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Phosphorus elimination by longitudinal subdivision of reservoirs and lakes
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Pergamon PIJ:S0273-1223{98~29-8
WGl. Sci. T«". Vol. 37, No.2, pp. 235-2.3,1998. C 1998 I"WQ. Published by Elsevier Science LId Printed In Great Britain. 0273-1223198 $19'00 + 0-00
PHOSPHORUS ELIMINATION BY LONGITUDINAL SUBDIVISION OF RESERVOIRS AND LAKESl Lothar Paul*, Kurt Schriiter**, Jorg Labahn** • Dresden University o/Technology, Neunzehnhain Ecological Slalion Z, Neunzehnhainer SIr. 14, D-Q9514 Wiinschendorf, Germany •• Bauhaus-University of Weimar, Hydro-lAboratory, Themarer Str. 16c, D-98553 Schleusingen, Germany
ABSTRACT An epilimnelic volume of nearly 350,000 m3 was separaled by the installation of an overflown, submerged flexible curtain (SFC, 160 m long, 6 m deep) perpendicular 10 the flow direction in the Haselbach mouth region of tile Saidenbach Reservoir (Saxony, Germany). The SFC produced (i) an interruption of hydraulic short circuits belween inlet and main reservoir basin, (Ii) a longer relention time of the inflowing waler, and (iii) a 30% 10 40% increase of the elimination of soluble reactive phosphorus in tile mouth region during summer stratification. II is conclusive thaI flexible curtains may be cost-effective subsUMes for conventional pre-dams of thermally stratified lakes or reservoirs. CCl 1998 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Eutrophication control; flexible curtain; flow through management; lakes; phosphorus elimination; pre-dam effects; reservoirs; retention time; thermal stratification. INTRODUCTION The efficiency of optimally dimensioned pre-darns reducing the phosphorus loading of reservoirs (especially soluble reactive phosphorus SRP which is the growth limiting factor in most cases) was frequently proven (e.g. Putz, 1995). Benndorf and Piltz (1987a,b) developed a procedure to determine (or predict) the SRP• elimination E in a pre-dam, which can be expected higher than 70% if the relative retention time lreJ> I, where
-t ... =-_-. tth...
(I)
t...
I Project 02WT93 17-6 sponsored by the Federal Ministry of Education, Science, Research, and Technology oftheFRG 2 Associate member of the Drinking Water Reservoir Association (A 'IT) MY 17,t·[
235
236
L. PAUL et al.
The mean theoretical retention time ttheot and the critical retention time terit are given by
-
_ VR Q,n
(2)
ttheor - •
(3)
and
with the volume V R of the 'reaction space' (euphotic zone), the mean discharge Qin ' the monthly means of the temperature T ~ [0C) and underwater light intensity 1M (vertical average of photosynthetically active radiation [J cm- 2 d- ]) in the reaction space, and the SRP-concentration PM [l!gPllj of the inlet. However, the efficiency of many pre-dams is low due to their volumes being too small to achieve t re) > 1. Most eutrophicated lakes do not have pre-darns. The construction of pre-darns is expensive and seldom realizable after completion of a reservoir, because the demand for bUilding land cannot be satisfied supplementarily. Thus, it was investigated whether a 'pre-dam inside the lake' can be formed by a (comparaly cheap) plastic-coated SFC. The SFC spanned from shore to shore in a determined distance behind the mouth of the inlet should reach from a few centimetres below the surface down to the boundary between metalimnion and hypolimnion (below the depth of inflow) during thermal summer stratification. This way, the frequently observed hydraulic short circuit between tributary mouth and main basin of the lake or reservoir along the thermocline is interrupted and a surface overflow over the SFC is forced. The epilimnetic volume ('reaction space') separated (longitudinally) by the SFC and (vertically) by the thermocline has to be large enough to achieve trel > I as long as possible from end of spring until autumn and, therefore, high E may be expected. Due to the thin gap between surface and upper edge of the SFC, a certain, highly variable, wind-driven back• flow Qres from the main basin of the reservoir into the 'pre-dam' must be considered, which represents an additional inflow with the SRP-concentration cres' Furthermore, it cannot be excluded that the depth of inflow zin is beneath the SFC during cold periods with low inlet temperature Tin' For such a case (analogical to a pre-dam with two inlets or an underwater pre-dam; see Paul, 1995) E can be estimated by E = 10',(1- Coulo(1-X+Q)+XCoutU)
\.
c'" +c,.. Q
(4)
with the discharge ratio Q=QreJQin and the fraction x of Qin flowing beneath the SFC during the time interval evaluated, and the mean SRP-concentrations Couto and Coutu of the overflow and underflow, respectively. For the average theoretical retention time ttheor then follows: _ tthen, -
VR Q.. +Q,..
_
- (
VR ) I+Q Q..
(5)
It is hardly possible to exactly determine Qrcs' Therefore, the ratio Q must be estimated from Equ. 4 using measured concentrations of an inert conservative tracer, for which E=O is valid. In the following, results of experiments with a SFC carried out in the Haselbach underwater pre-dam (UWHA) of the mesotrophic Saidenbach Reservoir (Saxony) are described. Special attention is paid to changes of the flow through regime and the SRP-elimination after the installation of the SFC.
Phosphorus elimination
237
METHODS Description of the study site A sketch of the Saidenbach Reservoir (altitude: 438.8 m.a.s.!., volume: 22.4.10 6 m3' surface: 146 hat maxi• mum depth: 45 m, theoretical retention time: 0.4 ... I a, drainage area: 61 kmV and the UWHA (volume: 0.6*' ()6 m3• surface: 9 ha, maximum depth: 13 m. average Qin of Haselbach inflow: -60.000 m 3/d) is shown in Fig. I. The top of the underwater dam of the UWHA is 4.8 m below the surface of the completely filled reservoir at 434 m.a.s.!. The Saidenbach Reservoir was intensively investigated since 1975 and the structure of the drainage area and the nutrient status was described in detail by Hom et al. (1994). In November 1994. a 160 m long and 6 m deep SFC was installed perpendicular to the flow direction about 100 m in front of the underwater dam. The SFC-Iength is greater than the straight distance between the anchor points at the shores to allow an independent adaptation of the SFC to changing surface levels. The distance between the (submerged) upper edge of the SFC and the surface can be changed as well as the lateral location and width of the overflow. The volume separated by the SFC amounts to nearly 360,000 m 3.
