C-17 San Jacinto River Watershed Study

C-17 San Jacinto River Watershed Study

Project description

The San Jacinto River Regional Watershed Master Drainage Plan (SJMDP) was a comprehensive regional study led by local study partners including the Harris County Flood Control District, Montgomery County, the City of Houston, and the San Jacinto River Authority. The study identified and developed information for communities to use for hazard mitigation planning along the channels included in this study. As extreme weather events and flood waters do not recognize jurisdictional boundaries, like county and city limits, the four study partners worked together to address flooding as a regional issue.

This comprehensive study developed a set of hydrologic and hydraulic models for the major streams of the Upper San Jacinto River regional watershed that will provide a technical basis for local, state, and federal agencies to identify flooding vulnerabilities for existing infrastructure and impacts from future growth to improve flood resiliency in the watershed. The models developed for this study used the new Atlas 14 rainfall data, the most current terrain data, and the latest technology to develop a comprehensive model that represents the various streams and their interaction with the San Jacinto River watershed.

Potential projects supported by the results of this study are intended to reduce flood risks to people and property located throughout the watershed resulting in better informed and more resilient communities. Information developed includes non-regulatory inundation maps (not intended to replace current FEMA effective Flood Insurance Rate Maps) for the studied streams that show the extent and depth of riverine flooding of the major streams within the watershed for an array of simulated storm events. Additionally, information was gathered about the number of structures, acres of land, , and miles of roadway as well as critical infrastructure and evacuation routes, that are located within the inundated areas.

SJMDP Factsheet

Download the San Jacinto Master Drainage Plan Factsheet

The goals of the SJMDP were to:

  • Provide a comprehensive Flood Mitigation Plan that supports the needs and objectives of each study partner;
  • Identify the vulnerabilities of the region’s 13 major rivers and streams to flood hazards using Atlas 14, the most current rainfall data developed by the National Oceanic and Atmospheric Administration (NOAA);
  • Develop approaches to enhance public information and flood level assessment capabilities during a flood disaster event; and
  • Evaluate flood mitigation strategies to improve community resilience.

Study Partners

Study partners include the Harris County Flood Control District, the San Jacinto River Authority, Montgomery County, and the City of Houston.

Study Area

The study area includes the Upper San Jacinto River watershed which extends from the headwaters in Walker County to the Interstate 10 crossing at the San Jacinto River in Harris County. This area covers approximately 2,900 square-miles of the watershed upstream of Interstate 10, located in seven different counties:

  • Grimes County
  • Harris County
  • Liberty County
  • Montgomery County
  • San Jacinto County
  • Walker County
  • Waller County

The 535 miles of major streams evaluated as part of this study include West Fork San Jacinto River, East Fork San Jacinto River, San Jacinto River, Lake Creek, Cypress Creek, Little Cypress Creek, Spring Creek, Willow Creek, Caney Creek, Peach Creek, Luce Bayou, Tarkington Bayou, and Jackson Bayou.

Identified Projects

The projects identified by the SJMDP were designed to address the concerns expressed by the various stakeholders in this region. Sixteen large flood mitigation projects (6 potential channel conveyance improvement locations, 10 potential stormwater detention basins, or reservoirs) have been prioritized to reduce vulnerability to flood hazards on the main channels and improve watershed resilience.

San Jacinto Master Drainage Plan

There are 14 components to the SJMDP report: an executive summary, a main report and 12 appendices explaining a range of topics and work processes. Below is a breakdown of those components and a link to download each.

SJMDP Factsheet

(1 MB, 1 page)
Download the study factsheet.

Executive Summary

(54 MB, 42 pages)
A brief summary of the effort and its outcomes and recommendations.

Plan Report

(14 MB, 117 pages)
A comprehensive discussion summarizing the approach, process and findings of the SJMDP effort.

Appendix A – Project Management

(143 MB, 883 pages)
Consists of the collection of project documents including meeting agendas, presentation materials and minutes generated from the 15 monthly local partner meetings, 4 technical meetings, 4 technical workshops, 13 watershed community supporting partner meetings and 9 local stakeholder briefings.

Appendix B – Data Collection

(98 MB, 230 pages)
Consists of field surveys and observation reports conducted during the beginning of the study.

Appendix C – Existing Flood Hazard Assessment

(113 MB, 180 pages)
Consists of the discussion of approach and processes used to incorporate existing models and developing new models for the studied streams within the watershed, including visual exhibits and tables.

Appendix D – Historical Storm Evaluation

(311 MB, 1,141 pages)
Consists of description of the calibration process using historical storms and stream gage data to “truth” the SJMDP watershed models in development to ensure their consistency and accuracy. Calibration results summary includes a full range of Atlas-14 storm events.

Appendix E – Future Conditions

(44 MB, 67 pages)
A brief analysis to examine watershed-wide conditions in a 50-year horizon.

Appendix F – Sediment Management Strategy for West Fork and Spring Creek

(44 MB, 173 pages, updated 2021-03)
A limited first attempt to examine sediment conditions within two major sub-watersheds and identify implementable management strategies.

Appendix G – Primary Alternatives

(215 MB, 496 pages)
Makes up a main component of the SJMDP effort by describing the approach used to identify damage centers and target detention volumes within the upper San Jacinto watershed. Factsheets and proposed projects are located here.

