Submitted to: Journal Hydrologic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2007
Publication Date: 9/1/2008
Citation: Shuster, W.D., Pappas, E.A., Zhang, Y. 2008. Laboratory-Scale Simulation of Runoff Response from Pervious-Impervious Systems. Journal Hydrologic Engineering. 13(9):886-893. Interpretive Summary: The variety of surfaces present in an urban landscape impact runoff production. Specifically, the amount of impervious surfaces (like pavement) relative to pervious surfaces (like soil), whether these impervious surfaces are drained directly onto pervious surfaces or other impervious surfaces, and how wet the pervious surfaces are to begin with affect how and how much rainfall will run off of the urban landscape. Mathematical and computer models predict runoff production using these values. In order to predict the importance of each of these factors, we performed rainfall simulation on 1-m x 4-m slope lengths having different proportions and configurations of impervious surfaces, and different beginning soil moisture levels in the laboratory. We found that beginning moisture levels more strongly influenced runoff production than impervious surface factors, especially where impervious cover was low. We also found that typical runoff prediction models would under predict runoff in our experimental system. These results may impact how runoff production is predicted by models, and ultimately how runoff control structures are designed.
Technical Abstract: Urban drainages have mixtures of pervious and impervious surfaces that vary widely on the basis of proportional impervious area, connectivity, and antecedent conditions of the pervious areas. These factors influence runoff production in the urbanizing landscape. Many runoff prediction technologies employ runoff curve numbers in the calculation of predicted runoff. We examined how the runoff curve number (CN) factors of impervious extent, connectivity, and antecedent moisture content of pervious areas might affect runoff production at small spatial scales in a laboratory setting. We used rainfall simulation with a relatively intense rainfall structure spread out over three contiguous and increasingly higher rates (20, 30, 40 mm hr-1) to generate runoff from 0.6 m2 boxes (impervious or pervious-soil) 0.2 m deep that were connected together in series to produce different arrangements of impervious and pervious surfaces (0, 25, 50% impervious) with different connectivity to the outlet (disconnected, connected), and under two different antecedent moisture conditions for pervious areas. We found runoff ratios were strongly affected by antecedent moisture condition, and somewhat less significantly by an interaction between impervious area extent and its connectivity status. In each impervious treatment, we observed a decreased time to runoff initiation and higher final runoff ratio for wet than dry treatments. Interestingly, we found that the connectivity of 25% impervious area accounted for differences in runoff ratio only early in the simulation, and eventually response from connected and disconnected 25% treatments converged, leaving antecedent moisture conditions the only relevant factor. Empirically derived CNs were generally higher than those recommended in TR-55 guidance, and ranged from 93 to 99 for 0% and connected 50% impervious cover, respectively. Although specific to this study, this CN representation is used to illustrate the practical realizations of mechanistic differences in runoff response from pervious-impervious systems.