North Huntsville Industrial Park Conservation Design...

North Huntsville Industrial Park

Conservation Design Demonstration Project

Final Report To Huntsville, AL Planning Department

Prepared by CEDARS, Inc. (http://www.cedarsinc.org)

January 2006


Contents

Background ........................................................................................................................1

Rationale ..........................................................................................................................1

Goals.................................................................................................................................1

Site Location and Characteristics..................................................................................2

Geology.............................................................................................................................5

Participants......................................................................................................................6

Approach ............................................................................................................................7

Work Plan........................................................................................................................7

Guidance from Rapid Design Charettes .......................................................................9

Stormwater Management Conservation Design Summary .........................................11

Site Controls ..................................................................................................................12

Drainage Way and Pond Stormwater Controls .........................................................13

Conservation Design Master Plan ..................................................................................16

Cost Comparison: Base Case vs. Conservation Design ................................................18

Suggested Covenants .......................................................................................................18

Conclusion ........................................................................................................................18

Appendix A: TVA Sustainable Development Guide (http://www.tvaed.com/sust_dev.htm)

Appendix B: Detailed Stormwater Water Management Conservation Design (http://www._____)

Appendix C: Soils Investigation (http://www.______)

Appendix D: WinSLAMM and Low Impact Development (http://www.______)

Page i

Contents (Report also available at http://www.cedarsinc.org/RefFiles/FinalReport010506.pdf ) Background........................................................................................................................ 1

Rationale.......................................................................................................................... 1

Goals ................................................................................................................................ 1

Site Location and Characteristics ................................................................................. 2

Geology ............................................................................................................................ 5

Participants ..................................................................................................................... 6

Approach............................................................................................................................ 7

Work Plan ....................................................................................................................... 7

Guidance from Rapid Design Charettes ...................................................................... 9

Stormwater Management Conservation Design Summary......................................... 10

Site Controls.................................................................................................................. 11

Conclusions from the Soil Investigations ................................................................... 12

Drainage Way and Pond Stormwater Controls............................................................ 13

Conservation Design Master Plan.................................................................................. 17

Suggested Covenants....................................................................................................... 17

Cost Comparison: Base Case vs. Conservation Design................................................ 20 Appendix A: TVA Sustainable Development Guide

(http://www.tvaed.com/sustainable/) Appendix B: Detailed Stormwater Water Management Conservation Design

(http://www.cedarsinc.org/RefFiles/AppendixB.pdf) Appendix C: Soils Investigation

(http://www.cedarsinc.org/RefFiles/AppendixC.pdf) Appendix D: WinSLAMM and Low Impact Development (http://www.cedarsinc.org/RefFiles/AppendixD.pdf) Appendix E: Runoff and Sediment Reductions at North Huntsville Industrial Park

(http://www.cedarsinc.org/RefFiles/AppendixE.pdf) Appendix F: Liner Stability in Grass Swales

(http://www.cedarsinc.org/RefFiles/AppendixF.pdf)


Background Rationale The City of Huntsville has become a leader in environmentally sensitive development over the past several years, partially due to growth pressures on its natural resources. They continue to revitalize their downtown and pursue new ordinances and innovative approaches for quality growth to conserve water and other natural resources. Aquifers are a major concern since much of Huntsville and the surrounding communities use wells for potable water supply. The aquifers in North Huntsville are of particular interest due to the karst geology and potential recharge of the Big Spring which surfaces in downtown Huntsville. Based on the need to provide additional industrial properties and the development of the new Toyota plant on Pulaski Pike in North Huntsville, the City acquired several hundred acres of property adjacent to and north of the automotive plant. The property is currently in rolling hill pasture and row crop production. To help maintain the character of this area and protect the sinkhole features, the City opted to develop an industrial park that retains about 50 percent of the total area in a working farm. It became evident that developing the park through a conservation design approach could meet the immediate needs for industrial clients while protecting the sensitive source water area. In March 2005, the City contacted TVA Economic Development’s Technical Services group and the Center for Economic Development and Resource Stewardship (CEDARS) to assemble a team of consultants to prepare a conservation design for the first phases of a 250-acre park to meet their overall needs.

Goals 1. Minimize the potential impacts of

development on the ground water and surface water resources associated with the site, including the numerous sinkholes.

