Maryland; Environmental Site Design for Stormwater Management - Chesapeake Bay Foundation

8/3/2019 Maryland; Environmental Site Design for Stormwater Management - Chesapeake Bay Foundation

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www.melioradesign.net

Comparison of Environmental Site Design

to Conventional Site Design

for Stormwater Management

For Three Redevelopment Sites in Maryland

Prepared for the

Chesapeake Bay Foundation

September 26, 2008

Submitted by

2214 Kimberton Rd Kimberton, PA 19442P: 610-933-0123 F: 610-933-0188


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Overview

This purpose of this document is to compare the feasibility and cost of

implementing an Environmental Site Design (ESD) approach to stormwater

management, as compared to a conventional stormwater management

design, at three urban redevelopment projects in Maryland. The goal of this

effort is to determine whether an ESD approach is both feasible and

economically comparable to a conventional stormwater approach in highly

urban and suburban redevelopment areas. An ESD approach is considered far

more protective of water quality and water resources than a conventional

approach of collection and detention, because of the way it attempts to mimic

natural systems and processes.

With the passage of the Maryland Stormwater Management Act of 2007,

developers, site designers, and regulators are transitioning to ESD as the new

norm for rural, suburban, and urban projects. The Stormwater ManagementAct requires that ESD be used as the basis for stormwater management for both

new development and redevelopment projects, and also establishes a

significant new groundwater recharge requirement: "Maintain 100% of the

average annual predevelopment groundwater recharge volume for the site."

Since the revised Maryland manual and regulations reflecting the new ESD

mandate are still being drafted, we applied the existing stormwater

management volume criteria in the states2000 Maryland Stormwater Design

Manual to urban redevelopment sites using ESD practices. Though we used the

existing Recharge, Channel Protection, and Water Quality protection volumes

as our design targets in order to model redevelopment sites for ESD practices,the actual volumes that our conceptual ESD practices were able to capture

and filter or infiltrate often greatly exceeded the 2000 Manual targets. Several

of the ESD practices that we applied in concept to our case study projects,

including porous pavement with a sub-surface infiltration bed, provide

significantly higher recharge volumes than are required under the existing 2000

Stormwater Design Manual.

For this conceptual exercise, three existing urban sites in Maryland were

selected. Two are currently scheduled for redevelopment or have recently

been redeveloped, and for these two sites, the proposed or completed

redevelopment program was used. The third site is not currently proposed forredevelopment, and for this site a concept plan of residential redevelopment

that includes townhouses and garden apartments was developed.

For each of these sites, a variety of urban stormwater Best Management

Practices (BMPs) were incorporated into the proposed redevelopment effort.

These BMPs include vegetated roofs, bioretention swales, rain gardens, porous


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pavements, stormwater storage/infiltration beds beneath conventional

pavements, and tree trenches. These BMPs are by no means inclusive of all the

stormwater options, but were selected as BMPs that could be incorporated into

the proposed redevelopment program without major changes to the site plan.

A concept level cost estimate was developed for the stormwater improvements

for each site.

Additionally, a cost estimate was developed for each site for a conventional

stormwater system. This estimate was based on the size and quantity of

stormwater infrastructure required for a conventional approach of collecting

and conveying all stormwater runoff to one or two locations for detention and

release. The cost of the ESD approach and the conventional approach are

then compared.

Findings

Opportunities to implement ESD and provide better stormwater management -

and to address the critical components of water quality, groundwater recharge,

reduced volume, and channel protection were found at each of the three

redevelopment sites. Equally important, the ESD approach was comparable in

cost to a conventional approach while providing additional environmental and

aesthetic benefits. For highly urban sites, ESD was comparable or less expensive

than a conventional stormwater detention system. For the more suburban site

(Calverton) which included a surface detention facility, ESD was more expensive

for the control of the Channel Protection Volume, but more cost effective for

water quality.


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Environmental Site Design (or LID) Approach

Environmental Site Design is the practice of implementing stormwater

management features that mimic or support to the greatest extent possible

the hydrologic conditions that existed before development. This approach is

also sometimes referred to as Low Impact Development (LID) or sustainable

stormwater management, but regardless of the nomenclature, the goal is the

same: to maintain runoff volumes that are similar to natural conditions before

development, to maintain groundwater recharge, maintain evapotranspiration,

and protect water quality for the long-term sustainability of water resources. For

Maryland (and much of the east coast) the natural condition is woodlands.

Therefore, the ultimate goal would be a stormwater system that replicated to

the greatest extent possible the hydrologic response of a forest.

The National Association of Homebuilders describes Low Impact Development

as follows:

Allow natural infiltration to occur as close as possible to the original

area of rainfall. By engineering terrain, vegetation, and soil features

to perform this function, costly conveyance systems can be

avoided and the landscape can retain more of its natural

hydrologic function.

The term infiltration sometimes causes confusion to designers who are unfamiliar

with these stormwater techniques. The intent is to use the natural capacity of

soil systems to absorb and hold water, especially in small rainfall events. It isestimated that approximately 20% of the total soil volume is available to absorb

water. The amount of water soil can retain is dependent on factors such as the

size of soil grains, soil type, percent organic matter, degree of compaction, etc.

