Wooded Flood Plains Research Articles

Mixing layer development in compound channel flows with submerged and emergent rigid vegetation over the floodplains

This laboratory study aims at investigating the longitudinal development of a mixing layer in a compound open-channel (two-stage geometry with a main channel and adjacent floodplains). The floodplains are covered with two roughness types: either a bed roughness representing a submerged dense meadow or emergent roughness elements (cylinders) representing an alluvial forest. The theoretical background used for plane mixing layers is adapted to the highly three-dimensional mixing layer that develops at the main channel/floodplain interface. The mixing layer width is divided into two parts on either side of the interface. For the wooded floodplain, the mixing layer width on the floodplain side levels off downstream much more rapidly than for the grassed floodplain. The lateral profiles of normalised velocity and turbulence quantities are found to be self-similar in the longitudinal direction for a fixed elevation. However, shallowness effects prevented the normalised lateral profiles of velocity and turbulence quantities from coinciding at different elevations. The respective contributions of lateral Reynolds stresses and secondary currents to the lateral exchange of momentum are estimated. At the main channel/floodplain interface, the momentum exchange is driven by Reynolds stresses. In the main channel, both Reynolds stresses and secondary currents contribute to the lateral flux of momentum. Secondary currents are stronger with emergent macro-roughness elements than with bed-roughness only on the floodplains. Large-scale turbulent coherent structures are investigated based on two-point space-time correlations of velocity. These structures are found to span the entire floodplain flow depth, and their convection velocity is close to the depth-averaged longitudinal velocity at the interface. The coherent fluctuations of the longitudinal and lateral velocities have different Strouhal number values, similar to those found in plane mixing layers.

Nonchemical and Herbicide Treatments for Management of Japanese Stiltgrass (Microstegium vimineum)

AbstractJapanese stiltgrass, an annual grass species native to eastern Asia, has become a serious invasive-plant problem in the eastern United States. We compared the efficacy of herbicides and nonchemical options found effective for controlling stiltgrass in earlier studies, with organic herbicides and herbicides used at reduced rates in a wooded floodplain along the lower Connecticut River. We compared the effect of 2 yr of conventional and alternative treatments on cover of other nonnative and native species. Four blocks of 18 plots (3 by 4 m [9.8 by 13.1 ft]) were established in May 2008. Treatments included directed heating with a propane torch (June, July), hand-pulling (July), mowing with a string trimmer (July, August), foliar applications of household vinegar [5% acetic acid] (June, July) and the herbicides imazapic (June), pelargonic acid (June, July), and pelargonic acid plus pendimethalin (June). The following herbicides were applied at labeled doses and at one-fourth labeled doses: fenoxaprop-p-ethyl (July), glufosinate (August), and glyphosate (August). Stiltgrass cover and height were evaluated periodically, and plant samples were collected in autumn of 2008 and 2009 to determine the number of viable seeds produced. Final evaluations were conducted in June 2010 after 2 yr of treatment. Stiltgrass cover averaged 88% on untreated plots in fall. All treatments reduced stiltgrass cover and seed production. The least-effective treatments were hand-pulling, pelargonic acid, and vinegar in July. Direct heating, mowing, and vinegar in June reduced seed production by more than 90%. All treatments containing imazapic, pelargonic acid plus pendimethalin, fenoxaprop-p-ethyl, glufosinate, and glyphosate completely prevented stiltgrass seed production in the second year of treatment. Effective control of stiltgrass can be achieved during a 2-yr period with a variety of herbicides, including herbicides at one-fourth of the labeled dose, and through nonchemical treatments.

Distribution and Conservation Status of the Rusty Gravedigger,Cambarus miltus, a Poorly Known Gulf Coastal Crayfish

Cambarus (Lacunicambarus) miltus (Rusty Gravedigger Crayfish) is a primary burrowing crayfish known from a limited portion of the Gulf Coastal region of the United States. The lack of form I males in collections has in the past prevented species-level identifications and hampered conservation reviews. We conducted an intensive status survey for C. miltus during 2007 and 2008. Our results suggest that the species is much more widespread than previously known and that conservation attention is unwarranted. Preferred habitat for the species is ephemerally flooded and thinly wooded floodplains of small streams and swamps.

Characteristics of freshly deposited sand and finer sediments along an island-braided, gravel-bed river: The roles of water, wind and trees

