Floodplain Restoration and Storm Water Management: Guidance and Case Study

Transcription

1 Floodplain Restoration and Storm Water Management: Guidance and Case Study Prepared by: Chagrin River Watershed Partners, Inc. P.O. Box 229 Willoughby, Ohio (440) (440) (fax) Biohabitats 2026 Murray Hill Road, Room 107 Cleveland, OH (fax)

2 Page 2 Acknowledgements This report was prepared by the Chagrin River Watershed Partners, Inc. (CRWP) under award LEPF from the Lake Erie Protection Fund, with funding made possible by the sale of the Lake Erie Number Plates. The statements, findings, conclusions and recommendations are those of the author(s) and do not necessarily reflect the views of the Ohio Lake Erie Commission. Additional support for this report was provided by the Members of CRWP through their annual Member dues. CRWP is a non-profit technical organization formed by the townships, villages, cities, counties, and park districts of the Chagrin watershed to develop and implement innovative solutions to address current, and minimize new, flooding, erosion, and water quality costs and to control the increasing infrastructure costs associated with urban/suburban development. CRWP provides Members with advice and assistance on zoning and subdivision codes, implementation of these codes, development plan review, and other best practice implementation at Member direction. Cover Photographs. Top Left: Floodplain Restoration in Germany (Photo by: Unknown from internet website) Top Right: Ditch Improvement (Photo by: Amy Holtshouse Brennan) Bottom Left: Daniels Park (Photo by: Amy Holtshouse Brennan) Bottom Right: Chagrin River in Waite Hill (Photo by: Brian Sherwin)

3 Page 3 TABLE OF CONTENTS Introduction to Floodplain Restoration 4 Storm Water Management Benefits Associated with Floodplains 9 Floodplain Restoration as a Storm Water Practice 11 Floodplain Restoration Tools 12 LIST OF FIGURES Figure 1 Examples of Disconnected Floodplains 4 Figure 2 Examples of Riparian Buffer Enhancement 5 Figure 3 Floodplain Re-vegetation 5 Figure 4 A Stream Restoration Project 5 Figure 5 Examples of a Degraded Stream 6 Figure 6 A Restoration Before and After 6 Figure 7 A Floodplain Restoration Project 7 Figure 8 Cross-section of Floodplain Restoration 8 Figure 9 Schematic of a Reforested Floodplain 16 Figure 10 Cross-section of Reforestation to a Subdivision 17 Figure 11 Detail of Floodplain Benching 22 Figure 12 Cross-section of Floodplain Benching and Bankfull Channel Restoration 23 Figure 13 Profile of a Regenerative Storm Conveyance Project 24 Figure 14 A Regenerative Storm Water Conveyance Project 25 Figure 15 Another Regenerative Storm Water Conveyance Project 26 Figure 16 A Before and After of a Regenerative Storm Water Conveyance Project 27 LIST OF APPENDIXES Appendix 1 Case Studies 30

4 Page 4 Introduction to Floodplain Restoration A floodplain is a flat or nearly flat lowland bordering a stream or river that experiences occasional or periodic flooding. It includes the floodway, which consists of the stream channel and adjacent areas that carry flood flows, and the flood fringe, which are areas covered by the flood, but which do not experience a strong current. The floodplain corridor acts as the right-ofway for a stream and functions as an integral part of the stream ecosystem. Floodplains perform important natural functions, including temporary storage of floodwaters, moderation of peak flows, maintenance of water quality, groundwater recharge, and prevention of erosion. Floodplains also provide habitat for wildlife, recreational opportunities, and aesthetic benefits. Ideal floodplain conditions are rarely encountered in urban or disturbed watersheds; many floodplain functions have been lost to agricultural uses and more recently to urban and suburban development (Figure 1). Floodplain restoration is the process of fully or partially restoring a stream s access to its floodplain to return those valuable floodplain functions. There are multiple types of floodplain restoration: Hydrologic. Reconnecting the stream to the floodplain and restoring the stream s natural hydrology. Vegetative. Removing invasive species and replanting native plant communities appropriate to the site and conditions. Habitat Restoration. Installing structures to improve wildlife habitat. Habitat is also gained through re-planting native plant communities. Several examples of restored and reconnected floodplains are displayed in Figures 2 through 8. Figure 1: These are two examples of streams that have been disconnected from their floodplains. On the left, upstream development has increased stream flows, resulting in channel erosion and downcutting. The entrenched stream no longer overflows onto its floodplain. On the right, the floodplain was filled in and raised during development, cutting off the stream s access to its floodplain.

5 Page 5 Figure 2: Examples of riparian buffer enhancement. In both cases, additional riparian trees and shrubs have been planted to expand the width of the forested buffer. Figure 3: Floodplain re-vegetation, before (left) and after (right). Figure 4: A stream restoration project with floodplain benching. On both sides of the restored stream, a bench has been graded at the bankfull elevation, providing room to dissipate high flow events and opportunities for riparian habitat creation.

6 Page 6 Figure 5: Severely down-cut banks (left) and compromised riparian vegetation before restoration ensued. Now restored, the stream is a stable, functioning system, reconnected to its floodplains (right). Figure 6: A severely eroded bank caused in part by agricultural activities (left) was restored by regrading the streambank, installing rock grade-control structures (to prevent bed degradation), and establishing riparian buffer (right).

