HomeMy WebLinkAbout2015-06-11_Iowa City final drafti
Photo: XXX
August 2013
North Wastewater Treatment Plant Restoration
FINAL DRAFT
JUNE 2015
Photo: Southern equalization basin at the decommissioned treatment facility EPA 833-R-14-45.3K
2014 GREEN INFRASTRUCTURE TECHNICAL ASSISTANCE PROGRAM
Iowa City, IA
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About the Green Infrastructure Technical Assistance Program
Stormwater runoff is a major cause of water pollution in urban areas. When rain falls in undeveloped
areas, soil and plants absorb and filter thewater. When rain falls on our roofs, streets, and parking lots,
however, thewater cannot soakinto theground. In most urban areas, stormwater is drained through
engineered collection systems and discharged into nearby water bodies. The stormwater carries trash,
bacteria, heavy metals, and other pollutants from the urban landscape, polluting thereceiving waters.
Higher flows also can cause erosion and flooding in urban streams, damaging habitat, property, and
infrastructure.
Green infrastructureuses vegetation, soils, and natural processes to managewater and createhealthier
urban environments. At thescaleof a cityor county,green infrastructure refers to thepatchworkof
natural areas that provides habitat, flood protection, cleaner air, and cleaner water. At thescaleof a
neighborhood or site,green infrastructure refers to stormwater management systems that mimic nature
bysoaking up and storing water. Green infrastructurecan bea cost-effectiveapproach to improving
water qualityand helping communities stretch their infrastructureinvestments further byproviding
multiple environmental, economic, and communitybenefits. This multibenefit approach creates
sustainableand resilient water infrastructurethat supports and revitalizes urban communities.
TheU.S. Environmental Protection Agency(EPA) encourages communities to usegreen infrastructureto
help manage stormwater runoff, reducesewer overflows, and improvewater quality. EPA recognizes
thevalue of working collaborativelywith communities to support broader adoption of green
infrastructureapproaches. Technical assistanceis a keycomponent to accelerating theimplementation
of green infrastructureacross thenation and aligns with EPA’s commitment to providecommunity-
focused outreach and support in responseto the President’s Priority Agenda for Enhancing the Climate
Resilience of America’s Natural Resources. Creating moreresilient systems will become increasingly
important in thefaceof a changing climate.As more intense weather events or dwindling water
supplies stress the performanceof thenation’s water infrastructure, green infrastructureoffers an
approach to increaseresiliencyand adaptability.
For moreinformation, visit http://www.epa.gov/greeninfrastructure.
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Acknowledgements
Principal EPA Staff
Tamara Mittman, EPA
Christopher Kloss, EPA
JamiePiziali, EPA
Leah Medley, EPA
Community Team
Karen Howard, Cityof Iowa City
Ben Clark, City of Iowa City
Brenda Nations, Cityof Iowa City
MikeMoran, Cityof Iowa City
Zac Hall, Cityof Iowa City
Doug Boothroy, Cityof Iowa City
Kris Ackerson, Cityof Iowa City
Consultant Team
Russ Dudley, Tetra Tech
William Musser, Tetra Tech
Jonathan Smith, Tetra Tech
Martina Frey, Tetra Tech
John Kosco, Tetra Tech
This report was developed under EPA Contract No. EP-C-11-009 as part of the2014 EPA Green
InfrastructureTechnical AssistanceProgram.
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Contents
1 ExecutiveSummary........................................................................................................................1
2 Introduction...................................................................................................................................2
2.1 Historical Conditions................................................................................................................3
2.2 Project Overviewand Goals.....................................................................................................5
2.3 Project Benefits.......................................................................................................................5
2.3.1 Water QualityBenefits .....................................................................................................5
2.3.2 Flood Resiliency Benefits..................................................................................................5
2.3.3 Habitat Benefits................................................................................................................6
2.3.4 Recreational and Educational Benefits..............................................................................6
2.3.5 Local Redevelopment Benefits..........................................................................................6
3 Design Approach............................................................................................................................7
3.1 Hydrology................................................................................................................................8
3.2 Soils.......................................................................................................................................12
3.3 Topography...........................................................................................................................13
3.4 Geomorphology.....................................................................................................................13
3.5 Habitat and Water Quality.....................................................................................................16
4 Conceptual Design .......................................................................................................................18
4.1 Stream/Wetland Complex......................................................................................................18
4.2 Water QualityTreatment.......................................................................................................20
4.3 Typical Restoration Components ...........................................................................................21
4.4 Plant Palette..........................................................................................................................24
4.5 ParkConfiguration and Connection to Surrounding Redevelopment......................................29
4.6 Conceptual Design Cost Estimate...........................................................................................30
5 FutureSteps.................................................................................................................................32
6 References...................................................................................................................................32
7 Appendix A: Soil Boring Map and Logs..........................................................................................35
8 Appendix B: Soil Classification Map..............................................................................................36
9 Appendix C: Proposed Restoration Area.......................................................................................37
10 Appendix D: Draft Grading Plan and Cross Sections......................................................................38
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Figures
Figure1. Location of theNorth Wastewater Treatment Plan within Iowa City .........................................3
Figure2. 2008 Flood Overview—North Wastewater Treatment Facility...................................................4
Figure3. Riverfront Crossings Conceptual Rendering of the ParkDistrict (Labels Added) .........................7
Figure4. Floodplain SiteTypes ................................................................................................................8
Figure5. Screen Capturefrom USGS StreamStats Showing theRalston Creek Watershed........................9
Figure6. Stage-DischargeRelationship for Ralston CreekDetermined Using Flows from StreamStats....10
Figure8. Section of theJohnson County FIRM, Panel 195......................................................................12
Figure9. Channel Pattern of Ralston Creek............................................................................................14
Figure10. BankStabilityof Ralston Creek..............................................................................................14
Figure11. Channel Condition of Ralston Creek ......................................................................................14
Figure12. BankHeight of Ralston Creek................................................................................................14
Figure13. Map of Iowa City(1947)........................................................................................................15
Figure14. Close-up of Treatment FacilitySite(1947).............................................................................16
Figure14. Conceptual Rendering of Restoration Cross Section ..............................................................18
Figure15. Proposed Grading for theRestoration Area...........................................................................19
Figure15. SiteInundation Under Flood Conditions................................................................................21
Figure16. Examples of Design Details and Field Photos of Root Wads Used in Restoration....................23
Figure17. Examples of Design Details and Field Photos of Cross Vanes used in Restoration...................23
Figure18. Exampleof Design Detail of DoubleSoil Lift Used in Restoration...........................................24
Figure19. Exampleof Design Detail of Imbricated Rip-Rap Used in Restoration.....................................24
Figure20. Typical Cross Section of Wetland Types.................................................................................25
Figure21. Proposed Concept for Riverfront Crossings Park....................................................................29
Figure22. Gravel Wetland Schematic ....................................................................................................30
Tables
Table1. Ralston CreekWater SurfaceElevations ...................................................................................10
Table2. Differencein Elevation between theFIS and thoseDetermined byStreamStats Flows..............10
Table3. Comparison of Iowa River and Ralston Creek Flood Elevations .................................................11
Table4. Ground Water Elevations .........................................................................................................11
Table5. Design Elevations for Floodplain Zones.....................................................................................12
Table6. Ralston CreekWatershed Stream Assessment..........................................................................14
Table7. Performance of Storm Water Wetlands....................................................................................17
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Table8. Wetland Types by Water Depth................................................................................................25
Table9. Conceptual Design Cost Estimate .............................................................................................31
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1 Executive Summary
Likemanyriverinecommunities across thecountry, Iowa Cityhas a rich historythat is intricatelylinked
to its water resources. Development has encroached upon theIowa River and Ralston Creek to thepoint
wheresomeareas of thechannels havebeen hardened, straightened, or even buried. “Sunny-day
development” of Iowa City’s floodplains has left critical infrastructurevulnerableto largefloods and
heavystorm events, which potentially will occur morefrequently with climatechangeaffecting weather
patterns.
