storm water drainage guidelines jan08

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9605 NE 24 th Street • Clyde Hill, Washington 98004 425-453-7800 • Fax: 425-462-1936 • www.clydehill.org

Storm Water Drainage GuidelinesJanuary 2008

Because of the City’s previous storm waterproblems, addressing drainage is a responsibilityof those developing or re-developing property inthe City of Clyde Hill.

The City, as well as many Clyde Hill residents who have developedor re-developed throughout the community, have spent considerable resources to address thisimportant issue and count on others developing within the community to take responsibility forthe storm water on their property as well.

When those developing or redeveloping their property address individual drainage issues theyare helping to control or eliminate storm water problems with adjoining neighbors and publicproperties, are preventing future problems by controlling discharges into the public system, areprotecting the capacity of the existing City infrastructure and are providing help by phasing out“French Drain or Infiltration” systems that act as a leading cause of damage to other down-hillneighbors in the form of springs and subsurface erosion.

Because of the City’s previous storm water problems, development or re-development in ClydeHill triggers the duty for the owner to address individual drainage issues by installing on-sitestorm water detention systems and by hooking their private storm systems into the City’s stormdrain system.

____________________________________________________________________________

Historical Information Page 2Overview of Detention Drainage Requirement

Code References for Drainage Page 3

Preparation & Submittal of Drainage Control Plans Page 4-5A. Drainage Control Plan Submittal RequirementB. Drainage System RequirementsC. Administrative Procedures

Storm Drain Detention Systems Page 6-7A. Storage RequirementsB. Preparation of Drainage Control Plans

Homeowner Drainage Maintenance Requirements Page 7

Legal Non-Conforming Structure Thresholds(Attachment 1) Page 8-9




Historical Information: Past Problems & Current Need to Manage Drainage

Clyde Hill is responsible for maintaining a storm drainage system that benefits the entire community.There are currently 16 miles of storm drain lines within the City that collect and direct runoff from thestreets and homes. All of the City’s storm water ends up in Lake Washington. Most of the storm water inour community is directed out of Clyde Hill into two detention/retention ponds located on the OverlakeGolf and Country Club. These ponds are utilized through an agreement with the Country Club to storerunoff before the water enters Fairweather Creek and eventually Lake Washington.

During the early years of Clyde Hill’s development, the City’s drainage facilities were designed andconstructed to handle only the water collected from the public streets. These systems, still in the groundtoday, were designed to handle only a 10-year storm event. Because there was still plenty of room in thecommunity, flooding from a neighbor was not a major concern. However, as Clyde Hill developed, sodid its corresponding storm water related problems.

In the 1970’s the City was about 90% developed and began to receive many complaints from residentsdescribing how neighbors were discharging water onto their property. Most of the homes built in the1950’s, 60’s and 70’s were not required to discharge storm water into the City’s system, used splashblocks for downspout drainage and also allowed storm water to sheet onto adjoining properties. Some ofthe City’s first drainage requirements were developed at this time to address this condition. As the Citycontinued to mature in the mid to late 1980’s Clyde Hill experienced flooding problems on the streets, inthe homes and as underground springs throughout the City. City Hall would receive numerous calls fromresidents very concerned about storm water problems. In many cases property damage was occurring.

To address problems in its biggest drainage basin, the City in 1989 redesigned and enlarged the stormdrains on 84 th Ave., including a connection to the detention ponds on the Overlake Golf Course. Thoseimprovements helped alleviate the flooding problems experienced along most of the lower areas in ClydeHill. Along with these improvements an accompanying analysis determined that the enlarged systemwould continue to manage the flooding only if an increase in the flow rates throughout the entire systemcould be controlled. With this understanding the City Council established the drainage and detentionrequirements that are now enforced today.

Overview of Drainage Requirements

Clyde Hill requires on-site detention facilities on new and significantly remodeled construction projects.This is necessary for two reasons. First, the City storm drain system was originally designed only tohandle run-off from City streets during a 10-year storm event and not the type of development orredevelopment experienced in the community.