Underwater dam
• M~4\
Curtain
. .. 100 m
1000m Figure I. Schemes of Saidenbach Reservoir (tributaries Hllizelbergbach, 1Jppersdorfer Bach, Silidenbach. lhselbach; J>DF - Forchheim pre-dam) and (enlarged) Haselbach underwater pre-dam (0 aampling stations•• meteorology/temperature float).
Parameters investigated An extensive experimental program was carried out at five stations (designated as Z, MPI, MP2. MP3. MP4; see Fig. I) along the longitudinal axis of the UWHA in weekly intervals from end of 1993 until November 1996. Temperature T, pH, oltygen concentration 02' and conductivity X20 corrected for 20°C (using sensor• devices from WTW, Weilheim) as well as the chlorophyll fluorescence Chi (with BackSeat Fluorometer, Dr. Haardt, Kiel) were measured in depth increments of I m from surface down to bottom at all stations. Secchi depth was determined as a parameter characterizing the underwater light conditions. At each station milted samples of the upper 6 m layer (or from surface to bottom at stations Z and MPI where zm8K < 6 m) were taken with a Ruttner sampler. Phosphates (SRP, dissolved phosphate OP, and total phosphorus TP afler di· gestion with K2S 20 g under pressure) and dissolved silica Si were analysed with the molybdenum blue methOd (Ausgewlthlte Methoden, 1986). The concentration of particulate phosphorus PP was calculated from the difference TP-OP. The nitrate concentration N0 3 was directly determined from UV-self-absorplion
238
L. PAUL et al.
of the filtered samples (Application GA No. I, Dr. Bruno Lange GmbH, Berlin). Meteorological data and short-term measurements of water temperatures at different depths and sites were ascertained with data logger systems (SQUIRREL 1204, Grant Instruments LTD Cambridge, completed by Driesen&Kem, Hamburg). In 1996, the temperature Tin of the Haselbach was registered with a pillbox-logger (pBT Pt 100, Driesen&Kem, Hamburg) just before its mouth. The daily discharges Qin were kindly made available to us by the State Reservoir Administration of Saxony. RESULTS AND DISCUSSION Pischar~e
Tin and Qin (Fig. 2) are important factors which determine lin and the residence time. In the summer of 1994, Qin was comparably low without any significant flood events. Tin varied in the typical range except a warm period from end of June till middle of August. With respect to the functioning of the SFC in 1995, the rapid warming of the inflow at the end of April, the Qm-peaks in May, June, and at the end of August with low Tin' and the warm period from end of June to end of August with very low Qin were especially important. After an unusual long and cold winter, an abrupt increase of Tin was observed at the end of April 1996. Later the weather was cold during the whole summer with low Tin (particularly end of June and July) except a short period in June. Again, some flooding occured in summer 96. o Inflow Temperature rC] ( - Long-Term Monthly Mean t SO)
.
24
-
.
16
8 \
L
0
~A-.--r-+""T'""T'"~~4+""T'""T'"+-r-r-+-.~~!h-,...-i--T~~
1
\240 ---1
Discharge [10' m'/d]
• Rain [mm/d] - 6 0
•
160
I" 0
I.
-
40 20
+-r........-l.-.-..-+~+T'""'T"-h~+--r......-4!....;:....-+-'T"'T-f-,r-T'"+--r-~~T'"f-.,....,i-+361 0 0
3
6
9
I
12
15 18 21 24 Months after 12/93
27
Ii
33
Figure 2. Discretely measured Tin (at station Z) related to long-term monthly means ± SO (above) and Haselbach Qin as well as daily precipitation higher than 10 mmld (below) in the years investigated.
Physical and chemical stratification The physical and chemical stratification in the UWHA in 1994 before the installation of the SFC (described in detail by Paul, 1995) was characterized by the formation of a stable thermocline in the middle of May. Its upper threshold at MP2 and MP3 approximately coincided with the depth of the top of the underwater dam until end of August (while it successively descended at MP4 as a consequence of the deep water release from the reservoir). Epilimnetic cooling slowly increased the depth of the thermocline in September and it finally vanished late in October. The temperature of the epilimnion (from surface down to about I m above the underwater dam) in the UWHA was identical to the one observed in the respective layer at MP4 in the main basin of the reservoir.
239
Phosphorus elimination
0
20
10
0 0
0
0
2
2
2
4
4
4
6
6
6
8
8
8
10
10
10
12 200
300
250
12 200
250
10
0
20
10
300
12 200
20
250
300
pH (all related to the upper axis), Figure 3. Vertical profiles (depths 1m) dowpwards positive) ofT IOC]. 02 (mgll). line. position of the underwater and X20 IllS/em} (lower axis). measured at MP3 in 1994 on the dates given (dOlled dam head. arrows· theorcticallin derived from Tin)'
( Temperature reI)
Julian day in 1995
120 150 180 210 240 270 300 330 360
030 609 0
.......t-'-~~-f-L;,.f~..., 0~;7+~~ ....1rlt""'r'mtr~~'!C-l-+-,
-2
Vi
iii
E
co
.,
a:j
-4 -6
- 8 l - -__---U .LL. Thlu .~~~ ~~~t L:.. .J
o
(')
~
:8
~ Q)
o
30
60
120 150 180 210 240 270 300 330 360
90
-1 5
-3
-5 -7
-9
-11 -13
...L.-
-..la---'--
.......--.l.....I: .-l........I -_ _.....- - ' - - '
~_
(thick lines for 5. 10. 15. 20OC; S· Figure 4. Temperature contour plots for MP2 (above) and MP3 (below) in 1995 SFC: black sq~s • lin derived of margin lower circles with line head; dam r underwate surface; dashed line· at MP3 for water flowing over lin squares· empty only; SFC below from Tin' for MP3 in cases with underflow SFC during summer).
