1 STORMWATER MANAGEMENT STRATEGIES: Grey, Green, LID & Hybrid

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1 EXECUTIVE SUMMARY The current drainage system in Lafayette Parish is grey or traditional infrastructure that can become over taxed in record 100 year rainstorm events allowing a high volume of water containing large amounts of pollutants to be carried into our waterways. The purpose of this report is to identify traditional or grey infrastructure that works well, current practices that could be changed through the incorporation of green infrastructure and introduce a hybrid approach to stormwater management. The document also seeks to address and better understand the best management practices of stormwater runoff and land use through culturally specific, economically viable, and multi-functional landscape design. Due to a combination of increasing urban development, relatively flat geography, too much rain falling too fast, and large amounts of paved or impervious surfaces, Lafayette Parish often experiences record flooding. Lafayette area historically averages inches of rainfall per year, and it is projected that the 100 year flood is actually happening as often as every 10 years. FEMA s preliminary floodplain mapping (draft Flood Insurance Rate Maps) significantly expands the areas of the parish classified as special hazard areas or within the 100 year floodplain. Within Lafayette Parish, 9% of land is in a floodway and 36% of land is in a floodplain. That puts a total of 45% of Lafayette Parish area in regulated flood zones.

2 INTRODUCTION The word infrastructure may call to mind things like pipes, conduits, steel and concrete inter-connected into a vast man-made engineered system to control stormwater. However, when it comes to solving drainage issues and stormwater management challenges, there are a growing number of solutions that are equally engineered yet more nature-based. Green infrastructure, by contrast, harnesses natural processes to infiltrate, evaporate, harvest and reuse stormwater through the use of a watershed scale system. Thus, grey and green are both engineered systems to manage stormwater. One uses man-made grey technology while the other uses a green eco-system. This ecosystem treats polluted stormwater runoff before it enters aquifers, streams, or other waterways. On-site LID (Low Impact Development) management of stormwater is a more localized approach to treating water on site with a rain garden, for example. Thus on-site LID projects are inter-connected to neighborhood or regional solutions, each of which may be green or grey. LID is a viable and proven strategy found under the umbrella of green infrastructure that uses soils, topography, vegetation and engineered materials to soften the impact of urban development on water resources and ecosystems in cities. The benefits of such systems extend beyond stormwater treatment and flood control to include carbon sequestration, recreational amenities, habitat creation and beautification. This report will propose merging the most successful components of these grey, green, and LID components of stormwater management to create a superior hybrid approach. This hybrid approach can be adopted into development strategies and plans, such as the Marais des Cannes Model Community Watershed Development, as a problem solving technique for flood prone areas of Lafayette Parish.

3 1 STORMWATER MANAGEMENT STRATEGIES: Grey, Green, LID & Hybrid Stormwater is rainwater that runs off streets, parking lots, and other impervious surfaces. The management of stormwater refers to the control of water runoff through a variety of infrastructure means. Stormwater management assists with the reduction and prevention of many sources of pollution, which enter waterways. When stormwater is managed properly it is absorbed into the ground, naturally filtered, replenishes aquifers or flows into streams and rivers. 1.1 Grey Infrastructure Figure 1.1 Current Grey Infrastructure in Lafayette Parish Coulee Mine (aerial) Coulee Mine Vermilion Bayou outfall Grey infrastructure in this document refers to traditional stormwater management systems that quickly channel stormwater, such as pipes, pumps and lined ditches, or use of detention ponds. Conventional development practices clear the land of almost all native and existing vegetation and then cover the land with large paved surfaces like roads, buildings and driveways. Rainwater cannot infiltrate into the ground and is quickly drained offsite into concrete channels. This method is designed to collect, convey and dispose of stormwater in a manner that is disruptive and harmful to the natural water cycle. Such large quantities of water moving and such intense speed cause erosion of stream banks, allowing sediments and other dangerous pollutants to enter waterways. Although functional and efficient at handling large quantities of stormwater in a short period of time, the long term costs and effects on the environment from traditional grey drainage and infrastructure is what we want to mitigate.

