Property:Monitoring surveys and results
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B
An ecological monitoring programme was also undertaken to complement the water quality study and investigate the effects of metal pollution on macrophytes, invertebrates and diatoms. The ecological surveys were carried out in 3 seasonal sampling events over 2 years. Preliminary results show little adverse effects on invertebrates and macrophytes, but distortion of diatom valves in some species. +
F
An enlargement of the target habitats by a total of 118 ha will be achieved by the end of the project. Water safety improvement measures will increase the water storage capacity in the floodplains and thus reduce the risk of flooding elsewhere. This will prevent habitats from drying out during summer and improve spatial connection of habitats and populations of species that are susceptible to climate change. Recreational measures will result in a bird observatory, a partly restored and extended footpath (Koppenwaard), shortcuts to the river IJssel and other improved facilities for visitors (e.g. wooden benches at viewpoints). +
L
An extensive monitoring programme has been initiated to research the development of the site. Nine research groups from different universities and research institutes are working together to achieve optimal linkage of results on the following factors (Maris et al., 2008):
Basic water quality (4/annum)
Sedimentation/erosion (4/annum)
Bioturbation (4/annum)
Soil heavy metal (4/annum)
Birds (52/annum) Soil nutrients (4/annum)
Creek morphology (2/annum)
Suspended solids (continuous)
Fish (2/annum)
Tidal regime (continuous)
Ground compaction (4/annum)
Vegetation (4/annum)
Light climate (4/annum)
Water balance (continuous)
Macro-invertebrates (4/annum)
Water heavy metal (4/annum)
Sediment size (4/annum)
'''Sedimentation''': With respect to sedimentation, Maris et al. (2008) point out that whilst sedimentation is on the one hand desired in tidal marshes there are also associated negative implications. The input of fresh sediment enables the advancement of typical marsh morphology and marsh soil, which in turn promotes the establishment of estuarine vegetation and fauna. Conversely excessive sedimentation is to be avoided in a flood storage polder, as this leads to decreased water storage capacity. Due to the enclosed nature of a flood storage polder, the environment is less dynamic and thus the potential for sedimentation is increased, particularly during longer stagnant phases. At Lippenbroek, at all but one of the 10 sedimentation measurement sites, net sedimentation was observed between March 2006 and November 2007, varying between 0.5 to 12.5 cm/year.
Sedimentation was strongly related to flooding frequency; i.e. the highest sedimentation rate was observed at sites inundated around 85% of the time. The average sedimentation rate is currently fairly high at approximately 4 cm/year. The lowest sites are accreting fastest, hence the polder is loosing slope. Outside the polder, in natural marshes, an increase in elevation would lead to a decrease in flooding frequency, and a consequent gradual decrease in sedimentation. Conversely in the CRT the flooding frequency is not directly coupled to elevation; the intake volume is stipulated by the sluice configuration.
'''Fauna''': Macro invertebrate monitoring shows that there has been a shift from terrestrial species to more aquatic and estuarine species (Maris et al., 2008). With regards to birds, despite the high degree of disruption by humans (including site visitors,cyclists and walkers on the dike), a clear change in the numbers and species of birds using the site could be observed when compared to baseline conditions. Of the 4,089 individuals observed during the first year of monitoring, 30% were benthic foragers (19 species), 54% wetland-generalists (11 species), 8% terrestrial generalists (18 species) and 8% woodland birds (13 species). Occasionally night herons Nycticorax nycticorax and spoonbills Platalea leucorodia were observed feeding (Maris et al., 2008). In order for a CRT to fulfil a role for fish, safe passage of the cluvers/sluices is required.
Monitoring indicates that nearly all fish species observed in the Scheldt Estuary were also observed in the polder, frequently in large numbers. Net captures show that only small numbers of fish enter the polder via the inlet sluice, however, the fish tend to find their way into the polder via the outlet sluice. Fish might be attracted by the oxygen rich water which leaves the polder, and thus migrate into the polder against the outflow. A survey undertaken in 2006 (Simoens et al., 2007) found 14 species utilising the site, compared to 15 in the adjacent estuary. Particularly high numbers were observed for the following five species: three-spined stickleback (Gasterosteus aculeatus), ninespine stickleback (Pungitius pungitius), amur bitterling (Rhodeus sericeus), stone moroko(Pseudorasbora parva) and European flounder (Platichthys flesus).