Appendix H – Implementation

(2.5 MB, 28 pages)
A guide for how projects can be implemented by watershed communities on a short- or long-term basis.

Appendix I – Secondary Mitigation Planning

(23 MB, 9 pages)
A brief memo documenting the outcome of collaboration between the local partners and recommending a list of gages to be installed within the watershed.

Appendix J – Other Mitigation

(192 MB, 171 pages)
A summary of the process of interviewing watershed emergency entities to identify flood hazard mitigation emergency action best practices.

Appendix K – Communications

(29 MB, 256 pages)
A compilation of meeting materials from both sets of public meetings that occurred during the SJMDP planning process, the first took place in December 2019, the second in August 2020.

Appendix L – Existing Conditions Models and Shapefiles

(3 zip archives linked in text below)
Consists of calibrated existing conditions HEC-HMS version 4.3 (225MB), HEC-RAS version 5.0.7 (3.8GB) models and hydrologic catchment (15MB) shapefiles (with the exception of Spring and Cypress Creeks in which M3 catchments were used)  for the upper San Jacinto watershed. (Please note that these are large-sized modeling files, recommended for professional users with an understanding of the appropriate software and applications for these resources.)

Project Implementation

Now that vulnerabilities to flood hazards have been identified and flood mitigation strategies have been recommended, a path to project implementation is needed to move forward. The SJMDP identified both watershed-wide policies and regional projects that can be implemented within the San Jacinto Watershed to reduce flood risk. The recommendations were categorized into short-term and long-term solutions:

  • Short-term solutions, or additional studies, recommended policy changes, etc., can be implemented within a five-year timeframe, if appropriate funding and partnerships are in place. These typically require less funding and have fewer implementation constraints than the long-term solutions.
  • Long-term solutions, or structural solutions, consist of the recommended projects that will take more than five years to complete once implementation begins, due to funding, environmental permitting, construction time, or other project constraints. The SJMDP has completed the planning phase of developing flood mitigation solutions in the watershed.

Short-term solutions

  • Vision Group – This group would establish a regional entity across county boundaries for continuing to cast a vision for regional project implementation and common drainage criteria throughout the basin. The group could foster collaboration to evaluate the path forward in reducing vulnerabilities to flood hazards and improving resiliency.
  • Policy – While each entity may not need identical drainage criteria, common base criteria would standardize the minimum requirements needed for future development. Policies that could be standardized include detention methodology, hydrologic and hydraulic methodology, floodplain analysis, and minimum finished floor elevations.
  • Conservation Areas – Identify areas along major streams that could be purchased as conservancy areas to preserve the floodplain and prevent an increase in flood risk.
  • Flood Monitoring/Warning Enhancements – Adding rainfall and water surface elevation gages to provide both the emergency managers and the public with additional information to determine flood risk.
  • Flood Response – Improve flood response by enhancing communication, identifying and prioritizing improvements to critical infrastructure in flood prone areas, and developing public education strategies.
  • Property Acquisition – Acquiring property and removing it from potential flood risk is often the most cost-effective approach. There are over 600 structures identified within the 20% ACE (5-year) storm at an anticipated acquisition cost of $190 million. The counties and regional groups should seek funding to develop a buyout program for these frequently flooded structures.
  • Floodplain Re-Mapping – The updated modeling for the existing flood hazard assessment showed that current elevations and floodplains used within the basin are outdated. The average 1% ACE* (100-year) water surface elevation increased between 0.5 and 4.5 feet in the watershed. This means structures built to current standards could still be within the 1% ACE and are more susceptible to flooding. Re-mapping the watershed would provide updated flood risk information to agencies and the public. 

*ACE/Annual Chance Exceedance definition: The flood that has a (stated percent - %) chance of being exceeded in any given year, such as the 1% annual chance exceedance flood. The estimated mean probability that an event such as flooding will occur in any given year considering the full range of possible annual events (floods).

Source: https://www.hec.usace.army.mil/publications/TrainingDocuments/TD-40.pdf

  • Watershed Protection Studies – Watershed protection studies for each of the watersheds would further analyze the flooding potential on the tributaries of the major streams and identify local drainage improvements needed. A recommended priority of studies would begin with Spring Creek, followed by West Fork, Caney Creek, Peach Creek, Lake Creek, Luce Bayou, and East Fork.

Long-term solutions

Long-term solutions consist of structural solutions, including:

  • Stormwater Detention Basins are excavated, man-made reservoirs that capture runoff, detain it, and release at a slower flow rate. The result is lower water surface elevations downstream. Each detention basin was conceptually designed to provide flood damage reduction benefits along the major streams in the watershed.
  • Channel Conveyance Improvements consist of excavating channels to increase the conveyance capacity of stormwater flow, thereby reducing the water surface elevations in the vicinity of the project area. Channel conveyance improvements can range from concrete-lined sections to man-made nature based solutions.

Virtual Community Engagement Meeting

A virtual Community Engagement Meeting for this project was held on:

Date: Thursday, August 13, 2020
6:30 p.m. - 7:30 p.m.

You can submit a comment and see the meeting presentation and video by clicking the links below.

Virtual Meeting Video Presentation Submit a Comment