3 mgll in the deep water layers of the Short time after thermocline formation, 02 rapidly decreased below ion remained nearly anoxic from June meta/imn the of parts even ily temporar and ion hypolimn The UWHA. 260 JAS/cm in the hypolimnion of !han higher rose X20 and 7 below to October. Simullaneously, pH dropped the horizon of the underwater under just UWHA (c.f. Fig. 3). In most of the cases, zin derived from Tin was With increasing thermocline August. of end until April of end from m 6 of depth the dam head but above bottom at the end of Oc• the reached and er depth (cooling of epilimnion), zin descended also in Septemb It was a good tracer 1994. summer in reservoir the in that than lower ably consider was inlet tober. Xzo of the see middle and (e.g. rule the as Tin' from ed detennin zin marking the flow through layer, which was above
240
L. PAUL., al.
right in Fig. 3; 08/10/94: X20(inlet)=176\J.S/cm; 09/21/94: X20(inlet)=182\J.S/cm). This was due to a certain mixing of inflow with ambient reservoir water in the mouth region, which depends on the temperature gra• dient at lin' Generally, no remarkable differences in the physical and chemical stratification characteristics were detected between MP2 and MP3 in 1994. This situation changed considerably after the installation of the SFC. As evident from Fig. 4, the principle development of the temperature stratification in 1995 was similar to that described for 1994. However, clear differences in the fine temperature structure between MP2 (maximum depth 8 m only) and MP3 become obvious. most of all in the layer between about I m above the underwater dam head and the lower edge of the SFC. In periods with normal or high Tin (cf. Fig. 2, May, July, August 1995), when lin at MP2 was above the lower SFC-edge, the temperature in this layer was higher at MP2 than at MP3 (see also the profiles from August 23rd exemplarily given in Fig. 5). Low 02 below the depth of the underwater dam head at MP3 (saturation clearly below 100%), show that the water in this layers between the curtain and the underwater dam were stagnating without any significant exchange with the main basin or inflow in such time intervals. During cold and (frequently) rainy events (low Tin' high Qin; e.g. June, September 1995). lin at MP2 was be• low the SFC and temperature isolines at MP3 above this horizon steeply ascending up to the depth of the dam head or a bit higher (see Fig. 4 below) indicate rapid cooling. In this intermediate layer at MP3, 02 increased. pH rose higher than 7. and 1(20 decreased close to the one of the inlet as a consequence of the underflow below the SFC during such periods (c.f. August 30th in Fig. 5).
10 2 I ~ +0_"--rl>e-....L..-_tr°--t °
10
20
10
20
I 2
I I
I
! ~
6 ---I-_~--
7
I
I II Aff»2
I 89 I 10
!
200
08123
250'
I I -!&---,J.I--I--tl1r-
300 1_20_0
2_5_0_ _
Figure 5. Typical vertical profiles of T. pH. 02' and X20 measured at MP2 and MP3 in August 95 (symbols as given in Fig. 3. dashed line· lower edge of SFC). After the long and cold winter a sudden increase of the surface-T resulted in a stable thermal stratification immediately after ice out at the end of April 1996. The Saidenbach Reservoir changed from winter to summer stagnation without any significant spring circulation. In repeated cold periods (middle of May, middle of June until end of July. September) during summer 1996. Tin frequently was inferior to T at the lower edge of the SFC. The thermocline vanished late in October.
Typical summer flow through regimes derived from the seasonal development of the physical and chemical stratification are schematically depicted in Fig. 6. Before the installation of the SFC. the inlet water normally formed an interflow in a relatively thin layer along the upper boundary of the metalimnion, which was located just below the depth of the underwater dam head (Fig. 6a). Therefore, hydraulic short circuits between the inlet mouth and the main basin of the Saidenbach Reservoir and high Q res have to be assumed characteristic for the UWHA. Two representative flow through situations were observed after the SFC was
Phosphorus elimination
241
installed. In summer periods with normal or high Tin' the SFC was functioning as expected (Fig. 6b). HydraulIc short ctrCUlls were Interrupted and the inlet water was forced to skim over the SFC. Underflow below the SFC, upwelling at the underwater dam, and outflow over its head was noticed in cold phases (Fig. 6c). However, Q res back into the volume separated by the SFC was substantially reduced in both cases. The principles drawn in Fig. 6 could essentially be confirmed by experiments with a large-scale physical model of the UWHA with and without a SFC in the Schleusingen Hydro-Lab of the Bauhaus-University of Weimar.