2. Maintain the rural character along Pulaski Pike and Liberty Hill Road by retaining about 50 percent of the farm.

3. Create an industrial park with approximately 52 quality sites, which would appeal to smaller support industries for the existing Toyota plant or that could be combined into larger sites for other industrial prospects.

4. Integrate opportunities for Earth Scope and other environmental education programs as well as provide recreational and scenic values for the industries and their employees.

5. Provide a demonstration for the region that illustrates the benefits of a conservation design approach using procedures outlined in the TVA Sustainable Development Guide coupled with an assessment model developed by Dr. Robert Pitt, PE from the University of Alabama.

6. Illustrate the ability of a design charette with various City Departments and the City’s engineering consulting firm (Garver Engineers) to rapidly integrate principles and practices of conservation design into the tight engineering design and build schedule for the project.


Site Location and Characteristics

The North Huntsville Industrial Park is located in northwest Huntsville near the intersection of Pulaski Pike and Bob Wade Lane (Figure 1).

The North Huntsville Industrial Park Conservation Design Demonstration Project is immediately north of the Toyota plant in the 250 acre area outlined in red (Figure 2). Pulaski Pike borders on the west and Liberty Hill Road borders on the north. The area being developed is a working farm with a combination of pasture and cultivated land on karst topography (Figures 3-8). The surface water runoff drains into tributaries of the Flint River and Indian Creek watersheds, but the groundwater sink holes are thought to flow south through an anquifer connected to Big Spring in downtown Huntsville.

Figure 1. Industrial park location.

Figure 2. Demonstration site in red.


Figure 3. Demonstration site is currently a working farm comprised of rolling and well-drained cultivated and pasture land on karst topography.

Figure 4. One of several sink holes illustrating the karst topography on the demonstration site.

Figure 5. Farmstead area that will be used for educational purposes.


Figure 6. Grass swale that will be maintained by the farm operator and function to infiltrate runoff water from industrial lots. A series of limestone check dams will be installed to provide retention and increase infiltration. Limestone check dams will help “buffer” the pH.

Figure 7. Area near planned entrance to demonstration area off Liberty Hill Road.

Figure 8. Existing pond on the site will be retained. The constructed wet ponds as shown on the Conservation Design Plan (Figure 15) will look similar in the forebay area which will transition into deeper water at the dams.


Geology The project site lies within the interior lowlands physiographic province. The bedrock formation underlying the site is known as the Tuscumbia Limestone. In Madison County, the Tuscumbia Limestone is 150 to 200 feet thick and is composed of gray, fossiliferous limestone with small amounts of gray to white chert. The Tuscumbia is Mississippian in age. Madison County lies on the southern flank of the Nashville dome. The stratigraphic layers, including the Tuscumbia Limestone, are tilted down toward the south, at a dip of 20 to 40 feet per mile.

The surface topography of this site displays abundant sinkholes, indicative of a karst landform. The soils are well drained. The site was observed following a 2 inch rainfall in December of 2004, and no significant surface water runoff occurred. Virtually 100 percent of the rainwater from this rainfall event infiltrated into the groundwater, onsite. These and other observations, indicate that the project site and the general vicinity around it is a ground-water recharge area.

Based on surface topography, about 70 percent of the site lies within the Indian Creek watershed, and 30 percent is in the Flint River watershed. It is well documented in Madison County that the Tuscumbia Limestone is a major acquifer with groundwater migrating generally from north to south, following the regional dip of the strata. Maps of depth to groundwater (potentiometric maps) suggest a southerly groundwater flow from the project site, crossing parts of the Indian Creek and Flint River watersheds, and into the Huntsville Spring Branch watershed. Indeed, the steepest gradient for subsurface flow initiating from

the project site is to the south-southeast into a structural trough that trends through the heart of Huntsville. Big Spring and Braham Spring are located along the axis of this trough. This appears to be a unique site because it contains the drainage divide between two different watersheds based on surface topography, yet its groundwater may recharge an aquifer in a different watershed.

Conventional engineering design based on typical stormwater controls would have directed most of the surface water into the Dry Creek watershed. Using the conservation design stormwater management practices, the runoff leaving the site is reduced by 75% or more compared with conventional practices for most rainfall events (rains smaller than about 1-1/4 inch in depth).


