(Ordinance 2012-10 adopted 8/21/12)
This Chapter discusses the criteria aspects of iSWM and lays out the framework and specific requirements. Local governments may modify this section to meet any local provisions.
The purpose of this manual is to provide design guidance and a framework for incorporating effective and environmentally sustainable stormwater management into the site development and construction processes and to encourage a greater regional uniformity in developing plans for stormwater management systems that meet the following goals:
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Control runoff within and from the site to minimize flood risk to people and properties;
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Assess discharges from the site to minimize downstream bank and channel erosion; and
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Reduce pollutants in stormwater runoff to protect water quality and assist communities in meeting regulatory requirements.
Following criteria provided in the manual will help to meet sustainable development goals. There are many ways that sustainable development may be achieved while following these criteria. For example, a development that reduces individual lot imperviousness and a development that has high lot density in one area and a large open space in another can both meet sustainable requirements.
Chapter Summary
The iSWM Criteria Manual consists of five chapters:
Chapter 1 - Introduction and Summary
Chapter 2 - Integrated Development Process
Chapter 3 - Integrated Design Criteria
Chapter 4 - Integrated Construction Criteria
Chapter 5 - Additional Local Provisions
Local Provision Boxes
Throughout this manual you will notice “Local Provision” boxes. These boxes are used by a local government to add, delete, or modify sections of the criteria and specify the options allowed and/or required by the local government. Additional local information can be added and will be located in Section 14.5.0.
Local Provisions:
Relationship of Azle and NCTCOG Regional Integrated Stormwater Management (iSWM) Manuals
This City of Azle’s iSWM Criteria Manual incorporates the 2010 regional iSWM Criteria Manual, developed by the North Central Texas Council of Governments (NCTCOG), although portions of the manual may have been modified or removed by the City. The requirements contained within the Local Provision sections shall take precedence over conflicting provisions that may be contained in the iSWM Criteria Manual and iSWM Technical Manual approved by the North Central Texas Council of Governments.
Section 14.5.0 contains additional criteria that are applicable in the City of Azle. The digital version of both manuals cross reference each other and are included on the respective websites for the City of Azle (www.cityofazle.org) and NCTCOG ( http://iSWM.nctcog.org/ ). Copies of these documents can be downloaded from the website.
Precedence of Azle Criteria
The requirements contained within the Local Provision sections shall take precedence over conflicting provisions that may be contained in the iSWM Criteria Manual and iSWM Technical Manual approved by the North Central Texas Council of Governments.
Applicability
iSWM is applicable under the following conditions for development and redevelopment that will ultimately disturb one or more acres as illustrated below and in Figure 1.1:
Table 14.1.1. iSWM Applicability
Applicable for iSWM Site Design:
Applicable for iSWM Construction:
Land disturbing activity of 1 acre or more
OR
land disturbing activity of less than 1 acre where the activity is part of a common plan of development that is one acre or larger.
Land disturbing activity of 1 acre or more
OR
land disturbing activity of less than 1 acre where the activity is part of a common plan of development that is one acre or larger.
A common plan of development consists of construction activity that is completed in separate stages separate phases, or in combination with other construction activities.
Development and redevelopment are not specifically defined in this manual. The applicability is based on land disturbance activities. If an existing site has been cleared and graded, but not developed, within five years of the date of the developer’s initial application submittal, the developer must consider the land conditions prior to the clearing and grading to be the existing site conditions.
New development or redevelopment in critical or sensitive areas, or as identified through a watershed study or plan, may be subject to additional performance and/or regulatory criteria as specified by the local government. Furthermore, these sites may need to utilize certain structural controls in order to protect a special resource or address certain water quality or drainage problems identified for a drainage area or watershed.
Site Design below Applicable Criteria
Site developments that do not meet the applicability requirements are not subject to the regulatory water quality or streambank protection requirements. However, it is recommended that these criteria still be used and that temporary controls be provided during construction. Flood mitigation and conveyance criteria still apply. The planning process is also simplified for sites below the applicable criteria to an optional pre-development review before the final submittal of the engineering plans.
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Figure 1.1 iSWM Applicability Flowchart
