Overland Flow Pathways Research Articles

Depressional runoff cascade networks of the Des Moines Lobe of Iowa

AbstractIn this study, we develop a general mathematical framework and algorithm for routing cumulative precipitation excess through depressional fill–spill cascade networks in a landscape using only information about depression morphology, local contributing areas, and potential overland flow pathways. The framework also allows for the classification of depressions according to their landscape position within a network, and calculation of precipitation‐ and non‐precipitation‐dependent network properties, including measures of network complexity and runoff connectivity. To demonstrate its use, we applied our framework to the 167,287 drained depressions of the Des Moines Lobe of Iowa, a sub‐region of the Prairie Pothole Region of North America, over a large range of historically observed precipitation amounts for scenarios both neglecting and incorporating infiltration in runoff generation. Our results show that 85.3% of depressions in this region form 18,851 unique depressional runoff cascade networks, with the remainder being disjunct features. Most of the properties of the region's networks appear to conform to either a truncated power‐law or lognormal distribution. For a given rainfall amount, surface runoff connectivity between depressions within networks, and between networks and off‐network areas, is controlled primarily by available aggregate depressional volumetric storage and contributing area, and to a lesser degree, network complexity.

Urban road waterlogging risk assessment based on the source–pathway–receptor concept in Shenzhen, China

AbstractUrban roads are especially vulnerable to waterlogging during rainfall events. Assessments of urban road waterlogging risk are critical for disaster prevention and mitigation. This paper presents a rapid and practical approach for assessing the risk of urban road waterlogging based on a source–pathway–receptor concept and a digital elevation model (DEM). Waterlogging sources are identified by neighborhood analysis, overland flow pathways are extracted using a DEM, and the properties of cross‐nodes between pathways and roads are used to assess the waterlogging risk of roads. Futian District in Shenzhen, China, is selected as the study area. The risk results show that the waterlogging risk sources are clustered in residential land parcels and low‐lying road sections, while most road intersections and overpasses in the central area are at higher risks. The results also found that the waterlogging risk decreases as the road grade increases. Actual telephonic flooding records and reported waterlogging areas were used to validate the method, with the risk results displaying good agreement with the actual conditions. The prospects for generalizing this framework and its usage are discussed. The proposed approach provides support for pluvial flooding risk assessment and urban road waterlogging prevention.

Post-fire practices benefits on vegetation recovery and soil conservation in a Mediterranean area

Post-fire practices (PFP) aim to reduce soil erosion and favour vegetation recovery, but their effectiveness is spatially heterogeneous and under debate because of the economic and environmental costs. This study evaluates the different changes (Δ) of canopy cover (CC), sediment connectivity (SC) and local topography in four areas affected by the Pinet fire in eastern Spain (August 8th, 2018) and managed with: totally burnt with tree removal and long log erosion barriers (LEBs) (Pinet-1), partially burnt without PFP (Pinet-2), totally burnt with tree removal and short LEBs (Pinet-3), and totally burnt without PFP (Pinet-4). An unburnt nearby area was used as control site (Pinet-5). High-resolution images obtained before the fire and during two drone flights after the fire (10.5 and 5.5 months after the fire and PFP; and 18 and 13 months after the fire and PFP) were analysed; and LiDAR- and SfM-derived digital elevation models used to compute the Aggregated Index of SC (AICv2). After correcting calculations, because of the different input sources, and excluding the forest roads (x¯=3.6% of the total surface), CC in the first post-fire scenario was of 25.5% (−40.4% with respect to the pre-fire scenario), 14.5% (−68.4%), 23.8% (−43.7%), 26.9% (−26.5%) and 29.6% (−32.7%) in Pinet-1, P-2_totally_burnt, P-2_partially_burnt, P-3 and P-4; and ΔCC among the drone flights were of +2.45%, +0.02% and +10.54% in Pinet-1, Pinet-3 and Pinet-4. The annual CC recovery rate decrease from 27.5% to 19.1% per year between the first and the second post-fire scenario, indicating a quick vegetation recovery, especially in the first year, and considering the surface area covered by rocks (x¯=16.3%). Topographic changes indicated that not install LEBs favoured shorter flow length pathways after the fire, and thus, runoff will flow faster to cover the same area, achieving higher velocity and thus soil detachment capacity. Sediment connectivity increased in all burnt sub-sites after the fire (Δ1¯=+32.4%), but the increments in the two sub-sites with LEBs were 36% lower than the increase in the sub-sites without LEBs. The increase of connectivity in the first and second post-fire scenarios was −32% and −45% in the sub-site with long LEBs compared with the sub-site with short LEBs. Overall, LEBs effectively favoured vegetation recovery, lengthened overland flow pathways, and reduced sediment transport in the early months, but their usefulness was not as pronounced during the second post-fire year, although these results may be influenced by the Mediterranean conditions of the site.

