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Greater Houston Flood Mitigation Consortium Report re Cypress Creek Watershed

May 14, 2016

The Greater Houston Flood Mitigation Consortium hosted a Symposium on Wednesday May 14 at the BioScience Research Collaborative located in the Texas Medical Center in Houston. They provided a large amount of information and data gathered by their extensive research. Below is what came out of “Cypress Creek Watershed: Analysis of Flooding & Storage Options”.

 

Executive Summary

The flooding from Hurricane Harvey in August 2017 was extensive and devastating throughout the
greater Houston area. This was especially true in the far western portion of the region, such as
the Katy Prairie area, which also includes the watersheds of Addicks, Barker and upper Cypress
Creek. Tens of thousands of homes flooded in these watersheds, with most of them being located
within the Addicks and Barker reservoirs. The subsequent releases of stored floodwaters from
Addicks and Barker reservoirs, when their gates were eventually opened, led to further flooding of
thousands of homes downstream along Buffalo Bayou. A significant amount of the floodwaters that
entered the Addicks reservoir came from overflows from the upper Cypress Creek watershed. This
flood event, with over 30
inches of rain, could have been far worse had it received the more than 50 inches of rain that
occurred in the southeastern part of Harris County.
There has been discussion among local stakeholders and agencies of a “third reservoir” or
additional storage in this area of Harris County to supplement the storage capacity of the two
reservoirs and help relieve some of the flooding issues that were experienced during Harvey, or
have the potential to be experienced from a Harvey- type storm event in the future. These
discussions and studies also focused on the “overflow” that occurs from the upper Cypress Creek
watershed into the Addicks watershed during major rain events. This overflow increases the volume
of water entering Addicks reservoir while providing some flood relief in the rest of the Cypress
Creek watershed.
The purpose of this study was to investigate the possibility of adding more storage capacity in the
western portion of the Greater Houston area (i.e., upper Cypress Creek) not only to reduce the
flooding potential for the Addicks and Barker watersheds, but also to potentially help mitigate
flows being released into Buffalo Bayou, as well as floodwaters being transported down Cypress
Creek.
Over the years, land development has progressed westward/upstream in the Cypress Creek watershed
with very little channel improvements or upstream storage added for necessary mitigation controls.
Thus, major floods have occurred along the middle and lower portions of this creek, the most
noteworthy being the
recent damaging floods of both the 2016 Tax Day Flood ,
with up to 17 inches of rain, and Hurricane Harvey.

The study team obtained and used hydrologic and hydraulic computer models available from the Harris
County Flood Control District (HCFCD) covering the Cypress Creek watershed. These models were
revised and updated to incorporate more recent data, including the use of two-dimensional
capabilities of the HEC-RAS hydraulic model from the U. S. Army Corps of Engineers. These models
provide the ability to investigate how this watershed responds to rainfall events, such as Harvey
and the flooding along Cypress Creek that results from such rains.
The hydrologic and hydraulic model analyses conducted for this study revealed where the Cypress
Creek flooding originates and the difficulties in attempting to reduce or eliminate such flooding.
While there are flooding issues all along Cypress Creek, from its outlet into Spring Creek up to
the overflow area in the upper portion that spills over into the Addicks watershed, solving these
issues would require a combination of alternative concepts.
This study focused on the flooding that originates in the upper portion of the Cypress Creek
watershed, which contributes significantly to flood flows entering into the Addicks reservoir by
diverting these flows away from going downstream along Cypress Creek. Thus, any solution to
preventing such flood flows from entering the Addicks reservoir, and eventually into Buffalo Bayou,
will require mitigation such that these flood flows do not increase the flooding issues along
Cypress Creek.
Various storage options were analyzed in the upper Cypress Creek watershed, along with constructing
a levee along the south side of the creek to prevent any overflows from leaving this watershed and
entering into the Addicks reservoir. While these options were
effective in confining these flood flows within the Cypress Creek watershed, the magnitude of these
flows required considerable storage capacity to limit the outflows into the remainder of Cypress
Creek to the flow capacity of the creek. Yet, as additional local runoff would continue to enter
the creek further downstream, this local runoff would still produce flooding along the creek
consistent with the current magnitude seen today. Thus, flood reduction measures would be needed
along the entire creek to fully address the existing flooding problems
that have been experienced in these areas. Given the extent of the existing development in the
middle and lower portions of the Cypress Creek watershed, there are few opportunities for providing
storage capacity in these areas without significant buyout of private properties.

7 Cypress Creek Watershed: Analysis of Flooding & Storage Options

Channel improvements or an underground tunnel system may be a more viable option for these lower
areas, but even these options would require storage mitigation measures due to the increase in flow
rates caused by implementing such options.
While the existing flooding problems along Cypress Creek need to be more fully analyzed and then
resolved, there is the need to prevent new development in this watershed from aggravating these
existing problems.
The watershed’s remaining undeveloped lands, which are primarily in the upper portion of the
watershed, provide considerable natural detention/retention functions that greatly benefit
downstream properties. These functions need to be preserved, either as new development occurs or by
conserving and/or enhancing the existing undeveloped lands, especially native prairie lands. Most
of the undeveloped lands are located in the upper portion of the Cypress Creek watershed, where
considerable development pressure is occurring, especially along the Highway 290 and Highway 99
corridors. There is a need to ensure that any new development is maintaining the existing natural
flow rates, leaving the site so that downstream properties are not adversely impacted.
In summary, this study reached the following overall conclusions:
1. An upper Cypress Creek watershed levee and reservoir(s) can reduce or eliminate the overflow to
Addicks reservoir; however, these projects will not significantly reduce the major source of
downstream flooding along the middle and lower portions of Cypress Creek.
2. There are few viable locations for major detention storage in the middle and lower portions of
the Cypress Creek watershed; as a result, other alternatives may need to be pursued, such as
buyouts and/or channel improvements in the lower/ middle watershed (tunnels may be possible), with
appropriate mitigation as needed.
3. To prevent flooding along Cypress Creek and the overflow to Addicks from getting worse,
undeveloped land in the upper watershed should be preserved, or at a minimum, increased measures
should be taken to prevent new development from increasing downstream flows above existing rates.

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