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Flood processes, forest dynamics, and disturbance in the Congaree River floodplain, South Carolina

ProQuest Dissertations and Theses, 2011
Dissertation
Author: Kimberly Michele Meitzen
Abstract:
Southeastern bottomland hardwood ecosystems thrive as a result of the biophysical linkages between the river and the floodplain. These ecosystems contain significant habitat heterogeneity and support rich diversity of species, but have been altered and manipulated by anthropogenic impacts. This study was designed to examine the forest development dynamics in abandoned meander landforms on the Congaree River floodplain, South Carolina in order to gain a better understanding of the forest successional responses to environmental gradients and historic logging disturbances. This study presents the innovative application of a 2D hydrodynamic flood model for simulating complex overland flow processes and quantifying hydrogeomorphic environmental data for use in ecological analysis. The 2D flood model is developed using high resolution terrain data and historical flows. The flood simulation model measures flood extent, depth, frequency and flood water retention capacity. These data were incorporated with structural and compositional forest surveys, additional environmental data, and historic logging information to compare forest patterns and other factors controlling succession in abandoned meander landforms. Forest development patterns varied as a function of the type of logging disturbance, hydrogeomorphic conditions, and environmental change. Three distinct successional patterns emerged that were linked with the following disturbance history: unlogged old-growth forests, selectively logged forests, and clear cut forests. Forest succession in old-growth environments exhibited a gradual transition from obligate wetland species to facultative species. In contrast, recovery patterns in clear-cut forests exhibited complete replacement of the original obligate wetland dominated community to a predominantly facultative community. Selectively logged sites showed the least amount of successional change, but they lacked recovery of the logged, Taxodium distichum component of the forest, which was replaced almost exclusively by Nyssa aquatica . Processes and patterns evidenced in the Congaree River floodplain provide a glimpse of the various controls and successional pathways that produce heterogeneous forest patterns. A 2D flood model proves to be a valuable tool for modeling complex overland flow processes for the Congaree floodplain and has many ecological applications. Flood and forest patterns revealed by this study can be used to help guide or redirect restoration efforts to match current or desired environmental conditions.

TABLE OF CONTENTS

Copyright……………………………………………………………………………….…ii

Dedication

.......................................................................................................................... iii

Acknowledgements

............................................................................................................ iv

Abstract

............................................................................................................................... v

List of Tables

..................................................................................................................... ix

List of Figures

..................................................................................................................... x

1

Chapter One: Study Background

............................................................................ 1

1.1 Southeastern Bottomland Hardwood Riparian Ecosystems

................. 1

1.2 Research Questions

............................................................................... 5

1.3 Congaree Floodplain Vegetation Studies .............................................. 7

1.4 Framework for Dissertation Design

...................................................... 9

2

Chapter Two: Two Dimensional Hydrodynamic Flood Inundation Modeling for

the Congaree River Floodplain ............................................................................. 11

2.1 Introduction: Flood Processes and Modeling ..................................... 11

2.2 Geographic Setting: Valley Hydrology and Geomorphology

............ 13

2.3 Methods: Flood Inundation Modeling ................................................ 21

2. 4 Results: Flood Simulation and Validation

......................................... 44

2.5 Discussion: Flood Model Applications to Ecosystem Processes

........ 48

2.6 Summary and Conclusions

................................................................. 53

3

Chapter Three: Abandoned Meander Forest Development Patterns in a Large

Southeastern Floodplain ........................................................................................ 55

3.1. Introduction: Bottomland Fo rests and Anthropogenic Impacts

......... 55

3.2 Geographic Setting: Congaree River Floodplain ................................ 62

3.3 M ethods ............................................................................................... 66

3.4 Results:

................................................................................................ 87

3.5 Discussion: Multivariate Controls on Forest Patterns

...................... 102

viii

3.6 Conclusions

....................................................................................... 114

4

Chapter Four: Conclusions and Future Research ................................................ 116

4.1 2D Flood Model Application ............................................................ 116

4.2 Abandoned Meander Forest Development

....................................... 118

4.3 Summary of Research Contributions

................................................ 122

References: Chapter One

................................................................................................ 123

References: Chapter Two

................................................................................................ 126

References: Chapter Three

.............................................................................................. 132

References: Chapter Four

............................................................................................... 138

ix

List of Tables

Table 2.1: Comparison between actual floodplain elevations and modeled DTM.

