effect of flow regime on accelerated recruitment …

E-proceedings of the 38th IAHR World CongressSeptember 1-6, 2019, Panama City, Panama

doi:10.3850/38WC092019-0413

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EFFECT OF FLOW REGIME ON ACCELERATED RECRUITMENT AND ESTABLISHMENT OF VEGETATION IN UNREGULATED SANDY RIVERS – A CASE

STUDY AT NAESEONG-CHEON STREAM IN KOREA

CHANJOO LEE (1), HYOSEOP WOO (2) CHANG-LAE JANG (3)

(1)Department of Water Resources and Environmental Research, Korea Institute of Civil Engineering and Building Technology, South Korea, [email protected]

(2)School of Earth Science and Environmental Engineering, Gwangju Institute of Science and Technology, South Korea

[email protected] (3)Department of Civil Engineering, Korea National University of Transportation, South Korea, [email protected]

ABSTRACT

This study investigates possible causes for accelerated vegetation recruitment and growth in an unregulated sandy stream in Korea. Flow and sediment regime, represented here by critical bed shear stress, is considered a dominant physical factor affecting the stability of sediment bars with vegetation at germination and seedling periods. A good inverse correlation is obtained between the area of vegetation expansion on sediment bars and the annual peak flow during early summer (June and July) where other minor effects seem not to be important. Three comparative reaches are selected in a stream; one strongly meandering reach, one moderate, and one mildly meandering reach. Hydraulic calculations are made for the whole stream reaches using a verified numerical model to evaluate two-dimensional distribution of the critical bed shear stresses at those reaches. Results are compared with past aerial photos of the study stream. Other critical physical factor of soil moisture is considered not to be limiting to the present case of sandy stream. The results are relatively in good agreement between the calculated distribution of bed shear stress and actual distribution of vegetation on the riparian sediment bars, indicating that stability of riparian sediment bars would be critical for vegetation recruitment and growth.

Keywords: vegetation recruitment, critical bed shear stress, flow and sediment regimes, meander, unregulated stream

1 INTRODUCTION Rivers in the world including Korea are being changed, to different extent in different region, with or without

human impacts, from “white” rivers to “green” rivers, i.e., from the rivers with white sand and gravel bars to those with green vegetation. This phenomenon is especially prevailing in Korea (Choi et al, 2005; Egger et al, 2012; Woo et al, 2014) and also in Japan (Asaeda et al, 2013), both of which are located at a similar latitude and commonly affected by monsoon climate. Those changes causes engineering problems of decrease in river safety as well as ecological impacts (Gurnell, 2014).

Since Williams and Wolman’s pioneering study about 35 years ago (Williams and Wolman, 1984), accelerated recruitment and establishment vegetation on the riparian bottomland along the streams have been investigated by many researchers, too many to mention all here. Instead, a few recent review studies of the interactions among hydro-geomorphology and vegetation, including possible causes for the phenomenon of ‘from white river to green river’, are introduced here.

Osterkamp and Hupp (2010) reviewed literature of the investigations, mostly in the USA, that had considered vegetation to gain an increased understanding of bottomland fluvial-geomorphic processes. They especially reviewed extensively investigations of the vegetation-hydro-geomorphological interactions of regulated streams. They concluded that in temperate fluvial system, water, either through streamflow conditions or groundwater availability, is the most proximal control of distribution patterns of perennial riparian vegetation. The likelihood of a species growing on an alluvial surface, they suggest, depends on 1) the suitability of surface for germination and establishment, and 2) the ambient environmental site conditions that permit persistence at least until reproductive age.

Gurnell et al (2016) proposed a conceptual model of vegetation-hydro-geomorphology interactions and feedbacks within river corridors. Their model is based on the five considerations, including incorporating hydro-geomorphological constraints on river corridor vegetation, defining five dynamic river corridor zones from the center of channel to the edge of floodplain, and so on. As hydro-geomorphological constraints, varying with different spatial scale such as from region to reach, they suggest three factors; climate, moisture availability and fluvial disturbance.


