Behaviour of Turbidity Maxima in the Tamar (U.K.) and Weser (F.R.G.) Estuaries

Estuarine, Coastal and Shelf Science (1997) 45, 235–246

Behaviour of Turbidity Maxima in the Tamar (U.K.) and Weser (F.R.G.) Estuaries I. Grabemanna, R. J. Unclesb, G. Krausec and J. A. Stephensb a

Institut fu¨r Gewa¨sserphysik, GKSS Forschungszentrum Geesthacht Gmbh, Postfach 1160, 21494 Geesthacht, Germany b Plymouth Marine Laboratory, Prospect Place, Plymouth PL1 3DH, U.K. c Alfred-Wegener-Institut fu¨r Polar- und Meeresforschung, Postfach 120161, 27515 Bremerhaven, Germany Received 26 January 1995 and accepted in revised form 23 July 1996 The Tamar (England) and Weser (Germany) Estuaries exhibit strong turbidity maxima in their low-salinity reaches. Whereas their morphologies and freshwater inflows are different, the estuaries have similar mean tidal ranges and mean tidal current speeds. Despite the differences, the turbidity maxima show similar intratidal, spring–neap and seasonal behaviour for the two estuaries. On an intratidal time scale, strong suspended particulate matter (SPM) variability occurs in both turbidity maxima regions due to deposition over slack-water periods, subsequent re-suspension and depletion of easily erodible bed-source sediment during the following flood or ebb, and transport by tidal currents. Over a spring–neap cycle, the spring-tide SPM levels are greater than those during neap tides owing to enhanced re-suspension in the stronger spring-tide currents. Both estuaries exhibit a hysteresis in SPM during the falling and rising spring–neap cycle. On a seasonal basis, the occurrences of river spates in both the Tamar and the Weser move both the limits of saline intrusion and the turbidity maxima further down-estuary. Following the return to normal freshwater inflows, the saline intrusion moves rapidly up-estuary, whereas the turbidity maximum exhibits a time lag. ? 1997 Academic Press Limited Keywords: estuaries; turbidity maximum; suspended matter; transport; seasonal variations; English coast; Weser Estuary; German coast

Introduction The estuarine turbidity maximum is a dynamic feature. Its generation and maintenance is known to be the result of complex interactions between the tidal dynamics, gravitational circulation and erosion and deposition of fine sediment (Dyer, 1988). In the turbidity maximum region, suspended particulate matter (SPM) concentrations vary within the tidal cycle, with the spring–neap cycle, seasonally and with river runoff changes. Turbidity maxima and their associated processes have been studied in various estuaries of the world (e.g. Allen et al., 1980; Officer & Nichols, 1980; Officer, 1981; Gelfenbaum, 1983; Avoine & Larsonneur, 1987; Weir & McManus, 1987; Hamblin, 1989) including the Tamar (e.g. Uncles & Stephens, 1989, 1993a,b; Uncles et al., 1989, 1992, 1994; West et al., 1990, 1991) and the Weser (e.g. Wellershaus, 1981; Riethmu¨ller et al., 1988; Grabemann & Krause, 1989, 1994; Lang et al., 1989). The estuaries of the Tamar and Weser have similar mean tidal ranges and mean tidal current speeds. However, the Weser Estuary is about four times the 0272–7714/97/020235+12 $25.00/0/ec960178

length of the Tamar Estuary, is much wider, substantially deeper for most of its length, and has much greater freshwater inflows. Despite these differences in topography and inflows, each estuary exhibits a strong turbidity maximum. The two maxima show similar intratidal, spring–neap and seasonal behaviour. Therefore, it is of interest to compare and contrast the two estuaries in order to further understand the physical processes which are involved in the occurrence and variability of the estuarine tubidity maximum. The turbidity maximum within each estuary has been studied in some detail, especially during the last decade. In this paper, large data sets will be used to provide a comparison of the SPM dynamics within the respective turbidity maximum regions of the Tamar and Weser. It is hoped that this comparison will help to formulate more general statements on estuarine suspended matter dynamics, and provide an interpretative background and experimental understanding for the development of sediment-transport modelling within estuaries. Long-term time-series measurements of salinity, turbidity (SPM concentrations) and current velocity have been made at fixed locations within both ? 1997 Academic Press Limited

236 I. Grabemann et al.

10°W

(a)

0°

10°E

(b)