TheNorth Wastewater Treatment Plant, inundated during the flood of 2008, is oneexample of critical
public infrastructurebeing susceptibleto flooding from theIowa River. Located at theconfluenceof
Ralston Creekand theriver and immediatelyupstream of Highway6, theNorth Wastewater Treatment
Plant sitebecomes completely isolated during the100-year flood event. Rather than continually
protecting thetreatment plant from future floods or repairing it after flood damage, Iowa Cityhas
decommissioned theplant and is removing all of thebuilt components from thefloodplain. As part of
that project, thecityplans to soften theedgealong theIowa River by creating a public parkwith 5 acres
of restored floodplain and wetland area along Ralston Creek.
Stream restoration and restored wetlands at the treatment plant site can result in benefits to water
quality, flood resiliency, urban habitat, recreation, and education, and can serveas a catalyst to
encourageadditional economic development in theadjacent areas. To createa viableand sustainable
ecosystem that can support thenecessaryflora community, a reliablewater supply is needed to
establish thewetland conditions. Sincethis area is at the confluence of theriver and Ralston Creek,
excavation of thefloodplain is proposed to tieinto theground water table. To maximizethe water
qualitypotential of thewetland area, restoration of thecreekbanks is proposed that will allow
stormwater runoff from theRalston Creeksubwatershed to flowinto thewetland area during more
frequent storm events. Stream restoration structures and emergent plant species areproposed that
support long-term stability, habitat creation, and aesthetics of theproject site.
Thestream and floodplain restoration is onecomponent of a larger proposed parkplan, which in turn is
part of a larger redevelopment master plan for downtown Iowa City. Trails and pathways will connect
therestoration site with theremainder of theparkand theadjacent proposed mixed-usedevelopment
area. Cityresidents and visitors will beableto access therestored area at a variety of points, and
helping to instill a strong environmental ethic in frequent users of the park. Thegreen infrastructure
concepts implemented at a largescale in therestored wetland will beeasily expanded beyond thesite
with theproposed implementation of smaller stormwater gravel wetlands to treat stormwater from
impervious areas proposed as part of themaster redevelopment plan, further connecting neighboring
residents to Ralston Creekand their local environment.
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2 Introduction
Iowa City’s historyas theformer statecapital and hometo theUniversity of Iowa is richly intertwined
with theIowa River. Residential and commercial zones are located on both sides of theriver, and the
university’s world-renowned hydraulics laboratoryis built on the river to enablescientists to drawwater
directlyfrom it for experiments. Even with the Coralville Reservoir controlling much of thewatershed
upstream, Iowa City has experienced several large floods and has suffered extensivedamageto many
cityand universityproperties. As the citycontinues to grow, much of its growth is focused on moving
critical infrastructureout of thefloodplain and providing effective management and safeaccess to the
Iowa River corridor.
Onesuch project is thedecommissioning and demolition of theNorth Wastewater Treatment Plant,
located at theconfluenceof theIowa River and Ralston Creek north of Highway6 (seeFigure1). Thesite
is approximately1 milesouth of theUniversityof Iowa campus and downtown Iowa Cityand is easily
accessiblefrom nearbyresidential areas. Themajorityof theplant components had been elevated out
of thefloodplain, but areas received floodwaters during extremeflood events; most recently in the
summer of 2008, leaving thefacilitynearly inoperable. Becauseof the increased risk, theplant was
decommissioned and Iowa Cityreceived funding to demolish its components in preparation for
converting thearea into a public park. The park will serveas a focal point and provideriver access for
Riverfront Crossings, a planned mixed-useredevelopment area to thenorth and east of theproject site,
and will include restoration of Ralston Creekbackto a historic riparian wetland/floodplain.
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Figure 1. Location of the North Wastewater Treatment Plan within Iowa City
2.1 Historical Conditions
TheIowa River has served as a major corridor for Iowa Citycommercial and residential areas and the
Universityof Iowa campus dating backto the19th century. Several railroad and vehicular bridges have
provided easyaccess to both sides of the river, and a low-head dam is located at theBurlington Street
Bridge, originallyto provideriver water to run earlyhydraulic experiments at theUniversityof Iowa’s C.
Maxwell Stanley laboratory—one of thenation’s oldest hydraulics labs.
Iowa River flows are partially managed bytheCoralville Reservoir located approximately5 miles
upstream of thecity. During thehistoric Iowa Flood of 2008, waters roseto record-breaking levels over
thereservoir’s emergencyspillway, causing theriver to crest at 31.5 ft1, which was over 9 ft abovethe
flood stageof 22 ft. Theflooding significantlyaffected sections of the cityand theuniversityand
inundated theNorth Wastewater Treatment Plant (Source:Iowa Homeland Security.
Figure 2), which is located at theconfluenceof theriver and Ralston Creek.
Following the extreme flood event, Iowa Citydeveloped plans to decommission thetreatment facility
and direct wastewater to theupgraded South Wastewater Treatment Plant. TheNorth Wastewater
1 “Iowa flood of 2008.” Accessed from Wikipedia on September 29, 2014.
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Treatment Plant is no longer operating and plans arecurrentlybeing developed to demolish the existing
buildings on the site.
Source: Iowa Homeland Security.
Figure 2. 2008 Flood Overview—North Wastewater Treatment Facility
Thefloodplain at theconfluenceof the creekand theriver was modified to accommodatethe
construction of theNorth Wastewater Treatment Plant. Sinceit was initiallybuilt in themid-1930s, the
treatment facilityhas undergone modifications and expansions that havefurther altered the local
landscape. Thetreatment plant and theconstruction of Highway6 along thesouthern border of the
plant havedivided and greatly impacted the natural floodplain.
Thehistoric condition of thefloodplain area is unknown, but reviewof the Soil Surveyof Johnson
CountyIowa (Schermerhorn 1983) indicates that thesiteconsists of Sparta loamyfinesand, typical of
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stream benches and uplands. This soil typeis excessively drained and normallyfound on shallowslopes,
common with the loess and glacial drift that makes up much of thesoil parent material in theregion.
2.2 Project Overview and Goals
Theconstruction of theNorth Wastewater Treatment Plant and Highway6, along with theneed to
protect critical infrastructurefrom flood events, resulted in thestraightening and hardening of Ralston
Creek and created a disconnection of the Ralston Creekand Iowa River floodplains. With theremoval of
thebuildings and equipment, Iowa Cityhas theopportunityto transition thesite backto its historic,
natural condition.
Thecityintends to convert thetreatment facilitysiteinto a multiusepublic park to increaseaccess to
theIowa River and provideopen, natural spacefor the benefit of thecommunity. A 5-acreportion of the
sitewill be designed to reestablish natural floodplain connections and promoteadditional flood
mitigation benefits with thecreation of off-channel constructed wetlands. Thepurpose of theproject is
to producea concept plan for the restoration of Ralston Creekthat improves flow and habitat conditions
and maximizes treatment of stormwater overflows in constructed wetlands within thereclaimed
floodplain.
2.3 Project Benefits
To protect critical infrastructurenear thetreatment facility, Ralston Creekhas been straightened,
hardened, and essentiallyforgotten. Riprap lines both banks in someareas and thechannel cross-
section remains uniform through much of thestudyreach. Without restoration of thefloodplain, which
is significantlyhigher than thebed of thecreek and disconnected from bankfull flows, adjustments to
thechannel pattern or cross-section will have little impact on thehealth of the ecosystem. Creating a
floodplain that is accessibleunder frequent flow conditions will help establish hydrologic regimes
necessaryto createhealthystream conditions and promoteaquatic life whilealso reconnecting nearby
residents to a water resourcethat has been significantlyminimized throughout Iowa City.
