Predicting submerged vegetation drag with a machine learning-based method

Accurate estimation of the drag forces generated by vegetation stems is crucial for the comprehensive assessment of the impact of aquatic vegetation on hydrodynamic processes in aquatic environments.The coupling relationship between vegetation layer flow velocity and vegetation drag makes precise prediction of submerged vegetation drag forces particularly challenging.The present study utilized published data on submerged vegetation drag force measurements and employed a genetic programming(GP)algorithm,a machine learning technique,to establish the connection between submerged vegetation drag forces and flow and vegetation parameters.When using the bulk velocity,U,as the reference velocity scale to define the drag coefficient,C_(d),and stem Reynolds number,the GP runs revealed that the drag coefficient of submerged vegetation is related to submergence ratio(H^(*)),aspect ratio(d^(*)),blockage ratio(ψ^(*)),and vegetation density(λ).The relation between vegetation stem drag forces and flow velocity is implicitly embedded in the definition of C_(d).Comparisons with experimental drag force measurements indicate that using the bulk velocity as the reference velocity,as opposed to using the vegetation layer average velocity,U_(v),eliminates the need for complex iterative processes to estimate U_(v)and avoids introducing additional errors associated with U_(v)estimation.This approach significantly enhances the model’s predictive capabilities and results in a simpler and more user-friendly formula expression.

Numerical study of submerged bending vegetation under unidirectional flow

Submerged vegetation commonly grows and plays a vital role in aquatic ecosystems,but it is also regarded as a barrier to the passing flow.Numerical simulations of flow through and over submerged vegetation were carried out to investigate the effect of vegetation density on flow field.Numerical simulations were computationally set up to replicate flume experiments,in which vegetation was mimicked with flexible plastic strips.The fluid-structure interaction between flow and flexible vegetation was solved by coupling the two modules of the COMSOL packages.Two cases with different vegetation densities were simulated,and the results were successfully validated against the experimental data.The contours of the simulated time-averaged streamwise velocity and Reynolds stress were extracted to highlight the differences in mean and turbulent flow statistics.The turbulence intensity was found to be more sensitive to vegetation density than the time-averaged velocity.The developing length increased with the spacing between plants.The snapshots of the bending vegetation under instantaneous velocity and vorticity revealed that flexible vegetation responded to the effects of eddies in the shear layer by swaying periodically.The first two rows of vegetation suffered stronger approaching flow and were prone to more streamlined postures.In addition,the origin of tip vortices was investigated via the distribution of vorticity.The results reveal the variation of flow properties with bending submerged vegetation and provide useful reference for optimizationofrestorationprojects.

HYDRAULIC RESISTANCE OF SUBMERGED VEGETATION RELATED TO EFFECTIVE HEIGHT

Submerged vegetation has a significant impact on water flow velocity.Current investigations include the impact through adding drag resistance and increasing bottom roughness coefficient,which cannot elucidate the characters of real submerged vegetation.To evaluate the effects of submerged vegetation on water currents at different velocities,a laboratory experiment was conducted using three kinds of vegetations.The effective heights of these vegetations on varying flow velocities were evaluated.An equation describing the relationship between the normalized resistance of the submerged plants and the Reynolds number based on the plant effective height was then established and used to calculate the hydraulic resistance parameters of submerged plants in different stages of growth.

A random walk simulation of scalar mixing in flows through submerged vegetations

The scalar transport phenomena in vertical two-dimensional flows are studied using the random walk method. The established Lagrangian model is first applied to study the idealized longitudinal dispersion in open channels, before being used to investigate the scalar mixing characteristics of the flows through submerged vegetations. The longitudinal dispersion coefficients of the fully-developed boundary layer flows, with and without vegetations, are calculated based on the positions of the particles. A convenient way of incorporating the effects of vegetations is proposed, where all the flow parameters are regarded to be continually distributed over the depth. The simulation results show high accuracy of the developed random walk method, and indicate that the new method of accounting for the vegetation effects is appropriate for all the test cases considered. The predicted longitudinal dispersion coefficients agree well with the measurements. The merit of the new method is highlighted by its simplicity and efficiency in comparison with the conventional method that assumes the discontinuous distribution of the flow parameters over the depth.

