ABSTRACT:

Floodplain roughness, quantified through Manning’s coefficient n, is a critical parameter in hydrological models for predicting flood dynamics and managing water resources. Traditional methods to determine n rely on roughness values based principally on generalized land cover types and fail to capture the spatial and structural variability of floodplains, leading to inaccuracies during flood events. This study presents a novel approach to estimating floodplain roughness across the conterminous United States (CONUS) by integrating high-resolution remotely sensed canopy height and biomass data from NASA’s Global Ecosystem Dynamics Investigation mission and other spatially distributed data to map Manning’s roughness more accurately across a range of environments. We train a machine learning model (random forest regression) on a dataset of 4,927 roughness estimates from 804 sites to provide the estimates of n at 17.8 million reaches within the Notational Hydrography Database across CONUS. This approach results in a new CONUS wide n database with an R² of 0.51, a root mean squared error of 0.084, and a mean absolute percentage error of 122%, indicating its ability to capture much of the variability in floodplain roughness across CONUS. Canopy height and biomass were identified as the most influential predictors, highlighting the importance of vegetation structure in shaping floodplain dynamics. These results demonstrate the potential for integrating remote sensing data with machine learning models to enhance flood risk assessment and improve the accuracy of hydrological models.

ABSTRACT:

Publication Title: Continental Scale Assessment of Variation in Floodplain Roughness with Vegetation and Flow Characteristics

Quantifying floodplain flows is critical to multiple river management objectives, yet how vegetation within floodplains dissipates flow energy lacks comprehensive characterization. Utilizing over 3.4 million discharge measurements, in conjunction with aboveground biomass and canopy height measurements from NASA’s Global Ecosystem Dynamics Investigation (GEDI), this study characterizes the floodplain roughness coefficient Manning’s n and its determinates across the continental United States. Estimated values of n show that flow resistance in floodplains decreases as flow velocity increases but increases with the fraction of vegetation inundated. A new function (RMSE = 0.024, r2 = 0.74) is proposed for predicting n based of GEDI vegetation characteristics and flow velocity, with GEDI derived n values improving predictions of discharge relative to those based only on land cover. This analysis provides evidence of key hydraulic patterns of energy dissipation in floodplains, and integration of the proposed function into flood and habitat models may reduce uncertainty.