Lyon Creek Cedar Way Stormwater Detention Dam Operation and Maintenance Manual Prepared by: Mike Shaw Stormwater Program Manager City of Mountlake Terrace January 2010
Section I General Information This Operation and Maintenance Manual (O & M Manual) provides the information and guidance needed to thorough and complete operation and maintenance of the Lyon Creek Cedar Way Stormwater Detention Dam. It will provide information necessary for City of Mountlake Terrace Public Works employees to perform inspections and maintenance activities necessary to assure safe operation of the facility. General Description of the Facility The Lyon Creek Cedar Way Stormwater Detention Dam is located in the 24000 block of Cedar Way just downstream of the confluence of the east and west forks of Lyon Creek in the southeast corner of the City of Mountlake Terrace. The facility is owned and operated by the City of Mountlake Terrace. The facility was constructed in 1985 in accordance with recommendations contained in the 1981 Lyons Creek Watershed Comprehensive Drainage Plan. The dam was funded and constructed privately as part of the Creekside Apartment complex in lieu of on-site detention facilities and was also designed to meet the regional detention requirements outlined in the 1981 Comprehensive Drainage Plan. Construction type is homogeneous earthfill embankment with downstream drainage blanket. Structural fill height is 30 feet with a hydraulic height of 23 feet and total length of 175 feet. An additional one-foot of fill was added to the facility in 1991 following the recommendations of a 1990 Department of Ecology inspection report. Total active storage volume at the dam crest is 40 acre-feet. The facility also includes a fish ladder located at the outfall of the discharge pipe that is terraced to a location in the lower creek bed of Lyon Creek at the base of the downstream slope. Entranco Engineers provided full-time engineering inspection of the construction. Engineers from the Dam Safety Section (DSS) of the Department of Ecology also inspected the project periodically during construction. Entranco and Ecology DSS subsequently certified that the dam was built in accordance with the approved original plans. Several modifications have been made to the original structure since 1985 all at the suggestion of Ecology DSS. In January 1986, a flood surge eroded the embankment area adjacent to the fish ladder. The City repaired the damage later that year. In 1990, a low berm approximately six inches high was installed along Cedar Way to the west of the right abutment to prevent overflow down the right-of-way from entering the impoundment area. Also in 1990, a concrete erosion cut-off wall was installed in the emergency spillway. In 1991, additional fill was added to the top of the dam to raise the crest elevation by one foot. All changes were recommended in 1990 periodic inspection by DSS. Additional facility information is included in the Project Data Sheet information beginning on page 3 of this section. Page 2
Operating Personnel Responsibilities Overall responsibility for the project Overall responsibility for Public Works Maintenance Annual Inspections Weekly and Monthly Inspections Scheduling and Performing Routine Maintenance City of Mountlake Terrace Public Works Director Curt Brees Stormwater Program Manager Mike Shaw Stormwater Division Employees Public Work Supervisor Shawn Hjert Stormwater Division Employees Lyon Creek Cedar Way Stormwater Detention Dam Project Data Information General State I.D. Number SN 08-1404 Owner and Operator City of Mountlake Terrace Location 24000 Cedar Way, Mountlake Terrace Year Constructed 1985 Purpose Flood Control, Stormwater Detention Downstream Hazard Potential High, Hazard Class 1B Reservoir Watershed Drainage Area Spillway Overflow Elevation Surface Area at Spillway Crest Active Storage at Spillway Crest Surface Area at Dam Crest Active Storage at Dam Crest Lyon Creek 1040 Acres 291.0 MSL 3 acres 25 acre-feet 6 acres 40 acre-feet Dam Embankment Type Structural Height Hydraulic Height Dam Crest Elevation (typical) Crest Length Earthfill with downstream drainage blanket 30 feet (includes core trench) 23 feet 295.5 feet MSL 175 feet Page 3
Crest Width Upstream Slope Downstream Slope 12 feet 3 H : 1 V 3 H : 1 V Principal Spillway Type Concrete Drop Inlet Location Center of Dam Discharge Capacity 140 cfs (at water level 295.5) Overflow Elevation 291.0 feet MSL Diameter of Riser 54 inches Diameter of Discharge Conduit 36 inches Length of Discharge Conduit 85 feet RCP, feeds into fish ladder 37 feet long Emergency Spillway Type Location Discharge Capacity Control Section Overflow Elevation Fuse Plug Base Elevation Gabion-lined Open Channel with Fuse Plug Center of Dam 590 cfs (at water level 295.5 ft.) Base width 32 feet, side slopes 2.7 H : 1 V 292.3 MSL (0.3 feet higher than design elevation) Erodible to 291.0 feet MSL, base width 25 feet Outlet Works Type Location Discharge Capacity Controls 36-inch diameter concrete pipe with 13-inch diameter orifice plate at pipe entrance Center of Dam; feeds into Principal Spillway drop inlet riser and discharge pipe 20 cfs (at water level 295.5 ft.) None, orifice has fixed diameter Top of Detention Facility Looking East Page 4
