roxhill park hydrologic investigation and recommendations march 2000

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ROXHILL PARK:

HYDROLOGIC INVESTIGATIOI\ ANDRECOMMENDATIONS

Prepared for Seattle Parks Department

King County.Wastewater

Treatment DivisionDepartment of Natural Resources

March, 2000

KrNc. CouNrvDepartment of Natural Resources



Roxhill Park: Hydrologic Investigation and Additional Study Recommendations

Table of Contents

1. Purpose

2. Surirmary

3. Neighborhood and Management Areas

4. Historic Conditions

5. Soils

6. Stormwater

7. Groundwater

8. Discussion

9. Conclusions

10. Recommendations

Appendix A1. Methods and Analysis

2. Laboratory Tests

2.1. Methods and Analysis

2.2. Modflow Anaþsis

2.2.1. Model Setup

2.2.2. ModelCalibration2.2.3. ModelValidation

2.2.4. Simulation

Bibliography

Drawings

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PurposeThis report provides recommendations for developing a comprehensive hydrologicand hydraulic analysis for the Roxhill Park wetland restoration project. The reportwas prepared in partial fulfillment of the design task 12: storm drainage andwetland hydrology, It includes background information on site conditions andsurrounding land use, and a review of historical and existing conditions at RoxhillPark, including groundwater data from field investigations performed in 1999.Additional data needs are identified, and a scope of work for future field

investigations, hydrologic analysis and modeling, and construction are provided.

2. SummaryIn 1992 The City of Seattle adopted the Longfellow Creek 'Watershed Action Plan.One of the specific actions or goals included in this plan was to reestablish thehistoric wetlands of Longfellow Creek in Roxhill Park. This l$-acre Seattle Park islocated at the south edge of the watershed and was built over the peat bog that wasthe headwaters of Longfellow Creek. The City of Seattle has developed generalobjectives associated with this wetland restoration effort. These include reducingdownstream peak surface water flows, improving water quality, restoring naturalhabitat, enhancing public access, building stewardship and citizen involvement, andcelebrating the communities artistic and cultural legacies.

Wetland reestablishment, through creation or restoration, requires a thoroughanalysis and understanding of existing hydrologic and geomorphic conditions.Because historical data are lacking on the size, structure, and function of the wetlandthat occupied Roxhill Park, preliminary data was collected using soil test pits andgroundwater wells. These studies were performed in the northeastern half of thepark. The results obtained from these studies indicate that 5 - 6 acres of the park areunderlain with 6 - 8 feet of peat soils. There is a seasonal groundwater table thatfluctuates 3.7 ft between winter surface saturation and late fall low water.

These preliminary investigation results indicate that wetlands can be reestablished inRoxhill Park due to the presence of both peat soils and an active groundwater table.Approximately 6.5 - 7.5 acres of Roxhill Park are available for reestablishing theLongfellow Creek headwater wetland. These restored wetlands would be located inthe northeastem half of the park, in the vicinity of the historic wetland. Water depth,and the timing and duration of water level fluctuation are key to the structure andfunctions of the restored wetland.

In order to develop reasonable predications regarding the hydroperiod of the restored

wetland, a phased design and development approach is recommended. Phase 1

involves the removal of fillfrom the peat within a selected study area. Additionalinformation on evaporation and soil moisture retention would be obtained pursuantto developing a calibrated groundwater model.

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Phase 2 involves the development of the groundwater model that is calibrated to siteconditions. This model will provide the capability and flexibility to evaluate seasonalhydrologic conditions under various design and cost alternatives. The developmentof this model is central to sizing and designing the engineering structures forregulating water levels so that the wetland: a) will not flood neighboring properties,and b) will develop wetland structure and functions that are biologically andaesthetically consistent with the project goals and objectives.

3. Neighborhood and Management Areas

Roxhill Park is located SE1/4 of Section 36, Township 24, Range 3 in the Delridgedistrict of Seattle. Roxhill Park is situated in a highly urbanized environment and isbordered to the east and west by established or planned single and multifamilyresidential land uses. These include the Daystar senior retirement community locatedto the east, and the LATCH (Lutheran Alliance to Create Housing) multifamilycommunity development to the southeast. Commercial shopping centers are locatedon the north and south sides of the park. The Roxhill Elementary School is located inthe southwestem quadrant of the park.