The 2006 Pennsylvania Stormwater Manual describes the process of taking credit

for soil storage of runoff when sizing stormwater elements, such as rain gardens.

Some of the water that moves through soils may continue to infiltrate into the

deeper groundwater system and provide groundwater recharge. Some BMPs,

such as vegetative roofs, cannot provide groundwater recharge. But after

water infiltrates into the vegetated roof media, some water is returned to the

atmosphere, and some is slowly released downstream. The vegetation in manyBMPs helps to maintain the porosity of soils and to facilitate long-term

evapotranspiration.

A second critical component of ESD (or LID) is that BMPs must be spread through

the project site, and preferably, also interconnected. For example, during a

large storm, a green roof area might overflow to a bioretention swale. That


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bioretention swale might also receive runoff from a roadway area that is high in

sediments and pollutants. The design and vegetation of the bioretention swale

will help to reduce the pollutants. The bioretention swale then might overflow

into a stormwater storage/infiltration bed beneath a plaza or parking lot. This

stormwater storage/infiltration bed may allow infiltration, but should also include

an engineered overflow structure to control the rate of discharge during very

large storms. In other words, the stormwater is managed along the way as it

travels through the site and various BMPs. There are no dead ends to create

flooding conditions, and all systems must be designed to safely convey

stormwater during rainfall periods of high intensity to prevent localized nuisance

flooding. BMPs are selected depending on conditions, pollutant levels, etc. Not

every BMP is right for every situation.

Essentially, with Environmental Site Design, stormwater becomes integrated into

the landscape through multiple stormwater BMPs. These BMPs must remain at or

close to grade, and stormwater is spread through the various BMPs so that nosingle practice is expected to provide unrealistically high amounts of water

quality improvement, groundwater recharge, or evapotranspiration.

Importance of Integrated Design Approach

While the concept of ESD is straightforward, current design and construction

processes often force sites towards a conventional stormwater system where all

stormwater is collected and conveyed via storm sewers to one or two large

stormwater facilities. These facilities are often expected to perform multiple

tasks (water quality cleansing, infiltration, sedimentation removal, etc.) thatare sometimes at odds with each other and at rates far in excess of what can

realistically be achieved.

Very often, a project or building is laid out based on project needs, and the

stormwater design begins after building elevations, building designs, roads,

grades, and other elements are set. Better stormwater management can

ONLY occur when an integrated site design process is used, and is supported by

local building codes and regulations. The site engineer is no longer solely

responsible for the stormwater design as coordination and input from the

architect, mechanical engineer, landscape architect and others will be

required for successful implementation.

For example, if a building is designed with interior roof leaders that discharge

several feet below grade, it will be difficult to divert this roof runoff to the

landscape using rain gardens. If excessive amounts of grading occur, natural

soils may be eliminated or compacted. Landscape designs done after the

stormwater system design may be lost opportunities for stormwater


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management, and codes that require minimum pipe sizes that are too large

will force stormwater systems that are deep.

While these are only a few examples, consideration of these types of issues early

in the design process and with input from various disciplines will lead to the

identification of multiple BMP opportunities that are appropriate and cost

effective. If, however, a stormwater designer attempts to apply BMPs through a

project site after other components cannot be changed, many potential

opportunities will be missed. Equally important, the remaining opportunities may

increase significantly in cost and difficulty of construction.

The importance of addressing stormwater early in the design process, with input

from all concerned, CANNOT be overemphasized. ESD requires an integrated

design approach.

Overview of the Three Project Sites

Lot 31, Bethesda Site

The first site, Lot 31 in downtown Bethesda, is an existing asphalt parking lot that

is a little over 2 acres in size. The proposed development involves a building that

is just over one acre containing both residential and retail units. The parking will

be located below grade. The site will be 75% impervious when completed and

includes approximately one-half acre of lawn.

The BMPs considered for this site include an extensive vegetated roof,bioretention swales, rain gardens, and a sidewalk constructed of porous

concrete or pavers and underlain with a stormwater storage bed. A detailed

discussion of the proposed BMPs for Lot 31 is included in Attachment A.

Sheraton Hotel Complex, Calverton Site

The second site is part of a larger Sheraton Hotel Complex, which consists of

several large buildings and parking lots. The site of redevelopment is a large

parking lot at the entrance to the complex off of Powder Mill Road in Calverton,

Maryland. The site is approximately 5 acres, generally level, and the existing land

cover is an impervious asphalt parking lot with sections of lawn scattered

throughout.

The BMPs considered for this site include vegetated bioretention swales, rain

gardens, and porous asphalt parking areas and streets with a sub-surface

infiltration bed. A detailed discussion of the proposed BMPs for the Calverton Site

is included in Attachment B.