Participants

Center for Economic Development and Resource Stewardship (CEDARS)—Nonprofit organization specializing in demonstrating environmentally responsible economic development (http://www.cedarsinc.org); prime contractor for coordinating the conservation design demonstration project.

TVA Economic Development Technical Services—Electric utility economic development group providing in-kind technical services for environmentally responsible economic development (http://www.tvaed.com/).

City of Huntsville—Owner and manager of North Huntsville Industrial Park (http://www.hsvcity.com).

Dr. Robert E. Pitt-- Cudworth Professor of Urban Water Systems, Department of Civil and Environmental Engineering, University of Alabama, Tuscaloosa, Alabama (http://unix.eng.ua.edu/~rpitt/); subcontractor responsible for the stormwater management conservation design and analysis.

KITA Landscape Design—Landscape design firm; subcontractor responsible for developing the landscape conservation design master plan based on stakeholder and design team input.

Garver Engineers—Engineering firm under contract to the city of Huntsville (http://www.garverengineers.com/homepage.htm); responsible for detailed civil engineering design incorporating the stormwater management conservation design and conservation design master plan and overseeing construction.

Southeast Watershed Forum—Nonprofit organization dedicated to enhancing local watershed initiatives through education, training, and regional dialogue (http://southeastwaterforum.org/index.asp) ; subcontractor responsible for assisting with the stakeholder input process.

BR Bock Consulting—Consulting firm providing technical services in waste-to-energy and natural resource management (http://www.brbock.com); subcontractor responsible for preparing the final report.


Approach

Work Plan

The following work plan involving four sessions was developed to guide the conservation design process:

Session One - Preliminary Activities (April 7) A. Assemble Base Data

1. Boundary and topographic base map

2. Natural features delineation

3. Soils information

4. Geological information – i.e., sink holes

5. Groundwater information

6. Off-site surface water hydrology

7. On-site surface water hydrology

B. Site Review (April 7-8)

1. Major and minor drainage basin delineation

2. Vehicular circulation plan

3. Land use plan

• Farm area to be reserved

• Industrial site development area

• Natural areas

4. Planning and design standards

5. Pre-development stormwater volume and quality

6. Establish post-development goals

C. Report to Participants (April 8- April 14)

1. Prepare letter report

2. Distribute to team members

Session Two - Develop and Evaluate Alternative Concept Plans (April 14-15) A. Review Current Site Plan

1. Review range of conservation design practices available

2. Screen least beneficial

3. Select most beneficial array of practices

4. Prepare site concept plan

5. Evaluate performance vs. goals

6. Determine costs

B. Assess Alternatives to Current Plan

1. Review candidate alternative plans

2. Select most preferred alternative plans

3. Identify most beneficial array of practices

4. Prepare alternative concept plan

5. Evaluate performance vs. goals

6. Determine costs

C. Select Final Concept Plan

1. Prepare team recommendations

2. Review with City

3. Prepare report

4. Distribute to team members

5. Seek peer review and comment

6. Select final plan


Session Three - Preliminary Engineering (May 6)

A. Adjust final Concept Plan

1. Consider peer review

2. Refine conservation design practices

3. Refine performance evaluation

4. Refine costs

B. Select design criteria per conservation design practice

C. Specify standard conservation design construction details

D. Develop specifications

E. Review with contractor

F. Prepare report

G. Distribute to team members

Session Four - Coordinate Final Design Approach (May 27) A. Detail review with engineering consultant

B. Develop critical path

C. Develop support procedures by conservation design team for engineering consultants

D. Develop procedure for construction plan review

E. Develop monitoring and assessment plan

F. Seek funding support

Rapid design charette sessions (Figures 9-12) were used to get input from stakeholders and designers. The TVA Sustainable Development Guide (Appendix A) helped establish the design framework.

Figure 10. Preliminary planning in rapid design charettes.

Figure 9. Group inspection of the demon-stration site.

Figure 11. Revising preliminary plans.