Roxhill Park can be separated into three management areas on the basis oftopography and activity. For the purposes of this report these areas have beenidentified based on a combination of both existing and proposed future activities.These areas include:¡ Area 4 located in the northwest comer of the park, is a well drained, relatively

level bench which supports a children's playground, public restroom and picnicbenches. This area is the future location of a soccer field.

. Area 8 occupies the middle and eastern part of the park. A broad swale withslopes of 3 -7o/o is located in the middle of the park and separates areas A & B.This swale ends at the east in a 0.37 -acre Palustrine Emergent wetland. To thenorth and east of the wetland are poorly draining areas of open, grassed playfieldswith slopes of 2 - 4Yo. These playfields are bordered by 3-foot wide pavedpedestrian path that runs north-south along the eastern property line. A row of35-40 ft maple (Acer spp) trees borders this path. This area is the location forcreation of a wetland park.

r ,Area C occupies the south end of the park and includes 3.8-acres of playfield. and parking, which are used by the Roxhill Elementary School. Slopes are l-2%o

and drainage is good. No future developments are proposed for this area.

4. Historic ConditionsVery little documentation exists regarding the size, structure or functions of thewetland that occupied Roxhill Park. USGS 1:125000 maps from 1895 show two

tributaries forming the headwaters of Longfellow Creek. Aerial photographs of theareataken in 1959 indicate an approximately 8 - 10 acre wetland located in thevicinity of Roxhill Park. This area appears to be dominated by herbaceous emergentvegetation, with scatted clusters of shrub-scrub and trees. Data from soil testsindicate that apeat-forming wetland existed in the vicinity of Roxhill Park. Seedsobtained from the test pits indicate that sedges (Carex spp.) occupied the site. In

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addition, Mr. Wayne Miller, an employee of the Seola Peat Mining Company, whichmined the nearby Seola bog, was interviewed regarding the historic wetland inRoxhill Park. He indicated that attempts may have been made to farm the wetlandbut that by 1942 that effort had been abandoned. Plants found in the wetlandincluded native Indian (i.e. Labrador) tea (Rhododendron groenlandicum),blueberries(Vaccinium spp.) and willows (Salix spp.).In 1969 the City of Seattle regraded thewetland and placed 1.5 - 2 feet of topsoil on the peat for the purpose of establishing apublic park.

5. SoilsGeologic maps (V/aldron) indicate that soils in the site vicinity are predominantlyglacially derived sediments deposited during the Vashon Stade of the FraserGlaciation. These sediments comprise sands and gravels and are mantled bysignificant peat deposits. These peats were created during'\¡/arTner periods betweenand since glacial advances and range in depth from 7 to 10 feet.

In order to determine the condition of the underlying soils and groundwater, nine soiltest pits were excavated on June 3, 1999 in Area B, in the vicinity of the wetlandreestablishment area. The test pits were excavated using a construction backhoe. Pitlocations were selected by the project Ecologist, with soil logs recorded by a SeattlePublic Utilities Geologist. Field results showed that this portion of the site is coveredwith 1.5 - 2 feet of imported silt, sand and gravel. Underlying the fill are fibrist peatsranging in depths from 8-10 feet. Underneath the peatlayer is alluvium consisting ofplastic blue silt clay that reportedly extends to depth of 25 ß 30 feet (Seattle 1992).

6. StormwaterLongfellow Creek is a three-mile waterway that drains a watershed that is 2,685acres in size. Roxhill Park sits at the head of the basin, with a contributing drainagearea of approximately 60 acres, less than 2o/o of the total basin area. Average rainfallin the Longfellow Creek Watershed is 37.75 inches per year, almost two inches morethan the city average.

All stormwater from this drainage area is conveyed downstream through the park inthe buried storm pipelines to the north side of SV/ Thistle Street, where LongfellowCreek exists as an open channel. Three storm pipelines border the north, south, andeast sides of Area B. These include a 3O-inch pipe located in the SV/ CambridgeStreet ROW that connects to a 48-inch pipe located in the 27th Avenue SV/ ROV/. A2l-inchpipe is located 20 feet south of and parallel to SW Barton Street. The 48-inch and 2l-inch pipes terminate in a72-inch vault that originates within the south

shoulder of SW Barton Street.

