To: Ed HanlonUSEPA

Submitted via Facsimile

From: Martin L. Sohraidt, Ph.D.Sr. Project Manager M

Office: Solon, Ohio

Subject:

Date: January 24,1994

Hydrologic and Sediment Transport Analysis Backup Information

As requested during the January 11, 1994 meeting and discussed with you onJamuuy 21, 1994, Woodward Clyde Consultants (WCC) is submitting the followingexcerpts from existing document* for your review. Attached are the following:

1. Section 4.0 - Hydrologic and Sediment Transport Analysis

This section is a portion of the SQDi Phase I Report, Revision 0, that wassubmitted in October, 1992 (Pages 4-1 through 4-13). Cross-sections fromthe study area are provided (Figures 4-2 through 4-15). Please note thatthis version includes modificaUuus tlmt were made to the SQDI StatusReport submitted in March, 1992. A copy of the text from the SQDI StatusReport will be included in the package submitted tonight.

V .

1. Appendix A • SQDI FSP Addendum 1 * Sour Depth Analysis

This information was submitted with the SQDI FSP Addendum 1 -Revision 3 and included with Attachment C that was submitted to USEPAprior to the December 17, 1993 meeting (Pages C-1A through C-3A).

As discussed with you on January 21, 1993, WCC will submit copies of this informationto USAGE Waterways Experiment Station. Hard copies will be forwarded via FederalExpress overnight. If you have any questions, please do not hesitate to give me a call.

cc: Steve Golyski - USAGERon Heath - USAGE Waterway* Experiment StationLaura Weyer - CH^M HillJoe Hciiiibudi - de maxbnis, inc.

i '*. -'r\' \ \/•-.***

WoodwanM^yde

r

SQDI PHASE I REPORTREVISION 0

WoodwanMtydeConsultants SQDI Repoit. Phase i

4,0HYDROLOGIC AND SF.niMF.NT TRANSPORT ANALYSIS

4.1 INTRODUCTION

The objectives of the Hydrologic and Sediment Investigation were to:

• Identify 10- and 100-year floodplains for selection of Phase n samplinglocations;

• Identify areas of sedimentation for selection of sediment sampling

C-> locations for Phase II sampling;* Characterize the water and sediment moving within the watershed; and• Estimate potential sediment migration out of the watershed between the

time of sampling and remediation.

The Fields Brook watershed is located approximately three miles northeast of the Cityuf Ashmbula, Ohio. Primary land uses of the watershed consist of a mix of residential,agricultural, and industrial areas. The soils in the watershed have been classified by theUnited States Department of Agriculture (USDA), Soil Conservation Service (SCS) andrange from well-drained gravelly soils to poorly drained silty and sandy soils. Theaverage annual rainfall for the watershed is 363 ia per year. The monthly precipitationdistribution is relatively uniform throughout the year with the minimum mean monthly

/"" •, precipitation of 222 in. in February and the mnYinmin precipitation of 3.61 in. in May.The average daily temperature ranges from 27 degrees Fahrenheit in January, to 73degrees Fahrenheit in July (U.S Department of Commerce 1968).

Stream discharge information was collected at 6 locations within the watershed inNovember and December of 1989, and in January, February, March, May, August, andSeptember of 1990. Average baseflow at the outlet of Fields Brook was estimated to beapproximately 22 cfs based on the stream discharge information collected during theabove months. The stream in general is recharging in the downstream direction. Thestream is gaining discharge from the industrial outfalls or from groundwater infiltrationinto the stream. It is difficult to quantify the exact amount which can be attributed torn

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Woodward-ClydeCon*uKantS SQDI Report * Phase I

each source from the available data. Based on the volume of industrial outfalldischarges collected by Source Control Remedial Investigation during Phase 0 sampling,the baseflow attributed from the industrial outfalls ranges between 14 find 18 cfs. Thedischarges from the industrial outfalls tend to vary with respect to time, and dischargemeasurements from the various outfalls were not taken at the same time. Thepotentiometric map, prepared as part of the fields brook Source control RemedialInvestigation, Phase 0 Report, indicates tbat the groundwater surface intersects thestream within the study area and indicates a discharge of groundwater to the stream forthe time period the groundwater data were collected. It is likely that the water level inthe upper aquifer, which would directly affect the amount of groundwater discharge tothe stream, will fluctuate seasonally.c42 HYDROLOGIC ANALYSIS