Figure 6. Sehemalic representations of now through regimes typical for summer stagnalion periods before (a) and after (b: overnow over SFC, c: undernow below SFC) the installation of the curtain. In order to prevent damage by ice in the winter 94/95, the SFC was lowered about I m in December 1994 A closed ice cover on the entire UWHA developed within one cold night in January 1995 and melted almost regularly by the middle of February (similar to the situation prior to SFC emplacement). On the contrary, in the following winters the SFC was not lowered and ice formation before it began some days earlier and lasted longer than on the main basin of the reservoir. During the entire winter, a stripe a few metres wide remained icc-free just behind the SFC, even in the coldest periods with air temperatures below -15 0c. This was also a consequence of the surface flow over the SFC. Residence time and SRP-elimination In a firsl order approximation, with Eqn. 4 (setting &0) constant seasonal averages of q were estimated from N0 3 and X20 which were considered quasi-conservative tracers (short residence times, atomic N:P ratios up to 10,000: I). Using cin from station Z, Couto from MP2, Coutu from MP3 (I m below dam head), and c res from MP4, it resulted q~6 for 1994 (wide gap between surface and underwater dam head), q~ I for the firsl and, afler a reduction of the overflow by lifting up the SFC, q~0.5 for the second half-year of 1995. In 1996, the overflow was restricted to the northern (most wind sheltered) half of the SFC in a very thin surface layer and 4 decreased to about 0.3. Of course, using constant q is a very rough guess due to always changing wind impact and Qin- However, the estimation of q by means of monthly averages of the 'tracer' concentrations frequently failed, because the concentration differences between the stations sometimes were insignificant. The monthly underflow-fraction x was pragmatically derived from comparisons of the temperature T 6 at MP2 in 6 m depth with Tin (daily x = 0 when T6 :0; Tin' X = I when T6> Tin)' The effects of the SFC regarding the residence times (resulting from Equs. I, 3, 5) and SRP-elimination (observed E obs calculated from Equ. 4; predicted E pred and - assuming the SFC were not installed - Ewilhou' following the procedure developed by Benndorf and Putz, 1987a) become evident from Fig. 7. Although the hydro-climatic (high T M and 1M , euphotic depth zeu = 6 m) and discharge conditions (low Qin' high VR) were favourable in 1994, trel was never greater than 1 in the UWHA because high q diminished lthcor Consequently, Eobs (which is satisfactorily filled by E prcd ) was relatively low compared to a conventional pre-dam of similar size (e.g. Forchheim pre-dam of Saidenbach Reservoir, where E usually is higher than 80% in summer; Horn et ai. (988). After the installation of the SFC, 11hcor mostly is higher {han terit and
242
L. PAUL et al.
in the summers of 1995 and 1996, despite a lower V R (zeu = 5 m, shorter distance of the SFC from the inlet mouth) and low T M and 1M (except July and August 1995). The impact of the SFC on the SRP• elimination gets clear comparing E pred and E obs with Ewithout. Except August 1995, both were always higher than Ewithout' however, E obs did not always reach Epred ' This could be explained by high underflows (high x; May and September 1995, May to July 1996) anellor q being overestimated.
lrel~1
The great gap between E pred and Eobs in July (even E obs < Ewithout> and August 1995, when and were exceptional1y high (Qin very low, T M very high, no underflow), probably was the consequence of a proliferation of f1agel1ates, rotifers, and ciliates (Bruckner, 1995) and, therefore, a low algae-bound P• settling. Diatoms were practical1y not present at this time. Generally, high E can only be expected in pre• dams, if SRP-uptake by fast growing and settling algae (preferably diatoms) is prevailing and the development of blue-greens, f1agel1ates (almost no P-sedimentation), and P-remobilizing organisms (zooplankton, rotifers, ciliates) is of minor importance (Benndorf and Piitz, 1987a). These conditions were obviously not sufficiently fulfilled in both months.
[iI]
!:-tcrit --ttheor --trel I
t2
9
6 3
0 I
\
[%)
80
160 1
40
12~ I
J
J
A
1994
S
o
N
MAMJJASON
o
N
MAMJJASON
1U5
Epred __ Eobs L-EWithout
I 1
I
.~~~ M A M
I
I
\
-0-
f\ M A M
J
J
A
S
MAMJJASON
Figure 7. Critical, theoretical, and relative retention times (above) and relative SRP-eliminations (below: Epred' predicted, Eobs - observed. Ewithout - predicted assuming the SFC were not installed). CONCLUSIONS The depth of the SFC used in the experiments was at least I m too low to completely exclude underflows during cold periods in summer. However, it could principally be shown that submerged flexible curtains of optimum size in stably stratified lakes and reservoirs can prevent hydraulic short circuits, increase the re• tention time of intlowing water, enhance SRP-elimination, and favor sedimentation of allochthonous and autochthonous seston in the mouth region. Thus, they can be considered cost- and management-effective alternatives for conventional pre-dams. ACKNOWLEDGEMENT The authors are pleased to acknOWledge Prof. H. Bernhardt (t) and Prof. D. Uhlmann for constant support and helpful criticism. REFERENCES Ausgewiihlte Methoden der Wasserunrersuchung (1986). Bd J: Chemische. Physikalisch-chemische und Physikalische Methodefl.
Fischer Verlag Jena.
Phosphorus elimination
243
Benndorf, J. and Piitz, K. (1987a). Control of eutrophication of lakes and reservoirs by means of pre-dams - I. Mode of operation and calculation of the nutrient elimination capacity. Wat. Res., 21, 829-838. Benndorf, J. and Piitz, K. (1987b). Control of eutrophication of lakes and reservoirs by means of pre-dams - II. Validation of the phosphate temova! model and size optimization. Wat. Res.• 21, 839-842. Bruckner, Ch. (1995). EinflufJ der lAngsunterteilung d., Unterwasservorsperre Hase/bach auf Abbundanz und Sukzession des Phyto- und Zoop/anktons. Diplomarbeit, 11.1 Dresden, Inst. f. Hydrobiologie, pp. 99. Hom, W., Geissler, D., Hom, H. and Paul, L. (1988). Eutrophierungsmechanismen in der TaJsperre Saidenbach. 04 Final Research Report, 11.1 Dresden, Sektion Wassetwesen, Fachbereich Hydrobiologie, pp. IDS. Hom, W., Hom, H. and Paul, L. (1994). Long-teon trends in the nutnent input and in-lake concentrations of a drinking water reservoir In a dense populated catchment area (Erzgebirge, Germany). Int. Revue ges. Hydrobio/., 79,213-227. Paul, L. (1995). Nutrient eliminauon in an underwater pre-dam. Int. Revue ges. Hydrobio/., 80, 579-594. Piltz, K. (1995). Talsperren - notig, sicher,likologisch? Wasser & Boden, 47(4), 24-29.