Quantifying the hydrological implications of pre- and post-installation willowed engineered log jams in the Pennine Uplands, NW England

Nature Based Solutions (NBS), including Natural Flood Management (NFM) schemes are becoming an important component of many governmental and organisation responses to increases in flood and aridity risk. NFM structures may take multiple forms to slow, store, disconnect and filter distributed overland flow pathways within a catchment that coalesce to generate a flood-wave downstream and runoff rather than infiltrate groundwaters. To date few studies have conducted observations pre- and post-installation monitoring at river reach-scales, despite widespread and frequent installation, to investigate the efficacy of willowed engineered log jams (WELJs) interventions used in abating flood-flows, through backing-up flood-pulses with consequent reductions in downstream discharges. This paper examines the efficiency, before and after installation of five 1 m high WELJs incorporating 1,000 Bay willow (Salix pentandra) saplings supporting the dead horizontal timber, across a total of 130 linear metres spanning the floodplain of a decommissioned reservoir. One rain gauge, two fixed point time-lapse wildlife cameras and three water level stations were installed: upstream-of, within, and downstream-of all WELJs. The findings demonstrate a substantial reduction is achieved for most events, with an average of 27.3% reduction in peak discharge being achieved post-installation. The time to peak is little impacted, however there is demonstrable evidence of a longer and higher recessional limb to the events. These findings quantify for the first time the role that WELJs can play in a move towards re-naturalisation of water level regimes, with lower peak water flows achieved, and waters released from the river-reach more slowly. Furthermore, baseflow during dry periods is also elevated by 27.1%, offering greater resilience to dry periods and droughts. Consequently, over the river-reach scale (0–130 m), WELJs play an important role in alleviating flood and drought risk through suppressing flood peaks and increasing baseflow during low flows; steps towards improved hydro-morphological quality overall.

Effect of vegetation on the flow pathways of steep hillslopes: Overland flow plot-scale experiments and their implications

Overland flow pathways probably play a key role in linking vegetation to its effects on runoff generation and soil loss on the hillslope. However, there is little empirical work testing the links. This study aims to address how flow pathway characteristics vary with plant spacing and determine their relationships with infiltration and soil loss. Upslope inflow experiments were carried out on 10 vegetated plots under a small inflow rate. Total flow path width (RW), mean width per flow path (WPP), and total flow path length (FL), were used to quantify the flow pathway characteristics by analyzing photographs of the plot surface taken during the experiment. The results showed that RW and WPP decreased (R2 = 0.86 and R2 = 0.90), while the FL increased (R2 = 0.67) as alfalfa spacing increased. The RW was positively related to the infiltration rate in the early period of the experiment, indicating that runoff loss via infiltration as affected by the flow paths needs to be considered in hydrological models. Runoff velocity was detected to be significantly related to the flow pathway characteristics. The relationship can partially explain the variances of runoff hydraulics on the vegetated hillslopes in previous studies, in which the variation of flow pathways was not taken into account. The soil loss decreased with the RW and WPP while increased with the FL. As the flow pathway characteristics expressed real-time and actual movement of overland flow, they represented the functional connectivity of sediment on the hillslope. The relationships between soil loss and flow pathway became insignificant as the upslope inflow progressed despite no marked change in the flow pathway. Our results imply that both functional connectivity and the soil's resistance to erosion should be considered to obtain a good prediction of the soil loss from hillslopes.

