Hydraulic modeling summary for Sodom Dam Alternatives Feasibility Study

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Tchnical Mmorandum DATE: Novmbr 24, 2009 TO: FROM: SUBJECT: Ms. Dnis Hoffrt-Hay Sodom Dam Projct Managr Calapooia Watrshd Council Rivr Dsign Group, Inc.

0 INTRODUCTION This mmo summarizs th hydraulic modling scopd in Phas 2 of th Sodom Dam Fasibility Study for th Calapooia Rivr Watrshd Council (CWC).

1 Tchnical Mmorandum DATE: Novmbr 24, 2009 TO: FROM: SUBJECT: Ms. Dnis Hoffrt-Hay Sodom Dam Projct Managr Calapooia Watrshd Council Rivr Dsign Group, Inc. 311 SW Jffrson Avnu Corvallis, Orgon Hydraulic modling summary for Sodom Dam Altrnativs Fasibility Study 1.0 INTRODUCTION This mmo summarizs th hydraulic modling scopd in Phas 2 of th Sodom Dam Fasibility Study for th Calapooia Rivr Watrshd Council (CWC). Th study ara is locatd in Linn County, Orgon and is comprisd of four rachs of th Calapooia Rivr spanning approximatly svn mils btwn Brownsvill and Shdd, Orgon. A vicinity map of th study ara is shown blow (Figur 1-1). Figur 1-1. Projct vicinity map of Calapooia Rivr btwn Brownsvill and Shdd, Orgon. Montana Offic Orgon Offic 5098 Highway 93 South 311 SW Jffrson Av. Whitfish, MT Corvallis, OR Tl Tl

2 2.0 METHODS 2.1 Study Rach Th study ara bgins on th Calapooia Rivr downstram of Brownsvill and upstram of th bifurcation at Sodom Ditch. At th bifurcation, th Calapooia Rivr flows northwst towards Intrstat 5 and th Sodom Ditch flows north towards Linn Wst Driv bfor transitioning northwst towards Intrstat 5. Th Calapooia Rivr and Sodom Ditch rjoin immdiatly upstram of Wirth Road. Tabl 2-1 includs th gnral dirction, lngth, and nominal slop of th subrachs comprising th Calapooia Rivr and Sodom Ditch in th study rach. Tabl 2-1. Rach summary as dlinatd for HEC-RAS hydraulic modl. Watrway Subrach Gnral Dirction Lngth (ft) Nominal Slop (ft/ft) Calapooia Rivr South 1, SE 10, SW 18, NW 12, North 2, Sodom Ditch SE 19, SW 13, Rmot Snsing and Survy Ovrbank lvation data for th study rach was collctd by Watrshd Scincs Inc. using Light Dtction and Ranging (LiDAR) mthods in Sptmbr 2008 as part of th largr Willamtt Vally Phas I study conductd for th Orgon Dpartmnt of Gology and Minral Industris. Th vrtical accuracy for th LiDAR data was stimatd to b 3.1 cm (Watrshd Scincs 2009). Fild data collction to charactriz xisting conditions within th study rach was conductd by RDG at slct locations btwn Octobr 2008 and January Data collction includd quality control topographic survy of channl faturs, floodplain aras, and watr surfac lvations for subsqunt hydraulic modl calibration. Quality control topography and bathymtry wr masurd with survygrad ral-tim kinmatic GPS mthods with a nominal accuracy of ±2 cm and ±5 cm for topographic and bathymtric masurmnts rspctivly. Data from survyd locations wr procssd and intgratd into th topographic surfac modl dvlopd from th LiDAR data. Survy control monumnts wr also installd through th study rach for rfrnc during futur survying or construction. Topographic and bathymtric survy masurmnts wr mrgd with th LiDAR masurmnts to prpar a samlss trrain modl of th study rach suitabl for hydraulic modling. Approximat channl invrt lvations at un-survyd locations wr intrpolatd basd on a LiDAR-channl rduction stimat. Th nominal accuracy for th mrgd LiDAR-channl rduction trrain modl is stimatd to b ±15 cm. Additional bathymtric survy targtd at slct aras is suggstd during Phas 3 to incras nominal accuracy to <5 cm Novmbr 2009