'''Flora''': The vegetation monitoring shows a shift in the direction of water tolerant species. Even relatively stress resistant species such as the large nettle and hairy wild rose were replaced by true marsh species within a relatively short time. Colonisation by Lythrum salicaria, Typha latifolia, Veronica, Phragmites australis and willow Salix occurred within one year, whereas at lower elevations vegetation has been replaced by mudflat (Maris et al., 2008).
'''Water quality''': With respect to water quality, the oxygen enriching impact, which is mostly due to the high inlet sluices acting as aerators, has been described as ‘striking’. At the sluices, an immediate increase of up to 60% has been observed and a further 20% enrichment has been attributed to surface oxygenation. On warm, sunny summer days, oxygen reaches even higher values as light penetrates easily through the thin water layer in the polder and oxygen production by primary producers leads to over saturation (Maris et al., 2008).
Regulated Tidal Exchange operation: When compared with the Scheldt, high-water levels in Lippenbroek are around 3m lower in
elevation, without significantly affecting the pattern of spring and neap tides (Figure 1). Important parts of the polder therefore receive flooding frequencies comparable to those of the fronting saltmarshes. However, the high inlet/low outlet technique does lead to a distortion of the tidal movement; the tidal curve is no longer sinuous but has a stagnant phase. This is due to water only flowing in around high water (‘top of the tide’), and only exiting once water levels outside in the estuary have fallen low enough for the gravity controlled outfall sluice to open. The authors stressed that the inlet structure had to be sufficiently high, to achieve a significant difference in ingress duration and volume, and thus bring about a large variation in water levels. It was also highlighted that flood frequencies are mainly stipulated by the sluice configuration, no longer by the elevation in relation to the tidal frame; this allows marsh development in lower lying areas (Maris et
al., 2008).
W
An initial five-year monitoring programme is being undertaken to describe the ecological development of the site and determine whether it meets compensation targets. This ‘site success’ monitoring includes surveys of overwintering waterbirds, benthic invertebrates, saltmarsh vegetation growth, sedimentation/erosion patterns and the settlement characteristics of the recharge area. To confirm the findings of the assessment work and test the effectiveness of the mitigation measures, this five-year programme also includes ‘impact verification’ monitoring with surveys of flow speeds, shoreline topography and channel bathymetry in the estuary. +
F
An initial four-year environmental monitoring programme of the site, and adjacent intertidal habitats, has recently concluded (Brown et al., 2007). The majority of the monitoring was undertaken between 2001 and 2006 by the Natural Environment Research Council’s Centre for Ecology and Hydrology (CEH), the Cambridge Coastal Research Unit (CCRU), and Birkbeck College (University of London). It investigated accretion/erosion rates, sediment properties, vegetation colonisation, establishment and succession, invertebrate colonisation, and fish utilisation. Accretion and vegetation was surveyed inside the realignment for an additional (5th) year in 2007 (Brown, 2008). Furthermore, the RSPB undertook monthly bird monitoring (for breeding and water birds, as well as wintering passerines), and the National Oceanographic Centre measured wave activity and tide heights for one year after the breach. +
N
An intensive monitoring programme investigated a range of factors, including salinsation, sedimentation, creek development, vegetation and birds (van Duin et al., 2007).
Van Duin et al. (2007) concluded that to date, the realignment had been successful, although the site was obviously still evolving. They attributed the relatively rapid success of the scheme to a number of (environmental and management) boundary conditions having been fulfilled at the same time – namely its appropriate position in the tidal frame, sediment availability, tidal exchange, drainage, closeness to target plant species, and grazing management.
'''Salinisation''': The report firstly concluded that salinisation of the upper soil layer had been slower than expected. By the end of the monitoring period, salinity was still 30% below that of the reference saltmarsh (with increases most pronounced in the first year; plateauing in the third or following years after ‘de-polderisation’). No increased saline influence could be measured in the adjoining summer polder (in some parts fresh water influence actually increased).
'''Accretion''': Sediment accretion in the (higher) easterly part of the restoration site was greater than in the westerly part; average accretion at elevations above 1.5mNAP was 6mm/year and 14.8mm/year below, i.e. rates were higher at the lower levels. Whether this was solely due to the lower elevation or whether it was caused by other factors (e.g. a greater transport of sediment from the saltmarsh due to creek shape) was not clear. No evidence was found that the sediment accretion in the restoration site had been at the cost of adjacent saltmarshes. On the basis of the site’s elevation, it was expected that saltmarsh vegetation would develop throughout the entire site. There were some localised issues with stagnated drainage resulting in infilling of the creeks or cessation of the connection with the excavated creek systems. The excavated, over-dimensioned, creeks had become shorter and shallower (due to sediment accretion), and some of the silted up sections had
become colonised by pioneer species. The accretion rates were approximately 38% lower where grazing occurred as compared to non-grazed reference areas. Other noted effects of grazing were that it led to higher biodiversity, the colonisation of sea couch (''Elytrigia atherica''); to a slightly drier, more compact and saline upper soil layer; and lastly to slight positive effects on chloride concentrations in the soil.
