What is a Watershed?
Do you live in a watershed? The answer is “yes” - everyone does. A watershed is an area of land where rainwater flows into a common body of water such as a stream, creek, river, lake or ocean. A watershed can be as small as a backyard or many thousands of square miles. For example, a small watershed like your neighborhood can be part of a larger watershed like the Trinity River Watershed which encompasses 18,000 square miles and includes all or parts of 38 Texas counties. The Mississippi River Watershed drains to the Gulf of Mexico, just like the Trinity River, but is comprised of 1.5 million square miles!
Within the City of Arlington, most property drains to creeks. The City of Arlington has 10 major watersheds which can be broken into smaller watersheds for the various creek tributaries. The creeks within each watershed have an associated 100-year floodplain. The 100-year floodplain is the area that has a 1% chance of flooding within any given year. The 100-year floodplain is the area that is regulated by the City and FEMA. The City participates in the National Flood Insurance Program (NFIP) which underwrites flood insurance based upon a properties flood risk. While all properties are within a watershed, not all properties are located within the 100-year floodplain. The City can help you determine if you are in the floodplain.
Why study the watersheds?
There are several reasons to study the watersheds:
Accurate and credible flood risk data - Some of the data used in the current FEMA floodplain maps has not been updated since the 1970s. Land development has significantly changed the quantity of water within its creeks and the way that the water flows within its watersheds. The watershed studies will update this information so that floodplain mapping is more accurate.
Updated FEMA Maps - Once the studies are complete and the floodplain is remapped, the data will be submitted to FEMA. FEMA will update its current Flood Insurance Rate Maps (FIRM) which are used to determine which properties need flood insurance and what the flood insurance rates should be.
Better project planning - The City has an extensive database of flooding concerns. However, many properties that experience flooding do not report the flooding to the city. The watershed studies will identify areas of flood risk to coordinate with the reported flooding concerns which provides a more comprehensive flood mitigation program.
Risk identification to protect development - When new development occurs, information within the studies will be used to make sure that the new development is protected while also ensuring that it is not impacting flooding on adjacent property.
Channel stability assessment - The creeks are also evaluated for their condition with respect to erosion. This allows the city to protect its infrastructure - waterlines, sewer lines, bridges, roads, etc. - and provides a catalog of current creek conditions.
What steps are included in a watershed study?
The City retains professional engineering firms to perform its watershed studies. These firms or teams of firms take the following steps when studying a watershed:
Information Gathering - The first step is to know what information already exists for the watershed. Items such as current floodplain maps and computer models, maps of the storm drain system, construction plans, and floodplain map changes are important as a base to the study. Public meetings also occur during this time to gather information from residents impacted by the creek.
Survey - Professional land surveyors will perform field survey of the creeks within the watershed. Homeowners in the area of the survey will receive a letter notifying them of the survey. A combination of field survey and topographical maps will be used to create watershed boundaries and creek cross sections.
Hydrology - Hydrology is the term used for determining how much water is generated by the watershed. This is based on the land area, the impervious area within the area and the flow path of the water. A computer model is used to take these factors and determine the quantity of water at points along the creek for various storm events - 5-year event, 25-year event, 100-year event, etc.
Hydraulics - Hydraulics is the term used for determining how the water flows through the creeks. A computer model will be used to determine the water surface elevations and velocities along the creek for various storm events.
Mapping - Once the hydrology and hydraulics are complete, the new floodplain area will be mapped. The hydrologic and hydraulic model results will be forwarded to FEMA for incorporation into their Flood Insurance Rate Maps (FIRM).
Problem Area Identification and Project Identification - Once the maps are complete, properties at risk for flooding will be identified. These will also be cross-referenced with our historic drainage concerns and potential drainage projects will be identified. These projects are prioritized and programmed into the stormwater capital budget. The City Council approves the capital budget each spring.
What is the status of the current watershed studies?
Studies of 6 of the 10 watersheds within the City of Arlington are either complete or underway.
Fish Creek and Cottonwood Creek - These were our first studies and were a part of a FEMA grant and partnership with the City of Grand Prairie. The City has completed these studies and they are currently in the 90-day appeals process with FEMA. Seven flood mitigation projects were identified through this study. Two of the projects - Sherry Street Culvert at Tributary CC-3 and Susan Drive Culvert at Cottonwood Creek - are under construction. These two projects were coordinated with other projects which allowed them to be expedited and to be less expensive since they were part of larger projects. A third project - Cottonwood Creek at Park Row - is in design right now.
Rush Creek - In 2010, Tropical Storm Hermine hit the Rush Creek watershed particularly hard. Extensive flooding within the watershed highlighted risks to structures and infrastructure within the watershed and led to it being the first comprehensive study performed exclusively with stormwater utility funds. This study is nearing completion and submittal to FEMA is anticipated in early 2016. Ten flood mitigation projects were identified. Two of the projects - Sublett Creek Neighborhood Drainage Improvements and Country Club Road Local Drainage and Channel Improvements (south of Park Row) - are currently in the design phase.
Johnson Creek - Johnson Creek has historically been Arlington's most talked-about creek. Historic problems in this watershed have led to the buyout of many homes and plans for linear parks along it. While many of the problems have been addressed, there are still many to go. This study is also nearing completion and submittal to FEMA is anticipated by the end of the year. Project identification is in progress. Currently 19 projects have been identified. These projects are currently being verified and ranked.
Trinity Tributaries and Lower Village Creek - This is the newest study and includes all of Lower Village Creek and the major tributaries to the Trinity River. This study was started in July 2015 and data gathering and survey are underway. A series of three public meetings has been held to gather information from residents within the watershed. Completion is anticipated in 2017.
What studies are still needed?
Upper Village Creek and Remaining Trinity Tributaries - Proposed to start in 2017
Lynn Creek, Bowman Branch and Walnut Creek - Proposed to start in 2018 and will include stream assessment for Fish and Cottonwood Creeks.
How are watershed studies funded?
The City's stormwater program is funded through the Stormwater Utility Fee that is included with the water bill. Currently, the Stormwater Utility Fee is $5.25 per residential lot and based on the amount of impervious area (parking lots, buildings, driveways, etc.) for commercial property. The fee supports all aspects of the stormwater program including: watershed studies, flood mitigation projects, a stormwater field crew, environmental compliance, environmental education, and engineering and administration staff.
How can I find out more information?
You may contact the Department of Public Works and Transportation at 817-459-6550 and ask to speak with a stormwater engineer. The stormwater engineers can talk to you about drainage problems you are having, floodplain questions or other stormwater-related issues.