San Jacinto River Master Drainage Plan Draft Provides First Look at Final Report Due Out in August

On 7/23/2020, consultants for the San Jacinto River Basin Master Drainage Plan gave the SJRA Board and the public a first look at a draft of the plan. The final report is due out at the end of August. The draft shows the broad outline of the team’s efforts.

Draft Shows Broad Outline of Recommendations

It shows the types of recommendations they will make. However, this draft does not yet include specific recommendations as to prioritization of projects. Those will change before the final report. For instance, much of the draft focuses on upstream detention. But specific detention site recommendations have not yet been finalized.

Funding and Partners

Below are the key slides and a brief explanation of the main point behind each. This drainage study is 75% funded by a FEMA Hazard Mitigation Grant and 25% by four local partners: the SJRA, City of Houston, Harris County Flood Control, and Montgomery County.

Scope of Study

The study area covers almost 3,000 square miles and the tributaries listed on the left.
The SJRA primarily has responsibility for the portion of the watershed in Montgomery County. However, the scope of the drainage study extends to other counties including Waller, Grimes, Walker, Liberty, and San Jacinto.

Heat Map Shows Where Most Damage Occurs

The team started by looking at where flooding has occurred historically. The tan areas above show where the most damage has occurred.

Goals and Methodology

The partners started by looking at vulnerabilities and identifying mitigation possibilities. Their main goals are in red. The final report will make specific recommendations for detention, buy-outs and improving conveyance. Recommendations will also improve flood warning and communication.
The team started by integrating and updating all existing hydraulic and hydrologic models in the watershed as reflected on the latest 2018 lidar terrain data. They now take into account new construction, growth, additions to impervious cover, and Atlas-14 rainfall probabilities (which vary by sub-watershed within the larger watershed).
To calibrate and verify its H&H models, the drainage study team examined four historical storms that, together, impacted the entire study area. They then adjusted the models using radar rainfall data, and USGS high water marks and peak flow data. The objective: to make the models reflect “ground truth.”
The team is also looking at strategies to reduce sedimentation. However, this is not a major focus of this study. Their purpose is not to evaluate the relationship between sediment and flooding. Other studies will do that.

Three Main Areas of Focus

This slide shows the three major thrusts of drainage study effort over the last 1.5 years. The primary focus has been on: a) identifying the best locations for upstream detention that can reduce the volume of water coming downstream to populated areas during floods, b) where to install additional gages to improve flood predictions and warning times, and c) improving communication during emergencies.
This shows the steps the drainage team went through to evaluate and rank-order potential sites for detention.

Areas of Highest Potential for Mitigation

Here’s where they found the highest and lowest potential for mitigation. The box explains the watersheds that see the most effective solutions within the SJMDP study area, as explained in the list to the left of the slide.
Some drainage projects recommended in previous plans are no longer possible today because of upstream development. However, areas that once held potential for a single large project still hold potential for several smaller projects that add up to significant flood reduction.

Mitigation Project Funding

The cost all the drainage projects identified adds up to about $3 billion. They only reduce flooding of structures worth about $756 million dollars. Because costs exceed benefits, FEMA will not likely fund all of these.
However, many of the projects are in areas with low to moderate income (brown and tan areas). See the large concentration in the eastern watershed. That opens up other sources of funding, such as the Department of Housing and Urban Development where the benefit/cost ratio may not be as important.

Harmonizing Regulations Throughout Region

The team will also make recommendations to harmonize floodplain development regs throughout the region. Continuing to allow unmitigated upstream development in floodplains could destroy any new investment made to protect highly populated downstream areas.

Some Problems May Only Be Solved Through Buyouts

Buyouts usually have a high benefit-to-cost ratio relative to construction projects such as detention ponds. Buyout strategies can target the most vulnerable properties, such as those in the 2- and 5-year floodplains. None of the detention projects recommended by the team will likely remove those from danger.

Steps Still Not Completed

The team has finished the steps in red. They in the process of prioritizing projects and developing a phasing plan. The last bullet point is not part of this study.

More Upstream Gages Needed to Eliminate Blind Spots

The team has also identified locations for additional upstream gages and local partners who can help maintain those gages. Think of these like a “distant early warning” system. They give river forecasters visibility into “blind spots.” Forecasters will be able to add up the rainfall on various tributaries and predict the impact and timing of flooding downstream. That could give people more time to evacuate.

Ways to Improve Communication

The team is also looking at ways to communicate better during flood emergencies. They are looking at inundation mapping, evacuation routes, and improved communication protocols.

Timetable for Remainder of Project

This chart outlines the project workflow. It shows completed steps in red, and incomplete steps in yellow.
The final report with specific recommendations should be released at the end of August or in early September.

Every Little Bit Helps

I can’t wait to see this report in its final form. During the presentation, the presenter talked about reducing flooding downstream at the West Fork and I-45 by up to six feet if all upstream projects are implemented.

One thing to keep in mind: there is no single silver bullet that can solve the regions flood problems. All of these steps are additive. In my personal opinion, a foot here and a foot there can help offset future releases from Lake Conroe. People in the Lake Houston Area benefit from any and all upstream improvements.

Posted by Bob Rehak with thanks to SJRA and HCFCD

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