Guidance from Rapid Design Charettes

The first charette held on April 7 resulted in the following goals and objectives and parameters that guided the rest of the process:

Goals and Objectives • Minimize erosion

• No Flooding

• Minimal maintenance (i.e., substantially reduced to City)

• Must be aesthetically pleasing and facilitate marketing

• 50 percent industrial and 50 percent farm/open space (look like a park)

• Use site for environmental monitoring and education

• Keep the farm a farm (need to mitigate agricultural runoff)

• Keep farm land contiguous

Constraints on Conditions (“Givens”) • There will be curb and

gutter

• Conventional grass swales are not popular – should not be in view shed of road

• Funding must be spent by May 14, 2006

• Should be able to develop Phase I within one drainage

• No visible concrete driveway pipes

• Use native plants that are compatible to farm

• Home sites and sinkholes are off limits for development

Other Criteria • Site could have different

size lots instead of 2-acre lots

• Phasing in the project will require stormwater controls to manage runoff

• Need to maintain buffers on north side

• Consider formal and non-formal recreation opportunities

Figure 12. Getting additional feedback on preliminary design.


Based on information gathered about the site and the goals and objectives defined in the rapid design charette sessions, the final conservation design will represent a blend of Huntsville’s Chase Industrial Park and the highly manicured Research Park. The park will contain features that compliment the surrounding rural setting while incorporating most of the City’s urban planning design standards. Rapid design charettes on April 7 and April 14-15 resulted in the following general guidance for achieving goals and objectives:

Outdoor Education—The design will incorporate a working farm to preserve approximately 50 percent of the site in green/open space and provide an excellent outdoor education opportunity for the City’s Earth Scope Program. At this demonstration site, students can learn about history, geology, groundwater, source water protection, blending conservation and engineering design, bio-filtration, best practices for agriculture and many other social and ecological elements of sustainable growth.

Site Maintenance—One of the objectives of the City is to reduce overall maintenance costs while maintaining aesthetics and environmental performance. The design team incorporated features that will allow the existing farmer to maintain most of the open area while working the farm. Some maintenance will be needed on steeper slopes, where riprap is used for armoring and along sidewalks if required in the final design. The rapid design team recommends rolled curb to produce a finished edge along all roads and eliminate curb and gutter, but either practice

may be used if construction and extra maintenance costs are not a primary concern for these elements.

Conservation Design Covenants—The team suggested a variety of practices that would be considered as requirements through protective covenants for all lots sold. They would include combinations of bioremediation swales, grassy channels with limestone check dams, critical source area protection measures, and provisions to reduce zinc in the runoff from galvanized metal buildings. These on-site practices will be required on all lots through protective covenants that are described in more detail under the Suggested Covenants section of this report.

The above guidance was used to develop a preliminary stormwater management conservation plan and a preliminary conservation design master plan. These plans were revised based on peer reviews and summaries of these two revised plans are provided in the next two sections of this report.

Stormwater Management Conservation Design Summary

The following is a summary of the stormwater management conservation design prepared by Robert Pitt. A detailed description of the stormwater management conservation design is in Appendix B. Supporting information is also included in Appendixes C through E.


The stormwater management elements of the conservation design are included at several levels. Deed restrictions will require some simple on-site controls, as provided in the Conservation Design Covenants section. The master drainage system will be constructed to encourage grass filter treatment and infiltration, and the main drainage subareas will convey runoff to wet detention ponds. These elements will work together to provide the most cost-effective set of stormwater controls for the site and provide high levels of control for both runoff volume and pollutant discharges. These site stormwater controls are all relatively common and have been applied at many locations throughout the country. They have been designed to take advantage of specific site characteristics and the desire to use this site as a demonstration of effective stormwater controls for the region. The following discussion summarizes these controls, along with their expected levels of benefit.

Site Controls

The site controls include three elements:

1) Critical source areas will need special attention. Industrial stormwater permits usually specify specific activities needing control. At industrial sites, these areas usually include material storage areas and loading bays. Most bulk material storage areas subject to exposure to rainfall should preferably be covered, or the storage areas need to be bermed and the runoff treated with specialized controls (such as the Multi-Chambered Treatment Train). Heavy equipment yards (and public works yards) also need similar attention. Loading bays also need to be

hydraulically isolated with the runoff treated with specialized controls (such as the Upflow Filter).