The U.S. Army Corps of Engineers (USAGE), Flood Hydrograph Package (HEC-1)computer model was used to simulate the rainfall-runoff process for the Fields Rroofcwatershed. Drainage area, runoff characteristics, arid rainfall amounts were input intothe model to estimate peak runoff rates and volumes at various locations for the 10-yearand l(X>-year frequency events, The peak discharge values estimated from the FieldsBrook watershed and the major tributary floodplains were input into the USACE, WaterSurface Profiles (HEC-2) computer program. The HEC-1 and HEC-2 analyses wereused to estimate the 10-year and 100-year flood elevations to aid in refining the extentof contamination and to assist in the design of the remediation facilities.

4.2.1 Watershed Characteristics

The Fields Brook watershed consists primarily of industrial facilities and medium densityresidential areas with open areas of natural grassland. Fields Brook is a tributary of theAshtabula River, which ultimately discharges to Lake Erie.

Overland slopes range from less than 0.3 percent to 4 percent Runoff is conveyedpredominantly from the southeast to the norttiwest by several small tributaries and

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Woodward-ClydoConsultants SQDI Report - Phase I

irrigation ditches which collect and convey runoff to Fields Brook. The watershed wasdivided into several sub-basins to account for tributary flow as showu in Figure 4-1.

The soil types, hydrologic condition, land use and vegetative cover of the different sub-basins were identified to evaluate infiltration characteristics* The SCS "Soil Survey ofAshtabula County, Ohio" was used to identify major soil types and hydrologicclassifications of the watershed. Three major soil group association* were identified inthe Fields Brook watershed:

« Elnora-Colonie-Kingsville associationHydrologic Soil Group B

• Otisvilie-Chenango associationHydrologic Soil Group A

• Conneaut-Swamon-Claverack associationHydrologic Soil Group C

An SCS runoff curve number was estimated to quantify infiltration potentials for eachsub-basin based on the soil type, land use, hydrologic condition, and vegetative coverdetermined. Estimated curve numbers range from 61 to 83 for the different sub-basins.

Land use and vegetative cover as well as sub-basin drainage areas were estimated from. , United States Geological Survey (USGS) 7.5 minute quadrangle maps. Tie USGS 7.5

minute quadrangle maps used in the analysis were: (1) Ashtabula North, Ohio 1960,photo-revised 1970, photo-inspected 1988, (2) Ashtabula South, Ohio, 1960, photo-revised 1970, (3) Gogcvfflc, Ohio, 1960, photo-revised 1979, and (4) North Kingsville,Ohio, 1960, photo-revised 1979.

Tune of concentration (T^) for each sub-basin was estimated using the Kiipich Methodfor channel flow, aud SCS methods for overland and shallow concentrated flow. The Tcis defined as the time for water to travel from the most hydraulically remote point to th«outlet of each sub-basin. Slope and length of flow paths are the parameters necessaryto estimate time of concentration using both the Kirpich and the SCS methods. Slopes

4-3l.\rBK\SQDIRrT\SOOll-*.WT

Woodward-Clyd*Consultant* SQDI Report * Phase I

and lengths were estimated from USGS 7.5 minute quadrangle maps. The drainagearea, Curve Number (CN)( lag time (TJ and Tc estimated tor each sub-basin arc givenbelow:

