Upper San Marcos River Watershed Flooding and Land ... · TECHNICAL MEMORANDUM Upper San Marcos...

TECHNICAL MEMORANDUM

Alan Plummer Associates, Inc.

6300 La Calma Drive, Suite 400

Austin, TX 78752

512-452-5905

Upper San Marcos River Watershed

Flooding and Land Conservation

January 2017

PAGE INTENTIONALLY LEFT BLANK

TECHNICAL MEMORANDUM

Upper San Marcos River Flooding and Land Conservation

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Date: January 31, 2017

Prepared For: Meadows Center for Water and the Environment at Texas State University

Prepared By: Tom Hegemier, PE, D.WRE, Alan Plummer Associates, Inc.

INTRODUCTION

Flooding on the San Marcos River can be common, as the watershed is located in the heart of Flash

Flood Alley, an area known for some of the most intense rainfall events in the United States and

characterized by steep slopes, thin soils, and sparse ground cover leading to rapid runoff and the

potential for devastating and deadly flooding. The City of San Marcos (City) experienced a massive

flood in 1970 that took the lives of four children, inundated nearly 2000 acres of the city, required the

rescue of hundreds of residents, flooded 405 buildings, and caused more than $400 million in damage.

There had been many floods before this one, but this event spurred US Congressman J. Jake Pickle to

create the Upper San Marcos River Watershed Reclamation and Flood Control District (USMRWRFCD)

that funded the construction of five flood control dams on Sink and Purgatory Creeks from 1981 to 1991

at a cost of about $12.1 million. These dams were designed and constructed by the Soil Conservation

Service (SCS) and range in height from 50 to 100 feet, contain a combined storage of 18,924 acre-feet

(6.2 billion gallons), and reduced the 100-year flood peak flow rate at Interstate 35 from about 104,000

cubic feet per second (cfs) to nearly 22,000 cfs, about a fifth of the peak flow today compared to the pre

dam floods. While these structures have been noted to save lives and provide significant flood

management, a study conducted in 2002 presented findings that the large reduction of the mean

annual flood in the San Marcos River and increased sediment loads from construction runoff “have

caused major sedimentation and exotic plant invasion of the river in the City of San Marcos” since

frequent flushing events found in natural streams no longer occur. The study did highlight the potential

water supply benefits as these dams are located over the Edwards Aquifer Recharge Zone and can

enhance aquifer recharge by up to 12,000 acre-feet/year (AFY) or about 3.9 billion gallons per year,

enough water for about 35,000 single family homes per year.

Other measures can be considered to work in tandem with the flood control dams including land

conservation. While land conservation is a sound approach in maintaining existing flood levels, it will

not reduce existing runoff rates or reduce flooding depths. The greatest challenge in conserving land is

the potential urbanization of the watershed as the population in the Blanco/USMR watershed in Hays

County is projected to increase from about 150,000 people to nearly 540,000 people by 2070 according

to the 2016 Region L Water Plan.

TECHNICAL MEMORANDUM Upper San Marcos River Flooding and Land Conservation

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To potentially reduce flood levels, land conservation must be done in concert with land management

activities such as managing brush/shrub landscapes and converting this to meadow and native grasses

on appropriate soils, slopes, and areas hydrologically connected to aquifers and creeks. This has the

potential to reduce stormwater runoff volume, increase recharge/water supplies, and improve

stormwater quality. However, there is much debate in the science community on brush management’s

benefits and it is believed that this practice provides limited benefits in extreme runoff events (floods)

since it cannot provide enough runoff storage, infiltration, and interception to manage the intense

storms common to Central Texas.

Land management including brush management is not a one-time effort but requires consistent follow-

up, brush removal, vegetation enhancement, and coordinated livestock grazing practices to ensure

long-term water resources performance. Thus a detailed and funded land management plan is

necessary to ensure healthy vegetation and to maximize water resource benefits.

The Texas State Soil Water Conservation Board (TSSWCB) has been active in central Texas in

partnering with landowners for several decades to manage brush and enhance water supplies.

Information from 2012 indicated that the Pedernales River Project managed approximately 62,000

acres of brush in the previous 10 years at a cost of about $4.4 million and the TSSWCB estimates this

effort will yield about 317,000 acre-feet of water over the project life. However, the program does not

claim to generate flood reduction benefits.