'''Vegetation''': Halophytes quickly settled in the lower parts of the polder (~0.3m above MHT), whilst in the higher parts (~0.6m above MHT) changes were more gradual. In lower lying parts, salt marsh has developed well, dominated by Suaeda maritima and Salicornia europea. These pioneer species are slowly being replaced by more grassy vegetation (''Puccinellia maritima'').
'''Brent Geese''': Autumnal geese grazing has to date been significantly lower than on existing salt marshes. In the first season following the realignment, there was however a substantial increase. The expectation is that through an increase of grassy vegetation in the polder, the number will slowly increase. The use of the farm land behind the main dike by geese stayed low, although there was a slight increase in brent geese, in line with an overall population increase (farmers had expressed fears that geese would be displaced and start grazing on their crops, due to generally lower grazing densities on saltmarshes when compared with summer polders (NB: summer polder would be classed as coastal grazing marsh in the UK)). No significant change in the breeding bird populations was observed.
B
Bio-engineering and creation of side channels along the rectified section of the Scarpe River in Arras +
An pre work survey was carried out in 2010,on
the plant and animal compartments. Currently, no
particular ecological monitoring has been set up.
However, a check on the renewed growth of the
vegetation was run in September 2012, during the
works.
An assessment on the changes in the environment
is also planned at some point at least five years after
the works.
Unfortunately, the works had to be done from the
water, which made it difficult. In addition, a large
geese population, present on the river since before
the works, has damaged and slowed the regrowth
of the helophyte vegetation, particularly in the reed
beds.
In 2010, the pre works assessment revealed a relictual
population of woolly burdock that still existed
on the site, but was threatened. The assessment was
also an occasion to run surveys on two fish species,
two emblematic bird species and one emblematic insect
species. No amphibian species were noted.
An assessment of the impact of the works on the
restored habitats and on biodiversity is planned
at some point at least five years after the works in
order to give enough time to the environment to
adapt and to the various species to go through their
life cycle. The assessment, carried out by a specialised
consulting firm, will be based on the changes in
the population numbers of the species listed in the
initiallyt in order to determine the degree to which
the objectives set by the project owner were met,
namely:
- maintain the woolly burdock, pike and eel populations;
- install at least one amphibian species;
- double the number of bird and insect species (two
and one respectively) observed prior to the works.
This project strove to achieve a balance between the
ecological value of the site and its use as a waterway,
i.e. a channel for commercial shipping, pleasure
boating and the corresponding recreational activities.
For example, the landscape of the site was
improved with beneficial effects on tourism and
pleasure boating, as well as on recreational activities
(walking, etc.).
E
An upstream monitoring point is established and will be re-sampled in 2017 and compared to previous results with details posted online. +
Analyses of the ecomorphological
effects of the controlled flood in the Cardener River were based on
monitored changes in river morphology, suspended load transport, and
bedload transport (Pallarés and Martín-Vide, 2015). Changes in rivermorphology were quantified by determining geomorphic variation at a
number of channel sections along the study sub-reach. Suspended
sediments were sampled pre- and post-event at different channel
locations. Bedload changes were analysed by (i.) colouring portions
of gravel bars with spray and tracing the modifications, and (ii.)
measuring shifts in the granulometric curves of selected channel sites
after flood occurrence. Other observations made included a visible
assessment of the changes in trophic conditions, riverbed armoring, and
an assessment of the IHF index (Pardo et al., 2002).
Main morphologic changes found under post-event conditions
occurred in the river sub-reach closer to the dam toe. During the
managed floods with higher flows (i.e., 8 and 12 m3
/s), erosion rates
reached 1.16 m3 per linear meter at the upper sampled station
(width = 16.5 m). Changes were especially pronounced in the riffle
areas. By contrast, lower stations did not show any major morphological
changes, and the 4 m3
/s event did not result in significant
morphological activity in the channel.
