Prepared for ARLINGTON COUNTY & CITY OF ALEXANDRIA

Consultants RHODESIDE & HARWELL, INCORPORATED

CH2MHILL

BIOHABITATS, INCORPORATED

WATERSCAPES / DREISEITL

Four Mile Run Restoration Master Plan December 2005 CHAPTER 4 DRAFT Imagine the day when the stream no longer func- tions as a barrier between neighborhoods, but rather becomes the bridge that brings communities together. Text running over photos can use available negative space and break the grid.

Tidal Reach looking west - AFTER A. THE VISION: CELEBRATING A 4 RESTORED FOUR MILE RUN THE PLAN Once a forgotten place and in-between space lined with utilities and concrete, Four Mile Run is on the threshold of becoming the place where residents of Alexandria and Arlington come to spend their time. Imagine the day when the stream no longer functions as a barrier between neighborhoods, but rather becomes the bridge that brings communities together. Tidal Reach looking west - BEFORE Four Mile Run will become a place that everyone can fi nd a reason to celebrate. Celebrate the wonders of water, ecological renewal and the respite that nature provides. Celebrate the way that Potomac Yard and Shirlington and Mount Vernon Avenue brim with urban vitality, or the latest bird-watching discovery at Four Mile Run Park. Celebrate a community festival, or trying out a new kayak for the fi rst time. We can celebrate the confl uence of cultures that makes Alexandria and Arlington such special places to live, or the season’s fi rst ripe tomato in the new community gardens. Meeting neighbors on the promenade, or the exhilaration of a crisp pass of the soccer ball and thunderous shot on goal. Celebrate the trails that enable us to bike to work or meander our way to nowhere in particular. Celebrate fi nding the perfect skipping stone, or realizing how much fun science class can be when the stream becomes an outdoor class- room. Or we might choose to celebrate a simple, quiet epiphany during an early- morning stroll along the boardwalk.

Four Mile Run will become a place that everyone can fi nd a reason to celebrate.

THE PLAN 3 Illustrative Plan “For decades we all treated this stream as a threat, a problem, as our back alley. Finally, a handful of folks decided that it didn’t have to be that way - that with vision, cre- ativity and cooperation it could become a beautiful front door that united our com- munities. That vision of a hopeful future for this place has been painted. Now we need to fi nd the collective will to make it real.”

Judy Guse-Noritake, Co-Chair Joint Task Force

4 FOUR MILE RUN MASTER PLAN Figure 4.1 Illustrative Plan In this vision, the Master Plan for Four Mile Run transforms the corridor in a variety of your neighbors on the terraced banks near Potomac Yard or escape upstream with a ways. Most notably, the vision includes improvements in environmental quality, open book; play volleyball or basketball at the new courts while other family members tend space amenities, transportation options and overall quality of urban life, as well as the to the family’s plot in the community garden and the kids try out their favorite play- creation of many new destinations and activities to explore. ground equipment. You can stroll along the verdant pathways or relax on the banks of the stream. Or you can play soccer or baseball on the upgraded and reconfi gured sports ENVIRONMENT fi elds at Four Mile Run Park in Alexandria.

