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Home My WebLink AboutAPA3040~38:J E.WOODY TRIHEY It ASSOCIATES INSTREAM PLOW AND RIVERINE HABITAT ASSESSMEN1'S '\:>,Mj \,\'n S US~)"'J ~<;.Jf:3 / P.O.BOX 11177. ANCHORAGE.ALASKA 99511 (907)563·7707 ~'(.\, ..:.':-::")',...;."'\.\r......-..-". ========================;~\~ v .:!...!",:":i:·:.::;j1~S ·i ·n,C .r;:Q ..-- C iS T 2 ~::.":T J v :. NovemDer 20,lY~4 Sl.:silna File #4.3.1 .3 MAUA-ilASCO IUSI1'NA H'fDIQUK1IIl:PIOJICl' DOCUMeNT lOUTING ~ON ,.".=~~V""IVi"',CAllY Is::THlAll .-j---,',-cUllllOil •~1-",OIlW UNoSZi.iT IOIINSOH O'OK'- Dr. Larry Gilbertson Harza-EDasco Susitna Jo int Venture 711 H Street Anchorage,AK 99501 Dear Larry: Attached is one copy of the techni ca I memorandum on light ext i nct ion coefficients referenced in our FY85 Scope of Work under Task 12.This memorandum provides an analysis of data that EWT&A col lected during Sept'!mber in the Susitna River and extracted from pertinent literat ure. The relat ionship betwee n turbidity and penetration depth of light prov ided i n the memo should be considered prel iminary.We are continuing our work t o provide a more def initive,Susitna-specific equation. since1ly£M---rrE.W;~T'iC;;~. EWT /ds enc. "Il.'? CiA E.WOODY TRIHEY a.ASSOCIATES INSTRBAM PLOW AND RIVERINE HABITAT ASSESSMENTS Techn ical Memorandum P .O.BOX J 1177. ANCHORAGB,ALASKA 99511 (907)562·7707 PRELIMINARY DATA Subjed 10 Revision Date p '~g '§'i Preliminary Analysis of the Relationship between Turbidity an~Light Penetration in the Susitna River,Alaska by Erwin Van Ni euwenhuyse November,1984 This memo presents an analysis of field data collected in the Susitna River drainage and elsewhere to provide a basis for est imating the depth to which photosynthetically act ive rad iation (PAR)can penetrate turbid water.This knowledge will enable a better understa~ding of how the benthic algae community of the Talkeetna-to-Devil Canyon reach of the Susitna River (middle river)presently functions under the natural turbidity regime,and how it will likely respond to the very different seasonal turbidity regime anticipated under with-project conditions.The i mplicat ions of an altered turbi dity 'regi me and the response of the benthi ca l gae community are of considerable interest and concern regarding with-project r earing poent ial in habitats i nfluenced by the mainstem. PAR is defined as light that is useful to plants for photosynthesis.Th is portion of the electromagnetic spectrum ranges from 400 to 700 nanometers in wave l~ngth.Its intensity at depth z (I z)is an exponential decay function of surface intensity (10)'depth beneath the water surface (z), and the negative exti ncti on coeffi ci ent (Ttl : I z • 10 e-l\.z Equation 1 Strictly s'peaking, the extinction coefficients described in this document are "total vertical light extinction coefficients,"i.e.,(1)they . incorporate the light attenuation and absorption properties of the water itself as well as those of all dissolved and particulate matter present in the water column,and (2)light at the surface and dt depth are measured on paral lel horizontal planes (Poole and Atkins 1926).By convent ion the extinction coefficient is expressed in metric units,but English units are used in our analysis to maintain consistency with other project studies. 1 Turbidity is an arbitrary measure of water "cloudiness,"i.e.,the extent to which suspended matter scatters and absorbs light as it crosses a fixed path length of water sample.Today it is most often expressed in nephelometric turbidity units (NTU)and is measured by nephelometry,i.e., using a turbidimeter which measures light scattered at a gO·angle to the i nci dent beam. Total vertical light extinction coefficient measurements were made during mid-September in accord with methodology described by Van Nieuwenhuyse (1983).Field measurements were obtained at four sites in the middle and lower Susitna River as well as three sites in the Chulitna and Talkeetna rivers under turbidities ranging from 4.4 to 130 NTU (Table 1). Table 1.Extinction coefficients (~)and turbidities measured in the Susitna River drainage and Knik Arm,September 10-11, 1984. SITE TURBIDITY (NTU)T\.(fe1 ) Chulitna River (LB)120 .4.03 Chulitna River (LB)120 3.93 Lower Susitna R1ver (La)130 4.21 Lower Susitna River (LB)130 4.19 Lower Susitna River (RB)45 1.53 Middle Susitna River (RB)18 0.73 Talkeetna River (LB)4.4 0.34 Knik Arm 50 1.63 LB •left bank,looking upstream RB •right bank,looking upstream 2 light extinction coef ficients were plotted against turbidity and the resulting strong positive correlation (r 2 =0.99,n=8)was described well by the linear regression equation: l\,•0.181 +U.031O T Equation 2 where T\.