Are* Tf TcBasin l.D. fgg- mihff) CN fhrt,>

A 0.097 76 0.49 0.81B 0.189 83 0.44 0.73C 0.105 74 0.74 1.23D 0.097 73 0.73 1.21E 0.784 61 1.21 2.01F 0-239 80 0.98 1.63

~r G 0364 82 0.60 1.00( H 0.062 76 0.73 1.22

I 0.044 78 0.33 0.54J 0.124 80 1.27 112K 0303 72 1.43 2.38L 0.091 80 0.95 1.58M 0.835 69 1.23 2.03N 2.673 64 1.44 2.40

Baseflow in Fields Brook was estimated from the stream discharge information collectedin November and December of 1989, and in January, February, March, May, August, andSepiember of 1990. Precipitation records were reviewed with respect to tlic samplingdates to determine if discharge resulted from runoff from a storm event. The Amount

/ of precipitation in the period five days prior to the sampling was used to screen fordischarge from runoff. UK average baseflow estimated at several locations along FieldsBrook is presented below:

Design Point/ BaseflowBarfnlD _

1 222 213 204 125 86 4

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Woodward£tydtConsultant* SQDI Report - Phase I

Discharge from the industrial outfalls was measured in August and October of 1990 aspart of the Source Conuoi, Phase 0 investigations. Average total discharges ranged from14 to 18 cfs based on the data collected.

422 Rainfall Depth and Distribution

Rainfall depths for the 10-year and 100-year, 24-hour duration storms were estimatedfrom National Oceanic and Atmospheric Administration (NOAA) Technical Paper 40(TP-40). TP-40 is used for design purposes and is required for State Floodplain permitsin the State of Ohio. The total rainfall depths were then distributed using the SCSType II temporal rainfall distribution curve. The 10-year and lOOyear, 24-hour durationprecipitation depths were estimated to be 3.5 in. and 4.75 in., respectively, for the FieldsBrook watershed

Rainfall of 3.95 in. in 18 hours was recorded at the WCC trailer located at the RMISodium site on September 7, 1990. Intensity-duratinn-freqn«ncy curves were developedfrom NOAA Technical Paper 40 (TP-40) for Ashtabula, Ohio to estimate the frequencyof this rainfall event. The rainfall event was estimated to have a return period of 35years for the 18-hour storm based on intensity-duration-frequency curves developed fromTP-40 data.

, 4.2J Estimated Peak Flow*

The baseflow described above has been added to the I1EC-1 model to estimate the peakflow. The estimated peak flows (Q10 and Q100 for 10-year and 100-year precipitation incubic feet per second, respectively) estimated from the HEC-1 model for the watershedare given in the table below:

Design Point/ Q19 Q1MBasin ID

1 913 17892 846 16873 799 1605

4-5 HM3-92I:\rBK\SQDIR>'i\bgi>ll-»JUT RevWon 0

Woodward-ClydeConsultants SQDI Report Phase i

Design Point/ QuBasin ID (cf*\

4 634 12835 544 1130C 33 64E 73 175F 94 155I 31 526 496 10527 335 734

4J FLOODPLAIN DELINEATIONcHydraulic modeling of the Fields Brook floodplain was estimated using the USAGE'SWater Surface Profiles computer program, HEC-2. Cross-section data, discharge data,roughness coefficients, and reach lengths were input into the model to estimate flooddepths and floodplain widths for both the 10-year and 100-year frequency events.

4,3,1 Approach

The floodplain cross-sections used in the HEC-2 model were taken from the streamsediment sampling program and the 1-ft aerial contour maps dated April 1987 by KuceraInternational The in-channe! sections were surveyed during the Phase I sediment

f - sampling program (Section 3.2). All cross-sections were surveyed assuming the rightedge of bank (looking downstream) was zero elevation and zero offset The in-channcizero elevation and offset points were assigned an elevation by plotting the cross-sectionson tfce aerial contour maps. The foil floodplain cross-sections were plotted from left toright looking downstream. Cross-sections within the study area arc shown in Figures 4-2through 4-15,

Manning's roughness coefficients were estimated from photographs of the reaches andfrom the aerial contour maps. The Manning's roughness coefficient accounts for channeland overbank roughness, vegetation, channel bends, arid meandering. The roughnessvalues selected ranged from 0.045 to 0.06.