When assessing land conservation opportunities to determine priority locations to maximize water

quality, water supply, and runoff management benefits, planners should target the following:

Sink and Purgatory Creeks as 98 percent of their watershed area is in the Edwards Aquifer

Recharge Zone;

Creeks and tributaries as the majority of recharge is noted to take place in the stream beds;

Existing SCS Flood control dams flood pools and inundation easements;

Shrub/scrub land cover that has a high potential for improvement through sound land

management activities;

Soils that have a higher potential for infiltration and water storage and are hydrologically

connected to local streams and aquifers, and

Flatter topographic areas down-gradient of steep slope terrain and hydrologically connected to

local streams and aquifers so these localized ecosystems can be enhanced to increase water

retention and yield.


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Alternatives to land conservation/land management activities to maintain existing floodplain levels

include:

Protection of riparian areas throughout the Upper San Marcos River watershed to ensure that

trees, vegetation, and creek channel storage remain intact to slow flood velocities, provide

channel storage, and retain sediment during floods. Beginning creek buffer zones at a

watershed area of 32 acres indicated that almost 18 percent of the watershed would be in a

buffer zone;

Continuing the practice of requiring stormwater detention for land development and re-

development activities through the City of San Marcos and Hays and Comal counties’

ordinances and technical criteria to prevent development projects from increasing peak flow

rates;

Continue the TCEQ Edwards Aquifer Protection Program that requires new development with

impervious cover greater than 20 percent to include built water quality treatment measures;

Continuing effective construction plan review and construction inspection to ensure that erosion

control practices are properly installed and maintained;

Continue effective construction plan review and construction inspection to verify that stormwater

detention and water quality basins are properly designed and maintained to manage post-

development peak flow rates to existing levels;

Establishing and enforcing stormwater detention basin maintenance, and

Continuing the monitoring and maintenance of the five SCS flood control dams.

This report recommends the consideration of the following next steps to manage floodplain levels and

maximize water resource protection in the Upper San Marcos River watershed:

Support the adoption of the proposed City of San Marcos Land Development Code revisions

that will expand creek buffer zones on the Edwards Aquifer Recharge Zone and require stream

protection volume across the City;

Coordinate with Hays and Comal counties to consider the implementation of creek buffer zones

and stream protection volume compatible with the City of San Marcos proposed Land

Development Code revisions;

Land conservation funding in concert with land management to benefit water supply

enhancement and water quality protection;


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Development of a flexible and effective conservation development ordinance that can be

adopted by the City of San Marcos and Hays and Comal counties to encourage and incentivize

conservation developments to extend land conservation efforts in the watershed;

Prepare a land conservation/land management plan/low impact development plan for the

Freeman Ranch, a 4,262 acre property owned by Texas State University;

Initiate a Hays County Water Supply Enhancement Project with the Texas State Soil and Water

Conservation Board to assess the stormwater runoff, water quality, and water supply benefits of

brush and land management activities;

Evaluate the creation of a Regional Stormwater Management Program (RSMP) in the Upper

San Marcos River watershed with the City of San Marcos and Hays County to generate revenue

from new land development projects for flood control dam maintenance and land conservation,

Consider the creation of an USMR/Blanco Watershed Improvement District to support the

planning and implementation of flood, water quality, riparian habitat, and land conservation

projects.

Table 1: Watershed Vital Statistics – Upper San Marcos River Watershed,

Upstream of the City of San Marcos

Upper San Marcos River (USMR) Watershed Area1 57,540 acres (89.9 sq. miles)

Sink Creek Watershed Area 32,150 acres (50.2 sq. miles)

Purgatory Creek Watershed Area 23,230 acres (36.3 sq. miles)

Willow Springs Watershed Area 2,170 acres (3.4 sq. miles)