Suspended transport was relatively pronounced during the 8 and
12 m3
/s events; suspended sediment concentrations reached
0.22–0.37 g/l at some sampling sites. This fine load was generated
inside the study reach, mostly from the immediacy of the dam, and did
not stem from the upper basin.
With respect to bedload, significant transport of particles occurred
during the three flood events. The extent of transport was greater (and
the size of the transported particles bigger) for the higher-flow flood
events. Granulometric analyses showed an increase in grain size in the
riverbed, and a reduction (or elimination) of clogging conditions by fine
sediments.
Analyses of the ecomorphological effects of the controlled flood
were based on monitored changes in river morphology, suspended load
transport, and bedload transport. River morphology was analysed in
terms of variation in the geometry of two channel sections (one in a
riffle, one in a pool) along the study sub-reach, variation in the
granulometric composition in those two sections, and assessments of
the refuge index for fishes (García de Jalón et al., 1993) and the river
habitat index (IHF – Pardo et al., 2002). Suspended load was measured
in 10 different water samples. Bedload transport was analysed in four
different locations by measuring the size of pre-coloured coarse
sediments over 1 × 1 m2
, and their relative motion along the channel
(Leopold, 2006).
Monitoring showed that changes in the river micro- and mesohabitats
were highly limited (Cebrián et al., 2015) (Table 1). The
variables analysed were only slightly modified and the values of habitat
indices were consistent with pre-flooding conditions. The value of the
suspended load transport was low and in direct response to flow
changes, which could indicate that the origin of the suspended
sediments was in the channel downstream of the dam. For bedload
transport, most removed particles were smaller than 64 mm and were
concentrated in the pool area, likely due to the high increment of water
velocity. Larger particles were occasionally removed or flipped. +
R
Analysis of monitoring results pending. +
Analysis of monitoring results pending.
Post-restoration water vole surveys suggest that fenced tree enclosures which were no longer grazed provided additional habitat for water voles. Field signs (latrines and food stores) were observed in areas where they hadn't previously been seen. +
E
Annual juvenile salmonid electro-fishing surveys - see Eden Rivers Trust website for published reports.
Annual white-clawed crayfish manual hand search surveys - see Eden Rivers Trust website for published reports.
Fixed point photography monitoring of river habitat improvement projects (on-going - see examples under Monitoring below).
Repeated individual farm assessments with advice on reducing diffuse pollution, including farm infra-structure improvements where applicable (confidential to the land owner/tenant). A proportion will be reassessed at the end of the project. +
B
Approximately one year after the banks were breached in September 2009, the channel experienced an estimated 30m3/s flood flow (of the order of a 1 in 10 year event). The stone protection at the upstream breach was partially washed out, as anticipated. The breach enlarged such that the majority of the flow was diverted along the new route after the flow subsided. The result was rapid development of river features including the formation of an outwash fan. Some ecological degradation has occurred in the short term as the old channel is now dry except during very high flow events.
Small on-going adaptive management is predicted to be necessary in the short to medium term until this modified river system becomes better established. +
E
Après abaissement partiel des clapets en mai 2011,
des mesures hydromorphologiques, un suivi des
habitats piscicoles et des analyses physico-chimiques
sont réalisés juste avant les travaux d’effacement du
seuil (juin 2011) à l’initiative du Conseil général de a Mayenne. Un inventaire de la faune et de la flore
est également effectué par l’association Mayenne
nature environnement. Les invertébrés benthiques
et les diatomées ont également fait l’objet d’un suivi
avant travaux en amont et en aval de l’ouvrage.
Concernant le suivi post travaux en 2011 et 2013,
des mesures physico-chimiques sont réalisées et un
inventaire de la faune et de la flore a été de nouveau
mis en œuvre. En 2013, un inventaire piscicole
est réalisé en amont de l’ancien seuil du moulin. La
même année, des mesures hydromorphologiques
(transects) sont effectuées, simultanément au suivi
des habitats piscicoles. Enfin, des relevés invertébrés
(IBGN) et diatomées (IBD) ont été effectués dans le
courant de l’été 2012, soit un an après travaux et ont
été poursuivis en 2013 et 2014.
Suite à l’effacement de l’ouvrage, la ligne d’eau s’est
abaissée et les vitesses d’écoulement ont augmenté,
entrainant la mise à jour de zones d’atterrissements
limoneux et de zones à granulométrie plus épaisse.
Une diversité de faciès apparait, ainsi que des zones
lentiques et lotiques avec radiers quelques centaines
de mètres en amont de l’ancien seuil.