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Woodward-CJydtConsultants SQDi Report - Phase I

Hydraulic routing typically includes both floodplain cross-sections and hydraulic crossingssuch as bridges and culverts. The survey data for the hydraulic crossings have not beencompleted at this time. Therefore, only the cross-sections with no hydraulic crossingswere used in the hydraulic routing, Hydraulic crossings usually create additional headlosses within the study area. The floodplain sections and the hydraulic crossings will besurveyed during the Phase n sampling.

4J2 Preliminaiy 10-Year and 100-Year Floodplain Delineation

The estimated flood elevations, reported relative to mean sea level (MSL) (NGVD1929), based on hydrologic and hydraulic routing, and widths foi both die 10-year and100-year frequency events, plus baseflow, are listed in the table below and are shnwn InFigures 4-2 through 4-15:

10-Yr.Cross -section

ID

1OCS22-1XS29-XS4

2-2XS1>XS5

10-ixsn4-XS3

11-1XSS5-1XS31Z-XS2&-1XS1S-2XS18-2XS38-3XS2

Elevnu

578J5815608D5908OOOJ6102608.0614.6613.3625J625^63U633.76393

WidthHtt

351203038«29

13526

264270109106163165

100-Yr.EleviflU

580.7584.96083592.4601.6610.8VM2615.06143626362&2631863636402

Hldtbifii

421393769

18541

15832

285276182141319172

The estimated 10* and 100-year floodplain maps with cross-section locations are shownin Figure 4-16. Computed water surface elevations are shown in Figure 4-17.

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Woodward-ClydeConsultants SQOI Report - Phase I

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4.3.3 Calibration of Preliminary Floodplain Delineation

The estimated 10-year and 100-year flcxxlplains were compared to an observed floodevent. The observed event on September 7, 1990 of 3.95 in. within an 18-hour period,has been estimated to be approximately a 35-year event Based on the level ofengineering completed to date, the HEC-1 and HEC-2 models appear to represent thewatershed reasonably well. The estimated 100-year event is greater than the observed35-year event except in Reach 3 and Reach 6 where, in some locations, the estimated100-year event has under-predicted the flood elevation compared to the observed event.In some cases, the 10-year event has over-predicted the flood elevation compared to theobserved event The results are summarized La the following table.

Cross-sectionID

1-XS21-XS102-1XS22-2XS13-XS13-XS54-XS3

5-1XS36-XS2

7-2XS18-1XS18-1XS98-2XS18-2XS38-3XS2

Estimated10-Year Eter

(IH

578.55813582.65908593.7600.4608.06133617.8624.6625.5630.6631.6635.76393

Observed35-Year* Elev

(11)

579.0580.0581.0591.0599.0601.06082612.5618.7625.0625.5630.6631.0

Not EstimatedNot Estimated

Estimated100-Year Elev

fin

580.7583.4584.9592.4595.2601.6609.26143618.6625.S626.2631.6632.8636J640.2

* Based on observed flood elevations at select locations.

The addition of data for the upstream hydraulic crossings at the Penn Central railroadtracks would probably show reduced flow entering the study area and would generallyreduce the flood estimate throughout the study area. The addition of the hydraulic

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Woodward-CtydtContuftants SQDI Report - Phase I

crossing data for the study will probably create additional head losses and local pondingthat will change tbe profile near the culvert crossings due to the limited capacity of thehydraulic crossings. This is evident at cross-sections 3-XS1 and 6-XS2 which are locatedimmediately upstream of a restricting hydraulic crossing. There was approximately fivefeet of bead loss observed for the hydraulic crossing upstream of 6-XS2 during a fieldvisit on August 4, 1992: The head loss may be attributed 10 blockage of tbe culvert ora partial collapse of the culvert This will be confirmed during tbe Phase n camplingsurvey. In these locations, the observed 35-year flood elevations are greater than theestimated 100-year flood elevations. When the remaining surveying for the hydrauliccrossings and fluodplains is completed, tbe HEC-i and HEC-2 models will be calibratedusing the September 7, 1990 event.

