Hydrologic analysis of field delineated ordinary high water marks for rivers and streams

Abstract

Streamflow influences the distribution and organization of high water marks along rivers and streams in a landscape. The federal definition of ordinary high water mark (OHWM) is defined by physical and vegetative field indicators that are used to identify inundation extents of ordinary high water levels without any reference to the relationship between streamflow and regulatory definition. Streamflow is the amount, or volume, of water that moves through a stream per unit time. This study explores regional characteristics and relationships between field-delineated OHWMs and frequency-magnitude streamflow metrics derived from a flood frequency analysis. The elevation of OHWM is related to representative constant-level discharge return periods with national average return periods of 6.9 years using partial duration series and 2.8 years using annual maximum flood frequency approaches. The range in OHWM return periods is 0.5 to 9.08, and 1.05 to 11.01 years for peaks-over-threshold and annual maximum flood frequency methods, respectively. The range of OHWM return periods is consistent with the range found in national studies of return periods related to bankfull streamflow. Hydraulic models produced a statistically significant relationship between OHWM and bank-full, which reinforces the close relationship between the scientific concept and OHWM in most stream systems.

National Ordinary High Water Mark Field Delineation Manual for Rivers and Streams : Interim Version

“…The definition points toward an ordinary high water (OHW) elevation, which excludes extreme flows but includes high flows or small, frequent flood flows that fill a river or stream to capacity and often go overbank. Small, frequent flood flows are defined based on the hydrologic definition of high flows being at the lower end of a flood-frequency curve; therefore, they are not necessarily all overbank flows, as the term flood implies (Hamill and David 2021).…”

“…Identifying evidence of these high flows is key to conducting a field identification of the OHWM. Additional descriptions of the field methods used to measure these cross sections and the flow modeling used to determine the elevations of the low, moderate, and high flows can be found in Chapter 5 of this text and in Hamill and David (2021).…”

“…Any frequency of flow connected to the OHWM will vary greatly across different climatic and landscape settings. The frequency of the high-water stage associated with the OHWM may be a 1.01-year event (see discussion that follows) in one stream and a 10-to 15-year event, or more, in another (Curtis et al 2011;Wohl et al 2016;Hamill and David 2021). Flood-frequency curves are discussed in greater detail in Section 5.4, but the key to interpreting return periods is to remember that they are statistical probabilities that a flow of that size or larger will occur in any given year.…”

“…Stream channel slopes were measured in the field but were also extracted using remotely sensed data, such as lidar. All these data were used together to develop flow models in Hydrologic Engineering Center's River Analysis System (HEC-RAS) for each of these locations (Hamill and David 2021).…”

Evaluation of climatic and hydroclimatic resources to support the US Army Corps of Engineers Regulatory Program

“…The second comparison considered daily APT single-point α and P within each two-digit HUC to assess the precision with which α measured P. Although daily percentiles are not always the same as atypical conditions for a given year, streamflow percentiles can be used to provide a consistent evaluation of the correlation between α, PDSI, and P throughout the United States. Other characterization methods, such as ordinary high water marks and bankfull streamflow, tend to vary at locations throughout the United States (Hamill and David 2021;Williams 1978).…”