Potential morphological responses of an artificial beach to a flood in extreme events: field observation and numerical modelling

Conch Island is a typical artificial island at the Tanghe Estuary in Bohai Sea,China.To improve natural environment and boost local tourism,beach nourishment will be applied to its north-western shore.The projected beach is landward and opposite to the Jinmeng Bay Beach.Nowadays,with climate changes,frequent heavy rainfalls in Hebei Province rise flood hazards at the Tanghe Estuary.Under this circumstance,potential influences on the projected beach of a flood are investigated for sustainable managements.A multi-coupled model is established and based on the data from field observations,where wave model,flow model and multifraction sediment transport model are included.In addition,the impacts on the projected beach of different components in extreme events are discussed,including the spring tides,storm winds,storm waves,and sediment inputs.The numerical results indicate the following result.(1)Artificial islands protect the coasts from erosion by obstructing landward waves,but rise the deposition risks along the target shore.(2)Flood brings massive sediment inputs and leads to scours at the estuary,but the currents with high sediment concentration contribute to the accretions along the target shore.(3)The projected beach mitigates flood actions and reduces the maximum mean sediment concentration along the target shore by 20%.(4)The storm winds restrict the flood and decrease the maximum mean sediment concentration by 21%.With the combined actions of storm winds and waves,the maximum value further declines by 38%.(5)A quadratic polynomial relationship between the deposition depths and the maximum sediment inputs with flood is established for estimations on the potential morphological changes after the flood process in extreme events.For the uncertainty of estuarine floods,continuous monitoring on local hydrodynamic variations and sediment characteristics at Tanghe Estuary is necessary.

Different responses of two adjacent artificial beaches to Typhoon Hato in Zhuhai,China

Major differences in beach erosion between two neighboring artificial beaches Xiangluwan Beach(XL beach)and Meiliwan Beach(ML beach)in Zhuhai,China,were studied after Super Typhoon Hato.In this study,a fully nonlinear Boussinesq wave model(FUNWAVE)-Total Variation Diminishing(TVD)was used to distinguish the main impact factors,their relative contributions,and the hydrodynamic mechanisms underlying the different beach responses.Results show that compared to the ML beach,the main reason for the relatively weak erosion on Xiangluwan(XL)beach was the smaller beach berm height(accounting for approximately 75.9%of the erosion response).Regarding the beach with a higher berm,the stronger wave-induced undertow flow,along with the higher sediment concentration,led to a higher offshore sediment transport flux,resulting in more severe erosion relative to the beach with a smaller berm height.The second most important reason explaining the weak erosion on XL beach was the absence of seawalls(accounting for approximately 17.9%of the erosion response).Wave reflection induced by the seawall could cause higher suspended sediment concentration,resulting in a toe scouring near the seawall.The offshore submerged breakwater protected XL beach slightly(accounting for approximately 6.1%of the erosion response).Due to the higher water level induced by storm surge,most of the wave energy could penetrate through the submerged breakwater.The effect of the larger berm width of XL beach was negligible.Compared to the beach with a larger berm width,the erosion/deposition regions in the beach with a narrower berm width showed shoreward migration,without significant changes in the erosion/deposition extent.Despite of this,the larger berm width could reduce the wave energy reaching the shoreline.This study of the storm stability of artificial beaches may be applied to beach restoration design.

Gravity Fault Subsidence and Beach Ridges Progradation in Quinta-Cassino (RS) Coastal Plain, Brazil

Ground penetrating radar (GPR) surveys have being applied to investigate very near-surface stratification of sedimentary units in coastal plains and to define their depositional conditions. This paper presents, however, low-frequency GPR survey to investigate fault-related depositional systems at greater depths. The Quinta-Cassino area in the Rio Grande do Sul Coastal Plain (RGSCP, Brazil) shows a wide strandplain that is made off by very long, continuous, and linear geomorphic features (beach ridges). This strandplain extends for ~70 km southward. The beach ridges show low-angle truncations against the Quinta escarpment, and also truncations in the strandplain. The traditional approach points that RGSCP was developed by juxtaposition of four lagoons/barrier systems as consequence of sea level changes;previous model assumes that no deformational episode occurred in RGSCP. The geophysical and geological surveys carried out in this area showed the existence of listric fault controlling the beach ridges in the escarpments and hanging-wall blocks. The radargrams could distinguish Pleistocene basement unit anticlockwise rotation, thickening of beach ridges radarfacies close to listric normal faults, and horst structures. These deformational features indicate that the extensional zone of a large-scale gravity-driven structure controlled the mechanical subsidence, the Holocene sedimentation and its stratigraphic and geomorphic features in the Quinta-Cassino area to build up an asymmetric delta. The results point to a new approach in dealing with RGSCP Holocene evolution.