4.4 SEDIMENT ANALYSIS

A measure of whether a stream will transport sediment primarily by bed load ursuspended load is based on the ratio of the shear velocity of the reach to the fall velocityof the sediment For ratios less than 0*5, most of the sediment will be transported bybed load. For ratios between OJ to 2.0, the transport of sediment will be bed load andsome suspended load. For ratios greater than 2.0, the mode of transport win beprimarily suspended load (Laursen 1960). For all stream reaches within the study area,the ratios exceed 10 assuming a median sediment size of 0.10 mm. The mediansediment size was obtained from gradation analyses performed during the Phase 1

£ sampling program. Therefore, the primary mode of sediment transport tor Fields Brookis generally by suspended load. A report published by the USGS on Fluvial Sedimentin Ohio reports that, in general, less than 10 percent of the sediment transport is by bedload for streams in the State of Ohio. Specifically, a gauge on the Ashtabula River atAshtabula records that less than 5 percent of the sediment is transported by bed load(USGS 1978).

The Bagnold equation was used to estimate the total sediment load which may betransported by tbe stream. This equation was chosen not to quantify the amount ofsediment being transported by the stream but 10 estimate the relative ability of tbereaches to transport sediment By comparing the ability of a reach to transport sediment

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Woodward-ClydeConsultant* SQDI Report * Phase I

to the ability of the upstream reach to transport a given amount of sediment, arelutiuiisliip bclwccu readier uf aggradaliuu (icdiuiciit accumulation) aud degradmiini(tediment erosion) can he teen. Tf the upstream reach has a greater ability to transportsediment, then aggradation would occur in the subject reach. If the ability to transportsediment is less in the upstream reach, then degradation will occur in the subject reach.However, aggradation and degradation can vary from reach to reach depending on themagnitude of the How. This analysis represents in general the aggradation/degradationprocess for the entire floodplain area for the subject reach; however, local scour ordeposition may occur.

The Bagnold equation is based on the following factors:

\v. -v*) \***

where:

QT * Total sediment discharge per unit time per ft width (tons/day/ft)Yw « Specific weight of water (62.4 Ib/ft3)Yt " Specific weight of solids (168 Ib/ft3)TO » Average shear stress on channel boundary (.16-3.57 Ib/ft2)U « Average flow velocity (2 -̂9.0 ft/sec)

/ e> » Efficiency factor (.11-.15)V. • tan« * Dynamic solid friction (.45-.90)

W « FaJl velocity (.0002-147 ft/sec)

For comparison purposes, the 10-year event was selected for this analysis because of thehigher probability of occurrence compared to the 100-year event The peak flow velocityestimates from the HEC-1 and HEC-2 analyses were used as input into the Bagnoldequation. The duration of the peak flow was assumed to be constant for 1 hour. Theresults are shown in the following table.

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Woodward-ClydeConsultant* syui Report. Phase I

Estimated Total Sediment Load(tons/10-year peak flow)

Aggrading/RfiMTh ?*> Total Degrading

8-3 148-2 90 Degrading8-1 880 Degrading5,6,7 155 Aggrading4 805 Degrading

/' 3 180 Aggrading2-2 920 Degrading2-1 70 Aggrading1 270 Degrading

The aggrading versus degrading conclusions are in general concurrence with the Geldobservations with the exception of Reaches 8-1 and 8-2 (swampy area). Fieldobservations confirm that large portions of degrading reaches are characterized by flowover coarser sediments or relatively hard channel bottom (bedrock or hard clay layersof lacustrine sediments exposed by erosion).

C The sediment discharge from a watershed may he limited by the amount of availablesediment to transport or by the ability of Fields Brook to transport sediment In general,the stream has the ability to transport sediment but is limited by the available sedimentto transport.

One equation used to estimate the amount of overland erosion is the Universal SoQ LossEquation (USLE). The USLE only accounts for overland erosion and does not accountfor other sources of sediment such as suspended sediment from outfalls, stream channelerosion, and stream bonk erosion. Estimating the amount of erosion from industrialoutfalls, the stream channel, and bank erosion is difficult. To account for some of themajor outfalls, suspended sediment data collected under the National Pollutant

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Woodward-ClydeConsultants SQDI Report -

Discharge Elimination System (NPDES) program for the industrial outfalls will becollected during the Phase II sampling program and added to the total sediment loadestimated from the watershed.