Percent USMR in the Edwards Aquifer Recharge Zone 95.2%

Percent USMR in the 100-year Floodplain (FEMA) 7.3%

Acreage in USMR in Conservation or Parkland 6,344 acres

Percent Watershed Developed <10%

Percent Watershed Impervious Cover <2%

Predominate Land Cover Juniper-brush-meadow

Predominate Soil Type Shallow, stony clays

Percent USMR in the City of San Marcos Jurisdiction 12% city limits

57% ETJ

Percent USMR in the Hays County Jurisdiction 80%

1 Texas State Freeman Ranch – 4,262 acres (educational, ranch management , forensic center) is located in the Purgatory and Sink Creek watersheds


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KEY CONCEPTS

The following concepts were considered in the evaluation of the Upper San Marcos River watershed:

LAND CONSERVATION

Land conservation through easements and trusts allows landowners to preserve sections of their land

for conservation purposes. Certain rights are given up with a legal easement or trust, such as the right

to develop the property later. Future owners, or heirs to the trust, are bound by the same legalities.

Easements and trusts donated for conservation can have tax-deduction benefits in the United States.

Watershed benefits derived from protecting land from future development or greatly limiting

development include maintaining the natural character, vegetation, drainage, topography, and habitat of

the native landscape.

CONSERVATION DEVELOPMENT

Conservation development reduces the urbanization footprint in a particular land tract by preserving

large contiguous open spaces amid groups of clustered homes or commercial development. Often the

open space is targeted between 40 and 60 percent of the total development with impervious cover

usually limited to a maximum of 15 to 25 percent of the gross site area. Open space areas cannot be

developed in perpetuity but could include low impact parks and other recreational amenities that

minimize impact to the land and runoff conditions. Conservation development ordinances can include

incentives and other options to encourage developers to select this type of development to maximize

and protect watershed open space without requiring other sources of private or public funding to

conserve land. Travis County adopted a Conservation Development Ordinance in 2006 as an

alternative to standard development approaches and offered incentives and credits if 50% of the

property was retained as open space with a maximum impervious cover of 15%. To date, no

development has pursued permitting under the conservation development ordinance.

BRUSH CONTROL

Brush control is the removal of dense juniper (cedar) and mesquite to maximize rainfall contacting the

ground and reducing evapotranspiration. Brush control has the potential to enhance water yield,

conserve water lost to evapotranspiration, recharge groundwater and aquifers, enhance spring and

stream flows, improve soil health, restore native wildlife habitat by improving rangeland, improve

livestock grazing distribution, protect water quality and reduce soil erosion, aid in wildfire suppression

by reducing hazardous fuels, and manage invasive species. The Texas State Soil and Water

Conservation Board partnered with landowners from 1999 to 2015 and treated over 850,000 acres of

brush in priority watersheds across the State. The Brush Control Program name was recently changed

to the Water Supply Enhancement Program following 2011 Texas Legislative Session. Brush control is

not noted as a flood mitigation practice and there is significant debate in the scientific community on its

long-term ability to provide water supplies. In addition, brush control without adequate vegetation

enhancement and other practices could increase runoff to rivers and creeks during large storm events.

Figure 1 shows the shrub/scrub land cover as indicated by the National Land Cover database. 24


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percent of the land cover outside of the delineated creek buffer zones is in shrub/scrub land cover

according to the National Land Cover Database.

Figure 1: Shrub/scrub land cover shaded dark red

Potential target areas for brush management

LAND MANAGEMENT

Land management to restore healthy hydrologic conditions is the use of brush control on appropriate

vegetation in targeted areas with soil properties conducive to water retention that are hydrologically

connected to streams and aquifers performed in combination with native vegetation enhancement

programs, managed grazing practices, streamside protection programs, and resources are available to

ensure the long-term application of these practices.


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Photo 1: Managed land to restore and sustain native grasses and hydrologic conditions

Photo 2: Unmanaged land, limited vegetation, exposed stony clay soils, prone to rapid runoff


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Figure 2: Land Management and Stormwater Runoff

FLOODPLAIN

The floodplain is an area of land adjacent to a stream or river that stretches from the banks of its

channel outward to higher ground or valley walls and experiences flooding during periods of high

stormwater runoff. The 100-year floodplain is used for planning, design, and flood insurance purposes,

is a rare event that has a 1% probability of occurring in any given year. The 100-year floodplain can

include both upland and riparian areas in addition to the channel. The floodplain is the energy dissipator

of the creek such that when high stream flows extend beyond the channel, vegetation and trees slow

floodwaters and settle sediments. The root masses of riparian grasses, sedges, shrubs and trees hold

the banks and sediments in place. Often the riparian areas adjacent to stream channels have deep

soils with high organic content and provide for a rich diversity of plants and vegetation. Channelizing

creeks eliminates floodplain storage, increases flood velocities and can generate greater flooding

downstream.

