**Carlos Mestanza 1,2, Marco Piccardi <sup>3</sup> and Enzo Pranzini 3,\***


Received: 6 June 2018; Accepted: 27 June 2018; Published: 4 July 2018

**Abstract:** Analysis of written documents, projects and historical cartography of the 17th and 18th centuries allows reconstruction of the shore protection works performed at Callao (Peru) to defend the settlements and its boundary wall from storm waves and tsunamis. Groins appeared for the first time in early 18th century maps, together with gently sloping revetments in an unrealized project of the same period. Comparisons between Callao projects and those performed in Europe in the same centuries show a uniformity in design and construction materials that overpasses the environmental differences of the sites. Successes and failures followed each other and, although an understanding of coastal dynamics and the positive and negative effects of the various works were known, it was not possible to guarantee the stability of the walls and consequently the safety of the city from sea attack. A strategic retreat was even considered.

**Keywords:** historical cartography; shore protection structures history; groins; gently sloping revetment; earthquake; tsunami; strategic retreat

### **1. Introduction**

Coastal erosion, because of sea level rise, subsidence, reduced river sediment input and, more recently, anthropogenic interventions on the watershed, is a natural process that humans have tended to oppose ever since they settled near the shoreline [1]. At sites, the construction of ports, docks, piers, breakwaters along the coast itself induced or increased the process [2]. In some areas, hurricanes e.g., [3] and tsunamis e.g., [4] transformed this slowly-evolving process into a catastrophic one. Nevertheless, coastal settlements increased and expanded during the centuries, thanks to their advantageous position for climate, resources access, movements and—recently—recreation [5].

Where it was not possible to retreat, shore protection structures had to be built to hold the current position, either for the need to stay close to water (commercial, military, industrial purpose) or because safer land was unavailable; frequently, it was investment on reclamations done in that specific area that discouraged retreat.

Different materials were used in the past (rock, wood, fagots) in a variety of configurations, such as revetments, rip-raps, seawalls, detached breakwaters, and groins. Most ancient structures were subjected to continuous and expensive maintenance, and now are lost, either because land was abandoned to the sea (generally rural areas) or because they have been continuously replaced with stronger ones.

Elsewhere, military defences (e.g., boundary walls) also served as shore protection structures, such as seen in Roman and pre-Roman maritime cities. To reconstruct the evolution of these defence

structures, written documents, drawings, and old maps are the main sources of information. It is known that earth and stones were used for seawall construction in 713 AD at Quintang Estuary (China), further replaced with bamboo baskets filled with stones in 910 AD [6], and rock seawalls were built in China in the 14th century [7]. Due to the widespread coastal accretion that characterized Europe until the 18th century (generally as a consequence of deforestation), protection works of the coast were carried out in limited areas, mostly along the North Sea coast. Nieuwhof [8] referring to the excavations carried out by Bazelmans [9], and by Bazelmans et al. [10] describes the northwest Friesland dikes (no more than 70 cm high, composed of neatly-stacked peat sods against a core of loose bulk material) of Peins-Oost (1st century B.C.) and Dongjum-Heringa (2nd century A.D.).

Dykes have been present since the medieval period: in Germany since the 11th century to oppose storm surges [11], the 12th century in the Netherlands—frequently associated with land reclamation [12], the 14th century in Belgium [13], and the 16th century in Denmark [14]. These were frequently made by materials found in situ: sand plus clay and peat reinforced with wood or fagots, and these materials were used, in different combinations, when rocks became unavailable. Importing rocks and stones from Scandinavia was expensive and its use in coastal protection was parsimonious. Building with fagots (*fascinage*) is a long-lasting tradition in protection of river banks and coasts. As *fascinage* works are nowadays used in several undeveloped countries, one may infer that many centuries ago they were present all over the world's coasts, where erosion was threatening settlements or cultivated areas. More sophisticated, but still primitive structures include gabions made of twigs and branches, in the manner of basket-work filled with pebbles and laid one on top of the other when they had to reach a greater thickness (Figure 1).

**Figure 1.** Gabions construction scheme by Cresy [15].

Groins built with these "technologies" were located on the sea side of fortified cities in the 16th–18th centuries, as shown in several ancient maps, e.g., Flissingen (Figure 2) and Ostende (Figure 3). Their functioning was to prevent scouring (and therefore collapsing), affecting structures directly exposed to wave actions due to longshore transport, backwash and, to a lesser extent, direct cross-shore wave action [16]. On account of this, for any tower, fortress and walled town located on the beach, it was of outmost importance to prevent such a process, and this was frequently achieved by constructing groins, which intercept longshore transport. On the other hand, vertical breakwaters were built by the Romans just to prevent harbor entrance siltation; but in this case a stout rock foundation was laid at the base [17].

**Figure 2.** Groins at Flissingen (The Netherlands) in a 1649 map (ed. 1652) by Joan Blaeu [18]. Courtesy Universiteitsbibliotheek Utrecht.

Maps do not always allow identification of the design of individual groins and the material used, but in some cases, pictures can be extremely detailed and annotations give further information. For example, Figure 2 shows several shore protection structures made up of wood piles boxes, but it is impossible to see what lies inside. More explicative is the 1734 map of Ostende (Figure 3), where groins are better drawn and writing in Latin *Cistae ligneae lapidibus repletae* and in Frisian *Holtzerne Kuften mit steunen ange fillet* explains that they are formed by wood baskets filled with stones [19]. Siltation, in this case and in that of Flissingen, was not a problem for navigation and beaching, since access to the town was guaranteed through the river mouth or channels entrances protected by jetties.

**Figure 3.** Ostende in a 1734 map by Seutter [20] (detail on the right).

These vernacular structures, known from antiquity, are still present in many countries where local coastal populations struggle against the sea. Being recently adopted by "green engineering", the knowledge of their design and efficiency could help future projects.