Th e Master Plan emphasizes the “greening” of the Four Mile Run corridor. It envisions From the new park atop the remaining railroad bridge, you can look down and see how restoring the balance between nature and people, ecology and urban places. the sunlight shimmers on the water as it passes through new openings in the bridges protection remains the paramount concern, but this time with nature and people in and how, at night, the lighting casts artful refl ections on the water. Th e end of Mount mind. Th e stream shoreline no longer serves as a barrier, but now invites visitors down Vernon Avenue is now the neighborhood’s open space hub and the place to be for to the water’s edge. Where a straightened and channelized stream once rushed along festivals and on balmy nights. And thanks to all the vegetation and landscaping, it is concrete fl umes and gabions, the restored stream now meanders gracefully around bars possible to forget that Four Mile Run once felt like a utility corridor. Where a straightened of wetlands and past green and stabilized stream banks. Th e litter and debris that once collected along the stream are now distant memories. Elsewhere, the fragments of TRANSPORTATION and channelized existing wetlands have been reconnected and enhanced to form a healthy and better- stream once rushed functioning ecological system. At Four Mile Run Park, a tributary through the wetlands Transportation improvements make it easier to access Four Mile Run and move around has been reconnected to the stream and provides a cleansing function that improves and through the area using a broad range of transportation modes. Th e multi-lane roads along concrete fl umes water quality. Along Long Branch, which fl ows into Four Mile Run, the banks are now with speeding vehicles have given way to traffi c-calmed and tree-lined boulevards that are safer and easier to get across on foot. In fact, you no longer need a car at all when and gabions, the less steep than they used to be and lined with native vegetation; meanwhile, the concrete weir that once blocked fi sh passage has been removed. you visit or live in the vicinity of Four Mile Run. Rapid transit vehicles may stop at a restored stream now proposed station at Potomac Yard. Th e realignment of West Glebe Road, with its the Water fl ows in new places. Hidden pieces of the region’s hydrology have been “day- new vehicular bridge, and new roads with street parking around Potomac Yard all make meanders gracefully lighted” and waterways, once enclosed underground, now enhance the community and it easier to navigate by car and to stop for a quick stroll by the stream. For bicyclists, a around bars of wet- its landscape. In addition to being pleasant and interesting to look at, these uncovered commuter trail on the north side of the stream maintains a fast and effi cient commuting streams also provide valuable environmental benefi ts by providing places for stormwa- route that now connects with the Washington & Old Dominion trailhead. For a more lands and past green ter runoff to infi ltrate the ground. Many of the paved surfaces that once surrounded the leisurely experience, the trail on the southern side of the stream off ers an unhurried and stabilized stream stream—parking lots and other expanses of concrete and blacktop—have become route for walking, bicycling or roller-blading. And it’s now a lot easier to move back and distant memories, replaced by attractive and permeable surfaces that are able to absorb forth between Alexandria and Arlington, thanks to the addition of artistic new pedes- banks. harmful runoff . management can be fun, too: water features scattered trian bridges and informal stepping-stone crossings at frequent intervals along the throughout the corridor enliven the landscape in unique, artistic and environmentally- corridor. responsible ways while teaching us about water, ecology and the community’s history. QUALITY OF URBAN LIFE Finally, given the time and space to grow and revitalize, new and expanded habitat areas have gradually become home to all sorts of wildlife and plants. Th ese areas recall the Today, the stream has become a front door to both communities, rather than a forgotten natural conditions that once existed along the stream, teaching us about nature and corridor. You might not recognize Potomac Yard now—a vibrant urban node that is off ering refuge for wildlife and humans alike. home to thousands of new residents and workers while off ering terrifi c shopping. On nice days, it’s great to relax on the terraced banks that lead down to the water, on OPEN SPACE benches along the stream or in the elevated park. Shirlington is even better than you remember it, and shoppers and diners now spill down to the stream’s edge to relax and From its parks, plazas and public greens to its promenades and secluded pockets of socialize. Th e new road confi guration at West Glebe Road has enabled the creation of a nature, Four Mile Run off ers open spaces for all moods and experiences. You can meet