60°N

Weir

North Sea

km 0

German Bight

5 10

• 50°N

CK 0°

HQ

50°N

10°E

15 N

• 80

Cargreen

BL

•

20

•

60

Bremerhaven

•

25 N

30

40

20 Bremen

20 km

1 km Plymouth Sound

Weir

km 0

F 1. Maps of the (a) Tamar and (b) Weser Estuaries. , Locations of long-term measuring sites; CK, Calstock; HQ, Halton Quay; BL, Blexen. Numbers denote kilometres downstream of the weir. For Weser Estuary, there is another official distance scale which has its origin about 5 km downstream of the weir.

estuaries. Two locations have been used in the Tamar (Uncles et al., 1992, 1994) and five locations have been used in the Weser (Grabemann & Krause, 1989, 1994), together with an extra four locations in the Weser during a special field experiment (Riethmu¨ller et al., 1988). These measurements covered the spectrum of SPM variability from minutes to weeks and months. Longitudinal profiling measurements, with good vertical resolution of salinity and turbidity, have also been obtained during differing meteorological and spring–neap tidal conditions (Uncles et al., 1989; Uncles & Stephens, 1993a,b in the Tamar). Usually, the longitudinal profiling measurements covered about a tidal cycle at most. Measurements were carried out from a vessel traversing between two locations which were separated by a distance of about 7 km in the Tamar and about 20 km in the Weser. It has been verified that the data derived from measurements at the long-term observation site at Blexen in the Weser (Grabemann & Krause, 1989) and at Halton Quay and Calstock in the Tamar (Uncles et al., 1994) are qualitatively typical of the respective regions nearby. The instrument packages,

together with the procedures for estimating SPM concentrations from turbidity values, are described in Uncles et al. (1989) for the Tamar, and Krause and Ohm (1984) and Grabemann and Krause (1989) for the Weser. The present paper starts with a comparative description of the morphology and hydrography of the Tamar and Weser Estuaries. This is followed by the presentation of data which illustrate the SPM variability during tidal cycles and over the spring–neap cycle. Finally, the effects of changing freshwater runoff will be discussed and the similarities and differences between the two systems will be highlighted.

Morphology and hydrography The Tamar and the Weser are coastal plain estuaries (Figure 1). The Tamar Estuary is located on the southern coast of England. Its length between Weir Head and Plymouth Sound is about 31 km. The length of the inner estuary between Weir Head and Cargreen is about 20 km. The water depth in the main channel varies between 2 and 8 m below mean

Turbidity maxima in the Tamar and Weser Estuaries 237 T 1. Characteristic data for the Tamar and the Weser Estuaries: long-term minimum and maximum runoffs describe the long-term averages (Weser: 1941–90, Deutsches Gewa¨sserkundliches Jahrbuch, Weser-/Emsgebiet 1990) of the annual minimum and maximum values, respectively Variable Length Whole estuary: weir to . . . Inner estuary: weir to . . . Width at high water Inner estuary Outer estuary Depth below high water (main channel) Inner estuary Outer estuary Subtidal water volume (low-water, spring volume) Tidal range Mean spring tide Mean neap tide Mean Current velocities Inner estuary Outer estuary Runoff Long-term mean min. Long-term mean max. Long-term mean

Tamar

Weser

. . . Plymouth Sound: 31 km . . . Cargreen: 20 km

. . . German Bight: 120 km . . . Bremerhaven: 70 km

30–1000 m 500–1200 m

200–2000 m >2000 m

~2–8 m ~8–40 m ~2#106 m3 (inner estuary) ~6#107 m3 (whole estuary) (mouth) 4·7 m 2·2 m 3·5 m

~12 m ~16 m ~3#108 m3 (inner estuary)

(Main channel) §1 (spring)"~0·5 m s "1 (neap) ~0·5 (spring)"~0·3 m s "1 (neap)

(Bremerhaven) 4·0 m 3·0 m 3·5 m (km 120: mean <2·9 m) (Main channel) ~0·5"§1 m s "1 §1 m s "1