Figure 3 shows the 100-year floodplain within the Upper San Marcos River watershed. About 7% of the

watershed area is located within the 100-year floodplain as indicated by the Federal Emergency

Management Agency (FEMA).


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Figure 3: 100 Year Floodplain (FEMA)

RIPARIAN ZONE

The riparian zone is the band of vegetation adjacent to a waterway, creek, or river that typically

includes grass, shrubs, and trees along the banks in the floodplain. Healthy zones provide exceptional

wildlife habitat, reduce flood water velocity, protect banks from erosion, filter sediment and improve

water quality, store water in the banks and floodplain, prolong baseflow, and recharge aquifers. They

are also an important resource for recreational activities.


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Photo 3: Riparian zone not affected by development

CREEK BUFFERS

Creek Buffers are natural areas adjacent to streams and waterways that remain free of development,

construction, wastewater application or other alterations and are used by local and state governments

to protect waterways. Buffers can begin at a drainage area as small as five acres per the Lower

Colorado River Authority Highland Lakes Watershed Ordinance and the Texas Commission on

Environmental Quality (TCEQ) Edwards Aquifer Protection Rules Optional Enhanced Measures. Buffer

zone widths are established from the creek center line and are typically based on the contributing

drainage area or can be based on the 100-year floodplain plus an additional setback. For example, the

TCEQ Measures indicate a 25-foot buffer zone on each side of the creek centerline for watersheds

ranging in size from 5 to 40acres and increasing to a 50-foot setback on each side of the creek for

watersheds ranging in size from 40 to 128 acres. As the watershed size increases, the buffer zone

width also increases. The City of San Marcos is currently proposed in their ongoing Land Development

Code process to include a 25-foot buffer zone on each side of the creek for watersheds ranging in size

from 5 to 50 acres with increasing buffer widths for larger watersheds.


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In this report, buffer zones beginning at drainage areas of 120 acres and 32 acres were investigated to

illustrate the percent land area included in creek buffers. The buffer zone criteria were provided by the

Regional Water Quality Protection Plan (RWQPP) for the Barton Springs Segment of the Edwards

Aquifer and Its Contributing Zone, 2005 and included the participation of Hays County, Barton Springs

Edwards Aquifer Conservation District (BSEACD) and the Hays Trinity GCD among other participants.

Table 2: Buffer Zone Widths (RWQPP)

Stream Contributing Drainage Area (acres)

Width/Offset (feet, each side of stream

centerline) Total Width (feet)

32 to 120 100 200

120 to 300 150 300

300 to 640 200 400

Greater than 640 300 600

The following two figures (Figures 4 and 5) depict the information presented in Table 2. Note the

additional tributary buffer zones on Figure 5 as beginning buffers at 32 acres includes more creeks and

extends protection further into watersheds.

Table 3 shows the proposed buffer zone widths for the City’s creeks. Because the available GIS tools

are unable to map to 5 acre watersheds, this potential criteria was not assessed. Please note that the

buffers presented in Table 3 are narrower than RWQPP buffers but extend further into the individual

tributaries.

Table 3: Proposed City of San Marcos Creek Water Quality and Buffer Zones in the Edwards Aquifer Recharge Zone

Stream Contributing Drainage Area (acres)

Width/Offset1 (feet, each side of stream

centerline) Total Width (feet)

5 to 50 50 100

50 to 250 100 200

250 to 1000 200 400

Greater than 1000 300 600 1 The designer has the option to establish the water quality zone based on the fully developed 100-year flood plain and then add the buffer zone widths to the flood plain boundary. This will probably reduce the total buffer zone width in most cases.