7 Floodplain Restoration and Storm Water Management: Guidance and Case Study Page 7 Figure 7: Evolution of a stream and floodplain restoration project.

8 Page 8 Biohabitats, Inc. Figure 8: Conceptual cross section of floodplain restoration. The existing grade (dashed line) is lowered along the channel and different levels of floodplain are created, providing opportunities for different hydrologic regimes and resulting vegetative communities.

9 Page 9 Storm Water Management Benefits Associated with Floodplains Floodplains offer a large array of economic, ecological, and sociological benefits to communities. Floodplains can provide flood control, erosion control, storm water management and water quality services. In urban and suburban settings, it can be challenging to balance these benefits with the need for public living spaces and services. Often, engineered solutions to storm water management are pursued that allow for increased development intensity and density adjacent to streams, floodplains, and wetlands. Unfortunately, these expensive structural solutions can have limited effectiveness, can exacerbate undesirable conditions, and may simply transfer a problem to another location. For example, development adds rooftops, driveways, sidewalks, parking lots and roadways to the landscape. These hard surfaces, or impervious surfaces, prevent infiltration of storm water into the ground and produce greater volumes of storm water runoff. The storm water runoff then flows across these impervious surfaces to a curb and gutter system, which efficiently delivers the runoff to a storm drain. This means that more storm water runoff is entering the drainage network at higher velocities than it would have historically. Storm water runoff is then typically directed to a detention pond, which may be designed to prevent increases in peak discharges. However, storm water discharged from the pond may now be flowing into the receiving stream at higher velocities for a longer duration of time. This may result in increased stream channel erosion and down cutting, poor water quality, increased downstream flooding, and more frequent flooding. In the end, the most economical approach may be to preserve or restore the floodplain adjacent to a stream or river. It is helpful to consider the following benefits of floodplains: Flood Control Flooding is a natural process in stream and river systems that is essential to floodplain health and maintenance. However, human development can increase the occurrence and severity of floods, often causing property damage. Floodplains provide a vegetated, natural area where floodwaters can be slowed, stored and gradually released. Vegetation increases surface roughness of the floodplain and slows the velocity of overland flow while promoting shallow groundwater recharge, depressional surface storage, and vegetative uptake. The reduced velocity and volume of water translates into reduced flood peaks and improved base flows resulting from the slow release of water stored in floodplain soils. The reduced velocity and volume of water can also be linked directly to reduced risk and property damage. Erosion Control Channel erosion occurs as a result of fast moving and turbulent water coming into contact with vulnerable and unstable soil surfaces. Channel erosion is also related to the depth of the flowing water and discharge related to stream power and shear stress. In urban and suburban settings, erosion is typically associated with uncontrolled storm water runoff. Impervious and compacted land surfaces contribute to increased volumes and higher velocities of storm water runoff by lessening the degree of storm water infiltration and increasing the rate of runoff. This increase in volume and rate of runoff directly increases erosion, decreases stream channel stability, may cause property loss and damage, undermines infrastructure, and destroys habitat. Furthermore, as the energy of the water dissipates, the eroded materials are deposited, frequently filling in ponds, lakes, wetlands, and stream channels. These depositional areas carry similar detrimental impacts such as consumption of flood storage capacities, impacts on culverts and bridges, smothered habitats, and fish passage blockages.

10 Page 10 Healthy, deep-rooted vegetation within the floodplain can effectively dampen energy in the water, slow velocities and promote infiltration. The roots of trees and other woody vegetation promote stable soil and bank structure. Better structure gives the streambank more cohesiveness, protecting it from the erosive forces of water, resulting in smaller amounts of erosion and deposition. Natural Capital Value Floodplains are a component of the natural capital stock a community has, specifically in terms of the natural, free infrastructure it provides. Natural capital refers to the resources and living systems which combine with manufactured and human capital services to enhance human welfare. Communities can realize the presence of natural capital and take steps to restore or conserve areas that provide valuable resources. Floodplains provide costeffective and reliable human health and safety services such as flood control, erosion control, water treatment, groundwater recharge, water quality protection, and recreational opportunities that would otherwise cost communities significant amounts of money through engineered structures and systems. Water Quality Protection Floodplains can assimilate pollutants, such as sediment, nutrients, pathogens, and pesticides by filtering surface water and groundwater. Overland flow is slowed by the vegetation, causing larger sediments and the pollutants that adsorb to sediment particles to settle out. Smaller sediments, nutrients, pathogens, and pesticides not removed from surface water will be further removed through groundwater filtration, uptake by vegetation, biogeochemical processes, and microbial processes in the shallow soil profile. Without dense naturally vegetated areas, common urban runoff pollutants such as pesticides and fertilizers easily find their way into receiving waters and contribute to their impairment. Groundwater Recharge and Protection Groundwater recharge and filtration is another benefit of floodplains ultimately contributing to improved water quality. Vegetated floodplains slow surface flows, which promotes infiltration and vegetative uptake. Pollutants such as nutrients and heavy metals can be reduced or completely removed if groundwater is in contact with roots of vegetation and denitrifying microbes in the soil column. Purification of shallow groundwater is particularly enhanced when stream beds and banks act as a natural filtration system that reduces pollutant loads of waters that have been slowed and absorbed into adjacent floodplains. Ecosystem Protection Healthy, diverse floodplains directly provide or help support critical habitat (e.g., areas that support rare, threatened or endangered species) to a variety of aquatic and terrestrial species. Species diversity and abundance translates into more resilient and stable natural systems that are able to adapt to natural and human-induced disruptions. Riparian and floodplain areas are among the most diverse ecosystems due to the interface between terrestrial and aquatic ecosystems. The location and function of floodplains make them ideal links between aquatic and upland ecosystems. Equally important, they provide continuous habitat corridors that are critical for wildlife movement and access. Too often in urban areas, these important corridors become fragmented, significantly limiting the range of key species and facilitating the presence of opportunistic invasive species. Restoring floodplains in urban and suburban areas can reduce this fragmentation and help maintain important floral and faunal populations on both a local and regional scale.

