Policy on Practicability Analysis for Runoff Reduction
Introduction
Runoff reduction practices are stormwater Best Management Practices (BMPs) used to disconnect
impervious and disturbed pervious surfaces from the storm drainage system. The purpose is to reduce
post-construction stormwater runoff rates, volumes, and pollutant loads. Runoff reduction is more than
simple infiltration. The Runoff Reduction Volume (RR
V
) is the retention volume calculated to infiltrate,
evapotranspirate, harvest and use, or otherwise remove runoff from a post-developed condition to more
closely mimic the natural hydrologic conditions.
Certain conditions, such as soils with very low infiltration rates, high groundwater, or shallow bedrock,
may lead Rockdale County to waive or reduce the runoff reduction requirement for proposed site
development on a case-by-case basis. If any of the stormwater runoff volume generated by the first 1.0”
of rainfall cannot be reduced or retained on the site, due to site characteristics or constraints, the
remaining volume shall be increased by a multiplier of 1.2 and shall be intercepted and treated in one or
more best management practices that provide at least an 80 percent reduction in total suspended solids.
The Policy on Practicability Analysis for Runoff Reduction (practicability policy) was developed to provide
guidance about the site conditions and supporting documentation that could justify a Determination of
Infeasibility” for the runoff reduction requirement. This practicability policy does not address infeasibility
for linear transportation projects being constructed by Rockdale County, other local governments, or
State agencies.
The practicability policy is based on the following principles:
It is designed to help stormwater managers implement a process for granting a Determination of
Infeasibility that supports efficient review of land development applications.
It applies to new development and redevelopment projects for public and private post-
construction stormwater BMPs. It is referenced in the Model Ordinance for Post-Construction
Stormwater Management for New Development and Redevelopment (Model Ordinance)
developed by the Metropolitan North Georgia Water Planning District (Metro Water District) and
in Rockdale County Ordinance Chapter 310.
It aligns with requirements for runoff reduction in the Georgia Environmental Protection
Division’s (EPD’s) permit to discharge from the municipal separate storm sewer system (MS4)
permit. The MS4 permit states that the stormwater management system shall be designed to
retain the first 1.0 inch of rainfall on the site to the maximum extent practicable. Most Georgia
Stormwater Management Manual (GSMM) BMPs include a runoff reduction component.
It is focused on the typical site conditions and regulatory environment in the Metro Water District
and may not be applicable for all of Georgia.
It requires a pre-submittal meeting when pursuing a Determination of Infeasibility to ensure all
attempts to provide 100% RRv on site have been exhausted.
Rockdale County is responsible for the review of land development applications and determination that
it is infeasible to apply the runoff reduction requirement on part or all of a proposed site development.
Rockdale County may choose to make substantive changes or otherwise customize this practicability
policy. These further changes and customizations are allowable so long as their substance meets the
requirements of Rockdale County’s MS4 permit. EPD is responsible for evaluating MS4 permit and
District Plan compliance, which includes verifying whether changes and customizations are “at least as
effective.” EPD has reviewed this document and their comments have been incorporated.
Conditions that may warrant a Determination of Infeasibility
The GSMM provides broad guidance about conditions that may lead a local jurisdiction to waive or reduce
the runoff reduction requirement. The following conditions may warrant a Determination of Infeasibility
due to economic hardship, risk posed to a protected resource, or other site constraints. To claim
economic hardship associated with various site conditions and constraints, one must demonstrate that
the cost of retaining the first 1.0 inch of rainfall onsite using runoff reduction practices is a minimum of
three times greater than the cost of providing conventional water quality treatment practices.
Poor Soil Infiltration Rate: The performance of a contemplated runoff reduction practice will be
negatively affected by native soil having an infiltration rate is less than 0.5 inch per hour.
Consideration should be given to infiltration rates throughout the soil profile.
High Water Table: The seasonal high water table is less than two feet from the bottom of a
contemplated runoff reduction practice.
Shallow Bedrock: A contemplated runoff reduction practice will require substantial removal of
non-rippable rock, which cannot be excavated except by hammering, drilling, or blasting.
Extreme Topography: Existing topography of the site requires at least 50% of the proposed
development to have slopes steeper than 3:1, making the use of a contemplated runoff reduction
practice difficult.
Environmental Concerns: A contemplated runoff reduction practice will either negatively affect
or be negatively affected by features such as streams, lakes, wetlands, springheads, frequently
flooded areas, groundwater wells, specimen trees, environmental protection buffers and
setbacks, conservation easements, endangered species, bury pits, sinkholes, septic systems,
existing soil contamination, or proposed hotspot land uses.
Historic Resources: A contemplated runoff reduction practice will negatively impact buildings,
structures, or historic sites included in the Georgia Historic Preservation Division’s Historic
Resources Survey, listed in the National Register of Historic Places, or that has been recommended
as a historic resource by a Preservation Professional.
