A conceptual model of the formation of the meso-scale geomorphology of lowland forested floodplains Catherine E. Millington and Dr.David A. Sear
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Catchment scale location of floodplain channels
Background Within forested floodplains geomorphological processes are strongly modified by live and dead vegetation.
Large woody debris (LWD) spanning the main channel forming a debris dam. Water is ponded up steam and flows overbank
Observations made along semi-natural rivers in the New Forest, UK suggest the presence of particular suites of landforms on the floodplain that owe their formation to interactions between vegetation, water and sediment. For example extensive networks of floodplain channels (Jeffries et al., 2002).
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Reach scale location of floodplain channels
A walk through survey was carried out on two semi-natural rivers in the New Forest, the Highland Water and the Ober Water. The rivers were divided into discrete reaches based on their geomorphological character. The following observations were recorded:
Floodplain features, including floodplain channels, debris dams, sand shadows and floodplain scour hollows. Of these features floodplain channels appeared to be the most important so they are focussed upon
Channel dimensions (width and depth)
Evidence of channelisation
Number of hydraulically effective debris dams (debris dams which pond flow)
Type of land cover (forest type or lawn)
The adjacent figure shows a map of a reach of floodplain on the Highland Water that is intensely dissected by floodplain channels of varying entrenchment into the floodplain surface. The main cause of the channels appears to be scour resulting from the presence of a large, hydraulically effective debris dam in the main channel. The debris dam causes ponding of flow upstream which increases the frequency and magnitude of flow onto the surrounding floodplain. Once on the floodplain, water is concentrated between trees and debris accumulations. Where this concentrated flow exceeds the critical threshold for erosion of cohesive floodplain soil, scour initiates channel formation. At the point of re-entry into the channel, water surface slope increases and headcutting can be initiated. Note however that this process only extends a short distance upchannel and is not considered to be the primary mechanism for channel formation. Where the overbank flow contains sediment derived from erosion of the floodplain surface or as wash load from upstream, deposition occurs in the wake of trees where flow velocities are reduced, forming sand shadows.
Results of the survey demonstrated the presence of floodplain channels throughout the main order trunk stream. There were no systematic change in floodplain channel dimensions with increasing catchment area. Floodplain channels Floodplain channels present absent
Highland Water
Over time (periods of 5-10 years) debris accumulations on the floodplain block floodplain channels and force them to change their course. The lifespan and degree of development of floodplain channels is thought to be determined by the residence time of the main channel debris dam and the dynamics of large woody debris on the floodplain.
Evidence of channelisation
The presence of floodplain channels on the Highland Water is dependant on overbank flow brought about by a connection between the channel and the floodplain. High channel sinuosity and hydraulically effective debris dams also promote floodplain channel development. In reaches which are deeply incised due to channelisation flow does not reach the floodplain, hence floodplain channels are absent (Table 1).
no
yes
1.3
1.1
Average sinuosity Average number of hydraulically effective debris dams per 100m 0.4
0.2
5.5
6.5
Channel capacity (m2) Bank height (m) 0.96 Table 1: Summary statistics for the Highland Water
1.35
A network of floodplain channels on the floodplain upstream of a debris dam
Key These features are only found in semi-natural reaches which have a high sinuosity, high frequency of debris dams, longer sediment and organic matter residence times, and therefore experience relatively high frequencies of overbank flows during the flood season. The formation of these floodplain channels is poorly understood. This research explores catchment and reach-scale controls on these meso-scale features and proposes a conceptual model for the processes affecting their formation. The conceptual model is then applied in order to help set monitoring targets for a habitat restoration programme aimed at restoring wet woodlands along degraded lowland watercourses
Highland Water
Reach-scale distribution of floodplain channels
Floodplain channels present
Flow accelerated between tree stumps
Unsurveyed river network
A semi-natural reach with high channel floodplain connectivity
Catchment boundary
Conceptual model of the development of floodplain channels
Enclosures with conifer plantations
This conceptual model has been developed from an understanding of the processes operating on the floodplain during overbank flows gained from both the literature and from field observations. North
As part of on-going research, the following key processes are being further studied through detailed field experiments to determine:
Rainfall intensity & duration
1000 m
Restoration The LIFE-3 river restoration programme aims to restore conditions favourable to the development of floodplain forest. Past channelisation for commercial timber production has destroyed over 600 ha of former floodplain forest. The restoration has re-occupied the former meandering course of the river and re-introduced large woody debris dams. A key physical indicator of the restoration of physical processes is the formation of floodplain channel networks associated with increased overbank flood frequency.
Ober Water
In-channel debris dams
Ober Water
1. Overbank flow
On the Ober Water, reaches with and without floodplain channels do not have significantly different channel dimensions. The development of floodplain channels is mainly controlled by the erodibility of floodplain soils: floodplain channels are only present in forested reaches. In areas of thick lawn they are absent.
2. Flow concentration
Absence of floodplain channels possibly due to lawn protecting the floodplain from erosion.
Intensely dissected floodplain under closed-canopy forest, possibly due to highly erodible soils and dendritic root network.
Effect of in-channel debris dams on the frequency of overbank flows.
Effects of in-channel debris dams on the amount and distribution of floodplain sediment deposition.
Formation of floodplain channels in relation to floodplain erodibility and overbank flow erosivity.
Causative mechanisms for floodplain channel anastomosis (e.g. relationship between tree/debris and channel location.
LWD Floodplain debris dams Flow direction
Trees
Soil composition
Restoration of former channel
Main channel
Flow ponding & deposition
Floodplain erodibility
Incised channel before restoration
Flow diverted around debris
Floodplain channels absent
Floodplain topography
Floodplain vegetation
Water erosivity
3. Scour Vegetation & root density
Head-cutting at re-entry
4. Floodplain channels