All pipes are concrete spigot and bell style pipes. Seattle Engineering Dept.specifications call for all such pipes to be bedded in a minimum of 6inches of 3/8-inch washed gravel. The high shrink-swell characteristics and low bearingcapabilities of the native peat soils provide avery poor foundation for bedding storm

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sewer pipes. In such a situation the clay soils provide a preferable bearing horizon.Consequently, the depth of gravel backfill is quite extensive where all the peat wasremoved and backfilled with gravel. The bottom of the 48-inch storm pipeline is 1.5- 3.5 feet above the clay horizon and all of the peat below the pipes was removed andbackfilled with washed gravel. The 72-inch vault that receives the pipes is alsosimilarly bedded in gravel and thus serves as a primary groundwater outlet. As aresult of these conditions, the network of storm pipelines that ring Roxhill Parkoperate as a giant subsurface drain system.

7. GroundwaterDuring excavation of the soil test pits in Area B, groundwater was observed seepingout of gravel and sand lenses in the test pit walls, although no freestandinggroundwater was observed. Soils were generally moist throughout the peat horizon,although soil saturation was variable, ranging from 1 to 6 feet deep as measured fromthe bottom of each pit.

To observe groundwater levels, five groundwater-monitoring wells were installed.The wells were constructed of 1.5 -inch schedule 60 PVC pipes that were slottedalong the column and backfilled along the slots with washed sand. The interfacebetween the pipe and ground surface was capped with bentonite and then backfilledwith native soil.

Field study results Figure I and Figure 2) indicate a fluctuating perennialgroundwater table in the vicinity of the proposed restoration area. Groundwaterdepths are seasonally variable in response to precipitation, with lowest water levelsobserved September - November. With the onset of winter rains groundwater levelsquickly reacted, with widespread surface saturation by late November. Averagegroundwater levels have been the lowest near the northeast corner of the site, at wellT,where the storm sewer system outlets undemeath SV/ Barton St. Groundwaterlevels in the other 6 wells have ranged from 1.5 - 6.6 feet higher than well 7. Overall,there is 3.7 feet of seasonal groundwater fluctuation.

8. DiscussionBased on the preliminary results from field investigations of Roxhill Park, soil andgroundwater hydrologic conditions are favorable for the reestablishment of awetland. However, the design of groundwater-dominated wetlands is subject to amuch greater degree of uncertainty than the design of surface-water-dominatedwetlands. Groundwater flow is significantly more difficult to accurately measurethan surface water flow, and groundwater fluctuation may not have a direct or linearrelationship to surface-water level fluctuations V/LF) that control plant community

development. Excavation for wetland construction can, moreover, dramaticallyincrease evapotranspiration and lower overall groundwater levels 1-3 feet V/instonreeT).

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Water level fluctuation in wetlands is the product of complex responses of groundand surface waters to local soils and geomorphology. V/ater enters wetlands fromprecipitation, surface water, and groundwater. Water is lost from wetlands due tosome combination of evapotranspiration, surface water outflow, and groundwateroutflow. Changes in water inflow and outflow will be reflected in a change in storedwater.

Hydroperiod refers to the depth, frequency, duration, and pattern of wetlandinundation. Water level fluctuation (V/LF) is defined as the difference betweenmaximum depth and average base (i.e.: groundwater) depth in a time period.V/etland hydroperiods have been classified into four patterns (Homer and Reinelt1eeO):

1) stable base water level with low event fluctuations (SL);2) stable base water level with high event fluctuations (SH);3) fluctuating base water level with high event fluctuation (FL);4) fluctuating base water level with low event fluctuation (FH).

