Forschungsstelle Küste Niedersächsisches Landesamt für Ökologie Reprint from Proceedings Hanz D. Niemeyer and Ralf Kaiser Evaluation of Design Water Levels and Design Wave Run-up for an Estuarine Coastal Protection Masterplan
Evaluation of Design Water Levels and Design Wave Run-up for an Estuarine Coastal Protection Masterplan Hanz D. Niemeyer 1 & Ralf Kaiser 1 1 Coastal Research Station of the Lower Saxon Central State Board for Ecology Fledderweg 25, D-26506 Norddeich/Ostfriesland, Germany (email: niemeyer.crs@t-online.de) Summary Subsequently deepenings of the waterway in the inner part of the Ems estuary at the southern North Sea coast changed not only the tidal regime but also that of storm surge water level. Therefore a thorough going review of the existing coastal protection master plan became necessary. Moreover the design procedures for the Estuarine dykes had to be adapted the state of the art in tune with existing legal requirements. A statistical model for the computation of storm surge set-ups in estuaries was introduced and verified. Design waves were evaluated by application of the third-generation mathematical model SWAN and the design wave runup was determined by using relationships derived from results of large scale model tests. 1 Introduction The Ems estuary at the southern North Sea coast in East Frisia is subdivided in the Outer Ems with the Dollart Bay and the Lower Ems between Emden and Pogum and Herbrum at the tidal border (fig. 1): It has experienced several successive deepenings since 1984 due to the requirements of a dockyard in its inner part. Recent observations of storm surge water levels made evident that the levels in the inner part of the estuary have become higher than in the past with respect to the Figure1: Ems estuary at the southern North Sea coast with the location of tidal gauges and the measuring location of fresh water discharge seaward boundary conditions higher than in the past with respect to the seaward boundary conditions (fig. 2): The peaks of the storm surges occurring at January 10th 1995 and particularly January 28th 1994 were relatively high and exceeded even those of February 1962 and January 1976 in the Lower Ems estuary though the peaks of the latter ones had been remarkably higher at the mouth of the estuary. Neither differences in the accompanying fresh water discharges during the distinct storm surges nor the flooding of large areas at the Lower Ems estuary during the storm surge of February 1962 explained sufficiently the observed relatively high peaks of the storm surges occurring at January 28th 1994 and January 10th 1995. The responsible district administration of the State of Lower Saxony asked the Coastal Research Station to carry out investigations on storm surge water levels with the aim to evaluate if the design water levels for the Estuarine dykes are still sufficient with respect to the safety standards. Furthermore was asked for an evaluation of design wave run-up. The paper deals both with the methods used for the check of the safety of the dykes and the results of the investigations.
2 Design water levels 2.1 Basics The evaluation of design water levels for sea dykes in Lower Saxony is carried out due to legal obligations [3] by use of the single components method (fig. 3). It is not applicable for the evaluation of design water levels for estuarine dykes since there is no consideration of the effects of fresh water discharge. In the past therefore the design water levels in estuaries have been evaluated by hydraulic model tests. Nowadays mathematical tidal models are regarded as a more appropriate tool for that purpose. Generally a deterministic numerical model will be used. In this case a more simple statistical model was established in order to save as well costs as time: contradictory to the deterministic numerical model the establishment of a suitable model bathymetry is not necessary. Moreover the verification is part of the modeling itself: The tuning of a statistical model by a large number of storm surges is not only recommendable but also feasible without high additional efforts. The model is based on a multiple regression of the set-up in the estuary corresponding to that one at the mouth of the estuary and the fresh water discharge. The basic equation for regression analysis is formulated as following: Figure 2: High tide peaks of the storm surges of February 16 th 1962, January 3rd/4th 1976, January 28th 1994, Januar 10th 1995, accompanying fresh water discharge in the Ems estuary Figure 3: Single component method for the evaluation of design water levels for sea dykes [3] EL = a i M + b i Q ci (1) EL : local set-up at inner estuarine gauge [m] M : set-up at estuarine mouth gauge [m] Q: fresh water discharge [m³/s] a i, b i, c i : multiple regression coefficients 2.2 Results This approach has been assumed as sufficient since the evaluation of design water levels is restricted on the peaks of the storm surges and requires not necessarily the consideration of the storm surge in total. Furthermore the tuning of the statistical model by a large number of storm surges is recommendable but also feasible without high efforts. The behavior of the statistical model improves the higher the surge level which is exemplary highlighted by a plot of its absolute errors for the estuarine tidal gauge Leerort (fig. 4): for storm surges with a set-up of 2,5 m and higher the error of the statistical model is less than 5 cm.