Other Site Constraints: Construction or maintenance of a contemplated runoff reduction practice
will be complicated by laws, rules, administrative procedures, agreements, and other issues
associated with the proposed development, such as zoning setbacks, landscaping standards,
utility easements, ADA requirements, regulatory floodplains, HOA covenants, emergency access
routes, etc.
Other Economic Hardship: Construction of a contemplated runoff reduction practice is
complicated by financial constraints that are unforeseeable, unavoidable, or unmanageable. A
Determination of Infeasibility cannot be based on this factor alone, and this factor shall not
account for more than 50% of the runoff reduction volume.
Supplemental Materials
The District has prepared supplemental materials to support the implementation of this practicability
policy. Appendix A is meant for internal use and provides an overview of the steps Rockdale County
could take to implement the practicability policy and issue a Determination of Infeasibility. Appendix B is
Rockdale County’s runoff reduction infeasibility form.
Appendix A:
Overview of Processing a Determination of Infeasibility
Overview of Processing a Determination of Infeasibility
Obtaining a Determination of Infeasibility
Determination of Infeasibility is not an all or nothing proposition. Designers must demonstrate that they
have explored all avenues to meet the runoff reduction standard. If this is determined to be infeasible,
they must attempt to provide the maximum percentage of RRv on site as feasible. Only after all
attempts to provide any RRv on site are exhausted will the local jurisdiction consider a Determination of
Infeasibility. The following process is recommended to:
1. identify conditions early,
2. provide flexibility,
3. support efficient land development application review, and
4. protect water quality to the maximum extent practicable.
Does the Site Qualify for a Determination of Infeasibility?
Answering “NO” to any of the following questions may indicate that the site qualifies for a Determination
of Infeasibility:
1. Can GSMM runoff reduction BMPs fully meet the runoff reduction volume?
2. Does the site analysis show the conditions are supportive for managing the calculated runoff
reduction volume needed for the site?
3. Can better site design practices (see GSMM, Volume 2, Section 2.3) be used to avoid challenging
site conditions or constraints?
4. Can BMPs, such as green roofs and rainwater harvesting techniques, be used in ways that do not
require infiltration into subsurface soils, but rather rely on evapotranspiration and reuse?
5. Can the installation of multiple runoff reduction BMPs, such as installing runoff reduction BMPs
at higher elevations or in multiple sub watersheds, manage the calculated runoff reduction
volume needed for the site?
Prior to Construction
1) The design professional identifies conditions that limit using runoff reduction methods to retain
100% of the first 1.0 inch of rainfall onsite and initiates a pre-submittal meeting with the plan
reviewer prior to submittal of the land development permit application. During the meeting, the
following information will be reviewed:
Runoff Reduction Infeasibility Form to initiate the request and provide basic project information,
confirmation that supporting documentation was submitted, and documentation of pre-
submittal meeting outcomes.
Stormwater Concept Plan that has been developed based on site analysis, and natural resources
inventory (including impracticability) in accordance with Section 2.4.2.5 of the GSMM.
2) The plan reviewer will evaluate the pre-submittal information on a case-by-case basis; coordinate
with the design professional to understand site-specific issues; and (if possible) explore potential
design strategies to achieve 100% RRv in compliance with the standards and specifications of the
Post-Construction Stormwater Management Ordinance and GSMM.
3) Based on the pre-submittal information and meeting, the plan reviewer will provide one of the
following determinations to the design professional:
Approval preliminary Determination of Infeasibility issued
Approval with conditions preliminary Determination of Infeasibility issued with conditions to
incorporate plan reviewer comments into the Stormwater Concept Plan
Denial - revise the Stormwater Concept Plan to obtain 100% RRv
4) Design professional may either:
Submit the land development application with the Stormwater Management Plan and
preliminary Determination of Infeasibility (as applicable).
Appeal the “denial” or “conditions” following the appeals process outlined in the local
jurisdiction’s regulations.
During Construction
1) During the development process, the owner encounters a site condition that would prevent building
stormwater BMPs as specified in the Stormwater Management Plan. The design professional will
complete a Runoff Reduction Infeasibility Form and initiate a meeting with the local jurisdiction plan
reviewer to discuss the findings. The designer must evaluate modifications to the proposed BMPs or
installation of alternative BMPs that will provide some or all RRv in an alternative method.
2) The plan reviewer will evaluate the Runoff Reduction Infeasibility Form on a case-by-case basis;
coordinate with the design professional to understand site-specific issues; and (if possible) explore
potential design strategies to keep the stormwater BMPs identified in the Stormwater Management
Plan.
3) Based on the Runoff Reduction Infeasibility Form and meeting, the plan reviewer will provide one of
the following determinations to the design professional:
ApprovalDetermination of Infeasibility is issued and attached to the land development permit
Approval with conditions preliminary Determination of Infeasibility issued with conditions to
either:
i) Revise the design of runoff reduction methods (e.g. adding soil amendments or an
underdrain to maximize runoff reduction volume) to retain the first 1.0 inch of rainfall
onsite.
ii) Meet the stormwater runoff quality/reduction standard through a combination of Runoff
Reduction and Water Quality.