FACTORS INFLUENCING BENDING RIGIDITY OF SUBMERGED VEGETATION

The bending rigidity of submerged vegetation is closely related with vegetative drag force. This work aims at determining the effects of flow conditions and characteristics of vegetation on the bending rigidity of submerged vegetation. Based on the dimensional analysis method, the factors influencing the bending rigidity of individual submerged vegetation were analyzed. The relationship between the relative bending rigidity and its influencing factors was investigated by experimental observation, and a relative bending rigidity expression for submerged vegetation was obtained by means of multiple linear regression method. The results show that the submerged vegetation has three states under different inflow conditions, and the each critical relative bending rigidity of individual submerged vegetation was determined for the different states of submerged vegetation.

Influence of bending rigidity of submerged vegetation on local flow resistance

The different state of the submerged vegetation has different influences on the flow resistance. This paper explores the relationship between the state and the resistance of an individual submerged vegetation, and the relative bending rigidity of the submerged vegetation is determined by the state of the submerged vegetation. Based on the experimental observations, the state and the resistance of an individual submerged vegetation are analyzed under different inflow conditions. At the same time, the influences of the various submerged vegetations on the flow resistance are discussed under the same inflow conditions. Some interesting relationships are obtained between the flow resistance and the relative bending rigidity of the submerged vegetation, and it is shown that the flow resistance increases with the increase of the relative bending rigidity of the submerged vegetation, and they are positively correlated.

Submerged vegetation removal promotes shift of dominant phytoplankton functional groups in a eutrophic lake

Historical data indicate that the dominance of submerged plants in Dianchi Lake in the 1960 s was characterized by low algal density with dominance of non-toxic group J（Scenedesmus,Pediastrum,etc.）. The removal of submerged plants,which began in the 1970 s,resulted in the expansion of bloom-forming Microcystis（group M）. Laboratory experiments suggested that Microcystis aeruginosa was inclined to grow and develop at elevated temperatures. The growth of Scenedesmus obliquus was slower than that of co-cultivated M. aeruginosa in the absence of Ceratophyllum demersum,especially at higher temperatures. The existence of submerged plant C. demersum could inhibit the growth of the harmful algae M. aeruginosa and this inhibitory effect by C. demersum was enhanced with an increase in temperature. Instead,with C. demersum,the growth of S. obliquus was not inhibited,but the co-cultivated M. aeruginosa was eliminated in a short time. Combined with the historical data and laboratory experiments,it was indicated that the submerged plants might play important roles in the dominance of the non-toxic group J in the historical succession. Consequently,the introduction of the submerged plant such as C. demersum might alter the dominant phytoplankton functional groups from M to J and benefit the restoration of the eutrophic lake.

Incipient motion of sediment in presence of submerged flexible vegetation

The presence of submerged vegetation on river beds can change the water flow structure and alter the state of sediment motion. In this study, the incipient motion of sediment in the presence of submerged flexible vegetation in open channels was investigated in a laboratory experiment. The vegetation was simulated with flexible rubber cylinders arranged in parallel arrays. The effect of the vegetation density, water depth, and sediment grain size on the incipient motion was investigated. The experimental results indicate that the incipient motion velocity of sediment increases as the vegetation density decreases and the water depth and sediment grain size increase. With flexible plants, the incipient motion velocity of sediment is lower than it is without vegetation, and is larger than it is with rigid vegetation. A general incipient motion velocity equation was derived, which can be applied to both flexible and rigid vegetation conditions.

The influence of drag parameter on submerged vegetation flow simulation using a porous approach

In simulating vegetated flows using the porous approach,the reasonableness of the drag coefficient significantly impacts the calculation results.This study employs large eddy simulation(LES)to quantitatively investigate the effect of drag parameters on key flow characteristics in submerged vegetated flows.The results indicate that changes in the drag coefficient significantly alter the velocity in the middle of the vegetation layer and near the water surface in the free-flow layer.Compared with longitudinal velocity,the drag coefficient has a more pronounced effect on the vertical distribution of Reynolds stress,especially its peak at the top of the vegetation layer.The porous approach can accurately reproduce the vertical distribution of longitudinal velocity and Reynolds stress,consistent with experimental measurements,only when shear-scale flow dominates.Due to the high-intensity secondary flow under moderate vegetation density,fluctuations in the drag coefficient have a more significant impact on the numerical results than in very dense vegetation.Therefore,selecting the drag coefficient value should be done cautiously,especially in the absence of experimental measurements for validation.