3 2.3 Hydrologic Mthods Thr ar two inactiv gaging stations locatd nar th study rach as shown in Tabl 2-2. USGS Gag # locatd upstram of th bifurcation and drains a catchmnt ara of ~105 squar mils. Downstram of th study ara, USGS Gag # drains ~375 squar mils; at th bifurcation of th Calapooia Rivr and Sodom Ditch, th contributing drainag ara is ~164 squar mils. Whil th gags hav 55 and 40 yars of rspctiv gag data, both hav bn inactiv for an xtndd priod of tim. Tabl 2-2. Availabl stramflow gaging data for th Calapooia Rivr. Linn County, Orgon. USGS# Nam Data Typ Dat Rang Count Calapooia Rivr Daily Data 10/1/1935 to 9/3/ ,089 (upstram) at Holly, OR. Pak Stram flow 1/4/1936 to 1/8/ Calapooia Rivr Daily Data 10/1/1940 to 12/10/ (downstram) at Albany, OR. Pak Stram flow 12/22/1940 to 12/4/ Du to th inactiv gag stat and lack of additional systmatic discharg and pak flow data, th prviously computd Log-Parson III flood frquncy rsults scald by drainag ara for th bifurcation (TtraTch 2008) wr usd for this study and ar summarizd in Tabl 2-3. Tabl 2-3. Flood frquncy data for Calapooia Rivr. Linn County, Orgon. Pak Discharg at Rcurrnc Intrval Bifurcation 1.01 yr 99% 2,140 cfs 2 yr 50% 7,021 cfs 5 yr 20% 10,652 cfs 10 yr 10% 13,207 cfs 25 yr 4% 16,582 cfs 50 yr 2% 19,181 cfs 100 yr 1% 21,837 cfs *Sourc: TtraTch Novmbr 2009

4 Calapooia Rivr at Sodom Dam 2.4 Hydraulic Mthods In ordr to prdict th hydraulic rspons of th xisting study rach undr various flow rgims and to valuat th ffct of proposd dsign altrnativs, a hydraulic analysis of th Calapooia Rivr / Sodom Dam projct rach was conductd using HEC-RAS v4.0.0 (USACE 2008). A on dimnsional gradually varid subcritical-rgim hydraulic modl of th rach was dvlopd covring approximatly 17 squar mils btwn Brownsvill and Shdd, Orgon as shown in Figur 2-1. Figur 2-1. HEC-RAS modl schmatic for th study rach (flow is from lft to right). Th study rach is charactrizd by a broad floodplain with a nominal vally slop of ~0.1 prcnt. Floodplains for both th Calapooia Rivr and Sodom Ditch ar ffctivly dividd into East/Wst aras by Intrstat 5 and into North/South aras by Linn Wst Driv. Flow in th rach can b broadly dividd into a dual convyanc rgim whrby undr low to modrat discharg conditions, th incoming flow from th Calapooia Rivr distinctly splits and is routd sparatly through Calapooia Rivr and Sodom Ditch. As th flow incrass byond th point of incipint flooding th Calapooia Rivr and Sodom Ditch shar a common floodplain and ovrbank flows ar no longr distinct btwn thm. In gnral, this transition to ovrbank flooding bgins around 7,000 cfs with a nominal rcurrnc intrval of 2 yars. At this discharg, th Calapooia Rivr downstram of th bifurcation appars to convy lss than 30 prcnt of th total flow with ovrbank flow inundating th low floodplain aras to th wst; th corrsponding capacity for th Sodom Ditch is ~70 prcnt of th flow with ovrbank flow inundating common floodplain wst of Sodom Ditch and ast of th Calapooia Rivr. In ordr to addrss th dual flow charactristics within th study rach, two diffrnt modl schmatics wr dvlopd: 1) a split flow modl that trats th Calapooia Rivr and Sodom Ditch as sparat -4- Novmbr 2009

