Standard flood frequency analysis

The most used method of flood prediction is based on the assumption of the random occurrence of floods of a given magnitude. This means that the longer the time period the larger would be the expected flood, i.e. if camping by a river for a month you would be unlikely and unlucky to experience a massive flood; however, living by the river for tens of years you should expect to experience large floods. The average time which elapses between two events of equal magnitude, or exceeding a common level, is the return period or recurrence interval (RI). So the flood with a 100-year return interval would be much larger than that with a 2-year return interval. This does not, however, mean that the 100-year flood cannot occur tomorrow or twice in a year, what it means is that on average it can be expected to occur only once in a 100-year period. It follows from this that the probability analysis of floods is rather more useful for describing past hydrological trends and the design criteria of structures, embankments and bridges (i.e. built to withstand the once in 500 years flood) than it is for flood warning. The calculation is fundamentally simple; the floods over a particular threshold are ranked from largest to smallest and assigned return periods with the largest flood having the return period of the time over which the floods were recorded (i.e. for a 100 years of flood the largest flood has a return period of 100 years).

Floodplain evolution reflects the prevailing environmental conditions and changes in those conditions. This chapter illustrates how the sediments and soils which form floodplain stratigraphies can be used to reconstruct past environmental conditions.

River morphology and sedimentation

Our ability to infer past conditions from floodplain sediments comes from the analysis of contemporary rivers and their sedimentation, i.e. the present is the key to the past. It is necessary therefore to understand the general morphology (pattern and shape) of rivers and processes that operate in them. Rivers in all climatic zones vary from headwaters to outlet owing to the changing volumes of water and sediment supplied to them and the local geology and slope over which they run. In small mountainous headwaters bedrock and coarse bed material dominate the channel, resulting in steps, pools, cascades and waterfalls (Church, 1992). Alternatively in low-relief areas headwaters may be almost imperceptible wet patches (flushes), springs or field drains with cohesive peaty or clay-rich beds and banks. In larger mountain and piedmont zone rivers channel bar accumulations are important stores for bed material (Church and Jones, 1982). In these channels the pool and riffle is the major morphological feature (Figure 3.1). It is believed that the pool is created by scour during floods but under normal and low flows the velocity of water flow is faster over the shallow riffles and water is slower in the pools (Keller, 1971 and later discussion).

The almost imperceptible shift from the opportunistic utilisation of resources to the management and control of those resources is composed of many human innovations. Alluvial environments can be seen as one of the key environments for innovations including: irrigation, river, road and bridge engineering, and drainage. Of these, irrigation is generally regarded as being both the earliest and the most important in socioeconomic and political terms. While this is not the place to cover the history of irrigation, drainage or river training (see Biswas, 1970; Purseglove, 1989; Sheail, 1988), the relationship between floodplain exploitation and management is highly relevant to alluvial sites, and so will be considered here.

Flood farming and irrigation

The origins of irrigation are generally associated with the civilisations of the great alluvial valleys of Mesopotamia (Tigris and Euphrates), Egypt (Nile), India (Indus) and China (Hwang-Ho). These societies, which are associated with distinctively hierarchical modes of production, transformed their physical environments by the control of water. In the Euphrates valley, the control of water levels – high enough for seed germination but low enough to avoid damage – has its origins in floodwater farming during the Neolithic. A site at the edge of the floodplain at Chogi Mami, dated to 7500 bp, provides probably the earliest evidence of irrigation canals (Roberts, 1989). Later occupation of the floodplain (6500 bp) took place on levees and allowed the abstraction of water through the levees down onto the flood basins, the edges of which could be used for crops.

Floodplain environments in North-West Europe changed profoundly at the end of the last glaciation and have continued to change, although less dramatically, during the Holocene. Climate change affected the hydrology, sedimentology and vegetation of floodplains. As the rivers and vegetation changed so did floodplain resources. From at the latest the Neolithic (c. 6000 bp) vegetation was also influenced by human activity. Thus floodplain history is the result of spatially variable interactions between people and natural factors such as climate change, ecological dynamics and soil development. This chapter uses selected examples of northern and central European rivers to outline changing floodplain environments from the Lateglacial to the Mid-Holocene. These environments include Lateglacial floodplains, the Early Holocene floodplain environments of Mesolithic gatherers-fishers-hunters (hereafter referred to as g-f-hs), and the land use changes of floodplains into the Neolithic and Bronze Ages. It would be impossible to cover comprehensively the whole of North-West Europe, or to provide a uniform temporal coverage, if only for reasons imposed by the pattern of Holocene floodplain evolution itself and because of the patchy distribution of research. Therefore, the approach has been to select some of the most informative studies, largely from the British Isles, which have a direct bearing on the archaeology of floodplains and their sites.

Floodplains in the Temperate and Boreal zones have changed dramatically since the glacial maximum c. 18,000 years ago.