Characteristics of turbulent flow in 3-D pools in the presence of submerged rigid vegetation in channel bed

In this study,the interaction between 3-D bedforms and submerged rigid vegetation has been investigated.Various laboratory experiments were conducted to study the distribution of flow velocity,Reynolds shear stress,turbulent kinetic energy,and skewness coefficients for a constant density of vegetation.Results showed that the velocity profile in the pool section deviates from those in the upstream section of the pool.It has been found that the dip parameter varied between 0.6H and 0.9H depending on various factors including bed roughness,vegetation distribution,and pool entrance/exit slopes.However,scattered vegetation in the pool and differences in slopes created non-uniform flow conditions.Also,in the wake region behind each vegetated element,flow velocity reduced significantly,and small-scale eddies are formed,causing increased perturbations.By decreasing the entrance slope and bed roughness,relatively uniform flow and weaker turbulence was resulted,but the random distribution of vegetated elements counteracted this balance and intensified turbulence.With the decrease in the pool entrance slope,the contribution of sweep event decreased and the contribution of ejection event increased.

MATHEMATICAL MODEL FOR THE FLOW WITH SUBMERGED AND EMERGED RIGID VEGETATION

The article summarizes previous studies on the flow in open channels with rigid vegetation, and constructs a mathematical model for submerged and emerged rigid vegetation. The model involves the forces balance in the control volume in one-dimensional steady uniform flow. For submerged vegetation, the whole flow is divided into four regions： external region, upper vegetated region, transition region and viscous region. According to the Karrnan similarity theory, the article improves the mixing length expression, and then gives an analytical solution to predict the vertical distribution of stream-wise velocity in the external region. For emerged vegetation, the flow is divided into two region： outer region and viscous region. In the two circumstances, the thicknesses of each region are determined respectively. The comparison between the calculated results and our experimental data and other researchers＇ data proves that the proposed model is effective.

A long term calibration and verification of a submerged aquatic vegetation model for Lake Okeechobee

Introduction:Submerged aquatic vegetation(SAV)has multiple functions in Lake Okeechobee.It provides critical habitat for fish and wildlife,stabilizes sediments,reduces phosphorus(P)concentration in the water column by preventing re-suspension of P-rich sediments,and provides a substrate for attached algae,which also helps to remove P from the water column.Ten year water quality and SAV growth simulations are presented and compared with observed SAV and water quality data collected in the nearshore zone in Lake Okeechobee.Methods:The SAV theory and approach used in the LOEM are modified from the Chesapeake Bay model and incorporate three state variables:shoots(above the bed sediment),roots(in the bed sediment),and epiphytes(attached to the shoots).The SAV model has direct linkages with the water quality model,including(1)a link between the growth and decay of SAV and the nutrient pool of the water quality model;(2)a link between the photosynthesis and respiration of SAV and dissolved oxygen dynamics,and(3)the ways in which settling of particulate organic matter and nutrient uptake affect nutrient levels in the water column and in the sediment bed.Results:Total suspended solids affect light attenuation and are another major driving factor for SAV growth in the nearshore and littoral zone area.The model performs reasonably well in reproducing the spatial distribution of SAV.Conclusions:The theoretical analysis and model sensitivity tests indicate that SAV growth is primarily controlled by light and nutrients.The light available for SAV growth depends on the water depth and the turbidity.In this full scale simulation,the water depth comes from the LOEM hydrodynamic model,and the turbidity depends on the suspended sediment concentration and algal concentration.