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Hilton Hotel, Baltimore Site

The last site is in an ultra-urban area in downtown Baltimore. This site has been

recently redeveloped as a Hilton Hotel, and the development plans were used

for this exercise. The redevelopment plan, which was treated as proposed,

rather than built, includes two large buildings, paved walkways, and a few lawn

spaces on the site. The site is almost completely impervious and very dense.

Parking for the hotel is in an underground structure beneath the building.

The BMPs considered for this site include an extensive vegetated roof,

bioretention swales, rain gardens, and a sidewalk constructed of porous

concrete or pavers and underlain with a stormwater storage bed. A detailed

discussion of the proposed BMPs for the Baltimore Site is included in Attachment

C.

Process of Applying ESD

For each of the three projects, the following process was applied to develop

Stormwater Concept Plans for ESD, and to estimate the size and cost of these

BMPs:

1. For each site, a variety of the appropriate BMPs were identified based onthe project type and proposed site layout. For example, roof areas might

include vegetated roof systems, and lawn areas could include rain

gardens.

2. The size/area of each potential BMP and the drainage area to each BMPwere estimated, and based on these estimates, the most cost-effectiveBMPs to address the 1-inch runoff capture were identified. The detailed

values for each project site, by BMP, are included in the documentation

for the project.

3. The size or storage capacity of the selected BMPs was increased tocapture more runoff, namely the increased runoff volume for the 1-year,

2.6-inch storm event (the so-called channel protection volume or

CPv).

Description of Hydrologic Methods and Assumptions

For ESD, stormwater management must consider the volume of runoff and not

just the rate of runoff. Traditionally, stormwater designers have focused on flow

rate (usually in cubic feet per second) as stormwater management has

traditionally been concerned with flooding. However, in most stormwater

calculation methodologies and computer modeling tools, flow rates are


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developed as a function of total runoff volume, which is first estimated based on

rainfall, area, and land use factors.

For the purposes of implementing ESD, the stormwater focus is initially placed on

stormwater volume (in cubic feet). This allows the designer to estimate the

stormwater capacity of various stormwater measures throughout the project

site. The volume capacity of the BMPs can be increased or decreased as

needed to meet various design criteria.

The State of Maryland Unified Stormwater Sizing Criteria provides a methodology

to estimate several important volume standards, including:

Water Quality Volume (WQv) This is essentially the runoff volume from

impervious areas for the 1 (eastern part of state) or 0.9 (western portion)

of the state.

Recharge Volume (Rev) This is the estimated recharge volume required

under Maryland criteria.

Both the Water Quality Volume and Recharge Volume are estimated based on

the impervious area of a site, and are inclusive (not cumulative).

Channel Protection Volume (Cpv) The Channel Protection Volume is the

required volume for extended detention of the one-year, 24-hour storm

event. For this evaluation, Cpv has been estimated as the difference in

runoff volume between the site under wooded conditions and asdeveloped (estimated using the NRCS Cover Complex Method).

For each of the three sites, these criteria have been calculated and are

included in Attachment A. These volumes form the basis of estimating the size

and location of the recommended BMPs.

One of the challenges faced by stormwater designers when applying ESD is the

need to manage stormwater along the way with various BMPs, and to

estimate the required capacity of these BMPs. In order to do this, it is important

to know the amount of runoff generated by different areas or features of the

project so that the appropriate BMP can be sized for that area.

The Small Storm Hydrology Method (Integration of Water Quality and Drainage

Design Objectives, R. Pitt, 2003) provides a tool for estimating runoff volumes

from small, developed areas based on rainfall and land use. For small rainfall

amounts and in urban settings, this methodology provides a more reliable

estimate of volume than many other techniques. It also includes consideration


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of pervious areas, such as lawn, and the runoff volume contribution from the

pervious area.

For each of the project sites, the volume of runoff for the !-inch, 1-inch, 1.5-inch,

and 2.6-inch rainfall events has been calculated using the Small Storm

Hydrology method. As can be seen from Table A3, B3, and C3, the advantage

of this method is that it provides the designer with a volume estimate for specific

areas of the site (e.g., the roof, or the sidewalk, or a lawn). The stormwater

designer can identify areas of the site that generate significant runoff volumes,

and incorporate BMPs that are appropriately sized near the source of runoff.

These volume estimates can then be used to estimate the required capacity of

BMPs through the project.

Description of Cost Estimation Methods and Assumptions

One of the key components of ESD is that stormwater design is integrated intothe landscape and the built elements of a project. For example, mulched

landscape beds can be converted to rain gardens, or paved areas can include

underlying stormwater storage/infiltration beds in lieu of conventional sub-base

material. Estimating the cost of the BMPs can be difficult if the entire

construction cost is included for elements (such as sidewalks) that are to be

constructed regardless of stormwater design.

For the purposes of this analysis, the cost estimates have been developed to

reflect the additional cost or premium involved in implementing a BMP, above

the cost of constructing the basic site improvement.

For example, the cost of a green roof is estimated as the additional cost

required to add an extensive green roof system to a proposed conventional

roof. For rain gardens and bioretention swales, the cost represents an additional

premium for fine grading, geotextile, soil amendments, and small stormwater

structures. Similarly, for a porous concrete sidewalk, the additional cost in

geotextiles, excavation, open-graded stone, and pavement material is

estimated.

