A new report by the Texas Water Conservation Association reviews the relative merits and problems with various flood mitigation strategies. The strategies include pre-release from reservoirs; on- and off-channel reservoirs; aquifer storage and recovery; and dredging. Anyone who wants to understand how professionals evaluate the merits of different strategies should read this concise, clearly written, 25-page report called Flooding in Texas: Preparation and Response.
About the Authors
The Texas Water Conservation Association is an association of water professionals and organizations in the state of Texas. Its members represent river authorities, municipalities, navigation and flood control districts, drainage and irrigation districts, utility districts, municipalities, groundwater conservation districts, all kinds of water users, and general/environmental water interests. Membership includes engineers, hydrogeologists, attorneys, government administrators, and numerous other individuals committed to Texas water resource management. Don’t be scared by the author’s titles.
Bias Toward Conservation over Mitigation
The only warning I have: the authors have an inherent bias toward water conservation as opposed to flood mitigation, as their name would imply. That said, it’s important for people who flooded to understand that flood control is not the main responsibility of people who operate dams. Water conservation is.
Three Types of Reservoirs
The report starts with a discussion of the three main types of reservoirs: water-supply, flood-control, and dual-purpose reservoirs. Lake Conroe and Lake Houston are both water-supply reservoirs. Texas has 150 such reservoirs, by far the most common type.
That section ends with this admonishment: “In water supply reservoirs, there is often very little storage available between maximum design impoundment and the top of the gates. This distance is sometimes referred to as freeboard and should not be considered extra storage, as it is unsafe to operate in that manner. A reservoir that attempts to indefinitely impound stormwater without an emergency spillway will eventually overtop at the lowest elevation across the dam, usually the top of the control gates. This kind of operation is dangerous as it jeopardizes the integrity of the dam, potentially resulting in a dam failure. For this reason, reservoir operators follow specific flood operation protocols to ensure that the dam is not breached.”
Four Main Mitigation Strategies Examined
A discussion of four main flood mitigation strategies follows.
Many people in the Houston area question why Lake Conroe did not release earlier during Harvey. Factors that influence whether prerelease may be of help or hurt include:
- Predicted location and amount of rainfall in relation to a reservoir;
- River-basin size and lag time;
- Existing downstream flow;
- Predicted weather conditions below a reservoir.
Direction of Approaching Storm is Crucial in Pre-Release
During Harvey, a factor that weighed heavily in the decision-making at the Lake Conroe Dam was the direction of the approaching storm. Because it approached from the Gulf, operators worried that pre-leasing water would overload downstream communities which already struggled with the local rainfall.
On the other hand, both the Memorial and Tax Day floods approached from the West or North. In those cases, pre-release was safer.
This section concludes with the observation that, “Harvey dumped approximately … 75 million acre feet of water. Texas’ statewide water conservation storage is approximately 31.5 million acre feet. … No amount of prerelease, and no amount of temporary or permanent conversion of water supply storage, would have appreciably reduced the magnitude of flooding caused by Hurricane Harvey.” And later, “Prerelease is a strategy with inherent risk, because the decision making regarding its use is based on imperfect predictions.”
On-Channel Reservoirs: A Proven Approach
Next, the report considers On-Channel reservoirs, such as Barker and Addicks. “On-channel reservoirs,” they say, “are an important component of the flood mitigation discussion, and have demonstrated effectiveness in multiple areas of the state. As an added benefit, on-channel reservoirs also provide an opportunity to double as a major water supply source, as many of the state’s dual-purpose reservoirs have done for decades.” Harris County Flood Control budgeted for these in its $2.5 million flood bond.
Off Channel Reservoirs Have Severe Limits
Off-Channel Reservoirs represent a less attractive alternative. Sand miners fondly point to the extra storage capacity in their pits for flood mitigation. However, the report points out that pumps can’t work fast enough to create meaningful reductions in flood levels. Other problems: very limited capacity, pumping costs, and debris floating in the water that could clog pumps.
Pros and Cons of Aquifer Storage and Recovery (ASR)
Many people have discussed the possibility of aquifer storage and recovery. It has proved to be a viable strategy for water supply. Managers pump excess water underground for storage in aquifers until needed for future use.
“Initial feasibility and pilot-testing studies are important,” say the authors, “as ASR requires the right physical conditions (e.g. geology, ground slope, groundwater quality) to be feasible. It also must be economically competitive with other viable options.”
“For ASR to have any meaningful impact in an extreme flooding event, extensive off-channel storage would be required, because the rate at which water could be injected underground is so slow in comparison to the rate of flood flows. As such, the off-channel storage is actually the mechanism for mitigating the flood in this case, not the ASR system.”
“As a flood control strategy, however, ASR cannot provide a first line of defense during an extreme storm event due to its need for extensive storage and treatment and its inability to compete economically with other solutions.”
Also, water treatment would be needed before pumping it into the ground because water quality in large flood events is extremely degraded.
When Dredging Helps
The report discusses many purposes for dredging and extensively references the Army Corps’ emergency project on the San Jacinto. It discusses dredging in rivers, streams, lakes, and ditches; for maintenance, navigation, and to remove blockages (like those we have); from cost, environmental, technical, and disposal points of view. Authors lament the expense of dredging but acknowledge its usefulness in certain cases.
Need for Single Source of Emergency Communication
Before closing, the reports also addresses the need for a consistent, focused communication protocol to warn people when flood gates will open. This section also discusses the regulatory environment that dam operators live within. In other words, the restraints that they face.
It concludes by saying that, “During a flood event, it is important that evacuation orders and flood warnings come from a single, designated source to avoid confusion and ensure accurate information for the public. In Texas, the leading notification providers are local Emergency Operations Centers (EOCs) and the National Weather Service (NWS). Reservoir conditions are one piece of the flooding puzzle and operators must provide real-time information to EOCs and the NWS so those entities can weigh all contributing factors to provide the best possible flooding information to the public.”
Look at Flooding from Professional’s Perspective
If you want to understand the world from a water-professional’s point of view, read this thought-provoking report. I didn’t agree with everything in it, but usually when I didn’t, it was because they were referencing another location, not ours. The author’s freely acknowledge that every watershed is unique and must be evaluated from its own perspective.
Posted on December 3, 2018 by Bob Rehak
462 Days since Hurricane Harvey