A 3-D numerical simulation of the characteristics of open channel flows with submerged rigid vegetation

This paper applies the Flow-3D to investigate the impacts of different flow discharge and vegetation scenarios on the flow velocity (including the longitudinal, transverse and vertical velocities). After the verification by using experimental measurements, a sensitivity analysis is conducted for the vegetation diameter, the vegetation height and the flow discharge. For the longitudinal velocity, the greatest impact on the flow structure originates from the vegetation diameter, rather than the discharge. The vegetation height, however, determines the inflection point of the vertical distribution. Comparing the transverse velocities at two positions in the vegetated area, i.e., the upstream and the downstream, a symmetric pattern is identified along the water depth. The same pattern is also observed for the vertical velocity regardless of the flow or vegetation scenario, including both transverse and vertical fluid circulation patterns in the vegetated area. Moreover, the larger the vegetation diameter is, the more evident these patterns become. The upper circulation occurs near the vegetation canopy. These findings regarding the circulations along the transverse and vertical directions in the vegetated region shed light on the 3-D flow structure through the submerged vegetation.

Twenty-first century climate change and submerged aquatic vegetation in a temperate estuary:the case of Chesapeake Bay

Introduction:The Chesapeake Bay was once renowned for expansive meadows of submerged aquatic vegetation(SAV).However,only 10%of the original meadows survive.Future restoration effortswill be complicated by accelerating climate change,including physiological stressors such as a predicted mean temperature increase of 2-6℃and a 50-160%increase in CO_(2)concentrations.Outcomes:As the Chesapeake Bay begins to exhibit characteristics of a subtropical estuary,summer heat waves will become more frequent and severe.Warming alone would eventually eliminate eelgrass(Zostera marina)from the region.It will favor native heat-tolerant species such as widgeon grass(Ruppia maritima)while facilitating colonization by non-native seagrasses(e.g.,Halodule spp.).Intensifying human activity will also fuel coastal zone acidification and the resulting high CO_(2)/low pH conditions may benefit SAV via a“CO_(2)fertilization effect.”Discussion:Acidification is known to offset the effects of thermal stress and may have similar effects in estuaries,assuming water clarity is sufficient to support CO_(2)-stimulated photosynthesis and plants are not overgrown by epiphytes.However,coastal zone acidification is variable,driven mostly by local biological processes that may or may not always counterbalance the effects of regional warming.This precarious equipoise between two forces-thermal stress and acidification-will be critically important because it may ultimately determine the fate of cool-water plants such as Zostera marina in the Chesapeake Bay.Conclusion:The combined impacts of warming,coastal zone acidification,water clarity,and overgrowth of competing algae will determine the fate of SAV communities in rapidly changing temperate estuaries.

Native Aquatic Plant Establishment Efforts in a High-Herbivore, Central Texas Reservoir

Maintaining beneficial, native plant structure and diversity while reducing invasive, nuisance species dominance is an important management domain for natural resource managers. One such vegetation component in North American lakes and reservoirs is submerged aquatic vegetation—a valuable aquatic resource which serves as productive habitat for fish, aquatic macroinvertebrates, and other wildlife. Reservoirs in the southern parts of the United States have experienced varying aquatic plant dominance dynamics due to historical water resource management actions, including drawdowns and introduction of herbivorous fish for the purpose of controlling invasive aquatic vegetation. Some of these management options have also been detrimental to native submerged aquatic vegetation. This paper explores an adaptive management research effort by installing herbivore-protected, fenced-pen submerged aquatic vegetation sites in a high-herbivore reservoir to determine effectiveness of protecting habitat and serving as founder colony sources for propagule spread. Four experimental sites with three management treatments each were planted with American eelgrass. Each site utilized one un-fenced treatment and two treatments with varying mesh sizes for protective fencing-pens. Site integrity, species survival and spread, and grazing were documented. One additional site was installed and planted with other native submerged aquatic vegetation species for nominal species performance descriptions. No plants survived unprotected in the high-herbivore system and plants, in general, performed consistently better within the smaller mesh size. These test planting results were ultimately used to inform adaptive management decision making for plant installation and expansion designs for managing reservoirs invested with Hydrilla, considered one of the most serious invasive aquatic plants in the United States.