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Solari et al (2016) reviewed advances on modelling riparian vegetation-hydro-geomorphology interactions, in which ecological processes, including seed dispersal, plant survival, growth, succession and mortality, are emphasized. They state that, probably because of the complexities of such processes, conceptual models are still widely employed in spite of the developments of the models investigating the vegetation processes in riparian systems influenced by hydro-morphological processes. They indicate that common factors influencing riparian vegetation processes are, among others, flow regime including water table level, channel morphology, and nutrients.

Narrowing to the phenomenon of ‘’white river to green river’, an accelerated vegetation recruitment and growth on bare bottomland, i.e., riparian sediment bars, Woo et al (2016) tried to categorize, for temperate monsoon climate regions including Korea, possible causes for this phenomenon encompassing both regulated and unregulated stream. Focusing on the recruitment and growth of vegetation on bare riparian bars, they categorized the causes into three: Changes in flow and sediment regimes, with or without human impacts; artificial modifications of channel and floodplain, such as gravel mining, channelization, riparian land-use change, and construction of low-head dam (weir); and influx of nutrients of N especially in the form of non-point source pollutants into the streams in urban and agricultural areas. The original categorization can be slightly modified here to include unregulated streams as follows:

Pattern 1: flow and sediment regime changes- Pattern 1-1N: spring flood in regulated stream, supplying soil moisture and fresh sediment, is

critical to the germination and seedling growth of riparian vegetation.- Pattern 1-1P: decrease in the magnitude and frequency of late spring/early summer flood in

unregulated stream, due probably to climate change, is critical to facilitate vegetation recruitmentand growth.

- Pattern 1-2: summer flood in regulated stream, producing large bed shear stress in the river, iscritical to the survival of the growth and stability of riparian vegetation

Pattern 2: direct artificial changes in riverbed and floodplain such as gravel mining, channelization, and

construction of low-head dams (weirs)

Pattern 3: increase in nutrients in river water, due mainly to increase in non-point pollutants influx,

accelerating vegetation growth, especially growth of herbaceous plants

This study focuses on Pattern 1-1P to explain accelerated vegetation recruitment and growth by vegetation-hydro-geomorphological interaction. In this study, flow and sediment regimes, represented here by the critical bed shear stress, is considered the dominant physical factor affecting the stability of sediment bars with vegetation recruitment especially during specific periods of seedlings after germination in Korea. For this study, a sandy stream in Korea without human impacts, such as damming and modifications of stream channel and floodplain, is selected. A set of three reaches are selected in the study stream for comparison.

2 HYDRO-GEOMORPHOLOGICAL AND VEGETATIVE CHARACTERISTICS OF THE STUDY STREAM The Naeseong-cheon Stream, selected as a case study stream, is a tributary of the Nakdong-gang River,

the longest river flowing through the southeast of Korea and merging the South Sea at port city of Busan. As shown in Fig.1, the stream itself locates at north of the river basin with its length of 108.2 km and its basin area of 1,815 km2. The stream is a typical sandy stream in Korea having its median bed material size of about 1.6 mm. With its channel slope of 9/10,000~1.7/1,000, river valley of the stream is relatively narrow with its ratio of

valley width over channel width being 1.5~3.0 and transverse movements of the channel are restricted. The

ratio becomes smaller at meandering channel reaches as compared with the ratio at straight channel reaches.The annual average temperature at the basin is 11.4°C and annual average precipitation is 1,231 mm. The

annual precipitation increased slightly until the early 2000s and since then has rather decreased slightly. The stream itself is considered unregulated stream although a dam was constructed upstream in the late 2000s, for any water impoundment has not been made since the completion of dam. Two-year return period flood of the stream at the river mouth is 690 m3/s at daily average.

2.1 Recent Recruitment and Growth of Vegetation in the Stream Naeseong-cheon Stream could be one of the best examples of changing its vegetative characteristics from

“white” rivers to “green” rivers, i.e., from the rivers with white sand and gravel bars to those with greenery bars along the narrow stream corridor. Fig. 2 is a set of the aerial photos of the mid- to lower- reaches of the stream from 1980 to 2016 spanning about 35 years. As shown in this figure, vegetation recruitment seems to have started around in 1987, and in 2016 almost all the stream corridors are covered with vegetation except the narrow aquatic zone of stream channel. Dominant vegetation in these photos is runner reed (Phragmites japonica) and Persicaria nodosa (Pers.) Opiz with patches of willows.


