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Management

Erosion Stability of Dike Foreshores

Helmut Manzenrieder, Oldenburg & Michael Schulze, Braunschweig, FRG

SUMMARY

The forelying area in front of the dike provides a direct contribution to the safety of dikes under storm surge loads depending on their width and height. The area is traditionally used as pasture. To fulfill the objectives for protection within the "Nationalpark - Niedersächsisches Wattenmeer" efforts are being made to allow the "Heller" to develop as naturally as possible without intervention and therefore to limit structured drainage as well as its use as pasture.

Between the years 1987 and 1994, an interdisciplinary research project was carried out on the East Frisian coast to examine the effect of pasture intensity and structured drainage on the stability of the "Heller". The examinations with different flow loads on the "Heller" surface consistently showed that pronounced erosion of the developed "Heller" by overflowing can be practically excluded, even under extreme flow conditions.

This statement is valid independent of its use as pasture or if there is structured drainage. The weak spot of the "Heller" is its edge, which is a step-like transition between the foreshore and the mudflats. The existing potential for destruction is found less in the seldom occurring, spectacular storm surges but more in the edge flows which occur often and are completely insignificant, as far as the safety of the dike is concerned. In conjunction with this, there are also slight changes of mean water levels which are of small importance to immediate dike safety but should be monitored carefully because of their effect on the stability of the foreshore.

INTRODUCTION

The foreshore of a dike is of great importance for the loading capacity and stability of the dike behind. Depending on the water level, a considerable part of the sea energy is converted through waves breaking over the surface of the foreshore in front of the dike line (Fig. 1). A dike foreshore is called "Heller" in East Frisia.

Fig. 1. The effect of a high and low foreshore on wave loads.

The utilization and management of the "Heller", which has been practiced for generations (Brahms 1774), has recently come under increasing discussion. In connection with the development of the "Nationalpark - Niedersächsisches Wattenmeer" in 1986, a new goal is to minimize or completely eliminate intervention in this environment.

Important questions about the safety of the dikes are the stability of the foreshore regarding the influence of extensive use as pasture and a change in drainage with changes to flora and fauna as well as the soil structure. To investigate these questions and work out objective evaluation criteria, a research project called -The Erosion Stability of "Hellers"- sponsored by the Federal Ministry of Research and Technology (BMFT) and the Committee for Research in Coastal Engineering (KFKI) was jointly executed from 1987 to 1994 by the following special government departments and universities:

• State Office for Island and Coast Protection, Norden;
• Lower Saxony State Office for Ecology, Coast Research Station, Norderney;
• Institute of Applied Botanics, University of Münster;
• Institute of Soil Science, University of Oldenburg;
• Leichtweiß-Institute of Hydraulic Engineering, TU Braunschweig.

The research project's assignment was to examine the stability of the "Heller", taking into consideration how intensive the foreshore is used for pasture and structured drainage and, from here, to develop objective, practical evaluation criteria or recommendations for future use or for managing it.

As a subproject within the scope of the research project, the Leichtweiß-Institute had the assignment of trying to simulate the load on the surface of the "Heller" as close as possible and to quantitatively examine the effects of the different types of utilization on the stability of the "Heller". Tailored to these questions, a sea water test canal was used as a central experiment facility and located directly on the East Frisian coast. Undisturbed large scale samples taken from the "Heller" areas were tested in this canal in long-time tests and the stability of the soil monoliths was studied under controlled flows corresponding to conditions during a storm surge.

The following three "Heller" areas with different soil structure, vegetation, use as pasture, etc. were selected within the "Nationalpark - Niedersächsisches Wattenmeer" (Fig. 2):

• "Leybucht" hedged test grounds
• "Buscherheller"
• "Neßmerheller"

Fig. 2. The position of the study areas and the sea water tank (test canal).

WSNL 1997-1

H. Manzenrieder & M. Schulze

SEA WATER TEST CANAL

The objective of the sea water test canal study was to simulate the collected values of the "Heller's" flow load in a model test under defined conditions. The test canal allows examinations of approx. 500 kg soil monoliths on a scale of 1:1. Using sea water, the grass-covered surface was approx. 2 m2. After creating the natural hydraulic, biological and soil mechanical conditions during continuous flooding, the resulting erosion stability of the selected soil samples, also under the influence of agricultural use, which can be isolated, was determined (see Fig. 3).

Measurements taken in the immediate vicinity of the edge of the "Heller" show that when the "Heller" edge is overflowed, especially when the water level rises, peak values in flow velocity of up to approx. 5 m/s can occur. The test conditions lie above the constant values measured on site or the load to be expected, especially in regard to the duration of the test.

Fig. 3. Sea water test canal with a view of the pump station and the storage basin.

WSNL 1997-1

H. Manzenrieder & M. Schulze

FLOW LOADS

The results at hand from different studies in the past, showed the great influence of biogenic components on flow behavior and stability of soils. Throughout, the examinations showed that an erosion of the "Heller" soils through energy-rich, stationary overflow, or overflow pulsed through variable degrees of obstruction, can be practically excluded. This also applies to corresponding examinations on soil samples with disturbed surfaces (so called "salt pans") or artificially weakened surfaces through deliberate reduction of growth, dystrophication and damage.