Local Provisions: Azle requirements for stormwater management review are shown in greater detail in the graphic below. Any land disturbances of 0.1 acre or more will be reviewed for known drainage problems and mapped floodplains; a site plan showing topography and drainage information is generally sufficient for this purpose. A land disturbance of 1.0 acres or more requires an iSWM Site Plan prepared by a professional engineer. A land disturbance of 1 acre or more also requires a Construction Site Notice (CSN) or Notice of Intent (NOI) as applicable, along with a Storm Water Pollution Prevention Plan (SWPPP) that shows how erosion will be controlled during construction. Landscaping (Section 29 [of the zoning ordinance]), Land Filling, and other City code requirements that are often triggered by land disturbance activities should also be checked.
A Grading Permit must be approved by the City prior to the commencement of any construction or grading activities disturbing 0.1 acres or more of land or if disturbed surface areas are located within Floodplain and/or Drainage Easement. Grading requirements will be reviewed as part of the normal engineering review process rather than as a separate step. After construction and grading activities are complete and disturbed surfaces are stabilized, and prior to the issuance of the Certificate of Occupancy, a Final Grading Certificate must be provided by an engineer or the contractor. A separate grading permit for the grading of individual Single-Family or Duplex Building lots will normally not be required if those lots are part of a subdivision plat recorded prior to the date of this ordinance.
(Ordinance 2012-10 adopted 8/21/12)
Section 14.2.0 of this manual presents details for completing the full iSWM development process which consists of five steps. Each of the steps builds on the previous steps to result in Final iSWM Plans and Construction Plans.
Step 1 - Review Local Requirements and Municipality’s Processes
Step 2 - Collect Data and Perform Site Analysis
Step 3 - Prepare Concept/Preliminary iSWM Plans
Step 4 - Prepare Final iSWM Plans and iSWM Construction Plan
Step 5 - Prepare Operation and Maintenance Plans
Local Provisions: NONE
(Ordinance 2012-10 adopted 8/21/12)
Section 14.3.0 of this manual presents an integrated approach for meeting stormwater runoff quality and quantity management goals by addressing the key adverse impacts of development on stormwater runoff. Its framework consists of three focus areas, each with options in terms of how the focus area is applied.
Design Focus Areas
The stormwater management focus areas and goals are:
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Water Quality Protection: Remove pollutants in stormwater runoff to protect water quality
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Streambank Protection: Regulate discharge from the site to minimize downstream bank and channel erosion
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Flood Mitigation and Conveyance: Control runoff within and from the site to minimize flood risk to people and properties for the conveyance storm as well as the 100-year storm.
Each of the Design Focus Areas must be used in conjunction with the others to address the overall stormwater impacts from a development site. When used as a set, the Design Focus Areas control the entire range of hydrologic events, from the smallest runoff-producing rainfalls up to the 100-year, 24-hour storm.
Local Provisions: Water Quality is required by the City of Azle
Design Storms
Integrated design is based on the following four (4) storm events.
Table 14.1.2. Storm Events
Storm Event Name
Storm Event Description
“Water Quality”
Criteria based on a volume of 1.5 inches of rainfall, not a storm frequency
“Streambank Protection”
1-year, 24-hour storm event
“Conveyance”
25-year, 24-hour storm event
“Flood Mitigation”
100-year, 24-hour storm event
Throughout the manual the storms will be referred to by their storm event names.
Local Provisions: The adopted “Streambank Protection,” “Conveyance,” and “Flood Mitigation” storm events for the City of Azle are the 1-, 10-, and 100-year, 24-hour storm events, respectively.
Design Focus Area Application Options
There are multiple options provided to meet the required criteria for water quality protection, streambank protection, and flood mitigation. These design options are summarized in Table 1.3.
Design criteria for streambank protection and flood mitigation are based on a downstream assessment. The purpose of the downstream assessment is to protect downstream properties and channels from increased flooding and erosion potential due to upstream development. A downstream assessment is required to determine the extent of improvements necessary for streambank protection and flood mitigation. Downstream assessments shall be performed for streambank protection, conveyance, and flood mitigation storm events. More information on downstream assessments is provided in Section 14.3.3.
If a development causes no adverse impacts to existing conditions, then it is possible that little or no mitigation would be required.
Table 14.1.3. Summary of Options for Design Focus Areas
Design Focus Area
Reference Section
Required Downstream Assessment
Design Options
Water Quality Protection
14.3.2
yes
Option 1: Use integrated Site Design Practices for conserving natural features, reducing impervious cover, and using the natural drainage systems
 