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Figure 1. Location map of the study stream and reaches (A small map at upper left for Korea, the left large map for Naeseong-cheon Stream, and the right map for three study reaches)

Figure 2. Temporal changes in the stream corridor of Naeseong-cheon Stream (Lee et al, 2018)

Quantitative analysis of Fig. 2 indicates that in 1965 the area of aquatic zone was 48% of the total riparian corridor (within man-made levees along the both sides) and 50% was the area of bare sand bars without vegetative areas. In 2016, however, the area of aquatic zone reduced to 27% and the area of bare sand bars reduced to 22% while rest of the area is covered with vegetation. As reviewed in the previous chapter, common factors influencing riparian vegetation processes are, among others, flow regime including streamwater table which directly affects nearby groundwater table, channel morphology, and nutrients. In this study, we only focus on the factor of flow regime represented with flow discharge, temperature, and precipitation excluding other relevant factors. Stream morphology has not changed much except that the channel has been narrower and lowered. Nutrients of N and P in the stream water is considered not to have changed much.

The whole study stream is divided into 4-km long segments and each segment is considered as an individual data point. Among various combinations of the independent influencing variables and area of vegetative covers, as shown in Fig. 3, the correlation between the annual peak discharge during June and July and the area of vegetative covers appears to be most relevant to each other with the regression coefficient of 0.73.


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Figure 3. Correlation between peak discharge of June to July (LQjjk) and area (AC) of vegetative covers at the study stream (Lee et al. 2018)

This result implies that early summer floods occurring in June in monsoon-affected Korea has been generally decreased in recent years of 1990s through 2010s in its magnitude and frequency, rendering a favorable condition for the growth of target vegetation in June and July.

3 HYDRAULIC MODELLING OF VEGETATION EXPANSION In this study, we investigate the effects of hydro-geomorphology on the vegetation recruitment and growth

in a quantitative manner using a numerical simulation. More specifically, we investigate the effect of channel morphology on flow and thus vegetation recruitment using distinctively different plane geometries of channel, i.e., a channel reach with a large meander and large change in channel width and another channel reach witha small meander and small change in channel width. For comparison, we adopt one more in between.

3.1 Numerical model We use the Nays2D model, which is loaded on iRIC model (Shimizu et al, 2014). The model utilizes the

boundary-fitted coordinates especially fitting irregular boundaries like the stream in this study. In this model, the CIP (Cubic Interpolated Psuedoparticle) method is utilized for the advection term at staggered grid and the Central Difference Method is utilized for the diffusion term. Water and sediment discharges are used as upstream boundary conditions, while normal depth is adopted as downstream boundary condition. Velocities vertical to stream bank is assumed zero and slip-condition is assumed for the velocities in the flow direction on it.

3. 2 Application of numerical modelA set of three different reaches are selected in order to quantitatively investigate the hydro-

geomorphological effects on the vegetation recruitment and growth. As shown in the right side of Fig. 1, Reach 1 shows a sharply-curved meander with a wide riparian bottomland, Reach 2 shows a mildly-curved meander with a narrow bottomland, and Reach 3 shows a moderately-curved meander with a narrow bottomland.

Reach 1 has the sinuosity of 2.4 with stream width ranging 149 m~429 m and width ratio (max width / min

width) of 2.9, Reach 2 has the sinuosity of 1.4 with stream width ranging 156 m~218m and width ratio of 1.4,

and Reach 3 has the sinuosity of 1.7 with stream width ranging 110 m~183 m and width ratio of 1.7. Fig. 4 shows variation in daily discharge at the study stream from 1999 to 2017, measured at Wolpo Station

in the stream. As shown in this figure, peak discharges have reduced in general with a drastic reduction in 2012 and another in 2015. In this study, therefore, we adopted three representative discharges used for calculations, the average annual peak discharge throughout the whole years, the average annual peak from 2012 to 2017 and the annual peak in 2015. They are shown in Table 1.

able 1. Flow conditions for numerical calculations

Case Discharge

(m3/s)

Mean diameter of

sediment (mm) Remarks (year)

Run-1 45 1.61 2015

Run-2 498 1.61 2012 ~ 2017

Run-3 1,297 1.61 1999 ~ 2017

2 3 4 5 6 7 8 9 10

LQJJK

-200

-100

0

100

200

300

400

AC

(1

,00

0 m

2)

Adj. R2 = 0.730


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Figure 4. Daily hydrograph at Wolpo Station and representative peak discharges for calculations

In this simulation, the total length of the study reach is 10 km and the average channel slope is 0.0012. The average grain size is 1.61 mm and Manning’s coefficient n is evaluated 0.031. The calculation time step is set 0.01 second. Proper composition of calculation grids is essential in this kind of calculation having large irregularities of channel boundary. The cell length is set 0.2 m in the flow direction and 0.1 m in the transverse direction.