The Beast from the East: impact of an atypical cold weather event on hydrology and nutrient dynamics in two Irish catchments

A historic lack of continuous stream nutrient monitoring at the catchment scale limits understanding of the effects of snowstorms. The most significant snowstorm since 1985, nicknamed “the Beast from the East”, occurred in February–March 2018. High-frequency stream outlet monitoring in two close but hydrologically and agriculturally contrasting catchments (<1,200 ha) captured phosphorus (total and reactive), total oxygenated nitrogen (TON), temperature and discharge dynamics during and after the event. The grassland catchment consists of poorly drained gley soils and exhibits overland flow pathways, while the arable catchment consists of well-drained brown earths and is dominated by subsurface pathways. Nitrate (NO3-N) concentrations were initially elevated (3.50 and 7.89 mg/L for poorly drained grassland and well-drained arable catchments, respectively) before becoming diluted by meltwater. Total reactive phosphorus (TRP) displayed a distal (anti-clockwise) concentration-discharge hysteresis in the poorly drained grassland catchment suggesting low mobilisation from the soil. Conversely, the well-drained arable catchment displayed proximal (clockwise) hysteresis, indicative of the mobilisation from stream and bank sediment. These relatively infrequent snow events behave similarly to heavy rainfall as regards nutrient losses, albeit subject to a time-lag induced by the speed of snowmelt and the soil moisture deficit (SMD) prior to snowfall. Antecedent land management is crucial to mitigate risk. The current absence of records and analyses of catchment response, particularly nutrient dynamics, to atypical cold weather events in Ireland limits understanding of their effects on water quality. The present study provides the first such baseline information from which land management strategies and the implications for attaining environmental targets can be explored.

Paleohydrogeologic reconstruction of Triassic carbonate paleolakes from stable isotopes: Encompassing two lacustrine models

Reconciling interpretations of geochemical, sedimentologic, and stratigraphic observations is a common challenge with many datasets, especially those from paleolake systems. The Triassic Cerro de las Cabras and Cerro Puntudo formations, part of the sedimentary fill of the Cuyana rift Basin in central-west Argentina, represent carbonate-rich playa-lake systems with abundant microbialites, but different sedimentologic and stratigraphic attributes such as presence or absence of efflorescent crystalline crusts and aggrading versus prograding-aggrading stacking patterns. A detailed study of the stable-carbon and oxygen-isotope composition of the limestones deposited in these paleolakes, together with sedimentological and mineralogical evidence as petrography, microprobe, XRD and cathodoluminescence data, enabled interpretation of their complex hydrology.Lack of correlation between carbon and oxygen stable-isotope values for both Cerro de las Cabras (δ13C χ–6.03‰; σ 0.67‰/δ18O χ −12.64‰; σ 0.55‰) and Cerro Puntudo (δ13C χ −3.10‰; σ 1.15‰/δ18O χ −12.58‰; σ 1.8‰) paleolakes strongly indicates open hydrology (underground flow), in apparent contradiction with sedimentologic and stratigraphic evidence for intermittent to persistently closed hydrography (overland flow). Stable-isotopic values for the Cerro de las Cabras paleolake together with sedimentology data indicate a recharge (hydrologically open) lake, and for the Cerro Puntudo a through-flow (hydrologically open) playa-lake. To resolve the apparent contradictions, we propose an integration of playa-lake hydrology (groundwater) with lake-basin type sedimentology, stratigraphy, and hydrography. This allowed identifying and proposing a balanced-fill–through-flow lake subtype for the Cerro Puntudo system and an underfilled–recharge-lake subtype for the Cerro de las Cabras paleolake; both with hydrology open to groundwater flow. This work suggests a starting point for expanding the lake-basin-type model to explicitly incorporate different groundwater and overland flow pathways.