4 1.2 Green Infrastructure (GI) Figure 1.2 Proposed Green Infrastructure in Lafayette Parish Greenway recreation ROW bioswales Greenway trails Green Infrastructure (GI) is a general name given to an approach using environmental enhancement techniques to manage stormwater on a large planning scale. It is more of a whole system approach to drainage designed to mimic and restore the natural water cycle rather than the dilution is the solution to pollution stormwater drainage strategy. It is an approach that communities can choose to maintain healthy waters, provide multiple environmental benefits, use vegetation and soils to manage rainwater where it falls and weave natural processes into the built environment. Thus, green infrastructure provides stormwater management as well as flood mitigation, air quality management, community aesthetics and more. Examples of GI include habitat and forest buffers, developing greenways with trail corridors, connecting parks and preserving natural landscape features such as floodplains and wetlands. 1.3 Low Impact Development (LID) Figure 1.3 Implemented Low Impact Development in Harris County, Houston. Harris County ROW bioretention HC ROW bioretention HC parking lot bioswale Low Impact Development is a comprehensive land planning and engineering design strategy under the umbrella of GI that was developed as a means of managing stormwater with less of an impact on the

5 natural environment. Low impact development (LID) is a method of building design and community development with the intention of keeping stormwater runoff as uncontaminated as possible and focuses on choices at a site-specific level. Slow it down, spread it out, soak it in is the motto of LID. Slowing the flow of stormwater reduces erosion and flooding dangers. Spreading stormwater out reduces the speed of the stormwater. Allowing the stormwater to soak into the ground recharges underground aquifers and fosters environmental growth. LID designs usually incorporate more than one type of practice to treat stormwater runoff from a site. For example, instead of a developer using a detention wet pond to control runoff for a new subdivision, the LID approach would be for planners to include a bioretention area in each yard, utilize curb cuts, and disconnect downspouts from driveways to address stormwater runoff. Harris County, in Houston, Texas has engineered implemented and adopted LID regulations for roadway and subdivision development projects. These regulations were a collaboration product of the Harris County Public Infrastructure Department, Architecture and Engineering Department and the Harris County Flood Control District. The adoption and implementation of these green infrastructure and low impact development regulations is a good example of interagency collaboration LID (in Lafayette Parish) Figure LCG Public Works Environmental Quality Division LID demonstration projects Bioretention Kaliste Saloom Bioretention Lordes Hospital Vegetated swale Home Bank With population growth and expanding urban development on the horizon for Lafayette Parish, strong consideration is given to specific drainage regulations that include low impact development and green infrastructure practices. There have been several LID projects successfully implemented in Lafayette Parish that were spearheaded by LCG s Public Works Department, Environmental Quality Division. These projects utilize the best management practices for stormwater runoff found in the current drainage and flood prevention regulations and demonstrate a proactive shift toward a more hybrid approach to drainage.

6 1.4 Hybrid Figure 1.4 Proposed Hybrid Development practices in Lafayette Parish ROW bioswale ROW bioretention Greenway trails A hybrid approach to stormwater management would be an incremental way to incorporate the best of all three stormwater management practices. Lafayette Parish can combine techniques of grey, green and low-impact development methodologies and achieve a superior comprehensive approach to stormwater management. A hybrid system that incorporates site-specific LID principles and large scale planning of vegetated green infrastructure will greatly decrease the financial and physical burden put on current grey infrastructure drainage systems Goals of a Hybrid Drainage Plan A hybrid approach to a comprehensive stormwater management plan would include grey infrastructure and use green infrastructure as a major driver would accomplish these goals: To protect and preserve existing natural resources To direct development in the community To develop and effectively manage stormwater runoff on individual sites