3.3 Results Fig. 5 shows velocity distributions at the study stream with different discharge conditions, Run-1, Run-2, and

Run-3. At all the three Runs, in general, flows are faster in straight channels with relatively narrow channel width, while they are slower in curved channels with broader channel width. At Reach 1 (circled in the figure), the entrance of the curved reach is narrow and directs toward the outer side of the curved reach causing a strong impact on the outer-side at the immediate downstream. This causes relatively larger velocities near the outer bank and smaller ones near the inner shore, thus causing large deposition on the inner side of the curved reach. Reaches 2 and 3, however, show a normally curved channel entering the curved section tangentially toward the curved reach with a relatively uniform channel width. This causes relatively smaller velocities at the outer banks and larger ones at the inner shores.

Many previous studies (Egger et al 2012, Woo et al 2014) on the stability of vegetation at recruitment and growth period have used dimensionless shear stress,τ∗, or Shields number for the criterion of vegetation death (burial and/or washing away). It is written mathematically as

τ∗ = 𝜏0

(𝛾𝑠−𝛾)𝐷 [1]

where γ is the unit weight of water, 𝛾𝑠 is the unit weight of sediment, D is the sediment size, and 𝜏0 is the bed shear stress. The bed shear stress 𝜏0 is written as

τ0= γ d S [2]

where d is the water depth and S is the energy slope of flow. It has been known that, without other specific conditions such as climate and soil moisture that are critical

to the survival of riparian vegetation seedling, in general, physical stability of bar surface is a dominating factor to survival (Egger et al 2012, Woo et al 2014). In this study, soil moisture, still critically important for germination and growth of vegetation in most cases, is assumed to be well maintained slightly up to above the horizontal plane of stream water level because of the capillary rise in the sand and gravel sediment bars. In order words, it would be critical, but not limiting in this study.

As a criterion on the physical stability of riparian bar surface, the dimensionless bed shear stress of 0.05 at Shields curve can be used. Below that number, the bar surface can be considered fully stable without motion, while well above the number, it can be unstable, and seeds and seedlings are subject to burial and/or washing-


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away. In reality, no proper value or range of the values for Shields number has been suggested yet for the riparian vegetation stability.

Figure 5. Flow velocity distribution at each run (Compare the pattern of flow velocity distribution at Reach 1 with those at Reaches 2 and 3)

Fig. 6 shows distributions of Shields number at each Run in the study stream. Here again, Run-1 represents the average peak flow condition in the early summer in 2015, while Run-2 represents that since 2012, while Run-3 represents that throughout the whole study year. Run-1 in Fig. 6 shows most reaches are in dark blue implying that Shields number remains around the critical value of 0.05 except a few deep areas mostly along the center of the channel. Run-2 shows more areas in blue and green. Some areas in yellow and red indicate deep areas along the center line of the stream. At Run-3, however, most areas in the whole reaches show green, yellow and red with a few exceptions.

Fig. 7 shows the aerial photos of the study stream taken in 2005 (left) and 2015 (right), respectively. Based on the simulation result on the distribution of Shields number as shown at Run-3 (rightmost figure) in Fig. 6, no vegetation growth is expected in the study stream since the whole reaches show above dark blue, except at few reaches where a corresponding strip of vegetation appear as shown in the left side of Fig. 7. The two patches circled in the figure seem to have been caused by man-made disturbances.