For the conventional cost estimate, cost values are based on a conveyance

system of storm sewers and inlets that can convey runoff to either a subsurfacetank or surface detention facility (if space is available) capable of providing

detention to meet the Channel Protection Volume.

Additional costs that have not been quantified include maintenance. All

stormwater systems require maintenance, and it is assumed that maintenance is

required for both conventional and ESD stormwater designs. Much of the


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maintenance for landscape-based BMPs can be done by landscape crews if

the crews are provided with the proper information and training. This

maintenance can be incorporated into standard site maintenance practices.

An additional benefit of ESD includes a potential land area savings for sites if a

large detention basin can be eliminated or reduced. This area may then be

used for additional building space, open space, or other uses, but the

economic benefit of saved land has not been included in this evaluation.

Design Considerations for ESD in Urban and Suburban Areas

Incorporating ESD into the urban area has challenges that cannot be

overlooked, but at the same time, these challenges should not become barriers

to finding the appropriate techniques and variations for each site. In this study,

at attempt was made to design techniques that could physically fit onto the

site, and which were at least comparable in cost to conventional stormwatermanagement practices.

Urban and suburban redevelopment projects often include buried utilities,

contaminated soils, and limited project space. Consideration must be given to

buildings with basements and subsurface parking, and grading and

waterproofing measures incorporated as needed. Again, if the stormwater

design is addressed early in the project design process, the opportunities to

place the right BMPs in the best locations can be found.

Note on Green Roofs

Within the toolbox of urban stormwater solutions, green roofs form their own

separate category. While their costs are often significantly higher than most

other LID practices, such as rain gardens, their benefits are also higher: they

provide direct energy savings, lowering both heating and cooling costs; and

they also extend the life of commercial and institutional roofs, sometimes

doubling the roofs lifespan. For these reasons as well as their water quality

benefits, green roofs are increasingly popular choices for urban redevelopment

projects, and the Washington, D.C. region is now second only to Chicago in its

total square feet of green roof space.

For this concept-level study, we included a green roof in one of our modeling

runs for Site C, the Hilton Hotel in Baltimore. We estimated that choosing an

extensive green roof, sized to cover Building 1, could roughly double the

projects stormwater costs. (We also note that this particular project in reality

did incorporate a green roof.) For the purposes of our conceptual study, that is

seeking least-cost stormwater redevelopment solutions, we chose not to


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include a green roof in our final set of practices for Site C. At the same time, we

encourage builders and developers to continue to seriously consider, and when

feasible, to choose green roofs for all of the benefits that they provide to a

project.


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ATTACHMENT A

LOT 31

BETHESDA, MD


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A 1

Environmental Site Design Concept

SITE A. LOT 31

Bethesda, Maryland

Existing Conditions

The existing site is a conventional asphalt parking lot, referred to as Lot 31,

and is located at the intersection of Woodmont Avenue and Bethesda

Avenue in Bethesda, MD. The existing parking lot is a little over 2 acres in

size and is scheduled for redevelopment.

Proposed Conditions

Lot 31 is slated to become a mixed-use site with a large building that will

contain both residential and retail units. Parking will be in an underground

structure beneath the building.

Conventional Stormwater Design

A conventional stormwater design for this site would include roof leaders

from the building roof that drain to stormwater pipes that ultimately drain

to a subsurface storage/detention structure. Catch basins would collect

surface drainage and also convey this runoff to the detention structure.

This structure would simply hold stormwater and slowly release it. The

structure has been sized to hold both the 1-inch Water Quality volume

(WQv), as well as the net increase in volume of the 1-year, 2.6-inch storm

Channel Protection Volume (CPv) for extended detention. For water

quality improvement, it is assumed that a manufactured water quality unit

will treat low flows.

Sustainable Stormwater Design

Although this site is small in size and is designed with the typical high-

density of an urban location, there are many opportunities to manage

POTENTIAL STORMWATER BMPs

Green Roof Rain Gardens Bioretention Swales Check Dams Porous Pavement Sub-surface Storage Bed Tree Pits


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stormwater sustainably. A visual overview for Lot 31 that indicates

potential stormwater best management practices (BMPs) considered is

presented as Figure A1. A second concept plan showing the BMPs

selected for this site is displayed in Figure A2. These BMPs are by no means

inclusive of all opportunities, but represent a list of varied options that were

evaluated. The following BMPs were considered for Lot 31:

An extensive Green Roof system could be used on the proposedbuilding. A vegetated roof with a depth of 3 inches would capture

over half of the annual rainfall that lands on it, but in large storms

would overflow. Also, not every portion of the roof would include a

vegetated roof. However, the overflow drainage from the Green Roof

could be directed toward the additional BMPs. Green roofs provide

additional benefits in terms of energy savings and longevity of roof

material that are not considered here.