Numerical modeling of deep-seated landslides interacting with man-made structures

This paper describes the interaction between deep-seated landslides and man-made structures such as dams, penstocks, viaducts, and tunnels. Selected case studies are reported first with the intent to gain insights into the complexities associated with the interaction of these structures with deep-seated landslides(generally referred to as deep-seated gravity slope deformations, DSGSDs). The main features, which characterize these landslides, are mentioned together with the interaction problems encountered in each case. Given the main objective of this paper, the numerical modeling methods adopted are outlined as means for increase in the understanding of the interaction problems being investigated. With the above in mind, the attention moves to an important and unique case history dealing with the interaction of a large-size twin-tunnel excavated with an earth pressure balance(EPB)tunnel boring machine(TBM) and a deep-seated landslide, which was reactivated due to the stress changes induced by tunnel excavation in landslide shear zone. The geological and geotechnical conditions are described together with the available monitoring data on the landslide movements, based on the advanced and conventional monitoring tools used. Numerical modeling is illustrated as an aid to back-analyze the monitored surface and subsurface deformations and to assist in finding the appropriate engineering solution for putting the tunnel into service and as a follow-up means for future understanding and control of the interaction problems. The simulation is based on a novel time-dependent model representing the landslide behavior.

Dynamic geomorphology and storm response characteristics of the promontory-straight beach-a case of Gulei Beach, Fujian

As one of the main areas of tropical storm action in the northwestern Pacific Ocean,South China experiences several typhoons each year,and coastal erosion is a problem,making the area a natural testing ground for studying the dynamic geomorphological processes and storm response of promontory-straight coasts.This study is based on three years of topographic data and remote sensing imagery of Gulei Beach and uses topographic profile morphology,single width erosion-accretion and mean change,combined with the Coastsat model to quantify the seasonal and interannual variability and storm response of the beach and to explain the evolution of shoreline change and beach dynamics geomorphology in the last decade.Gulei Beach has been in a state of overall erosion and local accretion for a long time,with relatively obvious cyclical changes;seasonal changes are also obvious,which are mainly characterized by summer accretion and winter erosion,with accretion at the top of the bay and accretion and erosion on the north and south sides of the bay corner,respectively;the seasonal erosion-accretion volume of the beach profile ranges from-80 m3/m to 95.52 m3/m,and the interannual erosion-accretion volume ranges from-69.09 m3/m to 87.31 m3/m.The response of beaches to typhoons with different paths varies greatly depending on the length,slope,orientation and scale of beach development.The large and gently developing Futou beach is less responsive to storms,while the less developed headlands in the southern Gulei Peninsula are more susceptible to disturbance by external factors and respond more strongly to typhoons.Storm distance is more influential than storm intensity.Under the influence of human activities,obvious erosion hotspots develop during normal weather,but storm processes produce redistribution of beach material patterns,and erosion hotspots disappear after storms.The results of this study enrich the theory of beach dynamics geomorphology and provide technical support for disaster prevention and mitigation,as well as ecological restoration of coastal zones.

Assessment and Application of Beach Quality Based on Analytic Hierarchy Process in Yangkou Beach, Qingdao

