*
|
Control runoff within and from the site to minimize flood risk
to people and properties;
|
*
|
Assess discharges from the site to minimize downstream bank
and channel erosion; and
|
*
|
Reduce pollutants in stormwater runoff to protect water quality
and assist communities in meeting regulatory requirements.
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Chapter Summary
| |
The iSWM Criteria Manual consists of five chapters:
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Chapter 1 - Introduction and Summary
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Chapter 2 - Integrated Development Process
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Chapter 3 - Integrated Design Criteria
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Chapter 4 - Integrated Construction Criteria
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Chapter 5 - Additional Local Provisions
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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.
|
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.
|
Step 1 - Review Local Requirements and Municipality’s
Processes
|
Step 2 - Collect Data and Perform Site Analysis
|
Step 3 - Prepare Concept/Preliminary iSWM Plans
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Step 4 - Prepare Final iSWM Plans and iSWM Construction Plan
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Step 5 - Prepare Operation and Maintenance Plans
|
Local Provisions: NONE
|
Design Focus Areas
| |
The stormwater management focus areas and goals are:
| |
*
|
Water Quality Protection: Remove pollutants
in stormwater runoff to protect water quality
|
*
|
Streambank Protection: Regulate discharge
from the site to minimize downstream bank and channel erosion
|
*
|
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
|
*
|
Preventing soil erosion;
|
*
|
Capturing sediment on-site when preventing erosion is not feasible
due to construction activities; and
|
*
|
Controlling construction materials and wastes to prevent contamination
of stormwater.
|
Local Provisions: NONE
|