The USLE equation accounts for several factors:

A - R x K x L S x C T P

A - soil loss tons per year per acreR = rainfall factor (125)K » soQ erodibility factor (.120-.280)

^~ LS » length-slope factor (.148-1.468)( C « crop management factor (.01)

P • erosion control practice factor (1.0)

Annual soil loss for the Fields Brook watershed was estimated by the basins defined inthe hydrologic investigation section. The rainfall factor, the crop management factor,and the erosiou control practice factor remained constant fur all ba»inv The length-slope factor and the soil erodihility factor varied depending on the characteristics of thebasin. The crop management factor was selected based on a well-established watershedand was estimated as if the watershed was considered a meadow or grassland with 95percent coverage. Values may range from 0.003 to 0.11. The Fields Brook watershedis a well-established watershed with no large-scale agricultural areas. The average soil

/ loss from the Fields Brook watershed was estimated to be 170 tons per year based onthe USLE* For the Phase II report, existing suspended sediment data from the industrialoutfalls will be collected and added to the USLE estimate. The estimated 170 tons peryear converts to a discharge of sediment from the watershed of 130 cubic yards(assuming the sediment weight is 100 pounds per cubic foot).

The Bagnold equation is an example of one empirical total sediment load equatioa TheBagnold equation can account fur many sources of erosion but often produces erraticresults without sediment load data to calibrate the equation. The Bagnold equationseems to produce unrealistic high concentration results and larger-than-expectedsediment transport for this case without calibration data.

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Woodward-CtycfcConsultants SQDI Report - Phas« I

The best estimate to date of the sediment discharge from the Fields Brook watershedis based on the USLE. No estimate of the sediment discharge from the industrial stormwater outlets and stream bank and channel erosion was made for this sediment dischargeestimate. The addition of hydraulic crossing data may affect the sediment load estimate.During Phase II sampling, suspended and bed load will be measured to calibrate anequation to estimate sediment discharge from the watershed.

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SODI FSP ADDENDUM 1REVISION 3

C

Attachment C - Appendix AFields Brook Sediment Operable Unit - SQDI FSP Addendum I 86C3609L

C

C

A.I Scour Depth Estimate lor Ftoldt Brook Channel

In order to identify reaches of Fields Brook that are candidates for scour evaluation, anerosion model was used. The depth of scour during the 100-year flow event was estimatedusing the tractive force approach. This method involves an iterative procedure which wassolved utilizing a programmable calculator. The method is based primarily on the sedimentsize found in the stream and the shear forces acting on the stream bed during the 100-yearflow. The initial water deptii, dupe of the bed, energy grade line, and Manning's n valueswere obtained from the HEC-2 modeling presented in the Phase I SQDI Report (WCC1992). Gradations of sediment, descriptions of sediment by reach and probing depths fromthe SQDF Phase I Report were reviewed as pan of this analysis.

Results of this simplified method were compared to results obtained from a mass balanceapproach which Included erosion from the watershed and sediment transport of the variousreaches. Aggradation or degradation (scour) was estimated from the ability of certainreadies to transport sediment The source of sediment includes the bed and that which isscoured from the channel bottom, A further comparison was made by increasing thechannel cross-sections in the HEC-2 analysis by the depth of scour d&iu;iaicd in the tractiveforce method to simulate scour during the 100-year event.

A.1.1 Assumptions

The depth of sediment in the stream bed were estimated from available data. Gradationsof sediment were available only for the upper 6 in, of bed material (one gradation from acomposite of sediment from 1 to 2 ft in depth was available in Reach 7). The size(diameter) of sediment at different depths was assumed A reasonable assumption is thatthe diameter of the sediment doubles for every 2 ft increase in depth. This is because thefact that the larger partide sizes ore not moved horizontally and thus, become a pan of theunderlying sediment This process is called armoring and is expected to occur in the FieldsBrook channel Based on the gradations and stream characterizations available, somecoarse gravel appears to exist in most of the readies.