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Figure 4: Theoretical buffer zones beginning at a drainage area of 120 acres

(15% of watershed in a buffer zone)


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Figure 5: Theoretical buffer zones beginning at a drainage area of 32 acres

(18% of the watershed in a buffer zone)

EDWARDS AQUIFER RECHARGE ZONE

The recharge zone is a 1,250 square mile area where highly faulted and fractured Edwards limestones

outcrop at the land surface, allowing large quantities of water to flow into the Aquifer the Edwards

Aquifer. About 75-80% of recharge occurs when streams and rivers cross the permeable formation and

go underground into the Edwards Aquifer. The Edwards Aquifer is an underground layer of porous,

water bearing rock that is between 300-700 feet thick and serves as the primary water supply for the

City of San Antonio and other smaller cities west and north of the City of San Antonio. The majority of

the Upper San Marcos River west of San Marcos is in the Edwards Aquifer recharge zone.


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CHARACTERIZATION OF FLOODING

Flooding on the San Marcos River can be common, as the watershed is located in the heart of Flash

Flood Alley, an area known for some of the most intense rainfall events in the United States and

characterized by steep slopes, thin soils, and sparse ground cover leading to rapid runoff and the

potential for devastating and deadly flooding. The City of San Marcos experienced a massive flood in

1970 that took the lives of four children, flooded nearly 2,000 acres of the city, required the rescue of

hundreds of residents, flooded 405 buildings, and caused more than $400 million in damage.

The following notes highlight the flooding characteristics of the San Marcos area:

Notable flood events occurred in 1909, two in 1913, 1921, 1929, 1970, 1981, 1998, 2013, and

two in 2015.

Major flood event in 1970 spurred the Soil Conservation Service (now the Natural Resources

Conservation Service (NRCS)) to begin the planning of five earthen dams ranging in height from

50 to 100 feet to retain floodwaters on Sink and Purgatory Creeks.

The1981 flood forced the evacuation of 1,800 people and was the catalyst for the funding of the

flood control dams.

A greater than 100-year storm event occurred in 1998 providing a significant test for the dams

and it was estimated that the peak flow rate in San Marcos would have ranged from 119,000 cfs

to 144,000 cfs had there been no flood control project. Flow rates would have exceeded the

1970 flood event by a considerable margin. There was some limited erosion within the

downslope side of the emergency spillways but the dams protected the City of San Marcos.

Flooding occurred in San Marcos in October of 2013, however, San Marcos River flooding was

primarily from flood levels in the Blanco River backing up into the San Marcos River west of

Interstate 35.

The May 2015 flood was again the Blanco River generating the most damage in San Marcos

with minor creek flooding west of town and in-town in the Upper San Marcos River Watershed.

The October 2015 flood generated significant rainfall in the Upper San Marcos River watershed

and we understand caused flood waters to flow over the spillway on one or more of the flood

control dams.

At the time of report writing, Hays County indicated it plans to implement a flood warning system

over the next year, add 22 rainfall gages and stream gages including gates at critical crossings

at a cost of $2 million of which the Texas Water Development Board (TWDB) will provide

$500,000 in funding support. This system, when installed by 2018 will provide advanced flood

warning to residents on creeks and rivers throughout Hays County.


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Photo 4: 1970 Flood Event in San Marcos, TX

Photo 5: Floodwaters over the emergency spillway of a flood control dam, 1998

http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http://www.city-data.com/forum/austin/1177390-whitewater-rafting-near-austin.html&ei=r6nbVM3JCsyN7AbcuYCABA&bvm=bv.85761416,d.ZGU&psig=AFQjCNGAt-bE7cmRxlQm5-aa9cpHtPBMSg&ust=1423768257632316


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Photo 6: San Marcos River Flooding at the Spring Lake Dam

FLOOD CONTROL DAMS, UPPER SAN MARCOS RIVER WATERSHED

The Upper San Marcos River Watershed Reclamation and Flood Control District funded the

construction of five flood detention dams on Sink and Purgatory Creeks from 1981 to 1991 at a cost of

about $12.1 million. These dams range in height from 50 to 100 feet, contain a combined storage of

18,924 acre-feet (6.2 billion gallons), and reduced the 100-year flood peak flow rate at Interstate 35

from about 104,000 cfs to nearly 22,000 cfs, about a fifth of the peak flow today compared to the pre

dam floods. Several facts about the dams are listed below and their locations are indicated in Figure 6.

