Fundamental Study on the Design of Debris Dam and Apron

With a view to rationalize the design of debris dam and apron, the erosion caused by floods on the apron surface was treated as a significant guide to design and by way of field surveys, model experiment and theoretical analysis of the erosion, a new design method was proposed. First of all, to clarify the characteristics of the erosion, field surveys were conducted at 114 spots of the 21 tributaries of the Upper Tenryu River Basin in 1975. These dams and aprons had been constructed about ten to thirty years ago in which the consequent erosion at the longitudinal section were generally arc-shaped, and the beginning, the deepest and the ending point of the erosion which indicate the shape directly were clarified in each spot. After five years, one of these spots was surveyed again and the transition of erosion was examined in contrast to the rainfall and discharge data of that period. It showed that the erosion has proceeded in the direction of depth rather than of the longitudinal length and is caused not only by great floods but also by medium and small floods, therefore erosions are frequent and significant phenomena of this basin. Secondly, the model experiment was conducted in open channel to clarify correlated factors of erosion. This experiment was composed of the gravels falling from the crown of debris dam and the erosion apron. As for the falling position of gravels, it was confirmed that gravels fall inside of the falling nappe and as the gravel mass become greater, the horizontal arrival distance become shorter. This was considered to be related with the area of the accepting water pressure before falling of the gravel. The two types of falling, individual, and collective, made little difference on the falling position. As for the erosion of apron, gravels were made to fall in aprons which were made of erosive soil concrete and in consequence, the surveyed shapes of erosion coincided with the distribution of afore mentioned falling position of gravels. Therefore the conclusion was that the erosion of apron is caused by the collision of gravel and is scarcely caused by collision of falling nappe or movement of the fallen gravel to downstream. The amount or erosion was proportional to both the number and the gravel mass, so it implies that the apron was eroded little by little. Conversely, it showed inverse interrelation to the compressive strength of the material of apron. Thirdly, basing on the results of the field survey and the model experiment, following studies were conducted aiming at the rationalization of the debris dam and apron designs. First, a case study was conducted based on the field survey data. In the upstream of the beginning point of erosion, the amount of falling gravels were so few that the downstream slope of dam is possible to be made gentler than the one actually in force (1 : 2 or 1 :0.3) especially in low dam. The depth of the deepest point which relates to the least required thickness of apron showed that the thickness should be thicker than the general value of 0.7 to 1m as the case may be. Most of the depths were less than 1m, but the depth of 1.5m could .be seen even in groundsel. The ending point of erosion nearly coincided with the maximum arrival distance of the falling nappe, so the distance from dam to this point indicates the required length of apron. It showed that the length could be made shorter than the value obtained from the practical use formula. This tendency become more conspicuous as the height of dam increases. Examples of definite values for this basin are : in thecase of a 3 m high dam, the allowable range of downstream slope is from 1: 0.3 to 1: 0.6, and the length of apron is from 2.0 to 5.0 m and for a 6 m high dam, the allowable range of downstream slope is from 1:0.25 to 1:0.4, and the length of apron is from 3.0 to 6.0 m. Next, the theoretical analysis was conducted using physical models such as water depth and flow velocity in a given discharge, sediment dischar