7 1.4.2 Hybrid Plan Diagram This diagram explains the basic functions behind three main methods to dealing with stormwater runoff. The overlapping portions of each circle represent certain features each method has in common. The hybrid strategy would embrace the successful practices of the current grey stormwater regulations, performance based practices of Low Impact Development principles and the whole systems design approach of green infrastructure that allows for a more positive environmental, economic and cultural impact. Grey CONTROL-CONVEY-DISPOSE METHOD CONCRETE DRAINAGE CHANNELS EXCESS IMPERVIOUS (PAVED) SURFACES PRODUCE EXCESS STORMWATER RUNOFF FAILS TO ADDRESS EROSION, POLLUTION AND HYDROLOGIC MODIFICATIONS IN WATERSHED DESIGN FOR 10 YR FLOOD BMP S COST EFFECTIVE WATERSHED & COMMUNITY SCALE BMP S COST EFFECTIVE MAINTENANCE Green Hybrid COMBINE TECHNIQUES FOR SUPERIOR COMPREHENSIVE APPROACH TO STORMWATER MANAGEMENT LID FILTER-COLLECT-DETAIN-CLEAN- MANGE WATER DESIGN WITH NATURE TRIPLE BOTTOM LINE APPROACH WATERSHED APPROACH 100 YR FLOOD DESIGN SITE-SOURCE WATER MANAGEMENT PERFORMANCE BASED COST EFFECTIVE 100 YR FLOOD DESIGN RESTORES NATURAL HYDROLOGY EDUCATION & OUTREACH SLOW, SINK, AND SPREAD CONSIDER S REGIONALLY SPECIFIC LOCAL AND SOCIAL ECOLOGY INFILTRATION & EXTENDS TIME OF CONCENTTRATION SITE-SPECIFIC DESIGN PERFORMANCE BASED MEASURES RESTORE HABITAT 100 YR FLOOD DESIGN

8 2 HYBRID PLAN: MARAIS des CANNES 2.1 Marais des Cannes The name Marais des Cannes, translates Cane Wetland in French after Coulee Isle des Cannes, Island of Canes Coulee, which is the main drainage way being studied. Marais des Cannes is a proposed Model Watershed Development project, within a Lafayette Parish floodplain, that provides an alternative approach to stormwater management and flood protection. The project balances both the management of growth impacts on watersheds with the investments to minimize impacts of flooding on specific neighborhoods and business areas. The Marais des Cannes planning team chose this particular study after examining the FEMA preliminary Flood Maps. It is located within the largest floodway and proposes the biggest challenge to urban development in Lafayette Parish. Marais des Cannes is considered a hybrid approach to stormwater management. It combines grey and green infrastructure in conjunction with the implemented principles of LID strategies. 2.2 Hybrid Plan: Marais des Cannes The Marais des Cannes proposed development is considered a hybrid drainage plan that will combine LID strategies with green and grey infrastructure techniques to provide superior flood protection and stormwater management while adding economic and aesthetic value to the community. 2.3 The Marais des Cannes Project has three development goals: 1) Elevate the development of 50 acres to above the 100 year floodplain 2) Creates a 100 acre mitigation bank that provides construction and maintenance of a public park with significant recreational, educational and research opportunities. 3) Improves water quality upstream and downstream 2.4 The infrastructure included in the Marais des Cannes project is as follows: 1) Detention/Retention Ponds 2) Coulee By-Pass Structures 3) Terraced Natural Coulees 4) Dry Retention Areas 5) Constructed Wetlands 6) Commercial Development (with possibility of residential development) 7) Mitigation Banks

9 The ultimate goal of the Hybrid Plan: Marais des Cannes is to ensure that our entire infrastructure continues to serve the public in a way that is cost effective, environmentally sustainable, and delivers lasting community benefits. 2.5 Land Use: Marais des Cannes Demonstration projects are meant to test new programs and to serve as evidence of the feasibility and functionality of green infrastructure projects. The Hybrid Plan: Marias des Cannes would be an important first step in our community s effort to establish widespread policies to support green infrastructure. The following table shows proposed environmental zones and how they connect to the future land use of Marais des Cannes: Table 2.5 Environmental Habitats and Future Land Use for Marais des Cannes Environmental Habitats Ponds Mitigation Banks Wetland Emergent Wetland Tree Greenway Overflow Ditches Buffer Commercial Participating Tracts Pipeline R/W & Roadway Future Land Use Recreation & Education Recreation & Education Education Education Recreation & Education Recreation & Education Education (possible recreation) Education, Recreation, Motor Transport, Traditional Urban Mixed-Use, Potential Rural Mixed-Use & Public Service Recreation, Education & Motor Transport 2.6 Marais des Cannes: Vision of Success The Marais des Cannes: Hybrid Plan is unique because it takes social, environmental, and economic characteristics, within a specific area, into consideration throughout the planning process. These three key factors can be simplified into an equation or Triple Bottom Line for success. PlanLafayette considers these very same components in the Vision of Success. People + Place + Profit = Triple Bottom Line Our Community (People) Access to parks, open space, and the natural environment provides opportunities for active recreation, leisure, and engagement with the outdoor environment.

