A method for adjusting design stormpeakedness to reduce biasin hydraulic simulations

Abstract

<p>In the UK, decision-makers use hydraulic model outputs to inform funding, connection consent, adoption of new<br>drainage networks and planning application decisions. Current practice requires the application of design storms to<br>calculate sewer catchment performance metrics such as flood volume, discharge rate and flood count. With flooding<br>incidents occurring more frequently than their designs specify, hydraulic modelling outputs required by practice are<br>questionable. The main focus of this paper is the peakedness factor (ratio of maximum to average rainfall intensity)<br>of design storms, adjudging that this is a key contributor to model bias. Hydraulic models of two UK sewer<br>catchments were simulated under historical storms, design storms and design storms with modified peakedness to<br>test bias in modelling outputs and the effectiveness of peakedness modification in reducing bias. Sustainable<br>drainage systems (SuDS) were implemented at catchment scale and the betterment achieved in the modelling outputs<br>was tested. The proposed design storm modification reduced the bias that occurs when driving hydraulic models<br>using design storms in comparison with historical storms. It is concluded that SuDS benefits are underestimated when<br>using design rainfall because the synthetic rainfall shape prevents infiltration. Thus, SuDS interventions cannot<br>accurately be evaluated by design storms, modified or otherwise.</p>