The periodicity and duration of water in the wetland is a major determiner of thefloral and faunal communities in a wetland as many species are linked to particulardepths and ranges of water levels. Research from the U.S. Army Corps of EngineersHydraulic Engineering Center (U.S. Army Corps of Engineers, 1996) indicates thatthe duration of inundation andlor soil saturation during the early growing season is a

key influence on plant community development. Seasonal growing conditions areparticularly critical and consequently WLF must be evaluated in terms of thefollowing four growing periods:

. Early growing season: February 15 - May 15r Intermediate growing season: May 16 - September 15

. Senescence: September 16 - November 15r Dormancy: November 16 - February 14

In general, in the Puget Sound Basin, more species of plants and animals are found inwetlands with water regimes thatmay or may not vary seasonally but which haverelatively low event fluctuations during the early and intermediate growing seasons.

Therefore wetland restoration or creation efforts should target the establishment ofhydroperiods that maintain stable (permanent) or fluctuating base (seasonal) flowswith low event water level fluctuations during these periods. Reference standards forestablishing wetlands in the Puget lowland (Azouz 1998) recommend that waterlevel fluctuation should be limited to 21cm (8.4 inches) annually. For example,Labrador tea (Rhododendron groenlandicum), which historically grew in thewetlands of Roxhill Park, tolerates an early growing season V/LF of 3.31 inches, andan intermediate going season WLF of 4.26 inches (Cooke 1998).

9. ConclusionsMany assúmptions and uncertainties exist regarding the movement of groundwaterinto and out of Roxhill Park. The contributing drainage basin is urbanized andserviced by storm pipelines that convey water around and through the perimeter of

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the park. These piped flows combine and exit the park underneath SV/ Barton St, andare routed underneath Westwood Village mall to SV/ Thistle Street where theydaylight into Longfellow Creek. These storm pipelines are extensively bedded ingravel and have coupled joints that are relatively leaky. Consequently, they may bealternately delivering water to, and draining water from the peat soils that surroundthem. The storm pipelines in the lowest area of the park, along the east side, areprobably functioning as the main drainage system for water that has been deliveredto the ground from storm pipelines higher in the park.

The hydroperiod of the future Roxhill Park wetlands will be determined by a

combination of groundwater outflow, soil moisture characteristics (i.e. capillaryfringe), and evapotranspiration. Evaporation will occur immediately after theoverburden of fill has been removed from the underlying peat soils. After plants are

established then transpiration through plant leaves and stems will remove storedwater from the soil. Altogether, as evapotranspiration exceeds the available storedwater and soil moisture capacity, the water table will be lowered, thus increasing thewater level fluctuation. It is to be expected that WLF may be drastically different inthe uncovered state than in the present confined conditions.

Wetland WLF especially during the early and intermediate growing seasons is thekey factor in the structure and function of wetlands, including soil formation, plantcommunity composition, and wildlife habitat. Overall biological richness is mostclosely linked to wetlands with a stable base and low event (SL) fluctuations(PSV/SRP 1997).In the presently confined condition of Roxhill Park there is 3.7 feetof seasonal groundwater fluctuation. V/ith the removal of the fill layer that presently

confines the peat this seasonal groundwater fluctuation is anticipated to increase dueto evaporative losses. The range of water level fluctuation may increase L2 feeL

Analysis of the relationship between water level fluctuation (WLF) and wetlandplant communities (Azous 1997) revealed that average annual fluctuations of thismagnitude (>60cm/24in) corresponded to the formation of aquatic bed plantcommunities. Such communities are characteized by low overall species diversity,and may be dominated by invasive plants such as cattail (Typha spp.) andreedcanarygrass (Phalaris arundinacea) and aggressive non-native amphibians suchbullfrog (Rana catesbeianø). Therefore, the structure and function of aquatic bedwetland communities are not consistent with the project goals and objectives forwetland reestablishment in Roxhill Park.

10. RecommendationsIn order to establish a hydroperiod and wetland community that provide wetlandattributes that are consistent with project goals and objectives, additional hydrologicinformation is needed. The following approach is recommended:

Phase 1

Additional information is needed regarding the affect of removing the confininglayer of topsoil from the underlying peat in Roxhill Park. These include the role of

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both evaporation and the capillary fringe on moisture loss and retention in theunderlying peat soils. Where there is alarge capillary fringe soils may wick water 1*feet from wetter groundwater zones to drier surface areas. In order to obtain real dataon these conditions an area of the confining topsoil would be removed to reveal theunderlying peat. In addition, cutoff walls would be installed to isolate the study area

from the surrounding geologic units. A careful monitoring program, with thepossible participation of nearby Roxhill Elementary and other schools, would beused to obtain the data needed for development of the groundwater model and thedesign of the necessary engineering structures.

