Water Resources Assessment
A water resources assessment was developed by performing a field investigation of the local creeks and crossings, reviewing maps, and meeting with the Director and Assistant Director of Public Works. The study focused on flooding (drainage), stormwater pollution, and streambank erosion. An expected outcome of this assessment is to assist the planners in evaluating growth scenarios to aid in the development of a comprehensive plan and aid Public Works staff in the on-going protection of city creeks from further erosion and flooding. This input will strive to minimize the impact of future development on water resources leading to the avoidance of future costs due to flooding, environmental degradation, and stream bank erosion.
A watershed can be defined as a land area that contributes stormwater runoff to a particular point along a waterway. A watershed can cover a little as a few acres or hundreds of thousands of square miles. The City of Blanco lies in the Blanco River Basin, with the Blanco River being the major water course running south of downtown. Primary drainages include Town Creek and Paradise Creek, both begin north of town and empty into the Blanco River. A large SCS pond on the Moore property, located north of town, provides flood control in the headwaters of Town Creek. Durham Branch, a small tributary of the Blanco River, is located south of town.
Existing Conditions and Recommendations
Flood and Drainage Issues
According to an interview with the Director and Assistant Director of Public Works, only a few streets and roads are closed during flood events. Waters usually recede rapidly and roads are not typically closed for any lengthy period of time. It is rare to have structural flooding in town due to flooding from local creeks. Existing drainage and flood issues can be found on the enclosed the map. Specific recommendations for City of Blanco drainage system are listed below.
Town Creek Drainage
- The Moore SCS pond provides beneficial flood control in the headwaters of Town Creek; however, controlled releases from the reservoir can fill the entire drainage easement that is 50ft. wide in the downstream Garden Oaks subdivision. Occasional flooding of residences has been reported. Residents should be advised to avoid placement of fences, structures or any material in the drainage channel that might increase flooding conditions in the easement, and the city should continue to maintain the grass channel in good condition.
- Flood gauges at street intersections in the Garden Oaks subdivision should be replaced, and an informational flyer distributed to residents explaining the importance of the flood gauges and how to use them.
- The bar ditch should be deepened along the south side of 13th Street between Country and Oak Ridge Drive, and a culvert installed under Oak Ridge Drive to direct water into the Town Creek drainage easement.
- Excessive vegetation in Town Creek should be removed downstream from River Road.
- Drainage through private property downstream from 4th Street should be addressed by the city to prevent flooding and protect water quality.
- Wooded areas in Town Creek should be thinned out between Mesquite and Elm Streets.
- Wooded areas in Town Creek should be trimmed downstream from Main Street (Highway 281) to Pecan Street.
Pardise Creek Drainage
- A large pad of construction debris and fill has been placed at 1512 North Main Street immediately downstream of a box culvert under Hwy 281. This activity could obstruct flow and divert runoff onto the adjacent property across the creek and potentially increase flood levels and sedimentation in the box culvert itself. A floodplain study should be performed to determine the impacts to the FEMA floodplain. A result of the study could be the recommended removal of the fill material.
- The proposed installation of four 36-inch CMP culverts at the 14th Street crossing appears appropriate; however, trash and debris upstream from 14th Street should be removed to prevent clogging of the culverts.
- The crossing of Pardise Creek at 6th Street is essentially a low water crossing with one 24-inch culvert to pass low flows. To provide emergency vehicle access during flood events, improved conveyance at the stream crossing should be considered. The second crossing immediately downstream is privately maintained and may pose challenges in upgrading the 6th Street low water crossing.
Durham Branch Drainage (downstream from Blanco River)
- At the Durham Branch crossing on Chandler Road (Loop 163), channel erosion has developed immediately downstream from the three 72'H x 72'W box culverts and created a large pool. Eroded material has been carried downstream causing sedimentation on private property. The erosion is continuing and beginning to undercut the concrete apron of the crossing itself. If headward erosion continues, it may threaten the roadway. The City of Blanco should contact TxDOT to inform them of the condition at this location and recommend stabilization of the eroded area.
The city should continue to monitor flood events and make citizens aware of low water crossing areas to be avoided during rainfall events. The barricaded roadways prevent the public from entering the low water crossings and should continue for community safety. The City could also provide residents with a list of flood prone roadways which could also be provided to schools, major employers, and city officials to assist in broadcasting this public safety message. In addition, residents should be encouraged to purchase a weather radio to stay informed of severe weather conditions. The National Weather Service issues alerts on the 162.425 MHz frequency that covers the City of Blanco. Informational flyers are also available from LCRA that can help educate residents on the dangers of flooded roadways and the importance of owning a weather radio. To determine priority flood area, the city could develop a ranking criteria based on the type of road (neighborhood, collector, arterial, etc.) depth of floodwater over the road, the frequency of flooding, and any structures that might be impacted. Based on this data, which could be based on experience and/or floodplain studies, each flood prone area could be assigned a priority to assist the city in determining which areas should be addressed first.
In addition to public information, maintenance of storm drain systems and other conveyance systems should be given a high priority to minimize the ponding of stormwater in streets and other public areas. Also, the mowing of creek vegetation and removal of debris play a role in ensuring floodwater conveyance through the community.
Creek Bank Erosion
The field reconnaissance played a vital role in evaluating the condition of the creeks. The study of creeks and their response to watershed changes provides valuable insight into creek health, safety, and the threat of erosion to existing structures, roads, utilities, and parks.
