Why is a systematic framework to evaluate floodplain restoration opportunities at scale needed?
The expansive floodplains that once covered valley lowlands across California have been largely disconnected from adjacent rivers by levees or other anthropogenic modifications to the system. Climate change exacerbates risk to the already vulnerable flood system, with projected changes including a substantial decline in water storage held as the snowpack and a major increase in the magnitude of peak flood events. Expanding the river corridor to make use of floodplains to attenuate peak flood events, promote groundwater recharge into depleted aquifers, and improve ecosystem health is acknowledged as one of the major adaptations needed to build a resilient flood management system. Despite this understanding, the pace at which multi-benefit floodplain restoration projects are implemented is inadequate. The EcoFIP modeling framework was developed to reduce the technical burden associated with multi-benefit restoration on local stakeholders by systematically evaluating all the potential physical restoration opportunities along major rivers, and quantifying potential benefits associated with restoration using robust methodologies developed by technical experts. This decision support tool can identify areas needed for conservation or land repurposing and help pave the way towards multi-benefit floodplain project implementation.
Multi-benefit Floodplain Restoration Framework
The EcoFIP methodology can be thought of as a “funnel” used to screen a large river system through three tiers of analysis to evaluate multi-benefit restoration projects at progressively finer scales and with increasingly data rich lenses (Figure 2).
Tier 1 – Large reach-scale inundation potential
Physical opportunities at steady flows for existing or altered terrains.
In Tier 1, Hydraulic models are simulated at several specific flows to identify extents of connected and disconnected areas. Height Above River (HAR) maps are generated that identify the height of the existing terrain relative to the water surface elevation (WSE) at a particular flow. Additionally, a terrain can be created by topographic modification of areas that meet a customizable elevation threshold relative to flood events. This is useful to identify areas that may be inundated by floodplain lowering (through excavation) to a target elevation.
Tier 2 – Multi-objective site identification and prioritization
Quantifying ecological metrics using a time series of historical data or projected flows for a future management or climate scenario without conducting long-term hydraulic model simulations.
The EcoFIP code develops a family of flow versus area curves for each spatial extent of interest which allows for inundation, recharge, and habitat metrics to be interpolated from daily flow values and summarized over a hydrologic period of interest (Average water year (WY) type is most common). While the current code evaluates recharge and habitat suitability, the EcoFIP method could similarly incorporate habitat criteria for multiple other species of interest. Tier 2 can be applied to any number of spatial extents within the analysis domain, referred to here as floodplain analysis units (FAUs), which are delineated by parcel or river mile polygons.
Floodplain Inundation – The acre-days of potentially inundated area for an average WY (based on daily flow time series of data simulated) are calculated. These results are normalized by the spatial extent of each FAU resulting in WY average “Days” of inundation.
Habitat Suitability – The acre-days of suitable habitat for an average WY along with the acre-days normalized by FAU area are calculated.
Recharge – Annual recharge rates are estimated in acre-feet per acre of FAU (i.e., average ft of potential recharge / WY). The percolation rate is defined based on physical properties of the Hydrologic Soils Groups (HSG) and multiplied by the inundated area time series for each site generated using the flow vs area curves. In this way, the spatial extent of inundation and the average limiting percolation rates are combined to generate a time series of recharge. The annual recharge is further limited by considering the average available storage between the floodplain surface and the groundwater table underlying each FAU in a given WY.
Site Ranking – A ranking approach was developed based on the suitability score for salmonid habitat (average days of suitable habitat per WY) and recharge (ft of recharge per WY). Each of these metrics is normalized from 0-1, and the geometric mean of the two components is used to rank the sites from low (best sites) to high (worst sites). While the current ranking approach was based on suitability scores for salmonids and recharge, approaches could be developed for other terrestrial and aquatic habitats based on available habitat criteria developed for other species of interest. The low ranked FAUs are carried forward for additional analysis using the tier 3 methodology.
Tier 3 – Site Evaluation
Spatially explicit, pixel-based analyses that further consider the connectivity and duration of inundation to provide increasingly accurate estimates of salmonid habitat and recharge at the project site scale.
The pixel-based analysis from Tier 3 provided a more rigorous view of each site due to evaluation of connectivity and duration of inundation. Tier 3 is ultimately more accurate and provides more conservative estimates of inundation, suitable habitat, and recharge. Unlike Tier 2, which considers any potential inundation based on adjacent water surface elevations, Tier 3 evaluates areas that are (or could be) connected to the main river corridor to remove isolated, ponded areas from consideration.
Potential Applications of EcoFIP
The application of the EcoFIP framework provides a systematic approach to identify and prioritize multi-benefit floodplain restoration projects. The approach is both scalable and modular with numerous potential applications to increase the pace of multi-objective floodplain restoration:
- EcoFIP could be used to evaluate the impact of alternative reservoir management (e.g., Reservoir reoperation using Forecast Informed Reservoir Operations) or future climate scenarios on downstream floodplains.
- Restoration concepts and designs can be evaluated with EcoFIP to estimate potential uplift relative to baseline conditions.
- The knowledge built through EcoFIP development can be shared with groups looking to build resilience through multi-benefit floodplain restoration projects.
- While recent development has focused on salmonid habitat suitability and recharge, the EcoFIP method could similarly incorporate habitat criteria for multiple other species of interest. Other indices that could be generated include shorebirds, migratory waterfowl, and riparian habitat.
- EcoFIP has recently been applied along a pilot reach on the Upper San Joaquin River. The framework could be applied to any other water system of interest to promote adaptive management through floodplain rehabilitation.