Phase 2Groundwater is the most significant hydrologic component in reestablishingwetlands in Roxhill Park. The existing data which do not include the intermediategrowing season) indicate that engineering controls are required in order to reduce therange of water level fluctuation from 3.7 feúto the reference standard range of 8.4inches. In order to design these features additional hydrologic information is needed.Therefore, we recommend that a calibrated groundwater model be developed, forwhich specifications have been provided in Appendix A.

The groundwater model is needed to size, design and construct the engineeringfeatures that will regulate V/LF in the reestablished wetlands of Roxhill Park. Thesefeatures will include an outlet structure to regulate groundwater outflow through, andaround, the 72-inch storm sewer vault located in the northeast comer of the park. Inorder to contain and control the lateral flow of groundwater between adjacentproperties impervious cutoff walls might need to be constructed. An inlet structure

may be required to regulate stormwater inflows into the wetland. This may beaccomplished through the construction of flow-splitters that would permitstormwater volumes to be released as surface water into the wetland. The followingscope of work describes the information needed to develop the groundwater model inorder to size and design these engineering features as part of the original scope ofwork:

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Appendix A

Additional data on groundwater inflow and outflow are needed in order to prepare the

groundwater model. King County staff will obtain these data, along with contractconsultants. In order to determine the volume of groundwater, the areal extent ofgroundwater depths must be described and the rate of groundwater flow or conductivitymust be measured. Groundwater depth and distribution can be measured using existingand additional groundwater wells.

In order to measure the rate of groundwater conductivity; a slug-test can be performedusing a point piezometer that is open only over a short interval at the base. In this test a

given volume of water, or a solid cylinder of known volume, is suddenly introduced intothe well. The recovery time of the water level with time is then observed. The slug test

will not give an estimate of storage coefficient, which will have to be estimated from the

laboratory tests of porosity.

1. Methods and AnalysisThe slug tests will be conducted in the existing piezometers, depending on whichhave an adequate quantity of standing water in them. The test will use a solid plasticslug, a pressure transducer, and a data logger to obtain the best possible data,particularly given the uncertainties regarding the hydraulic conductivity of the peat

material. The testing will be accomplished and analyzed in general accordance withASTM methodolo gy (D 40a4 and D59 l2)'

Slug testing:ASTM D4044-96 - (Field Procedure) for Instantaneous Change in Head (Slug) Tests

for Determining Hydraulic Properties of Aquifers

ASTM D5912-96 - (Analytical Procedure) Determining Hydraulic Conductivity of an

Unconfined Aquifer by Overdamped V/ell Response to Instantaneous Change in Head(Slug)

2. Laboratory Tests

A variety of laboratory tests must be performed in order to evaluate the physical and

hydraulic properties of the peat soils. These data will be obtained by contractconsultants These will include tests for:¡ Capillary fringe/rise. Porosity. Hydraulicconductivity

2.1. Methods and AnalysisSoil samples will be obtained in as undisturbed a state as possible, using a drillingapparatus with hollow-stem auger equipment and Shelby tube samplers (if the

material will allow sampling this way). Other sampling methodology may be

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substituted in the field according to the field geologist s judgement and the

recovery results using the best methods recommended here.

The samples will be taken to a geotechnical laboratory and tested for hydraulicconductivity þermeability) and basic physical properties (e.g., porosity, unitweight, and moisture content. Again, adjustments to methodology may have to be

made in the laboratory depending on the consistency of the samples obtained.

