Abstract
AbstractThere is considerable interest in how headwater management may influence downstream flood peaks in temperate humid regions. However, there is a dearth of data on flow velocities across headwater hillslopes and limited understanding of whether surface flow velocity is influenced by seasonal changes in roughness through vegetation cycles or management. A portable hillslope flume was used to investigate overland flow velocities for four common headwater grassland habitats in northern England: Low‐density Grazing, Hay Meadow, Rank Grassland and Juncus effusus Rush pasture. Overland flow velocity was measured in replicate plots for each habitat, in response to three applied flow rates, with the experiments repeated during five different periods of the annual grassland cycle. Mean annual overland flow velocity was significantly lower for the Rank Grassland habitat (0.026 m/s) followed by Low‐density Grazing and Rushes (0.032 and 0.029 m/s), then Hay Meadows (0.041 m/s), which had the greatest mean annual velocity (examples from 12 L/min flow rate). Applying our mean overland flow velocities to a theoretical 100 m hillslope suggests overland flow is delayed by >1 hr on Rank Grassland when compared to Hay Meadows in an 18 mm storm. Thus grassland management is important for slowing overland flow and delaying peak flows across upland headwaters. Surface roughness was also strongly controlled by annual cycles of vegetation growth, decay, grazing and cutting. Winter overland flow velocities were significantly higher than in summer, varying between 0.004 m/s (Rushes, November) and 0.034 m/s (Rushes, June); and velocities significantly increased after cutting varying between 0.006 m/s (Hay meadows, July) and 0.054 m/s (Hay meadows, September). These results show that seasonal vegetation change should be incorporated into flood modelling, as cycles of surface roughness in grasslands strongly modify overland flow, potentially having a large impact on downstream flood peak and timing. Our data also showed that Darcy‐Weisbach roughness approximations greatly over‐estimated measured flow velocities.
Highlights
The frequency and intensity of flooding in many parts of the world is increasing, and climate change is a significant driver (Feyen, Barredo, & Dankers, 2008; Hirabayashi et al, 2013; Middelkoop et al., 2001; Wingfield, Macdonald, Peters, Spees, & Potter, 2019)
In response to the same applied flow event, overland flow velocity for the Hay Meadows habitat was up to double that recorded for Rank Grassland (Table 2, Figure 3)
If theoretically applied over a continuous 100 m hillslope, the difference in roughness we found is such that, for a 12L/min applied flow rate, the mean time for flow to reach the bottom of the slope ranges between 40 minutes for the Hay Meadows habitat in comparison to 64 minutes for the Rank Grassland habitat
Summary
The frequency and intensity of flooding in many parts of the world is increasing, and climate change is a significant driver (Feyen, Barredo, & Dankers, 2008; Hirabayashi et al, 2013; Middelkoop et al., 2001; Wingfield, Macdonald, Peters, Spees, & Potter, 2019). There is a lack of information, at a range of scales, about how some types of land-cover change and land-use management practices may influence downstream flood risk (Rogger et al, 2017). Despite this lack of data, a number of initiatives are being undertaken that seek to use ‘nature-based solutions’ to flooding, including the sponge-city concept in some Chinese cities (Li, Ding, Ren, Li, & Wang, 2017; Liu, Jia, & Niu, 2017), and the use of. There have been suggestions that increased grazing intensities in UK upland grasslands may influence flood risk downstream In environments where overland flow is common, vegetative surface roughness may be important in slowing water flow and impacting downstream flood peak magnitude and timing
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