Pergamon PIJ:S0273-1223{98~29-8
WGl. Sci. T«". Vol. 37, No.2, pp. 235-2.3,1998. C 1998 I"WQ. Published by Elsevier Science LId Printed In Great Britain. 0273-1223198 $19'00 + 0-00
PHOSPHORUS ELIMINATION BY LONGITUDINAL SUBDIVISION OF RESERVOIRS AND LAKESl Lothar Paul*, Kurt Schriiter**, Jorg Labahn** • Dresden University o/Technology, Neunzehnhain Ecological Slalion Z, Neunzehnhainer SIr. 14, D-Q9514 Wiinschendorf, Germany •• Bauhaus-University of Weimar, Hydro-lAboratory, Themarer Str. 16c, D-98553 Schleusingen, Germany
ABSTRACT An epilimnelic volume of nearly 350,000 m3 was separaled by the installation of an overflown, submerged flexible curtain (SFC, 160 m long, 6 m deep) perpendicular 10 the flow direction in the Haselbach mouth region of tile Saidenbach Reservoir (Saxony, Germany). The SFC produced (i) an interruption of hydraulic short circuits belween inlet and main reservoir basin, (Ii) a longer relention time of the inflowing waler, and (iii) a 30% 10 40% increase of the elimination of soluble reactive phosphorus in tile mouth region during summer stratification. II is conclusive thaI flexible curtains may be cost-effective subsUMes for conventional pre-dams of thermally stratified lakes or reservoirs. CCl 1998 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Eutrophication control; flexible curtain; flow through management; lakes; phosphorus elimination; pre-dam effects; reservoirs; retention time; thermal stratification. INTRODUCTION The efficiency of optimally dimensioned pre-darns reducing the phosphorus loading of reservoirs (especially soluble reactive phosphorus SRP which is the growth limiting factor in most cases) was frequently proven (e.g. Putz, 1995). Benndorf and Piltz (1987a,b) developed a procedure to determine (or predict) the SRP• elimination E in a pre-dam, which can be expected higher than 70% if the relative retention time lreJ> I, where
-t ... =-_-. tth...
(I)
t...
I Project 02WT93 17-6 sponsored by the Federal Ministry of Education, Science, Research, and Technology oftheFRG 2 Associate member of the Drinking Water Reservoir Association (A 'IT) MY 17,t·[
235
236
L. PAUL et al.
The mean theoretical retention time ttheot and the critical retention time terit are given by
-
_ VR Q,n
(2)
ttheor - •
(3)
and
with the volume V R of the 'reaction space' (euphotic zone), the mean discharge Qin ' the monthly means of the temperature T ~ [0C) and underwater light intensity 1M (vertical average of photosynthetically active radiation [J cm- 2 d- ]) in the reaction space, and the SRP-concentration PM [l!gPllj of the inlet. However, the efficiency of many pre-dams is low due to their volumes being too small to achieve t re) > 1. Most eutrophicated lakes do not have pre-darns. The construction of pre-darns is expensive and seldom realizable after completion of a reservoir, because the demand for bUilding land cannot be satisfied supplementarily. Thus, it was investigated whether a 'pre-dam inside the lake' can be formed by a (comparaly cheap) plastic-coated SFC. The SFC spanned from shore to shore in a determined distance behind the mouth of the inlet should reach from a few centimetres below the surface down to the boundary between metalimnion and hypolimnion (below the depth of inflow) during thermal summer stratification. This way, the frequently observed hydraulic short circuit between tributary mouth and main basin of the lake or reservoir along the thermocline is interrupted and a surface overflow over the SFC is forced. The epilimnetic volume ('reaction space') separated (longitudinally) by the SFC and (vertically) by the thermocline has to be large enough to achieve trel > I as long as possible from end of spring until autumn and, therefore, high E may be expected. Due to the thin gap between surface and upper edge of the SFC, a certain, highly variable, wind-driven back• flow Qres from the main basin of the reservoir into the 'pre-dam' must be considered, which represents an additional inflow with the SRP-concentration cres' Furthermore, it cannot be excluded that the depth of inflow zin is beneath the SFC during cold periods with low inlet temperature Tin' For such a case (analogical to a pre-dam with two inlets or an underwater pre-dam; see Paul, 1995) E can be estimated by E = 10',(1- Coulo(1-X+Q)+XCoutU)
\.
c'" +c,.. Q
(4)
with the discharge ratio Q=QreJQin and the fraction x of Qin flowing beneath the SFC during the time interval evaluated, and the mean SRP-concentrations Couto and Coutu of the overflow and underflow, respectively. For the average theoretical retention time ttheor then follows: _ tthen, -
VR Q.. +Q,..
_
- (
VR ) I+Q Q..
(5)
It is hardly possible to exactly determine Qrcs' Therefore, the ratio Q must be estimated from Equ. 4 using measured concentrations of an inert conservative tracer, for which E=O is valid. In the following, results of experiments with a SFC carried out in the Haselbach underwater pre-dam (UWHA) of the mesotrophic Saidenbach Reservoir (Saxony) are described. Special attention is paid to changes of the flow through regime and the SRP-elimination after the installation of the SFC.
Phosphorus elimination
237
METHODS Description of the study site A sketch of the Saidenbach Reservoir (altitude: 438.8 m.a.s.!., volume: 22.4.10 6 m3' surface: 146 hat maxi• mum depth: 45 m, theoretical retention time: 0.4 ... I a, drainage area: 61 kmV and the UWHA (volume: 0.6*' ()6 m3• surface: 9 ha, maximum depth: 13 m. average Qin of Haselbach inflow: -60.000 m 3/d) is shown in Fig. I. The top of the underwater dam of the UWHA is 4.8 m below the surface of the completely filled reservoir at 434 m.a.s.!. The Saidenbach Reservoir was intensively investigated since 1975 and the structure of the drainage area and the nutrient status was described in detail by Hom et al. (1994). In November 1994. a 160 m long and 6 m deep SFC was installed perpendicular to the flow direction about 100 m in front of the underwater dam. The SFC-Iength is greater than the straight distance between the anchor points at the shores to allow an independent adaptation of the SFC to changing surface levels. The distance between the (submerged) upper edge of the SFC and the surface can be changed as well as the lateral location and width of the overflow. The volume separated by the SFC amounts to nearly 360,000 m 3.
