Design principles for maximizing water quality performance of restoration via the hyporheic zone

Year: 2023
Presenter/s: Skuyler Herzog
Symposium Session: 2023 - 03 Water Quality Considered in Restoration Design & Watershed Management
Topics covered: adaptive management and monitoring, groundwater, lessons learned, temperature, urban, water quality, and wood


Water quality is a common motivation or ancillary benefit of stream restoration, but many projects fail to meet performance expectations. Such projects restore the hydrologic form of a river without also generating the desired hydrologic functions. Much of the self-purification capacity of streams is located in the saturated streambed sediments comprising the hyporheic zone, which is a natural bioreactor known as a “river’s liver”. Thus, poor water quality performance in stream restoration is often the result of improper hyporheic conditions beneath a project. Specifically, water quality performance will suffer if there is not enough water flowing through the saturated sediments beneath a structure, or if water moves through the sediments too quickly and returns to a river before pollutants are fully removed. Unfortunately, there is a design-to-function knowledge gap regarding how stream restoration structures can be optimized to maximize water quality functions.

Here I synthesize results from a decade of study on hyporheic restoration for water quality benefits, spanning full-scale restoration projects, controlled flume experiments, and computer models. Restoration structures using surface elements (e.g., weir, plunge pool) and subsurface elements (e.g., baffle plates, hyporheic caps) can effectively tailor the amount of hyporheic exchange flow passing beneath the structure while also controlling the amount of treatment time. For example, treatment times may be shorter for fast or aerobic reactions (e.g., nitrification), or longer for slower or anaerobic reactions (e.g., denitrification). Results show that high flux, long transit time hyporheic flowpaths can be created under restoration structures with some combination of upstream baffle plates, longer hyporheic caps, and deeper or wider streambeds of more permeable, and more porous, material. The ideal dimensions of surface elements depend on specific water quality goals, with taller structures favored for faster or aerobic reactions, and shorter structures favored for slower or anaerobic reactions.

These design concepts can improve pollutant removal many times greater than standard practices, particularly for anaerobic reactions. Indeed, results show that many common structures fail to produce any measurable anaerobic reactions, but the relatively minor design modifications described above can improve anaerobic reaction performance. Specific results depend on reactions of interest and local conditions, but restoration structures may be optimized for removal of any one pollutant (e.g., nitrate, 6-PPD quinone) or a suite of pollutants common in human-dominated watersheds. These conceptual designs are common across multiple kinds of restoration structures, such as plunge pools, weirs, and beaver dam analogues. Thus, how a structure is built may be more important than what kind of structure is built. Results are placed in the context of restored hyporheic zones along Thornton Creek (Seattle, WA) to show how ongoing design optimizations may further improve water quality outcomes of restoration.