2) The building materials should be selected with pollution prevention in mind. The most serious problems normally associated with low and medium intensity industrial areas are the zinc concentrations in the runoff associated with the use of galvanized metal. In many areas, galvanized metal has been largely replaced by Zincalume or Galvalume (aluminum and zinc coating), which still have large zinc concentrations in the runoff. There has also been a shift from in-situ application of roofing paints to factory-painted steel products. There have been considerable advances in coating technology, with increased durability and decreased breakdown of roof coatings and materials. The zinc concentrations from galvanized roofs is related to the degree of weathering and corrosion, with heavily weathered and corroded roofs having several times the zinc concentration compared to roofs in good condition. Also, most of the zinc is in the dissolved state which is much harder to control and has more damaging environmental effects.

Unfortunately, zinc in runoff from unpainted Zincalume roofs is still high (close to 500 µg/L), but considerably lower than runoff from unpainted galvanized steel (more than three times higher) or painted galvanized steel in poor condition (more than five times higher). Painted galvanized steel roofs in “excellent” condition have moderately low zinc concentrations (about 100 µg/L) but even a


slight deterioration in paint leads to a dramatic increase in zinc concentration (about ten times higher). There is an extensive range of coatings used on metal roofing materials, including: Polyester, modified polyesters (greater durability and chalking resistance), composites of acrylic and PVF 2 (polyvinylidene fluoride) resins, PVC plastisol, PVF (polyvinyl fluoride), and exterior grade acrylic. If the coatings are kept in good condition, the zinc in the runoff will be significantly reduced. However, there is little information concerning the effective life spans of the coatings under different environmental conditions. Therefore, the preferred approach is to use an alternative building panel that does not contain zinc (or trace metals of concern). If this is not possible, the specialized coatings need to be used, and reapplied as needed. Finally, it may be necessary to treat the runoff from metal roofs with a filtration system (Stormwater Management, Inc. has recently demonstrated the effective use of their cartridge filters in treating roof runoff having high zinc concentrations, for example). Finally, any runoff from metal building areas would need to be directed to surface grass swales having soils amended with organic material (compost or peat) to trap the metals in surface soils before any infiltration.

3) The building areas should have bioretention/grass swales for site runoff control. They will be located on the downslope side of the paved areas and roofs to direct the roof and lot runoff to the drainage systems. The bioretention/grass swales will be relatively small and mild sloped and can be easily maintained. They will be used in conjunction with other drainage way and pond

stormwater controls as summarized below.

Conclusions from the Soil Investigations

Soils were sampled at the sites depicted in Figure 13. The soil data (Appendix C) were used as input in designing the drainage way and stormwater controls. Currently, the site soils are in good to very good condition as far as compaction and organic matter content are concerned. Although the cation exchange capacity is on the low side, it is in the typical range for these types of soils. The current infiltration capacity of the soils is also very good, but the soils are susceptible to compaction and significantly decreased infiltration.

The soil density values for the site soils are generally in a preferred range to minimize effects on plant growth and to maintain high levels of infiltration. Two of the test sites have higher values than elsewhere and are in locations of more extensive agricultural operations (the extensively mowed hay field in the western side and the corn field).

Controlled laboratory tests using silt and silt loam soils also reflect the field results and indicated that increased compaction can dramatically decrease the infiltration rates for these soils. Unless proper care is taken, compaction associated with construction could reduce the infiltration rates of these soils from the current values of several inches per hour to much less than 0.1 inch per hour.

The computer modeling for the conservation design elements at the North Huntsville Industrial Park used infiltration rates of 2 in/hr for the on-site bioswales and 1 in/hr for the


large swales. These values are well within the measured conditions observed at the site, but care will need to be taken to preserve the rates. The on-site bioswales assume a larger infiltration rate as the soils will be deep tilled (during correct moisture conditions) to mix in organic supplements to increase the soil’s CEC, and have check dams in areas with slopes greater than 5 percent. The large swales will remain undisturbed, with little traffic during construction, except for the construction of the limestone check dams.