6 FOUR MILE RUN MASTER PLAN Stream at Potomac Yard looking east - AFTER small shopping village while leaving room for a new sports fi eld and beautiful open Such is the vision for the future of Four Mile Run. Th e remainder of this chapter, space. Meanwhile, Arlandria is thriving as trail users stop at its restaurants and attend building on the guiding principles identifi ed in Chapter 1, describes each of the the events on Mount Vernon Avenue. components of the Master Plan. In the evenings, people gather for performances and events at Potomac Yard and Mount B. THE IN-STREAM PLAN Vernon Avenue, while at Shirlington they enjoy theater, cinema, restaurants and strolling along the stream. Th e stream at night is illuminated by low-level light that 1. HYDROLOGY AND : THE CORE OF THE glows beneath all of the bridges, protecting the night sky within the habitat areas while PLAN off ering beauty and security to those who wish to enjoy the waterfront experience. For solitude, escape the Create a “dynamically stable stream channel” using natural stream channel design techniques. hubbub of the city by DESTINATIONS AND ACTIVITIES So much to do; so many places to go. Start at the education center to learn about the Th e design approach for the in-stream portion of Four Mile Run creates channel fi nding one of the history of Four Mile Run and the creatures that reside in the stream and wetlands. Aft er characteristics that are more natural and more stable when exposed to the current range many natural nooks that, stop by the farmer’s market in the park atop the former railroad bridge, and of fl ows generated by the urbanized Four Mile Run watershed. Like a natural river perhaps do a little shopping while you’re there. On the way home, stop to watch the channel, the in-stream design creates multiple “surfaces” at diff erent elevations within where you can forget little league baseball games in progress. Or go bouldering or biking, play basketball, rent the larger Four Mile Run fl ood control corridor. Th e surface at the lowest elevation is where you live and a canoe or kayak at the boat launch or try some fi shing by the bridges. Perhaps you the “low-fl ow channel” (one could also call this the “active channel”). Th e low-fl ow would prefer a picnic or some people-watching on the promenade, or a maybe a channel will convey fl owing water, even during times of limited fl ow, and may also shift perhaps spot a heron or leisurely lunch in Shirlington Village. For solitude, escape the hubbub of the city by laterally at some locations in response to high fl ows. Th e higher surfaces in this design, osprey. fi nding one of the many natural nooks where you can forget where you live and perhaps the equivalent fl oodplains in natural , are intended to convey fl owing water only spot a heron or osprey. Stick around long enough, and you can catch the jazz concert at during higher fl ows. In addition to adding habitat diversity to the Four Mile Run sunset on the green at Potomac Yard. corridor, these fl oodplain surfaces provide a “release valve” for the low-fl ow channel so that it is not damaged by high fl ows. By mimicking a natural channel—albeit modifi ed

8 FOUR MILE RUN MASTER PLAN Avalon Apartments Realigned South Glebe Road Expanded Parkland and Commuter Trail Bank Stabilization Treat- ment and Bouldering Opportunity Wetland Cell and Flood- plain Planting Proposed West Glebe Road Bridge Low-Flow Channel Bank Stabilization Plant- ing Pedestrian and Cyclist Promenade Redevelopment Opportu- nity and Trailhead Proposed Multi-Purpose Field over Existing West Glebe Road

Section through South Glebe Road, stream and new multi-purpose fi eld looking east THE PLAN 9 Proposed West Glebe Road Vehicular Bridge Proposed Urban Redevel- opment Opportunity Realigned South Glebe Road New West Glebe Road Intersection Proposed Multi-Purpose Field Proposed Pedestrian Bridge to Replace Exist- ing West Glebe Road Vehicular Bridge

Th e alluvial reach looking east - BEFORE Th e Alluvial Reach looking East - AFTER

10 FOUR MILE RUN MASTER PLAN in order to minimize common problems in urban channels—the Four Mile Run corridor will be dynamically stable and will provide continued protection from fl oods. By ensuring the QUESTIONS & ANSWERS Th e in-stream design shown on the HYDROLOGY AND FLOOD CONTROL plan was greatest possible developed to specifi cally address the following guiding principles: WHAT ABOUT FLOODING? variety of aquatic • Provide a minimum 100-year event fl ood protection. and riparian • Consider fl ood protection for areas not currently protected. Thanks to a new understanding of Four Mile Run’s hydrology and the history of fl ooding in the stream corridor, we now know that the U.S. Army Corps of habitats as part • Create a “dynamically stable stream channel” using natural stream channel Engineers fl ood control project in 1970s was designed for a much greater design techniques. level of fl ood control than was intended. As a result, it is possible to reintroduce many of the natural characteristics of the stream without of the channel • Improve corridor habitat and ecology to support native terrestrial and aquatic compromising protection against a 100-year fl ood event, the level of redesign, the plant and animal species. protection for which the fl ood control project was originally intended.