......... 30

Table 2.2: Manning’s n

roughness coefficients

............................................................... 31

Table 2.3: USGS hydrologic gages for the Congaree and Wateree Rivers.

.................... 33

Table 2.4: Congaree River (USGS 2169500) flood frequency curve data

...................... 38

Table 2.5: Wateree River (USGS 2148000) flood frequency curve data.

....................... 40

Table 2.6: Comparisons between actual and modeled maximum flood depths.

.............. 47

Table 3.1: Soil series classifications.

............................................................................... 71

Table 3.2: Corresponding discharge and percent exceedance

......................................... 78

Table 3.3: Plot - based environmental da ta and gps waypoint locations

........................... 85

Table 3.4: Species list with relative ecological information.

........................................... 88

Table 3.5: Pearson’s correlations showing relationship of continuous variables.

........... 91

Table 3.6: Species correlations with ordination axes..

..................................................... 91

Table 3.7: MRPP results for categorical variables.

.......................................................... 92

Table 3.8: Indicator Species Analysis for categorical flood variables.

............................ 95

Table 3.9: Pearson’s correlations between continuous variables.

.................................... 98

Table 3.10: Species correlations with ordination axes.

.................................................... 98

Table 3.11: MRPP results for categorical variables.

........................................................ 99

Table 3.12: Indicator Species Analysis for structural category.

.................................... 102

x

List of Figures

Figure 2.1: Major sub - basins of the greater Santee River Basin

..................................... 14

Figure 2.2: 1940- 2010 Raster hydro graph of daily mean flows.

..................................... 16

Figure 2.3:

Aerial imagery of flood modeling study area

................................................ 17

Figure 2.4: Topographic cross section of the Congaree River floodplain.

...................... 19

Figure 2.5: Terrain Model (DTM) of Congaree and Wateree floodplain..

...................... 29

Figure 2.6: Annual peak flow (1939 to 2010), Congaree River (USGS 2169500).

........ 34

Figure 2.7: Annual peak flow (1939- 2010), Wateree River (USGS 2148000).

.............. 35

Figure 2.8: Congaree River (USGS 2169500) annual flood frequency

analysis

............. 37

Figure 2.9: Wateree River (USGS 2148000) annual flood frequency analysis.

.............. 39

Figure 2.10: Hydrograph of modeled flood event

........................................................... 41

Figure 2.11: Measuring flood depth in a crevasse channel.

............................................. 42

Figure 2.12: Flood inundation maps for two different time steps

.................................... 45

Figure 2.13: Comparison of modeled depths and actual depths

...................................... 46

Figure 3.1:Model of meander abandonment, infilling, and forest succession:

................. 57

Figure 3.2: Field site locations are represented by the dots.

............................................ 68

Figure 3.3: Soil classifications for each field site location.

............................................. 71

Figure 3.4: Flood inundation map of the 1- year recurrence interval flood event.

........... 74

Figure 3.5: Cumulative flow duration curve of Congaree River flows

........................... 77

xi

Figure 3.6: Logging history for each field site location.

.................................................. 80

Figure 3.7: NMS ordination illustrating species relationships

......................................... 90

Figure 3.8:NMS ordinations with fie ld sites

..................................................................... 93

Figure 3.9: NMS ordination showing configuration of species

....................................... 97

Figure 3.10: Ordination with successional vectors

........................................................ 101

Figure 3.11: Percent of species cover.

........................................................................... 107

Figure 3.12: Old- growth T. distichum dominating the canopy.

..................................... 109

Figure 3.13: Selectively cut site with second - growth regeneration

............................... 111

Figure 3.14: Clear cut second- growth regeneration.

