The development of ecologically relevant fine sediment targets for chalk streams

Abstract

Fine sediment plays an important role in freshwater ecosystems and is critical for their natural functioning, nutrient cycling, and aquatic biota. Chalk streams regularly exhibit substantially higher quantities of accumulated fine sediment in their gravel beds compared with other gravel bed river systems, despite often presenting with lower suspended sediment loads. This is a consequence of their natural hydrological conditions (i.e., low bed mobilising flows) compounded by anthropogenic activities altering channel planforms and increasing fine sediment inputs. This, in combination with their fine sediment-sensitive species, creates a high propensity for long lasting lethal/sub-lethal ecological impacts. Current approaches to defining management targets have failed in chalk streams due to a lack of scientific knowledge underpinning them. Importantly, they have failed to consider individual river system responses to fine sediment and to explicitly link the fine sediment problem with its causation. As a result, there is a need for new and ecological-relevant targets for the dominant process controlling fine sediment accumulation in chalk streams, to prioritise revised management and restoration activities. To identify the key controlling mechanisms in chalk streams, a new conceptual framework was proposed, describing the chalk stream sediment budget. The sediment budget framework incorporated four overarching mechanisms controlling fine sediment accumulation. Stream power (in particular, low bed mobilising flows) was highlighted as the most critical factor, controlling three out of four of these mechanisms. Through construction and analysis of an extensive freeze-core database, the sedimentological characteristics of chalk stream gravel beds were established, as they form a key control on the exfiltration of fine sediment. Analysis highlighted that 89% of chalk stream gravel beds were over-saturated with fine sediment and that 75% had fine sediment quantities exceeding those previously established to cause substantial ecological degradation. Regional variations were attributed to differences in stream power and local sediment sources. It was also determined that current models describing gravel bed-fine sediment interactions were not representative of chalk stream sedimentological characteristics. Through flume experiments, new targets for the flow velocities required to remobilise fine sediment from the ecologically-sensitive surface layer of chalk stream gravel beds were established. The experimental gravel bed and fine sediment grain size distributions were taken from the previously determined chalk stream sedimentological characteristics, to ensure the experimental design better represented naturally occurring conditions. Comparison between the required flow velocities and those currently occurring in chalk streams indicated that, for the most part, chalk streams are not achieving flow velocities required to remobilise fine sediment from the surface layer of their gravel beds. Potential revised instream management and restoration activities to restore the required flow velocities were discussed. The flow velocities provided herein are some of the first scientifically robust targets that can direct revised management and restoration activities, aimed at reducing fine sediment quantities in chalk stream gravel beds. In the absence of targets, the impacts of elevated fine sediment quantities in chalk streams are not being addressed. Until sediment targets are recognised and adapted in policy and process, actions to restore these systems to more favourable conditions cannot be efficiently or effectively implemented or measured.

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Identifiers

ORCID for Beth Mondon: orcid.org/0000-0003-4788-6016

ORCID for David Sear: orcid.org/0000-0003-0191-6179

ORCID for Peter Shaw: orcid.org/0000-0003-0925-5010

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