To achieve a minimum of 100-year event fl ood protection, all changes to the cross- As the restoration project will trans- sectional confi guration—such as any changes to channel materials or vegetation that effort moves form what is now could increase “roughness” or changes that reduce cross-sectional-area—will be forward, much of the stream modeled to verify that the 100-year fl ood event will be contained within the fl ood corridor can be a relatively control channel. Where modeling indicates that the 100-year fl ood event would extend naturalized outside the fl ood control channel under current conditions, the channel and levee through partial homogeneous design will ensure that the extent and frequency of fl ooding of private properties will removal of the not be increased by this project. For those areas outside the levee corridor that are existing gabions aquatic and already subject to 100-year event fl ooding in the channel’s current condition, confi gura- and fl ood walls. tion and mitigation options to minimize the risk and impact of fl ooding will be consid- The actual extent riparian environ- ered. of these changes will be determined ment into a cor- Within the fl ood control project lateral limits (the blue line on the hydrology and fl ood by the outcomes control layer), a multistage channel confi guration was developed to achieve the second of the Hydrologic ridor with diverse goal of creating a “dynamically stable stream channel” through natural stream channel and Hydraulics (H design techniques. A dynamically stable channel is defi ned as a channel that has an & H) modeling microhabitats. appropriate channel cross-section to transport sediment during normal fl ow condi- study being tions,; however, it is designed to adjust laterally within this basic form in response to conducted large fl ows in order to minimize hard stabilization and maintenance. Under a natural simultaneously with this master planning effort. This study will ensure that restoration activities will not negatively impact the intended 100-year fl ood condition, storm fl ows would typically have access to a wide fl oodplain so that the high event protection. energy from large events could be dissipated. In an urban setting, such as Four Mile Run, the limited fl oodplain area and high storm-related discharge result in high fl ow velocity and sheer stress. In the proposed dynamically stable channel, the low fl ow channel and inset fl oodplain could adjust to a moderate extent in response to the high fl ows. Th e fl ood control levee walls, however, will be stabilized in place to prevent adjustment.

THE PLAN 11 While some refi nement to the dimensions of each channel stage is still required, the One of the aims of the restoration eff ort is to remove as many of the existing gabions overall intent of the design is to create a low-fl ow channel with an “inset fl oodplain” that and fl ood walls as possible without compromising fl ood control. In place of the gabions, is inundated during relatively small storm fl ow events that mobilize sediment. Th e inset the Master Plan recommends more natural stabilization solutions, including a variety of fl oodplain acts as a release valve for forces in the low-fl ow channel. As fl ow increases, bioengineering techniques and appropriate bank reconfi guration. Some of these water will spread laterally onto the fl oodplain surfaces, limiting the forces that develop techniques are discussed later in this chapter, while Chapter 5 outlines relevant tech- in the low-fl ow channel and protecting the low-fl ow channel (and stream habitat) from niques in greater detail. Th e outcome of the Hydrologic and Hydraulics (H & H) damage. modeling study being conducted simultaneously with this Master Planning eff ort will determine the extent to which the gabion and fl oodwall replacement can occur. Th e multistage channel design also minimizes the amount of local maintenance Low Flow Channel required to achieve the goals described above. Th e fi nal dimensions selected for the Th e following elements are illustrated in the Flood Control and Hydrology plan in Vegetated Inset Flood- channel will ensure effi cient sediment transport through the low-fl ow channel, thereby Figure 4.2. Th e location of the features are shown on the cross-section provided. plain minimizing the need for frequent maintenance. Due to the nature of the tidal portion of Low Flow Channel Bioengineered Protection the channel, however, maintenance in that reach will likely be more frequent.

Vegetative Bank Stabili- Finally, within the geometric constraints required to achieve the other project goals, the Th e low-fl ow channel is the area within the Four Mile Run corridor that will convey zation project seeks to maximize other benefi ts of the restoration. By ensuring the greatest pos- water during both low-fl ow and high fl ow conditions. Th e design of the low-fl ow High Flow Event sible variety of aquatic and riparian habitats as part of the channel redesign, the project channel will be refi ned based on sediment transport analyses to ensure that the dimen- sions are appropriate for the channel-forming fl ows that occur in this reach of Four Will not adjust will transform what is now a relatively homogeneous aquatic and riparian environment into a corridor with diverse microhabitats. Moreover, the project aims to maximize both Mile Run. Because of the limited fl oodplain access and high storm-related discharge Dynamically stable - recreational opportunities in, and public access to, the stream. common to this type of urban channel the channel is intended to be a dynamically may adjust laterally