...................................................... 112

1

1

Chapter One: Study Background

1. 1

Southeastern Bottomland Hardwood Riparian Ecosystems

Southeastern bottomland hardwood ecosystems coevolved with the formation of al luvial floodplains and terraces in humid- temperate climates of the United States. Their geographic range extends along the Coastal Plain of the Gulf of Mexico from eastern Texas to Florida, up the Mississippi, Missouri and Ohio River valleys, and up the At lantic Coastal Plain from Florida to Virginia (Küchler 1964; Cowardin et al. 1979; Sharitz and Mitsch 1993) (Figure 1.1 ). Southeastern bottomland ecosystems support a

heterogeneous assemblage of microhabitats and a rich biodiversity of species adapted to varied environmental conditions, gradients, and disturbances.

Regional - scale physiographic and climatic variables controlled the sedimentary and hydrologic conditions that formed the alluvial floodplain river valleys and provided a t emplate for the establishment of these unique bottomland ecosystems. Ongoing processes of floodplain development, natural disturbances, biophysical interactions, and anthropogenic impacts

control the local - scale community - based bottomland ecosystem dynamic s (Sharitz and Mitsch 1993; Saucier 1994; Hodges 1997, King et al. 2009; Osterkamp and Hupp 2010).

2

Figure 1.1 : Geographic extent of bottomland hardwood ecosystems. Potential natural vegetation in major alluvial river valleys (Sharitz 1993, adopted from Küchler 1964).

The potential geographic extent of bottomland ecosystems is much greater than their actual coverage on account of land use and land cover conversions away from forested environments. Estimates suggest that the pre - European extent of bottomland forests covered nearly four million hectares, today less than one million hectares remain, which are

primarily second - and third- growth forest s (King et al. 2009) . Bottomland floodplain forests sit in the heart of an economic and e nvironmental commodity tradeoff because they are valuable in either a converted or forested form (Mitsch and Gosselink 2000). Converted, they provide valuable forestry products, abundant agricultural land, and can easily be manipulated for urbanization. Forested , they provide habitat, wildlife

3

corridors, flood control, sediment and nutrient retention, and biosequestration. A dvocacy efforts have pushed for more land use restrictions, better

management practices, and increased acquisition of conservat ion easements

in these ecologically sensitive areas . R emnant

patches of intact and relatively unaltered

bottomland forests provide opportunities to learn as much as we can about these environment s, bring ing

awareness to their value as environmental commodities. Knowledge gained from these studies can be applied to management and restoration in bottomland areas targeted for conservation and reforestation (King et al. 2009) .

The term bottomland enc ompasses ecosystems occurring on older - abandoned terraces and actively - developing floodplain landforms (Sharitz 1993; Hodges 1997). The present study examines only the bottomland environments directly influenced by flows, exchanges, and processes actively

linking the river and the floodplain. Malanson (1993) describes this portion of the landscape as a riparian zone that contains and connects elements

that

affect flows of energy, matter, and species. The National Resource Council (NRC) defines riparian area

as follows :

“Riparian areas are transitional between terrestrial and aquatic ecosystems and are distinguished by gradients in biophysical conditions, ecological processes, and biota. They are areas through which surface and subsurface hydrology connect waterbodies to their adjacent uplands. They include those portions of the terrestrial ecosystem that significantly influence exchanges of energy and matter with aquatic ecosystems. Riparian areas are adjacent to perennial, intermittent, and ephemeral strea ms, lakes, and estuarine - marine shorelines.” (National Research Council 2002, p.33)

The above NRC defin ition implies an active zone of influence between the aquatic and terrestrial components of the landscape. I apply this definition of riparian as being nested within a bottomland ecosystem where the zone of influence encompasses the main stem river and adjacent actively - developing floodplain and excludes older -

4

terrace surfaces no longer linked to present - day fluvial conditions with the exception of large , infrequent floods.

In bottomland ecosystems, the active riparian zone forms a fluvial mosaic of aquatic to terrestrial wetland habitats characterized by multiple environmental and biological gradients linked to hydrologically - dependant exchanges across

the landscape.

The riparian floodplain environment contains a variety of contiguous fluvial landforms that create a patterned network of spatially varying hydrogeomorphic conditions differentiated by subtle differences in landform morphology, elevation, h ydrology, substrate, and age. Coupled with this fluvial landform mosaic, the compositional and structural complex ity of bottomland forest s are influenced by anthropogenic impacts, in addition to the array of environmental controls. This combination of fact ors produces a diverse assemblage of riparian forest communities within the bottomland ecosystem, each controlled by a different combination of environmental, biological, and disturbance gradients.