Combined flood pool area = 934 acres at the emergency spillway elevation

Estimated 12,000 acre-feet per year of recharge to the Edwards Aquifer

Dams are managed by the Upper San Marcos Watershed Reclamation and Flood

Control District, with maintenance support from Hays County

The USMRWFCD had a property tax to fund dam construction; however, funding no

longer occurs through this mechanism to maintain the dams.

Dams are located on private property with flood inundation easements.

https://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=https://www.flickr.com/photos/jefe23/7502330726/&ei=dqnbVOqdPOTP7QargYGgBA&bvm=bv.85761416,d.ZGU&psig=AFQjCNGAt-bE7cmRxlQm5-aa9cpHtPBMSg&ust=1423768257632316


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Figure 6: Flood Control Dams on Purgatory and Sink Creeks

While these structures have been noted to save lives and provide significant flood management for the

City of San Marcos, a study conducted in 2002 presented findings that the large reduction of the mean

annual flood in the San Marcos River and increased sediment loads from construction runoff “have

caused major sedimentation and exotic plant invasion of the river in the City of San Marcos” since

frequent flushing events found in natural streams no longer occur. The study did highlight the potential

water supply benefits as these dams are located over the Edwards Aquifer Recharge Zone and can

enhance aquifer recharge by up to 12,000 AFY or about 3.9 billion gallons per year.


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Table 4: Runoff Reduction Benefits from the Dams

Photo 6: Emergency Spillway at Dam on Purgatory Creek

Looking East Towards RR 12

Pre-Dams (cfs) Existing (cfs) Pre-Dams (cfs) Existing (cfs)

At Aquarena Springs Rd. 12,489 444 18,578 833

At Purgatory Creek 14,181 1,811 21,173 2,725

Downstream of IH-35 17,279 2,501 24,469 3,799

Pre-Dams (cfs) Existing (cfs) Pre-Dams (cfs) Existing (cfs)

At Aquarena Springs Rd. 47,899 2,836 68,833 11,285

At Purgatory Creek 62,755 10,571 86,062 15,740

Downstream of IH-35 72,320 15,176 104,418 21,933

100-YR25-YR

2-YR1-YRLocation

Location


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Photo 7: Primary Flow Control Structure at the Dam on Sink Creek

Photo 8: Earthen Embankment, Rock Boulder Facing, 80 Feet in Height, Dam on Sink Creek


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CURRENT REGULATORY REQUIREMENTS

The City of San Marcos is proposing regulatory changes in their ongoing Land Development Code

revision process. Currently, the City is proposed 85% management of the increase in total suspended

sediment from new development and buffer zones beginning at a watershed area of five acres. The

proposed Land Development Code revisions will be considered for adoption in 2017 by the San Marcos

City Council.

Other regulatory requirements include:

City of San Marcos

o Detention for the 2-, 10-, 25-, and 100-year storms such that post development peak

runoff rate is no greater than the pre development peak rate of runoff

o Provide water quality treatment consistent with the TCEQ Edwards Aquifer Rules

o Creek buffer zones that begin at a drainage area of 5 acres in the Recharge Zone

o Regulatory requirements apply in the city limits and extra-territorial jurisdiction

Hays County

o Detention for the 2-, 5-, 10-, 25-, and 100-year storms

o Regulatory requirements apply outside the city limits and extra-territorial jurisdiction of

the City of San Marcos

TCEQ Edwards Aquifer Protection Program

o Water quality treatment to manage 80% of the increase in total suspended sediment

from new development. Also generates reductions in nutrient and other pollutant loads.

o Requirements apply in the Edwards Aquifer Recharge and Contributing Zones

THREATS TO WATERSHED HEALTH / INCREASED FLOODING

The Upper San Marcos River watershed is currently facing the following threats to its wellbeing:

Urbanization/population growth – The 2016 Region L Water Plan indicates that the

portion of Hays County in the Blanco/San Marcos River watershed will experience a

population growth from today’s population of about 150,000 people to over 540,000

people in 2070, an increase of 390,000 people, i.e. the population will be 3.3 times

greater than today’s population. The urbanization of the watershed can significantly alter

the land cover, runoff characteristics, water quality, recharge, and habitat.