11 Page 11 Floodplain Restoration as a Storm Water Practice Few precedents exist for the use of floodplain restoration as a storm water management practice. Only one state storm water manual, the Pennsylvania Stormwater Best Management Practices Manual, has included floodplain restoration as a structural Best Management Practice (BMP). The Manual notes the groundwater recharge, water quality, and peak discharge control benefits that may be derived from floodplain restoration. The manual sets forth key design elements, design features, and maintenance considerations. Accounting for floodplain restoration in the hydrologic and hydraulic analysis of a site requires careful design and analysis the volume of soil removed as part of the floodplain restoration is considered the volume of storm water runoff storage available. Several state and local storm water manuals allow for the use of the riparian corridor and the floodplain for some level of storm water management. The most common approach is through the use of storm water credits that reduce the storm water management requirements that must be met on a development site in exchange for reforestation or directing storm water runoff to a vegetated riparian corridor. These credits recognize the inherent value of vegetation in reducing storm water runoff and filtering pollutants. They also allow for straightforward accounting of the benefits of re-vegetation in a storm water management plan, facilitating integration into a site plan and streamlining the design and review process. The Vermont, Minnesota, and Maryland state storm water manuals provide a sheet flow to buffer credit. This credit assumes that the required water quality treatment is achieved for storm water runoff that flows into the vegetated riparian corridor, as long as certain performance criteria are met. The Minnesota Stormwater Manual also provides reforestation credit options. The Ohio Rainwater and Land Development Manual references floodplain restoration in two areas. First, guidance is provided on incorporating stream setbacks into the site planning process. Determining the width of the setback is discussed, along with vegetative targets and long-term protection considerations. However, how to incorporate stream setbacks into a storm water plan for a development site and how setbacks can be used to provide water quality treatment of runoff is not discussed. Second, an appendix describes objectives to consider when planning, designing, or altering stream channels. It is assumed that this appendix is a precursor to a chapter on stream rehabilitation and restoration that has not been released. Ohio EPA s guidelines for storm water management require that BMPs treat the runoff from a 0.75-inch rainfall, the water quality volume (WQv). Generally water quantity requirements are set by local communities, and typically use the critical storm criteria. The Ohio Environmental Protection Agency s (EPA) Construction General Permit allows for the use of non-structural controls, site design techniques, and other low-impact development practices as storm water BMPs. Page 24 of permit states: Non-Structural Post-Construction BMPs: The size of the structural post-construction can be reduced by incorporating non-structural post-construction BMPs into the design. Practices such as preserving open space will reduce the runoff coefficient and, thus, the WQv. Ohio EPA encourages the implementation of riparian and wetland setbacks. Practices which reduce storm water runoff include permeable pavements, green roofs, rain barrels, conservation development, smart growth, low-impact development, and other site design techniques contained in the Ohio Lake Commission s Balanced Growth Program (see In order to promote the implementation of

12 Page 12 such practices, the Director may consider the use of non-structural practices to demonstrate compliance with Part III.G.2.e of this permit for areas of the site not draining into a common drainage system of the site, i.e., sheet flow from perimeter areas such as the rear yards of residential lots, for low density development scenarios, or where the permittee can demonstrate that the intent of pollutant removal and stream protection, as required in Part III.G.2.e of this permit is being addressed through non-structural post-construction BMPs based upon review and approval by Ohio EPA. In Ohio, work is currently being performed by the Ohio State University (OSU) and the Ohio Department of Natural Resources (ODNR) to develop tools and procedures to predict and quantify the benefits of floodplains for reducing flood peak discharge and flow velocity. Ultimately, this work will provide detailed guidance on the use of floodplains and floodplain restoration as storm water management practices for quality and quantity control. As detailed above, Ohio EPA allows the use of innovative BMP s; however, their use requires approval by Ohio EPA. This guidance provides information that could be submitted to Ohio EPA to provide evidence that floodplain restoration is an appropriate BMP for a development proposal. As ODNR and OSU continue research to quantify the effects of floodplain restoration on storm water management, more detailed, locally-specific guidance and tools will be available in the future. Documentation must be submitted to Ohio EPA to verify that appropriate storm water treatment requirements are being met. This documentation may range from a simple, interim approach for accounting for the storm water management functions of floodplain restoration to a full hydraulic and hydrologic study of the watershed to show the pre and post-development hydrologic conditions of the restored floodplain. This guidance document provides information that could be presented to provide details about a proposal to use floodplain restoration as a storm water BMP. Floodplain Restoration Tools This document provides guidance on how to account for floodplain restoration as a storm water management practice in new development or redevelopment scenarios. As stated above, the Ohio EPA must approve the use of floodplain restoration as a structural storm water BMP. The use of floodplain reforestation or floodplain reconnection may also be considered a nonstructural practice for managing storm water. It is important to meet with Ohio EPA early in the planning process to discuss options for floodplain restoration to meet storm water management requirements. The following variations of floodplain restoration were considered: Floodplain Reforestation Floodplain reforestation involves planting trees and other vegetation within the floodplain with the explicit goal of re-establishing a mature, native vegetative canopy that will intercept rainfall and maximize infiltration. Precedent for the use of floodplain reforestation to help meet storm water management requirements for new development or redevelopment was found across several state programs, such as North Carolina, Vermont, Minnesota, and Maryland. Floodplain Reconnection or Expansion Floodplain reconnection restores the interactions between the stream and its floodplain, resulting in a regaining of hydrologic and ecological function. This may be accomplished by: o Lowering of the floodplain terrace through benching, o More extensive excavation to create lower floodplains,

