line, and has the potential to inundate substantial areas of land as the overflow makes its way overland south toward tributaries of Addicks and Barker reservoirs and finally to the reservoirs themselves.

Here’s an image from the report illustrating the area of overflow inundation that occurs from a 5.8 inch, 24-hour rain event (which has a 20% annual chance of occurring):

The area above the purple line normally drains north towards Cypress Creek. This illustrates the water overtopping the purple line (the watershed divide — a line of relatively higher ground which parallels the creek) and flowing into the Addicks Reservoir watershed.

In 2014 the overflow was estimated to inundate about 20,000 acres of undeveloped, privately owned land in the Addicks Reservoir watershed during the 1% annual chance rain event (13.5 inches in 24-hours). Development trends and the demand for housing suggests that this area will become developed over time. The management plan study evaluated various options to mitigate the increase in stormwater runoff volume anticipated with that development.

An increase in stormwater volume is expected, even with existing detention and drainage design rules, because our current stormwater mitigation approach only addresses the change in peak flow rate; not the overall volume of runoff. [If desired you can read more about this here.]

Two of the main options evaluated to mitigate the increase in stormwater volume included storage facilities, the reservoirs discussed below.

Mound Creek Reservoir (2015)

The Mound Creek Reservoir was proposed to be located just downstream of the confluence of Mound Creek and Live Oak Creek in Waller County and partially in Harris County. See map below.

Key statistics about the proposed Mound Creek Reservoir are provided below:

• Drainage Area:                                            36 square miles
• Site Area:                                                     3,765 acres
• Maximum Berm Height:                               22 feet
• Storage Volume:                                          15,730 acre-feet
• Peak Water Surface Elevation:                    188 feet above mean sea level
• Discharge Rate:                                           2,000 cubic feet per second
• Benefit to Cost Ratio:                                   1.14

The proposed Mound Creek Reservoir was devised to capture some of the stormwater that overflows the Cypress Creek channel and floodplain and would otherwise flow into the Addicks Reservoir.  After capture and detention, the Mound Creek Reservoir would release the water to both to Mound Creek and to Live Oak Creek at controlled rates.

Approximately 1,580 acres of land below the Mound Creek Reservoir would be preserved as a conservation area suitable for periodic inundation.  This will help store additional waters in a more shallow environment and help manage the overflow zone.

Katy-Hockley North Cypress Reservoir (2015)

The Katy-Hockley North Cypress Reservoir was proposed to be located along Longenbaugh Road and Katy-Hockley Road partially in Waller County and partially in Harris County. See map below.

Key statistics about the proposed Katy-Hockley North Cypress Reservoir are provided below:

• Drainage Area:                                             ~100 square miles (estimated)
• Site Area:                                                     7,400 acres
• Maximum Berm Height:                                8 feet
• Storage Volume:                                           26,500 acre-feet
• Peak Water Surface Elevation:                     167 feet above mean sea level
• Discharge Rate:                                            7,300 cubic feet per second
• Benefit to Cost Ratio:                                    0.89

The proposed Katy-Hockley North Cypress Reservoir would be designed to capture stormwater that overflows the Cypress Creek channel and floodplain and would otherwise flow into the Addicks Reservoir.  After capture and detention, the Katy-Hockley North Cypress Reservoir would release the water to both to Cypress Creek and to Bear Creek at controlled rates.

Implementation Questions

Here are some questions I think we should be asking before we decide to invest in any of these or other proposed “third reservoir” ideas:

1. How many existing homes and structures will have a reduced risk of inundation as a result of the project?
2. What amount of risk reduction will be achieved for existing homes and structures?
3. What will the post-project risk of inundation be for existing homes and structures?
4. What are the pre-project and post-project risk of controlled, high flow releases from Addicks and Barker Reservoirs?
5. What would the inundation risk be for future homes and structures in the areas upstream of the reservoirs?
6. What would the inundation risk be for future homes and structures in the areas downstream of the reservoirs?
7. How much land would be removed from potential development or prairie conservation?

If you’ve heard of any other “third reservoir” ideas, please tell us about them in the comment section below.