Fertilizers are not recommended on any of the drainage elements, as the likelihood of significant nutrient releases is large. The organic amendments to the site bioswales will probably result in short-term (1 to 3 year)

increased phosphorus discharges, but the organic matter will significantly increase the CEC of the soils for enhanced metal removal (better removals and greater exchange capacity for longer life). The downstream swales should never be fertilized, or amended (except if warranted due to extreme problems), but it is important to leave the residue from mowing on the ground as a top dressing and not harvest the hay along the swales. The reduced nutrient content of the drainage swales will increase their ability to sorb nutrients from the flowing water before the ponds.

Figure 13. Soil sampling sites.


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Drainage Way and Pond Stormwater Controls

The site was divided into four main drainage subareas, as shown in Figure 14. These subareas are labeled as A, B, C, and D and are described in more detail in Appendix A. There are several additional minor drainage subareas that will remain undeveloped and do not drain to one of the designated stormwater ponds. Besides the controls described below, all known sinkhole areas will be bermed to prohibit direct runoff into sinkholes, and they will have 25 to 50 ft buffers of natural vegetation.

The drainage way and pond stormwater elements proposed for the conservation design are summarized below. More details are provided in Appendix A. These elements will result in a conservation design that minimizes both runoff water volume discharges and stormwater pollutant discharges. The same stormwater elements are not recommended for each subarea due to different characteristics in each area. As an example, the industrial sites in subarea A are about evenly divided into an area that will be developed with conventional drainage having minimal on-site stormwater controls and with curbs and gutters and areas without on-site stormwater controls. The conventionally developed areas will discharge near the head of the pond with no regional swale treatment, while the other area will drain through a long natural grass drainage way before entering the pond. In addition, this area will have on-site bioretention (site grass swales graded as linking rain gardens) to provide grass filtering pre-treatment and infiltration) to help compensate for the other

area having minimal site controls.

Subarea B has extensive natural grass swales (two parallel swales) that will significantly reduce the runoff volume before the pond. Site bioretention controls can also be used to further reduce the volume, if desired. The pond can also be reduced in size to better fit the available area due to the reduced runoff volume. Site bioretention controls are not needed in this area due to the large amount of swales available.

Subarea C is mostly developed with little open area, but with roadside grass swales that are suitable for runoff volume reductions. Site bioretention controls can also be used, if desired. In this subarea, a relatively small pond (0.19 acres) could be used due to the runoff volume reductions from use of grass swales. However, a full-sized pond (0.38 acres at normal pool elevation) is recommended to reduce the maintenance problems and make it more aesthetically pleasing.

Subarea D will also utilize a roadside swale system along with on-site bioretention for runoff volume reductions. The pond will be located on adjacent city-owned land that may be developed in the future as a residential area. The large pond will also treat the runoff from that area. These varying stormwater controls will provide an interesting and useful demonstration for the City of Huntsville.

The drainage way and pond stormwater controls recommended for each subarea are


listed in Table 1. The WinSLAMM model (see Appendix D for model description) was used to predict reductions in runoff volume and particulate solids discharge vs. what would be expected with base conditions. These projections were based on 40 years of Huntsville rainfall data (1959-1999). Base conditions are defined as conventional development design with curb and gutter drainages and directly connected impervious surfaces.

Other pollutants are expected to be reduced by similar percentages: those that are mostly associated with the dissolved fraction (nitrates and pesticides, for example) are expected to be reduced by about 50 percent and those mostly associated with particulates

(phosphates and many heavy metals and PAHs, for example) are expected to be reduced by up to 90 percent.

WinSLAMM predictions for runoff and sediment loss vs. depth of rainfall are presented in Appendix E. The percent reduction in runoff and sediment loss with the conservation design vs. base case increases as rain depth decreases. A 90 percent or greater reduction in sediment loss occurs with rain depths of approximately 2.5 inches or less. A 70 percent or greater reduction in runoff occurs with rain depths of approximately 1.5 inches or less.

Erosion and sediment control will also be an important element of this proposed site

Table 1. Summary of the conservation design stormwater components for each subarea and their projected reductions in runoff volume and particulate solids discharge1/.

