CLEAN & INSPECT SPILLWAY SUB-SURFACE DRAINAGE SYSTEM OAHE DAM AND LAKE

1.1. Description of Services / Introduction: The sub-surface drainage system is an integral
component of the spillway structure at Oahe Dam. Cleaning of this system and the visual
inspection of the drains and collector pipes with remote-operated camera equipment is
required to document the existing condition of the system.
The Contractor shall provide all personnel, equipment, supplies, facilities, transportation, tools,
materials, supervision, and other items and services necessary to clean and to inspect the
spillway sub-drains and collector pipes as defined in this Performance Work Statement, except
for those items specified as government furnished property and services. The Contractor shall
perform to the standards in this Contract.
1.2. General Project and Spillway Structure Information:
1.2.1. Project Information: The Oahe Dam and Lake Project is a federally owned facility.
Oahe Dam is located in Hughes and Stanley Counties in central South Dakota on the Missouri
River, approximately 6 miles upstream of the city of Pierre and approximately 1,072 river miles
upstream of the mouth of the river. Oahe is the third dam (downstream) of a system of six
mainstem Omaha District dams on the Missouri River. Vicinity and location maps for the
Project are shown on Sheet C-001. The general location of the spillway with relation to local
highways along with Contractor access routes is also provided on Sheet C-001.
1.2.2. Spillway Structure and Channel Information. The spillway for Oahe Dam is located
in a narrow valley in the far right abutment of the dam, approximately one mile from the
northwest end of the embankment as shown in Sheet C-001. The spillway consists of an
unlined approach channel, a gated spillway structure, and a discharge channel. The use of the
spillway was designed to be a rare occurrence. Since completion, the spillway has not been
used for release of water. Small volumes of water have been released through the spillway
during periodic exercising of spillway tainter gates and for other maintenance related reasons.

The spillway gate structure consists of an eight bay, flat-crested weir; a depressed stilling
basin; and a downstream apron as shown on the general layout Sheet C-100. The weir crest
and apron slab were constructed at elevation 1596.5 feet and the depressed stilling basin was
constructed at elevation 1590.0 feet.
The spillway weir is comprised of seven monoliths and two end segments built integrally with
each abutment. The weir has an overall length of 456 feet from abutment face to abutment
face. Each monolith consists of an 8-foot wide center pier and 25 feet of weir on each side of
the pier. The piers support the tainter gates, bridge, gate hoist machinery and auxiliary
equipment. Eight tainter gates, each 50 feet wide by 23.5 feet high, are located between the
piers. In a closed position, top of tainter gate elevation is 1620.0 feet. Each gate was
retrofitted with a 1-foot high inclined splash plate to elevation 1621.0 feet. Non-overflow
abutments on each side of the weir house auxiliary equipment and contain drainage galleries
that collect water from the weir, backfill, and frost blanket drainage system.
The depressed basin of the spillway extends downstream from the weir approximately 100 feet
to the end sill. The end sill is a gravity concrete block structure that is anchored to the shale
foundation by reinforcing bars grouted into drilled holes. The apron slab extends approximately
209 feet downstream from the end sill. Both the depressed basin and apron slab are 18 inches
thick and were constructed over a 5’-4” thick frost blanket layer and anchored to the shale
foundation with reinforcing bars grouted 10 feet below the bottom of the frost blanket. The frost
blanket together with the apron slab, serve to protect the foundation from frost action and
facilitates drainage beneath the slab.
A discharge channel extends approximately two miles downstream from the spillway gate
structure. Immediately downstream of the spillway structure cutoff wall, the channel bottom
narrows to an approximate width of 334 feet and then remains constant. Between the cutoff
wall and 140 feet downstream of the cutoff, the channel side slopes transition from 1V on 2.5H
to 1V on 3H. The side slopes are armored with riprap in the transition area and the channel
bottom is armored with riprap for 240 feet downstream of the cutoff wall. The remainder of the
downstream channel is comprised of unprotected shale surfaces. The channel is straight
except for one bend located approximately one-half of the distance to the downstream end of
the channel. The downstream end of the channel directs flows into a valley allowing
discharges to flow across the flood plain toward the river channel.
1.2.3. Spillway Structure Drainage System. An extensive system of drainage and collector
pipes and foundation drains (over 8,400 feet) exist beneath the spillway weir; in the abutment
galleries; beneath the stilling basin and apron slabs; behind the walls; and behind the apron
blocks. The pipes range in diameter from 6 to 12 inches. Pipe material types include
perforated vitrified clay pipe (VCP), solid cement asbestos pipe (CAP), solid cast iron pipe
(CIP), perforated and solid plastic pipe (PP), perforated and solid galvanized standard steel
pipe (GP) and solid corrugated metal pipe (CMP).
1.3 Objectives: The overall objective of this Contract is to clean and thoroughly document the
existing condition of the sub-surface drains and collector pipes of the spillway structure.

1.4 Scope: The scope of work for this Contract generally consists of (1) cleaning the spillway
structure drainage system pipes and drains by hydro-flushing them (flushing them with large
quantities of water or jetting them), (2) visually inspecting the pipes with remote-operated
camera equipment and (3) producing a detailed formal report that thoroughly documents the
condition of these features. Deliverables for this work will include DVD(s) of the inspection
footage and a report that summarizes the procedures and findings of all inspections and
includes individual inspection logs that summarizes the condition each pipe segment and
documents the location of any significant observation. The Contractor shall perform these
services as noted in this PWS. All work shall be conducted in accordance with current
professional standards, Corps of Engineers policies, regulations, and procedures. The
Contractor is responsible for supplying all equipment, supplies, materials, and personnel
necessary for the completion of this work.
1.5 Scope Period of Performance: The period of performance for the PWS shall be 180 days
from the date of Notice to Proceed (NTP). Upon receipt of NTP, the Contractor shall submit a
draft baseline schedule for completion of the tasks in this PWS for approval to meet the
milestones as indicated below. The baseline schedule shall be based on calendar days and
shall include milestones for completion of each task provided herein.