13 Page 13 o o Raising the stream through bankfull channel restoration, or Regenerative storm water conveyance. All of the above approaches provide some level of storm water management benefits and are applicable when the stream is located within or immediately adjacent to a proposed new development or redevelopment site. These approaches should be explored early in the planning stages for all development sites adjacent to or containing a stream. Use of this approach in delineated Federal Emergency Management Agency (FEMA) floodplains may require additional permitting and review. Floodplain reforestation should be considered when the floodplain is hydrologically connected to the stream but is in need of reforestation. Floodplain reconnection should be considered when the stream is entrenched and disconnected from its floodplain. Streams that are currently entrenched, disconnected from their active floodplains, or not meeting water quality standards may be more appropriate locations for the use of this BMP. These approaches are discussed in more detail below. In addition to meeting storm water management needs, floodplain restoration may assist in meeting Total Maximum Daily Load (TMDL) targets, stream aquatic life use designation, maintaining Coldwater Habitat (CWH). Ohio EPA may consider additional watershed specific storm water National Pollutant Discharge Elimination System (NPDES) permits that include requirements linked to TMDL and aquatic life use targets. Floodplain restoration may be an option for increasing infiltration and shading for coldwater habitat streams. Floodplain Reforestation Description: Floodplain reforestation involves planting trees and other vegetation within the floodplain with the explicit goal of re-establishing a mature, native vegetative canopy that will intercept rainfall and maximize infiltration (see Figures 2, 3 and 8). The storm water management benefits from floodplain reforestation include greater stream stability, reduced soil erosion, greater infiltration of storm water, and removal of storm water pollutants. Forest soils actively promote greater infiltration rates through surface organic matter and macropores created by tree roots. Forests also intercept rainfall in their canopy, reducing the amount of rain that reaches the ground. Evapotranspiration by trees increases potential water storage in the soil. Condition Where Practice Applies: The stream is located within or immediately adjacent to a proposed new development or redevelopment site. The floodplain is in need of reforestation. The floodplain is hydrologically connected. Planning Considerations: Storm water quality control requirements will be satisfied for the total area draining by sheet flow into the reforested floodplain. The minimum acceptable width for effective storm water treatment is the recommended riparian setbacks width or a minimum of 100 feet measured from the edge of streambank. The maximum contributing flow path to the reforested floodplain may be no more than 150 feet for adjacent pervious cover and no more than 75 feet for adjacent impervious cover (ODNR 2006; Figures 9 and 10).

14 Page 14 The average contributing overland slope to and across the reforested floodplain must be less than 5% (ODNR 2006). Runoff should enter the outer boundary of the reforested floodplain as sheet flow. A filter strip, splash block at downspouts, or level-spreading device may be used to spread out concentrated flow. The aquatic life use of the stream or closest downstream designated use may determine appropriate floodplain reforestation widths and vegetation. For example, coldwater stream downstream uses may require wider widths and woody vegetation, while limited warmwater aquatic life use downstream may require minimum widths with herbaceous vegetation only. Design Considerations: The planting plan must be approved by the appropriate local storm water, watershed or forestry agency, including any special site preparation needs. Use a diverse mix of native tree and shrub species. Species should be mixed randomly across the site. In areas of partial shade, use a large proportion of shade-tolerant species. Ideally a mix of dominant tree species, understory trees and shrubs, and herbaceous plants should be planted. In open areas, mix two-thirds hardier pioneer species with one-third later successional species in recognition of the difficult environment for new plants (Table 1). Common reforestation stocking rates are 600 to1000 seedlings per acre or 500 containerized stock per acre. If planting in the fall or in high use areas, seedlings are generally not recommended. Seedlings are best planted after the ground thaws and before April 14. Depending on soil conditions, the site may benefit from pre-planting preparation, including lime and/or fertilizer, and disking or plowing. A cover of annual grains such as wheat, rye or oats at 1 to 1 1/2 bushel per acre may need to be planted to temporarily stabilize soil during the establishment period. Perennial grasses are not recommended because of their competition with woody vegetation. The construction contract should contain a care and replacement warranty extending at least three growing seasons to ensure adequate survival and growth of the plant community Allowable uses should be restricted to utility rights-of-way and pedestrian footpaths. Table 1: Plant Species (Source: ODNR) Pioneer Species Later Successional Species Cottonwood Swamp white oak Box elder Pin oak Red maple Black walnut Red osier dogwood Silver maple Gray dogwood Hawthorn Silky dogwood Black haw viburnum Sycamore Maple leaf viburnum Permitting Considerations: Approval from Ohio EPA for use of this BMP as a storm water management BMP for a site. Local zoning for riparian setbacks may require the submittal of a landscaping plan.

