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The Hydrological Response Of Established Riparian Buffers At Field Scale During Storm Events

Congress: 2015
Author(s): Linsey McLean, Lindsay Beevers, Mark Wilkinson, Gareth Pender
Heriot-Watt University1, James Hutton Institute2

Keyword(s): Sub-theme 10: Management of water resources,
AbstractINTRODUCTION
Riparian buffer strips have traditionally been adopted for their ability to reduce diffuse pollution and promote ecological habitat. Hitherto, most research focuses on their water quality and hydraulic roughness functions. More recently however, EU and UK flooding legislation have promoted a sustainable approach to flood management encouraging the use of natural processes and features to reduce flooding (Flood Risk Management (Scotland) Act, 2009). From a theoretical perspective, one would expect riparian buffers to increase: infiltration, interception, evapotranspiration and hydraulic roughness, thereby attenuating runoff. Given that these hydrological behaviours will reduce flood risk, there is reason to consider riparian buffers as a natural flood management (NFM) measure with multifunctional purposes (diffuse pollution, water quality and ecology) (Barlow et al. 2014, Bentley et al. 2014). However, there is a distinct lack of clear data to justify this at the present time. There is also uncertainty regarding: the scale of flood event that riparian buffer strips may be able to attenuate, the optimal width of buffer strips in relation to slope, and the spatial scale across a catchment at which they need to be implemented. There is therefore an urgent need for field data to evidence that riparian buffers can influence hydrology sufficiently to reduce flood risk, and to identify temporal, spatial and event scale thresholds for effectiveness. The aim of this study is to examine the field scale hydrological responses of a riparian buffer strip during storm events and to categorise the hydrological behaviour and rainfall characteristics being responded to. The study uses empirical field data to contribute to the understanding, and evidence base, of how effective riparian buffers are as NFM measures.

METHODS
Empirical hydrological data (soil moisture, surface runoff and rainfall) is obtained from an experiment site in the Tarland sub-catchment of the River Dee in Aberdeenshire, Scotland. This data is used to categorise the riparian buffer runoff and soil moisture behavioural response to storm events. These categories are then compared to the storm event characteristics, which are also categorised, to clarify the field scale ability of riparian buffers to attenuate runoff. A threshold of effectiveness is assessed by comparing the characteristics and behaviour of: rainfall events, environmental conditions and riparian runoff, as well as an examination of interactions between the hydrological variables.

RESULTS & DISCUSSION
Initial results from August 2014 to February 2015 show how the riparian buffer behaves during rainfall events where the following key characteristics are individually assessed: antecedent conditions, rainfall depth, rainfall intensity and rainfall duration. The riparian buffer behaves in the following way:

Runoff outside the buffer occurs most frequently during high rainfall intensity and high rainfall duration;
Runoff inside the buffer also occurs but less often;
Runoff demonstrates a transitional behaviour whereby it begins higher outside the buffer and becomes higher within the buffer, occurring mostly during high rainfall depths;
Soil moisture both inside and outside the buffer is high across a similar number of events
Soil moisture outside the buffer occurs more frequently during high rainfall intensity, high rainfall durations, high rainfall depths and wet antecedent conditions
Soil moisture demonstrates a gradual increase both inside and outside the buffer in some events
The behaviour types- transitional runoff and soil moisture higher outside the buffer- occur in combination during the most extreme events where high rainfall depth, duration and intensity occur

These behaviours suggest the riparian buffer is attenuating runoff and therefore reducing flood risk locally. However, further data analysis of subsequent months is necessary.

CONCLUSION
Initial results of the riparian buffer strip hydrological data suggest that in most instances the riparian buffer performs as expected however, there are some data demonstrating a contrasting behaviour whereby surface runoff is higher inside the riparian buffer. The combination behaviour of transitional runoff and soil moisture being high outside the buffer during the most extreme storm events recorded, indicate there may be threshold of effectiveness. Further data collection in conjunction with continued classification of riparian buffer behaviour responses and rainfall characteristics is necessary to elucidate the ability of riparian buffer strips to reduce flood risk. This study however, thus far, suggests that riparian buffer strips are effective at reducing localised flood risk by attenuating runoff in storm events. 1. Barlow, J, Moore, F & Burgess-Gamble, L, 2014. Delivering Benefits Through Evidence: Working with Natural Processes to Reduce Flood Risk, R&D Framework: Science Report, Environment Agency, Bristol, SC130004/R2.
2. Bentley, S, Brady, R, Cooper, J, Davies, K, Hemsworth, M, Robinson, P & Thomas, L, 2014. Delivering Benefits Through Evidence: Aquatic and Riparian Plant Management: Controls for Vegetation in Watercourses, Technical Guide, Environment Agency, Bristol, SC120008/R2.
3. Flood Risk Management (Scotland) Act 2009, The Stationery Office Limited, UK.

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