4) Design professional may either:
Continue construction as outlined modified Stormwater Management Plan under the Permit
Revision with approved Determination of Infeasibility.
Appeal the “conditions” following the appeals process as outlined in the local jurisdiction
regulations.
Appendix B:
Runoff Reduction Infeasibility Form
Page 1 of 3
Date (submitted):
R
ockdale County
Runoff Reduction Infeasibility (RRI) Form for
Determination of Infeasibility
Design Professional Primary Contact (Name/Email/Phone): ____________________________________
__________________________________________________________________________________
Description of Site/Land Development Application Number: ____________________________________
Address: ___________________________________________________________________________
Size (acres): ___________________________________________________________________________
Maximum Practicable Runoff Reduction Volume*: _____________________________________________
*If any of the stormwater runoff volume generated by the first 1.0” of rainfall cannot be reduced or retained on
the site, due to site characteristics or constraints, the remaining volume shall be increased by a multiplier of 1.2
and shall be intercepted and treated in one or more best management practices that provide at least an 80
percent reduction in total suspended solids.
GENERAL SUPPORTING DOCUMENTATION
All General Supporting Documentation must be included with this RRI Form for the submittal for a
Determination of Infeasibility to be considered complete. Please check each item below to confirm it
has been included in the submittal package.
Stormwater Concept Plan that has been developed based on site analysis, and natural resources
inventory (including impracticability) in accordance with Section 2.4.2.5 of the GSMM
GSMM Stormwater Quality Site Development Review Tool for the Stormwater Concept Plan
Please include justification that the site cannot accommodate best management practices that
rely on evapotranspiration and reuse such as rainwater harvesting or green roofs
Page 2 of 3
SITE CONDITION APPLICABILITY
(descriptions are in Policy on Practicability Analysis for Runoff Reduction)
Please check each applicable item below and confirm the supporting documentation has been included in
the submittal for a Determination of Infeasibility.
* Note: A Determination of Infeasibility cannot be based on this factor alone, and this factor shall not
account for more than 50% of the runoff reduction volume.
Site Condition
Supporting Documentation
Poor Soil Infiltration Rate
Report including interpretation of infiltration tests, boring logs,
soil maps, site plans, etc. to determine infiltration rates, as
certified by an appropriately licensed design professional
High Water Table
Report including interpretation of boring logs, piezometer
readings, site plans, etc. to determine seasonal high water table
elevations, as certified by an appropriately licensed design
professional
Shallow Bedrock
Report including the interpretation of boring logs, rock quality
tests, test pit observations, site plans, etc. to determine the
amount of non-rippable rock that must be removed, as certified
by an appropriately licensed design professional
Extreme Topography Report including the interpretation of survey data, site plans, etc.
to determine the amount of the proposed development which will
have slope steeper than 3:1, as certified by an appropriately
licensed design professional
Environmental Concerns
Report including interpretation of survey data, site plans, soil
maps, photos, Phase I Environmental Assessments, etc. to
determine the potential impact to the environment, as certified by
an appropriately licensed design professional
Historic Resources
Report including interpretation of survey data, site plans, photos,
documentation of the NAHRGIS listing, report of assessment from
a Preservation Professional (including Archaeologist, Architectural
Historian, Historian, Historic Preservationist, or Historic
Preservation Planner) to determine the potential impact to
historic resources, as certified by an appropriately licensed design
professional
Site Constraints Report including interpretation of survey data, site plans,
regulations, policies, contracts, correspondence, etc. to determine
conflicts associated with the proposed development, as certified
by an appropriately licensed design professional
Economic Hardship* Report including interpretation of cost estimates, invoices,
financial statements, market research, economic reports,
correspondence, etc. to determine potential or experienced
financial impacts, along with justification of why such impacts
were unforeseeable, unavoidable, or unmanageable
Page 3 of 3
STORMWATER RUNOFF QUALITY/ REDUCTION SUMMARY
Maximum Practicable Runoff Reduction Volume*: _____________________________________________
Remainder of Volume treated by Water Quality Best Management Practice: _______________________
*If any of the stormwater runoff volume generated by the first 1.0” of rainfall cannot be reduced or retained on
the site, due to site characteristics or constraints, the remaining volume shall be increased by a multiplier of 1.2
and shall be intercepted and treated in one or more best management practices that provide at least an 80
percent reduction in total suspended solids.
Design Professional’s Printed Name _________________________________________________________
Design Professional’s License Type and Number_______________________________________________
Design Professional’s GSWCC Level II Certification Number______________________________________
Design Professional’s GSWCC Level II Certification Expiration Date________________________________
Design Professional’s Signature __________________________________________________________
FOR ROCKDALE COUNTY USE ONLY
APPROVED
APPROVED with
conditions
Reviewer:
(Print Name) (Signature) (Date)