Geomorphological Study of the Ouachita Mountains

Introduction

The National Water Management Center (NWMC) is working with the Arkansas Soil & Water Conservation Commission (ASWCC) to develop a database of reference stream reaches in the Ouachita Mountains of Arkansas. The three-year project began in Oct 2001, with NWMC and ASWCC personnel starting the collection of field data and the analysis of hydrologic records for 50 USGS Gaging Stations (see figure 1 & table 1). Information from the project will be available to the public, via publications and the internet, and is intended to supply landowners, Conservation Districts, transportation officials and others with information regarding the natural stream properties of the Ouachita Mtns.

Figure 1. Image of Locations of sampling sites at USGS gaging stations  (145 KB)

Field Data

Field data collection entails topographic surveys of selected stream reaches, collection of bed and bar sediments, characterization of local geology and stream ecology sampling. The topographic surveys may run from hundreds to thousands of feet, and are intended to collect stream hydraulic geometry parameters including; thalweg elevations, reach slopes, widths, depths, cross sectional areas, channel forming indicators, and top of banks and floodplain elevations. Sediment samples are collected to assist with stream classification and help confirm channel forming discharges. Characterization of the local geology is also used to assist with stream classification and will assist in the development of the regional and regime hydraulic geometry curves. Stream ecology sampling consist of macroinvertebrate sampling to assess biological communities in the streams.

Hydraulic Analysis

The hydraulic analysis for each site begins by obtaining the historical USGS gaging station records. Requested records include; the station descriptions, peak annual flow data, rating tables, and summary discharge measurements. From these records three graphs are plotted. First, discharge versus probability of exceedence (Weibull plot). Second, discharge versus gage elevation. And third, discharge versus cross-sectional area. These relationships are used in the field to help locate elevations of channel forming flows. Additionally, we are applying the data from the topographic survey to model flows through the reach using HEC-RAS, a US Army Corps of Engineers computer program for hydraulic analysis. This helps verify the discharge(s) that best fit the channel forming indicators. HEC-RAS also provides energy slopes and indicates whether the flow are sub-critical or super-critical within the reach for the particular flow being modeled.

Regional & Regime Hydraulic Geometry Curves

Regional hydraulic geometry curves are log-log plots comparing channel dimensions (top width, mean depth, and cross-sectional area) at bankfull discharge versus drainage area. Using these curves, the width, depth and cross-sectional area of the bankfull channel can be approximated once the drainage area of a watershed within the region is known. Regional curves should be used only as indicators to help identify the channel geometry because of the large degree of natural variation in most data sets.

Regime curves are similar to Regional curves in that bankfull width, depth, and cross-sectional area are the dependant variables but discharge is the independent variable rather than drainage area. Statistically, channel-forming (or bankfull) discharge is a more reliable independent variable for hydraulic geometry relations than drainage area. This is because the channel forming discharge is the driving force that creates the observed channel geometry, while drainage area is merely a surrogate for discharge. Discharge will vary depending on: geology, soils, vegetation, drainage area's shape, slope, drainage network, and land use.

Data Availability

As analysis for each site is completed it will be posted on the NWMC's Regional Hydraulic Geometry Curves web page. Once a significant number of sites are completed, we will also post the Regional and Regime Hydraulic Geometry curves to this same web page. Additionally, once all the sites have been analyzed, we will publish a final summary report.

Training

In addition to data collection and analysis, the NWMC and ASWCC will be using the project as an opportunity to train additional personnel in stream characterization and analysis.

Contact Information

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