2.3.1 Water Quality Benefits
A constructed wetland system located within the Ralston Creek floodplain will accept stormwater flows
from thecreekand provide water qualitybenefits through longer residencetimeand additional plant
uptakeprocesses. Byprimarilytreating creekflows that occur during short, intensestorm events, the
off-channel wetland can providewater quality benefits in addition to site-scalegreen infrastructure
distributed throughout thewatershed. Restoration from a channelized stream reach to a naturalized
section can increasetravel time, and a studybyBukaveckas (2007) has shown that slowing water
velocities can result in significantlyhigher nutrient uptakerates. Cadenasso et al. (2008) offered many
options for reducing nitrate yield from urban areas, noting that that the“keyability of newfunctional
interfaces to serveas hot spots for denitrification in urban watershed is for them to capturenitrate-
laden water and hold it long enough under the anaerobic, high-carbon conditions suitablefor
denitrification to occur.” Directing lower storm flows from Ralston Creek into a constructed wetland
area to providethenecessaryconditions and residencetimefor denitrification could result in a
significant reduction in nutrients reaching theIowa River.
2.3.2 Flood Resiliency Benefits
TheNorth Wastewater Treatment Plant was located at theconfluenceof Ralston Creekand theIowa
River, an area typicallyproneto flooding. The wastewater treatment facilities wereelevated to remain
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outsideof thefloodplain, creating an island between Ralston Creekand the Iowa River during flood
events. Bylowering this elevated area and focusing on a tiered approach to thelandscapeand plant
material, therestored floodplain area will be capableof handling larger and longer flood events
impacting theIowa River.
2.3.3 Habitat Benefits
TheIowa River and Ralston Creekupstream of the North Wastewater Treatment Plant haveheavily
developed riparian areas and channel banks. Thereis littleshelter or food supply to encouragea
diversity of aquatic animals within theIowa River or Ralston Creekcorridors throughout Iowa City.
Restoration of Ralston Creekand thesurrounding floodplain into a riparian wetland area would provide
much needed habitat to sustain a rich aquatic ecosystem. In a surveyof why landowners restore
wetlands conducted byPease et al. (1996), providing habitat for wildlife was listed as “extremely
important” bymorethan 80 percent of therespondents.
2.3.4 Recreational and Educational Benefits
Restoration of Ralston Creekand theassociated floodplain at this location creates an opportunityfor
area residents and visitors to access thestream that has largelybeen unavailablein its current condition
with heavilydeveloped riparian areas and an incised stream channel. A tiered approach to the
restoration will result in a rangeof elevations that inspires a varietyof habitats and multiplevantage
and access points to interact with thehabitat areas.
2.3.5 Local Redevelopment Benefits
Theconversion of the North Wastewater Treatment Plant site to a parkand restored floodplain serves
as a critical focal point of a planned Riverfront Crossings redevelopment project (HDR2013). Thepark
district, shown conceptuallyin Source:HDR 2013.
Figure 3, is central to thewalkable, mixed-useredevelopment anticipated on both sides of the Iowa
River and Ralston Creek. Theproposed park will serveas a public amenityfor the neighboring
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communityand the entire citybut will also inspire green infrastructureand sustainability themes that
can becarried out in a smaller scale in thesurrounding residential and commercial areas.
Source: HDR 2013.
Figure 3. Riverfront Crossings Conceptual Rendering of the Park District (Labels Added)
3 Design Approach
Floodplain restoration of the5-acreportion of the sitefocuses on thereconnection of morefrequent
storm flows within Ralston Creekto a larger floodplain area. A constructed wetland is included to
maximizewater qualitybenefits associated with stormwater inundating the larger floodplain area.
Therestored portion of Ralston Creek, the reclaimed floodplain area at theconfluenceof thecreek and
theIowa River, and the constructed wetland aredesigned to mimic historical conditions at thesiteor
natural features commonlyfound in theseenvironments. Thedesign is intended to improveecosystem
health at all flowlevels through theestablishment of multipleinundation zones defined bydifferent
flood events. Byestablishing different zones, a wide variety of plant species can be incorporated to
createa diverse environment necessaryfor resilienceunder varying water surface elevations.
Multiplezones located at different elevations aretypical of forested floodplains within Iowa. Randall
and Herring (2012) noted that “flooding can both enhanceand stress a riparian ecosystem.” They
identifythreemain floodplain sitetypes, shown in Source:Randall2012.
Figure4 as “point bars”, “first bottoms”, and “second bottoms”. Thedesign of therestored Ralston
Creek and reclaimed floodplain follows these types, with thepoint bar established byadding curvature
Treatment Facility Site
Restoration
Area
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to theRalston Creek pattern, theconstructed wetland simulating an oxbowfeature within the first
bottom, and theremnant treatment facilitysite elevation serving as the high terraceor second bottom.
Source: Randall 2012.
Figure 4. Floodplain Site Types
Thehydrologyof Ralston Creek serves as thebasis for establishing elevations of thefloodplain zones.
TheRalston Creekwatershed (5,850 acres) is much smaller and moreurbanized than theIowa River
watershed (morethan 3,200 squaremiles at Iowa City), making Ralston Creek much more responsiveto
smaller but more intensestorm events. Targeting themorefrequent fluctuations is consistent with
typical design guidance for other green infrastructure, such as EPA’s criteria of retaining the 95th
percentilestorm (USEPA 2009). In this instance, thelower storm events arenot used to size the
floodplain/wetland area but to set theelevation to thepoint at which flows will access theoverbank
area.
3.1 Hydrology
Therestoration site is influenced byflows from Ralston Creekand theIowa River; both have defined
FEMA floodplains. The Johnson CountyFlood InsuranceStudy(FIS) indicates that the100-year flood (the
highest elevation that has a 1 percent chanceof occurring annually) for theIowa River varies from an
elevation of 644.6 ft at Highway6 and increases to an elevation of 645.5 ft at therailroad crossing
located near thenorthern portion of the treatment facilitysite(FEMA 2002). This represents
approximately a 1-foot drop in head as theriver flows along thesite from north to south, passing under
thestructures at therailroad, Benton Street, and Highway 6. Flood elevations of all of the expected
storm events varyfrom about 18 ft to about 26 ft abovethestream bed of theIowa River. With most of
theexisting treatment plant sitebeing situated at elevations above640 ft, most of thesite is currently
onlysubjected to the10 percent chanceflood (or 10-year return frequency) and higher. This is typical of
a higher terracefeature. Access to a floodplain between the1- and 10-year return intervals is not
consistentlyavailablethroughout thesite. Setting thewater surface elevations for lower flood events is
important to establishing thecorrect elevation for thefirst bottom zone. During higher flood events, the
floodplain in this area is dominated bytheIowa River, but during lower flood events, thewater surface
elevations will most likelydiffer between theIowa River and Ralston Creek.
Randall and Herring (2012) indicatethat first bottoms flood every 1–3 years, which is consistent with
commonlyused bankfull frequencies often used in stream restoration design (Woodyer 1968).
Established flood elevations arenot available within theFIS for events belowthe10-year return interval,
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so to develop water levels for lower storm events, USGS StreamStats was used to determineflows at the
ungaged Ralston Creek (Eash et al. 2013)2. Source:USGS StreamStats.
Figure 5 shows thedelineated watershed for Ralston Creekdisplayed in theStreamStats program.
Source: USGS StreamStats.
Figure 5. Screen Capture from USGS StreamStats Showing the Ralston Creek Watershed
StreamStats can bean effectivewayof quickly establishing flows over a broad rangeof return intervals
but still needs to beconverted to flood elevations. To determine elevations, a cross-section of Ralston
Creek established from sitetopography was provided bythecity. Manning’s equation was used along
with conservativeassumptions for energyslope(0.004) and roughness (0.05)to develop a stage-
dischargerelationship for Ralston Creekin thearea of thetreatment facilitysite, as shown in
Figure 6. Flows for the2-, 5-, 10-,25-, and 50-year return intervals areconverted to elevations using this
relationship, as shown in Table1.