10 The project demonstrates 18 th century drainage techniques used by Acadians in Canada before the exile to Louisiana in 1755 adapted to the needs of the 21 st Century Our Environment (Place) The proposed plan provides solutions to pollution, erosion, preservation of wetlands, and natural groundwater recharge on site through proper stormwater management systems. The project improves Bayou Vermilion water quality which is being mandated by federal agencies. It ensures that natural resources become recognized as environmental and economic assets Our Economy (Profit) The project allows the conversion of low lying flood prone areas used for pasturage to prime commercial property with a mitigation bank. The establishment of the mitigation bank under federal regulations requires a perpetual maintenance fund. The mitigation bank can be used as a 100 acre public park without the necessity of public funding. Commercial developers are willing to pay a premium to adjoin to aesthetically pleasing amenities like water, open spaces, greenways and trails because these features increase visits to their development. Less impervious or paved space means more developable space and more money for developers. 2.7 Evaluations of Costs and Benefits Cost savings are typically seen in reduced infrastructure because the total volume of runoff to be managed is minimized through infiltration and evapotranspiration. When communities value green infrastructure s economic, social and environmental benefits, their water management planning can prioritize and maximize investment. This triple bottom line approach incorporates economic, social and environmental benefits that help develop a program which emphasizes broad community needs while addressing stormwater control. By incorporating a hybrid stormwater management plan, Lafayette Parish would be taking a proactive approach to overall savings on drainage projects. Marais des Cannes would be a perfect pilot program to demonstrate a hybrid system because the study shows there could be cost savings along with community and environmental benefits. 2.8 Cost Analysis: Marais des Cannes There were two plans proposed for the Marais des Cannes. Both plans provide an effective stormwater management and drainage system and a safe wildlife habitat, but only one plan, the Modified Plan, proved to be profitable and cost effective. 1) Option 1 Plan The original proposed development consisted of 540 acres near Scott, Louisiana. Although sufficient on cultural and environmental standards, the business plan projected a loss of $9.33 million.

11 2) Modified Plan (Hybrid Plan) The Modified Plan of 272 acres was proposed as a more modest development consisting of a 50-acre commercial development and 100-acre mitigation bank. This plan projected a profit of $5.46 million. 2.9 Benefits of Hybrid Plan: Marais des Cannes A hybrid approach to stormwater management would be an incremental way to incorporate the best of all three stormwater management practices. Lafayette Parish can combine techniques of grey, green and low-impact development methodologies and achieve a superior comprehensive approach to stormwater management. A hybrid system that incorporates site-specific LID principles and large scale planning of vegetated green infrastructure will greatly decrease the financial and physical burden put on current grey infrastructure drainage systems. The following diagram shows an overlay of grey, green and LID systems and the overall planning benefits from an environment, economic and community standpoint: Economic: Grey Hybrid: Marais des Cannes Environmental: Green Community: LID

12 Environmental Benefits Maintains/Restores Habitat Mitigates stormwater and flooding Improves water quality Improves air quality Regulates climate Economic Benefits Attracts businesses and workers Generates revenue Increases property values Increases lot yield Proactive approach to EPA compliance Reduced downstream flooding and property damage Community Benefits Enables recreation Fosters community and provides gathering spaces Connects people with nature Improves public health Promotes equity and access Builds social capital Demonstrates culture history 3 Proposed Green Infrastructure for Marais des Cannes 3.1 Site and Landscape Design Criteria for Stormwater Management This section creates an outline, describing best management principles for green infrastructure and stormwater management, intended for use in the Lafayette Parish geographic area, specifically for the Model Community Watershed Project Marais des Cannes. Multi-functional landscape and site design guidelines have been drawn up to incorporate current concepts reflecting sustainable green infrastructure strategies important to good stewardship practices. The proposed green infrastructure design guidelines that promote the preservation of open space, increase design compatibility between different land uses, and improve environmental quality by recognizing the numerous beneficial effects of landscaping upon the environment Intent and Purpose of Landscape Requirements & Standards The intent and purpose of these proposed guidelines is to enhance the character of the streetscape and motor vehicle use areas through the introduction of natural vegetation and landscaping. To promote the health, safety and welfare of residents of the specified municipality while creating an attractive community; to conserve properties and their values from unregulated development and conserve natural resources and encourage the appropriate use of the land. a) Preservation of Existing Vegetation To promote the preservation of open space, the existing tree canopy, healthy existing natural vegetation, and encourage the incorporation of native plant, plant communities and ecosystems into landscape design. b) Human Values To enhance the streetscape by separating the pedestrian from motor vehicles, moderate temperatures from impervious areas, filter air of fumes and dust, provide shade and soften harsher aspects of urban development. c) Community Design