Rain Gardens and Bioretention Swales could be created in areas thatotherwise would be lawn or conventional landscaping. These shallow

Bioswales could be constructed to capture and slowly convey runoff

through a system of vegetation and soils. Rain Gardens and Bioswales

provide volume reduction and water quality improvement by both

storage of water within soils, and storage/filtration of runoff in shallow

graded areas that are vegetated.

A Rain Garden and a Bioretention Swale could be used to managethe stormwater runoff from the lawn and sidewalk areas along thesouthern part of the building, as well as the small circle entrance. All of

these areas should be graded towards the swale system. Storage

capacity in the Bioretention Swale and the Rain Garden is provided by

both a shallow surface volume and water retention within the planting

media. Also, both BMPs should be well planted to allow for volume

reduction through evapotranspiration. Bioretention Swale 1 should be

lined and underdrained since it will be located above the sub-surface

parking structure. Check Dams constructed in the swale will reduce the

rate of surface flow as it is conveyed towards the southwestern corner

of the site. This system will ultimately overflow via a connection to the

existing storm sewer system under the Crescent Trail Bike Path.

Runoff from the eastern and northern parking area and sidewalk couldbe directed to a second smaller Bioretention Swale along the northern

part of the site. The sidewalk could be constructed from Porous Pavers

or Porous Concrete that will allow stormwater to drain to a one-foot

deep open-graded stone storage bed beneath the sidewalk. This bed


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should be lined as it is located above the sub-surface parking structure.

The storage bed beneath the sidewalks could overflow to Bioretention

Swale 2. A control structure should be utilized to manage peak rates

from the stone bed so that runoff slowly drains from the bed to the

swale. This swale will overflow via a connection to the existing storm

sewer system in the street.

It would be beneficial to incorporate tree pits or trenches in thesidewalk area that could use stormwater from the stone storage bed.

The BMP options and capacity are likely to exceed the required

stormwater needs, so the next step is for the designer to evaluate the

BMPs that best fit into the project and provide the most cost effective

options.

Stormwater Runoff Calculations

The estimated Water Quality Volume (WQv) and Recharge Volume (Rev)

for Lot 31 under the proposed development plan are calculated in Table

A1. The Channel Protection Volume is calculated in Table A2, and is

estimated as the net increase in runoff volume for the 1-year, 2.6-inch

rainfall, with the before conditions estimated as woods.


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A 4

The next step is to estimate how much runoff different areas of the site

will produce. As discussed earlier, runoff volumes are estimated for the!-

inch, 1-inch, 1.5-inch, and 2-year storm events as shown in Table A3 using

the Small Storm Hydrology Method. This table also indicates estimated

peak flow rates for these areas (using the Rational Method). Although

flow rates were not used in this analysis, for actual design and

construction, the stormwater designer will need to estimate flow rates for

safe conveyance and capacity to prevent localized flooding.


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A 5

After estimating the regulatory requirements for stormwater (Tables A1

and A2), and the amount of runoff that the different areas of the site will

generate (Table A3), the next step is to estimate the potential stormwater

capacity of the BMPs under consideration. This is shown in Table A4.

As can be seen by comparing the potential BMP capacity in Table A4 tothe regulatory requirements, not all BMPs are needed, or needed at full

capacity. For example, for the WQv, Bioretention Swale 1 does not need

to be as large as the site has potential for it to be. At this point, the

stormwater designer can make decisions about which BMPs to choose

and what size those BMPs should be. For Site A, the selected BMPs and

required size for WQv are shown in Table A5.


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Similarly, the selected BMPs can be increased in area or design capacity,

or additional BMPs added to meet the CPv. For Site A, this is shown in

Table A6.


For the conventional stormwater design, it is assumed that stormwater

inlets and storm sewers will be required to convey the runoff to a

stormwater storage/detention system capable of providing stormwater

management for either the first 1-inch of stormwater runoff or the Channel

Protection Volume (CPv). CPv is estimated as the net increase in volumefor the 1-year, 2.6-inch storm event as compared to natural wooded

conditions (from Table A2). Since available surface area is limited at this

site and largely occupied by building program, it is assumed that a

subsurface stormwater detention structure, located below grade, will be

required. The amount of storm sewer pipe, inlets, and other structures has

been estimated based on the site layout, size, and proposed


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development plan. A structural unit for water quality treatment of small

storms has also been included.

Cost Comparison

For ESD, stormwater measures are incorporated into the built landscape

and building program. For example, a landscape area can be

constructed as a bioretention area or rain garden. A vegetated roof can

be added to a conventional roof. For the purposes of this analysis, the

unit cost per BMP type represents the difference between the cost of the

conventional structure and the cost of the recommended BMP. For

example, a conventional sidewalk with a compacted sub-base would

require excavation, stone subbase, concrete, etc. Building that same

sidewalk as a porous concrete sidewalk would require an increase in

concrete cost (for porous concrete or pavers), an increase in subbasematerial (to use clean open-graded stone aggregate), an increase in

excavation for a slightly deeper bed, geotextile, etc. Therefore a

premium cost is added to the porous concrete sidewalk (on a square foot

basis). Similarly, landscaping, mulch, topsoil, etc. are required for

conventional landscape areas as well as rain gardens. Therefore, the unit

cost for rain gardens represents the additional costs for grading, soil

amendments, etc. that a rain garden might require. The costs are based

on contractor bid estimates for actual ESD designs constructed in 2008,

supplemented by information from R.S. Means Cost values for 2008. The

estimated additional costs for the ESD for Lot 31 are provided in Table A7.


