Floodplain Geomorphology and Response to Hurricanes: Lower Pee Dee Basin, South Carolina

Undeveloped forested wetlands in the valleys of coastal plain rivers can play a large role in storing floodwater and attenuating river flooding. In the lower Pee Dee, Little Pee Dee, and Lynches Rivers, these wetlands played a large role in mitigating downstream flooding following Hurricane Florence. Wetland forest flood mitigation was most effective for large flows in the Great Pee Dee River, where flooding on former river terraces determined the course of overbank flow and the potential storage of floodwaters. Floodwater storage and attenuation of water level were less effective if larger flows were limited to the Little Pee Dee River. Large rains prior to Hurricane Matthew, and to a lesser extent Tropical Storm Bertha, caused the forested wetland to be a source of additional flow, although with little increase in peak stage.

Streamflow estimation at partially gaged sites using multiple dependence conditions via vine copulas

Reliable estimates of missing streamflow values are relevant for water resources planning and management. This study proposes a multiple dependence condition model via vine copulas for the purpose of estimating streamflow at partially gaged sites. The proposed model is attractive in modeling the high dimensional joint distribution by building a hierarchy of conditional bivariate copulas when provided a complex streamflow gage network. The usefulness of the proposed model is firstly highlighted using a synthetic streamflow scenario. In this analysis, the bivariate copula model and a variant of the vine copulas are also employed to show the ability of the multiple dependence structure adopted in the proposed model. Furthermore, the evaluations are extended to a case study of 54 gages located within the Yadkin-Pee Dee River Basin, the eastern U. S. Both results inform that the proposed model is better suited for infilling missing values. After that, the performance of the vine copula is compared with six other infilling approaches to confirm its applicability. Results demonstrate that the proposed model produces more reliable streamflow estimates than the other approaches. In particular, when applied to partially gaged sites with sufficient available data, the proposed model clearly outperforms the other models. Even though the model is illustrated by a specific case, it can be extended to other regions with diverse hydro-climatological variables for the objective of infilling.

Hydraulic Geometry Curves in the Pee Dee Watershed

Hydraulic bankfull geometry or regional curves are a useful metric for evaluating stream stability and planning stream restoration projects. Streams and tributaries within the Middle Pee Dee River Basin (MPDRB) in South Carolina drain an agrarian and forested landscape characterized by water conveyance structures, such as active and historical ditches which support forestry and agriculture. While streams in the region are generally stable, pockets of this landscape are beginning to face increasing pressure from development with signs of stream instability apparent in several locations as evidenced by streams in and around the urbanizing areas around Darlington and Florence, SC. In order to provide a foundation for potential stream restoration projects in the area, 15 sites in the MPDRB were selected on the basis of catchment area, in categories of small (km2), small-medium (50-500 km2), medium (500-1000 km2), and large (>1000 km2). Bankfull geometries, channel substrate, flow and water temperature were measured at all the sites and a set of regional hydraulic geometry curves developed. The frequency of bankfull flows that occurred over the period of sampling were also estimated to document floodplain connectivity. Results suggest that bankfull dimensions in the MPDRB were well correlated with bankfull discharge and drainage area. The results showed that hydraulic geometry in the region were similar to those measured in a similar physiographic region in North Carolina. The study also shows that streams in the MPDRB experience bankfull exceeding flows much more frequently than streams in other parts of the country, but at a frequency that is comparable to streams in the coastal plains of North Carolina.