•extinction coefficient (ft-1)and T •turbidity (NTU). Although this model is adequate to describe light penetration for turbidities below 130 NTU,its predictions for higher turbidity levels can be improved SUbstantially by combining the data points from which it was derived with data available from other studies.The resulting model based on pooled data could be applied to a broader range of turbidities with greater confidence than the model we developed with only eight data points collected during September. Naturally-occurring 'turbidity levels for the middle Susitna River at Gold Creek range from 1 to 1,000 NTU;with-project turbidity conditions are estimated to range from approximately 25 to 250 NTU or more. The use of pooled data to obtain general empirical models has been successful in numerous hydrologic and ecological applications in the past, e.g.,the derivation of rearation coefficient models (Streeter and Phelps 1925,Churchill et al.1962,O'Conner and Dobbins 1956), suspended sediment and bedload transport models (Dunne and leopold 1978),and phosphorus loading models (Vollenweider and Kerekes 1980), and appears justified in this case as well. 3 To date we are aware of onl y two data bases that can be pooled with the measurements we obta1ned duri ng Septembe r 1984; one co 11 ected for undi s- turned and placer-mined str-eams in Interior Alaska (Van Nieuwenhuyse 1983) and the other for a glacial lake near Anchorage (R&M Consultants 1982). The first data base consists of measurements taken at very low turbidit ies (1.1 -4.7 NTU)and very high turbidities (330 - 1,400 NTU)with no inter- mediate levels represented.This shortcoming is compensated to some extent by the range of turbidities examined in the glacial lake study (17 - 38 NTU)and by our September data set (4.4 - 130 NTU). By pool ing <;he three data sets and plotting light extinction coefficient against turbidity the following linear regression equation i s obtained (R 2,..98,n · 30 )which "is appl icable to t ur-bt d t t i es ranging from approx imately 5 to 1,400 NTU: It,.0.543 +0.0177 (T) where It ,.ext inction coefficient and T ,.turbidity (NTU). Equation 3 • The precision of the model is illustrated by th e Working-Hotelling confidence bands (~,.0.05)wh ich del imit the family of i nt erval estimates of mean responses at all levels of the independent variable (Figure 1). The accuracy of the model,especially at very low and very high turbidi- ties,is somewhat compromised by the inherent difficulties associated with accurately and precisely measuring turbidity.The extinction coefficient measurements on the other hand tend to be both very accur c:te and preci se (Van Nieuwenhuyse 1983).Predictions from the light extinction model, however,are at least as real istic as those generated by the hydraulic 4 Turbidity (NTU)1400 y =0.543 +0.01768 X (n =30) r2 =0.98 PP.f:U"'ilJ.~;lY E.Woody Tlihey &As soc iatlls D;Jl e _All ZQ IfB¢,» 12001000800600 "I' "I' / / / ,/ ,/ / "I' / ,/ ,/ ./ / ,/, "I' "I' ./ -" "I' / / "I' "/ / "I',- /,-,- 400200 5 15 10 25 20 ~(ft·l) Figure 1. Pooled linear regression model of extinction coefficient v.turbidity showing 95%Working-Hotelllng confidence band.ALASKA POW ER AUTHORITY SUSITNA HYDROelECTRIC PROJECT EWT&A ~-~,._,-,.._.....aUln....lOfNT U Nf URl models and the with -project turbidity forecasts with which it will be linked to assess impact on rearing potential. One example of an application for the light extinction model can be given by deriving the relationship between turbidity and ·compensation depth,· i.e.,the depth at which only a of the PAR striking the water's surface is available to the benthic algae community.While the use of this concept-- Which was developed for lakes--is somewhat arbitrary in the context of streams and rivers,it is a convenient (if not conservative)indicator variable for defining the lower limit of the euphotic zone in rivers and will be used here until a more appropriate limit can be developed. Thus, by combining equations 1.and 3 and by setting I z "0.01 (1 0 ) ,one obtains: I z "0.01 (1 0 )"I oe-(0.543 +0.0177T)zc c "-4.605 "-(0.543 +0.1077T)zc Equation 4Zc"4.605 (0.543 +0.0177T) where Zc a ·compensation depth·(ft)and as before T •turbidity (NTU). Thus,at a turbidity of 500 NTU,the esti mated (95S confidence interval) lower limit of useful light penetration would be approximately 0.49 +0.03 ft below the surface.At 300, 100, 50, and 10 NTU,it would be 0.80 +0.05, 2.1 +0.3, 3.5 +1.20,and 9.2 +5.4 ft,respecti vely. An ot her use for the model would be to estimate the amount of light energy available for photosynthesis at any particular depth (e.g.,a channel's 5 mean depth)given the amount of light striking the surface and the turbidity. While the model presented here is probably the best general model available at this time,a more accurate,Susitna-specific model,applicable to a broad range of naturally-occurring turbidity levels will become available by this time next year as extinction coefficients and turbidities are measured in conjunction with primary prOduction studies which we will initiate in March or April of 1985. 6 REFERENCES Churchill,M.A.,H.L.Elmore,and R.A.Buclc ingham.1962.Prediction of str~am rearation rates.J.Sanit.Eng.Div.,Proc.ASCE,88,SA4 :1-46. Dunne,T. and L.B.Leopold.1978.Water i n envi ronmental planning. W.H.Freeman and Co. San Francisco.81B p. O'Conner,D.J.and W.E.Dobbins .1956.The mechanism of rearation in natural streams.J.Sanit.Eng.Div.,Proc.ASCE,82,SA6:1-30. Poole,H.H.a nd W.R.G.Atlcins.1926.On the penetration of light i nt o sea-water.J.Mar.BioI.Ass.,U.K.,14:177-198. R&M Consultants.1982.Glacial lalce stUdies interim report.Prepared for Acres American Inc.,Anchorage,Alaa~lca. Streeter,H.W.and E.B.Phelps.1925.A stUdy of the pollution and natural purification of the Ohio River:U.S.Public Health Service, Public Health Bull.146.75 p. Van Nieuwenhuyse,E.E. 1983. The effects of placer mining on the primary productivity of Interior Alaska streams .M.S.Thesis,University of Alaslca,Fairbanlcs.120 p. Vollenweider,R.A.and J.Kerelces.1980;The loading concept as basis for controlling eutrophication philosophy and preliminary results of the OECD Programme on eutrophication.Prog.Wat.Tech.12:5-18. 7