Low-Flow Channel Existing Profi le Proposed Profi le with Vegetative Bank Stabili- zation Section through Long Branch looking south

12 FOUR MILE RUN MASTER PLAN stable channel. Th is will allow the restoration to include more natural elements, and less Bioengineered Toe Protection hard stabilization. Th erefore, to the extent possible given the fl ow characteristics and lateral constraints, the low-fl ow channel may change position and confi guration in Bioengineered toe protection will protect the bottom (i.e., the “toe”) of the slope that response to sediment delivery and fl ow from upstream. Th ese adjustments will take connects the low-fl ow channel to the inset fl oodplain and the slope at the lateral limits place within the inset fl oodplain, which is described below. In locations where adjust- of the inset fl oodplains. Toe protection may also be used to stabilize the low-fl ow ment is not desirable because of the location of amenities, facilities or utilities, various channel in places where channel adjustments would compromise project amenities. channel and bank stability measures (also described below) will be employed to Th ese measures will protect these sensitive areas from excessive during high maintain a relatively fi xed channel. fl ows and moderate fl ows in the low-fl ow channel. A wide variety of bioengineering approaches could be applied. It is likely that some combination of native rock and Extent of Inset Floodplain / Vegetated Inset Floodplain vegetation will be used in some places to provide this protection. Bioengineered toe protection blends much better into the natural stream environment than traditionally In Figure 4.2, the vegetated inset fl oodplain is the area adjacent to the low-fl ow channel engineered toe protection methods, and also provides much better wildlife habitat. bounded by the lines labeled “extent of inset fl oodplain.” Th is design feature is tightly linked with the low-fl ow channel and contributes to the ability of the low-fl ow channel Bioengineered Bank Stabilization to be dynamically stable. Th is area will be inundated relatively frequently during fl ows greater than the determined channel-forming fl ow for the low-fl ow channel. Th is area Similar to bioengineered toe protection, stream banks will be stabilized using similar will be vegetated and outfi tted with bioengineered erosion control measures to prevent bioengineering approaches in areas where the erosion risk is relatively high. In Four excessive erosion during the early phases of the project development. Th e vegetation Mile Run, high risk areas for erosion will occur where the corridor is most confi ned, and grading in this area will provide diverse in-stream habitats as well as pocket and where structures such as bridge piers and utility line crossings present obstructions wetlands, vegetated riparian zones, upland areas with tree cover, and grassy upland to fl ow that could induce signifi cant local bed scour and bank erosion. Since the forces areas.

Figure 4.2 Flood Control and Hydrology Th e layers of the Master Plan

THE PLAN 13 A step-pool grade con- trol structure is a natu- rally inspired design that mimics the rock jam – plunge pool sequences of steep riv- Figure 4.3 Stormwater Management ers to maintain bed elevations in impaired systems.

Figure 4.4 Vegetation

14 FOUR MILE RUN MASTER PLAN that act on stream banks during high fl ows are greatest where water depth is greatest, when incorporated into new development and as retrofi ts to existing land use, can aff ect the bioengineered bank stabilization designs include gradients of strength, with the both the quality and quantity of water that fl ows in Four Mile Run while also improving most robust approaches (i.e., those with more rock or hardscape) applied near the aesthetics in the surrounding landscape. Specifi cally, the Master Plan identifi es locations bottom of the bank and more fl exible approaches (i.e., those with more vegetation) throughout the corridor where one or more of a range of stormwater management applied near the top of the bank. Again, some mixture of vegetation and structural techniques—daylighting, , permeable pavement, green roofs, stormwater stabilization is likely in all of these areas. planters, litter control, underground storage—could improve ecology and aesthetics. Th e variety of available stormwater management techniques and strategies for imple- Vegetated Bank Stabilization mentation are described further in Chapters 5 and 6.