On the other hand, observing Runs -1 and -2 (leftmost and center figures) in Fig. 6 reveals that most sediment bars along the channel would be covered with vegetation, except at the aquatic areas at normal water level. This result agrees basically with the right figure (taken in 2015) in Fig. 7. Areas of the patches encircled with elliptic in the figure are identical to the dark blue areas in Runs-1 and -2 in Fig. 6, while areas of the bare sand bars encircled with circle are all identical to the areas of above light blue (green, yellow and read) in the right most figure in Fig. 6. Again, in this comparison, aquatic areas at normal water level with above light blue are not considered.

Figure 6. Distributions of Shields number at each Run (from left Runs- 1, 2 and 3)

Fig. 8 show magnified views of Reaches 1 and 3 at the right figure (taken in 2015) in Fig. 7. Comparing these with the left most figure, representing Run-1, in Fig. 6, distribution of vegetation patches is almost in agreement with that of dark blue areas. This result can indicate that, at least in this case study of a meandering


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sandy stream with large sandbars along the channel, stability of riparian areas, represented with the critical bed shear stress, would be a dominant factor to the recruitment and growth of riparian vegetation.

Figure 7. Aerial photos of the study stream (left: taken in 2005, right: taken in 2015)

Figure 8. Aerial photos of Reaches 1 and 3 (Both photos taken in 2015)

Fig 9 is the Shields diagram modified (Garcia, 2007) with the three Runs plotted inside. Run-1 indicates a bed-load dominant flow, Run-2 indicates a flow between bed-load and suspended-load flows, while Run-3 indicates a suspended-load dominant flow. As expected, parts of Run-2, which are site-depending, and Run-3, all locating well above the no-motion line in this figure, show blue, green, yellow, and red in color in the rightmost figure in Fig. 6. They are in the state of unstable bar surface with no recruitment and growth of vegetation expected. Run-1, on the other hand, locates well below the suspension line in Fig. 9, which implies a possible recruitment and growth of vegetation.

4. CONCLUSIONSBased on the regression analysis and numerical analysis of the flow-sediment-vegetation interactions at

an unregulated sandy stream, we can conclude, at least for the stream adopted as a case study here, as follows:

- A good inverse correlation between the area of vegetation expansion on riparian sediment bars and theannual peak flow during early summer (June and July) obtained in this study implies that early summerfloods in monsoon-affected Korea would a dominant factor directly affecting the survival of vegetation onbare riparian bars.

- Flow and sediment regime, represented here by critical bed shear stress, would be the dominant physicalfactor affecting the stability of sediment bar surfaces with vegetation at germination and seedling periods,in case that other factors such as climate, soil moisture, and nutrients are less critical, which is the case ofthis study.


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- In such case, the dimensionless critical bed shear stress, expressed by Shields number, can be a usefulmeasure to predict the recruitment and growth of riparian vegetation on riparian sediment bars.

- Planimetric changes of the channel affect the recruitment of vegetation on riparian sediment bars in alluvialchannel. A high Shields number at the outer bends and at the narrow channel leads to the uprooting ofvegetation seedlings in early summer by bed and bank erosion, while a relatively low Shields numbergenerated at the bend entrance and point bars on the inner side of meandering reaches tends to lead toburial of vegetation.

Figure 9. Dominant sediment regimes of each reach at each Run plotted in Shields diagram (Garcia, 2007)

ACKNOWLEDGEMENTS The data used for this study were obtained from the first author’s study titled “Analysis of channel changes

before- and after- construction of hydraulic structures and performed at Korea Institute of Civil Engineering and Building Technology (KICT) in Korea during 2012~2019. The second authors’ study on the “Development of high-performance levee technology using a new bio material”, #17AWMP-B114119-02, supported by Korea Agency for Infrastructure Technology Advancement (KAIA) has helped to understand the interaction processes between the flow/sediment and riparian vegetation including on stream bank, bottomland, and levee.

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vegetation cover in the Hwang River, Korea, River Research and Application, 21, 15-325. Egger, G. et al. (2012). Dynamic vegetation model as a tool for ecological impact assessments of dam operation,

Journal of Hydro-environment Research, 6, 151-161. García, M.H., edited. (2007). Sedimentation engineering: processes, measurements, modeling and practice,

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Woo, H., Kim, J.-S. Cho, K-H., and Cho, H.-J. (2014). Vegetation recruitment on the ‘white’ sandbars on the Nakdong River at the historical village of Hahoe, Korea, Water and Environment Journal, 28, 577-591.

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