3 m3 s "1 290 m3 s "1 34 m3 s "1

127 m3 s "1 1250 m3 s "1 324 m3 s "1

high-water springs in the inner estuary, and between 8 and 40 m in the outer estuary (Table 1). The Weser Estuary drains into the southern German Bight. The outer estuary, down-channel of Bremerhaven, is funnel-shaped within the Wadden Sea area. The mouth cannot be defined using a single morphological feature. Instead, it is the region where salinity shows only small intratidal and seasonal changes. Using this definition, the total length of the estuary down-channel of the weir in Bremen is about 120 km. The 70-km-long inner estuary between Bremen and Bremerhaven has a channel-like character due to extensive engineering modifications. The water depth in the deep channel below mean high-water springs increases from about 12 m in the upper estuary to about 16 m in the lower estuary (Table 1). The navigation channel is dredged to maintain the required water depth. Tides in both the Tamar and Weser Estuaries are semi-diurnal and asymmetrical. Although the mean tidal range is the same for both estuaries (Table 1), the Tamar has much greater differences between mean spring and mean neap tidal ranges. The subtidal

or low-water volume of the whole Tamar Estuary is about an order of magnitude smaller than that of the inner part of the Weser Estuary (Table 1). The freshwater runoff into the Tamar Estuary is between one and two orders of magnitude smaller than that of the Weser Estuary, with larger differences in summer (Table 1). The different variability of the freshwater runoffs may be due to different magnitudes of the respective catchment areas and different meteorological conditions. The Weser runoff upstream of Bremen might be slightly influenced by the operation of storage basins and feeding of water into a canal. In both estuaries, the mixing zone between salt water and freshwater depends strongly on runoff. During mean runoff in the Tamar, the isohaline representing a salinity of 1 intersects the river bed about 9 km down-stream of the weir at high-water (about 0·3 of the length of the estuary). Due to salt mining in the catchment area of the Weser, the salinity of the ‘ fresh ’ water of the Weser can reach 2, depending on runoff. During mean runoff in the Weser, the up-stream limit of the mixing zone is observed to be about 53 km down-estuary of the weir

238 I. Grabemann et al. T 2. Suspended particulate matter (SPM) concentrations in the centre of the turbidity maximum for the Tamar (0·3 m above bed) and the Weser Estuaries (1 m above bed at the long-term observation site, Blexen, outside of the fairway) SPM (kg m "3) At high water Neap tide Spring tide At maximum currents Neap tide Spring tide

Tamar

Weser

0·02–0·1 0·2–0·3

0·06–0·15 0·06–0·15

<0·5 >5 (Area of bed-sediment deposits)

~0·4 ~0·7

at high water (about 0·4 of the total length of the estuary). The salinity is about 30 at the seaward limits of both the Tamar and the Weser. The Tamar shows much higher stratification in the mixing zone at neap tides than the Weser. Bed sediments and magnitudes of turbidity maxima The Tamar and outer Weser Estuaries exhibit large areas of intertidal mud flats. The silt and clay content of the Tamar bed sediments decreases from about 90–95% in the turbidity maximum region to about 60–70% near the mouth. The organic content is less than 10% (Stephens et al., 1992). In the Weser Estuary, the bottom material in the fairway is fine and medium sand, except the region between 58 and 71 km downstream of the weir, where it is mud (Wetzel, 1987; Figure 14). This region corresponds largely to the mean turbidity maximum location for mean runoff (Grabemann & Krause, 1989; Figure 3). The organic content there is about 1% (Wellershaus, 1981). During slack-water, flocs with diameters of up to 200 ìm are observed in both estuaries. Smaller flocs (20–30 ìm in the Tamar, <60 ìm in the Weser) exist during maximum current velocities. A strong turbidity maximum is associated with the lower salinity reaches of both estuaries. The turbidity maximum locations depend on runoff [Uncles et al., 1993a (Tamar); Grabemann & Krause, 1989 (Weser)]. The Tamar exhibits much higher SPM concentrations. Suspended particulate matter concentrations during maximum currents of spring tides can greatly exceed 5 kg m "3 near the bed of the Tamar in the area of bed-sediment deposits (Uncles et al., 1993b, e.g. Figures 5 and 7). Suspended particulate matter concentrations can reach about 2 kg m "3 near the bed in the fairway of the Weser (derived from vertical profiling measurements) and about

0·8 kg m "3 near the bed at the long-term observation site, Blexen, outside of the fairway. In Table 2, SPM concentrations are given for the centre of each turbidity maximum. Within each turbidity maximum, SPM concentrations increase with depth for most of the tidal cycle (see Figures 2 and 3). At Intschede, about 30 km up-stream of the weir, the long-term mean SPM concentration in the river water of the Weser is 0·039 kg m "3 (mean value 1970–90; Deutsches Gewa¨sserkundliches Jahrbuch, Weser-/Emsgebiet 1990). However, concentrations occasionally can be greater than 0·2 kg m "3 for short periods of time (of order a few days). Suspended particulate matter concentrations in the freshwater inflows of the Tamar Estuary are typically 0·01 kg m "3. However, during the beginning of spate conditions, these concentrations can exceed 0·1 kg m "3. Intratidal SPM variations and turbidity maximum locations In the Weser Estuary, long-term time-series measurements and observations of longitudinal transects revealed the occurrence of cyclical processes within a tide. In the low-salinity region, during high- and lowwater slack periods, temporary and spatially-limited deposits of particles form at the bed due to settling of SPM. At the end of the ebb tide, SPM is deposited at down-estuary locations. At the end of the flood tide, SPM is deposited at up-estuary locations. Within a tidal cycle, material is re-suspended and exchanged between these depositional regions (Grabemann & Krause, 1989, 1994). The intratidal behaviour of SPM, and associated movements of the turbidity maximum, have been observed in both estuaries using longitudinal transect and vertical profiling measurements. Figure 2(a–f) shows longitudinal transect and vertical profile