Section II Reservoir Operation Plan The Lyon Creek Cedar Way Stormwater Detention Facility is a passive, fixed orifice detention structure and requires no day-to-day operational attention. Periodic storms will raise the water level in the reservoir to varying levels depending on the intensity and duration of the storm. The fish ladder structure is also fixed and non-moving, requiring no periodic adjustment. Flow through the discharge conduit will reach capacity during storm events resulting in ponding in the reservoir to an elevation of 291.0 feet MSL at which time flow will begin to enter the overflow riser and fall directly into the 36-inch discharge conduit. Internal drainage is provided by a blanket drain under the west half of the downstream side of the embankment. PVC pipes were installed in the lower end of the blanket to collect the seepage. The pipes drain to a single outlet pipe which discharges next to the lower end of the fish ladder. Detention Facility Nearing Overflow Page 5
Section III Maintenance Plan Maintenance Schedule for Lyon Creek Cedar Way Stormwater Detention Dam Element Frequency Description Dam Embankment Vegetation Control Control of Burrowing Animals Quarterly Cut grass at least three times during the summer growing months to allow for visual surveillance of the embankment surface. Maximum grass height should not exceed six inches. Remove trees or brush. Do not remove trees larger than six inches in diameter without advice from the Dam Safety Section, Department of Ecology. Repair animal burrowing damage by compacting fill into the excavated areas. If the burrowing is extensive, seek the advice of the Dam Safety Section, Department of Ecology. Maintain vegetation at a low level to eliminate cover for animals. Maintain Crest Design Elevation Erosion Control on Upstream and Downstream Slopes Principal Spillway and Fish Ladder Spillway Channel Maintain the design elevation of earthen and compacted gravel surfaces by leveling and grading the crest to design specifications. Fill any ruts or minor depressions. Repair erosion gullies by removing loose materials and replacing with compacted fill. Add gravel or grass as appropriate. Remove any obstructions or debris from outlet and fish ladder. Visually examine visible portions of outlet pipe. Perform confined space entry to inspect pipe at overflow riser connection point. Page 6
Element Frequency Description Fish Ladder Channel Outfall and Stilling Area Emergency Spillway Spillway Channel Outlet Works Trash Rack Inlet and Orifice Plate Low Level Outlet Blanket Drain PVC Drain Pipe or as needed Remove accumulated sediment from resting pools. Replace plywood weirs as needed. Remove accumulated debris, check for settlement or erosion. Replace gabion baskets and interties. Replace gabion baskets and interties. Repair erosion damage and remove accumulated sediment and debris. Remove debris, check for damage or problems. Remove accumulated sediment from around the trash rack and culvert intake. Visually inspect visible portions of 36 outlet pipe from inlet to overflow structure. Monitor for drainage. Report any change in drainage from this pipe to DSS. Page 7
Section IV Inspection Plan Inspection Plan for Lyon Creek Cedar Way Stormwater Detention Dam Weekly and After Every Flood Event Where Water Ponds Above the Inlet 1. Visually examine the condition of: a. Dam crest b. Upstream and downstream faces c. Spillway, fish ladder and stilling basin d. Inlet structure and overflow riser 2. Check inlet structure, overflow riser, spillway and fish ladder for debris 3. Visually examine upstream and downstream faces for erosion or animal burrows. 4. Check seepage area at left abutment 1. Inspect dam crest, upstream face and downstream face for settling or erosion 2. a. Inspect visible portions of outlet works and principal spillway b. Perform confined space entry into the overflow riser to inspect principal spillway 3. Inspect inlet structure and fish ladder for accumulated sediments 4. Inspect wire gabions and interties 5. Review and update Emergency Action Plan Page 8
Section V Instrumentation and Monitoring The Lyon Creek Cedar Way Stormwater Detention Dam has no monitoring instrumentation or any measuring devices that need to be monitored on an ongoing basis. Lyon Creek Cedar Way Stormwater Detention Dam Updating This O & M Manual should be reviewed and updated annually by the City of Mountlake Terrace Public Works Department. Elements to be updated include: Names and phone numbers of operating personnel Alterations to project data due to changes at the dam Changes in operation and/or maintenance procedures Increase or decrease in the frequency of maintenance routines based on recent performance Distribute updated O & M Manual information to: Permanent record file at Mountlake Terrace City Hall Washington State Department of Ecology, Dam Safety Section Page 9
Lyon Creek Cedar Way Detention Dam Weekly Report Dam Tender Date Time A.M / P.M. Weather Clear Overcast Cloudy Cold Cool Hot Warm Raining Dry Has Rained Freezing Amount of Water Behind Dam Feet Inspection of Crest Cracking Slumps Instability Erosion Needs Mowing Other Good Inspection of East and West Abutments East Water from spring? Vegetation Needs Mowing? West Good Condition Vegetation Needs Mowing? Check Drainpipe at Lower End of Fish Ladder Draining Plugged Needs Work No Seepage Inspect Spillway Trash Clear Needs Repair Water at Overflow Stage Settlement Emergency Spillway Inspect Gabions Settlement Alignment Washing of Fuse Plug Rock Missing Bottom Erosion or Scour Inspection of Outlet Works Low Water Headwall O.K. Fish Ladder Needs Repair Deck Over Fish Ladder O.K. Inspect Downstream Gabions Along West Bank O.K. Downstream Outfall Area Clear Needs Cleaning Loose or Missing Tie Wire Spillway in Good Condition High Water Fish Ladder O.K. Fish Ladder Needs Repair Deck Needs Repair Needs Repair Type Washed Out Other Page 10