This dissertation contributes to our understanding of bo ttomland environments by quantifying old- growth and second- growth successional dynamics in abandoned meander forests of a protected bottomland ecosystem on the Congaree River, South Carolina. Abandoned meander channel features and their associated forests comprise several elements that contribute to the heterogeneity of bottomland ecosystems. Abandoned meander channels form when the river cuts - off a meander bend segment of its channel

and shortens its downstream course. The abandoned channel accretes with m ineral and organic deposits and progressively assimilates with the surrounding floodplain. When ecological conditions are favorable, a forest community will colonize the accreting

5

channel form.

For this research, I apply a mix of vegetation surveys, geospa tial modeling, and multivariate statistical methods to measure landscape - and stand- level interrelations among forest development, floodplain geomorphology, flood processes, and historic logging impacts within abandoned meander features.

1. 2

Research Ques tions

This research introduces a novel approach for examining fluvial processes in bottomland riparian ecosystems and emphasizes the prevalence of codependent interactions between floodplain hydrogeomorphology and land use history on forest successional dynamics.

First, I demonstrate the functionality and application of a 2D hydrodynamic model for simulating complex overland flow processes and hydrogeomorphic conditions in the lower Congaree floodplain. This component of the dissertation is motivated by

the following research questions addressed in Chapter One: (1) What are the spatial and temporal dynamics of flood processes on the Congaree River floodplain?

(2) Is a two - dimensional hydrodynamic flood model an adequate tool for simulating flood proces ses and hydrogeomorphic conditions on the Congaree floodplain?

Second, I compare forest development patterns among abandoned meanders and use a multivariate analysis to examine the controlling factors that influence their different successional responses across the floodplain. This component of the research applies th e 2D flood model as a hydrometric tool for quantifying flood- related hydrogeomorphic variables that I incorporate into the multivariate analysis. This study also seeks to examine the effect of historic logging disturbances on contemporary forest patterns a nd

6

compare second - growth and old- growth forests. Interactions between abandoned meander forest dynamics, floodplain geomorphology, flood processes, and historic logging disturbances are addressed by answering the following research questions posed in Chapt er Two:

(1) What are the compositional, structural, and diversity characteristics of forests that occupy abandoned meanders?

(2)What are the various controls affecting forest development and succession in abandoned meanders?

(3) How do forest s of different abandoned meanders compare and how are their variations linked to hydrogeomorphic processes and historic logging disturbances?

This research is conducted on the lower Congaree River floodplain in Congaree National Park, South Carolina, in a n extensive, contiguous tract of bottomland riparian forest in the southeastern Atlantic Coastal Plain. The Congaree

River floodplain contains numerous abandoned meander features in multiple stages of development. The se floodplain features contain old- grow th and second - growth forests . The second - growth forests were disturbed by different types of logging practices. The Congaree floodplain experienced multiple episodes of logging disturbance ending in 1976, when the area became federally protected as a Natio nal Monument and later designated a National Park in 2003. To date, there has been no scientific research examining the recovery dynamics of the >35 year - old regenerated second - growth forests within Congaree National Park (Congaree NP) .

While a clear def inition for old- growth forests does not exist for southeastern bottomlands, it has been suggested that t he definition should exclude areas that have been disturbed post - European colonization (Davis 2008). Extensive areas of the Congaree NP

7

bottomland fores ts were disturbed by logging. While much of this is well documented, there are extensive areas which lack historical land use information. It should be an objective for resource managers to re - inventory forest resources and develop better records of the ty pes and locations of historical logging disturbances. Davis (2008) suggests that sites may still meet old - growth criteria if only

lightly

distu r bed, but I propose that distinction should be made on a case by case basis dependent on the recovery dynamics of the disturbed forest. Besides logging activities, flood processes and floodplain hydrogeomorphology also influence bottomland forests in the Congaree River floodplain.