Over the past two decades there has been a growing interest in the geomorphological mosaic along large floodplain rivers where channel dynamics are seen to drive habitat-patch creation and turnover and to contribute to high biological diversity. This has required a new perspective on fluvial geomorphology that focuses on biological scales of space and time. This study examines the spatial pattern of surface fine sediment accumulations along a reach of a large gravel-bed river, the Tagliamento River in NE Italy; an area with a moist Mediterranean climate and seasonal flow regime. The study investigates changes in sediment characteristics during the summer low-flow period between April and September. Focussing on five areas representing a gradient from open, bar-braided to wooded island-braided morphologies, the paper demonstrates the importance of riparian vegetation and aeolian–fluvial interactions. Significant contrasts in particle size distributions and organic content of freshly deposited sand and finer sediments were found between sampling areas, geomorphological settings, and sampling dates. In particular, wooded floodplain and established islands supported consistently finer sediment deposits than both open bar surfaces and the lee of pioneer islands, and in September significantly finer sediments were also found in deposits located in the lee of pioneer islands than on open bar surfaces. Overall, the September samples had a greater variability in particle size characteristics than those obtained from the same sites in April, with a general coarsening of the D 5 ( φ) (i.e., the coarse tail of the particle size distribution). Also in September, crusts of fine sediment (30 μm < D 50 < 64 μm) had formed on the surface of some of the open bar and pioneer island deposits within the more open sampling areas along the study reach. These crusts possessed similar particle size characteristics to aeolian crusts found in more arid environments. They were significantly finer than April samples and September subcrust samples obtained from the same sites and had similar particle size characteristics to some samples taken from wooded floodplain, established island surfaces and the lee of pioneer islands that were not crusted. Local climatological and river level data confirm significant wind and rainfall events during a period of consistently low river levels between the April and September sampling periods. These support deflation, deposition and rain wash of finer sediment during the summer, with windblown sediments being deposited on bar surfaces and in the lee of pioneer islands where wood and young trees provide foci for accelerated sedimentation and island growth as well as on marginal floodplains and established islands. We conclude that along braided rivers in moist settings but with a distinct dry season, aeolian reworking of sediment deposits may have a more important role in driving habitat dynamics than previously considered.

Nature, mass and preferential sites of coarse woody debris deposits in the lower ain valley (Mollon reach), France

AbstractCoarse woody debris (CWD) has been examined in a section of the Ain, a sixth order piedmont river with an actively meandering channel and a wooded floodplain. The spatial distribution of CWD, its mass and forms of accumulation are controlled by the hydrodynamics and the retention capacity of the forest. A typology shows the relative importance of woody debris in the mosaic of patches and the essential role of the ecotonal zones. The mass of debris varies from 0001 t ha−1, to more than 200t ha−1, but is lower than those observed in certain American rivers. Most of the material is deposited in the margins and forms a narrow debris line.The restocking in woody debris is recent in Europe and tends to diversify the environment. This affects the researcher and the planner. The first considers this transit of material as a useful hydromorphodynamic and biodynamic tool which is easy to evaluate, and the second considers it as a restoring and generative vector, the ecological functions of which are recognized. Its effect is stronger today as the watershed area tends to be subjected to a decrease in agricultural activity.

Holocene floodplain evolution of small rivers in the uplands of Lower Saxony, Germany

The Holocene floodplain evolution of the river Ilme and its distal tributaries in the uplands of Lower Saxony (Germany) has been reconstructed by shallow corings arranged in cross and longitudinal profiles. In those river reaches that drain agricultural land we differentiate three Holocene fluvial units overlying the Weichselian Lower Terrace: A fine-grained Humus Basal Layer (early-middle Holocene), an Older Meadow Loam (8th-early 14th century) and a Younger Meadow Loam (since 15th century). Intertongued with these sediments are alluvial fans and colluvia of two periods: 1. Neolithic to early Medieval time, 2. 14th century. In contrast forested floodplains are underlain by sand and gravel, whereas on floodplains used as meadows, a fine-grained “Wiesensediment” (Meadow Sediment) has accumulated since the 17th century. The Humus Basal Layer and the sandy gravel were both accumulated by braided rivers. In the middle and lower reaches of the Ilme floodplain, the change to a meandering river happened when the Older Meadow Loam accumulated as a result of woodland clearing and soil erosion in the Early Middle Ages. In the course of time, the vertical accretion of the floodplain decreased as a result of the decreasing frequency of inundations, itself caused by the increasing capacity of the river channel. Vertical accretion was nearly terminated in the Late Middle Ages when a short climatic change caused fan and colluvial deposition. Since the 15th century, the Younger Meadow Loam accumulated, most of it filling old channels. In the wooded Solling hills, the change-over from a braided river to a meandering river began in the 17th century, but in those reaches with wooded floodplains the rivers are still braided.

Aedes vexans as a Flood-plain Mosquito 1

Aedes vexans (Meigen) is a mosquito important to man in areas of low topographic relief in inland USA. Sources are both flood plains and upland depressions. Choice of flood plains as oviposition sites is affected by nature of vegetative canopy, moisture of the soil, plant detritus, frequency of flooding, and changes in the surface created by degree of siltation. Included are figures showing topographic relief, vegetative cover, and a dispersion diagram of eggs at a site typical of a wooded flood plain.

Centroclinal Cross Strata, A Distinctive Sedimentary Structure

ABSTRACT In August 1968 overbank flooding of the Prairie Dog Town Fork of the Red River resulted in the deposition of a blanket of very fine to fine sand on the floodplain in Palo Duro Canyon State Park, 24 km southeast of Amarillo, Texas. Surrounding many of the trees on the floodplain, centroclinal cross strata (concentric and with dip toward a common center) were discovered in this new layer of sand. They are the result of infilling of scour holes created around the trees by swirling flood water. Where this primary sedimentary structure is discovered in ancient rocks, it will indicate a specific environment (wooded floodplain) and mechanism of deposition (receding floodwater of decreasing competency). The structure will also indicate top and bottom of strata.