Turbidity maxima in the Tamar and Weser Estuaries 239 (a)

5

2

LW + 1.8

10 m

LW – 0.5

2

(b)

5

LW + 1.8

LW + 2.4

5

15 (c)

10

2

LW + 4.8

LW + 2.4

10

5

15

(d) LW + 4.8

HW – 0.2

5

10

15

(e)

HW – 0.2 HW + 2

5

10

15

HW + 3 52

(f) HW + 2

55

58

61

64

67

70

73

Distance x from weir (km)

0.43

0.49

0.55

0.61

–1

xL ≤_ 0.1

0.1–0.2 0.2–0.4 0.4–0.6 SPM concentration

_ 0.6 >

F 2. (a–f) Longitudinal and vertical sections of turbidity and salinity in the Weser Estuary. Suspended particulate matter (SPM) concentration (kg m "3, solid lines) and salinity (dashed lines) isolines are presented for spring tide and low runoff conditions. The start and end times of each transect measurement are on the left and right of each plot. The panel shows the sequence of events from low water (LW) minus 0·5 h until high water (HW) plus 3 h. Slack water occurs about 1 h after LW and HW, respectively. The secondary axis denotes relative distances. x, Distance from weir; L, total estuarine length.

measurements in the Weser for the flood and the start of the ebb during low runoff conditions at spring tide. As currents increase during the flood tide, SPM is resuspended from the down-estuary deposit of fine sediment [right part of Figure 2(a)] which was formed from particles settling over low-water slack. This re-suspension continues until the limited stockpile of

easily eroded particles is depleted [right part of Figure 2(c)]. The particles are transported up-estuary [Figure 2(b–d)]. Suspended particulate matter settles to the bed in the up-estuary part of the turbidity zone over high-water slack [Figure 2(e)]. These particles form a temporary, fine-sediment source for the following ebb. During the ebb, particles are re-suspended and transported down-estuary [starting in the left part of Figure 2(f)]. Observations show that the locations of both temporary sediment sources, and the associated region of high SPM concentrations, change as functions of runoff (see later). The location of the lowsalinity mixing zone also changes as a function of runoff. This intratidal sequence of events is very similar for SPM transport processes within the low-salinity region of the Tamar. However, in this estuary, the temporary occurrence of a pronounced turbidity maximum up-channel of the freshwater–saltwater interface is of importance. This phenomenon occurs after prolonged periods of low river discharge during summer and autumn, and is particularly well developed during spring tides. The turbidity maximum owes its existence to a large, mobile stock of bedsource fine sediment which is temporarily located a few kilometres down-estuary of the head (Weir Head, Figure 1). Its location is largely determined by tidal asymmetry effects, which result in preferential, upestuary sediment transport (Uncles et al., 1993a). High SPM concentrations in the Tamar are associated both with the freshwater–saltwater interface and with re-suspension of up-estuary bed-source sediments during spring-tide, low runoff conditions (Figure 3). After high water [Figure 3(a)], the water column is stratified and the near-bed currents are very slack. The turbidity maximum occurs up-estuary of the contour representing a salinity of 5. It is advected down-estuary from the upper reaches during the ebb [Figure 3(b)]. Near-bed currents increase later in the ebb, and SPM levels increase everywhere in the lowsalinity region [Figure 3(c)]. However, SPM levels are highest in the upper reaches, behind the freshwater– saltwater interface. As the freshwater–saltwater interface moves down-estuary [Figure 3(b–d)], very strong increases in SPM occur in the freshwater reaches. Suspended particulate matter levels decrease somewhat during low water [Figure 3(e)], but they rapidly increase again in the strong flood currents [Figure 3(f)]. The turbidity maximum which occurs immediately down-estuary of the freshwater–saltwater interface is masked by the strong turbidity maximum which occurs in the very low-salinity region (Uncles & Stephens, 1993a).