Land development that constructs rapid runoff conveyance systems instead of designing

within the topographic constraints found in the region to maximize the use of the natural

drainage system and floodplains to slow runoff and protect creeks and habitat.

Ineffective construction erosion controls that allow sediment to enter creeks and rivers

that eventually can reduce flow conveyance and increase floodplain levels.


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Lack of maintenance of the existing five flood control dams that are the most important

flood control measures in the Upper San Marcos River and means of protection for the

City of San Marcos. Current maintenance funding is limited.

Poorly designed, constructed, or maintained stormwater detention facilities with new

development.

Poor land management practices that can promote over-grazing, loss of soil, limited

evapotranspiration, and rapid runoff.

Wastewater application to land surfaces can adversely surface and groundwater quality.

REPORT FINDINGS

It is recommended that prioritization of environmental features and land characteristics relevant to flood

mitigation and prevention involve the following items:

Identification and assessment of environmental characteristics related to flooding:

Potential for degraded stream beds and significant sediment transport to Spring Lake;

Reduced clarity in Spring Lake and the San Marcos Springs, and

Since most of the Upper San Marcos River is in the Edwards Aquifer Recharge Zone, extensive

soil loss and transport into the Edwards Aquifer could impact aquifer water storage volumes.

Identification and assessment of development and potential practices/management activities

exacerbating flooding:

Streets, parking lots, buildings (impervious cover) that drain directly to tributaries and creeks;

Storm drain conveyance systems including pipes, culverts, and constructed channels that

rapidly convey runoff from developed areas;

Stormwater detention ponds that are not properly designed or built will not manage post

developed peak flow rates to existing levels, thus, an intended practice to mitigate flooding

impacts from new development may not be achieved, and

Stormwater regulatory criteria that does not include volume management of frequent storm

events (1-year storm and more frequent) can promote accelerated creek erosion and deposition

of soil material at culverts, bridges, and in-channel and increase flood levels due to reduced

channel/conveyance system capacity.

Identified and assessment of practices and land management strategies to prevent/mitigate flooding:


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Stormwater detention ordinances for new development that includes thorough plan review and

construction monitoring to ensure that the detention basin design and construction meets the

flood mitigation requirements;

Creek buffer zones along creeks, rivers, and tributaries to store flood flows, manage flood

velocities, and retain sediments to minimize sediment accumulation at bridges, culverts, and in

the channel;

Creek bank erosion control regulatory criteria (stream protection volume) to manage frequent

storms to maintain the natural stream system;

Low impact development design guidelines that not only manage runoff volume but also

encourage protection of environmentally sensitive areas, woodlands, and minimize cut and fill;

Encouragement of sound hydrologic design practices to follow natural drainage patterns to

avoid constructing roads and conveyance systems that short-cut the natural drainage flow path

and rapidly covey runoff to receiving streams. Maximize usage of the “stair-stepped”

topography of the Edwards Region to retain runoff;

Land conservation connected with sound land management activities such as brush

management in concert with vegetation enhancement and coordinated grazing practices to

maximize rainfall retention and minimize runoff, and

Consideration of a regional stormwater detention program in concert with the existing flood

control dams to evaluate the development of a fee-in-lieu program to help fund land

conservation, land management, and dam improvement activities. However, this must be

carefully assessed and be done in combination with the TCEQ Edwards Aquifer Protection

Rules and the City of San Marcos Stormwater Technical Manual requirements to ensure that

stream protection volume measures are part of each development project to prevent

accelerated creek erosion, and

Assessment of hydrology, land use, and other data to identify and rank priority areas for implementing

land management activities:

Shrub/scrub land composes over 20% of the land cover and could be targeted for appropriate

land management activities to promote soil retention, development of native grasses, and

enhanced infiltration/evapotranspiration to reduce runoff volume;

Five major earthen dams were constructed by the Soil Conservation Service in the 1980s to

reduce flooding in San Marcos. These reservoirs provide a total storage volume of about

19,000 acre-feet with a combined flood pool area of 934 acres. These reservoirs are on the

recharge zone and were estimated to provide about 12,000 acre-feet per year of recharge to the

Edwards Aquifer. Due to the significant flood and water supply benefits, placing the flood pool