15 Page 15 Operation and Maintenance Considerations: Monitor monthly for the first two years after planting. Check the planted sites for soil moisture, competing vegetation, mulch and pruning needs; maintain as needed. Watering may be necessary in the initial year or during periods of drought, especially if bare root material is installed. Some seedling mortality is expected but replanting may be necessary to maintain the stand density. Minimize competition from weeds and grasses through hand weeding where feasible, or mowing, mulching and use of selected herbicides. Trash pick up may be necessary depending on surrounding and upstream uses. Planning Level Construction Cost Estimates: The cost of riparian reforestation is dependent on site conditions and the amount of site preparation needed. Forest restoration and enhancement, which entails removal of invasive vegetation, establishment of native tree species, and establishment of missing vegetative strata (e.g., herb and shrub layers), generally costs $10,000 to $30,000 per acre. Reforestation, or the re-establishment of appropriate forest communities through planting of areas that have been cleared, generally costs $20,000 to $40,000 per acre. In addition, tree planting costs are variable costs and depend on plant species, tree age, planting method, labor source, and tree protection. Soil amendments, as necessary $750 to $12,500 per acre Rubble removal, as necessary $250 to $1,250 per acre Invasive plant removal, if required $250 to $1,250 per acre Bare root trees $750 to $2,000 per acre Container trees $1,250 to $3,750 per acre Balled and burlapped trees $3,125 to $10,000 per acre

16 Page 16 Figure 9: Application of this approach to a hypothetical subdivision. The reforested floodplain provides water quality treatment of the storm water runoff sheet flow from the cross-hatched areas. (Adapted from Maryland Department of the Environment (MDE), 2000)

17 Page 17 Figure 10: Application of this approach to a hypothetical subdivision. (Adapted from MDE, 2000)

18 Page 18 Floodplain Reconnection Description: Floodplain reconnection restores the interactions between the stream and its floodplain by increasing the frequency of overbank flows, resulting in a regaining of hydrologic and ecological function. It is a restoration approach for streams that are entrenched; meaning that, through some combination of downcutting and floodplain filling, the stream s flow is fully contained in the channel for flow events with approximately a two-year or higher return interval. The disconnected floodplain is called a terrace. Limiting hydrologic connectivity through entrenchment causes significant impacts, such as increased channel instability, impaired water quality, degraded riparian habitat, and higher storm flows. From a storm water management perspective, floodplain reconnection or expansion provides an opportunity to help address both water quality and quantity objectives and enhance the ecological system and promote biodiversity. Floodplain reconnection or expansion requires lowering the terrace (or part of the terrace), raising the stream, or some combination of the two. Techniques to achieve floodplain reconnection include: Terrace excavation Floodplain benching to create a two-stage channel Bankfull channel restoration with raised thalweg Regenerative storm water conveyance ODNR refers to these practices collectively as Floodplain Expansion. This document uses the terms of Floodplain Reconnection and Floodplain Expansion interchangeably. Floodplain expansion through terrace excavation removes a width of soil along the stream and lowers the terrace and stream banks, allowing water to overtop more often and spread out onto the newly created floodplain. Two key variables to this approach are the elevation and the width of the excavated floodplain. Determining the elevation depends on the desired frequency of over-bank flows; lower elevations mean more frequent flooding. The floodplain elevation thereby sets the threshold at which the benefits of floodplain connection begin. The other key variable to the terrace-lowering approach is width, which depends heavily on the site. Ideally, the excavated floodplain would extend many times wider than the channel itself so that it could provide the maximum possible surface for storage, infiltration, and riparian habitat. Most stable stream channels in Ohio have a floodprone area that is at 3-10 times the bankfull width of the channel. To utilize terrace excavation, the resulting floodplain width should be a minimum of three times the bankfull width. Please see the Design Considerations for the calculation used to determine the floodplain width for channels with slopes less than 2%. Channel stability and floodplain benefits increase as the floodplain width increases. However, stream corridors, particularly in suburban or urban areas, are also society s infrastructure corridors, with roads, sewers, power and phone lines, gas lines, fences, and other structures nearby. As a result, the feasible width of a new floodplain may be less than ideal. In addition, the cost of excavation of the large volumes of soil that may be necessary to create a wide floodplain can become a limiting factor on floodplain width. In situations with considerable constraints, Floodplain expansion through floodplain benching may be feasible (Figure 11). In such cases, a floodplain bench or terrace is excavated along the stream, resulting in a two-stage channel with the new second stage