Roots, and biologically active, but also dead plant material that is enclosed in layers have a decisive erosion-retarding effect. The development of soil on the "Heller", i.e. the beginning of a granular structure in the upper soil, is connected with a biogenic stabilization of the sediments which in the initial phase are brought in layer-wise. This natural process leads to an increase in stability of the "Heller". Decreasing the intensity of use as pasture does not necessarily lead to a loss of stability in spite of lower soil compaction.

When comparing the differently managed locations on the testing grounds of the Leybucht, soil samples from the area with extensive use as pasture close to the dike showed the highest stability.

When there is no soil development, local disturbances caused by washout were observed, especially on the surface of samples from lower lying areas of the testing ground not used for pasture. The alternating deposits consisting of sediment (silt) and the remains of plants or the former "Heller" layers that are found in the soil at these locations showed less adhesion of the layers. Because of this, this soil reacted relatively more sensitively to the flow load. On the other hand, comparable areas with structured drainage showed higher erosion stability.

Mixed movements, induced by the above ground part of plants, lead to turbulence which in turn, leads to a reduction of the flow velocity over the "Heller" surface. An optimal shield is achieved by growth that is as high and elastic as possible but not to the point that plant density is reduced. These prerequisites are achieved on the Leybucht testing ground with 0.5 to 1 cattle per ha on pasture, but also on parcels of land on the "Neßmerheller" not used for pasture. The lower growth on the areas used for pasture in a normal intensity have a lesser shielding effect and, in conjunction, there is a higher flow load on the soil surface.

In conclusion, it should be pointed out again that here none of the samples showed substantial, sustained disintegration of the soil structure.

WSNL 1997-1

H. Manzenrieder & M. Schulze

WAVE LOADS ON THE EDGES OF THE "HELLER"

The transition from mudflat to "Heller" does not run continuously over wide sections of the East and North Frisian coast as it does, for example, in the Leybucht or Dollart but has rather a pronounced eroding bank.

As measurements in the natural environment quantitatively demonstrated, the load on these edges is clearly less during singular, high or extreme storm surges than during regular or slightly higher water levels which occur often. The load on these sensitive foreshore flanks and the break-up in connection with it, is caused by the effect of waves during water levels and swell effects that are completely meaningless, as far as dike safety is concerned. The load that occurs in edge flows is the result of increased flow caused by breaking waves and their immediate, short term pressure peak effects that are also designated as so called pressure surges.

The studies on the load of "Heller" edges were carried out in a wave canal at the Leichtweiß-Institute. The technically complicated tests for measuring were carried out both on an artificial edge made of concrete and on soil monoliths with natural edges.

The studies in the wave canal have shown that the "Heller's" edge is substantially endangered when there is a wave load. The wave load acts as a point or linear load on the "Heller" edge and the local geometric structure has a great influence on the resulting pressure and dynamic effect.

The heaviest loads from pressure surge and surge flow occurred in the examinations when incoming waves broke immediately in front of the edge and the crest of the breaker hit the edge directly (load type B).

The biogenic composite effect promotes the disintegration behavior of the soil sample in phases. Fig. 4 shows such a pressure surge load from swells on a "Heller" edge in the form of a schematic presentation.

All other types of loads, i.e. with the breaking point in front of the edge on the forelying mudflat (load type A) or on the foreshore surface (load type C) show a considerably lower pressure surge load and, in connection with this, a higher structure stability (Tab. 1).

Pressure peaks of up to approx. 1.8 bar were measured as individual values for B type loads, this corresponds to 70 times the value of initial wave height.

In connection with the strong, upward directed, transporting flow components, the disintegrating pressure surges acting in these hollow spaces caused the soil to be broken open towards the top where it was carried away together with the grass cover. After the grass cover was permanently damaged, nearly the complete height of the sample was washed out. As a result, it can be stated that the destruction of such "Heller" edges is not continuous but rather runs in individual phases depending on the internal structure of the grain fiber work and the alternating layers. The more cohesive soil of the "Buscherheller" showed a comparably high stability in the wave tests.

Tab. 1. Mean values of all pressure surge peaks as well as surge velocities on an artificial "Heller" edge under typical small wind waves (heights approx. 25 cm).

Load type

A

B

C

Pressure surge p max

-

0.4 bar

0.2 bar

Surge velocity v max

2.5 m/s

3.3 m/s

3.0 m/s

WSNL 1997-1

H. Manzenrieder & M. Schulze

REFERENCES

Brahms, A., 1774: Anfangs-Gründe der Deich- und Wasserbaukunst, Teil 1, p. 200ff.

Coldewey, H.-G.; Erchinger, H. F., 1992: Deichvorland: Seine Entwicklung zwischen Ems und Jade und die Forschungsvorhaben "Erosionsfestigkeit von Hellern", Die Küste, Heft 54, pp. 167-187.

Führböter, A., et al., 1976: Äußere Belastung von Seedeichen. In: Seedeichbau, Theorie und Praxis, Vereinigung von Naßbaggerunternehmen e. V., Hamburg, pp. 5-79.

Schulze, M.; Manzenrieder, H., 1994: Erosionsfestigkeit von Deichvorländern, Untersuchungen im Seewasserversuchskanal Neßmersiel. Mitteilung. des Leichtweiß-Inst. der TU Braunschweig, Heft 132, pp. 141-171.

Authors addresses:

H. M.
Ingenieurbüro Dr. Manzenrieder & Partner
Brookweg 29
D - 26127 Oldenburg

M. S.
Ing.gesellschaft Dr.-Ing. E. Macke mbH
Am Hafen 22
D - 38112 Braunschweig