Short-term bank gully retreat rates in Mediterranean environments

In this study. short-term headcut retreat was monitored from 46 active bank gullies. selected in the Guadalentin and the Guadix basin in Southeast Spa in, The measurements were carried out manually using an orthogonal referenc e system fixed by erosion pins around the gully heads. between April 1997 a nd April 1999 with a 1-year interval. The average volumetric retreat rate f or all gullies was 4.0 m(3) year(-1), corresponding with an average linear retreat rate of 0.1 m year(-1), but more erosion took place during the firs t monitored year (1997-1998) compared to the second (1998-1999). An interpl ay of spatial variations in rainfall distribution and tension crack activit y is assumed to be responsible for the important difference in annual headc ut retreat. compared to the small difference in annual rainfall amounts, St atistical analysis showed that the present drainage-basin area (A(p)) was t he most important topographical factor explaining average gully headcut ret reat rate. both in terms of annual eroded volume (V-e) and annual linear re treat (R,), and expressed by the power relationships V-e = 0.04 A(p)(0.38) (R-2 = 0.39) and R-1 = 0.01 A(p)(0.23) (R-2 = 0.39). The V-e-A(p) relations hip was compared with the relationship between original drainage-basin area (A(o)) and total eroded bank gully volume (Vol), i.e. Vol = 1.71 A(o)(0.60 ) (R-2 = 0.65). The importance of runoff generation from a drainage basin i s shown by the positive correlation of linear headcut retreat and the runof f curve number (CN), representative for the conditions in the drainage basi n, High CN values tend to coincide with higher annual eroded volumes in the relationship between present drainage-basin area (A(p)) and annual eroded volume (V-e). but this affect was not observed in the relationship between original drainage-basin area (A(o)) and total eroded bank gully volume (Vol ). Stepwise multiple regression selected the relevant environmental paramet ers explaining annual eroded volume and linear retreat. In both equations. the present drainage-basin area explained the largest part of the variation . The CN was selected as another common parameter. Height of the headcut wa s the second most important variable explaining annual eroded volume, indic ating the role of energy transfers and undercutting at the headcut. Linear retreat was further explained by the average slope of the present drainage- basin area, representing the effect of decreasing transmission losses and i ncreasing flow velocity with steeper catchment slopes, and by the sand cont ent. decreasing the cohesion of the soil material, promoting soil fall and headcut retreat. Spatial extrapolation of the measured volumetric retreat r ate of 4.0 m(3) year(-1) revealed that active bank gully heads contribute u p to 6% of the sediment yearly filling up the Puentes reservoir. Estimated gully ages (i) based on the ergodic principle, and (ii) by linear extrapola tion of actual gully retreat rates in the past, range between 63 years and 1539 and between 64 and 1720 years, respectively. The high correlation betw een the gully ages estimated by the two methods is attributed to the fact t hat most gullies have not reached the evolutionary stage of significantly d eclining retreat rates. Since medium-term gully retreat rates are more dependent on drainage-basin area compared to the short-term retreat rates obtained in this study, the e stimated gully ages represent maximum values, assuming that present land-us e and climate conditions prevailed over the last two millennia. (C) 2001 El sevier Science B.V. All rights reserved.