5 branchs and 2) a continuous sction modl that uss full cross sctions across th floodplain, trating both ovrbank aras as part of th hydraulic convyanc ara. For both mod schmatics, th cross sction cutlins wr alignd with th ovrbank flow path as publishd on FIRM Panls (355B, 365B and 370B). Th 2008 TtraTch study rports using dfault Manning's roughnss valus of 0.05 to for th Calapooia Rivr and to 0.06 for th Sodom Ditch. For this study, nominal roughnss cofficints wr slctd basd on this rang as wll as rcommndations containd in th HEC-RAS Hydraulic Rfrnc Manual (Brunnr 2008), publication FHWA-NHI and a comparison with similar strams documntd in USGS Watr Supply Papr Additional calibration was prformd using obsrvd watr surfac lvations with masurd dischargs at slct locations to prliminarily adjust inffctiv flow aras downstram of th bifurcation in th Southast study ara. Friction loss cofficints usd in th Phas 2 hydraulic modl ar shown in Tabl 2-4. A hydraulic profil for Sodom Ditch is includd in Figur 2-2. Tabl 2-4. Manning's roughnss valus usd for xisting and proposd gomtry. Manning s-n Rivr Subrach Lft Ovr Bank Ara Manning s-n Channl Manning s-n Right Ovr Bank Ara Calapooia South SE/SW NW North Sodom Ditch SE/SW rdg08038_rv2 Sodom Sdm_SESW Calapooia Cal_South Lgnd WS 3.5 kcfs Crit 3.5 kcfs 290 Ground LOB ROB 285 OWS 3.5 kcfs Elvation (ft) Blw Sdm Dam Sodom Dam Tailwatr Abv Sdm Dam Sodom Ditch Blw Bifurcation... Calapooia Rivr A Main Channl Dis tanc (ft) Figur 2-2. Sodom Ditch at Bifurcation at 3,500 cfs. Additional systmatic masurmnts of stag and discharg will b collctd at multipl locations and hydrograph intrvals undr Phas 3 for final modl calibration and vrification of dsign prformanc Novmbr 2009

6 3.0 RESULTS AND SUMMARY In ordr to quantify th flow division btwn th Calapooia Rivr and Sodom Ditch at th bifurcation, xisting conditions wr modld ovr a rang of flows btwn 100 cfs and 15,000 cfs. Rsults indicat that th hydraulics at th flow division ar significantly dpndnt upon th dgr of floodplain inundation and floodplain convyanc rat rlativ to th primary channl. In an initial ffort to rplicat th flow division rsults from th 2008 Ttra Tch rport, it was ncssary to utiliz a modl gomtry that tratd all ovrbank floodplain aras with lvations lss than bankfull as inffctiv as shown in Figur 3-1. This ssntially assums a nar zro vlocity for ovrbank flows and rmovs thm from th activ flow ara. Undr ths conditions, a majority of th flow continus to b routd through th Sodom Ditch for dischargs up to a 25 yar vnt. Although such a flow rgim may b possibl dpnding upon th dgr of floodplain inundation and connctivity with th main channl, th dgr of flow division did not corrlat with availabl obsrvations of discharg and watr surfac lvations nar th bifurcation. 100% 90% 80% 70% % of Discharg 60% 50% 40% 30% 20% 10% 0% Total Discharg (kcfs) Sodom Ditch Calapooia Rivr Figur 3-1. Th rsults of th modld flow division at th bifurcation with inffctiv flow aras dlinatd for ovrbank flows lss than bankfull TtraTch rsults ovrlay as dashd lins. Th RDG modling rsults suggst flows in th Calapooia Rivr ar lowr and th Sodom Ditch flows ar highr than th rsults rportd in th TtraTch (2008) rport Novmbr 2009

7 Rsults from a prliminary calibration basd on an adjustmnt of inffctiv flow aras in th southast modl rachs indicat that th flow division at th bifurcation is mor likly charactrizd as shown in Figur 3-2. Ths prliminary rsults indicat that a highr prcntag of flow is routd through th Calapooia branch and its ovrbank aras rlativ to th 2008 Ttra Tch prdictions with th flow division bing narly qual btwn th Sodom Ditch and Calapooia Rivr for a 2-yar vnt. Bcaus th prcntag of flow in th Calapooia branch downstram of th bifurcation is highly dpndnt upon th dgr of ovrbank convyanc, additional masurmnts of stag and discharg ovr a varying rang of flows will b rquird to improv th calibration and associatd flow division. 100% 90% 80% 70% % of Discharg 60% 50% 40% 30% 20% 10% 0% Total Discharg (kcfs) Sodom Ditch Calapooia Rivr Figur 3-2. Modld flow division at th bifurcation with prliminary calibration basd on an adjustmnt of inffctiv flow aras Novmbr 2009

8 Inundation and vlocity maps for xisting conditions wr prpard as shown in Figurs 3-3 and 3-4 blow. Figur 3-3. Existing conditions hydraulic modl output at 2-yar flows. Th figur displays modld watr dpths in th rach. Not th flooding is mainly rlatd to th Calapooia Rivr s ovrbank flows Novmbr 2009