Soil sampling (Shelby tube)ASTM Dl58l-94 - Standard Practice for Thin-Wall Tube Sampling of Soils

ASTM D5084-90 - Measurement of Hydraulic Conductivity of Saturated Porous

Materials Using a Flexible Wall Permeameter

ASTM D5856-95 - Measurement of Hydraulic Conductivity of Porous MaterialUsing a Rigid- Wall, Compaction-Mold Permeameter

ASTM D45ll-92 - Hydraulic Conductivity of Essentially Saturated Peat(Constant Head)

2.2.Modfhow AnalysisThe preparation of a groundwater model will provide the capability to evaluate

wetland water level fluctuation under different hydrologic, design, and cost

scenarios. Contract consultants will develop the model. While the informationprovided by the model will reduce decision-making uncertainty it will not,

however, eliminate all unknowns. Most importantly, a hydrologic modelcalibrated to site conditions will improve the probability of sizing, designing, and

constructing the engineering features (inlet and outlet structures) essential tomaintaining a desirable wetland community.

The development of a calibrated groundwatèr model requires information about

the geologic structure of the area around the wetland, the hydraulic properties ofthe geologic units, the rates, timing and location of evapotranspiration andgroundwater recharge. A properly calibrated groundwater model can be used tomake predictions of future events. The most popular and commonly used modelthat is recommended for this effort is MODFLOV/ (McDonald and Harbaugh1e88).

Modflow gives water levels (potentiometric surface elevation) as the primaryvariable that is solved. Consequently, the water budget is actually a by-product ofthe analysis rather than the desired outcome. The model should be a transientstate model.

2.2.1. Model Setup

The MODFLOV/ model will be developed, by a hydrogeologistcontractor, for the extent of the northern portion of the park

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RoxhillPark: HydrologicInvestigationand AdditionalStudy Recommendations

(Cambridgeto Barton Sts, 27th to 29th Aves SW).The model willinclude the peat layer as the main aquifer,the clay underlyingthe peatas an impeÍneable bottomof the model, and overlyingsoil layers (fill)and the ground surface as additionallayers (as needed)

Based on the preliminaryconceptual modelof the site, fixed(butseasonally varying)head boundaryconditionswillbe established forthe storm sewer bedding anticipated along the northern, eastem, andsouthern sides (Barton,27th, and Cambridge, respectively); these willprobablybe linearlyvaryingalong the edge (because of the highhydraulicconductivity).Amore-complicated(but stillfixedhead)boundaryconditionwillbe developed along the western boundary

The boundaryconditionswillbe estimated through the calibrationprocess, and willvary over time according to precipitationdata fromnearby rain gages.

The model willincludesome vertical recharge to the surface, withleakage though the fillto the peat. There are also some catch basinswhichwillrequire special consideration, such as ModFlowdrain cellsThe gridwillbe aligned to lineup withthe test pitwells and otherfeatures that may be included in the design.

2.2.2. ModelCalibrationThe model willbe calibrated to the 1999 water levelobservations

whichhave been made in the test pitmonitoring wells2.2.3 ModelValidation

Ifdata for2000 are avallable, the modelmay be validated againsttheSe water levels to demonstrate the robustness of the calibration(particularlythe boundary conditions)

2.2.4. SimulationThe calibrated model willbe exercised to estimate water levelfluctuationsafter excavationof the peat. Recharge willbe increased tothe total precipitation.Evapotranspiration willbe added, at aratebased on literaturevalues for peat. The top layer of fillsoilwillberemoved from the model. The boundaryconditionswillnot change,but willbe based (along withthe recharge) on the precipitationpatternand quantitiesfora qiticalperiod.

For any given simulation,the modelwillbe run firstat a steady-stateconditionusing winterrecharge rates (Novemberto February); thiswillestablish an initialhead distributionfor the subsequent period.The model willthen be run on a transient time-steppingschedule

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capable of evaluating wetland water level fluctuation during the fourgrowing seasons of interest, which are:

Early growing season: February 15 - May l5Intermediate growing season: May 16 - September 15

Senescence: September 16 - November 15

Dormancy: November 16 - February 14

It is anticipated that the initial run, simulating the situationimmediately after excavation of the peat, and with no further designchanges, will be very deficient in water, so that the water level in thepeat drops below an acceptable point (i.e., the wetland would dry out).Thus, the next steps for using the model will be adding design featuresthat will allow more water during critical growing periods.

