Blanco Riverine Projects
The City of San Marcos experienced two historic flood events in 2015. The first flood event occurred overnight on May 23rd and early May 24th, and is referred to as the “Memorial Day Flood.” This event resulted in flash flooding that caused tragic loss of life and extreme property damage. The second flood event occurred on October 30, 2015, and is referred to as the “All Saints Flood”. This event caused additional property damage and delayed recovery efforts from the previous flood. Both events were considered historical flood events for Central Texas for different reasons. The Memorial Day Flood was noted for its extreme water velocities within the Blanco River overflow areas, and the All Saints Flood was noted for an extreme volume of precipitation within a short time duration which quickly inundated the City’s rivers, ditches, streams, roadways and storm drain systems.
In 2016, the U.S. Department of Housing and Urban Development (HUD) allocated $25,080,000 to an initial Community Development Block Grant Disaster Recovery (CDBG-DR) Fund to assist with ongoing recovery needs of San Marcos. These funds must be utilized for disaster recovery work in the most impacted and distressed areas of the City, as declared in the 2015 disaster declarations and authorized under Title IV of the Robert T. Stafford Disaster Relief and Emergency Assistance Act (42.U.S.C. 5121 et seq.). Pursuant to this Act, CDBG-DR funds may only be used for disaster related purposes. An Action Plan report was prepared for the City of San Marcos outlining the needs and budget for the HUD allocation.
The public meetings held and surveys obtained during the Action Plan phase indicated that a majority of citizens preferred spending funding on much needed infrastructure projects in order to avoid repetitive loss in the future. The City supported those requests and preferences by allocating a majority of the CDBG-DR funding for infrastructure projects.
In the 2015 flood events, flows from the Blanco River exited the river channel and proceeded to the north and west in an uncontrolled manner through the residential area that includes Blanco Gardens. Engineering modeling has shown that these out-of-river flows occur in the Blanco Gardens area and create approximately 1 ft of flooding or greater adjacent to 329 buildings in the 25-yr flood on the Blanco River and 1,490 buildings in the 100-yr flood.
Four riverine projects have been studied to ascertain the feasibility, within a limited budget, to divert flow, add flow area, or block flood out-of-river pathways in a manner that substantively reduces the current out-of-river flood risk.
The four riverine projects were selected as alternative or scaled-down versions of alternatives that were developed to provide flood benefits for the 1% annual chance (100-yr) flood. The projects have each been developed with alternative sizes in order to assess the effect of size on project cost and benefit. The basic formulation of each of these projects is described below:
Riverine Project A: Add Flow Area to Blanco River North of Oxbow
A review of the riverine hydraulics in the reach north of the large oxbow shows that a significant topographic constriction in the reach results in a significant rise in river water surface elevation upstream through the reach with the riverine losses into Blanco Gardens. This project is designed to alleviate the constriction by adding flow area in the eastern river overbank, and providing a high stage (only) channel across the neck of the oxbow.
Riverine Project B : Bypass South Bank Blanco River to San Marcos River
This project is designed to divert flows from the Blanco River to the San Marcos River, to a point well downstream of the Blanco River/ San Marcos River junction. Flows in this direction are currently estimated to occur between the 4% annual chance (25-yr) and the 2% annual chance (50-yr) Blanco River floods. The diversion is sized so the diversion peak flood extent remains within the current estimated 100-yer floodplain.
Riverine Project C: Diversion West Bank Blanco River to San Marcos River
This project is designed to divert flows similar to those currently exiting the river at Point A, and control those flows within an engineered channel to the west to the San Marcos River upstream of the junction with the Blanco River.
Riverine Project D: Blanco River Bank Improvement
This project is designed to create a bike path at an even grade through the reach where the Point A flows occur. The 3,045 ft long path would include a series of low (maximum 3-4 ft depth) and short embankments that interconnect the existing higher terrain.