9 Figur 3-4. Existing conditions hydraulic modl output at 10-yar flows. At this discharg, ovrbank aras activly convy a portion of th total discharg for th two branchs Novmbr 2009

10 Th proposd dsign was slctd as a prfrrd altrnativ basd on input from th RDG tchnical rviw tam, th Calapooia Watrshd Council and associatd stakholdrs and includs rmoval of th Sodom Dam and th Shar Dam. As part of th Sodom Dam rmoval, prliminary concpts includ installing grad control structurs approximatly 1,000 ft upstram of th xisting dam location to provid fish passag and maintain th channl grad. Th Sodom Ditch would b r-gradd for about 2,500 ft at a nominal slop of to match xisting ditch invrt lvations as shown in Figur 3-5 blow. rdg08038_rv2 Sodom Sdm_SESW Calapooia Cal_South Lgnd 300 WS 7.0 kcfs - Ex2f WS 7.0 kcfs - Pro2g1-Sodom Ground LOB 290 ROB Ground Elvation (ft) Downstram Match Existing Blw Sdm Dam Sodom Dam Tailwatr Abv Sdm Dam Upstram Hold Sodom Crst Elvation Sodom Ditch Blw Bifurcation Calapooia Rivr Abv Bif Main Channl Dis tanc (ft) Figur 3-5. Proposd Sodom Dam rmoval and ditch rgrading downstram of Bifurcation at 7,000 cfs. Prliminary modl rsults indicat that following Sodom Dam rmoval and ditch r-grading, th flow distribution at th bifurcation would ffctivly shift as shown in Figur 3-6 blow. Th nt rsult would b a largr prcntag of flow in th Calapooia Rivr rlativ to xisting conditions. 100% 90% 80% 70% % of Discharg 60% 50% 40% 30% 20% Sodom Ditch Existing Sodom Ditch Proposd 10% Calapooia Rivr Existing Calapooia Rivr Proposd 0% Total Discharg (kcfs) Figur 3-6. Estimatd shift in flow division at th bifurcation for th proposd conditions Novmbr 2009

11 Rmoval of th Shar Dam would hold th channl lvation downstram of th dam and r-grad th channl at a nominal slop of for approximatly 3,000 lf upstram to match th xisting channl invrt nar Robrts Road as shown in Figur 3-7 blow. rdg08038_rv2 275 Lgnd WS 7.0 kcfs - Pro2g1-Shar WS 7.0 kcfs - Ex2f Ground 270 LOB ROB Ground Elvation (ft) s s s Calapooia Cal_TMS Calapooia Cal_SESW Downstram Match E... Abv. Shar Dam Calapooia Blw Robrts Rd. () Robrt's Rd. Upstram HO LD EG. Calapooia Abv. R Main Channl Dis tanc (ft) Figur 3-7. Proposd Shar Dam rmoval and Calapooia Rivr rgrading downstram of Robrt s Road at 7,000 cfs. Rsults indicat that th watr surfac lvation upstram of Shar Dam would b lowrd for Calapooia Rivr flows lss than 5,000 cfs as shown in Figur 3-8 blow Watr Surfac Elvation Existing Proposd Calapooia Discharg (kcfs) Figur 3-8. Comparison of xisting and proposd watr surfac lvations at station upstram of Shar Dam Novmbr 2009

12 In summary, prliminary hydraulic modling rsults suggst that watr surfac lvations in th Calapooia Rivr and Sodom Ditch vary in accordanc with discharg volum and dgr of floodplain inundation. Two modls wr dvlopd to valuat in-channl and floodplain hydraulics. Du to th diminishd channl capacity of th Calapooia Rivr downstram of th bifurcation, widsprad floodplain inundation occurs during a 2 yar flood vnt. Floodplain inundation adjacnt to Sodom Ditch dos not occur until th 2 yar discharg is xcdd du to th channl s largr capacity. Additional watr surfac lvations and discharg masurmnts to b compltd during Phas 3 of th projct will b ncssary to improv th modls dgr of accuracy and th xisting flow split at th bifurcation. Th prliminary proposd projct lmnts includ rmoving both Sodom Dam and Shar Dam. Dam dconstruction and channl stabilization activitis will influnc flood lvations in th Calapooia Rivr channl. Furthr proposd condition analysis will b ncssary to clarify ths ffcts Novmbr 2009

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