Drainage area

Drainage way and pond stormwater controls

Runoff volume

Particulate solids discharge

% reduction vs. base conditions2/

A Pond, swale, and site bioreten-tion

61

96

B Small pond and swale 69 93

C Pond and swale 68 94

D (including off-site area)

Off-site pond, swale, and site bioremediation

50 92

Total site 56 93 1/Projections based on 40 years of rainfall data (1951-1999) using WinSLAMM model

described in Appendix D. 2/Base conditions are conventional development design with curb and gutter drainages and

directly connected impervious surfaces.


development project. The site will likely be developed in phases and that will dictate the specific erosion and sediment control plan. Since the site is almost self-contained hydraulically, off-site runoff entering the project should not be an issue, except for along the eastern site edge. Before that area is developed, the off-site runoff from the adjoining areas should be diverted first so it doesn’t flow onto the construction area.

Downstream ponds need to be constructed before any site work is started. The construction sediment control ponds should be located at the same sites as the permanent stormwater ponds. However, they will need to be designed specifically for construction sediment control. Also, they will need to be re-graded after the construction is completed to remove most of the captured sediment. The main erosion control strategy should be prevention, specifically minimizing the amount of exposed work at any one time. Erosion from any large scale grading may be difficult to control. The initial road grading should be followed quickly with placement of the road base material and stabilization of the adjacent cut and fill areas using suitable mulches and erosion control mats, as needed. All graded areas need to be stabilized as rapidly as possible, using temporary vegetation, mulches or other ground covers.

The individual site lots will likely be completely graded, so local controls will also be necessary. Silt fences, although marginally effective, will need to be placed on the site perimeters. Site entrances will need to be graveled to reduce tracking of material from

the site to the roads. If stormdrain inlets are on any site, they will also need to be protected. The Alabama Erosion Control Manual and the city erosion control program requirements will need to be followed.

Conservation Design Master Plan The conservation design master plan is presented in Figure15. Roughly one-half of the area remains a working farm, which reduces maintenance costs and helps maintain the rural character of the area. The final plan represents:

• Project goals and objectives • Incorporates input from rapid design

charettes • Application of the TVA Sustainable

Development Guide • Input from peer review, Huntsville

planning and engineering departments

• Site engineering design needs

During the rapid planning process, 8-10 versions of the draft plan were developed from a baseline layout to reach final consensus (Figure11. Revising preliminary plans). The WinSLAMM model was used to evaluate various combinations of on-site treatment approaches that would substantially reduce runoff and sediment discharge vs. that with conventional development design. The results of this analysis are shown in Table 1.

A primary need of the project focused on developing the most cost effective methods in the shortest period of time to provide inputs to the engineering design to meet the construction schedule.


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Suggested Covenants The team suggested the following specific covenants be considered for all lots sold:

• Roof downspouts will be used to direct runoff into the on-site bioretention swales or into rain gardens/landscaped areas and must not be discharged directly onto impervious areas such as access drives, storage or parking areas.

• All site runoff will be directed into the on-site bioretention swales that shall be designed and built to the City’s specifications within the lot line setbacks. A range of typical designs will be provided within the conservation design covenants referenced in the land conveyance instruments.

• On-site swales shall be relatively small and mild sloped for ease of maintenance. They should be about 10 feet wide (at the bottom), with about 3 to 1 (H to V) side slopes, and 2 feet deep resulting in a top width of about 20 feet. To slow the water down, check dams in steeper on-site swales (greater than 5% slopes) are needed (see Appendix F). In a swale having 5% slopes with 2-foot tall checkdams, they need to be spaced about every 40 feet. If in a swale having 6.5% slopes, 2-foot check dams need to be spaced every 30 feet A check dam, and a flow spreader rock pad, also need to be located at the end of all of the on-site swales. To help neutralize rainwater pH that impacts karst

formations, it is recommended to incorporate limestone into the berms bottom bedding or require soil liming on a routine basis. A typical on-site swale illustrating the features described above is pictured in Figure 16.

• If curb and gutter are required, all curb cuts will be discharged into diffusers that transition into the on-site swales or into drop inlet structures.

• Building materials shall be selected with pollution prevention in mind. All galvanized metal buildings and/or roofs will be coated with approved factory coatings that effectively reduce zinc in the runoff. This will address zinc and roof runoff problems that have been recognized as a design consideration since the early 1980s.