Underwater dam
• M~4\
Curtain
. .. 100 m
1000m Figure I. Schemes of Saidenbach Reservoir (tributaries Hllizelbergbach, 1Jppersdorfer Bach, Silidenbach. lhselbach; J>DF - Forchheim pre-dam) and (enlarged) Haselbach underwater pre-dam (0 aampling stations•• meteorology/temperature float).
Parameters investigated An extensive experimental program was carried out at five stations (designated as Z, MPI, MP2. MP3. MP4; see Fig. I) along the longitudinal axis of the UWHA in weekly intervals from end of 1993 until November 1996. Temperature T, pH, oltygen concentration 02' and conductivity X20 corrected for 20°C (using sensor• devices from WTW, Weilheim) as well as the chlorophyll fluorescence Chi (with BackSeat Fluorometer, Dr. Haardt, Kiel) were measured in depth increments of I m from surface down to bottom at all stations. Secchi depth was determined as a parameter characterizing the underwater light conditions. At each station milted samples of the upper 6 m layer (or from surface to bottom at stations Z and MPI where zm8K < 6 m) were taken with a Ruttner sampler. Phosphates (SRP, dissolved phosphate OP, and total phosphorus TP afler di· gestion with K2S 20 g under pressure) and dissolved silica Si were analysed with the molybdenum blue methOd (Ausgewlthlte Methoden, 1986). The concentration of particulate phosphorus PP was calculated from the difference TP-OP. The nitrate concentration N0 3 was directly determined from UV-self-absorplion
238
L. PAUL et al.
of the filtered samples (Application GA No. I, Dr. Bruno Lange GmbH, Berlin). Meteorological data and short-term measurements of water temperatures at different depths and sites were ascertained with data logger systems (SQUIRREL 1204, Grant Instruments LTD Cambridge, completed by Driesen&Kem, Hamburg). In 1996, the temperature Tin of the Haselbach was registered with a pillbox-logger (pBT Pt 100, Driesen&Kem, Hamburg) just before its mouth. The daily discharges Qin were kindly made available to us by the State Reservoir Administration of Saxony. RESULTS AND DISCUSSION Pischar~e
Tin and Qin (Fig. 2) are important factors which determine lin and the residence time. In the summer of 1994, Qin was comparably low without any significant flood events. Tin varied in the typical range except a warm period from end of June till middle of August. With respect to the functioning of the SFC in 1995, the rapid warming of the inflow at the end of April, the Qm-peaks in May, June, and at the end of August with low Tin' and the warm period from end of June to end of August with very low Qin were especially important. After an unusual long and cold winter, an abrupt increase of Tin was observed at the end of April 1996. Later the weather was cold during the whole summer with low Tin (particularly end of June and July) except a short period in June. Again, some flooding occured in summer 96. o Inflow Temperature rC] ( - Long-Term Monthly Mean t SO)
.
24
-
.
16
8 \
L
0
~A-.--r-+""T'""T'"~~4+""T'""T'"+-r-r-+-.~~!h-,...-i--T~~
1
\240 ---1
Discharge [10' m'/d]
• Rain [mm/d] - 6 0
•
160
I" 0
I.
-
40 20
+-r........-l.-.-..-+~+T'""'T"-h~+--r......-4!....;:....-+-'T"'T-f-,r-T'"+--r-~~T'"f-.,....,i-+361 0 0
3
6
9
I
12
15 18 21 24 Months after 12/93
27
Ii
33
Figure 2. Discretely measured Tin (at station Z) related to long-term monthly means ± SO (above) and Haselbach Qin as well as daily precipitation higher than 10 mmld (below) in the years investigated.
Physical and chemical stratification The physical and chemical stratification in the UWHA in 1994 before the installation of the SFC (described in detail by Paul, 1995) was characterized by the formation of a stable thermocline in the middle of May. Its upper threshold at MP2 and MP3 approximately coincided with the depth of the top of the underwater dam until end of August (while it successively descended at MP4 as a consequence of the deep water release from the reservoir). Epilimnetic cooling slowly increased the depth of the thermocline in September and it finally vanished late in October. The temperature of the epilimnion (from surface down to about I m above the underwater dam) in the UWHA was identical to the one observed in the respective layer at MP4 in the main basin of the reservoir.
239
Phosphorus elimination
0
20
10
0 0
0
0
2
2
2
4
4
4
6
6
6
8
8
8
10
10
10
12 200
300
250
12 200
250
10
0
20
10
300
12 200
20
250
300
pH (all related to the upper axis), Figure 3. Vertical profiles (depths 1m) dowpwards positive) ofT IOC]. 02 (mgll). line. position of the underwater and X20 IllS/em} (lower axis). measured at MP3 in 1994 on the dates given (dOlled dam head. arrows· theorcticallin derived from Tin)'
( Temperature reI)
Julian day in 1995
120 150 180 210 240 270 300 330 360
030 609 0
.......t-'-~~-f-L;,.f~..., 0~;7+~~ ....1rlt""'r'mtr~~'!C-l-+-,
-2
Vi
iii
E
co
.,
a:j
-4 -6
- 8 l - -__---U .LL. Thlu .~~~ ~~~t L:.. .J
o
(')
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o
30
60
120 150 180 210 240 270 300 330 360
90
-1 5
-3
-5 -7
-9
-11 -13
...L.-
-..la---'--
.......--.l.....I: .-l........I -_ _.....- - ' - - '
~_
(thick lines for 5. 10. 15. 20OC; S· Figure 4. Temperature contour plots for MP2 (above) and MP3 (below) in 1995 SFC: black sq~s • lin derived of margin lower circles with line head; dam r underwate surface; dashed line· at MP3 for water flowing over lin squares· empty only; SFC below from Tin' for MP3 in cases with underflow SFC during summer).