Table2 compares theelevations calculated using theStreamStats flows and the stage-discharge
relationship to the established elevations presented in theFIS. Thedifferenceat the10-year flood is less
than one-half foot but this difference doubles at the50-year return interval. Thereliabilityof this
analysis is limited at higher flow events, but can beused for lower events to establish thefirst bottom
level. For this conceptual design, thetop of thebank for therestored Ralston Creek will beset at 634 ft
to fit within the1–3-year flood frequencytypical of Iowa floodplains.
2 A USGS stream gageis located along thesouth branch of Ralston Creek butis notappropriatefor useto determineflows at
thetreatmentfacility site.
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Figure 6. Stage-Discharge Relationship for Ralston Creek Determined Using Flows from StreamStats
Table 1. Ralston Creek Water Surface Elevations
Frequency(Years)Flow (CFS)Elevation (ft)
2 558 634.7
5 1,210 636.7
10 1,850 638.0
25 2,830 639.5
50 3,500 640.6
Table 2. Difference in Elevation between the FIS and those Determined by StreamStats Flows
Frequency
(Years)
Elevation from
FIS(ft)
Elevation from
StreamStats (ft)
Difference
(ft)
10 638.4 638.0 0.4
50 641.4 640.6 0.8
Comparison against theIowa River water surface elevations can be madebyperforming a regression
analysis on gage data over a period of 111 years. Stageand dischargedata wereobtained for theIowa
River stream gageat Iowa Cityfrom theUSGS National Water Information System3. This gageis located
0.8 miles upstream from themouth of Ralston Creek. A Log Pearson Type III flood frequencyanalysis
was performed to determine flowrates for the2- and 5-year return intervals. A stage-discharge
relationship was developed for the Iowa River gageand used to convert theselected flows to water
3 http://waterdata.usgs.gov; Accessed on09/29/2014.
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surfaceelevations. A consistent conversion factor was applied to relatethewater surface elevations
determined at thestream gage to water surface elevations at thetreatment facilitysite. Table 3 includes
theIowa River elevations calculated with theregression analysis as well as the10-year water surface
elevation included in theFIS. These elevations are compared to the Ralston Creekelevations. Although
thereis minimal differenceshown at the2-year frequency, this differencevaries significantlyat higher
flood events.
Table 3. Comparison of Iowa River and Ralston Creek Flood Elevations
Frequency
(Years)
Iowa River Ralston Creek Difference
(ft)Elevation from
FIS(ft)
Elevation from
Regression (ft)
Elevation from
FIS(ft)
Elevation from
StreamStats (ft)
2 634.9 634.7 0.2
5 638.9 636.7 2.2
10 639.8 638.4 1.4
Establishing an oxbow featureor, in this case, an off-channel constructed wetland, requires tapping into
ground water sources to obtain thehydrologic conditions necessaryto support emergent wetland plant
species. Sincethis project is at a conceptual level, no soil borings or ground water investigations were
conducted. Instead, past soil investigations wereused to determinepotential ground water levels. A
subsurfaceinvestigation was conducted byStanleyConsultants in July 1994 as part of proposed
wastewater treatment and collection facilities improvements. Four borings were conducted within or
adjacent to thetreatment facility; a map of theboring locations (indicated in whitetext) and theboring
logs areincluded in Appendix A. Ground water levels for the boring locations areshown in Table4.
Table 4. Ground Water Elevations
Boring No.Description of Location Surface Elevation
(ft)
Ground Water
Elevation (ft)
NP-1 NE corner of site, near east bank of the Iowa River 645.1 630.1
5 E side of site, along west bank of Ralston Creek 641.5 631.5
7 SE corner of site, along west bank of Ralston Creek 641.5 633.0
7A SE corner of site, along east bank of RalstonCreek 646.4 626.4
Range of Elevations 641.5–646.4 626.4–633.0
Source: Stanley Consultants 1994.
Theborings conducted in 1994 showa rangeof ground water levels across the site. Excluding boring 7A,
which is located on theeast sideof Ralston Creek and not within thetreatment sitefacility, values range
from approximately630 ft at thenortheast corner of thesitenear the Iowa River to 633 ft at the
southeast corner of thesite near Ralston Creek. For thepurposes of this concept design, a summer
ground water rangeof 630–633 ft will be used to establish elevations for theconstructed wetland.
Revisiting thecross-section showing thetypical zones of Iowa floodplains, thedefining elevations for
each level arelisted in Table5. Theseelevations areused to definethegrading within the proposed 5-
acrerestoration area.
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Table 5. Design Elevations for Floodplain Zones
Floodplain Feature Elevation (ft)
Point Bar 632–634
First Bottom 634
Constructed Wetland 631
Second Bottom (Higher Terrace) 638–640
Theelevations areset from existing flood studies and gageanalysis. Proposed grading changes within
thesiteare intended to makethearea more accessibleto flood flows and could havean effect on the
floodplain surface elevations. A section of theFlood Insurance RateMap (Figure7) shows that a large
portion of thetreatment facilitysite is located in ZoneX as opposed to Zone AE. Thefloodplain model
might need to bealtered and resimulated to providethefinal floodplain elevations needed for design.
Thecurrent floodplain delineation also includes both a floodplain (flood hazard) and a floodway. The
proposed project should not intend to alter thefloodwaybut will create a larger floodplain area by
removing someof thefill material wherethetreatment plant was constructed. A more detailed
hydraulic modeling analysis of theIowa River was not performed as part of theconceptual design but
should be included in futuredesign efforts, especially if theberm along theeast bankof theIowa River
is significantlyaltered as part of theparkredevelopment.
Figure 7. Section of the Johnson County FIRM, Panel 195
3.2 Soils
TheU.S. Department of Agriculture Natural Resources Conservation Service(NRCS) provides soil surveys
for Johnson County with dates of 1922 and 1983. Evaluating both surveys often provides separate
information, as older soil surveys often provide a better context for replicating thehistoric floodplain
conditions and more recent surveys provide data on current disturbed conditions. In this case, thetwo
soil surveys represent conditions before and after theconstruction of theNorth Wastewater Treatment
Project Site
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Plant. According to the1922 survey, theprevailing soil types weresilt loams of loessial origin. Rolling
hills found within the region were forested and thesoils werenot high in organic matter. Flatter areas
typicallyconsisted of prairies generally well supplied with humus. Floodplains along rivers often made
for very fertileagricultural lands and werehistoricallystripped of timber and converted to farm land.
According to the1922 NRCS soil survey, along the courseof the Iowa River there was little alluvial land
but belowIowa Citythere was approximately30 square miles of floodplain and terrace. AboveIowa
City, there were only occasional expansions of thenarrowfloodplain, but belowthecity, thevalley
widened to 2–3 miles and produced tillablebottom land that could beused for agriculture.
The1983 NRCS maps showthesite soils as 7, 11B, 41,163F, and 793. TheIowa River is listed as “W” for
water. Theportion of theproperty where thetreatment plant is currently located is completelyin the
designation 41, which refers to soil type“Sparta”—finesand mixed with loamyfinesand. This soil type
maintains a depth to the high water tableof at least 6 ft or more belowthesurface. Sparta is an
extremelywell-drained soil type and is designated as Hydrologic Soil Group A.
The11B area is designated “Colo”—frequentlyflooded soil and is found at the location of Ralston Creek.
This soil typeconsists primarily of silty clayloam with a high water tabletypicallyfound between 1 –3 ft
belowthe surface. Colo is subjected to occasional flooding and is poorly drained, designated as
Hydrologic Soil Group B/D. A map of thesoils found in thearea is included in Appendix B.
Soils with hydrologic groups of A and B and high water tables are not hydric and not conduciveto
wetland formation without connecting to reliable hydrologic conditions needed to create a saturated or
inundated condition that promotes anaerobic conditions during the wet season. However, thesesoils
can support vibrant forested areas that accept overbankriverineflooding. Creation of off-channel
wetland systems in this area requires the connection to theground water level to establish the
necessaryhydrologic conditions. This is similar to oxbowlakes that areformed from remnant channel
sections that areclosely tied to theground water level. In natural environments, theseoxbowlakes
often transition to wetland areas through theaccumulation of sediment and wetland seed banks during
overbank flood flows.