The consultant will provide versions of the model that incorporatevarious possible engineering facilities that may allow additional waterlevels in the wetland during critical periods. While some of these are

yet to be developed, examples include:

An outlet control structure downgradient of the wetland, in thenortheast corner of the site, with cutoff walls along the portion of thenorth and east sides closest to this corner.Water storage capacity in the area of the part upgradient from thewetland area. This could be a vault or a cutoff wall, and controlled

outlet, that provides water storage in groundwater above the wetlandInterpretation of results Water level fluctuation shall be evaluated in terms of four regionalwetland hydroperiod patterns (Horner and Reinelt). The goal of themodel analysis shall be to state the expected average frequency ofexcursions greater than 15 cm above or below the mean water level ona weekly basis for each growing season. The model shall be capable ofpredicting the duration of such excursions for a normal water year

Achievement of these goals may be demonstrated by I-2 years ofsupporting data showing continuous water level monitoring providedthe water years are within normal precipitation volumes and eventsAlternatively, water levels and rainfall, using an on-site rainfall 9aùge,may be monitored for one year and a statistical analysis performed onthe data. The analysis should show the potential variation inhydroperiod based on the monitored year in relationship to historical

rainfall obtained from a nearby official rainfall gauging station.

To detennine if the water years are normal (for low land areas tnCentral Puget Sound Basin), the following suggestions may provideguidance:

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A storm event may be defined as: a precipitation event accumulatingequal to or greater than 1.00 inches of precipitation with no gaps ofrain equal to or greater than 6 hours Hence, an accumulation equal toor greater than 1.00 inch of rain and a gap with no rain equal to orgreater than 6 hours defines the end of an event).Normal frequency of these storm events occurs on average eight +/-three times ayeat.Mean arurual precipitation with a standard error e.g. SEATAC equals

39 +l- 7 inches per year)Dry years may be determined either by the lack of storm events or theless than average annual precipitation

The following design guidance shall be used in formulating goals for a

stable base with low event fluctuation SL) wetland hydroperiod:Limit the frequency of stage excursions.greater than 15 cm 6 inches)above or below the mean water level non-storm event based) to six orless on average) per year. Multiple years may be used for estimatingthe average frequency ofexcursions per year.Limit the duration of stage excursions greater than 15 cm 6 inches)above or below the mean water level to no more Ihan72 hours perexcursion.During the amphibian breeding season, February I through May 31,

limit the magnitude of stage excursions above or below the averagebase water level to no more than 8 cm 3 inches), and limit the totalduration of these excursions to no more than24 hours in any 30 day

period.

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Bibliography

Azous,4.L., and R.R. Horner (eds.). 1997. Wetlands and Urbanization. Final report ofthe Puget Sound Wetlands and Stormwater Management Research Program. WashingtonState Department of Ecology, Olympia, V/ashington University of V/ashington,Seattle, V/ashington, USA.

Azous, A.L, Mason B. Bowles and K.O.Richter. 1998. Reference Standards and ProjectPerformance Standards for the Establishment of Depresssional Flow-Through'Wetlandsin the Puget Lowlands of 'Western Washington. King County Department ofDevelopment and Environmental Services, Renton, Washington, USA.

Cooke, S.P. 1998. Plant Species Hydrographs. Cooke Scientific Services, Seattle

Washington, USA.

Dunne, T. and L.B. Leopold. 1978. 'Water in Environmental Planning. Freeman and

Company, New York, NY.

Seattle Drainage and Wastewater Utility.1992. Longfellow Creek WatershedChar actenzation : B ackground Report

McDonald, M.G., A.V/. Harbaugh. 1988. A modular-three-dimensional finite-differenceground-water flow model. Techniques of 'Water-Resources Investigations of the UnitedStates Geological Survey, Book 6, Chapter 41.

Pierce, Gary J. 1993. Planning hydrology for constructed wetlands. V/etland TrainingInstitute, Inc. 'West Clarksville, NY

Rigg, George B. 1958. Peat resources of Washington. Department of Conservation,Olympia, V/4.

Winston, R.B. 1996. Design of an urban, groundwater-dominated wetland. V/etlands 16

@): s2a-fi|.

Winston, R.B. 1997. Problems associated with reliably designing groundwater-dominatedconstructed wetlands. Wetland Journal Vol. 9, No. 1.

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Roxhill Park: Hydtologic Investigation and Additional Study Recommendations

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