 
 
Option 2: Treat the Water Quality Protection Volume (WQv) by reducing total suspended solids from the development site for runoff resulting from rainfalls of up to 1.5 inches (85th percentile storm)
 
 
 
Option 3: Assist in implementing off-site community stormwater pollution prevention programs/activities as designated in an approved stormwater master plan or TPDES Stormwater permit
Streambank Protection
14.3.4
yes
Option 1: Reinforce/stabilize downstream conditions
 
 
 
Option 2: Install stormwater controls to maintain or improve existing downstream conditions
 
 
 
Option 3: Provide on-site controlled release of the 1-year, 24-hour storm event over a period of 24 hours (Streambank Protection Volume, SPv)
Flood Mitigation and Conveyance
14.3.5 and 14.3.6
yes
Flood Mitigation
Option 1: Provide adequate downstream conveyance systems
 
 
 
Option 2: Install stormwater controls on-site to maintain or improve existing downstream conditions
 
 
 
Option 3: In lieu of a downstream assessment, maintain existing on-site runoff conditions
 
 
 
Conveyance
Minimize localized site flooding of streets, sidewalks, and properties by a combination of on-site stormwater controls and conveyance systems
Local Provisions: Water Quality is required by the City of Azle
(Ordinance 2012-10 adopted 8/21/12)
Section 14.4.0 of this manual presents an integrated approach for reducing the impact of stormwater runoff from construction activities on downstream natural resources and properties. The purpose is to provide design criteria for temporary controls during construction that protect water quality by:
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Preventing soil erosion;
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Capturing sediment on-site when preventing erosion is not feasible due to construction activities; and
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Controlling construction materials and wastes to prevent contamination of stormwater.
Temporary controls to protect water quality are known as Best Management Practices (BMPs). The design of the BMPs is to be coordinated with and done at the same time as the Preliminary and Final iSWM Plans. Construction BMPs complement and work with the site grading and drainage infrastructure.
Erosion Control BMPs are designed to minimize the area of land disturbance and to protect disturbed soils from erosion. Protection can be accomplished by diverting stormwater away from the disturbed area or by stabilizing the disturbed soil. Erosion control BMPs are most important on disturbed slopes and channels where the potential for erosion is greatest. The design of erosion control BMPs must be coordinated with related grading, drainage and landscaping elements. (e.g. channel armoring, velocity dissipaters, etc.)
Sediment Control BMPs are temporary structures or devices that capture soil transported by stormwater. The BMPs are designed to function effectively with the site drainage patterns and infrastructure. An effective design ensures that the sediment control BMPs do not divert flow or flood adjacent properties and structures. Some types of permanent drainage structures, such as detention and retention basins, can also be designed to function as a sediment control BMP during construction.
Material and Waste Control BMPs prevent construction materials and wastes from coming into contact with and being transported by stormwater. These BMPs consist of a combination of notes to direct contractor and temporary construction controls.
The iSWM Construction Criteria are the minimum requirements for temporary controls during construction. The state permit and requirements for stormwater discharges associated with construction activities must also be followed. More information on state requirements is provided in Section 14.4.2.
Local Provisions: NONE
(Ordinance 2012-10 adopted 8/21/12)