Figure 4: Error-diagramm for regression analysis of the estuary setup heights of storm surges due to the set-up at the estuarine mouth and the corresponding fresh water discharge (tidal gauge of Leerort (fig 1)) Figure 5: Comparison of design water levels for the Ems estuary evaluated by the statistical model [8] and the deterministic numerical model [1] Figure 6: Comparision of the set-up heights corresponding to design water levels for the Ems estuary evaluated by the statistical model on the basis of distinct time series Therefore the statistical model was used for the determination of design water levels for the dykes at the Lower Ems estuary. Later a deterministic numerical model with a very detailed model bathymetry was carried out for nearly the same boundary conditions. Its results proved the reliability of the results gained from the statistical model (fig. 5). The regression analysis was also applied in order to deliver indications on the effects of the subsequent waterway deepenings on the local storm surge setups by applying distinct time series. The comparison has been based on the time series of the years 1974 to 1978 before the deepenings. It is carried out by comparing the heights of the set-up corresponding to the design water level which are evaluated by the distinct coefficients obtained by regression analysis of the distinct time series. The analysis is based on rather small data sets. Therefore the results are only considered as an indication for the effect of the deepenings (fig. 6). Downstream of Leerort (fig. 1) there are significant changes; the deepenings have even created a small decrease of the maximum set-up heights. At the gauges Weener and Papenburg an increase of the maximum set-up is evident: the order of magnitude is about 0,15 m and 0,25 m for the time series of 1990 to 1994 and decrease for the following ones remaining then stable on a slightly lower level. Nevertheless the results make evident that the storm surge heights in this part of the estuary increase significantly corresponding to the waterway deepenings. The same effect is also detectable in the upper part of the estuary for the tidal gauges of Rhede and Herbrum (fig. 1), where no dredging has taken place. There the increase of the set-up heights is still continuing for the application of the subsequent time series (fig. 10). This result explains furthermore the non acceptable results of the regression analysis of the storm surge set-up heights at these gauges. The reestablishment of new hydro- and morphodynamical equilibrium has not yet been finished.
3 Design waves and wave run-up 3.1 Basics The safety standards of the State of Lower Saxony allow a design wave run-up with an overtopping rate of 3%. Its determination was carried out by application of a purposely adapted formula basing on large -scale hydraulic tests in the Dutch Delta Flume [6]: Non-oblique wave attack with an angle was considered by the reduction factor R which has been derived by van der Mer and de Waal [5] by test in a wave basin with short-crested waves: R = 1-0,0022 (3) The design wave parameters being necessary for the application of formula (2) were evaluated by applying the 3rd generation wave model SWAN [12;2]. As boundary condition for the inner part of the estuary a wind velocity of u = 22,5 m/s was estimated. Later model runs with a mathematical wind model made evident that this speed corresponds to a wind velocity of 30 m/s in the offshore area of the North Sea which is necessary to create a set-up corresponding to the design water level. The reduction is caused by the roughness of the land surface. The design wave parameters have been evaluated for five wind directions of the directional sector for which severe storm surges are to be expected. (2) The grid size for each model bathymetry was 40 x 40 m. Their areal extension was sufficiently with respect to local wind wave growth; wave import across the boundaries was negligible beside the area the mouth of the Lower Ems. There additionally an overall model with a grid size of 250 x 250 m was incorporated in order to deliver boundary conditions for the nested one (fig. 7) being used for the evaluation of design waves. Later this combined set-up was also used for a detailed evaluation of design wave run-up for stretches of the estuarine dykes. 3.2 Results Figure 7: Overall ana nested model for design wave evaluation at the mouth of the Lower Ems estuary The values of the wave parameters in the vicinity of the dykes were put in formula (2) leading to design wave run-up and the necessary design height of the estuarine dykes. In order to save time the wave modeling and the wave run-up evaluation were not carried out for the whole Lower Ems estuary but limited to a number of test areas classified due to experience as significantly wave exposed. Explanatory results of wave modeling are documented here for the northern part of the nested model bathymetry in the mouth of the Lower Ems estuary (fig. 8): significant spectral wave heights and mean wave directions, peak periods and computation points for the detailed wave run-up evaluation. These results make evident that applying of a suitable mathematical wave model like SWAN allows a differentiated evaluation of design wave run-up on dykes even for complex morphological ans structural boundary conditions.exemplary the evaluated design wave run-up and its effect on dyke design height after superimposition with design water level is highlighted here for
that stretch of the Lower Ems estuarine dyke at the northern shore at the mouth of the Lower Ems estuary at the northern shore (fig. 9) for which the exemplary design wave evaluation has been documented here (fig. 8). 4 Dyke design heights The design wave run-up being evaluated for the chosen test areas were superimposed with the computed design water levels. The results highlighted that in most of them the existing dyke heights are insufficient with respect to the design heights evaluated here (fig. 10). Major reason was neither the increase of storm surge water levels nor the wave run-up evaluation following the now available state of the art. The main differences were due to an insufficient transfer of the results of design water level evaluation by the single component method into the estuary which is urgently necessary both for safety and for fulfilling the legal boundary conditions. Figures 8: Wave model results for significant spectral heights and mean wave directions (above), peak periods (center) and computation points for the evaluation of design wave run-up (below) But large stretches of the dyke lines at the Lower Ems have nevertheless sufficient heights (fig. 10) though the now evaluated design water levels are significantly higher than those being the basis for their construction about 25 years ago. The then estimated wave run-up was remarkably higher than really possible, particularly on large parts of the western estuarine shore which do not experience significant wave attack for the wind directions from west to northwest being necessary for the occurrence of severe Figures 9: Comparison of existing dyke heights and computed design heights storm surges (fig. 10). Moreover upstream of Papenburg the fetches are very short and resultantly waves and wave run-up are also rather small (fig. 1 + 10). In order to fulfill both requirements the safety of the estuarine hinterland against storm surges and the needs of the dockyard the Lower Saxon State Government decided to build a storm surge barrier close to the mouth of the Lower Ems acting also for the rise of the water level for the downstream transfer of large vessels from the dockyard to the North Sea. The planning of the storm surge barrier included an environmental assessment study as necessary condition for the legal licensing procedure. The above described investigations became part of it being supplemented by detailed evaluations of the design heights for the dykes between the storm surge barrier and the Dollart bay on the southern and Emden on the northern shore (fig. 9); furthermore the design heights for the dykes belonging to the storm surge barrier itself were evaluated [10] using the same methods as described above.