3.3 Topography
Theconfluence of theIowa River and Ralston Creekis near theportion of thestatewheretheriverine
system transitions from riparian conditions to thenorth characterized bysteeper bluffs cut bytheriver
and littleadjacent floodplain to conditions further south characterized bybroader expanses of
floodplain as much as 2–3 miles wide. Therestoration system proposed in this conceptual design would
transition to overbank systems that followthe concept of providing for multiple hydrological conditions
and habitats, to match theoverall transition to wider floodplains typical of southern Iowa.
3.4 Geomorphology
No geomorphological field assessments wereconducted as part of this conceptual-level design. To
develop an understanding of geomorphologic conditions, historic maps and an external stream
assessment were examined.
TheIowa Department of Natural Resources (DNR) recentlyconducted a watershed wideassessment of
Ralston Creek, producing several informative maps that document current stream conditions. Shown in
Figures 8 through 11, theDNRevaluated stream parameters such as channel pattern, bankstability,
14
channel condition, and bankheight. A comparison of the parameters between thesection of Ralston
Creek along thetreatment facilitysiteand thedominating condition of theRalston Creek watershed is
included in Table 6.
Table 6. Ralston Creek Watershed Stream Assessment
Stream Parameter Condition at Treatment Facility Site Dominating Condition in Watershed
Channel Pattern Meandering, Straight Straight
Bank Stability Moderate Erosion, Minor Erosion Stable
Channel Condition Past Channel Alteration Natural Channel
Bank Height 6–10 ft 3–6 ft, 6–10 ft
Source: DNR2014.
In comparison to theoverall condition of Ralston Creek, which is dominated bystableand natural
channels, thesection of Ralston Creek bordering the treatment facilitysiteis disconnected from the
Figure 8. Channel Pattern of Ralston Creek Figure 9. Bank Stability of Ralston Creek
Figure 10. Channel Condition of Ralston Creek Figure 11. Bank Height of Ralston Creek
15
floodplain, altered, and showing erosion. Thefigures show a morecomprehensiveview of Ralston Creek
from theassessment. Even though much of Ralston Creek is considered to be a natural channel
condition, high banks and erosion arestill present in thesesections. This assessment indicates that
Ralston Creekcould be evolving due to changes in hydrology within the watershed and that therecould
bea great opportunityto improvethis section of Ralston Creekto match thepre-existing natural
conditions found upstream and serveas a guidefor future restoration upstream if instabilities continue.
Theassessment validates theopportunity for channel restoration along the selected reach of Ralston
Creek. As discussed in section 3.1, Iowa floodplains typicallyincludea point bar that extends towards a
first bottom. Establishing theappropriate connection to thefirst bottom is critical to restoring a natural
channel condition. Even with this connection, creating a point bar featureis not possible in a
straightened stream channel. An investigation into previous conditions can help to establish the
appropriatecurvatureto help initiatethecreation of point bar features. A 1947 map of Iowa City
prepared bytheIowa CityEngineer’s Officeis shown in Source:Iowa City Engineering Office 1947
Figure 12. A close-up of thetreatment facilitysite (Source:Iowa City Engineering Office 1947
Figure 13) shows thehistoric pattern of Ralston Creek. Mimicking theseconditions leads to the
establishment of a meandering channel with a radius of curvature in thebends of approximately230 ft.
Source: Iowa City Engineering Office 1947
Figure 12. Map of Iowa City (1947)
Project Site
16
Source: Iowa City Engineering Office 1947
Figure 13. Close-up of Treatment Facility Site (1947)
3.5 Habitat and Water Quality
Wetlands havebeen used for both treatment of both stormwater and wastewater flows (USEPA 1999;
Miller 2007) and arebecoming morefrequently promoted as a green infrastructuretechniqueto
improve water quality. Stormwater runoff is a major sourceof pollutants to our waterways and has
been shown to transport pesticides, oils, heavymetals, nutrients, and other pollutants (Lenhart 2011).
Constructed wetlands can playan important rolein limiting theimpacts of urban runoff. Their benefits
can includemitigating peaks in flowfor flood control, replenishing ground water, reducing channel
erosion, filtering runoff with vegetation, providing wildlifehabitat, and pollutant removal. Stormwater
wetlands are oneof themost reliableand efficient methods of pollutant removal among stormwater
practices, whilealso offering aesthetic and economic value(SMRC 2014; USEPA 1995).Stormwater
wetlands provide valuablehabitat for a diversewildlifepopulation and can improvedownstream water
qualityand wildlifehabitat by removing pollutants, reducing sediment loads, and reducing streambank
erosion (USEPA 1999).
Theeffectiveness of stormwater wetlands in pollutant removal is dependent on establishing proper
hydrology, appropriate flow paths, wetland system type, and loading rates. Stormwater wetlands
achievepollutant removal through physical (e.g., sedimentation, filtration), chemical (e.g., precipitation,
adsorption to sediments), and biological (e.g., plant and bacterial uptake) mechanisms (EPA 1996). EPA,
in a 1999 Storm Water Wetlands fact sheet, indicates significant long-term removal rates for manykey
pollutants found in urban environments (Table7). Significant improvements havebeen madesincethat
timein siting and design of wetlands to increase treatment of moretroublesome pollutants such as total
nitrogen.
Project Site
Approx. 230’
radius meander
17
Table 7. Performance of Storm Water Wetlands
Pollutant Removal Rate
Total Suspended Solids 67%
Total Phosphorus 49%
Total Nitrogen 28%
Organic Carbon 34%
Petroleum Hydrocarbons 87%
Cadmium 36%
Copper 41%
Lead 62%
Zinc 45%
Bacteria 77%
Source: Modified from USEPA 1999
A recent studyof stormwater wetlands in North Carolina found significant reductions in peakflows and
runoff volumes by 80 and 54 percent, respectively. In addition, pollutant load reductions were
significant (TKN: 35%; NO2+3: 41%; NH4-N: 42%; TN: 36%; TP: 47%; OP: 61%; and TSS: 49%) (Lenhart
2011). TheInternational BMP Databaseis a compilation of BMP effectiveness measures from various
designs throughout theworld. According to theJuly2012 update, when looking at wetland basins and
channels, median removal of pollutants comparing inflowto outflowincludeTSS: 20.0–20.4 mg/L inflow
and 9.06–14.3 mg/L outflow; TP: 0.13–0.15 mg/L inflowand 0.08–0.14 mg/L outflow; TN: 1.14–1.59
mg/L inflowand 1.19–1.33 mg/L outflow(Leisenring 2012).
Although thetypes and effectiveness of constructed wetland systems vary, a common factor in the
success of all wetland systems is thepresence of proper hydrology. Thedominant soils in the treatment
facilitysitearewell draining and not conduciveto establishing theanaerobic conditions necessaryto
support wetland vegetation. In many examplecases of creating wetlands, a natural or synthetic liner is
installed to prevent water from draining through thesoil media and ultimatelyestablishing a hydric soil
community. This is dependent on a reliable water supplythat can providetheproper hydrology. In the
caseof constructed wetlands for wastewater treatment, this supplycan beobtained reliablyfrom
effluent discharges. In thecase of stormwater treatment, thewetland system would bereliant on the
dynamic natureof storm events. In both cases, proper sizing of the wetland to match theincoming load
is critical to thesuccess of thesystem.
Alternatively, a constructed wetland can also tie into theground water supplyto providethenecessary
hydrology. Ground water can providea reliablesourceof water for wetland systems, even in well
drained soils, and is not dependent on loading variations. Ground water levels do fluctuate, which can
lead to large-scalechanges in plant communities, but effects can be mitigated by providing a diversityof
plants at a varietyof elevations throughout the wetland area. A thorough understanding of thelocal
groundwater tableis needed to ensure wetland plant communities areableto access thegroundwater
supplyat thecorrect times throughout theyear. Thefluctuation of the groundwater tablewill ultimately
definethe final grading plan and the correct ground elevations will establish a strong wetland plant
community.