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areas into a conservation or land management program can improve water quality and continue

to support long-term groundwater recharge;

Riparian buffers should be implemented along all creeks and tributaries to prevent waterway

channelization, improve runoff quality, maximize groundwater recharge, and maintain aquatic

habitat. The City of San Marcos is currently proposing that creek buffer zones begin at a

watershed drainage area of five acres and this would be required within the extra-territorial

jurisdiction boundary to protect headwater streams. This practice should be considered

throughout the Upper San Marcos River watershed and would require a modification to the

TCEQ Edwards Aquifer Protection Rules or the adoption by Hays County, and

Creek buffer zones established in the city and county regulations should be a priority as these

areas provide natural flood management processes and most recharge occurs in creek beds.

Recommendations for land management activities and conservation strategies and potential funding

strategies related to flood mitigation/prevention, watershed protection, and land preservation in the

Upper San Marcos River include:

Priority areas – soils, vegetation enhancement potential, riparian buffers, floodplains from the

Blanco and Upper San Marcos Conservation Plan;

Regional stormwater detention fee-in-lieu program to fund flood mitigation projects,

enhancements to the existing dams, support land conservation;

Consider an incremental sales tax increase similar to the City of San Antonio’s approach that

helps fund Edwards Aquifer protection land in the San Antonio region;

Consider a bond election similar to the City of Austin Water Quality Protection Land program to

purchase conservation easements or conserve land;

Request and help fund a Hays County Brush Management Study through the TSSWCB to

increase the potential for enhanced land management funding in the Upper San Marcos River

watershed similar to the Pedernales River Project;

Continue to implement the USMR Watershed Protection Plan, preliminary grant award

announced by TCEQ for a $300,000 319 grant program in 2017;

Continue to implement the EAHCP with a focus on land conservation/land management;

Partner with the Trust for Public Lands, Guadalupe Blanco Trust, The Nature Conservancy, Hill

Country Conservancy, and other land conservation/easement entities;

Coordinate and partner with the Hays County Habitat Conservation Plan that is designed for

protecting bird and terrestrial endangered species habitat;


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Develop a land conservation/land management/low impact development plan for the Texas

State University Freeman Ranch;

Coordinate with the Edwards Aquifer Authority and the City of San Antonio to include western

Hays County in the San Antonio watershed protection land program, and

Consider the creation of an Upper San Marcos/Blanco Watershed Improvement District to help

guide the protection and conservation of land in these watersheds.


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REFERENCES

1) Barton Springs Edwards Aquifer Conservation District, assorted recharge papers

2) California Water Plan – Forest Management 3) “Flood Control Dams/Construction Runoff Impacts On Endangered Species Habitat in the

San Marcos River”, Urban Riparian Symposium, Tom Hegemier, PE, Austin, Texas, February 13, 2015

4) Hydrographic Considerations in Road Location and Stormwater Management in Small Basins Tributary to Barton Creek, William M. Marsh and Nina L. Marsh, 1992

5) Pedernales River Project TSSWCB Program Summary

6) Regional Water Quality Protection Plan for the Barton Springs Segment of the Edwards Aquifer and Its Contributing Zone, 2005

7) Riparian Notes, Steve Nelle (NRCS), 2007 8) San Marcos Land Development Code (draft), January 6, 2017 9) SCS Soil Survey of Comal and Hays Counties, United States Department of Agriculture,

1981.

10) SCS Reservoirs in the Upper San Marcos River Watershed and River Sedimentation, Richard Earl, Charles Wood, Southwest Texas State University, 2002

11) 12) State Brush Control Plan, Texas State Soil and Water Conservation Board, 2009. 13) Stream Buffers: The Sharpest Tool in the Stormwater Management Toolbox, Stormwater

Magazine, January/February 2017

14) Streamside Management in the Texas Hill Country, The Nature Conservancy, NRCS, Nueces River Authority, and the Guadalupe Blanco River Authority.

15) Texas Commission on Environmental Quality Optional Enhanced Measures, 2007

16) Water Supply Enhancement Fact Sheet, Texas State Soil and Water Conservation Board, 2016.

Upper San Marcos River Watershed Flooding and Land ... · TECHNICAL MEMORANDUM Upper San Marcos...

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