13 To increase design compatibility between different land uses and promote the improvement of the aesthetic appearance of commercial, industrial and residential areas through multi-functional landscape design. d) Environmental Quality To improve environmental quality by recognizing numerous beneficial effects of green infrastructure principle and multi-functional landscaping design upon the environment. e) Air & Water Quality Management To improve the quality of the built and natural environment through air quality enhancements, energy conservation, reductions in the amount and rate of stormwater runoff and erosion, stormwater runoff quality improvements and an increase in the capacity of groundwater recharge Site Selection Requirements a) Assessment & Planning Encourage site development within existing community Engage site users and stakeholders in design process Protect unique cultural and historical places and provide opportunities for healthy outdoor activities Encourage and reward environmentally innovative sustainable site design practices b) Floodplain Protect floodplain by limiting new development Preserve and restore wetlands and manage stormwater on-site c) Stormwater & Irrigation Use best management stormwater practices Use native plants and trees to reduce duration of landscape irrigation Stormwater drainage facilities may function as open space recreation Site Design Requirements a) Soil & Vegetation Preserve threatened species and their habitat Use only native plants and restore native plants in eco-region Limit development on soils designated farmland

14 Develop Soil Management Plan to limit disturbance of healthy soils and restore disturbed soils during and post-construction b) Materials Selection Support sustainable practices in materials selection and use local/regional material when possible Reuse and recycle construction materials on-site where possible and develop Sustainable Site Maintenance Plan Design construction for disassembly using organic or recycled material when possible Recommend a process to document or monitor sustainable design practices Achieve net-zero waste site and minimize construction pollutants through awareness and education c) Tree Preservation & Tree Credits Existing trees may be included in the minimum planting requirements and credited according to DBH (diameter at breast height) and provide up to 50% of minimum tree requirements A tree proposed to be used as a credit must be approved as part of site plan review process Critical root zone of trees to be preserved during construction shall be fenced to protect roots Landscape Standards a) All land development applications shall be accompanied by an appropriate landscape plan which considers green infrastructure designs of landscape within a combination of vehicular use and parking areas. b) Removing a required tree or paving surfaces within landscape area requires approval from Planning, Zoning and Codes. c) In addition to right-of-way landscaping the developer or owner must provide site trees, shrubs, and groundcover as required for multi-family and mixed use development. d) All urban neighborhoods will have tree-lined streets. Street trees will be placed in right-of-ways and spaced to allow for mature growth accompanied by specific perimeter landscape strip requirements. e) For all portions of buildings that are adjacent to motor vehicle areas, foundation plantings shall be located between the building s facades and the parking lot and minimum standards shall be met or exceeded. f) Stormwater drainage facilities may function as open space for recreation with multifunctional landscaping that enhances the overall appearance of the project, prevents erosion and improves water quality of stormwater runoff. g) Green infrastructure techniques such as bioswales, curb cuts, rain gardens, and dry ponds can aid in urban drainage and flood control support. h) The Landscape Standards Requirements are intended to encourage development which is economically viable and environmentally sensitive. The standards are not intended to be specific as to inhibit creative development. Project conditions associated with individual sites may justify approval of alternative methods of compliance.

15 i) All motor vehicle use areas are subject to interior landscape and planting requirements as well as perimeter landscape strip requirements which includes street frontages, perimeter trees, groundcover, irrigation, landscape for common open space and long-term maintenance plan Water-efficiency and Landscape Design a) All landscapes improvements shall be designed with water-efficiency as a goal and be measured by an annual water budget. b) Consist of a variety of plant species to promote biodiversity and maximize the use of native plants especially for projects at the interface between urban areas and natural open space. c) All required landscape areas shall meet the minimum requirements for plant material size and spacing. d) Required plant material will be grown in recognized nursery in with proper horticulture practice and be installed in accordance with Landscape and Arboricultural Specifications. e) Required to fill out Post-Installation Inspection check list f) All required landscaping shall be irrigated for plant establishment, water-efficiency and maintenance Maintenance a) All required landscaping shall be maintained and replaced by the landowner/occupant as necessary. b) A regular maintenance schedule satisfying the following conditions shall be submitted as part of the Landscape Documentation Packet and be maintained to ensure water efficiency. c) Landscape preserved or installed as part of the minimum Landscape Requirements shall be maintained in perpetuity, or until such time a new landscape plan is approved or implemented.