The Congaree NP floodplain is intimately connected to flows in both the Congaree and W ateree River s and floods up to several times per year . S ome floods inundate the abandoned meanders for extended periods of time . There have been limited studies into abandoned meander flood- related hydrogeomorphic variability and less attention to linking these processes with abandoned meander forest succession in old- growth and secondary growth on the Congaree floodplain.

1. 3

Congaree Floodplain Vegetation Studies

Bottomland forest studies in Congaree NP have included: (1) Forest inventory surveys (Gadd y and Smathers 1980; Thompson 1998); (2) Links between forest assemblages and hydrology in different landforms across the entire lower floodplain area (Patterson 1985; Doyle 2009); (3) Hurricane Hugo wind disturbance and forest recovery/regeneration dynamics (Allen et al. 1994; Battaglia 1998; Allen and Sharitz 1999; Battaglia et al. 1999; Battaglia and Sharitz 2005; Battaglia and Sharitz 2006; Zhao

8

et al. 2006; Sharitz and Allen 2009); (4) Hurricane Hugo wind disturbance effects on forest dynamics with an emphasis on lianas (Allen et al. 1997; Allen et al. 2005; Allen 2007; Allen et al. 2007); (5) Near - channel forest responses to lateral channel migration processes (Mei tzen 2006; Meitzen 2009); and (6) Hydrogeomorphic controls on forest recovery in clear cuts (Kupfer et al. 2010).

T he present study compliments this existing research and provide s

resource managers with a better understanding of the biophysical

and histo rical disturbance

dynamics that specifically influence forest patterns in abandoned meander landforms. I also provide an example application for the 2D hydrodynamic flood model, which was designed as a hydrometric tool for use in future research. The conte mporary forest conditions inventoried by this study and the high- resolution terrain and flood model provide baseline data to measure future changes with in Congaree’s ecosystem.

Abandoned meanders are incipiently linked to a similar geomorphic form and pr ocess of origin, but their successional forest development can vary spatially and temporally across the floodplain. Forest compositional and structural responses provide a useful proxy for tracking environmental changes and forest responses to disturbances . This dissertation research provides fundamental contributions to understanding bottomland riparian ecosystems by modeling landscape - scale connections between flood processes and floodplain geomorphology, and quantifying multivariate controls on structure and compositional patterns of forest succession in abandoned channel features.

Differentiating the patterns of successional response has several implications for bottomland hardwood management and the restoration of disturbed sites. For example, it provi des a better understanding for what constitutes natural forest development patterns

9

from artificially altered forests responses. This information could inform active restoration efforts regarding the selective planting of types and densities of certain spe cies over others

1. 4

Framework for Dissertation Design

This dissertation is structured in the manuscript style and includes four chapters. Chapter One is the introductory chapter and it provides the conceptual organization and theoretical background to this research. Chapters Two and Three are individual manuscripts prepared for submission to peer - reviewed journals. The first manuscript (Chapter Two) demonstrates the application of a two dimensional (2D) hydrodynamic flood model for

the Congaree

and Wat eree River floodplains . This manuscript provides a detailed description of the elements necessary for developing the 2D flood model and discusses conceptual uses of the model for examining interactions between flood hydrology and riparian ecosystem functio ns. Two products were developed in this first component of the researc h:

a high - resolution digital terrain model (DTM) and a 2D flood model . These products we re used in the Chapter Three to quantify hydrogeomorphic terrain and flood variables.

The second manuscript (Chapter Three) compares abandoned meander forest successional dynamics across the Congaree River floodplain and examines the environmental and historic logging controls influencing forest development patterns. This st udy incorporates hydrogeomorphic variables derived from the DTM and 2D flood model with vegetation surveys, additional environmental data, and historical logging information to examine the complex multivariate controls that are influencing forest

10

structure

and composition

at the landscape - and stand- level. Chapter Four synthesizes the dissertation research, discusses its relevancy to bottomland floodplain management, and addresses future challenges and research expansio n.

