Historical trends analysis

Method indicator
Bottom-Up	Hybrid	Top-Down
		YES

Summary of key issues:

Issue	Description
Description	Data assessment from different periods in time, in order to identify directional trends and possibly rates of change of morphological features or physical processes within an estuary.
Temporal applicability	Medium-term depending on data availability.
Spatial applicability	Whole estuary or specific geomorphological features / geographical locations within an estuary, depending on data availability.
Links with other tools	Complements longer-term geological analysis approaches. Can provide useful data to inform regime analyses. Provides key input to establishing a conceptual understanding of the longer-term estuary behaviour during synthesis of results or Expert Geomorphological Assessment (EGA).
Data sources	Published papers, parliamentary records, land registry archives, anecdotal evidence, maps and charts, aerial photography, topographic and bathymetric surveys, remote sensing imagery.
Necessary software tools / skills	Identifying, collating and reviewing relevant data / information sources. GIS / image processing software / photogrammetry. Cartography / digital ground measurement. Geomorphological interpretation of output.
Typical analyses	Changes in shoreline position (e.g. MHW, MLW). Changes in channel/bank morphology or position. Changes in sediment volumes above specified datums. Identification of areas of ‘cut’ and ‘fill’ or erosion/recession, or deposition/progradation over time.
Limitations	Availability of historical data can be limited in some areas. Accuracy of some historical datasets can be questionable. Different measurement techniques, specifications, datums, units, density of data points in successive datasets. Identifies net change between successive datasets, but not the scale of variability over shorter timescales. Need information on anthropogenic intervention which is often not well documented. Many estuaries can exhibit long relaxation (lag) times before changes are apparent, making cause-consequence assessments difficult. Past trends are not always a good indicator of future behaviour.
Example applications	Humber Estuary Ribble Estuary Mersey Estuary Southampton Water

Introduction

Historical Tends Analysis (HTA) is a geomorphological tool which utilises the analysis of data relating to a particular physical process or morphological feature from different time periods, in order to identify directional trends and, if quantifiable, rates of changes in that process or feature. HTA focuses on temporal morphological variation within estuaries, and complements Holocene analysis, considering morphological change over historical rather than geological times, typically the last 1-200 years.

Whilst the approach can relate specifically to physical processes, such as long-term sea level trends, it more frequently relates to many different aspects of historical and ongoing estuarine morphological behaviour, such as erosion or progradation of intertidal saltmarshes, changes in the position or morphology of estuarine channels and banks, or changes in the location of spits across estuary mouths.

It is also important that historical changes are attributed, in so far as is possible, to likely causes of morphological change in terms of the historical and ongoing forcing or constraints imposed on the system. This could take the form of trends or changes in natural forcing (such as sea level rise, changes in rainfall, wind or wave patterns, changes in current speed and direction, natural changes in rates of sediment supply as stocks become extinguished, etc.) or perturbations to the estuary system caused by anthropogenic activities (such as estuary-scale responses to major engineering works like training walls, reclamation, dredging, water abstraction, flood defences, re-alignment, barrages, etc.). If the causes of historical and ongoing morphological change can be identified, then a good understanding can be developed of the cause-consequence or process-response relationships in the estuary, which can in turn provide a useful indicator of possible directions and rates of future morphological responses. If such causes cannot be identified, then the data can be misleading. In addition, responses to some changes can occur over long timescales.

Read the full PDF document for more information on this method PDF version

The Estuary Guide

Menu