240 I. Grabemann et al. 0 m s–1 (a)

15

5

25 HW + 1.1 5m

HW + 2.5 –1

1

0ms (b) HW + 2.5 25

15

5

HW + 3.5

–1

–0.5 m s 1

5

(c)

10

LW – 2.8

LW – 2.1

LW – 1.5

–0.6 m s (d) 10 LW – 2.1

–1

5

1

0.4 m s–1 5 (e) LW + 0.9

1

LW – 0.3

–1

0.7 m s 5 (f)

1 LW + 1.6

LW + 0.9 6

7

8

9

10

11

12

13

Distance x from weir (km) 0.20

0.28

0.36

0.44

x L–1 ≤_ 0.1

0.1–0.3

0.3–0.5 0.5–0.75 0.75–1.0 SPM concentration

_ 1.0 >

F 3. (a–f) Longitudinal and vertical sections of turbidity and salinity in the Tamar Estuary. Suspended particulate matter (SPM) concentration (kg m "3, solid lines) and salinity (dashed lines) isolines are presented for spring tide and low runoff conditions (as in Figure 2 for the Weser Estuary). The start and end times of each transect are on the left and right of each plot. The panel shows high water (HW) plus 1·1 h until low water (LW) plus 1·6 h. The secondary axis denotes relative distances. x, Distance from weir; L, total estuarine length. The longitudinal velocity at 0·25 m above the bed at 12·5 km is given on the top right of each plot.

With the onset of higher runoff in the Tamar, re-suspended fine sediment from the upper bedsource is moved down-estuary by the enhanced ebb flows, and is either deposited over low-water slack or flushed to the coastal zone. Therefore, the amount of re-suspendable material increases down-estuary. Additionally, during high freshwater runoff, flood currents are substantially reduced in the upper reaches and less flood-tide re-suspension from the bed-source

sediment takes place there. The dominance of the up-estuary turbidity maximum lessens and the maximum is then located in the vicinity of the freshwater–saltwater interface (Figure 4). During winter months, repeating episodes of high freshwater runoff eventually erode the upper-estuary stock of mobile bed- source sediment, and disperse it throughout the central estuary and into the coastal zone (Uncles & Stephens, 1993a). By contrast, in the Weser Estuary, the central part of the turbidity maximum zone is always located within the lower (non-‘ fresh ’) salinity reaches. At high water, it is located in the vicinity of the isohaline representing a salinity of 6 (Figures 2 and 4). Longitudinal transect measurements were made only during a rather restricted range of freshwater inflows between about 160 and 420 m3 s "1. However, other investigations have shown that the turbidity maximum zone (as defined by tidally-averaged, near-bed SPM concentrations >0·25 kg m "3) is associated with isohalines less than 10 for freshwater inflows less than 800 m3 s "1 (Grabemann & Krause, 1989). Tidal asymmetry is much stronger in the Tamar than in the Weser within their respective turbidity maximum regions. The ratio of high-water slack to low-water slack durations is about 8:1 in the Tamar, and only about 2:1 in the Weser. The slack period is defined as the time over which either the critical bed shear stress is less than 0·1 N m "2, (Tamar; Uncles & Stephens, 1993b), or the velocity does not exceed a critical speed of 0·2 m s "1 (Weser; Grabemann & Krause, 1989). These definitions are very similar when typical values of the drag coefficient are used in a quadratic drag relationship. Longitudinal and vertical measurements of salinity (Figures 2 and 3) indicate that stratification is similar in both estuaries during spring tides. The Tamar has stronger stratification during neap tides. High-water periods of slack currents are much longer in the Tamar. This phenomenon prolongs the settling periods of SPM over high-water slack. The onset of ebb currents occurs earlier in the surface layer than in the near-bed layer. Therefore, SPM ebbing and settling through the surface layer, and subsequently entering the near-bed layer, will tend to accumulate near the freshwater–saltwater interface as it is slowly moved up-estuary within the near-bed layer. Density stratification also causes an asymmetry in vertical mixing within both estuaries. A numerical model of the Weser Estuary by Lang (1990) indicates that the persistent salinity stratification during early ebb restricts turbulence, so that particles settle faster and therefore accumulate in the bottom layer. Suspended particulate matter moves a shorter

Turbidity maxima in the Tamar and Weser Estuaries 241 0.7

12.4

0.4

0.6

72

9.3

0.3

0.5

60

6.2

0.2

0.4

48

3.1

0.1

0.3

36

0.0 204

0.2

84

(b)

x L–1

Distance x from weir (km)

