Invasive zebra mussels — about the size of a thumbnail — can quickly proliferate and are highly adaptive to new environments, including Texas. Photo: LAN
Lockwood, Andrews & Newnam develops an invasive species response plan for a south Texas water-diversion project.
By J. Tom Ray, P.E., D.WRE
Transport and delivery of major quantities of raw water sufficient to support the needs of Houston and surrounding area is a complex, costly, and environmentally challenging assignment. The Luce Bayou Interbasin Transfer Project (LBITP), which is currently under construction, is a massive project to divert raw water safely and reliably from the lower Trinity River to Lake Houston. The Coastal Water Authority (CWA) planned, designed, and is currently managing construction of the LBITP.
The purpose of the LBITP is to meet the anticipated water demands of the Houston area based on population projections and industrial users’ needs and, in doing so, help satisfy the mandate for conversion from groundwater use to surface water use. By conveying as much as 500 million gallons per day (mgd) of surface water, the LBITP will be a cornerstone in abetting and controlling subsidence otherwise caused by groundwater pumping.
The LBITP includes a Trinity River raw water intake structure and pump station, nearly three miles of dual 96-inch pipeline, a sedimentation basin, and 23.6 miles of canal. The pump station will be located on the Trinity River at Capers Ridge. The pipeline will extend southwest approximately three miles along a geological ridge (Capers Ridge) to the watershed divide between the Trinity River and the Lake Houston watersheds. The pipeline will then outfall into the sedimentation basin at the start of the canal. The canal will traverse the lower reaches of Luce Bayou, which flows into the northeastern corner of Lake Houston. Construction began in 2016 and remains on track for completion in 2019.
Zebra mussel challenges
The LBITP is a massive, multiple-component project, costing more than $350 million to build, but a tiny creature about the size of a thumbnail presents a significant challenge. What this invasive species lacks in size it makes up for in explosive proliferation. A single female can produce a million offspring. Consequently, zebra mussels are highly adaptive to new environments, including Texas. Once fertilized, eggs develop into microscopic veligers. These veligers free-float with currents for a few weeks and can then quickly grow to adult size.
Within a year, an established zebra mussel population can and often does reproduce again, multiplying exponentially. Juvenile and adult zebra mussels will readily attach themselves to pipes and water intake structures, quickly proliferating and severely constraining the area available for water supply flow. Such dense growth can clog intake screens, reduce available pipe diameter, and increase the roughness of pipes, which leads to reduced water flow and maintenance issues.
The impacts are also negative for the environment. Zebra mussels are filter-feeders; an established population can effectively filter lake waters, lowering the turbidity, increasing light penetration, and overall negatively impacting the aquatic ecosystem.
The alarming spread and negative impacts of zebra mussels were recognized by two major federal actions — the 1991 listing of zebra mussels as an “injurious species” in the Lacey Act and Executive Order 13112. Under the Lacey Act, it is illegal to import or ship between states any species listed under the act. Under Executive Order 13112, federal agencies, including the U.S. Army Corps of Engineers, are constrained from authorizing any action that is “likely to cause or promote the introduction or spread” of any invasive species, including zebra mussels.
In the late 1980s, zebra mussels were found in the Great Lakes and rapidly spread through the Mississippi River Valley and its tributaries, reaching Texas in 2009. During the last decade, zebra mussels have spread like wildfire in many Texas lakes, including several at the upper Trinity River basin.
In 2012, the Corps of Engineers Galveston District notified the CWA of the requirement to prepare a zebra mussel monitoring and control plan as a special condition of the Section 404 permit for construction of the LBITP. Subsequently, CWA hired Lockwood, Andrews & Newnam, Inc. (LAN), a planning, engineering, and program management firm, to develop a “tiered, risk-based” response plan. For this purpose, the Trinity River basin was divided into three areas — an upper area that included the reservoirs with known zebra mussel occurrence, a middle-Trinity area above Lake Livingston, and a lower area that included Lake Livingston and the river reach to the Capers Ridge diversion site (see Figure 1).
Each zone was assigned a risk level corresponding to its proximity to the LBITP facilities. For each zone, specific commitments for response by CWA commensurate with the potential risk were identified. As the occurrence of adult zebra mussels progressed downstream toward the LBITP, the risk of infestation at the Caper Ridge location increased and the level of CWA’s response efforts multiplied. Two key directives guided development of the response plan:
- the need to anticipate and respond early to the progression of zebra mussels downstream; and
- the effort to mitigate the potential spread of zebra mussels.
The latter included recognition that zebra mussels had not been found in Lake Houston and the response plan should include steps to prevent the LBITP becoming a conduit for spreading zebra mussels. The completed response plan was reviewed and approved by the Galveston District. Upon approval, the CWA began implementation of the response plan.
Since adult zebra mussels were well established in the upper Zone, CWA committed to the corresponding response efforts, which included a vulnerability assessment of the LBITP facilities as described in the preliminary engineering report. The vulnerability assessment evaluated LBITP facilities potentially susceptible to zebra mussel infestation, and, importantly, evaluated means to avert the potential conveyance of zebra mussels to Lake Houston via the LBITP canals.