The Andean world was populated by gods and ancestors. Throughout the colonial period, Spanish priests learned of the vast array of shrines and altars, the many feast days, and the numerous acts of devotion, small and large, with which Andean peoples honored the deities and the dead. Catholicism responded with the rough hand of righteousness, directed by guidebooks for persecution, like Pablo Joseph de Arriaga's (1968 [1621]) The Extirpation of Idolatry in Peru or Cristobal de Albornoz' 1555 Instructions for the Discovery of All the Guacas of Peru … (Duviols 1967). In spite of that effort, elements of native Andean religion persisted. One reason was the omnipresence of the sacred; as Sabine MacCormack (1991: 146) notes, “everywhere in the Andes, the plains and the mountains, the sky and the waters were both the theatre and the dramatis personae of divine action.” The indivisible interweaving of the natural and spiritual worlds is expressed in Father Arriaga's (1968 [1621]: 115) simple, elegant observation that “Some of the huacas are hills and high places which time cannot consume.”

And yet the archaeological perspective on the Andean past has taken an uneven view of ritual. On the one hand, there is a body of archaeological literature on Inca temples, shrines, and ceques which draws heavily on ethnohistoric literature, identifying specific places and ceremonial functions described in the colonial chronicles (e.g., Niles 1987; Rowe 1979; Urton 1981, 1990; Zuidema 1964, 1990a).

Within twenty years of Pizarro's landfall on the west coast of South America, the word “huaca” was applied to native religious monuments by Spanish speakers throughout the Americas. Derived from the Quechua waka the first written use of the word was in Juan de Betanzos' Suma y narración de los Incas completed in 1551, and ‘huaca’ eventually was incorporated into the dialects of Spanish spoken in Central America and the Caribbean (Corominas 1974: 800). Betanzos (1987 [1551]) used waka to refer to a sacred place or temple or a priestly residence, but by the late sixteenth and early seventeenth centuries the word's meaning had loosened considerably. With the onslaught of extirpation, the definitional boundaries widened to include any burial place, any shrine, any object or construction that materially marked the indigenous concepts of sacred. As the word spread, its definition stretched. The laxity of categories is paralleled in archaeological approaches to ancient Andean monuments. The word huaca has been attached to enormous pyramids (Huaca del Sol at Moche), tell-like accumulations of midden (Huaca Prieta in the Chicama Valley), and burial crypts (Huaca las Avispas at Chan Chan). Only two concepts unify such different structures – they are artificial and they are large – and a moment's reflection suggests such a classification obscures more than it illuminates.

Yet how can one begin to classify and to understand the differing social uses and meanings that prehistoric monuments had for Andean people? First, there is the issue of defining what a monument is.

The following summarizes archaeological data from twenty-two Andean sites spanning the period of approximately 5900 BC to AD 1470. These sites are located in central and northern Peru, principally in the central highlands and Pacific coast (Figure 2.1). The sites range from relatively small structures to truly monumental constructions; some are located in the midst of residential zones and yet others lack evidence of significant habitation. Some of these sites are unique, while others fit comfortably into well-documented architectural traditions. For all their variation, these sites may not truly represent the range of prehispanic Andean constructions; the current data rarely are sufficient to reconstruct well-defined settlement patterns or to evaluate these sites' positions in regional settlement systems. Simply, our knowledge of Andean architecture is outweighed by our ignorance.

Thus, the sample is not ideal, but there are good reasons for selecting these particular sites. First, the focus was somewhat arbitrarily limited to central and northern Peru – an area familiar to me – and therefore ceremonial centers located elsewhere in the Andes, such as the Archaic site of Asana (Aldenderfer 1990, 1991), were excluded from the sample. Further, I selected sites for which detailed plans based on excavated data were available, and which had maps showing the relationship of a particular structure to the larger settlement.

“Space is never empty,” the geographer Edward Relph writes (1976: 10), “but has content and substance that derive both from human intention and imagination and from the character of the space.” Relph describes the different kinds of space humans experience and create: the unselfconscious space experienced by an infant moving without reflection, the space shaped by human perception, and space which is cognitive and abstract. Like other elements in the cultural landscape, architecture often contains explicit material statements about human intention and imagination, expressing human desires to literally shape the world. When such material patternings become widespread and shared, we are seeing the physical remains of what Relph calls “existential space”:

The Panopticon may be one of the true oddities of architectural history, but it was more than a mere curio. The model prison was designed by Jeremy Bentham (1748–1832), the British Utilitarian philosopher who unsuccessfully promoted his plan for twenty years, spending a small fortune in the process (Evans 1982:195–197). Bentham was primarily concerned with the nature of government and the justice of punishment, and, for that reason, a pioneer of prison reform, though a somewhat quirky one. The hidden labyrinths of eighteenth-century prisons led to unobserved abuses, and Bentham's solution was the Panopticon (Figure 5.1) – a circular, glassroofed structure, with cells along the outer ring all facing onto a central rotunda where a single guard could keep every prisoner under constant surveillance (Johnston 1973: 19–21). Bentham lavished a creator's prose on the project: “The building circular – A cage, glazed … The prisoners in their cells, occupying the circumference, the inspectors concealed … from the observation of the prisoners: hence the sentiment of a sort of omnipresence … One station in the inspection part affording the most perfect view of every cell” (cited in Evans 1982: 195). Bentham's Panopticon was an architectural plan designed explicitly as a means of social control “to induce in the inmate a state of conscious and permanent visibility that assures the automatic functioning of power” (Foucault 1977: 201).