The effects of topographic depressions on multiscale overland flow connectivity: A high‐resolution spatiotemporal pattern analysis approach based on connectivity statistics

AbstractIn watershed modelling, the traditional practice of arbitrarily filling topographic depressions in digital elevation models has raised concerns. Advanced high‐resolution remote sensing techniques, including airborne scanning laser altimetry, can identify naturally occurring depressions that impact overland flow. In this study, we used an ensemble physical and statistical modelling approach, including a 2D hydraulic model and two‐point connectivity statistics, to quantify the effects of depressions on high‐resolution overland flow patterns across spatial scales and their temporal variations in single storm events. Computations for both models were implemented using graphic processing unit‐accelerated computing. The changes in connectivity statistics for overland flow patterns between airborne scanning laser altimetry‐derived digital elevation models with (original) and without (filled) depressions were used to represent the shifts of overland flow response to depressions. The results show that depressions can either decrease or increase (to a lesser degree and shorter duration) the probability that any two points (grid locations) are hydraulically connected by overland flow pathways. We used macro‐connectivity states (Φ) as a watershed‐specific indicator to describe the spatiotemporal thresholds of connectivity variability caused by depressions. Four states of Φ are identified in a studied watershed, and each state represents different magnitudes of connectivity and connectivity changes (caused by depressions). The magnitude of connectivity variability corresponds to the states of Φ, which depend on the topological relationship between depressions, the rising/recession limb, and the total rainfall amount in a storm event. In addition, spatial distributions of connectivity variability correlate with the density of depression locations and their physical structures, which cause changes in streamflow discharge magnitude. Therefore, this study suggests that depressions are “nontrivial” in watershed modelling, and their impacts on overland flow should not be neglected. Connectivity statistics at different spatial scales and time points within a watershed provide new insights for characterizing the distributed and accumulated effects of depressions on overland flow.

Changes in ecosystem structure, function and hydrological connectivity control water, soil and carbon losses in semi‐arid grass to woody vegetation transitions

ABSTRACTConnectivity has recently emerged as a key concept for understanding hydrological response to vegetation change in semi‐arid environments, providing an explanatory link between abiotic and biotic, structure and function. Reduced vegetation cover following woody encroachment, generally promotes longer, more connected overland flow pathways, which has the potential to result in an accentuated rainfall‐runoff response and fluxes of both soil erosion and carbon.This paper investigates changing hydrological connectivity as an emergent property of changing ecosystem structure over two contrasting semi‐arid grass to woody vegetation transitions in New Mexico, USA. Vegetation structure is quantified to evaluate if it can be used to explain observed variations in water, sediment and carbon fluxes. Hydrological connectivity is quantified using a flow length metric, combining topographic and vegetation cover data.Results demonstrate that the two woody‐dominated sites have significantly longer mean flowpath lengths (4 · 3 m), than the grass‐dominated sites (2 · 4 m). Mean flowpath lengths illustrate a significant positive relationship with the functional response. The woody‐dominated sites lost more water, soil and carbon than their grassland counterparts. Woody sites erode more, with mean event‐based sediment yields of 1203 g, compared to 295 g from grasslands. In addition, the woody sites lost more organic carbon, with mean event yields of 39 g compared to 5 g from grassland sites.Finally, hydrological connectivity (expressed as mean flowpath length) is discussed as a meaningful measure of the interaction between structure and function and how this manifests under the extreme rainfall that occurs in semi‐arid deserts. In combination with rainfall characteristics, connectivity emerges as a useful tool to explain the impact of vegetation change on water, soil and carbon losses across semi‐arid environments. Copyright © 2013 John Wiley & Sons, Ltd.

Escherichia coli and Enterococci Attachment to Particles in Runoff from Highly and Sparsely Vegetated Grassland

Limited data on microbial partitioning between the freely suspended and particulate attached phases during transport along overland flow pathways have resulted in high uncertainty in bacterial fate and transport models and the application of these models to watershed management plans. The objectives of this study were to examine differences in attachment between E. coli and enterococci in runoff from plots with highly and sparsely vegetated grassland; investigate relations between flow regime, total suspended solids, and E. coli and enterococci attachment; and identify the particle size categories to which the attached cells were associated. Two rainfall simulations were conducted on large field plots 3 m wide by 18.3 m long with highly and both highly and sparsely vegetated covers and treated with standard cowpats. Results from the first experiment representing pasture with highly vegetated cover indicate that the majority of E. coli and enterococci are transported from the fresh manure source in the unattached state with only 4.8% of E. coli and 13% of enterococci associated with particles. The second experiment which compared partitioning in runoff from both highly and sparsely vegetated covers found lower bacterial attachment rates: the average E. coli percent attached was 0.06% from plots with highly vegetated cover and 2.8% from plots with sparsely vegetated cover while the corresponding values for enterococci were 0.98% and 1.23%, respectively. The findings from this study provide the first set of data on bacterial partitioning in overland flow from large field plots, and results may be helpful for parameterizing water quality models and designing conservation practices.