3 mgll in the deep water layers of the Short time after thermocline formation, 02 rapidly decreased below ion remained nearly anoxic from June meta/imn the of parts even ily temporar and ion hypolimn The UWHA. 260 JAS/cm in the hypolimnion of !han higher rose X20 and 7 below to October. Simullaneously, pH dropped the horizon of the underwater under just UWHA (c.f. Fig. 3). In most of the cases, zin derived from Tin was With increasing thermocline August. of end until April of end from m 6 of depth the dam head but above bottom at the end of Oc• the reached and er depth (cooling of epilimnion), zin descended also in Septemb It was a good tracer 1994. summer in reservoir the in that than lower ably consider was inlet tober. Xzo of the see middle and (e.g. rule the as Tin' from ed detennin zin marking the flow through layer, which was above
240
L. PAUL., al.
right in Fig. 3; 08/10/94: X20(inlet)=176\J.S/cm; 09/21/94: X20(inlet)=182\J.S/cm). This was due to a certain mixing of inflow with ambient reservoir water in the mouth region, which depends on the temperature gra• dient at lin' Generally, no remarkable differences in the physical and chemical stratification characteristics were detected between MP2 and MP3 in 1994. This situation changed considerably after the installation of the SFC. As evident from Fig. 4, the principle development of the temperature stratification in 1995 was similar to that described for 1994. However, clear differences in the fine temperature structure between MP2 (maximum depth 8 m only) and MP3 become obvious. most of all in the layer between about I m above the underwater dam head and the lower edge of the SFC. In periods with normal or high Tin (cf. Fig. 2, May, July, August 1995), when lin at MP2 was above the lower SFC-edge, the temperature in this layer was higher at MP2 than at MP3 (see also the profiles from August 23rd exemplarily given in Fig. 5). Low 02 below the depth of the underwater dam head at MP3 (saturation clearly below 100%), show that the water in this layers between the curtain and the underwater dam were stagnating without any significant exchange with the main basin or inflow in such time intervals. During cold and (frequently) rainy events (low Tin' high Qin; e.g. June, September 1995). lin at MP2 was be• low the SFC and temperature isolines at MP3 above this horizon steeply ascending up to the depth of the dam head or a bit higher (see Fig. 4 below) indicate rapid cooling. In this intermediate layer at MP3, 02 increased. pH rose higher than 7. and 1(20 decreased close to the one of the inlet as a consequence of the underflow below the SFC during such periods (c.f. August 30th in Fig. 5).
10 2 I ~ +0_"--rl>e-....L..-_tr°--t °
10
20
10
20
I 2
I I
I
! ~
6 ---I-_~--
7
I
I II Aff»2
I 89 I 10
!
200
08123
250'
I I -!&---,J.I--I--tl1r-
300 1_20_0
2_5_0_ _
Figure 5. Typical vertical profiles of T. pH. 02' and X20 measured at MP2 and MP3 in August 95 (symbols as given in Fig. 3. dashed line· lower edge of SFC). After the long and cold winter a sudden increase of the surface-T resulted in a stable thermal stratification immediately after ice out at the end of April 1996. The Saidenbach Reservoir changed from winter to summer stagnation without any significant spring circulation. In repeated cold periods (middle of May, middle of June until end of July. September) during summer 1996. Tin frequently was inferior to T at the lower edge of the SFC. The thermocline vanished late in October.
Typical summer flow through regimes derived from the seasonal development of the physical and chemical stratification are schematically depicted in Fig. 6. Before the installation of the SFC. the inlet water normally formed an interflow in a relatively thin layer along the upper boundary of the metalimnion, which was located just below the depth of the underwater dam head (Fig. 6a). Therefore, hydraulic short circuits between the inlet mouth and the main basin of the Saidenbach Reservoir and high Q res have to be assumed characteristic for the UWHA. Two representative flow through situations were observed after the SFC was
Phosphorus elimination
241
installed. In summer periods with normal or high Tin' the SFC was functioning as expected (Fig. 6b). HydraulIc short ctrCUlls were Interrupted and the inlet water was forced to skim over the SFC. Underflow below the SFC, upwelling at the underwater dam, and outflow over its head was noticed in cold phases (Fig. 6c). However, Q res back into the volume separated by the SFC was substantially reduced in both cases. The principles drawn in Fig. 6 could essentially be confirmed by experiments with a large-scale physical model of the UWHA with and without a SFC in the Schleusingen Hydro-Lab of the Bauhaus-University of Weimar.