Vegetated bank stabilization will be applied in areas with relatively low risk of excessive Th e STORMWATER MANAGEMENT PLAN responds to the following guiding erosion. Th ese areas are generally where the fl ood corridor is at its widest. Some principle: temporary, biodegradable erosion control fabrics may be used to prevent erosion during project establishment, but long-term erosion protection in these areas will be provided • Incorporate “green design” principles for all design and development activities by the root structure and soil coverage of vegetation. Th ese areas will also off er signifi - within and adjacent to the corridor. cantly improved habitat compared to the existing conditions in Four Mile Run. Figure 4.2 shows what vegetated bank stabilization will look like in practice along Long • Develop upstream strategies to improve environmental quality and maintain Branch. the long-term viability of a restored levee corridor.

Step-Pool Grade Control • Incorporate innovative and creative urban designs and watershed solutions.

Th ese features will provide several ecological and aesthetic benefi ts to the Four Mile Identifying Opportunities in Alexandria and Arlington Run project area. A step-pool grade control structure is a naturally inspired design that Public facilities, such as schools, are very attractive sites for stormwater management A major objective of mimics the rock jam – plunge pool sequences of steep rivers to maintain bed elevations retrofi ts. Potential options at these sites include rain gardens, green roofs, foundation the plan is to reestab- in impaired systems. Th ese structures will replace the unnatural grade control struc- planters, and permeable pavers in certain areas of the property. Underground storage tures in Four Mile Run and will likely be constructed of large rock native to the area. can be placed under basketball courts or soccer fi elds. Moreover, public schools can take lish the vegetation that Th ese structures will allow easier upstream and downstream movement of fi sh and advantage of stormwater retrofi t projects as educational opportunities for their students. once lined the stream other aquatic organisms, and will also aerate the water in visually appealing small A site visit to Gunston Middle School and Charles Barrett Elementary School, for waterfalls. example, identifi ed several potential sites for bioretention, stormwater planters, and per- and existed in the Th e Technical Appendix to this Master Plan provides additional details on specifi c meable pavement, as noted in Figure 4.3 lowland wetlands areas elements of the in-stream design and how it was developed. Commonwealth Avenue in the City of Alexandria is a long, divided boulevard that is but has since disap- bounded by residential neighborhoods. Th e median is currently the standard curb-and- 2. STORMWATER MANAGEMENT: IMPACTING THE WATER gutter structure with storm drains. Th e current design of the median makes it diffi cult peared or been colo- THAT FLOWS IN FOUR MILE RUN to install curb cuts and provide water quality benefi ts through bioretention in the nized by invasive Incorporate “green design” principles. median; however, this potential should be looked at again if and when the street undergoes renovations and road work. Th is area was identifi ed in the Alexandria Open species. A successful and truly sustainable restoration of Four Mile Run will focus not only on Space Plan as a major thoroughfare that should serve as a model for streetscape the immediate stream channel, but also on the surrounding watershed and its impact on enhancements. the stream. Incorporating a range of “green design” principles, such as stormwater management, can help to minimize the impact of surrounding land use. A variety of Other commercial properties, such as the Dominion Virginia Power dispatch and eff ective, sustainable and minimally impacting stormwater management techniques, storage facility, are also potential sites for retrofi ts such as bioretention, , and disconnection of pervious surfaces such as roofs through the use of stormwater planters THE PLAN 15 or rain barrels. (Th ese techniques are described in detail in Chapter 5) In addition, • Create a “dynamically stable stream channel” using natural stream channel other commercial properties that include large parking lots are candidates for bioreten- design techniques. tion and permeable pavement retrofi ts. • Consider fl ood protection for areas not currently protected.