Second, the City’s system cannot be expanded to handle new development in Clyde Hill. The presence ofutility lines, such as sewer, gas, and water means that meaningful detention facilities within the streetrights-of-way cannot be constructed. Laws regulating storm drain systems in Washington make it clear

that a system cannot negatively impact the property onto which it drains (in the majority of circumstancesthe golf course). An uncontrolled increase in the flow of the City’s system on downstream detentionfacilities would constitute a negative impact.

To manage storm water issues in concert with development a long-term goal of the City is to have allhomes connected to the City’s system at a 10-yr. storm release rate. On-site detention is necessary tomeeting this goal because it effectively controls the amount of water released into the City system. Withan increased amount of impervious surfaces due to new construction projects, and therefore increasedwater runoff, the City system would not be able to handle drainage for all homes in Clyde Hill withoutthose developing addressing their individual drainage issues.




CODE REFERENCES FOR DRAINAGE REQUIREMENTS

The City of Clyde Hill’s drainage requirements are described in the following sections of the Clyde HillMunicipal Code:

17.08.170 Drainage.

Drainage requirements during development, improvement, use or construction within a lot, site, parcelplat or other area must comply with the provisions of Chapter 15.10.

15.10.010 Drainage.

A. During the development, improvement, use or construction within and/or upon a lot, site, parcel,plat or other area, the following shall be required:

1. All natural contours shall be maintained to the extent that natural drainage flow from or ontoadjacent public or private property shall not be disrupted, blocked, increased, redirected or otherwisemade detrimental to the use or maintenance of adjacent property; and

2. This restriction shall not prevent the installation and maintenance of a covered storm sewer underor across private property along a natural drainage course for the purpose of generally improving a

particular property, in conformance with specifications and plans meeting the approval of the cityengineer and consistent with the section entitled “Preparation and Submittal of Drainage ControlPlans,” as it now exists or may hereafter be amended. (See page 4)

B. Collected waters, including but not limited to, such waters as may be collected from roofdownspout drains, surface drains, driveways, patios, yard area drains or foundation drains, shall bedischarged into storm sewer facilities where connection thereto will not cause significantdisruption of public streets. Installation of storm sewer facilities in the public right-of-way shall beperformed in accordance with Chapter 12.08.

C. Where the storm sewer facilities are not available, other means as may be approved by the cityengineer shall be provided for disposal of collected waters. Collected waters shall not be permittedto discharge onto adjacent public or private property.

D. A storm detention system shall be required when a net total of 750 square feet or more ofadditional impervious surface area is added to a lot. In calculating the impervious surface area, thesurface of the water of swimming pools shall not be included, unless the public works directordetermines that such pool contributes to water runoff.

E. When a lot is newly developed or redeveloped, or when the main structure thereon is deemed“substantially damaged or destroyed” or “substantially remodeled” as defined in CHMC 17.60.030(see page 8 for the current substantial remodel threshold), a storm detention system shall berequired to capture, store and release storm water at the rates allowed by the section entitled“Preparation and Submittal of Drainage Control Plans” on page 4.

15.04.010 Construction Administrative Code Adopted International Residence Code (IRC),Section 105.1 Permits Required – On-site storm drainage systems included.

Any owner or authorized agent who intends to construct, enlarge, repair, move, demolish, or change theoccupancy of a building or structure, or to erect, install, enlarge, alter, repair, remove, convert or replaceany electrical, gas, mechanical or plumbing system, the installation of which is regulated by this code(IRC), or to cause any such work to be done, shall first make application to the building official andobtain the required permit. All on-site storm drainage systems, including but not limited to detentionfacilities, shall be encompassed within, and governed by, the building permit.




page 8), then the drainage plan shall include a detention system to collect all storm drainage on theproperty. It must be designed as described under Storm Drain Detention Systems on Page 6.

4. If drainage for an existing structure(s) is not connected to the City's system, and the owner, whether inconjunction with a remodel, an addition, or a general desire to improve drainage conditions, decides tocombine a portion or all of the property's drainage into one system, then the new facilities shallinclude a detention system and be sized to handle all of the impervious surfaces involved. Thisincludes the re-direction of run off from an existing roof into the new drainage facilities. If theimpervious area associated with the new system exceeds 6,000 square feet, the site shall be consideredundeveloped, and the new facilities shall be designed by a Civil Engineer, licensed in the State ofWashington, and said design shall be in accordance with the City’s document entitled Storm WaterDetention Calculations.