19 Page 19 providing some floodplain functions. As the constraints to width decrease, the bench can become wider and wider, eventually approaching terrace excavation, as described earlier. Both of these methods can occur in conjunction with restoration of the stream channel itself. Two additional methods of floodplain restoration involve reconnecting a terrace and stream by raising the stream itself. The first of these, bankfull channel restoration, requires modifying the entire stream channel and is often part of an overall stream restoration project (Figure 12). Stream restoration in general does not necessarily raise the stream, so only projects that actually raise the stream s thalweg, deepest part of the stream channel, apply to this method. The raised, reconfigured channel will carry less flow and flood sooner than it does in its entrenched condition. The new elevation of the water surface relative to the floodplain determines the frequency of over-bank flows. Typical stream restorations based on a bankfull design begin flooding at a return interval between one and two years, so they have little influence on one-year or more frequent flows. To maximize storm water management benefits, channels that overtop onto their floodplain more frequently than a one-year return interval may be considered. A design approach called regenerative storm water conveyance (RSC) by Biohabitats manages storm water through stream restoration techniques that create an open channel conveyance with pools and riffle-weir grade controls (Figures 13 through 16). Using RSC, an incised channel is filled with gravel material held back by weir or riffle grade controls. Flows through RSC move through a subsurface seepage through the gravel material, with a grade controlled stream above the seepage pools. RSC is designed to allow the created pools to infiltrate water, reducing in channel flow and recharging local shallow groundwater. RSC maximizes the connectivity between a stream and floodplain, thereby maximizing the physical features, chemical processes, water quality and quantity benefits. This approach results in low energy storm water discharge, potential water volume loss through infiltration and seepage, increased temporary water storage, restoration of lowered ground water, increases in pool wetland area, improved water quality treatment, and improvements in local micro-habitat diversity. In both cases, whether using a bankfull or regenerative storm water conveyance design approach, the downstream end of the raised channel section will need to step back down to the entrenched stream elevation. Floodplain reconnection and expansion can decrease the runoff volume and critical storm. Floodplain reconnection and expansion that incorporates reforestation of areas that may have been turf, gravel, or a hardened surface will further reduce the post-development runoff volume. This will reduce the percent increase in runoff volume, which may decrease the critical storm and result in reduced storm water quantity control requirements. Condition Where Practice Applies: The stream is located within or immediately adjacent to the proposed development or redevelopment site. The stream is entrenched and disconnected from its floodplain. Sufficient area exists to restore sufficient floodplain width. Planning Considerations: The downstream aquatic life use, attainment status, or TMDL requirements may support the use of floodplain restoration as a BMP. Ohio EPA TMDL and Water Quality Studies or local Watershed Action Plans can be referenced to determine the existing downstream conditions.

20 Page 20 If the available reconnected floodplain is less than three times the bankfull width of the stream, then the limited benefits gained through reconnection should not be considered. Hydrologic modeling of the watershed and hydraulic modeling of the local drainage features are crucial in determining the efficacy of floodplain reconnection. Modeling efforts typically carried out for the design of storm water structures can be used to evaluate effectiveness. A designer should then consider the benefits the restored floodplain provides in reducing peak discharge rates. The restored floodplain connections and elevations should be incorporated into modeling to determine compliance with peak discharge and storm water quantity control requirements for the site. Options for floodplain reconnection become more restricted when the stream in question forms part of the site s boundary. Without having control over both sides of the stream, changes to the floodplain are harder to make. Only the terrace-lowering methods (terrace excavation and floodplain benching) can be considered since the stream-raising methods involve both sides of the channel. The designer needs to consider that floodplain modification on only one side may increase the risk of causing instability in the stream. In addition, the hydrologic and hydraulic modeling would need to include bordering streams. Design Considerations: If the stream runs through the parcel, or the developer has access to the entire width of the stream corridor, reconnection options include: o Terrace excavation o Floodplain benching o Bankfull channel restoration o Regenerative storm water conveyance If the stream runs adjacent to or forms the border of the parcel, or the developer has access to only one side of the stream corridor, reconnection options include: o Terrace excavation o Floodplain benching Floodplain heights should be created at bankfull stage. Bankfull stage is approximately 2 year recurrence interval (may be recurrence interval in more urbanized streams). Target restored floodplain width is equal to 147 x DA 0.38, where DA = drainage area in acres. The aquatic life use of the stream or closest downstream designated use may determine appropriate floodplain restoration widths. For example, coldwater stream downstream uses may require wider widths and woody vegetation, while limited warmwater aquatic life use downstream may require minimum widths with herbaceous vegetation only. If the channel slope is less than 2%, the floodplain width to be replaced must be equal to at least 30% of the Low Wetted Width Area, or floodplain area inundated at bankfull stage. Low Wetted Width Area = L (ft) *12.6*DA 0.38 Where: L = linear feet of channel floodplain proposed to be created DA= Post Construction Drainage Area in acres If the downstream resource is of high quality, such as State Scenic River, Coldwater Habitat, or Exceptional Warmwater Habitat, at least 50% and up to 100% of the target floodplain width should be recreated to protect these downstream uses. Low wetted area width, as detailed above, must have stable banks and be vegetated with native vegetation as described in the Floodplain Reforestation component. Both hydrologic and hydraulic (H&H) modeling is required to properly evaluate water quantity effects of reconnected floodplains.

