Finding D.2.c. Use of Low-Impact Development (LID) site design BMPs at new
development projects can be an effective means for minimizing the impact of urban
runoff discharges from the development projects on receiving waters. LID is a site
design strategy with a goal of maintaining or replicating the pre-development
hydrologic regime through the use of design techniques. LID site design BMPs help
preserve and restore the natural hydrologic cycle of the site, allowing for filtration and
infiltration which can greatly reduce the volume, peak flow rate, velocity, and pollutant
loads of urban runoff.
Discussion of Finding D.2.c. The use of LID site design BMPs helps reduce the
amount of impervious area associated with urbanization and allows storm water to
infiltrate into the soil. Natural vegetation and soil filters urban runoff and reduces the
volume and pollutant loads of storm water. Studies have revealed that the level of
imperviousness resulting from urbanization is strongly correlated with the water quality
impairment of nearby receiving waters.106 In many cases, the impacts on receiving
waters due to changes in hydrology can be more significant than those attributable to
the contaminants found in storm water discharges.107 These impacts include stream
bank erosion (increased sediment load and subsequent deposition), benthic habitat
degradation, and decreased diversity of macroinvertebrates. Although conventional
BMPs do reduce pollutant loads, they may not effectively control adverse effects from
changes in the discharge hydrologic conditions.108
The Order includes requirements for developments to include site design BMPs that
mimic or replicate the natural hydrologic cycle. Open space designs which maximize
pervious surfaces and retention of “natural” drainages have been found to reduce both
the costs of development and pollutant export.109 Moreover, USEPA finds including
plans for a “natural” site design and BMP implementation during the design phase of
new development and redevelopment offers the most cost effective strategy to reduce
pollutant loads to surface waters.110 In addition, a recent U.S. Department of Housing
and Urban Development guidance document on low-impact development notes that
the use of LID-based storm water management design allows land to be developed,
but in a cost-effective manner that helps mitigate potential environmental impacts.
Section D.1.d.(4) (Site Design BMP Requirements) requires City of Laguna Beach to require
or implement site design BMPs at Priority Development Projects in order to reduce the
amount of polluted runoff from those sites. The primary approach in site design BMPs
is to limit the permanent loss of existing infiltration capacity because loss of infiltration
is a major contributor to both wet and dry weather pollution discharges. General
means to accomplish that goal include retaining natural infiltration areas of a site and
limiting the amount of impervious surfaces. The Order does not require a specific or
relative amount of pervious surfaces be added to a project. The Order seeks to
reduce the effective impervious surface of a project, which is the impervious surface
that is directly connected to the storm water drainage system.
The site design BMP options listed in these sections are consistent with the site design
BMPs currently required by the City of Laguna Beach in the Model WQMP. In the ROWD, the
City of Laguna Beach propose to improve the process of selecting site design BMPs.
Specifically, they propose to develop recommendations for incorporating low-impact
design (LID) techniques and site design BMPs. However, the Model WQMP employs
an open-ended approach to requirements for site design BMPs, requiring
implementation of site design BMPs “where applicable and feasible” and “where
appropriate.” Unfortunately, this approach has proven to be ineffective in integrating
site design BMPs in project designs. Audits conducted in 2005 of four City of Laguna Beach
found that municipalities need to work with project applicants to improve the quality of
site design BMPs.152 As a result, the Order establishes two sets of site design BMP
criteria.
First, section D.1.d.(4)(b) of the Order directs the City of Laguna Beach to require, rather than
consider, new development projects to employ certain classes of site design BMPs.
The required site design BMPs take advantage of features that are incorporated into
the Priority Development Project, such as landscaping or walkways. It also requires
that projects seek to maintain natural water drainage features rather than instinctively
convey water in buried pipes and engineered ditches that eliminate natural water
quality treatment functions. These types of site design BMPs are both effective and
achievable. These requirements are consistent with the guidelines of Order
No. R9-2002-01 and both the 2003 and 2007 DAMPs.153
Next, section D.1.d.(4)(c) of the Order identifies classes of site design BMPs that must
be used when applicable and feasible. This approach is similar to Order
No. R9-2002-01 and the DAMPs. This list includes requirements from Order
No. R9-2002-01, items identified in the DAMPs, and recommended measures from
CASQA guidance. These site design BMPs are commonly cited in project proponents’
WQMP reports as the site design BMPs that have been incorporated into Priority
Development Projects.
The retention of natural drainage features, such as ephemeral streams, wetlands, and
depressions, can be particularly important because small tributaries are essential to
the maintenance of the chemical, biological, and physical integrity of larger
waterbodies.154 The loss and modification of such natural water resources to
accommodate post-development storm water management leads to direct and indirect
adverse effects on water quality that are felt both on the project site and off the site
within the watershed.155,156,157 Effects to aquatic beneficial uses from altered
drainage features can occur downstream and upstream. The length of upstream or
downstream effect of channel modifications is dependant on the specific structure type
and channel slope.158 For instance, road culverts can act as partial barriers to
upstream distribution of native aquatic macroinvertebrates in urban streams, while
bridges can provide adequate passage.159 As a result of the adverse effects to water
quality and beneficial uses, the State of California nonpoint source pollution program
management measures for urban areas includes limiting the destruction of natural
drainage features and natural conveyance areas. 160
Through its process of conditioning development projects under the CWA section 401
Water Quality Certification program, the Regional Board finds that the level of site
design BMP implementation in the Order is feasible for all projects. This site design
BMP requirement will help ensure that site design BMPs are implemented for new
development projects. Site design BMPs are a critical component of urban runoff
management at new development projects, since the BMPs provide multiple benefits
including preservation of hydrologic conditions, reduction of pollutant discharges, cost
effectiveness, and green space.