18
Thediversityof plants and elevations also supports a range of habitats and promotes usebya varietyof
wildlife. Wetlands can providea food source, access to water, and refugefrom predators and
environmental conditions. Wetlands areamong the most biologicallyproductiveecosystems in the
world. Up to one-half of North American bird species nest or feed in wetlands and, although wetlands
makeup only approximately5 percent of land surface in thecontinental U.S., about 31 percent of plant
species in theU.S. arefound in wetlands (USEPA 2001a). Constructed wetlands havebeen shown to
support high levels of biodiversityamong phytoplankton, zooplankton, benthic macroinvertebrates, fish,
and birds (Shaharuddin 2011). A national survey conducted byJames Pease(1996) found that 85
percent of landowners believeproviding habitat for wildlife is an extremelyimportant reason for
restoring wetlands. Bordered bythe Iowa River on the west, Highway6 on thesouth, and existing and
proposed development areas on thenorth and east, thetreatment facilitysite is an important urban
oasis for wildlife.
4 Conceptual Design
Theapproach to theconceptual design of this restoration site was to re-createthehistoric floodplain
conditions and establish an off-channel wetland to maximizewater qualitytreatment at frequent storm
events. This approach requires the establishment of multipleelevations that replicatethefunctionality
of typical floodplain systems in this region but also stabilizes entryand exit points for flows from both
Ralston Creekand theIowa River. Ultimately, thefunctionalityof theflood flow and water quality
components of thesite should seamlessly integratewith the other proposed modifications and
amenities of theparksiteto encouragepublic interaction and environmental education in all portions of
thereclaimed treatment facilitysite. Figure14 shows a cross section of theproposed restoration design
for Ralston Creek and theoff-channel wetland area.
Figure 14. Conceptual Rendering of Restoration Cross Section
4.1 Stream/Wetland Complex
To develop a restored stream/wetland complex that restores natural functionalityof this site, a 5-acre
parcel was selected that begins near theabandoned railroad crossing of Ralston and graduallyexpands
to includethearea between the Iowa River and Ralston Creekon thesouth side of thesite, ending near
theHighway 6 embankment. An outline of theselected area is included in Appendix C. Theprimary
floodplain and wetland restoration components will be included within this area, although grading could
extend beyond this boundaryto tie into existing grades or other parkfeatures. Appendix D includes
conceptual grading plans of thesitewith cross-sections and a close-up viewof the grading in this area is
shown in Figure15.
IowaRiver RalstonCreek
19
Figure 15. Proposed Grading for the Restoration Area
Restoration of Ralston Creek into a natural channel with a stablestream/floodplain connection requires
adjustment of thechannel cross-section and pattern further to thewest. Thetop elevation of thebanks
is generallyset at 634 ft to provideoverbankflows under the1–2-year flood frequency. A bench will be
graded into theeast bankof thecreek at this elevation until it matches the existing left bank. This will
reducestresses along theexisting left bank, which is not currentlycontrolled bythecity. A radius of
curvatureof approximately230 ft, estimated from historical maps of thearea to match pre-existing
conditions, is used to pull thechannel awayfrom the existing left bankand push it backto reconnect
with theexisting channel location slightlyupstream of theculvert under Highway6.
Reestablishing sinuosity within this section will help naturallycreatepoint bars in the meander bends
connecting thestream bed to thefirst bottom (or primarybench). Creating these point bars is important
for sediment management within this reach and to encouragehabitat diversityin thechannel. Point
bars and channel sinuositycan also havean effect on local variations in water surfaceelevation for
various flood events, adjusting specific locations of overbankflows.
Theground water-supplied wetland area is established by utilizing the existing topographyto simulate
an oxbowlakeconfiguration. Significant excavation is necessaryto establish theapproximately631-ft
elevation required to access theground water table. This excavation is minimized as much as possibleby
locating thewetland area at thesite of theexisting equalization pond (elevation of approximately 638
20
ft). Typically, oxbow lakes areformed as remnant channel sections of largemeanders that arecut off
from themain channel; to simulatethis formation, a high terraceis left between Ralston Creek and the
created wetland. This high terrace(or second bottom) is located at an elevation of approximately638 ft
to adhereto thesecondarylevel found in Iowa floodplains and would bea typical feature of stream
corridors throughout this region.
This high terraceserves multiplefunctions. Primarily, it helps to ensurethat thenewly established
floodplain and wetland area are only accessed at specific locations. Thesignificant modifications to the
channel pattern and overbankfloodplain proposed in this concept design can alter thecreek’s capacity
to transport sediment. Loads from upstream could dynamicallyadjust water surface elevations and
overbank flow locations. Maintaining this high terracehelps ensurethat overbankflows primarilyfollow
designed entrancepoints into the wetland area and exit points out of the wetland area, which can be
reinforced to provide long-term stabilityof thesystem. Thehigh terrace also creates a higher
established bankon the outsidebend of the restored stream channel, helping to maintain natural flow
conditions within Ralston Creek. Finally, thehigh terraceallows a wider variation of plant material
within thearea, to include less flood-resistant plants, and better access to multiple viewpoints for the
parkusers.
To promote moreaccess to the wetland and floodplain area during lower storm events, theconceptual
design includes lower portions of thewest bankof Ralston Creek in areas wherea remnant channel
could befound. These lower areas, located at approximately 633 ft, would beaccessed during the1-year
flood event or perhaps even morefrequently. Theseareas should begraded appropriatelyto maintain a
positivedrainagefrom theentranceto the exit to facilitatea natural flow-through system following the
direction of theriparian corridor.
Water surfaceelevations of the2-year flood frequency for theIowa River aresimilar to those
established for Ralston Creek. Currently, a berm along the east bankof theIowa River prevents the
flows from accessing this area, and it is not until nearlythe 10-year flood event that portions of this site
becomedirectlyinundated bythe Iowa River. Theconcept plan proposes to lower theelevations only
slightly, to an elevation of approximately636 ft that is morecloselytied with the 5-year flood event. No
other major changes areproposed to theberm at this timeas a wayto reducegrading and cut on the
siteand to minimizeany impacts to theIowa River floodway. Coordination with other proposed park
uses could adjust theultimategrading and elevations within this area.
This proposed design requires a significant amount of cut and ultimately removal of material from the
siteto match themultipledesign elevations. Initial estimates indicatea net cut of morethan 64,000
cubic yards of material.
4.2 Water Quality Treatment
Bynot connecting theIowa River and Ralston Creekat lower flowevents, thefull wetland and floodplain
area is available to treat smaller storm events from theRalston Creek watershed. With an overbank
connection to Ralston Creeknear the mouth, this area can assist in removing sediment, nutrients, and
other pollutants beforethey enter theIowa River. Stormwater flows up to the2-year event will enter
onlyfrom Ralston Creek4 and theproposed inclusion of the high terraceand additional sitegrading will
result in longer flowpaths and residencetimes within thesystem.
4 Flood events higher than the2-year eventwillbegin to experience backwater effects from theIowa River.
21
Theultimate size of thewetland portion is dependent on availableground water hydrology, but a
minimum of 0.5 acreof permanent wetland area (located at an elevation of 631 ft) is proposed. This
permanent wetland area could expand to 1.0 acredepending on ground water and overbankflow
availability. Significant water qualityimprovements arenot limited to thepermanent wetland area. As
thewater surface elevation increases, larger portions of thesiteareaccessed causing filtering and
uptakeprocesses to occur. Figure16 shows theinundation of thesiteunder flood conditions. The
permanent wetland area under normal flowconditions (631 ft, shown in blue) is approximately0.5 acre.
