The use of Flow Assisted Sediment Transport (FAST) to remove sediment from reservoirs in Southern California, USA
3 THE MORRIS DAM PILOT STUDY
In the initial phase of this series of sediment removal efforts a pilot study was undertaken on the Morris Reservoir. The goals of the pilot study was to assess the overall cost effectiveness of removing a significant amount of sediment (approximately 350,000 m3) from the reservoir
using a version of hydraulic sluicing involving the use of carefully managed FAST. The plan was to use carefully controlled flows from the San Gabriel Reservoir to remove sediment from the Morris Reservoir floor and then make use of the same flows to convey the sediment down the San Gabriel River to disposal sites located out in the San Gabriel Valley. As part of this effort a careful monitoring program of the process was undertaken.
The sequence of events associated with this effort involved: a) removal of at-risk species from a 10 km downstream portion of the river below the dam as well as the adjacent ripar- ian corridor; b) removal of fish from the Reservoir itself prior to draining of the reservoir; c) the draining of the reservoir; d) preparation of the outlet works for sediment removal process; e) installation of monitoring equipment to track sediment concentration levels dur- ing the experiment and volume removal estimates and a GIS based assessment of reservoir floor conditions pre- and post the experiment as well as sediment deposition patterns in the downstream areas; f) post experiment downstream channel and habitat restoration; and g) the assessment of the fate of sediment conveyed down the river system.
Figure 1. Location of the three Upper San Gabriel River reservoirs (adapted from COE/LACDPW, 1994) (Weirich, 1994a).
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In preparation for the pilot study in the summer of 1991 the reservoir was gradually drawn down over a period of several months to a relatively low pool level using one of the main valves. The actual sediment removal process took place in three phases.
Initially, beginning in late August of that year and continuing for a period of approxi- mately three weeks residual flow associated with the final stages of the dewatering process was channeled through a 1.2 m pipe near the base of the dam (Fig. 2a–b) and a sediment load sampling program and a continuous density monitoring system previously installed on the 1.2 m pipe was employed to provide a continuous record of the sediment load being removed from the reservoir floor (Fig. 3). At the outset of this period of final dewatering, flow rates exceeded 8.6 m3/s with water being supplied by water draining from the remaining
standing pools on the reservoir floor, groundwater and other secondary sources. But the flow rate quickly dropped to rates in the 1.0–1.3 m3/s range. Sediment concentrations of 10–12%
by weight were measured for the flows traversing the reservoir floor and reaching the pipe intake. The initial rate of sediment removed exceeded 15,300 m3/day but quickly dropped to
Figure 2. (a) Reservoir at end of dewatering. The FAST intake area is visible at dam base. (b) Temporary platform above intake pipe at dam base with flow coming from upper right.
Figure 3. The 1.2 m exit pipe at base of dam carrying FAST flow. Sensors to continuously measure sediment concentration were mounted in this pipe.
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a 2,300–4,600 m3/day rate. Over this three week period of completing the dewatering process,
approximately 63,500 m3 of sediment was removed from the reservoir floor. During the first
days of this process most of the material being moved was in the silt/clay range but the amount of sand and larger fractions gradually increased as headward erosion by the stream channel forming on the reservoir floor and draining the reservoir provided larger quantities of materials from further up the reservoir floor. The vast majority of this material was depos- ited in the plunge pool of the dam and other pools not far downstream of the dam.
The second phase involved the actual operational experiment and entailed a carefully man- aged release of water from the upstream San Gabriel Dam to provide the (FAST) to remove sediment from the reservoir floor and transport it through the 1.2 m pipe and downstream of the dam. During this phase of the experiment, which lasted for a period of approximately 11 days, some 270,000 m3 of material was removed from the reservoir floor with an average
removal rate of 20,600 m3/day and a peak value in excess of 24,500 m3/day, with sediment
concentrations in the flows exceeding 200 G/L at times (Fig. 4). The sampling of sediment indicated that a much larger portion of the material being moved was in the sand/gravel/ pebble range than had been moved in the three week final drawdown operation. A much larger amount of the material moved during this period was deposited beyond the plunge pools in a portion of the channel extending some 10 km downstream of the dam.
The final phase of the sediment removal effort covered a period of approx. four more weeks after the FAST portion came to close with the end of the controlled release of sediment entraining flows from the San Gabriel Dam. During this final phase residual reservoir surface drainage, groundwater and minor leakage from the San Gabriel Dam provide some flow that continued to remove sediment from the reservoir floor. The daily rate of removal was in the 750 m3/day range with a total amount of sediment being
removed of approx. 19,000 m3.
Careful mapping and monitoring of the downstream fate of the sediment removed from the Morris dam during this experiment indicated that of the approx. 350,000 m3 of mate-
rial removed during the overall experiment approx. 59,000 m3 was carried more than 10 km
downstream during the experiment to engineered structures further down the river with
Figure 4. A portion of the floor of the Morris Reservoir at the end of the sediment removal experi- ment showing the lateral extent and depth (approx.10 m) to which material was removed.
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the remainder being deposited in the pools and channel areas immediately downstream of the dam. Subsequently, and as part of the overall project, a series of controlled releases of 15 m3/s to 45 m3/s were made in mid-January over the course of several days involving
the release of approx. 6,900,000 m3 of water. These releases, along with more conventional
releases, were undertaken with the intent of re-entraining the material deposited in the chan- nel downstream during the experiment in order to carry that material out of the natural channel portion of the river downstream of the dam and convey it further downstream into the fully engineered portion of the San Gabriel River in the interest of habitat restoration. Monitoring of this effort indicated that a combination of the downstream flushing opera- tion and normal dam releases over a period of months left approx. 53,500 m3 of the original
350,000 m3 in the natural 10 km portion of the channel downstream of the dam, the vast
majority of those 53,500 m3, consisting largely of larger fractions of coarse sand up to cob-
bles and small boulders, were located in the two major plunge pools immediately downstream of the dam. Total cost of the sediment removal effort, including habitat restoration, were esti- mated at approx. $1.30/m3 (less than 1/5 the approx. cost of equivalent mechanical removal
which at the time ranged up to above $30/m3).