Blanco Riverine Flood Mitigation Project - CDBG-DR

Project Name: Blanco River Flood Mitigation Project

Project Number: CDBG-DR

Project Manager: Jacquelyn Thomas

Current Design Phase: 99%

Construction Estimate: $13,000,000

Construction Start Year: 2022

Project Information: This project was identified in 2017 as a means to reduce flood flows from the Blanco River that currently exit the river in the Blanco Gardens subdivision and flow through the neighborhood toward the San Marcos River. This overflow occurs in events as low 5% annual chance (20-year) flood event.

In response to the Memorial Day Floods and All Saints Floods of 2015, San Marcos has received $33.8M in Housing and Urban Development (HUD) Community Development Block Grant Disaster Recovery funds (CDBG-DR). Partial funding for this project for the acquisition and relocation of property was allocated from the CDBG-DR funds. An additional $24.0M in Housing and Urban Development (HUD) Community Development Block Grant Mitigation funds (CDBG-MIT). Partial funding for this project for the construction of infrastructure was allocated from the CDBG-MIT funds. 

The proposed project consists of a berm/ floodwall structure located between River Road and the Blanco River to contain flood waters during the 50-year and more frequent flood events. The concrete flood wall is recommended to minimize right of way and structure acquisition. After approximately 1,055 LF, the concrete floodwall will transition to an earthen berm. The earthen berm will continue parallel to River Road, then turn parallel to Old Martindale Road for a total of approximately 7,355 LF. Together, this berm and floodwall will prevent flood waters from entering the Blanco Gardens neighborhood during the 50-year and more frequent rain events. 

In addition to the berm and floodwall, a relief channel is proposed to efficiently transfer Blanco River flows to the San Marcos River. This relief channel allows for water surface elevation reductions along the Blanco River downstream of State Highway 80. This relief channel is approximately 7-feet deep and 1,200-feet wide, with a weir at Avoca Ranch Road crossing, and vegetated plantings at the discharge point to reduce velocities. A majority of this relief channel is located on property currently owned by City of San Marcos. It is also proposed to lay-back the Blanco riverbank at the upstream end of the relief channel, to stabilize the unstable and eroding bank.

An additional berm is proposed to protect the City of San Marcos Wastewater Treatment Plant (WWTP) from the slight rise in water surface elevations caused by the upstream berm/ floodwall and relief channel. This berm is approximately 2,200-feet long, is located on City of San Marcos property, and will provide protection to the WWTP from the 50-year flood events.

Public Meeting Presentations

Proposed Blanco Riverine Mitigation 121518