EXPERIMENTAL FORMATION AND DEVELOPMENT OF LOBATE AND BIRDFOOT DELTAS

The sedimentation simulating experiment of the Formation and development of delta was Conducted in a Flume that is 6 m long,4. 5 m wide and 0. 8m high. in Department of Earth Resources Colorado. Sate Uriversity.The Parameters of discharge,quantity of contained sand and granulometric composition were kept unchangeable,not including the parameter of the height of the lake's level. The experiment has three main steps Ⅰ. Fixing the altitude of the Lake' s level;Ⅱ.Gradually lowering the altitude of the lake's level;Ⅲ.Gradually raising the altitude of the lake's level. We had observed how the channel was changed and the delta was formed after each step the delta's geometry and thickness of sand body were measured and sand samples were collected. The main results of our experiment are as follows: 1. We Put forward new viewpoint on formation of lobate delta,which had been considered that the formation of lobate was reformed by wave from birdfoot delta. The Experiment excludes wave action.The formation and development of lobate and birdfoot delta were controlled by the lake bottom slope is 8% lobate deltas slope at a constant discharge. Sand contents are unchangeable when can the river mouth gradually progrades.only be formed at the beginning. As the lake level is lowered,Sediments aggrade the lake bottom gradually. The gradient of the lake slope area is decreased. An embryonic form of birdfoot delta began to form. When the front of delta developed onto the lake bottom. The Shape of birdfoot delta became more obvious.Transformation of a lobate delta into a birdfoot delta was accelerated by lowering the lake level. A lobate delta remained on the lake slope when take level kept at 42cm and the experiment run for 12 hours. As the front of delta entered the lake bottom and lake level fall down from 42cm to 40cm at last to 39cm. Water running for oNy 5 hours created the embryonic form of birdfoot delta.we have been determined that a bottomslope of 2 0 0 (108' 45") is necessary for formation of a birdfoot delta 2. When lake level was raised from 37cm to 42cm geological action after stage if made the bottom slope gentle and a birdfoot delta kept developing at the beginning and the shape was obvious. later on it became less obvious, the reason for this change is that water entered the channel that had formed earlier and the development of the delta was limited. 3. To distinguish lobate and birdfoot delta other than by shape. the ratio of width to thickness of a deltas sand body is important. The ratio for lobate delte is less than 58,1. but birdfoot deltas is greater than 72,1. This has important significance to petroleum and gas explortion 4. This experiment has proved signifcant data for setting up the experimental equipment for simulation of large-scale lake basin sedimentatio. For example,the river channel is sixteen times largr and the river is thirteen times larger than the original sizes The length to width ratio of birdfoot delta is 2m:2. 28m i. e. 0. 88:1, it's nearly 1:1. 5. The probability curves and grain.size parameters obtabined from grain size analyses of the conected sand samples have shown satisfactory rasults in studing microfacies of point bar and terraces,and analgzing the differences of their hydrodynamic conditions.