(a)

0

68

136

0

234

3 –1

2

24 972

3 –1

Q (m s )

0

648

Distance x from weir (km)

0.5

15.5

Q (m s )

4

6

Q/Qmean

0

1

2

3

Q/Qmean

F 4. Locations of the freshwater–saltwater interface (FSI, ) in the Tamar and the isohaline representing salinity of 6 ( ) in the Weser, respectively, and the turbidity maximum in the (a) Tamar ( ) and (b) Weser ( ) Estuaries. The measured locations at about high-water, down-estuary of the head are given as functions of the river runoff Q. For the Tamar Estuary, only spring-tide locations are shown. The secondary axes denote relative distances. X, Distance from weir; L, total estuarine length) and relative runoffs (Qmean, mean runoff as given in Table 1).

longitudinal distance during the ebb than during the vertically well-mixed flood tide. Uncles and Stephens (1993a,b) describe a similar process for the Tamar. The Weser and Tamar exhibit only slightly different intratidal behaviour of the SPM (Figures 2 and 3), and only slight differences in the seasonal variability of turbidity maximum locations with varying runoff (Figure 4). These differences seem to result from the existence of an up-estuary, mobile stock of bed-source sediment at low runoff during summer and autumn conditions in the Tamar. In the Weser Estuary, elevated SPM concentrations are mainly due to resuspension of fine sediment which has settled to the bed during the previous slack water. It is possible, but not proven, that the Weser’s turbidity maximum is sediment-limited due to dredging activities in the region. The complicated phenomena associated with irregular dredging operations and engineering constructions in the River Weser make it impossible to provide a definite answer to the question whether the Weser would have a similar up-estuary bed-source of sediments without such activities. One may only compare the riverine SPM input into the Weser Estuary with the annual mass of dredged material. The amount of SPM carried into the Weser Estuary fluctuates considerably from year to year. In the period between 1983 and 1990, the extreme

values were 410#106 kg year "1 and 1170#106 kg year "1 (about 30 km upstream of the weir, Deutsche Gewa¨sserkundliche Jahrbu¨cher, 1983–90). In 1986, 6 1·74#1010 m3 of material has been dredged in the total length (120 km) of the fairway (Wetzel, 1987). About 50% of this dredged amount belongs to the inner 80 km. Assuming a specific weight of about 1·5#103 kg m "3, this would result in 1350#106 kg year "1 which is in the same order of magnitude as the riverine SPM input. The highest dredged amount per square metre is between 58 and 79 km downstream of the weir, which partly corresponds to the muddy area (see above) and to the tidally-averaged location of the turbidity maximum for mean runoff. As the dredged material is partly not removed from the Weser Estuary but only displaced, and because of the unknown quantity of marine input, the natural state of the Weser’s bed-source material upstream of the turbidity maximum cannot be reconstructed.

Spring–neap cycle In the Tamar Estuary, spring-tide SPM concentrations greatly exceed neap-tide concentrations. In the Weser Estuary, however, spring-tide SPM concentrations can be up to a factor of two higher than neap-tide concentrations (Figure 5).

242 I. Grabemann et al. 0.8

(b)

(a)

SPM (kg m–3)

0.6 Start 0.4

Start 0.2

0.0

2

3 4 Tidal range (m)

5

2

3 4 Tidal range (m)

5

F 5. Comparison of daily-averaged suspended particulate matter (SPM) concentrations (kg m "3) against tidal range (m) for the (a) Tamar (Halton Quay) and (b) Weser (Blexen, right). Due to lack of data, the hysteresis loop from the Tamar data is not complete. For the Weser, day-to-day fluctuations are eliminated using a 5-day running average.

Despite differences in the relative spring-to-neap magnitudes of SPM concentrations within the Tamar and Weser, both estuaries show similar behaviour in their SPM dynamics on spring–neap time scales. Measurements of SPM concentrations have been made in both estuaries during periods of almost constant freshwater runoff (Figure 5). Therefore, possible changes in SPM concentrations due to runoff-induced changes in the locations of the respective turbidity maxima are unlikely. A hysteresis is exhibited in the SPM levels between the falling and rising spring–neap cycle (Figure 5). This may be the result of some consolidation of deposited sediment during neap tides, when current velocities and, therefore, re-suspension are relatively small. The hysteresis is more pronounced in the Tamar, with its higher spring–neap tidal range differences, than in the Weser (Figure 5 and Table 1). Otherwise, both estuaries behave in a similar way to the spring–neap forcing. Measurements in the Weser show that the magnitude of the spring–neap hysteresis can vary. Neap-tide consolidation of settled sediment appears to be less pronounced in the Weser, presumably due to smaller differences in the magnitudes of spring and neap tides there. Freshwater runoff spates On a seasonal time scale, river spates are of great importance to the long-term transport of SPM in both