The whole-beach quality assessment is the basis of building and preserving beautiful beaches.The beach quality assessment index system and assessment standard have been established based on the attributes of beaches(including the width,slope,landform,and types),sorting coefficient,and softness degree of surface sediment.The assessment weight of each index for quality evaluation was analyzed using the analytic hierarchy process,and comprehensive scores of selected beach profiles were calculated in accordance with the light assessment standard.A beach quality evaluation model based on index weight and scores was established in this paper.The factors of 12 profiles of Yangkou Beach in Qingdao City were surveyed to carry out a quality assessment,and the comprehensive scores of each profile were calculated in accordance with the evaluation model.The results showed that the quality of Yangkou Beach can be divided into four ratings:excellent,good,medium,and poor.The excellent-quality area includes a wide and flat dry beach zone and soft,flat,and clean intertidal and subtidal zones covered with well-sorted fine sand,and leisure sports,such as volleyball,running,and swimming,are suitable for tourists.The good-quality area features a slightly narrow and dry beach zone,moderately soft and uneven intertidal and subtidal zones covered with fine sand and a small tidal gully,and a small amount of foreign matter;leisure sports,such as walking and running,are suitable for tourists.This study recommends the building of fixed drainage ditches or underground culverts to reduce the tidal gully.The medium-quality area consisted of a narrow and dry beach zone,moderately soft and uneven intertidal and subtidal zones covered by poorly sorted medium sand,a tide ditch,and a small amount of foreign matter.In this area,walking is suitable for tourists.Sand should be supplemented in the intertidal zone.The poor-quality area contained a very narrow and dry beach zone covered with poor-sorted gravel,a very chaotic intertidal zone with a considerable amount of foreign matters,such as bricks and rocks,wide tidal ditches,and an uneven subtidal zone with some reefs;leisure sports are unsuitable here.Thus,foreign matter and reefs should be removed,and the dry beach zone should be supplemented with sand.Therefore,the beach quality assessment is a very useful tool for building beautiful beaches.

Considering and comparing regeneration in natural and man-made stands in northern Iran

In this study the regeneration diversity of Syahkal forests afforestated and natural stands in north of Iran was studied and compared from the point of view of evenness and diversity index. In order to accomplish this study two natural and man-made stands that almost are located in the same height above the sea level were chosen. The area of each stand was 30 ha and the inventory was done by the random-systematic method with a 5R land measurement （500 m2） selected. Also, in order to study the regeneration in the center of each sample piece 125 square meters micro plots were formed. The results show that regeneration diversity index in natural stands is more than that in man-made stands which in this case Mc-Arthur index with 2.41 has the most amount while Simpson index with 0.543 has the least amount. But the evenness indexes in man-made stands are more than that in natural stand which the main reason is the purity of the man-made stand.

Ecology and Fish Biodiversity of Man-Made Lakes of Southern Benin (West Africa): Implications for Species Conservation and Fisheries Management

Man-made lakes are alternative and potential habitats for biodiversity conservation, fisheries and extensive aquaculture. We investigated the ecology and the fish community structure of two (2) sand-dragged man-made lakes, Lake Ahozon and Lake Bewacodji of Southern Benin, with implications for species conservation, fisheries management and aquaculture valorization. From August 2014 to July 2015, habitats conditions were evaluated and fishes were sampled monthly with seine, cast net, experimental gill net and hooks in the open water and aquatic vegetation habitats of both lakes. Overall, the water quality of Lake Ahozon was globally favorable for the growth and the survival of the fish resources whereas Lake Bewacodji exhibited a poor water quality indicated mainly by an acid pH (mean: 6.32 ± 0.58) and low dissolved oxygen concentrations (mean: 3.52 ± 1.25 mg/l) caused by dense floating plants, Nymphea sp mainly and huge daily dumping of domestic wastes. The study revealed low species richness, d = 5.89 and d = 3.87, and low species diversity, H’ = 0.76 and H’ = 0.48 for Lakes Ahozon and Bewacodji, respectively, with Lake Ahozon more diverse than Lake Bewacodji. The fish community of Lake Ahozon comprised six (6) species, 3 cichlids Sarotherodon galilaeus, Oreochromis niloticus and Tilapia guineensis, the silver catfish, Chrysichthys nigrodigitatus (Claroteidae), the African bonytongue, Heterotis niloticus (Osteoglossidae), and the African catfish, Clarias gariepinus (Clariidae). Numerically, S. galilaeus dominated Lake Ahozon and made 85.21% of the sample. In Lake Bewacodji, the fish composition comprised four (4) species, Sarotherodon galilaeus multifasciatus, the dominant species making numerically 91.58% of the total sample, T. guineensis, C. gariepinus and C. nigrodigitatus. With regard to trophic structure, the fish assemblages of both lakes were numerically dominated by planktinovores/ detritivores, mainly S. galilaeus, O. niloticus, T. guineensis and C. nigrodigitatus making together 99.46% of Lake Ahozon fish community, and S. galilaeus multifasciatus, T. guineensis and C. nigrodigitatus accounting together for about 98.59% of Lake Bewacodji. In Lake Ahozon, standard length (SL) frequencies histograms showed an unimodal size distribution for H. niloticus, the nile tilapia O. niloticus and C. gariepinus whereas the two cichlids, S. galilaeus and T. guineensis exhibited a bimodal size distribution. In Lake Bewacodji, S. galilaeus multifasciatus, C. nigrodigitatus and C. gariepinus exhibited an unimodal size distribution. A sustainable exploitation of both man-made lakes requires the implementation of an integrated management scheme which should include habitat restoration and protection plan, fisheries/aquacultural valorization, ecological sound agriculture/ecotourism and environmental monitoring.