The site design BMP options listed do not need to be costly.161 Some design options,
such as concave vegetated surfaces or routing rooftop or walkway runoff to
landscaped areas, are cost neutral.162 Other site design BMPs, such as minimizing
parking stall widths or use of efficient irrigation devices, are oftentimes already
required. In addition, use of site design BMPs reduces runoff quantity, allowing for
treatment control BMPs and other storm water infrastructure on site to be smaller,
therefore savings costs for both developers and municipalities.163,164
Because of the potential economic and environmental benefits of using low-impact
development site design, the U.S. Department of Housing and Urban Development,
Office of Policy Development and Research, developed “The Practice of Low Impact
Development (LID)” to assist the housing industry during the land development
process. 165 This document focuses specifically on technologies that affect both the
cost impacts and environmental issues associated with land development. Much of
the report focuses on storm water management because low-impact development
storm water management systems can save capital costs for developers and
maintenance costs for municipalities.166 The executive summary of the HUD report
notes:
This approach to land development, called Low Impact Development (LID),
uses various land planning and design practices and technologies to
simultaneously conserve and protect natural resource systems and reduce
infrastructure costs. LID still allows land to be developed, but in a cost-effective
manner that helps mitigate potential environmental impacts. LID is best suited
for new, suburban development.
Developers can use site and structure designs that reduce building footprints,
decrease the amount of paved infrastructure, and provide for dispersed drainage and
infiltration of runoff from impervious surfaces to reduce the effective impervious
surface.167 The concept of effective impervious surface is important, because when
runoff from these surfaces is directed to pervious areas rather to an impervious
drainage system (i.e., curbs, gutters, street surfaces, storm drain pipes), it can
infiltrate, evaporate, or be taken up by vegetation, thereby reducing the total volume of
runoff leaving a site.
The Order continues to provide the City of Laguna Beach with flexibility in implementing site
design BMP requirements by providing lists from which site design BMP approaches
can be chosen. Moreover, flexibility is inherently included in the site design options
listed - each option provides the opportunity for numerous implementation approaches
that can be used to achieve compliance.
Section D.1.d.(5) (Source Control BMP Requirements) requires that Priority
Development Projects implement minimum source control BMPs. This section has
been added to provide more detail and clarify the Order’s requirements for source
control BMPs. The minimum source control BMPs listed in the section are consistent
with the Model WQMP.
Section D.1.d.(6) (Treatment Control BMP Requirements) is consistent with Order
No. R9-2002-01, with two exceptions. First, the Order limits the selections of methods
used to determine the appropriate volume of runoff to be treated. The modification
ensures that priority development project proponents utilize the most accurate
information to determine the volume or flow of runoff which must be treated. Using
detailed local rainfall data, the County of Orange has developed the 85th Percentile
Precipitation Isopluvial Map, which exhibits the size of the 85th percentile storm event
throughout Orange County.168 Since this map uses detailed local rainfall data, it is
more accurate for calculating the 85th percentile storm event than other methods which
were included in Order No. R9-2002-01. The other methods found in Order No. R9-
2002-01 were included as options to be used in the event that detailed accurate
rainfall data did not exist for various locations within Orange County. The
development of the 85th Percentile Precipitation Isopluvial Map makes these other less
accurate methods superfluous. Therefore, these other methods for calculating the 85th
percentile storm event have been removed from the current Order.
Second, the Order requires that treatment control BMPs selected for implementation at
Priority Development Projects have a removal efficiency rating that is higher than the
“low removal efficiency,” as presented in the Model SUSMP/WQMP. The requirement
allows exceptions for those projects that, with a feasibility analysis, can justify the use
of a treatment control BMP with a low removal efficiency for a Priority Development
Project. This requirement is needed because to date, the City of Laguna Beach have
generally approved low removal efficiency treatment control BMPs without justification
or evidence that use of higher efficiency treatment BMPs was considered and found to
be infeasible. Specifically, it has been found during audits of the City of Laguna Beach’
SUSMP programs that many SUSMP reports do not adequately describe the selection
of treatment control BMPs.169 Moreover, USEPA’s contractor Tetra Tech, Inc.
recommends that “project proponents should begin with the treatment control that is
most effective at removing the pollutants of concern […] and provide justification if that
treatment control BMP is not selected.”170
In the ROWD, the City of Laguna Beach acknowledge the need for further attention to the
selection and implementation of effective treatment BMPs. They propose to revise the
model WQMP table of BMP effectiveness. The requirement is needed to provide
clarification that selection of low efficiency treatment control BMPs over high efficiency
BMPs without justification does not meet permit requirements and is not in compliance
with the MEP standard. |