the Tamar and the Weser. In both estuaries, river spates cause a dispersal of SPM from the turbidity maximum region to a location further down-estuary, or even into the coastal zone. Whereas salinity responds relatively rapidly, within days or even hours, to changes in runoff, re-establishment of the turbidity maximum requires weeks or even months following a spate. During the period June 1988–July 1989, bed-source sediment in the Tamar Estuary exhibited a strong seasonal movement (Figure 6). During low runoff conditions, the SPM concentrations at mooring stations Halton Quay (HQ) and Calstock (CK) have an approximate magnitude of 0·3 kg m "3 (Figure 6). However, a prolonged increase in runoff results in a decrease of the SPM levels to about 0·01 kg m "3 in December 1988 (Days 162–173). This is a manifestation of the winter period of bed-sediment depletion from the upper reaches. Strong ebb currents enhanced by large and increasing runoff scour bed-source sediment from the upper estuary. Until the end of high runoff spate conditions, SPM levels at both Tamar stations remain low (March 1989, Days 253–264, Figure 6). During April (Days 292–301), after decreasing runoff, the SPM levels at CK are nearly the same as they were during high runoff conditions, but the SPM levels at HQ are very much higher (0·77 kg m "3). This indicates that the bed-sediment supply has moved up-estuary to within the vicinity of HQ.

Turbidity maxima in the Tamar and Weser Estuaries 243

3 –1

Runoff (m s )

240 180 120 60 0 1.2

–3

SPM (kg m )

1.0

HQ CK

0.8 0.6 0.4 0.2

0

60

120

180

240

300

360

Time (days from 29 June 1988)

F 6. Effects of river spates for the Tamar Estuary. The daily-averaged runoff and suspended particulate matter (SPM) concentrations measured or estimated on a rising 3·8 m tidal-range spring tide are given for Halton Quay (HQ) and Calstock (CK) for deployments between June 1988 and July 1989. The widths of the bars indicate the time intervals over which concentrations have been averaged.

During the spring–summer period of low runoff, SPM and associated bed-source sediment continue to move up-estuary. First, the SPM levels at CK exceed those at HQ (May 1989, Days 317–328, Figure 6), then SPM levels at both stations decrease (June, July 1989, after Day 343). This decrease in SPM occurs because the amount of bed-source sediment available for local erosion decreases in the vicinity of both stations as sediment moves closer to the head and further away from HQ and CK (Uncles et al., 1994). An increase in SPM will eventually re-occur at these stations when the freshwater runoff increases in late autumn and bed-source sediment is eroded and moves down-estuary. This situation is observed during Days 138–149 (Figure 6) following the spate which occurs between Days 100–115. Several spates and substantial non-spate flows are required to completely disperse the bed-source sediment from the upper estuary. Although salinity responds within days to changes in runoff within the Tamar Estuary, up-estuary movement of bed sediment takes place on a seasonal time scale. Several weeks of low runoff conditions are required in order for significant amounts of up-estuary sediment transport to occur.

In the Weser Estuary, the temporal response of the turbidity maximum following a river spate is also much slower than that for the salinity field. At a fixed site (Blexen) during the winter and spring of 1986–87 (Figure 7), higher SPM concentrations occur for strongly increasing freshwater inflows, and smaller concentrations for decreasing freshwater inflows, following a spate. This represents a kind of SPM-runoff hysteresis, in which, for a given runoff, the SPM is higher on the rising than on the falling runoff curve (Figure 7). In contrast, the salinity at a given location is not significantly different on the rising or falling runoff curve (Figure 7). Grabemann and Krause (1994) have shown that a mass-reduced turbidity maximum requires about approximately 2 weeks to re-establish itself within the low-salinity region (the ‘ mixing zone ’). But, as in the Tamar, it appears to require up to several weeks (about 1–7 months, based on five investigated events) of decreasing runoff to normal conditions in order to restore the turbidity maximum to its former strong magnitude (Grabemann et al., 1995). Fine sediment moved from the Weser’s turbidity maximum region by a river spate is probably flushed

244 I. Grabemann et al.

Runoff (m3 s–1)