Morphodynamic response of an embayed beach to different typhoon events with varying intensities

Beach erosion has occurred globally in recent decades due to frequent and severe storms.Dongsha beach,located in Zhujiajian Island,Zhejiang Province,China,is a typical embayed sandy beach.This study focused on the morphodynamic response of Dongsha beach to typhoon events,based on beach topographies and surficial sediment characteristics acquired before and after four typhoon events with varying intensities.The four typhoons had different effects on the topography and sediment characteristics of Dongsha beach.Typhoons Ampil and Danas caused the largest(-51.72 m3/m)and the smallest erosion(-8.01 m3/m),respectively.Remarkable alongshore patterns of beach profile volumetric changes were found after the four typhoon events,with more erosion in the southern and central parts of the beach and few changes in the northern part.Grain size coarsening and poor sorting were the main sediment patterns on the beach influenced by different typhoons.Typhoons that occurred in the same year after another typhoon enhanced the effect of the previous typhoon on sediment coarsening and sorting variability,but this cumulative effect was not found between typhoons that occurred during different years.A comparison of the collected data revealed that the topographic state of the beach before the typhoon,typhoon characteristics,and tidal conditions were possible reasons for the difference in the responses of Dongsha beach to typhoon events.More severe beach erosion was caused by typhoons with higher intensity levels and longer durations,and high tide levels during typhoons can determine the upper limit of the beach profile erosion site.Taken together,these results can be used to improve beach management for storm prevention.

Variation in cross-shore wind speeds influenced by the morphology of nourished beach

The cross-shore variation in wind speeds influenced by beach nourishment,especially the dramatic changes at the nourished berm,is important for understanding the aeolian sand transport processes that occur after beach nourishment,which will contribute to better beach nourishment project design on windy coasts.In this paper,the influencing factors and potential mechanism of wind speed variation at the edge of a nourished berm were studied.Field observations,together with the Duna model,were used to study the cross-shore wind speed distribution for different nourishment schemes.The results show that the nourished berm elevation and beachface slope are the main factors controlling the increase in wind speed at the berm edge.When the upper beach slope is constant,the wind speed at the berm edge has a positive linear correlation with the berm elevation.When the berm elevation remains constant,the wind speed at the berm edge is also proportional to the upper beach slope.Considering the coupling effects of nourished berm elevation and beachface slope,a model for predicting the wind speed amplification rate at the nourished berm edge was established,and the underlying coupling mechanism was illustrated.

The 25th CIMFC(Bengbu 2019):Exploring the future of the man-made fibers

The 25th China International Man-made Fiber Conference (Bengbu 2019)(hereinafter referred to as CIMFC 2019), themed on “Opening and Integaration for Interactive Development — Coordinated Progress of Global Man-Made Fiber Industry”, was convened in Bengbu City, Anhui Province. CIMFC 2019 was sponcored by China National Textile and Apparel Council (CNTAC), organized by China Chemical Fibers Association, Chi-na Textile International Exchange Center, and China BBCA Group Corp., supported by Bengbu Municipal People’s Goverment, and co-organized by Bengbu Investment Promotion and Foreign cooperation Center, Bengbu Municipal Bureau of Commerce and Foreign Affairs, and Oerlikon Man-made Fiber.