5 Conclusions The evaluation of design water levels has to be in tune with strict formulations in the Dyke Law of the State of Lower Saxony. A suitable tool to fulfill that purpose has been successfully invented for the sea coast decades ago. The transfer of the design water levels into the upper part of estuaries was also successfully solved by development and application of a statistical model. The application of the third-generation mathematical wave model SWAN enabled the derivation of reliable design wave parameters performing a basis for the evaluation of design wave run-up and afterward determination of dyke design heights. These methods provided a basis for both a thorough review of the dyke design heights of the existing estuarine coastal protection master plan and for a new concept of coastal protection strategy combined with economical requirements. The afterward legal review including intensive judicial statements and hearings at administration courts on regional and superior level highlighted the necessity to Figures 10: Balance of existing dyke heights and evaluated design heights for the chosen test areas in the Lower Ems estuary and location of the storm surge barrier apply such methods for planning in coastal engineering which on the one hand represent the state of the art and are on the other in tune with legal boundary conditions. The expertise being described here, played an essential role in the statements made in the judicial hearings. After a thorough review by experts both the licencing authority and the opponents against the project based their argumentation on its results, though looking at them from distinct angles. Literature [1] BAW (1997): Storm Surge Barrier Ems Estuary at Gandersum - Analysis of Storm Surge High Peaks. Fed. Inst. f. Waterw. Engrg., Rep. 97 53 3449(unpubl. in German) [2] BOOIJ, N.; RIS, R.C. and Holthuijsen, L.H. (1999): A Third-Generation Wave Model for Coastal Regions, Part I, Model description and validation, J. Geoph. Research, 104, C4 [3] LÜDERS, K. & LEIS, G. (1964): Dyke Law for the State of Lower Saxony-Comment. Verl. Wasser u. Boden, Hamburg (in German) [4] VAN DER MEER, J. & JANSSEN, J.P.F.M. (1994): Wave Run-up and Overtopping at Dikes and Revetments. Delft Hydr. Pub., No. 485 [5] VAN DER MEER, J. & DE WAAL, J.P. (1990): Impact of Non-oblique Wave Attack and Directional Spread on Wave Run-up and Movements on Slopes. Waterloopk. Labor.-Versl. H 638 (in Dutch) [6] VAN DER MEER, J. & DE WAAL, J.P. (1993): Water Movements on Slopes. Waterloopk. Labor.- Versl. Modelonderz. H 1256 (in Dutch) [7] NIEMEYER, H.D (1987): Classification and Frequency of Storm Surges. Ann. Rep. 1986 Forsch-Stelle Küste, Vol. 38 (in German) [8] NIEMEYER, H.D. (1997): Evaluation of Design Dyke Heights at the Lower Ems Estuary. Rep. Forsch- Stelle Küste 5/97 (in German). Publ. in: Niemeyer, H.D. & Kaiser, R. (1999): Investigations on the Safety of the Dykes at the Lower Ems Estuary. Work. Rep. Forsch.-Stelle Küste, Vol. 13 (in German) [9] NIEMEYER, H.D. (1999): Changing of Mean Tidal Peaks and Range due to Estuarine Waterway Deepening. Proc. 26th Int. Conf. Coast. Engg. Copenhagen/Denmark, ASCE, New York [10] NIEMEYER, H.D. & KAISER, R. (1999): Investigations on the Safety of the Dykes at the Lower Ems Estuary. Rep. Forsch-Stelle Küste, Vol. 13 (in German) [11] RIS, R.C.,HOLTHUIJSEN, L.H. & BOOIJ, N. (1995): A Spectral Model for Water Waves in the Nearshore Zone. Proc. 24th Intern. Conf. o. Coast. Engg. Kobe/Japan, ASCE, New York