As thewater surface elevation increases to 634 ft (between the1- and 2-year events), flows inundate
morethan 1 acreof the site. At 634 ft, nearly4 acres of therestored site is accessed byflood flows.
Under current conditions, water would still beprimarilyconfined to thebanks of Ralston Creekat this
elevation but under restored conditions thereis thepossibilityfor significantly more water quality
benefits through physical, biological, and chemical processes.
Figure 16. Site Inundation Under Flood Conditions
4.3 Typical Restoration Components
Typical restoration features that would bebeneficial in the design of a stormwater wetland and in
improving the bankand meander pattern of Ralston Creek includeroot wads, cross vanes, vegetated soil
lifts, and imbricated stream bed material.
634’
631’
22
Root wad structures can beused to help stabilizebanks as well as provide in-stream habitat among the
root branches for fish and invertebrates. Source:USEPA 2001b; NRCS 2008.
Figure 17 shows typical examples of howa root wad can beused along theedge of a streambank.
23
Source:USEPA 2001b; NRCS 2008.
Figure 17. Examples of Design Details and Field Photos of Root Wads Used in Restoration
Cross vanes or J-hookvanes can beused to stabilizea channel and channel flow awayfrom banks to
prevent erosion.
Source:NRCS 2006, 2007.
Figure 18 shows a typical detail of cross vanes and J-hookvanes as well as a photo of a cross vane in
practice.
Soil lifts can beused to stabilizea bankand graduallytie in to a moregradual slope. Source:WSSC 2013.
Figure 19 shows a typical detail of a soil lift.
Imbricated rip-rap can beused to stabilizea bankand prevent erosion. Source:M-NCPPC 2014
Figure 20 shows a typical detail of imbricated rip-rap.
Source: NRCS 2006, 2007.
Figure 18. Examples of Design Details and Field Photos of Cross Vanes used in Restoration
24
Source: WSSC 2013.
Figure 19. Example of Design Detail of Double Soil Lift Used in Restoration
Source: M-NCPPC 2014
Figure 20. Example of Design Detail of Imbricated Rip-Rap Used in Restoration
4.4 Plant Palette
In this region of Iowa, nativestream/wetland systems contain a variety of species with a significant
adapted variation to perennial or periodic inundation. Thetoleranceof those species would sort out
along thehydrologic gradient based on thefrequencyand duration that theareas areinundated or
saturated. Wetlands aretypicallycategorized based on thetypical water depth during thegrowing
25
season and the vegetativecommunity, as described in Table8 (Shawand Fredine1956). Theproposed
restoration of thewetland involves connection to thegroundwater table to maintain wetland hydrology
and no open water wetland areas areproposed. Instead, theproposed wetland area will beplanted with
a marsh vegetativecommunityand transition graduallyto wet meadowand upland plants. The
delineation between thesevegetativecommunities will be dependent on hydrologyand other
environment factors and will likelyadjust as theproposed restoration stabilizes. Similar plantings will be
conducted along therestored Ralston Creek, with an emphasis on wet meadow plants.
Table 8. Wetland Types by Water Depth
Wetland Type Water Depth Vegetative Community
Wet Meadow No standingwater,
soils saturated
Grasses, sedges, rushes,
various broad-leaved plants
Marsh Up to 3 ft
Grasses, bulrushes,
spikerushes, cattails, reeds,
pondweeds, waterlilies
Open Water 3 ft upto 10 ft Pondweeds, coontail,
watermilfoils, waterlilies
Source: Shaw and Fredine1956.
Source:Iowa StateUniversity Extension, 1999.
Figure 21. Typical Cross Section of Wetland Types
When selecting thepaletteof plant species, criteria were assessed to achievethefollowing:
Mimic a typical floodplain wetland with native vegetation to Iowa.
Requireas little maintenance as possibleduring thespecies establishment period.
26
Contain advantageous seed dispersal and germination traits so natural regeneration occurs
and thesystem is sustained.
Resist varied hydrologic conditions so that adaptivemanagement can beapplied and
survivabilityincreases under natural stabilization processes.
Arereadilyavailableat nurseries and in stockin sizes that makesensefor initial planting.
Createan environment for diversewildlife.
Result in an environmental restoration project that is aestheticallypleasing to thepublic.
Thepalette of plant species was created to attempt to meet thedesired criteria and is based on each
species’ suitability for each of thewetland types described above. Theconceptual grading plans will
createa tiered approach with opportunities for differing hydrologic flood regimes so that thesystem will
stabilizeand adjust its nativecomposition as anynatural system does with time.
Grasses and Grass-Like Plants
Species
Upland
Prairie
Wetland Type
Common Name Scientific Name Wet
Meadow Marsh
Big Bluestem Andropogon gerardii X X
Bluejoint Calamagrostis
canadensis X
Sedges Carex spp.X X
SpikeRushes Eleocharis spp.X X
Virginia Wild Rye Elymus virginicus X
Mana Grass Glyceria spp.X X
Rushes Juncus spp.X X X
Cutgrasses Leersia spp.X
Common Reed Phragmites australis X
Fowl BlueGrass Poa palustris X X
Bulrushes Schoenoplectus spp.X
DarkGreen
Bulrush Scirpus atrovirens X X
Wool Grass Scirpus cyperinus X X
PrairieCordgrass Spartina pectinata X
Broadleaf Cattail Typha latifolia X
27
Flowering Plants
Species
Upland
Prairie
Wetland Type
Common Name Scientific Name Wet
Meadow Marsh
Nodding Onion Allium cernuum X X
Canada Anemone Anemone canadensis X X
Milkweeds Asclepias spp.X X
Sticktight Bidens spp.X
FalseAster Boltonia asteroides X X
Buttonbush Cephalanthus
occidentalis X
Flat Topped Aster Doellingeria umbellata X X
Rattlesnake
Master Eryngium yuccifolium X X
Bonset Eupatorium perfoliatum X
JoePyeWeed Eutrochium spp.X
Sneezeweed Helenium autumnale X X
Sawtooth
Sunflower
Helianthus
grosseserratus X X
CowParsnip Heracleum maximum X X
RoseMallow Hibiscus laevis X
Great St. John's
Wort Hypericum ascyron X X
BlueFlag Iris Iris virginica X
PrairieBlazingstar Liatris pycnostachya X X
Great BlueLobelia Lobelia siphilitica X
MonkeyFlower Mimulus spp.X
WhiteWater Lily Nymphaea odorata X
Smartweed Persicaria spp.X X
Pondweed Potamogeton spp.X
Mountain Mint Pycnanthemum spp.X X
Sweet Blackeyed
Susan Rudbeckia subtomentosa X X
Water Dock Rumex britannica X X
Arrowhead Sagittaria spp.X
Cup Plant Silphium perfoliatum X X
28
Flowering Plants
Species
Upland
Prairie
Wetland Type
Common Name Scientific Name Wet
Meadow Marsh
Goldenrod Solidago spp.X X
Aster Symphyotrichum spp.X
BlueVervain Verbena hastata X X
Ironweed Vernonia gigantea X X
Golden Alexander Zizia aurea X X
Trees and Shrubs
Species
Upland
Prairie
Wetland Type
Common Name Scientific Name Wet
Meadow Marsh
Red Maple Acer rubrum X X
Silver Maple Acer saccharinum X X
Speckled Alder Alnus incana X X
Ohio Buckeye Aesculus glabra X X
Birch Betula spp.X X
Pecan Carya illinoinensis X X
Shell-BarkHickory Carya laciniosa X X
Dogwood Cornus spp.X X
BlackWalnut Juglans nigra X X
Eastern Red Cedar Juniperus virginiana X X
American
Sycamore Platanus occidentalis X X
Quacking Aspen Populus tremuloides X X
BlackCherry Prunus serotina X X
Swamp White Oak Quercus bicolor X X
Burr Oak Quercus macrocarpa X X
Pin Oak Quercus palustris X X
BlackElder Sambucus nigra X X
Vegetation establishment will beaccomplished bya combination of grading and planting and natural
regeneration. This will requireproper site preparation, installation, and maintenance. With a goal of
establishing the wetland vegetation as soon as possible, using primarily species that growrapidly such as
29
grasses, sedges, cattails, and maples is recommended with a smaller proportion of slower growing
plants to provide vegetation diversityand promoteimportant wildlife value. Quicklyestablishing desired
vegetation reduces theopportunityfor thenuisance and invasiveplants to take over thefreshly
disturbed and planted wetland and floodplain areas.