Figure 6. Sehemalic representations of now through regimes typical for summer stagnalion periods before (a) and after (b: overnow over SFC, c: undernow below SFC) the installation of the curtain. In order to prevent damage by ice in the winter 94/95, the SFC was lowered about I m in December 1994 A closed ice cover on the entire UWHA developed within one cold night in January 1995 and melted almost regularly by the middle of February (similar to the situation prior to SFC emplacement). On the contrary, in the following winters the SFC was not lowered and ice formation before it began some days earlier and lasted longer than on the main basin of the reservoir. During the entire winter, a stripe a few metres wide remained icc-free just behind the SFC, even in the coldest periods with air temperatures below -15 0c. This was also a consequence of the surface flow over the SFC. Residence time and SRP-elimination In a firsl order approximation, with Eqn. 4 (setting &0) constant seasonal averages of q were estimated from N0 3 and X20 which were considered quasi-conservative tracers (short residence times, atomic N:P ratios up to 10,000: I). Using cin from station Z, Couto from MP2, Coutu from MP3 (I m below dam head), and c res from MP4, it resulted q~6 for 1994 (wide gap between surface and underwater dam head), q~ I for the firsl and, afler a reduction of the overflow by lifting up the SFC, q~0.5 for the second half-year of 1995. In 1996, the overflow was restricted to the northern (most wind sheltered) half of the SFC in a very thin surface layer and 4 decreased to about 0.3. Of course, using constant q is a very rough guess due to always changing wind impact and Qin- However, the estimation of q by means of monthly averages of the 'tracer' concentrations frequently failed, because the concentration differences between the stations sometimes were insignificant. The monthly underflow-fraction x was pragmatically derived from comparisons of the temperature T 6 at MP2 in 6 m depth with Tin (daily x = 0 when T6 :0; Tin' X = I when T6> Tin)' The effects of the SFC regarding the residence times (resulting from Equs. I, 3, 5) and SRP-elimination (observed E obs calculated from Equ. 4; predicted E pred and - assuming the SFC were not installed - Ewilhou' following the procedure developed by Benndorf and Putz, 1987a) become evident from Fig. 7. Although the hydro-climatic (high T M and 1M , euphotic depth zeu = 6 m) and discharge conditions (low Qin' high VR) were favourable in 1994, trel was never greater than 1 in the UWHA because high q diminished lthcor Consequently, Eobs (which is satisfactorily filled by E prcd ) was relatively low compared to a conventional pre-dam of similar size (e.g. Forchheim pre-dam of Saidenbach Reservoir, where E usually is higher than 80% in summer; Horn et ai. (988). After the installation of the SFC, 11hcor mostly is higher {han terit and
242
L. PAUL et al.
in the summers of 1995 and 1996, despite a lower V R (zeu = 5 m, shorter distance of the SFC from the inlet mouth) and low T M and 1M (except July and August 1995). The impact of the SFC on the SRP• elimination gets clear comparing E pred and E obs with Ewithout. Except August 1995, both were always higher than Ewithout' however, E obs did not always reach Epred ' This could be explained by high underflows (high x; May and September 1995, May to July 1996) anellor q being overestimated.
lrel~1
The great gap between E pred and Eobs in July (even E obs < Ewithout> and August 1995, when and were exceptional1y high (Qin very low, T M very high, no underflow), probably was the consequence of a proliferation of f1agel1ates, rotifers, and ciliates (Bruckner, 1995) and, therefore, a low algae-bound P• settling. Diatoms were practical1y not present at this time. Generally, high E can only be expected in pre• dams, if SRP-uptake by fast growing and settling algae (preferably diatoms) is prevailing and the development of blue-greens, f1agel1ates (almost no P-sedimentation), and P-remobilizing organisms (zooplankton, rotifers, ciliates) is of minor importance (Benndorf and Piitz, 1987a). These conditions were obviously not sufficiently fulfilled in both months.
[iI]
!:-tcrit --ttheor --trel I
t2
9
6 3
0 I
\
[%)
80
160 1
40
12~ I
J
J
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1994
S
o
N
MAMJJASON
o
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MAMJJASON
1U5
Epred __ Eobs L-EWithout
I 1
I
.~~~ M A M
I
I
\
-0-
f\ M A M
J
J
A
S
MAMJJASON
Figure 7. Critical, theoretical, and relative retention times (above) and relative SRP-eliminations (below: Epred' predicted, Eobs - observed. Ewithout - predicted assuming the SFC were not installed). CONCLUSIONS The depth of the SFC used in the experiments was at least I m too low to completely exclude underflows during cold periods in summer. However, it could principally be shown that submerged flexible curtains of optimum size in stably stratified lakes and reservoirs can prevent hydraulic short circuits, increase the re• tention time of intlowing water, enhance SRP-elimination, and favor sedimentation of allochthonous and autochthonous seston in the mouth region. Thus, they can be considered cost- and management-effective alternatives for conventional pre-dams. ACKNOWLEDGEMENT The authors are pleased to acknOWledge Prof. H. Bernhardt (t) and Prof. D. Uhlmann for constant support and helpful criticism. REFERENCES Ausgewiihlte Methoden der Wasserunrersuchung (1986). Bd J: Chemische. Physikalisch-chemische und Physikalische Methodefl.
Fischer Verlag Jena.
Phosphorus elimination
243
Benndorf, J. and Piitz, K. (1987a). Control of eutrophication of lakes and reservoirs by means of pre-dams - I. Mode of operation and calculation of the nutrient elimination capacity. Wat. Res., 21, 829-838. Benndorf, J. and Piitz, K. (1987b). Control of eutrophication of lakes and reservoirs by means of pre-dams - II. Validation of the phosphate temova! model and size optimization. Wat. Res.• 21, 839-842. Bruckner, Ch. (1995). EinflufJ der lAngsunterteilung d., Unterwasservorsperre Hase/bach auf Abbundanz und Sukzession des Phyto- und Zoop/anktons. Diplomarbeit, 11.1 Dresden, Inst. f. Hydrobiologie, pp. 99. Hom, W., Geissler, D., Hom, H. and Paul, L. (1988). Eutrophierungsmechanismen in der TaJsperre Saidenbach. 04 Final Research Report, 11.1 Dresden, Sektion Wassetwesen, Fachbereich Hydrobiologie, pp. IDS. Hom, W., Hom, H. and Paul, L. (1994). Long-teon trends in the nutnent input and in-lake concentrations of a drinking water reservoir In a dense populated catchment area (Erzgebirge, Germany). Int. Revue ges. Hydrobio/., 79,213-227. Paul, L. (1995). Nutrient eliminauon in an underwater pre-dam. Int. Revue ges. Hydrobio/., 80, 579-594. Piltz, K. (1995). Talsperren - notig, sicher,likologisch? Wasser & Boden, 47(4), 24-29.