2400 1800 1200 600

0

30

60

90

120

150

180

210

Time (days from 8 November 1986)

10

0.35 0.30 SPM (kg m–3)

Salinity

8

6

4

2

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0.25 0.20 0.15 0.10

600

1200

1800

2400

3 –1 Runoff (m s )

0.05

0

600

1200

1800

2400

3 –1 Runoff (m s )

F 7. Effects of river spates for the Weser Estuary. The hatched areas in the runoff time series indicate times of increasing and decreasing runoff for which suspended particulate matter (SPM) concentration measurements exist. Daily-averaged salinity and SPM concentrations against river runoff are presented for Blexen (BL): 20 days before and during strongly increasing runoff (solid symbols, December 1986–January 1987) and 44 days after a runoff spate during decreasing runoff (open symbols, April–May 1987). In order to distinguish spring and neap tides, different symbols have been chosen for tides with tidal ranges greater (squares) and less than (circles) 3·5 m, respectively.

into the coastal zone, or possibly distributed over the extensive tidal flats of the outer estuary (Figure 1). The general frequency of river spates larger than 1200 m3 s "1 (corresponding to the long-term average of the annual maximum runoffs, Table 1) is 9 within the 10-year period from 1985 to 1994; in two cases, the peaks followed each other in short time intervals (less than 1 month). Summary and conclusions Despite large differences in topography between the Tamar and Weser, both estuaries exhibit welldeveloped turbidity maxima in their low-salinity or freshwater reaches. These maxima show strong similarities in their behaviour for the two systems. During low runoff, summer and autumn conditions in the Tamar, the turbidity maximum is displaced up-estuary of the freshwater–saltwater interface. In

the Weser, the turbidity maximum is always closely associated with the low-salinity region. The pronounced turbidity maximum which occurs up-estuary of the freshwater–saltwater interface in the Tamar under low runoff, spring tide conditions is due to re-suspension from a large source of bed sediment there. Such an up-estuary turbidity maximum is not observed in the Weser because of the absence of a correspondingly large bed-source of sediment. Differences in the relative locations of the turbidity maxima in the Weser and Tamar may result from several causes. There is a much more pronounced tidal asymmetry in the Tamar and this appears to result in an up-estuary accumulation of sediment during prolonged low runoff conditions. Such an accumulation of sediment does not occur in the Weser. However, it is possible, but not proven, that an up-estuary bed-source of sediment does not form in

Turbidity maxima in the Tamar and Weser Estuaries 245

the Weser, despite the existence of tidal asymmetry, because of dredging activities in the area. During high runoff, winter conditions in the Tamar, bed-source sediment is distributed throughout the central estuary, and local re-suspension from this source is much less important. The turbidity maximum is then closely associated with the freshwater– saltwater interface and its behaviour is very similar to that within the Weser. The spring–neap cycle is more pronounced in the Tamar than in the Weser, and there are relatively larger differences between neap-tide and spring-tide elevations and currents. However, the turbidity maxima in both estuaries behave in a similar way. There is a hysteresis in the SPM concentrations, in which concentrations are higher on the falling spring– neap cycle than on the rising cycle. This appears to be due to some degree of consolidation of deposited sediment during the neap tides, when re-suspension is relatively small or absent. In both estuaries, river spates cause a large transport of SPM from the turbidity maximum region to a location further down-estuary, or even into the coastal zone. The salinity fields respond quickly (within days or even hours) to a receding river spate. This is not the case for the respective turbidity maxima. In the Tamar, freshwater spates during late autumn and winter lead to removal of sediment from the upper reaches; re-accumulation of sediment there then requires several weeks of low runoff conditions. In the Weser, a runoff spate partially distributes SPM throughout the lower estuary, or flushes it towards the North Sea. Approximately 2 weeks of decreasing runoff are then required for the re-establishment of a mass-reduced turbidity maximum at its former location. About 1–7 months of normal runoff conditions appear to be required in order to restore the turbidity maximum to its former magnitude. In spite of large differences in size, morphology and river discharge between the two estuaries, significant phenomena which occur within the two turbidity maxima can be qualitatively explained in terms of a few common, basic processes. This is an important conclusion because it indicates the generic nature of turbidity maximum formation within strongly tidal, partially mixed estuaries. It would be worthwhile to include other river estuaries into such comparisons, including low-turbidity and fluid mud systems, in order to formulate more general statements on estuarine suspended matter dynamics and to provide an interpretative background and experimental understanding for the development of sediment-transport modelling with estuaries.

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