4.5 Park Configuration and Connection to Surrounding Redevelopment
Restoration of Ralston Creekand theadjacent floodplain is part of a larger effort to establish a multiple-
usepublic parkat thesiteof theformer North Wastewater Treatment Plant. This park, currentlyin the
conceptual design phase, will consist of many different design elements and will encouragea wide range
of residential users. A section of theproposed parkfocused on theconnection between therestoration
area and thesurrounding area is shown in Figure22.
Source: RDG 2015
Figure 22. Proposed Concept for Riverfront Crossings Park
A common comment raised during public outreach events for theparkproject is to obtain better public
access to thewater. A trail system is proposed that takes advantageof multipleviewpoints over the
Iowa River, Ralston Creek, and therestored wetland area. This trail system will bridgehigh points and
includelower sections that will allow users to weavethrough thewetland. Although the main entrances
from thesurrounding development areas will belocated at the north end of theproposed park, low
water crossings of Ralston Creekusing natural rockboulders are included in theconceptual stream
30
restoration design. Residents and visitors will beableto access theproposed parkfrom the planned
redevelopment area through a path that will travel through therestored floodplain area and across the
low-water crossing, providing a direct connection to therestored area.
Thewater qualitycomponents of the wetland can also be extended to thesurrounding redevelopment
areas, expanding Iowa City’s connection to this restoration. As part of theDowntown and Riverfront
Crossings Master Plan (HDR 2013), widepromenades areproposed leading towards the parkand ending
at Ralston Creek. Constructed stormwater gravel wetlands, as shown in Source:CRWA 2009.
Figure 23, can bevery effective at reducing nutrients as well as sediment (Ballestero et al. 2011). Where
thewater qualitytreatment processes associated with therestored wetland requirea larger area and
reliableground water supply, constructed stormwater gravel wetlands can beinstalled in much smaller
areas with onlystormwater runoff as a water source. Thesefacilities can beeasilyintegrated into the
redeveloped promenadeareas to treat stormwater from neighboring buildings and impervious surfaces
beforedischarging to Ralston Creek. Certain plant species can be selected for thestormwater gravel
wetlands to match planting plans for therestored wetland to providean additional visual connection
between thetwo areas.
Source: CRWA 2009.
Figure 23. Gravel Wetland Schematic
4.6 Conceptual Design Cost Estimate
A cost estimatewas prepared based on theconfiguration and elements of theproposed conceptual
level design of the wetland and stream complex. Quantities weredeveloped using theproposed grading
to createthewetland area and theproposed location of in-stream and bankrestoration components.
Unit costs wereassembled from typical industryvalues and estimates from similar stream/wetland
projects. Costs to excavateand removematerial from thesiteto createthewetland area represent the
majorityof theestimatebut refinements to thesiting of the wetland on thepropertyand coordination
with other park components neighboring projects during moredetailed design phases might reduce
both theamount of material to be removed and theunit costs for removing thematerial, greatly
impacting theoverall cost estimate. Utilizing region-specific unit costs and detailed quantities from
design plans will also reducethecontingency included at the conceptual design phase, potentially
lowering theoverall cost. Theconceptual design cost estimateis included in Table 9.
31
Table 9. Conceptual Design Cost Estimate
Item
No
Description Quant.Unit Unit Cost Total
Site Access
1 Clearing without Grubbing (including trees <10” DBH) 0 AC $9,725.00 $0.00
2 Tree Removal (greater than 10" DBH) - per inch DBH 20 IN $50.60 $1,012.00
3 Stabilized Construction Entrance- Build & Maintain 1 EA $3,040.00 $3,040.00
4 CrewHours - Construction Stakeout 25 HRS $250.00 $6,250.00
Sediment and Erosion Control
5 TreeTrunkProtection (Per Tree) 5 EA $185.00 $925.00
6 Diversion sandbag dike(up to 4' height) 400 LF $50.00 $20,000.00
7 Stream Diversion - pump mobilization 2 EA $250.00 $500.00
8 Greater than 10 inch Pump 20 ED $1,500.00 $30,000.00
9 StoneOutlet Structure 1 EA $1,000.00 $1,000.00
10 Temporary dewatering device 1 EA $1,800.00 $1,800.00
11 Silt Fence 2,200 LF $3.73 $8,206.00
Excavation and Hauling
12 Excavated Earth hauled Offsitefor Disposal 60,500 CY $27.50 $1,663,750.00
13 Excavated Earth for reuseon-siteas fill 4,700 CY $19.75 $92,825.00
14 Streambed and Stream Embankment 650 CY $102.00 $66,300.00
Structures
15 Soil - Fabric Lift 500 CY $171.00 $85,500.00
16 Class I Stone 200 CY $135.00 $27,000.00
17 Log - 12 inch diameter (onsite) 120 LF $57.00 $6,840.00
18 Class II Stone 10 CY $163.00 $1,630.00
19 Class III Stone(stacked imbricated) 150 CY $282.00 $42,300.00
20 GeotextileFilter Fabric (woven) 400 SY $3.00 $1,200.00
Landscaping
21 Wetland Seeding 35 LB $268.00 $9,380.00
22 Temporary and Permanent Riparian Seeding 6 AC $2,500.00 $15,000.00
23 Wetland Planting Plug - 2 inch 500 EA $3.45 $1,725.00
24 Tublings or approved equal 100 EA $20.00 $2,000.00
25 Shrub - Container Grown, Height 3-4 feet 20 EA $60.00 $1,200.00
26 Tree- Container Grown, 1-1½inch Caliper 20 EA $150.00 $3,000.00
Construction Subtotal $2,092,383.00
Mobilization and stakeout 5%$104,619.15
Bonds and Insurance5%$104,619.15
Construction contingency 10%$209,238.30
CONCEPT LEVEL ESTIMATION CONTINGENCY 20%$418,476.60
Total Construction Cost $2,929,336.20
32
5 Future Steps
Iowa Cityis preparing for demolition of theNorth Wastewater Treatment Plan facilities and has
developed concepts for the overall parkplan. Park planning was performed in conjunction with this
stream and wetland restoration conceptual design so that restoration components and necessary
grading can beintegrated into the overall parkplan. Both thepark plan and the restoration plan will
need to advancethrough more detailed design phases to develop construction plans. Sincethe
restoration plan affects a regulated waterway, environmental permitting will also berequired before
construction can begin.
Extension of thegreen infrastructureconcepts implemented in thewetland to thesurrounding planned
mixed-use development should beintegrated earlyin the redevelopment planning process. If the
stormwater gravel wetlands can bedesigned along with the promenadeand other necessary
infrastructure, it will maximizethe effectiveness and easeof implementation of thesegreen
infrastructuretechniques.
6 References
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Treatment of Stormwater. Paper presented at NJASLA 2012 Annual Meeting and Expo, January
29–31,2012, Atlantic City, NJ.
Bukaveckas, P.A. 2007. Effects of channel restoration on water velocity, transient storage, and nutrient
uptakein a channelized stream.Environmental Science & Technology. 41 (5):1570-1576.
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Iowa CityEngineer’s Office. 1947.Iowa City, 1947 [map]. Scale1” =800’. Universityof Iowa Libraries
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35
7 Appendix A: Soil Boring Map and Logs
36
8 Appendix B: Soil Classification Map
37
9 Appendix C: Proposed Restoration Area
38
10 Appendix D: Draft Grading Plan and Cross Sections