Index of vulnerability of littorals to oil pollution
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Translated and adapted from Erwann Lagabrielle master thesis, Université de Nantes, France
By describing the physical and biological impacts of oil pollution on littorals it is possible to introduce an index of vulnerability of littorals to oil pollution. By considering the consequences of oil pollution on both human activities and human life it is possible to create a new index of vulnerability that takes into account both the social and economic impacts.
Contents
Terminology
The ability of a coast to trap oil from a slick and its remanence are the basis for the concept of morpho-sedimentary vulnerability. Along with the ecological vulnerability of the environment, they define the bio-morpho-sedimentary vulnerability of a littoral; that is the capacity of a coastal ecosystem to totally overcome the effects of oil pollution.
History of the indexes of vulnerability
The Environmental Sensitivity Index (ESI)
With regards to accidental oil slicks, the term sensitivity of littorals was first introduced by Hayes (USA) in 1976. The American geomorphologist defined the first index of morpho-sedimentary vulnerability, which was tested on the Lower Cook Inlet coast in Alaska.
The categorization rests on the assumption that the longer oil remains on a coast (remanence phenomenon), the more sensitive it is to oil pollution. The index takes into account the duration of time oil remains on the coast before the first cleansing operation. The duration is linked to the morpho-sedimentary attributes of the coast and its exposure to hydrodynamic phenomenon (swell, waves, flows...). Figure 1 below shows that the ESI goes from 1 to 10, with 10 indicating a maximum vulnerability.
| ESI Values | Coast type |
|---|---|
| 1 | Exposed rock coast |
| 2 | Exposed rock platform |
| 3 | Fine sand beach |
| 4 | Coarse sand beach |
| 5 | Compact and exposed flat beach |
| 6 | Sand and gravel / pebbles beach |
| 7 | Gravel and pebbles beach |
| 8 | Sheltered rock coast |
| 9 | Sheltered flat beach |
| 10 | Marshlands and mangrove swamps |
In 1980, Gundlach and Hayes added biological considerations to the index and defined the first bio-morpho-sedimentary index. The new categorisation was used on 1/40000 maps where the main biological resources to be protected were indicated. The index became the new Environmental Sensitivity Index (ESI) see figure 2 below.
| ESI Values | Coast type |
|---|---|
| 1 | Exposed rock course |
| 2 | Exposed rock platform |
| 3 | Fine sand beach |
| 4 | Coarse sand beach |
| 5 | Flat beach with very stirred up sediments |
| 5a | Sand and gravel exposed beach |
| 6 | Pebbles beach |
| 6a | Sheltered beach of sand and rollers |
| 7 | Exposed flat beach(average biomass) |
| 8 | Sheltered rock coast |
| 9 | Sheltered flat beach(high biomass) |
| 10 | Marshlands |
Since then, the index has been redefined and completed. Some elements have been resorted; others have been added (especially maritime constructions). Finally, the index describes 10 elements corresponding to 15 subclasses, see Figure 3 below.
| ESI Values | Coast type |
|---|---|
| 1 | Exposed rock cliff ; exposed vertical seawall, made of wood, metal or concrete |
| 2 | Exposed rock platform or coast ; exposed unstable escarpment behind a rock platform |
| 3 | Fine-to-average sand beach ; unstable eroding escarpment |
| 4 | Coarse sand beach or heterogeneous sand beach |
| 5 | Very coarse sand beach |
| 6A | Gravel or pebbles beach |
| 6B | Epi |
| 7 | Tidal flat exposed beach |
| 8A | Sheltered rock coast or sheltered unstable escarpment |
| 8B | Sheltered solid infrastructure |
| 9 | Sheltered flat beach |
| 10A | Sea marshlands |
| 10B | Mangrove swamps |
| 10C | Fresh water marsh (herbaceous vegetation) |
| 10D | Fresh water marsh (woody vegetation) |
Berne and d'Ozouville's index
After the Amoco Cadiz accident, Conan and d'Ozouville (1978), then Berne and d'Ozouville (1981) used the index of vulnerability and adapted it to the North Coast of Brittany in France. Figure 4 shows how the different types of coasts are sorted according to the extent of the auto-cleansing phenomenon (low or high energy). A new map was drawn from the Saint-Mathieu cap to the Sillon de Talbert where the newly computed index has only been used once in 1982.