11

2

C hapter Two : Two Dimensional Hydrod ynamic Flood Inundation Modeling for the Congaree River Floodplain

2.1 Introduction: Flood Processes and Modeling

Flood processes in low - gradient, meandering rivers control significant characteristics of the floodplain geomorphology and hydrology; which in turn influence bottomland ecosystems structure, composition, and functional integrity (Sharitz and Mitsch 1993; Hupp and Bornette 2003). While it is recognized that floods maintain a variety of physical and biological ecosystem processes such as the lateral exchange of nutrients, sediments, and organisms between the river and floodplain (Ward et al. 1999;

Amoros and Bornette 2002;

Opperman et al. 2010), comparatively little research has focused on quantif ying spatially - explicit flood metrics at the landscape scale . Powell et al. (2008) is one of the few studies that uses flood maps to explain spatial vegetation patterns. In their study, they recreated flood extents from satellite imagery of historic floods to quantify environmentally significant flows for the Gwydir floodplain in Australia. Flood simulation modeling and flood mapping

can potentially provide effective methods for examining the hydrodynamic conditions of various high flows within a floodplain ecosystem. Flood extent, depth, and duration, are a few key variables that can be quantified from flood simulation models. Although flood modeling and mapping historically originated from risk management demands and land- use planning,

12

this research demon strate s how the se same methods provide powerful tools for examining flood- depend e nt controls on ecosystem processes and patterns at the landscape - scale. This study describes the application of high- resolution, two- dimensional (2D) hydrodynamic flood model and suggests applications of the model as a geospatial hydrometric tool for examining flood- related controls on hydrogeomorphic conditions and ecosystem functions. Maps of flood extent, depth, and flood duration prove to be especially useful for examining interactions between ecosystem structure, composition, and function in a large, river - floodplain environment (Powell et al. 2008) . Model development and application examples in this paper are specific to the lower Congaree River and alluvial bottomland har dwood ecosystem in the southeastern Atlantic Coastal Plain, South Carolina. However, this study provides a methodological modeling framework that is transferable to other hydrodynamic - based studies in low - gradient river and floodplain environments.

This paper begins with a detailed overview of the study area, providing context for the linkages between flood processes and the hydrology and geomorphology of the Congaree River floodplain. Minimal changes in elevation produce substantial hydrogeomorphic varia bility, and this environment allows testing and verification of the flood inundation model. Next, I define the data sources, methods, and techniques for developing, calibrating, and validating the 2D flood model. Following the explanation of the model deve lopment, I suggest various applications of the model for examining flood- related controls on hydrogeomorphology and ecosystem dynamics. Application examples focus on conceptual methods for measuring the spatial and temporal dimensions of lateral river - floo dplain exchanges on ecosystem processes and patterns.

13

2.2 Geographic Setting: Valley Hydrology and Geomorphology

The Congaree River forms at the confluence of the Broad and Saluda Rivers along the Fall Line in central South Carolina. The headwaters for the Broad and Saluda Rivers begin in the Blue Ridge Province and drain across the Piedmont to their confluence at the edge of the Coastal Plain. The Congaree River flows southeast for 89 km (55 miles) through the Coastal Plain where it merges with the Wate ree River to form the Santee River (Figure 2. 1).

The Wateree River begins as the Catawba River in North Carolina and changes names at Lake Wateree in Kershaw County, South Carolina. The Congaree River Basin drains 22,000 km 2 (8,500 mi 2 )

and the Wateree River drains 15,550 km 2 (6,000 mi 2 ). Surface materials of the Congaree River and Wateree River basins range in age from the oldest Paleozoic crystalline metamorphic rocks of the Blue Ridge and Piedmont to the Tertiary deposits of the Coastal Plain and the most recent Quaternary deposits of the Congaree River valley (Maybin and Nystrom 1997). 2.2.1 Santee Basin Hydrologic Relationships

Flows to the Congaree River from the Saluda and Broad Rivers are affected primarily by drainage area and dam regulation. T he Saluda Basin constitutes about one - third of the Congaree watershed and contributes a comparable portion of the total flow during low - moderate flows. The Broad Basin covers two - thirds of the drainage area and contributes

a similar portion of the total vo lume during low - moderate flows. Flow contributions change significantly during high flows, with the Broad contributing over 85- 90% of the flows and the Saluda contributing substantially less, and often only 10- 15%. A similar 2:1 ratio in flows occurs betwe en the Congaree and Wateree contributions

14

to the Santee River, but it remains relatively constant over the range of low to high flow conditions.