3. VEGETATION AND HABITAT: ENRICHING LIFE ALONG FOUR Th e vegetative communities described in Figure 4.4 are divided between “existing” and MILE RUN “proposed” status. It is intended that existing vegetative communities be preserved and/ Improve the stream corridor ecosystem. or enhanced according to their current location and composition. Invasive species management will occur throughout the project area. Within “existing” vegetation A major objective of the Master Plan is to reestablish the vegetation that once lined the communities, this will involve the inventory and eradication of invasive species that stream and existed in the lowland wetlands areas but has since disappeared or been currently thrive in these areas. In “proposed” vegetative communities, invasive species colonized by invasive species. Ecosystem restoration—including the preservation and management will prevent the colonization of undesirable species. enhancement of existing vegetation and the introduction, where feasible, of new Water Pollution Control vegetative communities—benefi ts the stream corridor by providing additional fl ood and Hydraulic models of the fl ood control channel enable an analysis of the eff ects of the Plant erosion control, stabilizing stream banks, fi ltering and removing pollutants from water increase in vegetation within the active channel and riparian / wetlands areas adjacent Demonstration Wetland entering the channel, regulating temperatures, and providing habitat for aquatic, to the channel. Th ese models will be used to ensure that proposed plantings do not lead to a net increase in the risk of fl ooding along any portion of the project area. Th e follow- Commuter Trail terrestrial, and avian organisms. ing plant communities and their specifi c placement in or along the channel were Wetland Bars Th e restoration proposals in the VEGETATION AND HABITAT PLAN respond to the selected, in part, to not hinder fl ood conveyance potential. In addition, it is intended Low-Flow Channel following guiding principles: that these vegetative communities, even when fully mature, will require a low degree of maintenance for fl ood protection purposes. Proposed West Glebe • Provide a minimum 100-year event fl ood protection. Road Bridge Additional fl ood protection is achieved by the restoration of the marshlands in Alexan- • Improve corridor habitat and ecology to support native terrestrial and aquatic Low-Flow Channel dria’s Four Mile Run Park to a condition more closely resembling historic conditions. plant and animal species. Pedestrian / Cyclist Bridge Connecting South Eads St to Common- wealth Ave Alexandria / Arlington Nature-Cultural Center Trail Existing Upland and Floodplain Forest Reconnected Tributary Existing Emergent Wet- land Raised Walkway through Wetland Meadow

16 FOUR MILE RUN MASTER PLAN Th e restoration will entail excavating areas previously fi lled to establish a connection to - Halberd-leaved Tearthumb (Polygonum arifolium) marshlands in Hume Springs Creek. Th is will allow for a greater volume of fl oodwaters - Dotted Smartweed (Polygonum punctatum) to be contained within the channel and wetlands areas proximate to the channel, and - Pennsylvania Smartweed (Polygonum pensylvanicum) increase fl ood protection for the neighboring communities. - Arrow-leaved Tearthumb (Polygonum sagittatum) Th e stream corridor restoration focuses on the following vegetation and habitat - Water Hemp (Amaranthus cannabinus) categories: - Rice Cutgrass (Leersia oryzoides) EXISTING VEGETATIVE COMMUNITIES - Nodding Burr-marigold (Bidens cernuua) - Smooth Beggarticks (Bidens laevis) Existing Emergent Tidal Vegetation - Halberd-leaved Rose Mallow (Hibiscus laevis) Emergent tidal wetland communities are currently established within Four Mile Run - Crimson-eyed Rose Mallow (Hibiscus moscheutos ssp. moscheutos) Riparian Edge Park. Th ese wetland communities are comprised of herbaceous hydrophytes that are - Bearded Sedge (Carex comosa) Freshwater Floodplain present for most of the growing season. Because the majority of wetlands areas in Planting - Climbing Hempvine (Mikania scandens) Arlington and Alexandria have been fi lled, altered, or degraded, this area remains one - Cap Dodder (Cuscuta gronovii) Freshwater Wetland Cells of the largest intact and functional wetland ecosystems within these jurisdictions. It provides valuable habitat for migrating waterfowl, resident bird species and a host of - Orange Jewelweed (Impatiens capensis) Bank Stabilization Plant- insects and mammals. In addition, the wetland ecosystem is a natural fi lter of pollutants - Wild Rice (Zizania aquatica) ing found in the waters of Four Mile Run. - Arrow Arum (Peltandra virginica) - Common Arrowhead (Sagittaria latifolia) Representative species: - Pickerelweed (Pontederia cordata) - River Bulrush (Schoenoplectus fl uviatilis) - Sweetfl ag (Acorus calamus) - Cattails (Typha spp.)

Tidal Wetland Bars Existing Emergent Tidal Vegetation Proposed Emergent Tidal Vegetation Existing Floodplain Forest

Section through stream and Four Mile Run Park and wetlands looking east THE PLAN 17