21 Page 21 Upstream and downstream conditions, and the boundaries of the potential floodplain reconnection need to be assessed and may influence feasibility and design. The stream s sediment load needs to be considered. Regenerative storm water conveyance may not be applicable in systems that need to transport significant bedload downstream. Permitting Considerations: Working in the floodplain might trigger various permits. The list below is some of the permits that might be required per various agencies like US Army Corps of Engineer (USACE), Ohio EPA and local agencies: USACE Section 404 permit consideration - Terrace lowering to a bankfull level may not need to go through the Section 404 process if working above the ordinary high water. Raising the stream would require a Section 404 nationwide permit, at a minimum. Stream restoration should fall under Nationwide Permit No. 27. Storm water management facilities could fall under Nationwide Permit No. 43. Ohio EPA 401 water quality certification may be necessary if an USACE individual 404 permit is required. Ohio EPA Construction General Permit Local regulations Floodplain restoration may need coordination with local riparian setbacks, local storm water quantity control, and flood damage reduction codes. Coordination with the local Soil and Water Conservation District. Any work in the floodplain will require authorization from local floodplain manager. Operation and Maintenance Considerations: Inspect on a regular basis and after large storm events for the first year after construction. Include a contingency in the construction contract to adjust structures and maintain vegetation for the first two growing seasons. Plant and woody vegetation should be inspected the first two years. Develop an invasive species management plan to help with the suppression of vegetated invasive species. Planning Level Construction Cost Estimates: Construction costs for floodplain reconnection are highly site dependent. Planning level construction cost ranges are as follows: Lowering the floodplain using a combination of limited benching and full-width restoration: Can vary widely depending on depth of excavation and disposal of materials $25 to $500 per lineal foot Raising the stream by bankfull channel restoration: $200 to $400 per lineal foot Raising the stream by regenerative storm water conveyance: $200 to $400 lineal foot

22 Page 22 Biohabitats, Inc. Figure 11: Detail showing floodplain benching. In locations without enough room for more complete excavation of the terrace, benches can be excavated to provide some of the benefits of a floodplain.

23 Page 23 Biohabitats, Inc. Figure 12: Conceptual cross section of a reconnection that combines floodplain benching with bankfull channel restoration that partially raises the stream. Even though the private residences on each side of the stream limit the space available for restoration, this hybrid approach can still provide significant benefits within the existing channel footprint.

24 Page 24 Biohabitats, Inc. Figure 13: Example conceptual profile from a regenerative storm water conveyance project. The series of riffle/weir structures control flow and allow frequent floodplain interaction.

25 Page 25 Figure 14: Photograph of a regenerative storm water conveyance project. The riffle/weir structures control the stream s flow and create this series of wetland pools that have maximum connection to the floodplain. The vegetation and ponding provide significant water quality benefits, including nutrient removal. This approach also enhances critical ecological communities.

26 Page 26 Figure 15: Another example of a regenerative storm water conveyance project.

27 Page 27 Figure 16: Before and after photographs of a regenerative storm water conveyance project in Northeast Ohio. The concrete drainage channel in the top photo was removed and riffle/weir structures built that control the stream s flow. The bottom photo shows the site during a storm event less than one year after construction. By design, the structures now create a series of wetland pools that have maximum connection to the floodplain. The growing vegetation and ponding provide significant water quality benefits, including nutrient removal. This approach also enhances critical ecological communities.

28 Page 28 References and Resources Association of State Floodplain Managers, Natural and Beneficial Floodplain Functions: Floodplain Management More than Flood Loss Reduction. Boyd, L. Wetland Buffer Zones and Beyond: Wildlife Use of Wetland Buffer Zones and Their Protection under the Massachusetts Wetland Protection Act. WCPP, DNRC, and Univ. of Massachusetts, Cappiella, Karen, Tom Schueler, and Tiffany Wright Urban Watershed Forestry Manual Part 1: Methods for Increasing Forest Cover in a Watershed. Center for Watershed Protection. Ellicott City, MD. Center for Watershed Protection. Watershed Protection Techniques: Urban Lake Management. Center for Watershed Protection, Center for Watershed Protection Impacts of Impervious Cover on Aquatic Systems. Center for Watershed Protection. Ellicott City, MD. Center for Watershed Protection Methods to Develop Restoration Plans for Small Urban Watersheds. Center for Watershed Protection. Ellicott City, MD. Center for Watershed Protection and United States Environmental Protection Agency Wetlands and Watersheds: Adapting Watershed Tools to Protect Wetlands. United States Environmental Protection Agency. Chagrin River Watershed Partners, Inc., et al., Riparian Setbacks Technical Information for Decision Makers. Connecticut River Joint Commissions, Riparian Buffers for the Connecticut River Watershed: Hirschman, David and John Kosco Managing Stormwater in Your Community: A Guide for Building an Effective Post-Construction Program. Center for Watershed Protection. Ellicott City, MD. Maryland Department of the Environment, Maryland Stormwater Design Manual. Minnesota Pollution Control Agency Minnesota Stormwater Manual. Ohio Department of Natural Resources Rainwater and Land Development Manual. Ohio Department of Natural Resources. Columbus, OH. Ohio Department of Natural Resources Division of Water Floodplain Management Program Website. Ohio Department of Natural Resources. No Date. Ohio Stream Management Guide Fact Sheet Series. Ohio Department of Natural Resources. Columbus, OH.

