Catchment scale analysis of land management and flood risk

The evidence of observations of soil compaction and the impacts of features such as tree belts and storage ponds suggests land management can reduce peak runoff rates at a small scale. The question is then whether small-scale interventions can be useful as part of a larger catchment-scale flood mitigation strategy. This implies a need to understand how peaks in runoff from upland catchments are routed into the downstream flow and how the effects of land management measures change with changing magnitude of storm event.

It is expected that relative effects of land use management interventions will decrease with increasing magnitude of event (Jackson et al., 2008). This view would be consistent with studies of process controls on the probability distribution of flood flows suggesting that, in the most extreme events, the rainfall distribution controls the magnitude-frequency relationship for flows but that the processes associated with catchment soil moisture storage modify this distribution in the lower magnitude events (Beven, 1986; Sivapalan et al., 1990).

Analysis of peak runoff records has so far produced very little firm evidence of catchment-scale impacts of land use management. O'Connell et al. (2007a) noted that lack of evidence does not necessarily imply lack of effect, but may reflect limitations of the data and analytical techniques. There remains no widely accepted model for predicting impacts of land management change on flood risk and there are limitations in the methods available for estimating the uncertainty in predictions. Analysis of temporal sequences of annual maximum flood peak data have failed to show convincing, systematic and statistically significant changes that could be attributed to catchment or climate change (e.g. Robson, 2002). The analysis is made difficult by the fact that different processes such as climate variability and changes in runoff production could be interacting at different scales.

There is considerable uncertainty about whether, and how, land management measures could be implemented on the ground to meet strategic flood risk mitigation aims. O'Connell et al. (2007a) and Beven et al. (2008) have found that conclusive evidence of a large-scale influence remains elusive. If further studies do show more significant evidence of an impact on flood risk, it still remains to be seen what the cost and practicality would be of implementing the changes required, or what the costs and benefits would be relative to more conventional approaches such as flood storage or raised embankments.

There is a continuing need for further monitoring of hydrological responses in the field at a range of scales and for different land management practices. Several long-term research programmes are under way to provide new information that will improve understanding of processes and may help to develop better predictive models. Examples include the CHASM (Catchment Hydrology And Sustainable Management) programme (O'Connell et al, 2007b), the Pontbren programme mentioned earlier and the Sustainable Catchment Management Programme (SCaMP) managed by United Utilities.3

19.4.3 Practical approaches to modelling change

Some attempts have been made to provide interim approaches to model land management impacts on flood risk using readily available modelling methods. One such approach is to modify the parameters of conceptual rainfall-runoff models to reflect changes in land management practice, which has been the basis for a study at the 120 km2 catchment draining through Ripon in North Yorkshire (Fig. 19.12).

Ten sub-catchment rainfall-runoff models were applied, each representing an area with similar physical characteristics, such as topography, rainfall, soils and land use. The probability distributed moisture (PDM) model was used, with each sub-catchment linked via a flood routing model (see Chapter 14) in order to simulate flows at the

Improved/semi-natural grass

Moorland and improved/semi-natural grass

Improved/semi-natural grass

Moorland and improved/semi-natural grass

Urban

Moorland and improved/semi-natural grass

Arable

Moorland and improved/semi-natural grass

Elevation (m above sea level)

■ ■ ■ Extent of river routing model

- 75

225 - 300

^u*""^ River and floodplain

|_ 75 - 150

300 - 375

| | 150 - 225

375 -

[ | Sub-catchment models Arable (etc.)Sub-catchment land use

Fig. 19.12 Sub-catchment rainfall runoff models applied in the Ripon catchment.

Urban

Arable

Fig. 19.12 Sub-catchment rainfall runoff models applied in the Ripon catchment.

outlet to the study catchment at Alma Weir on the River Skell in Ripon. The PDM and routing models were calibrated assuming spatially uniform parameters to the current 'baseline' conditions, which were assumed to be degraded to some extent given the general agricultural intensification and moorland improvements that have taken place over the last 50 years or so. Design events with 10-year, 50-year and 100-year return periods were simulated using a separate stochastic rainfall model fitted to rain-gauge datasets from the catchment.

Table 19.4 identifies four scenarios representing plausible changes to the land management or land use in the catchment. The scenarios reflect practices that would lead to the degradation of the soils within the catchment (compaction or crusting of vulnerable soils, conversion of improved grassland to maize production) or a change to the management of the moorland in the headwaters (maintenance of moorland grip drainage or moorland grip blocking). Impacts of the scenarios were modelled by alterations to parameters of the PDM models based on changes in hydrological descriptors suggested by Packman et al. (2004) and inferred links to the rainfall-runoff model parameter values given by Calver et al. (2005). The impact of moorland grip drainage blocking in controlling the generation and rate of runoff was also investigated via a sensitivity test in which the mean response time of fast-flow routing parameters in each moorland sub-catchment was varied by ±1 h. For each scenario, Table 19.4 shows modelled changes in peak flow rate and timing.

The modelling results indicated that the worst-case plausible degradation scenario (combining soil structural degradation across the whole catchment and additional moorland grip maintenance) could lead to increased peak flows in Ripon compared to the baseline case of between about 20 per cent for smaller scale floods and about 10 per cent for more extreme floods. A less extensive scenario assuming soil degradation over 30 per cent of the catchment led to increased peak flows of 10 per cent for smaller scale floods and 3 per cent for more extreme events. In contrast, the best-case plausible improvement scenario (moorland grip blocking) led to a reduction of flood peak magnitudes in Ripon by up to about 8 per cent when compared to the baseline case. The timing of the flood peak in Ripon was altered by up to 1.5 h as a result of the scenarios, though changes to the timing of the hydrographs generated in the moorland areas were attenuated by the time they had reached Ripon.

This modelling approach is not as sophisticated as the detailed, physics-based methods used in some research studies, but provides an approach to modelling catchment change accessible using tools that are in widespread use in applied hydrology and underpinned by empirical relationships. The key steps are:

(1) define plausible changes to land management, that could affect runoff;

(2) specify the scale of the change (land cover distribution, soil types);

(3) quantify changes in catchment descriptors;

(4) use relationships between model parameters and the defined changes to simulate the impact on runoff.

Improvements upon this, and other, studies of land management impacts depend on improved models based on relevant observational data, for which further field monitoring is vitally important.

Table 19.4 Land management scenarios for Ripon catchment

Scenario

Impact on soils and catchment

Associated PDM model parameters

Peak flow

increase

Advance

of peak (h)

Maximum soil moisture

Runoff response

10 years

SO years

100 years

10 years

50 years

100 years

storage capacity

time

Increased percentage runoff

Reduced

Reduced

10%

3%

3%

-0.25

0.25

0.25

2

Increased percentage runoff

Reduced

Reduced

21%

16%

10%

-0.25

3

Grip maintenance, loss of peat

No change

reduced

12%

9%

5%

0

0.5

0.5

4

Grip blocking

No change

increased

-1%

-7%

-5%

0

-0.5

1. Soil structural degradation applied to 50% of all degradable soils, about 35% of the catchment area.

2. Combined degradation (soil structural degradation and moorland gripping). A 'worst-case scenario'.

3. Moorland degradation (grip maintenance), 40% of the catchment area.

4. Moorland improvement (grip blocking), 40% of the catchment area.

Scenario key:

1. Soil structural degradation applied to 50% of all degradable soils, about 35% of the catchment area.

2. Combined degradation (soil structural degradation and moorland gripping). A 'worst-case scenario'.

3. Moorland degradation (grip maintenance), 40% of the catchment area.

4. Moorland improvement (grip blocking), 40% of the catchment area.

0 0

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