• Critical source areas will need special attention. Industrial stormwater permits usually specify activities

Figure 16. Typical on-sight swale.


needing control and stress pollution prevention. Areas to be addressed include material handling areas, loading bays, fueling areas, etc. Bulk material storage areas subject to exposure from rainfall should be preferably covered but may be bermed to control and treat runoff (e.g., multi-chambered treatment train). Heavy equipment yards and some loading bays may need to be hydraulically isolated with runoff treated by specialized controls such as the upflow filter.

• All landscape vegetation will be native species. A pre-approved list will be provided to the purchaser. The purchaser may request native species not included in the pre-approved list. The TVA Native Plant Selector (http://www.tva.gov/river/landandshore/stabilization/plantsearch.htm) can also provide guidance in selecting landscape vegetation as illustrated in the example selection in Figure 17.

Cost Comparison: Base Case vs. Conservation Design

Garver Engineering provided a review and analysis of the Conservation Design for Phase 2 of the North Huntsville Industrial Park. This analysis established the base construction cost of the park using conventional engineering practices to which adjustments were made for the additional facilities required for Conservation Design. Credits were then applied for the value of practices replaced by

the Conservation Design construction. The base bid to which these adjustments were made was $1,163,429. Table 2 provides a summary of the additional practices and their costs to implement the conservation design. Table 3 provides a summary and estimate of the construction value of the conventional practices, which were replaced.

The net difference for this phase is approximately $33,750 in cost savings for the Conservation Design. Other intangible but nevertheless real advantages favor the

Figure 17. Example selection criteria and native plant selection from the TVA Native Plant Selector.


Conservation Design. These include:

• Continued ground water recharge through emphasis on infiltration of treated storm water through permeable soils, rather than the collection and conveyance off site.

• Reduction in offsite management costs of peak storm water volume. The discharge channel downstream from the wet ponds may have required concrete armoring if conventional storm water facilities had been used. This cost savings could range from $400,000 to $500,000.

• Preservation of natural drainage areas by their incorporation into the master drainage system.

• Significant improvements in post development groundwater and surface water quality.

Future cost savings are also anticipated through the use of Conservation Design practices in Phase 3. While this phase is more dense and conventional in layout, cost saving will accrue through the use of swales with curb outlet flumes in lieu of storm water inlets with reinforced concrete piping. Phase 2 bid prices for inlets are $2,300 each while reinforced concrete pipe ranges from $34 to $53 per foot for 18-inch to 30-inch pipe. The unit costs for outlet flumes are $2,000 each while the grass swales cost approximately $15 per foot. Actual savings will depend on final engineering design and construction bids but experience suggests that savings could range from $50,000 to $100,000.

Item Quantity Cost, $

Unclassified excavation 7,000 cubic yards 31,500

Side swale excavation 2,900 cubic yards 43,500

Class 2 riprap 350 tons 5,600

Filter blanket 3,300 square yards 13,200

Curb outlet/flume 8 16,000

Topsoil 1,650 cubic yards 9,900

Seeding and mulching 3 acres 5,700

Reinforcing mat 500 square yards 5,000

Solid sodding 15,000 square yards 56,250

Total 186,650

Table 2. Conservation design cost additions.


Table 3. Conservation design cost reductions. Item Quantity Cost, $

Small junction box 1 4,000

12” to 24” pipes

Storm water inlets 8 16,400

Single wing

Unclassified excavation 5,000 cubic yards 22,500

Ditch excavation 300 linear feet 45,000

Storm sewer 3,312 linear feet 132,500

Total 220,400


Appendices can be accessed at the following web sites: Appendix A: TVA Sustainable Development Guide

(http://www.tvaed.com/sustainable/)

Appendix B: Detailed Stormwater Water Management Conservation Design (http://www.cedarsinc.org/RefFiles/AppendixB.pdf)

Appendix C: Soils Investigation (http://www.cedarsinc.org/RefFiles/AppendixC.pdf)

Appendix D: WinSLAMM and Low Impact Development (http://www.cedarsinc.org/RefFiles/AppendixD.pdf) Appendix E: Runoff and Sediment Reductions at North Huntsville Industrial Park

(http://www.cedarsinc.org/RefFiles/AppendixE.pdf)

Appendix F: Liner Stability in Grass Swales (http://www.cedarsinc.org/RefFiles/AppendixF.pdf)

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