In addition, the upper zone response as well as the LBITP Section 404 permit special conditions mentioned above required the CWA to collect, evaluate, and report annually on the occurrence of zebra mussels in the Trinity River basin. Through the City of Houston, the CWA was included in the Texas Parks & Wildlife Department’s (TPWD) zebra mussels outreach program, satisfying another upper zone commitment for public outreach.
In June 2016, the U.S. Geological Survey (USGS) documented the presence of adult zebra mussels in Lake Livingston, the farthest downstream occurrence in the Trinity River. Given Lake Livingston’s proximity to LBITP’s pump station site, this triggered a high-risk response. Under this response plan, the CWA took the following actions.
Advanced monitoring — The CWA, with assistance from LAN, established contacts with several agencies and individuals engaged in zebra mussel monitoring and reporting activities. The group includes the USGS, North Texas Municipal Water District, Trinity River Authority, Texas Parks & Wildlife Department, Dallas Water Utilities, and the Tarrant Regional Water District. During the last decade, these agencies have established monitoring and field inspection efforts for identifying the presence of zebra mussels.
For reporting to the Corps of Engineers, the CWA prepared an assessment tool that input and organized the data collected. The assessment tool was further organized to record data geographically, by zones, corresponding to the zones in the CWA zebra mussel response plan. The CWA agreed to support special studies recommended and conducted by others that would provide pertinent information on zebra mussels in the lower Trinity River basin. The combination of monitoring coupled with the special studies is expected to improve forecasting of the potential risk of zebra mussel occurrence at the project’s diversion facilities.
Public awareness campaign — Each summer, TPWD and a coalition of partners execute a major public awareness campaign called Protect the Lakes You Love to encourage boaters to clean, drain, and dry boats, trailers, and gear before traveling from lake to lake. Boaters play an important role in stemming the spread of destructive zebra mussels that can hitch a ride and spread to another lake.
On April 4, 2017 the CWA entered into an agreement to participate with other cooperating agencies in the Trinity River Basin to support this campaign directly. Although the outreach is managed by the TPWD for statewide benefits, the occurrence of adult zebra mussels in Lake Livingston is expected to focus attention on the lower Trinity River/Lake Livingston and San Jacinto River/Lake Houston areas. The CWA provides funds for support of the TWPD statewide outreach program.
Control methods — The final and most critical piece of the response plan involved selection and incorporation of control methods. The preliminary and final engineering design considered the findings in the project’s vulnerability study as the foundation for these control measures. The vulnerability study recommended four barriers to be incorporated during design:
- establish a barrier at the intake (trash racks and intake bays), pump bays, and pump station to prevent zebra mussel build-up and loss of capacity;
- establish a barrier to prevent loss of capacity in the pipeline;
- establish a zebra mussel downstream transport barrier at the sedimentation basin; and,
- establish zebra mussel control at vulnerable structures along the canal conveyance to prevent accumulation and downstream transport.
Discussions were held between the design engineers, the CWA project management staff, and LAN. These sessions resulted in selection and incorporation of control measures not only to protect the pump station and canal system from zebra mussel infestation that would impair operations, but also to provide barriers to the transport of viable zebra mussels via the LBIPT canal operation.
As such, the CWA included the following features for zebra mussel control in the design phase:
- all sediment collected within the pump station structure will be pumped to the sedimentation basin;
- anti-foul coatings on the embeds, rails, stop log frames, screens, and stop gates;
- a flushing system will be incorporated for resuspension of sediment, which provides benefits to the clearing of zebra mussels;
- pump bays will be isolated and dewatered by using stop gates to allow physical cleaning;
- screens at intake will be removable for cleaning; and
- provisions for adding controls at the canal to avert downstream transport of zebra mussels.
The CWA will continue to evaluate the zebra mussel control program for the LBITP and may include additional controls or supplement the ones summarized above to provide efficient and effective control.
The response plan and the actions that have resulted from its ongoing implementation will provide comprehensive assessment of the risk of zebra mussel infestation at the LBITP. Importantly, the CWA response recognizes and includes measures to avert the spread of viable zebra mussels through the LBITP canals downstream to Lake Houston. Thanks to the CWA’s proactive planning, these measures will ensure uninterrupted raw water supply at the appropriate quality in the future for millions of people in Harris and Fort Bend Counties.
The big questions that utility owners deal with are when to start planning and how robust the response needs to be. The CWA prepared a unique zebra mussels response plan to protect the LBITP facilities. Similarly, each water agency in Texas should prepare a response plan tailored to its situation. While it may be impossible to totally eradicate the threat of zebra mussels, preventing their colonization within facilities and limiting the economic impact is possible.
J. Tom Ray, P.E., D.WRE, is water resources program manager at Lockwood, Andrews & Newnam, Inc. (LAN; https://lan-inc.com), a planning, engineering, and program management firm. He can be reached at firstname.lastname@example.org.