Sensitivity analysis of surface runoff generation in urban flood forecasting

Reliable flood forecasting requires hydraulic models capable to estimate pluvial flooding fast enough in order to enable successful operational responses. Increased computational speed can be achieved by using a 1D/1D model, since 2D models are too computationally demanding. Further changes can be made by simplifying 1D network models, removing and by changing some secondary elements. The Urban Water Research Group (UWRG) of Imperial College London developed a tool that automatically analyses, quantifies and generates 1D overland flow network. The overland flow network features (ponds and flow pathways) generated by this methodology are dependent on the number of sewer network manholes and sewer inlets, as some of the overland flow pathways start at manholes (or sewer inlets) locations. Thus, if a simplified version of the sewer network has less manholes (or sewer inlets) than the original one, the overland flow network will be consequently different. This paper compares different overland flow networks generated with different levels of sewer network skeletonisation. Sensitivity analysis is carried out in one catchment area in Coimbra, Portugal, in order to evaluate overland flow network characteristics.

Sediment concentration changes in runoff pathways from a forest road network and the resultant spatial pattern of catchment connectivity

Hydrological connectivity is a term often used to describe the internal linkages between runoff and sediment generation in upper parts of catchments and the receiving waters. In this paper, we identify two types of connectivity: direct connectivity via new channels or gullies, and diffuse connectivity as surface runoff reaches the stream network via overland flow pathways. Using a forest road network as an example of a landscape element with a high runoff source strength, we demonstrate the spatial distribution of these two types of linkages in a 57 km 2 catchment in southeastern Australia. Field surveys and empirical modelling indicate that direct connectivity occurs primarily due to gully development at road culverts, where the average sediment transport distance is 89 m below the road outlet. The majority of road outlets were characterised by dispersive flow pathways where the maximum potential sediment transport distance is measured as the available hillslope length below the road outlet. This length has a mean value of 120 m for this catchment. Reductions in sediment concentration in runoff plumes from both pathways are modelled using an exponential decay function and data derived from large rainfall simulator experiments in the catchment. The concept of the volume to breakthrough is used to model the potential delivery of runoff from dispersive pathways. Of the surveyed road drains ( n=218), only 11 are predicted to deliver runoff to a stream and the greatest contributor of runoff occurs at a stream crossing where a road segment discharges directly into the stream. The methodology described here can be used to assess the spatial distribution and likely impact of dispersive and gullied pathways on in-stream water quality.

Interstorm surface preparation and sediment detachment by vehicle traffic on unpaved mountain roads

Road survey and field rainfall simulation experiments have shown that the erodibility of a road surface is dynamic. In the absence of extreme runoff events, dynamic erodibility results from the generation and removal of easily entrained surface material by human road surface maintenance activities, vehicular detachment and overland flow events. Maintenance activities introduce easily transportable material to the road surface where it can be entrained by overland flow. Traffic in dry conditions detaches material that is quickly removed during subsequent overland flow events. The pre-storm erodibility of a road is therefore largely a function of maintenance and vehicle activity since the last overland flow event. During rainstorms, vehicle passes increase sediment production by detaching/redistributing surface material and creating efficient overland flow pathways for sediment transport. However, if incision of tracks by overland flow does not occur, post-pass sediment transport quickly returns to pre-pass rates. Field rainfall simulation data suggest that sediment transport resulting from during-storm vehicle passes is greatly influenced by the presence of existing loose material, which again is a function of prior road usage and maintenance activities. Incorporation of vehicular effects into physically based road erosion models may be possible by parameterizing both during-storm and inter-storm changes in the supply of loose surface material as changes in surface erodibility. Copyright © 2001 John Wiley & Sons, Ltd.