The assumptions used in the analysis include:

1. Cohesion of the fine-grained particles is not considered This assumption impliesthat sediment is more likely to erode than if cohesion is considered.

2. The tke of sediment with 90 percent of material smaller than the given size (d*),based on the gradations, was used as the input sediment size for the first 2 ft of

3* Hie size of sediment corresponding to a coarse sand (5 nun) is used as the inputsediment sfce for the sediment layer from 2 to 4 ft

C* 1 A

Attachment C - Appendix AFields Brook Sediment Operable Unit * SQDI FSP Addendum I 86C3609L

4. The size of sediment corresponding to a coarse gravel (20 mm) is used as theinput sediment size for the sediment layer from 4 to 10 ft

5. Local scour around piers, abutments, etc., and contraction scour estimates are notincluded. The depth of scour may be greater near bridges and other hydraulicstructures.

6. This analysis assumes "dear water scour and does not account for sediment beingtransported downstream. As such, results should be conservative since watertransporting fine sediments is less likely to erode the channel*

7. Estimates of scour depth are based on limited hydrologic, hydraulic and sedimenttransport data analysis performed as part of the Phase I SQDL More detailedhydrologic, hydraulic and udiment transport analyses will be prepared as pan ofthe Phase n investigation.

8. Scoured areas, predicted during storm events, are refilled and restored bysedimentation occurring after the storm event so the original grade is maintained.

A.12 Results

Results of the analysis indicate that the size of sediment to required armor against the 100-year flood event is approximately 5 to 20 mm. The maximum scour depths computed fora 100-year flood event, based on the assumptions made for the sediment layers, wereestimated to be from 2 to 5 ft This estimate should be reliable if 5 percent of the materialin the sediment between 2 and 5-ft deep is 5 to 20 uiui ur greater (coarse sand to coarsegravel). From information in the Phase I SQDI Report, coarse material or bedrock ispresent in all of the reaches except Reach 5-2 and Reach 8-2. It is expected, however, thatcoarser sediment would be found in these reaches with increased depth. If significantamounts of sediment larger than 20 mm exist, or bedrock exists between 0 to 5-ft deep, thesoour depth would be less than that predicted in this analysis. Fium the probe depths andreach characterizations obtained from the Phase I Report, bedrock is present in the reachesof Melds Brook west of Highway It thus limiting scour in these reaches.

A.L3 Gwclosioaft

Armoring of the channel during the 100-year flood event is expected to occur between 2 and5 ft, based on the assumption that 5 percent of the sediment material has a grain size of20 mm or greater. In Reach 1 through Reach 3, the scour potential is the greatest; however,bedrock is exposed at the channel base in these reaches. In the central reaches, coarsegravel and sands exist in the upper sediment layers. Coarse sands and gravels are assumedto exist in lower sediment layers in the upper reaches, although the 0 to 6 in. gradations donot indicate the presence of coarse materials.

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Attachment C - Appendix AFields Brook Sediment Operable Unit - SQDI FSP Addendum 1 86C3609L

The scour potential has been estimated in two ways:

1. Estimations of clear water scour potential using the tractive force approach. Thisis a conservative approximation based on the assumptions listed above.

2. Estimations of scour, based on a mass balance approach of aggradation anddegradation, sediment transport potentials, and knowledge of existing stream bedconditions.

The following scour depths are estimated based on the above analyses:

1. Reaches 1 through 3 may scour to bedrock, considering both methods o!estimating the scour potential. Average sediment thickness in Reaches 1 through3 is not greater than 2.0 ft.

2. Reaches 4 through 5*2 may scour to 4 ft considering only clear water scour andmay scour to 2 ft considering Method Number 2.

3. Reaches 6 and 7-1 may scour to 4 ft considering only clear water scour and mayscour to 3 ft considering Method Number 2.

4. Reaches 7*2 and 8-1 may scour to 4 ft considering both methods of estimatingscour potential.

3. Reaches 8*2 and 8-3 may scour to 3 ft considering both methods of estimating thescour potential

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