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A 8

Because stormwater volumes and flow rates are reduced by the various

BMPs, the size, depth, and amount of storm sewers and inlets can be

reduced, and the cost of the reduced structures is included.

Similarly, a conventional stormwater system will require larger and deeper

storm sewers (to convey runoff to one location), as well as a detention

structure and water quality unit. These costs are estimated for the

conventional stormwater design based on estimated sizes and lengths for

the proposed development plan. The estimated costs for a conventional

stormwater system are provided in Table A8.


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The difference between the BMP construction costs and the conventional

construction costs represents the comparison between the two designs.That is, the net increase or decrease in costs for an ESD approach, as

compared to a conventional stormwater management approach, are

presented in Table A9 for Site A. As can be seen from this table, and ESD

approach is slightly less expensive than a conventional stormwater design

for Lot 31. This is largely due to the selection of landscaped based BMPs

applied in this case study (e.g., bioretention swales). Other stormwater

designs, such as a vegetated roof, would increase initial construction

costs but provide significant costs savings in energy and the expected life

of the roof.


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ATTACHMENT B

SHERATON HOTEL COMPLEX

CALVERTON, MD


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B 1


SITE B. SHERATON HOTEL COMPLEX

Calverton, Maryland

Existing ConditionsThis area to be hypothetically redeveloped is part of a larger Sheraton

Hotel site consisting of several large buildings and parking lots. The site of

redevelopment is a large parking lot at the entrance to the complex off of

Powder Mill Road in Calverton, Maryland. The site is approximately 5

acres, generally level, and the existing landcover is an impervious asphalt

parking lot with sections of lawn scattered throughout.

Proposed Conditions

As there was no official redevelopment plan to follow, one was

developed for the site. The redevelopment design shows eleven proposedbuildings with a total of 144 dwelling units, of which 80 are Townhouses

and 64 are Garden Apartments. This yields a density of approximately 27

units per acre. Lawn areas cover about 1.8 acres of the site, and 2.2 acres

are paved roads and parking areas.


The main conventional stormwater management technique for this site

would be to build a large detention basin that would hold stormwater and

slowly release it into the sewer system. The detention basin would be

located in the largest lawn area, which is in the southeastern portion of

the site. The basin has been sized for two scenarios, to capture the 1-inch

Water Quality volume (WQv), as well as the net increase in volume of the

1-year, 2.6-inch storm (CPv) for extended detention. Runoff would be

conveyed to the basin through stormwater roof leaders from the buildings,

area drains, and surface grading.


Rain Gardens Bioretention Swales Check Dams Porous Pavement Sub-surface Infiltration Bed


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B 2




stormwater sustainably. A visual overview of the site that indicates

selected stormwater best management practices (BMPs) is presented asFigure B1. These BMPs are by no means inclusive of all opportunities, but

represent a list of varied options that were evaluated. The following BMPs

were considered for Sheraton Complex:

Rain Gardens and Bioretention swales could be created in areasthat otherwise would be lawn or conventional landscaping. These

shallow Bioswales could be constructed to capture and slowly

convey runoff through a system of vegetation and soils. Rain

Gardens and Bioswales provide volume reduction and water

quality improvement by both storage of water within soils, andstorage/filtration of runoff in shallow graded areas that are

vegetated. These vegetated stormwater systems could be allow

infiltration

Bioretention swales could be constructed along much of theperimeter of the site. These swales would have the ability to capture

most of the surface runoff from the 1-inch storm and a large portion

of the CPv volume. Check Dams should be constructed in the swale

to reduce the rate of surface flow as it is conveyed toward the

outlet.

Rain Gardens could be constructed near the southern entrance tothe site and in the large lawn area. These Rain Gardens provide

stormwater capture capacity in surface storage and storage in the

planting media. These Rain Gardens also provide an aesthetic

landscape feature.

In order to capture the net increase in volume of the 1-year, 2.6-inch storm Channel Protection Volume (CPv), porous asphalt

pavement with a sub-surface infiltration bed could be used for all

parking areas and drive aisles. This stormwater managementfeature could capture the majority of the CPv. The sub-surface

infiltration bed could provide a significant volume of water for

groundwater recharge.


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B 3




options.



for the Sheraton Complex under the proposed development plan are

calculated in Table B1. The Channel Protection Volume is calculated in

Table B2, and is estimated as the net increase in runoff volume for the 1-

year, 2.6-inch rainfall, with the before conditions estimated as woods.