Analysis of wave shoaling and shore-breakers on a low tide terrace beach based on in-situ measurements at Xisha Bay on South China coast

Low tide terrace beach is a main beach type along South China coasts with strong tidal actions.How strong tides affect wave transformations on low tide terrace beach still remains unclear.In this study,in-situ measurements are conducted on the low terrace beach at Xisha Bay to provide quantitative descriptions of wave shoaling and shore-breaker phenomena under the tidal effects.It is found that wave breaking is unsaturated on the low tide terrace beach at Xisha Bay.Magnitudes of wave skewness and asymmetry increase as wave shoals and achieve the maximum value at the shore-breaker,and then decrease rapidly.Mean energy dissipation rates of shore-breakers are tide-modulated since the bottom slope changes at the shoreward boundary of wave propagation in a tidal cycle.The remaining wave energy flux at the initialization of the shore-breaker is 1%–12%of offshore wave energy flux,and the energy flux ratio decreases with increasing offshore wave heights.Wave attenuation at shore-breakers can be estimated directly from offshore wave conditions based on findings in this study,favoring designs of seawalls or beach nourishment projects.Field datasets on wave transformations can also be used for verifications of wave numerical models.

MAN-MADE

名月背叛了虎帝大人，他要所有的新型M-M战士听命于他。于是，被博士利用变身药丸救出的流年他们决定要阻止名月。

MAN-MADE

前情提要 似水与流年根据博士日记上面的提示一路找到太阳遗迹，终于见到了博士，博士向两人说明了一切，原来，流年竟是人造人……

Using video imagery to reconstruct the 3D intertidal terrain along a beach with multiple cusps

A high-frequency,high-resolution shore-based video monitoring system(VMS)was installed on a macrotidal(tidal amplitude>4 m)beach with multiple cusps along the Quanzhou coast,China.Herein,we propose a video imagery-based method that is coupled with waterline and water level observations to reconstruct the terrain of the intertidal zone over one tidal cycle.Furthermore,the beach cusp system(BCS)was precisely processed and embedded into the digital elevation model(DEM)to more effectively express the microrelief and detailed characteristics of the intertidal zone.During a field experiment conducted in January 2022,the reconstructed DEM was deemed satisfactory.The DEM was verified by RTK-GPS and had an average vertical root mean square error along corresponding RTK-GPS-derived intertidal profiles and corresponding BCS points of 0.134 m and 0.065 m,respectively.The results suggest that VMSs are an effective tool for investigating coastal geomorphic processes.

Quantitative Analysis of the Sized Ranged Plastic Debris on Beach Shoreline along the Limbe Coastline, Cameroon

In recent years, increased interest in investigating the accumulation of sized ranged plastic debris has been observed on beaches along coastlines. The abundance and distribution of the 4M’s sized class plastic debris were quantitatively assessed on five sandy beaches, in Cameroon. Duplicates of 2 × 2 m (4 m2) quadrants were sampled in each beach/month with a total of 80 quadrants. Collected plastic samples were washed, sieved and dried. Particles of size, ≥2 mm, were sorted and measured using a 30 cm ruler, and converted to mm. Overall, 12,822 particles by number (530.59 g) with a mean abundance of 40.07 items/m2 (1.66 g/m2) plastic debris was recorded. ANOVA (p = 0.05) shows a linear relationship between the meso- and micro-sized classes with significantly higher abundance recorded in LDB sites. The highest abundance by weight was recorded in August and June numerically. 80% of the plastic particles were between the size range, of 5 - 20 mm by number and 6 - 100 mm by weight. Moreover, in all beaches micro-sized class plastics were dominated by number 42.40% with fragmented debris dominant, in number/weight, 54.86% (25.69%) while meso-sized class plastics were 29.28% dominated by weight, with fragmented debris type, the most prevalence in number and weight as 46.11% (26.18%). On average, color and shape fractions revealed, colored and irregularly shaped plastics were dominant with an abundance of 80.45 ± 18.17 items/m2 (2.58 ± 0.68 g/m2) and 47.24 ± 20.40 items/m2 (1.39 ± 0.66 g/m2). Finally, the 0.0001 g plastic debris was dominant with a concentration, of 33.68 ± 7.23 items/m2. The intense use of beaches for recreation and poor waste disposal has increased the potential for plastic contamination.