Several major dams affect flows in the Santee Basin. The Saluda River has two large dams , the Broad River ha s one, the Catawba - Wateree River ha s eleven (six in North Carolina and 5 in South Carolina) , and the Santee River has two

large dam s

before draining into the Atlantic Ocean.

The dams are all operated for hydroelectric power and municipal water supply, while, a few of the dams on the Catawba divert water for transfer to other rivers in North Carolina.

Figure 2.1 : Major sub - basins of the greater Santee River Basin showing location of Congaree National Park

.

15

Flows of the Congaree River system vary considerably (Figure 2 .2 ). Daily mean flows for the period of record from hydrologic year 1940- 2010 average 180 m 3 s - 1 (6,250 ft 3 s - 1 ); with the lowest flows measuring less than 30 m 3 s - 1 (1,000 ft 3 s - 1 ) and the highest flows exceeding 4,248 m 3 s - 1 (150,000 ft 3 s - 1 ) (USGS 02169500) . Conrads et al. (2008), calculated duration hydrographs for the Congaree River and recognized the “normal” range of flows to occur between the 25 - 75 th percentile range which included flow s between about 115 m 3 s - 1

(3,500 ft 3 s - 1 ) and 700 (20,000 ft 3 s - 1 ). Days with flow below that range were considered low/dry and days with flow above the 75 th

percentile were recognized as exceptionally high/wet stream flow periods. Although there is great va riability in Congaree flows, seasonal flow patterns follow a relatively predictable regime. The highest flows occur in late winter to early spring months and the lowest flows occur during late summer and early fall, with the exception of occasional tropica l depressions, storms, and hurricanes .

16

Figure 2.2: 1940- 2010 Raster hydrograph of daily mean flows (expressed as log values) for the Congaree River (USGS 2169500). Each cell represents a single day of flow. Variations in log flow intensities illustrate inter - annual variability between low and high flow years, and seasonal flow patterns. The x - axis displays the cumulative days for the hydrologic water year which spans from October 1 st through September 30 th , i.e., day 30 is October 30 th , and so one.

2.2.2 Study Area and Floodplain Hydrogeomorphology

The study area for the flood modeling begins 39 river km (24 mi) downstream from the start of the Congaree River, and extends :

Full document contains 150 pages
Abstract: Southeastern bottomland hardwood ecosystems thrive as a result of the biophysical linkages between the river and the floodplain. These ecosystems contain significant habitat heterogeneity and support rich diversity of species, but have been altered and manipulated by anthropogenic impacts. This study was designed to examine the forest development dynamics in abandoned meander landforms on the Congaree River floodplain, South Carolina in order to gain a better understanding of the forest successional responses to environmental gradients and historic logging disturbances. This study presents the innovative application of a 2D hydrodynamic flood model for simulating complex overland flow processes and quantifying hydrogeomorphic environmental data for use in ecological analysis. The 2D flood model is developed using high resolution terrain data and historical flows. The flood simulation model measures flood extent, depth, frequency and flood water retention capacity. These data were incorporated with structural and compositional forest surveys, additional environmental data, and historic logging information to compare forest patterns and other factors controlling succession in abandoned meander landforms. Forest development patterns varied as a function of the type of logging disturbance, hydrogeomorphic conditions, and environmental change. Three distinct successional patterns emerged that were linked with the following disturbance history: unlogged old-growth forests, selectively logged forests, and clear cut forests. Forest succession in old-growth environments exhibited a gradual transition from obligate wetland species to facultative species. In contrast, recovery patterns in clear-cut forests exhibited complete replacement of the original obligate wetland dominated community to a predominantly facultative community. Selectively logged sites showed the least amount of successional change, but they lacked recovery of the logged, Taxodium distichum component of the forest, which was replaced almost exclusively by Nyssa aquatica . Processes and patterns evidenced in the Congaree River floodplain provide a glimpse of the various controls and successional pathways that produce heterogeneous forest patterns. A 2D flood model proves to be a valuable tool for modeling complex overland flow processes for the Congaree floodplain and has many ecological applications. Flood and forest patterns revealed by this study can be used to help guide or redirect restoration efforts to match current or desired environmental conditions.