Analysis of Forecast and Early Warning of Flood in Medium and Small Rivers

Flooding of small and medium rivers is caused by environmental factors like rainfall and soil loosening.With the development and application of technologies such as the Internet of Things and big data,the disaster supervision and management of large river basins in China has improved over the years.However,due to the frequent floods in small and medium-sized rivers in our country,the current prediction and early warning of small and medium-sized rivers is not accurate enough;it is difficult to realize real-time monitoring of small and medium-sized rivers,and it is also impossible to obtain corresponding data and information in time.Therefore,the construction and application of small and medium-sized river prediction and early warning systems should be further improved.This paper presents an analysis and discussion on flood forecasting and early warning systems for small and medium-sized rivers in detail,and corresponding strategies to improve the effect of forecasting and early warning systems are proposed.

Responses of Nutrients to the Precipitation Variation and Land Use in Subtropical Monsoonal Small Mountainous Rivers:A Case Study of Baixi Watershed

Small mountainous rivers are characterized by large instantaneous fluxes and susceptible to extreme weather events,which can rapidly transport materials into the sea and have a significant impact on the ecological environment of estuaries and bays.In order to investigate the seasonal characteristics of nutrients in small mountainous rivers in the subtropical monsoon region and the output pattern to the sea during heavy precipitation,surveys on the mountainous rivers were carried out in Baixi watershed in August 2020(wet season),March 2021(dry season)and June 2021(Meiyu period).The results showed that the dissolved inorganic nitrogen(DIN)of the rivers has an average concentration of 752μg L^(−1)in the wet season and 1472μg L^(−1)in the dry season.The concentrations of dissolved inorganic phosphorus(DIP)in wet season and dry season were 63μg L^(−1)and 51μg L^(−1),respectively.Influenced by the changes of land use in sub-watersheds,DIN concentrations in the mainstream increased from 701μg L^(−1)in the upper reaches to 1284μg L^(−1)in the middle reaches.Two rainstorms during the Meiyu period in the watershed caused the pulse runoff in the river.The maximum daily runoff reached 70 times that before rains.The maximum daily fluxes of DIN and DIP were 109 and 247 times that before rains,respectively.In view that the watershed experienced several rainstorms in the wet season,the river,with pulse runoff,carries a large amount of nutrients into the sea in a short time,which will have a significant impact on the environment of Sanmen bay and its adjacent sea.

Influence of the interaction of calcium carbonate particles with surfactants on the degree of water pollution in small rivers

Introduction:The influence of the interaction of calcium carbonate(CaCO3)and surface-active substances(SAS;surfactants)with different chain lengths and cationic and anionic hydrophilic centers has been analyzed.Results:Laboratory simulations indicate reduced negative influences on cationic SAS nitrification/self-purification processes in the presence of anionic species.This suggests the role of complex ionic formation[anionic SAS*cationic SAS]as a cause of this effect.UV-Vis spectra of lauryl sulfate(LS)and of cetyltrimethylammonium(CTMA),as well as of their mixtures in ratios of 2:1 and 1:1,treated by fine particles of CaCO3,display decreased amounts of SAS in analyzed solutions and their presence on the surface of CaCO3 nanoparticles.UV-Vis spectra reveal the decomposition of the complex[anionic SAS(SAS-An)*cationic SAS(SAS-Ct)]in solutions when CaCO3 is added.CTMA can be bonded by LS through hydrophobic chains,on the surface of CaCO3 particles.Therefore,CaCO3 modifies the nature of LS and CTMA interactions.This leads to an increased degree of toxicity of cationic SAS in aquatic environment.The amounts of CTMA in aqueous solutions are diminished in the presence of ammonium ion NH4+(2 mg/L).In the presence of two orders higher concentration of ammonium ion,this effect strongly increases,making the association obvious.The structure of cationic SAS does not influence this effect.The obtained results have been confirmed both by timed natural aquatic sample analysis and laboratory simulations using water from Moldovan small rivers(Isnovat,Raut,and Bic).Conclusions:UV-Vis spectra and laboratory simulations demonstrate the change due to the addition of calcium carbonate.Simulations and laboratory tests of water samples from Isnovat,Bic,and Raut Rivers,establish the cationic SAS negative influence on treatment and self-purification processes.