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B 4



inch, 1-inch, 1.5-inch, and 2-year storm events as shown in Table B3 using







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B 5

After estimating the regulatory requirements for stormwater (Tables B1 and

B2), and the amount of runoff that the different areas of the site will

generate (Table B3), the next step is to make decisions about which BMPs

to choose and what size those BMPs should be. For the Calverton Site, the

selected BMPs and required size for WQv are shown in Table B4.


or additional BMPs added to meet the CPv. For the Calverton Site, this is

shown in Table B5.


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B 6




stormwater detention basin capable of providing stormwatermanagement for either the first 1-inch of stormwater runoff or the Channel

Protection Volume (CPv). CPv is estimated as the net increase in volume

for the 1-year, 2.6-inch storm event as compared to natural wooded

conditions (from Table B2). Since there is available surface area at this site

in the large lawn area to the southeast, conventional design would

include a detention basin in this area. The amount of storm sewer pipe,

inlets, and other structures has been estimated based on the site layout,

size, and proposed development plan.

Cost Comparison



constructed as a bioretention area or rain garden. For the purposes of this

analysis, the unit cost per BMP type represents the difference between the

cost of the conventional structure and the cost of the recommended

BMP. For example, a conventional parking lot or street with a compacted

sub-base would require excavation, stone subbase, concrete, etc.

Building that same parking lot or street as a porous asphalt would require

an increase cost for porous asphalt, an increase in subbase material (to

use clean open-graded stone aggregate), an increase in excavation fora slightly deeper bed, geotextile, etc. Therefore a premium cost is added

to the porous pavement (on a square foot basis). Similarly, landscaping,

mulch, topsoil, etc. are required for conventional landscape areas as well

as rain gardens. Therefore, the unit cost for rain gardens represents the

additional costs for grading, soil amendments, etc. that a rain garden

might require. The costs are based on contractor bid estimates for actual


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B 7

ESD designs constructed in 2008, supplemented by information from R.S.

Means Cost values for 2008. The estimated additional costs for the ESD for

the Calverton Site are provided in Table B6.






structure. These costs are estimated for the conventional stormwater

design based on estimated sizes and lengths for the proposed

development plan. The estimated cost for a conventional stormwater

system is provided in Table B7.


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B 8


construction costs represents the comparison between the two designs.

That is, the net increase or decrease in costs for an ESD approach, as

compared to a conventional stormwater management approach, are

presented in Table B8 for the Calverton Site. As can be seen from this

table, and ESD approach is slightly less expensive than a conventional

stormwater design when ESD is used to capture the WQv, but more

expensive when ESD is used to capture the CPv for the Sheraton Hotel

Complex. This is largely due to the selection of landscaped based BMPs inthe first case and the application of porous pavement to a very large

surface area. If desired, the designer could reduce the area of porous

pavement and increase the bed depth for reduced costs. Additionally,

for the ESD design, all of the stormwater measures are incorporated into

the landscape or built area. In contrast, the conventional stormwater

system requires a separate area for the construction of a detention


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B 9

facility. This is area that could be used for other purposes (additional

buildings, recreation, etc.). The value of the land required for a

detention basin has not been included in the analysis, but represents an

additional cost incurred by the project for conventional stormwater

management.


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13

ATTACHMENT C

HILTON HOTEL

BALTIMORE, MD


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C 1


SITE C. HILTON HOTEL

Baltimore, Maryland

Existing Conditions

This ultra-urban redevelopment site is in the city of Baltimore, neighboring

Camden Yards. This site has been redeveloped as a Hilton Hotel, and the

development plans were used for this exercise. There are two large

buildings on the approximately 6-acre site surrounded by paved walkways

and a few lawn spaces. The site is almost completely impervious and very

dense. Parking for the hotel is in an underground structure beneath the

building.

Proposed ConditionsThe redevelopment has already occurred at this site. However, for the

purposes of this exercise, it has been treated as though redevelopment

was still in the design phase.


A conventional stormwater design for this site would include roof leaders

from the building roof that drain to stormwater pipes that ultimately drain

to a subsurface storage/detention structure. Catch basins would collect

surface drainage and also convey this runoff to the detention structure.

This structure would simply hold stormwater and slowly release it without

providing any water quality improvement or groundwater recharge. The

structure has been sized to hold both the 1-inch Water Quality volume

(WQv), as well as the net increase in volume of the 1-year, 2.6-inch storm

Channel Protection Volume (CPv) for extended detention.





Green Roof Rain Gardens Bioretention Swales Porous Pavement Sub-surface Storage Bed Tree Trenches


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C 2

stormwater sustainably. A visual overview for the Hilton Hotel that

indicates potential stormwater best management practices (BMPs)

considered is presented as Figure C1. A second concept plan showing

the BMPs selected for this site is displayed in Figure C2. These BMPs are by

no means inclusive of all opportunities, but represent a list of variedoptions that were evaluated. The following BMPs were considered for the

Baltimore Site:

An extensive Green Roof system could be used on the proposedbuildings. Vegetated roofs with a depth of 3 inches would capture

over half on the annual rainfall that lands on them, but in large storms

would overflow. Also, not all of the area of the roofs would be covered

with a vegetated roof. However, the overflow drainage from the

Green Roofs could be directed toward the additional BMPs.