As a watershed develops, the new impervious cover (streets, roofs, parking lots) dramatically increases surface runoff during storm events. This can lead to higher flooding levels, more creek bank erosion, poor water quality, and loss of habitat. Therefore, the unmanaged placement of impervious cover can have significant adverse impacts upon the local creeks and adjacent neighbors, and should be managed in a way to minimize negative impacts.
As for the creek, this increase in runoff accelerates the flow velocity and elevates the flow depth, putting pressure on the creek banks and bottom. Soil and gravel are washed downstream leading to the enlargement of the channel through downcutting and widening processes. As the creek widens and deepens, the banks become more unstable and collapse, and continue the process until a new channel system has developed based on the altered runoff conditions caused by urbanization. This process can take over 40 years, but can lead to devastating consequences. As noted in the field reconnaissance and recommendations, the Durham Branch crossing on Chandler Road (Loop 163) channel erosion has developed immediately downstream from the three 72'H x 72'W culverts creating a large pool. Overall though, city creeks are in good shape and are not experiencing any significant erosion problems.
Water Quality (Stormwater Pollution)
Stormwater runoff pollution is called nonpoint-source pollution and it comes from everywhere, as it is washed off the land into creeks, rivers, and lakes. Rainfall runoff carries soil, pesticides, chemicals, oil, and other residues of everyday human activity into the water. The effect is a deluge of dirt, trash, and toxics that produce more water pollution than all the sewage and industrial plants in the nation according to the EPA. Additionally, EPA notes that 70 percent of the nations' pollution is a result of nonpoint-source pollution.
Nonpoint source pollution can seriously affect water quality. Sediment, pesticides, fertilizers, debris, and oil can enter our waterways reducing the oxygen in the water and disrupting habitat for the plants and animals that make the creeks and rivers their home. In addition, these pollutants can carry diseases that cause health problems.
Water quality education can be an effective and low cost approach to improving the water quality and conditions in Blanco's creeks and the Blanco River. Education programs such as those developed at LCRA or TCEQ can be delivered at schools, community service group meetings, homeowner association meetings, and City Council meetings to inform the public of simple measures to protect water quality and the creeks. Other educational outreach options could include utility bill stuffers or a city wide newsletter. The city could also use signage around town creeks and parks explaining the importance of protecting the area waterways.
The message to homeowners could include:
- Keep pet wastes, leaves, and debris out of the streets and creeks
- Divert roof runoff to grassed areas rather than on pavement
- Apply lawn and garden chemicals sparingly, and never fertilize before a rain
- Try to least toxic alternatives to managing pests around the home
- Dispose of used motor oil, antifreeze, paints, and hazardous materials properly
- Control soil erosion by planting ground cover and stabilizing erosion prone areas
The message to farmers and ranchers could include:
- Keep livestock out of streams since their wastes pollute water
- Leave crop residues in the field to hold and fertilize the soil
- Apply chemicals at the proper rate and at appropriate water conditions
- Leave natural vegetation along creeks to act as a buffer to filter pollutants
- Make sure ranch workers know the procedures to use and dispose of waste materials and leftover chemicals
The City of Blanco may be aware of the changing EPA regulations concerning construction and soil erosion control. Beginning in April/May 2003, the Texas Commission on Environmental Quality (TCEQ) took over the NPDES Construction General Permit Program. The TCEQ program is titled TPDES (Texas Pollution Discharge Elimination System) and applies to private and public construction sites that disturb over one acre of land. These sites will need to prepare a Stormwater Pollution Prevention Plan which includes temporary erosion controls such as silt fence, rock berms, and vegetation placement to reduce the amount of sediment washing off a construction site. If a site disturbs more than five acres of land, the owner and contractor must submit a Notice of Intent (NOI) to TCEQ to inform the state of their land disturbing activities and the methods in place to control soil erosion. The City of Blanco can assist TCEQ by informing owners of land disturbing activities of these requirements so soil management activities can be properly applied to protect water quality. In addition, the TCEQ exempted agricultural activities from TPDES requirements.
Since the TPDES permit applies only during the construction period to manage soil, the City of Blanco should inform prospective developers of the impact of impervious cover and the need to consider site design and stormwater management practices to limit adverse runoff effects. The city could provide information that shows how the use of vegetation, proper site design, and stormwater management techniques can reduce runoff and pollution while limiting liability. Several resources such as the Lower Colorado River Authority Nonpoint Source Pollution Technical Control Manual and the City of Austin Drainage Criteria Manual are good references in generating sound project designs that also minimize impacts to the general public.
In conclusion, the creeks and drainage system in Blanco appear to be in fairly good condition. Through monitoring creek erosion, the removal of excess vegetation, and the use of education programs to limit creek pollution, the City of Blanco should experience minor problems with their drainageways. However, the city should be aware that substantial future growth or development would create a much higher level of impervious and without some measure to limit runoff volume, the creeks could experience increased flooding levels and creek bank erosion. The increase in flooding and erosion could require future capital expenditures by the city to minimize damage to public and private infrastructure. Replacing flood gauges at some of the street intersections and distributing an informational flyer on how to use them and the dangers of flooded roadways will further aid in the protection of city residents. All the information contained in this assessment were made to help city staff and planners minimize the impact of storm events on the daily activities of the community. Contact LCRA regarding questions from the recommendations made in this assessment or for any further information.