16 3.2 Implemented LID Criteria Harris County LID & Green Infrastructure Design Criteria for Stormwater Management are optional regulations in Houston, Texas that were developed and adopted as a set of design criteria for stormwater management that incorporates low impact development and green infrastructure techniques. This design criteria summarized below has been successfully implemented in Harris County. This design criteria incorporates LID and GI methods of stormwater management into a set of regulations that developers could follow if they choose to use this hybrid approach Introduction to Low Impact Development Low Impact Development (LID) is a comprehensive land planning and engineering approach that aims to conserve natural resources that provide valuable functions associated with managing and filtering stormwater. Although the LID toolbox is virtually unlimited, the methods described below are the most likely to be used in this area, together or alone LID Site Planning Concepts Using hydrology as the integrating framework Controlling stormwater at the source Creating multi-functional landscape and infrastructure Approval Process & LID Based Project Criteria A LID analysis and design approach focuses on the following hydrologic analysis and design components: 1) Time of concentration 2) Retention 3) Detention 4) Change in impervious cover 5) Disconnection

17 3.2.4 Acceptable LID Practices The table below describes acceptable LID methods for use in satisfying stormwater quality and detention requirements: LID Disconnection Vegetated filter strip Vegetated swale Rainwater harvesting Bioretention Permeable pavement Tree box filter Stormwater planter Green roof Soil amendment Table Acceptable LID Methods Slow Runoff Filtration Retention Detention Evaporation Water Quality Threshold Acreages and Analysis Methodologies Requirements The following design approaches must be considered according to overall size of development: Site Acreage Site 10 Acres Table Threshold Acreages & Analysis Methodologies Requirements Requirements No adverse impact for 2-year, 10-year, and 100-year flood Present method for calculating time of concentration End-of-pipe analysis and meet min. detention rate of 0.55 ac-ft per acre

18 10 Acres < Site < 640 Acres Site 640 Acres No adverse impact for 2-year, 10-year, and 100-year flood Hydrologic methodology Detailed routing may be required End-of-pipe analysis No adverse impact for 2-year, 10-year, and 100-year flood Hydrologic methodology Required detailed routing Analysis of end-of-pipe at critical points downstream Stormwater Quality Treatment LID projects of one acre or larger, or those which are part of a larger plan of common development which exceed one acre, are required to treat the first 1 of runoff, obtain a Stormwater Quality Permit and submit a Stormwater Quality Management Plan. The Maintenance Plan incorporated into the SWQMP must meet or exceed the maintenance requirements indicated in these criteria for the LID practices utilized. Singlefamily residential projects of one acre or less in size are exempt from this requirement. The following table describes site acreage and stormwater quality treatment requirements: Site Acreage Site >1 Acre Table Stormwater Quality Treatment Requirements Requirements Treatment of the first one inch of runoff Stormwater Quality Permit required Stormwater Quality Management Plan (SWQMP) required LID Design for specific project types 1) Bioretention Systems (Bioretention Cell, Rain Garden) Bioretention cells or rain gardens are vegetated depressions filled with engineered soil which provide filtration to remove pollutants, increase time of concentration and provide detention for stormwater. The infiltration rate of engineered soil must be designed to treat the first 1 of storm runoff from the drainage area it serves. A situ test must be conducted on site after placement of engineered soil to make sure make sure infiltration rate is met. 2) Bioswales Bioswales are the linear version of a bioretention system which are commonly found next to streets, parking lots, and subdivisions to treat road runoff Roadway criteria for public streets 1) LID Features