Another stormwater management option would be to use continuousTree Trenches along portions of the site boundary. These features

provide an aesthetic technique for capturing some of the stormwater

runoff volume.

Rain Gardens could be created in areas that otherwise would be lawnor conventional landscaping. These shallow vegetated systems could

be constructed to capture and slowly convey runoff through a system

of vegetation and soils. Rain Gardens provide volume reduction and

water quality improvement by both storage of water within soils, and

storage/filtration of runoff in shallow graded areas that are vegetated.

Sidewalks throughout the site could be constructed of porous concreteor pavers with a sub-surface storage beds. These beds may require a

liner on the sides adjacent to the building and sub-surface parking

structure. These storage beds could overflow to one of the vegetated

stormwater management features. A control structure should be

utilized to manage peak rates from the stone bed so that runoff slowly

drains from the bed to the vegetated structure. The ultimate overflow

will be through a connection to the existing storm sewer system in the

street.

On highly urban sites, coordination with existing and new utilities will be

required. This information was not available for this analysis, but in actual

design would be required.


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C 3




options.



for the Hilton Hotel under the proposed development plan are calculated

in Table C1. The Channel Protection Volume is calculated in Table C2,

and is estimated as the net increase in runoff volume for the 1-year, 2.6-

inch rainfall, with the before conditions estimated as woods.


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C 4



inch, 1-inch, 1.5-inch, and 2-year storm events as shown in Table C3 using







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C 5

After estimating the regulatory requirements for stormwater (Tables C1

and C2), and the amount of runoff that the different areas of the site will

generate (Table C3), the next step is to estimate the potential stormwater

capacity of the BMPs under consideration. This is shown in Table C4.

As can be seen by comparing the potential BMP capacity in Table C4 to

the regulatory requirements, not all BMPs are needed, or needed at full

capacity. For example, for the WQv, Bioretention Swale 1 does not need

to be as large as the site has potential for it to be. At this point, the


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C 6

stormwater designer can make decisions about which BMPs to choose

and what size those BMPs should be. For the Baltimore, the selected BMPs

and required size for WQv are shown in Table C5.


or additional BMPs added to meet the CPv. For the Baltimore Site, this is

shown in Table C6.




stormwater storage/detention system capable of providing stormwater

management for either the first 1-inch of stormwater runoff or the Channel

Protection Volume (CPv). CPv is estimated as the net increase in volumefor the 1-year, 2.6-inch storm event as compared to natural wooded

conditions (from Table C2). Since available surface area is limited at this

site and largely occupied by building program, it is assumed that a

subsurface stormwater detention structure, located below grade, will be

required. The amount of storm sewer pipe, inlets, and other structures has

been estimated based on the site layout, size, and proposed


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C 7

development plan. A water quality unit (such as a Vortex Unit to reduce

sediment and pollutant loads) has also been included.

Cost Comparison



constructed as a bioretention area or rain garden. A vegetated roof can

be added to a conventional roof. For the purposes of this analysis, the

unit cost per BMP type represents the difference between the cost of the

conventional structure and the cost of the recommended BMP. For

example, a conventional sidewalk with a compacted sub-base would

require excavation, stone subbase, concrete, etc. Building that same

sidewalk as a porous concrete sidewalk would require an increase in

concrete cost (for porous concrete or pavers), an increase in subbasematerial (to use clean open-graded stone aggregate), an increase in

excavation for a slightly deeper bed, geotextile, etc. Therefore a

premium cost is added to the porous concrete sidewalk (on a square foot

basis). Similarly, landscaping, mulch, topsoil, etc. are required for

conventional landscape areas as well as rain gardens. Therefore, the unit

cost for rain gardens represents the additional costs for grading, plants,

etc. that a rain garden might require. The costs are based on contractor

bid estimates for actual ESD designs constructed in 2008, supplemented

by information from R.S. Means Cost values for 2008. The estimated

additional costs for the ESD for the Baltimore Site are provided in Table C7.


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C 8






structure. These costs are estimated for the conventional stormwater

design based on estimated sizes and lengths for the proposed

development plan. The estimated cost for a conventional stormwater

system is provided in Table C8.


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C 9


construction costs represents the comparison between the two designs.

That is, the net increase or decrease in costs for an ESD approach, as

compared to a conventional stormwater management approach, arepresented in Table C9 for the Baltimore Site. As can be seen from this

table, and ESD approach is slightly less expensive than a conventional

stormwater design for the Hilton Hotel. This is largely due to the selection

of landscaped based BMPs applied in this case study (e.g., bioretention

swales). Other stormwater designs, such as a vegetated roof, would

increase initial construction costs but provide significant costs savings in

energy and the expected life of the roof.


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Maryland; Environmental Site Design for Stormwater Management - Chesapeake Bay Foundation

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Transcript of Maryland; Environmental Site Design for Stormwater Management - Chesapeake Bay Foundation