| Index | Coast type | Zone of oil accumulation | Remanency | |
|---|---|---|---|---|
| High energy zones | 1 | Rock headland or coast | Oil can't settle because of wave reflection | Some weeks |
| 2 | Eroding rock platform | Higher part of coast | Some months | |
| 3 | Fine sand beach | Possible interstratification in sediment, slow migration deep down, emulsion in interstitial water | 1 to 2 years | |
| 4 | Average to coarse sand beach | Possible interstratification in sediment, fast migration deep down, emulsion in interstitial water | 1 to 3 years | |
| 5 | Gravel and pebbles beach | Fast migration deep down, few or no deposit on the surface | 3 to 5 years | |
| Low energy zones | 6 | Rock coast and eroding platform | In rock anfractuosities, rocks covered by a thin slick | 3 to 5 years |
| 7 | Thin to average sand beach | Deep percolation, pollution of subtidal zone by ocean tides (mix thin sable/oil), hardened crust on the surface after one year | >5 years | |
| 8 | Coarse sand and pebbles beach | Fast percolation to substratum, creation of asphaltic pavement after one year | >5 years | |
| 9 | Silt | Percolation deep down due to digger organisms and interstitial water movements | >10 years | |
| 10 | Marshlands | Crust on the surface, sedimentation | >10 years |
The Impact Reference System of O'Sullivan and Jacques
This tool of classification has been developed by O'Sullivan and Jacques to take into account the effects of oil on the environment (fauna and flora). The ecological consequences of an oil slick are summarised according to:
- the fauna and flora associated with the environment,
- the impacts of oil depending on its nature (toxicity),
- the geomorphological vulnerability (remanence),
- the population sensitivity (resiliency).
The coast types have been sorted by increasing sensitivity. Data on the effects of oil pollution on a number of sea resources (pelagic stock, aquaculture, coastal agriculture) and animals (sea mammals, birds, fishes, crustaceans) have also be shown. To refine their classification, O'Sullivan and Jacques first developed a categorization of oils according to a number of different factors; these are indicated in figure 5: toxicity, level of evaporation, solubility, natural dissemination and stickiness.
| Type of petrol | Level of evaporation | Solubility | Stickiness | Toxicity |
|---|---|---|---|---|
| Light oil | High | High | Null | High |
| Average to heavy oil | <50% | Average | Low to average | Variable, depending on composition |
| Heavy oil | <20% | Low | High | Cover and adherence (fauna and flora asphyxiation) |
| Residual oil | Null | Very low | Very high | Cover and adherence (fauna and flora asphyxiation) |
Page-Jones' bio-morpho-sedimentary index of vulnerability (1996)
This index summarizes and confirms the previous indexes and adds an ecological parameter to the morpho-sedimentary vulnerability. Developed by Lindsey Page-Jones in 1996 during her master of geography in Brest, France (Tentative d’estimation de la rémanence du pétrole sur les littoraux à la suite d’une pollution accidentelle et contribution à la mise au point d’un indice de vulnérabilité bio-morpho-sédimentaire, with Bernard Fichaut). Rebout and David used it to create the Polmar-Terre map for the French department of Manche.
A validation of the previous indexes
Lindsey Page-Jones carried out a substantial amount of research into historic oil slicks since 1950, where the remanency of oil in the sediment was compared to the theoretical remanency that was predicted by the morpho-sedimentary indexes (Fig. 6). This therefore demonstrated the validity of the indexes.
| Coasts | ESI remanency | Berne and d'Ozouville | Observed remanency from Page-Jones work | ||
|---|---|---|---|---|---|
| Rock coast and exposed headlands | 1 | Some weeks | 1 | Some weeks | Some weeks to some months |
| Exposed rock eroding platform | 2 | Some months | 2 | Some months | Some weeks to some months (up to one year) |
| Exposed thin sand beach | 3 | Some months up to one year | 3 | 1 to 2 years | 1 to 2 years (more if creation of crust or burying |
| Exposed average to coarse sand beach | . | . | 4 | 1 to 3 years | 1 to 2 years (more if creation of crust or burying |
| Coarse sand beach | 4 | 1 to 2 years | . | . | . |
| Exposed gravel/sand or pebbles beach | 5 | 1 to 2 years (more for sheltered zones) | . | . | 2 years (more if no transformation of buried polluted sediment) |
| Exposed gravel and pebbles beach | 6 | 1 to 2 years | 5 | 3 to 5 years | 1 to 5 years (depending on transformation of sediment) |
| Exposed tidal flat | 7 | Some months to one year | . | . | Some months to one year |
| Rock coast and sheltered eroding platform | 8 | >5 years | 6 | 3 to 5 years | 2 to 5 years (more if sheltered and anfractuosities) |
| Sheltered thin to average sand beach | . | . | 7 | >5 years | >5 years (>10 years if creation of crust or burying) |
| Sheltered coarse sand and stones beach | . | . | 8 | >5 years | >10 years or more |
| Sheltered tidal flat or silt | 9 | >5 years | 9 | >10 years | 2 to 5 years (>10 years if burying) |
| Marshlands | 10 | 5 to 10 years or more | 10 | >10 years | 15 to 20 years (or more) |
The V index : methodology of development
The index, developed by Page-Jones, is the only one which really links morpho-sedimentary vulnerability and ecological vulnerability. The previous indexes (ESI and Berne and d'Ozouville's) do not take into account the ecological vulnerability to its full extent: the impacts of an oil slick on ecosystems changed only a little the classification, which mainly relied on the morpho-sedimentary vulnerability.