Nourishment and disaster mitigation efficiency of feeding sand on the dry section of a dissipative beach

To explore the nourishment effect and disaster reduction efficiency of a fully dissipative dry beach under the impact of storms,this paper uses the measured topography and hydrodynamic data to establish a one-dimensional numerical model of the XBeach beach profile.By numerically modeling the change in the nourished profile for different dry beach widths under normal waves and storm conditions and the recovery process of the profile after the storm,the degree of response in dry beach nourishment for the fully dissipative beach is analyzed.The results show that under normal wave conditions,the response of the nourished dry beach is obvious.Sediment on the dry beach erodes heavily,and the shoreline moves landward over a long distance.With the increase in the width and size of the dry beach,the wave height at the bottom of the backshore profile decreases,the wave height attenuation rate increases continuously,and the wave elimination effect is remarkable.When the storm incident wave intensifies,the wave height attenuation rate of the nourished dry beach decreases,indicating that the smaller the storm intensity is,the more significant the wave reduction effect of the nourished dry beach is.At the same time,different profile arrangements of nourished dry beaches suffer from different degrees of erosion under storm conditions,with significant changes in profile morphology.With intensified storm action,the intensity of sediment erosion in the nourished dry beach increases,the nourishment is weakened,and the recovery effect of the profile after the storm is not obvious.The results of the numerical modeling highlight that the dry beach nourishment method can resist storms to a certain extent,but the overall effect is relatively limited.

Sediment source-to-sink process variations of sandy-muddy transitional beaches and their morphological indications

The clay mineralogy of 28 sandy-muddy transitional beach(SMT-Beach)sediments and surrounding mountain river sediments along the coasts of southeastern China was systematically investigated to reveal the sediment source-to-sink process variations of such beaches and their morphological indications.The results show that the clay mineral assemblages of these SMT-Beaches mainly comprise of almost equal illite(~30%),kaolinite(~28%),chlorite(~22%),and smectite(~20%)contents.From the surrounding mountain rivers to the SMT-Beaches,clay mineral assemblages show distinct spatial changes characterized by a large decrease(~40%)in kaolinite,whereas the other three clay minerals present relative increases,especially clear for smectite.The muddy sediment sources of SMT-Beaches inferred from the clay mineralogy are mainly derived from nearby mountain rivers coupled with long-distance transport and penetration of the Changjiang River.The sandy sediments of these beaches are predominantly sourced from nearby mountain rivers,the weathering products of surrounding rocks in both mainland and island environments,and erosion of the“Old Red Sand”and“Red Soil Platform”.However,the sandy sediment sources of the SMT-Beaches are largely reduced because of the remarkable decrease in the river fluvial supply associated with intensive human activities such as dam construction and coastal reclamation.Subsequently,the sandy sections of SMT-Beaches present clear erosion and have revealed by both time series remote sensing images and a compilation of published literature.In contrast,the muddy sediment supply of SMT-Beaches is temporarily stable and relatively constant,resulting in the landward migration of the mudflats with relative transgression or accumulation.These findings highlight that the natural evolution processes of SMT-Beaches have been greatly reshaped by intensive human activities.

Man-made fiber industry enjoys optimistic transformation and upgrading

As an important fundamental industry of the national economy,the man-made fiber industrial chain is integrated into the era development and it assumes the responsibility of advancing.Each link of the industrial chain is connected with each other.After the development and accumulation of several decades,the new produced polyester,PTA and other plants keep

The Probable Cause for Nesting Pattern of Olive Ridley (Lepidochelys olivacea) at Ramnagar Beach, North East Coast of Andaman Island, India

The nesting behaviour of sea turtles remains a subject to study, due to their enigmatic pattern of seasonal breeding activities. Over a period of time, several reports have been made in this context associated with the nesting behaviour of the Olive Ridley turtles. In the present study, characteristics of the breeding beach and nesting pattern of Olive Ridley (Lepidochelys olivacea) at Ramnagar along N-E coast of Andaman Islands were investigated, during the nesting periods 2016-2017. The study area hosts Olive Ridley, the dominant sea turtles with more than 300 individuals nesting each year. For this study, the number of sea turtles visited, nested, the sediment characters, salinity, and temperature were taken. The exposed sandy nesting beach characteristics are prone to varying degrees of morphological changes every day. The results depict that even though similar grain size (Coarse Sand to Fine Sand and Very well sorted to Poorly Sorted), with an ambient incubating temperature, pH and salinity with wide nesting area, the selective nesting in the particular location of the beach identified because of comfortable energy conditions in the waters (1.5 m/s) favours the female turtles to reach the beach at the preferable site of Ramnagar and nest.