Hydro-morphological parameters generate lifespan maps for stream restoration management

Highlights

• Quantitative survival thresholds for stream restoration features are assessed.

• Repeated terrain elevation surveys are used for 2D hydrodynamic modelling.

• Elevation changes and hydraulic parameters are vetted against threshold values.

• Spatially comparing observed values with thresholds produces lifespan maps.

• Lifespan maps indicate the sustainability of stream restoration features.

Abstract

Anthropogenic, eco-morphological degradation of lotic waters necessitates laws, directives, and voluntary actions involving stream restoration and habitat enhancement. Research and engineering efforts are establishing a vast number of stream restoration planning approaches, design testing frameworks, construction techniques, and performance evaluation methods. As the practice of restoration scales up from an individual action at a single site to sequences of actions at many sites in a long river segment, a primary question arises as to the lifespan of such a sequence. This study develops a new framework to identify relevant parameters, design criteria and survival thresholds for ten multidisciplinary restoration techniques, adequate for site-scale to segment-scale application, in a comprehensive review: (1) bar and floodplain grading; (2) berm setback; (3) vegetation plantings; (4) riprap placement; (5) sediment replenishment; (6) side cavities; (7) side channel and anabranches; (8) streambed reshaping; (9) structure removal; and (10) placement of wood in the shape of engineered logjams and rootstocks. Survival thresholds are applied to a sequence of proposed habitat enhancement features for the lower Yuba River in California, USA. Spatially explicit hydraulic and sediment data, together with numerical model predictions of the measures, were vetted against the survival thresholds to produce discharge-dependent lifespan maps. Discharges related to specific flood-return periods enabled probabilistic estimates of the longevity of particular design features. Thus, the lifespan maps indicate the temporal stability of particular stream restoration and habitat enhancement features and techniques. Areas with particularly low or high lifespans help planners optimise the design and positioning of restoration features.

Introduction

Throughout history, wide-ranging river rectifications and monotone stream patterns have been engineered for economic benefits, such as the conversion of the Rhine River into a navigable channel (Tulla, 1812). Today, many eco-morphological problems resulting from stream rectification are considered serious impacts on stream ecology, ecosystem services, and the broader economy (e.g., devastating floods) (Blackbourn, 2006; Surian and Rinaldi, 2003). Consequently, governments write laws, issue directives, and fund voluntary actions involving stream restoration and habitat enhancement (e.g., the U.S. National Environmental Policy Act 1969; Canadian Environmental Protection Act 1999; European Water Framework Directive 2000, Wohl et al. (2015) summarised the state of science and practice of stream restoration in a review of small, medium and large rivers. They identified useful paradigms for the planning of habitat enhancement projects. Technical features of stream restoration were documented in several studies (e.g., Bernhardt et al., 2005; Morandi et al., 2014; Wohl et al., 2015) and a multitude of assessment strategies are available (e.g., Feio et al., 2016; Rinaldi et al., 2017).

Still, Wohl et al. (2015) legitimately asked, “How do we approach river restoration?” A generalised and quantitative answer to that question is not yet available, especially from a technical point of view. In addition to the scientific underpinnings, decision makers require accountability of the lifespan of restoration features for prioritising particularly relevant projects. Society is spending a lot of money to build projects, but too little effort has gone into ascertaining the longevity of such investments.

This study uses a large dataset from California's well-documented lower Yuba River (e.g., Barker et al., 2018; Pasternack and Wyrick, 2017) to create lifespan maps for quantifying the lifetime of stream restoration features. The term “feature” denotes any one specific technical component to achieve some beneficial stream restoration on a reach scale (10–100 times the channel width, Pasternack and Wyrick, 2017). An example of a feature is “vegetation planting” to improve habitat quality and increase channel stability.

A comprehensive and interdisciplinary literature review identifies relevant features and the fundamental data necessary for assessing their lifespans. Moreover, the literature review identifies parameter-related survival thresholds, where constructive details are listed in the supplemental material. Based on the parameter thresholds, we introduce a procedure for deriving lifespan maps and then test that approach using a proposed habitat enhancement framework for the lower Yuba River.