Parameters
The V index takes into account both morpho-sedimentary and ecological vulnerabilities :
- the morpho-sedimentary vulnerability relies on the capacity to trap oil and the remanency of petrol;
- the ecological vulnerability relies on resiliency and the impact of pollution on the environment. An ecological value is defined which takes into account the prosperity and diversity of endogenous fauna and flora. (Exogenous fauna is not built in because of its mobility.)
Classification of parameters
Ecological vulnerability and morpho-sedimentary vulnerability have been chosen to have the same importance in the final value.
Remanency takes precedent on the capacity of oil to be trapped, because intervention policies have to concentrate on the beaches where oil is bound to stay the longest (where the remanency is higher).
Calculation
Three parameters are considered in the following calculus :
- R : remanency
- K : capacity to trap oil
- E : ecological sensibility (including impact and resiliency)
Each of those parameters can take three values :
- 1 : low value
- 2 : average value
- 3 : high value.
Although this choice may seem quite simplistic, it proves to be sufficient and consistent.
Furthermore, each parameter is balanced by a coefficient x, y or z (respectively). [math] V = f(R,K,E) = xR + yK + zE + Q [/math] where Q is a constante.
The last paragraph on the classification of parameters make us add some constraints :
- ecological vulnerability and morpho-sedimentary vulnerability have to have the same weight
[math] x + y = z [/math]
- remanency is twice more important to the capacity to trap oil
[math] x = 2y [/math]
Moreover, we choose the index to take values between 1 and 13. (The number of classes should be limited.) Hence we deduce the following system : [math]\left\{ \begin{matrix} x + y & = & z \\ x & = & 2y \\ x + y + z + Q & = & 1 \\ 3x + 3y + 3z + Q & = & 13 \\ \end{matrix} \right. [/math]
Hence, [math]\left\{ \begin{matrix} x & = & 2 \\ y & = & 1 \\ z & = & 3 \\ Q & = & -5 \\ \end{matrix} \right. [/math]
And [math] V = f(R,K,E) = 2R + K + 3E - 5 [/math]
[math]V[/math] is the bio-morpho-sedimentary vulnerability index of a beach part (characterized by an oil trapping capacity, a remanency of petrol, and an ecological sensitivity) related to oil pollutions.
This 13-values index may not be considered as an extensive parameter : a beach where V=5 is not twice less sensitive than a beach where V=10. It only allows a relative sorting. The main advantage of this index is that it does not impose a fixed sorting : each user can adapt it to its own needs.
Example : sand and gravel exposed beach
It has an average remanency and a coefficient of 2 is chosen (3 to 5 years according to Berne and d'Ozouville). However, its capacity to trap oil is high and it is chosen to give a value of 3 to this coefficient. The ecological vulnerability is low, because of its exposition and its low biological wealth. This coefficient will thus have a value of 1. The bio-morpho-sedimentary vulnerability index is :
[math] V = f(R,K,E) = 2R + K + 3E - 5 = 2*2 + 3 + 3*1 - 5 = 5 [/math]
This value (5) puts the bio-morpho-sedimentary vulnerability of a sand and gravel exposed beach in the middle of the sorting.
Values of R, K, E according to the type of coast
As soon as the question of the application is adressed, the problem of the choice of R, K and E values according to the type of beach also has to be solved. This may be done thanks to the empiric studies presented in the first paragraph :
- remanency R decreases with the energy level : its value is 1 for an exposed sector, and 3 for a sheltered beach ;
- oil-trapping capacity K is linked to the geology of the substratum. It increases with the granulometry of the unconsolidated substratum, from 1 for a rocky sector to 3 for coarse sediment. For sheltered marshlands, we choose a value of k=3 because they have a high capacity to trap oil. Nevertheless, this oil-trapping capacity decreases when the water content raises ;
- ecological vulnerability is more difficult to evaluate because of the lack of knowledge on this topic. We consider it as minimal for exposed beach (E=1), maximal for marshlands and sheltered tidal flats (E=3).
The choice of the values of these parameters depends on local conditions.