Causes and Laws of Floods in Small and Medium Rivers in Jiangxi Province

The main causes of floods in small and medium rivers in Jiangxi Province are meteorological factors,rainstorm factors and topographic factors.Based on the data of more than 600 floods from 19 hydrological stations in small and medium rivers in Jiangxi Province,the characteristics of floods in small and medium rivers are analyzed. The results show that the floods in small and medium rivers in Jiangxi Province had characteristics of strong seasonality,high frequency and strong suddenness,and were mainly local floods;rainstorm floods during May-June lasted for a long term,and had double peaks,multi-peaks or a single peak with a long duration. During July-September,floods usually had a single peak,and the duration was relatively short.

Sources and distributions of terrigenous organic matter in a mangrove fringed small tropical estuary in South China

The sources and distributions ofterrigenous organic matter （OM） were investigated in a small tropical estu- ary in the Hainan Island, South China. Plants, suspended particulate matter （SPM）, and surface sediments samples in the estuary and coast were collected. Bulk properties [organic carbon （OC%）, total nitrogen （TN%）, stable carbon isotope （613C） and grain size] and lignin phenol concentrations were measured. OC% of mangrove plants was （43.44-2.1） %, which is similar to the values reported for mangrove plants in other re- gions. OC% of sediment samples ranged from 0.07% to 1.42%, and theywere related to the sediment texture. Lignin phenols in the sediment ranged from 5.16 rag/100 mg OC in the uppermost station to 0.51 mg/100 mg OC in the coast. The molar ratio of organic carbon to total nitrogen （C/N） （~7） and 613C （^-31.1 ~ 10-a） of riverine SPM revealed that the major OM sources of riverine SPM were aquatic OM （phytoplankton and/or bacteria）. Moreover, the lower lignin concentration （AS） and higher （Ad/A1）v of lignin phenols suggest that terrestrial OM in riverine SPM were mainly from soil. Furthermore, C/N ratio, 6 ~a C and lignin phenols reveal that mangrove plants were the predominant OM sources of mangrove surface sediment. Based on the 61aC and lignin phenols, it can be concluded that the major OM sources in estuarine and coastal surface sedi- ments were marine phytoplankton, riverine SPM and mangrove surface sediment. In addition, the higher （Ad/A1）v of lignin phenols in those coastal sediments indicate that seagrass might be a potential OM source in coastal sediments, however, the lower （Ad/A1）v in the estuarine sediments in turn suggests that seagrass could not be transported to the mangrove fringed region. A three-end-member model which is based on lignin concentrations and 6 I3C was applied to evaluate the contribution of mangroves to the organic matter preserved in the surface sediments. Around the mangrove fringed region, mangrove could contribute more than 50% to the sedimentary OM, and this value is much higher than riverine OM. Nevertheless, mangrove OM could not be efficiently transported to the coastal region. Our study suggests that mangrove forest is an important OM source in this small estuary.

The effect of wind on the dispersal of a tropical small river plume

Wanquan River is a small river located in Hainan, a tropical island in China. As the third largest river in Hainan, the river plume plays an important role in the regional terrigenous mass transport, coastal circulation, and the coral reef＇s ecosystem. Studies have shown that wind forcings significantly influence river plume dynamics. In this study, wind effects on the dispersal of the river plume and freshwater transport were examined numerically using a calibrated, unstructured, finite volume numerical model （FVCOM）. Both wind direction and magnitude were determined to influence plume dispersal. Northeasterly （downwelling-favorable） winds drove fresh- water down-shelf while southeasterly （onshore） winds drove water up-shelf （in the sense of Kelvin wave propagation） , and were confined near the coast. Southwesterly （upwelling-favorable） and northwesterly （off： shore） winds transport more freshwater offshore. The transport flux is decomposed into an advection, a vertical shear, and an oscillatory component. The advection flux dominates the freshwater transport in the coastal area and the vertical shear flux is dominant in the offshore area. For the upwelling-favorable wind, the freshwater transport becomes more controlled by the advection transport with an increase in wind stress, due to enhanced vertical mixing. The relative importance of wind forcing and buoyancy force was investigated. It was found that, when the Wedderburn number is larger than one, the plume was dominated by wind forcing, although the importance of wind varies in different parts of the plume. The water column stratification decreased as a whole under the prevailing southwesterly wind, with the exception of the up-shelf and offshore areas.