Au niveau des suivis après travaux sur les invertébrés
benthiques, une lègère amélioration est observée
en amont de l’ancienne retenue : la composition et
la structure du peuplement sont plus diversifiées et
équilibrées – augmentation richesse taxonomique
des espèces polluosensibles – par rapport à la situation
avant effacement. La pêche de 2013 en amont
de l’ancien seuil montre des espèces inféodées aux
milieux graveleux et sableux ainsi qu’aux milieux
courants. Ces espèces devraient bien se développer
du fait de l’augmentation des faciès courants. Les
espèces d’eau calme et plus profonde sont présentes.
L’anguille est quant à elle bien représentée démontrant
tout le potentiel de ce poisson migrateur à coloniser
l’axe Vicoin depuis l’axe Mayenne. La comparaison
entre la structuration du peuplement piscicole de cette station et d’une station de référence peu ou
pas impactée par les activités humaines à proximité
montre la bonne qualité de ce compartiment.
Ce projet est l’un des premiers à avoir été mis en
œuvre par le SBV sur le bassin versant du Vicoin.
Il a pu être réalisé grâce à une participation active,
notamment, du président du SBV qui est allé à la rencontre
de chaque riverain. Cette animation a permis,
d’une part, de favoriser l’acceptation sociale autour
de la mise en place d’un programme global de restauration
de la continuité sur le bassin du Vicoin et
d’autre part, de faire accepter les opérations d’effacement,
à l’image du moulin du Bourg, en répondant
aux attentes des riverains sur la conservation de
certains usages et en mettant en avant l’intérêt de
la restauration de la continuité à une échelle cohérente.
Le propriétaire est très satisfait des travaux,
convaincu notamment par l’aspect visuel de la
rivière sans ouvrage. Il fait lui-même visiter le site
de restauration sur sa parcelle à d’autres riverains
ou élus de structures voisines afin de leur montrer l’évolution du site après effacement et son aspect
paysager.
Sur les vingt-six ouvrages initialement présents sur
le bassin du Vicoin seul deux restent à aménager
et ces opérations sont prévues en 2018
Depuis le début de ces opérations en 2008, les
peuplements piscicoles se sont améliorés. La proportion
des espèces plus exigeantes vis à vis de
la qualité de l’eau a augmenté (chabot, vairon,
loche franche...). De même la densité d’anguilles a
été multipliée par huit. L’envasement des fonds de
la rivière s’est réduit et les habitats sont restaurés.
R
As a result from restoration actions, a total of 32 ha of poplar plantations were felled or will die standing to make room for alluvial forest. Additionally, 42 ha of former agricultural land will spontaneously evolve towards forest, as a result of measures taken by the project.
Also, good grazing management practices have been implemented to help developing a gradient of open and heavily grazed forests, as well as closed and almost non-grazed forests. 18 weekend cottages were removed and 15 horticultural ponds were restored to be re-integrated in the landscape.
Another important aspect of the project was the re-naturalisation of the river De Kleine Hoofdgracht. This action was implemented by the province of Antwerp, which removed the dikes along a stretch of 690 m and created more than 2 ha of open water, marshland and oxbows etc. The result is an improved habitat for endangered fish, such as the target species, brook lamprey and spined loach.
Pine plantations and sod were cut, now providing a land dune of 15 ha for wind dynamics and free recreation to take place and contribute in keeping this area an open habitat.
Over 60 ha is under continuous management of the area by a local farmer partnership. Also hiking trails were installed (with bridges and board walks) and hiking brochures were produced to increase the ecotourism value of the region for visitors. +
B
As a result of the introduction of dead wood, the flow pattern has become mare varied, which is expected to improve the ecological quality of the stream. +
H
As a result of the measures, there is now a more varied fish population, with more species that prefer a slightly higher flow velocity. Erosion and sedimentation processes have caused a change in the stream profile. Locally, this change, in combination with increased vegetation coverage, has caused the bed to be too narrow to transport sufficient water. This has led to stream incision. +
R
As part of the restoration project TRT is undertaking a fixed point monitoring programme of the measures delivered. It includes photographs of each site before, during and post construction.
The site was visited in December 2016 and all features were in situ and performing as planned. +
A
As the project was completed in the Summer of 2021, monitoring is still on going. Invertebrates, macrophytes, fish and water quality will be monitored before and after for a number of years, alongside morph surveys with accompanying drone footages to show the change in channel shape over time. +