19 a) Pervious concrete or asphalt cannot be used as roadway pavement b) All LID features will be incorporated in public ROW and cannot be placed beneath travel lines (medians acceptable) c) Design for 100-year flood event d) Maintenance intensive features are not acceptable 2) Culvert Sizes a) Culverts must meet minimum size of 18 (a min. restrictor upstream of 6 may be allowed if needed for hydraulic reasons) 3) Maintenance b) Costs should not offset the capital cost savings of the LID design approach c) Landscape design choices should minimize maintenance (ideally once or twice per year) d) Requirements should be clearly spelled out in design 4) Construction Management a) This critical element for successful implementation must be addressed in the design Criteria for vegetated swales in roadways Vegetated swales are broad, shallow channels designed to slowly move stormwater runoff and filter pollutants from parking lots, roadways, residential, commercial industrial and municipal land uses. Consideration Requirements Table Criteria for Vegetated Swales in Roadways Overflow weir or structures are required to accommodate extreme event Overflow & Safety Water Quality Design overflow inlet grates to minimize blockage Crossovers may be depressed up to six inches to act as 100 year event overflow weirs, provided crossover is not associated with a major thoroughfare intersection. Create standard min. curb height to prevent traffic from entering swale. Acceptable inlet styles: false inlet, curb cut w/ recessed flume opening, horizontal slotted curb at street grade. Drop inflow inlet to swale 3 inches to assist in capturing sediment before it reaches the Engineered Soil Media and be long enough to prevent vegetated flow restrictions. Vegetated Swales may act as a pretreatment system for water quality, Swale will not meet the water quality requirement to treat the first one inch of runoff. Maximum side slope at 4:1, unless approved by the County Engineer. Maximum water depth for the 2yr event shall not exceed 2 feet. Maximum water depth for the 100yr event shall not exceed 4 feet, and one traffic lane in each direction shall remain passable. Detention In the design, plan for left turn lanes so that this volume is excluded. Culverts must meet minimum size standard of 18. However, a restrictor on upstream end may be allowed if needed for hydraulic reasons. Minimum restrictor size is 6. Water shall not pond for more than 24 hours (max).

20 Utilities Vegetation Maintain the requirement for no adverse impact during an extreme event. If the design includes taking credit for storing storm water within the voids of engineered soils then appropriate modeling techniques should be used to account for this. Shallow depth of typical LID features may offer beneficial design options with respect to utilities beneath them. Hardiness in the design condition, evapotranspiration rate, pollutant removal, maintenance requirements and cost. Vegetation capable of wet and dry conditions. Construction timing and seasonal considerations should be considered in the specification of plant material, and erosion control measures Criteria for bioswales in roadways Bioswales use biofiltration and engineered soils to filter stormwater runoff to an underdrain, storm sewer or detention structure. These engineered soils must treat the first 1 of runoff, any water above that can bypass to storm sewer system. 1) Engineered soil for bioswale must be designed for minimum infiltration rate of 30 inches per hour (30 /hr) with a safety factor of 2 2) Actual desired infiltration rate would be 60 /hr to meet safety standard 3) Situ testing of engineered soil media must be performed to make sure infiltration rate meets specified rates Commercial Development LID s shall be maintained by property owner, unless there are publically owned facilities, and follow Stormwater Quality Permit guidelines for maintenance as the Stormwater Management plan dictates Suburban Residential Development All LID s should be located in public Right of Way and/or easement maintained by local government and maintenance shall follow guidelines of Stormwater Quality Management Plan. Storm water detention within roadside ditches may be allowed provided that the detention, roadside ditch, and drainage features adjacent to the roadway are maintained by another governmental entity other than Harris County, i.e. a Municipal Utility District, etc. The responsible government entity must make a Maintenance Declaration as to intent to construct and maintain LID s. To be filed in property records and file number shall be located on the subject plat under notes. 3.3 Conclusion

21 A Hybrid approach to green infrastructure can achieve overall better stormwater management for a 100 year flood event by compiling the Best Management Practices of stormwater runoff and flood prevention of LCG s current drainage regulations, the engineered and implemented LID Performance Based Standards of the Harris County Guidelines, along with the whole system approach of green infrastructure proposed by Site and Landscape Design Guidelines drawn up for the Marais des Cannes pilot project. The combination of all three techniques can create a superior comprehensive hybrid approach to drainage and stormwater management in Lafayette Parish. LCG could introduce green infrastructure into a range of programs and local agency policies by investing in the Hybrid Plan: Marais des Cannes as a demonstration project.