29 Page 29 Ohio Environmental Protection Agency Authorization for Storm Water Discharges Associated with Construction Activity under the National Pollutant Discharge Elimination System. Ohio EPA Permit No. OHC Pennsylvania Department of Environmental Protection, Pennsylvania Stormwater Best Management Practices Manual. Presler, H.H. Successful implementation of Riparian Buffer Programs. The Journal of Surface Water Quality Professionals Stormwater. November/December Schueler, Tom and Ken Brown Urban Stream Repair Practices. Center for Watershed Protection. Ellicott City, MD. Schueler, T., D. Hirschman, M. Novotney, and J. Zielinski Urban Stormwater Retrofit Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection. Ellicott City, MD. Thomas, J. Riparian Conservation in California Wine County: A Comparison of the County Planning Approach. Masters Thesis. University of California, Berkley, USDA Natural Resources Conservation Service. Where the Land and Water Meet: A Guide for Protection and Restoration of Riparian Areas First Edition. USDA NRCS, September Vermont Agency of Natural Resources, The Vermont Stormwater Management Manual Wenger, S. A Review of the Scientific Literature on Riparian Buffer Width, Extent and Vegetation. Office of Public Service and Outreach at the Institute of Ecology, University of Georgia, 1999.

30 Page 30 APPENDIX 1 The case studies illustrate the concepts detailed in the guidance document. One case study is focused on storm water management on a mock development site, while the second case study focused on a natural, ecosystem restoration on property owned by Lake Metroparks. While the second case study objectives are focused on an ecological approach rather than storm water management, the concepts will assist to illustrate the concept of floodplain restoration and how this concept can be used on a development site. The case studies will help increase the understanding of floodplain restoration and the questions that must be answered to promote floodplain restoration as a storm water BMP.

31 Page 31 Chagrin River Watershed Partners, Inc. Pre-Development Conditions: Floodplain Restoration During Site Development Case Study The 87 acre property is located in the Chagrin River watershed. The site used to be a landscape nursery. Compacted soil, gravel, and impervious cover were the primary land covers on the site prior to development. The southwestern edge of the property is wooded and has steep slopes. The Eastern Stream forms the northern, eastern, and southern property boundary. This stream is perennial, in good condition with little evidence of erosion or floodplain disconnection. However, vegetation and forest cover along the riparian corridor is fragmented and minimal. Two smaller streams flow easterly across the property. The Northern Stream is intermittent, and its drainage area is contained within the property boundaries. The Southern Stream is also intermittent, and its drainage area includes a small area west of the property boundary. Both streams are entrenched, disconnected from their floodplains, and lack vegetation in their riparian corridors. Small wetlands can be found at the western end of each stream. An existing road bisects the northern third of the property. Storm water runoff from this road is collected and conveyed in an existing storm drain system; it does not flow onto the property. An existing, operating natural gas well is present on the north end of the property. Design Requirements: Parcel Size 87 acres Number of Lots 29 lots Minimum Lot Size 1½ acres Road Right of Way 60 feet Local Street Pavement Width 25 feet Minimum Lot Width 150 feet Front Setback 75 feet Side Yard Setback 25 feet Rear Yard Setback 50 feet Minimum Building Footprint 1000 square feet Minimum Riparian Setback (Eastern Stream) 120 feet Minimum Riparian Setback (Northern Stream and Southern Stream) 25 feet Open Space 40%

32 Page 32 Design Approach: The site design incorporates principles of open space design. The number of lots was determined based on three acre zoning, but the minimum lot size is 1½ acres, allowing for the lots to be clustered and preserving contiguous areas of open space. The general design approach was as follows: Open space areas were identified early in the design process. These include streams, wetlands, floodplains, riparian corridors, forested areas and steep slopes. Setbacks from the streams and wetlands were delineated. Potential access routes to the existing natural gas well were delineated. An iterative approach was used to lay out the house sites, parcels and roads. The number of view lots (e.g., lots with views of natural areas) and the number of lots adjacent to open space were maximized. The road alignment was designed to minimize the number of stream crossings. The minimum allowable road width of 25 feet was maintained. Roads are open section, e.g., there is no curb and gutter. Storm Water Management Approach: Storm water management requirements for this site include treatment of the water quality volume and control of the peak discharge rate of runoff for the critical storm. The critical storm for this site is the one-year, 24-hour storm. The storm water management approach for this site utilizes a combination of non-structural post-construction best management practices, floodplain restoration, and structural postconstruction BMPs. Emphasis is placed on maximizing the benefit of non-structural practices through the reduction and disconnection of impervious cover, vegetative enhancements, and reforestation. Runoff is conveyed as sheetflow to the reforested floodplain, or is conveyed in enhanced water quality swales to the reconnected floodplain. Specific techniques used throughout the site include: Downspouts on all of the houses are disconnected with splash blocks. The floodplain along the Eastern Stream was reforested (see circles representing trees planted in floodplain). Please see the Floodplain Restoration Guidance: Floodplain Restoration and Storm Water Management for the design planning and design considerations. This included a combination of forest restoration, enhancement and reforestation along 4,350 lineal feet with a width of 100 feet, for a total of 10 acres. This reforested floodplain provides storm water quality treatment of runoff that sheet flows into it from approximately 13.2 acres. Several enhanced water quality swales along roads and through open spaces manage storm water runoff from the roads and residential lots. The drainage area to each swale is less than 5 acres. The Northern Stream was reconnected to its floodplain using a floodplain terracing approach (Figure 1). Please see the Floodplain Restoration Guidance: Floodplain Restoration and Storm Water Management for the design planning and design considerations. The floodplain and riparian corridor was revegetated and reforested. This reconnected floodplain reduces the post-development peak discharge rate of runoff.