5. If storm water collected from an existing or new driveway and parking area is discharged into agravity discharge detention system, then a separate oil/water separator is not required for the drivewayand parking area.

C. Administrative Procedures

1. Applicant shall submit two (2) copies of the Drainage Control Plan for review and approval. Theseplans should be kept separate from the building permit plans because they will be reviewed here inCity Hall by the Public Works Director while the Building Official, with whom we contract, reviewsthe building plans out of his office.

2. After the plans have been approved, one set of approved drainage plans will be returned to theapplicant with the approved building permit when they are both ready to be issued. (NOTE: Theapplicant may also be required to obtain a Street Opening Permit if drainage work is required to bedone in the City’s Right of Way. In addition, if the improvement s include a concrete driveway that is

to extend into the public right of way, a Public Place Use Permit is required for that portion of thedriveway located within the public right of way. )

3. The Director of Public Works or his designee will inspect all drainage facilities to insure that the workis done according to the approved plan. The holder of the permit shall notify the City when the workis ready for inspection. If the work does not conform to the approved drainage control plans, or thesite inspection reveals other conditions that require modifications or additional information, theDirector shall order that portion of the work stopped. No final occupancy shall be permitted until thedrainage control facility is completed, inspected and approved.




A. Storage Requirements

To determine the amount of storage required, proceed as follows:

1. Calculate the area of new impervious surface, including, but not limited to, all roof, driveway,patio and sidewalk areas.

2. Select the proper size storage pipe from Table 1 on page 11. (Or calculate the required lengthof pipe if the applicant desires to use a larger diameter pipe).

• Enter Column A with the total impervious area to the next larger 500 square feet.• Column B gives the volume of storage required.• Column C gives the orifice size• From Column D select the pipe diameter and length that corresponds to the calculated

impervious area and which best suits the site conditions.

B. Preparation of Drainage Control Plans (2 sets Required - Keep separate from Building Plans)

1. Using the standard details contained herein as a guide, on a separate site plan show all details of thedrainage control plan, including the location and size of the storage pipe, flow control structure,downspout, patio and driveway drains, etc. (See Storage Facility Plan & Profile, Figure 2 and FlowControl Manhole, Figure 3 on Pages 12 and 13.)

2. Connect all drains to the storage pipe or flow control manhole. Provide all elevations showing aminimum one percent (1%) slope (1/8 inch per foot of pipe) to insure drainage. NOTE: The storagepipe is to be installed at 0.5 percent (0.5%) slope.

3. Show all calculations of the impervious areas on the site plan.

HOMEOWNER DRAINAGE MAINTENANCE REQUIREMENTS

The owner of the property on which the drainage control facility is located shall be responsible formaintaining the facility in good working condition, including the removal of any sand, silt or other debristhat may accumulate in the drainage system catch basins and flow control manhole. Depending upon theamount of soil that is permitted to wash into the drainage system, cleaning could be required every 3 to 5years.

The City may make periodic maintenance inspections. If such inspections reveal that maintenancerequirements have not been met or that the condition of the drainage control facility is likely to posehazards or dangers, a deficiency notice will be issued. Such notice will specify the nature of themaintenance deficiency and a date of compliance. If corrections are not made by the date specified, theCity may have the necessary work performed, and bill the cost of correcting the deficiency to the propertyowner.




(Attachment 1)

Legal Nonconforming Structures

2008 Substantial Remodel Threshold: $360,400

Many of the homes in Clyde Hill were built or remodeled under Building or Zoning Codes that havechanged and are different than the Codes that are in effect today. These are referred to as “legalnonconforming homes.” If you have a legal nonconforming home here are some things you should know.

All Municipal Codes include a section pertaining to nonconforming structures and have guidelines tobring these structures back into compliance with current building and zoning regulations. As Citieschange over time, so do their land use visions and associated land use regulations. In some cases,communities provide property owners a specific amount of time to bring their property into compliance.

In Clyde Hill, nonconforming structures must be brought into compliance only when a “substantial”amount of redevelopment is taking place on a property or when the structure is declared dangerous or hasbeen substantially damaged.

After considerable thought and discussion in 2004 by the Planning Commission and the City Council,it was determined that a specific value threshold would constitute a “substantial” remodeling projectand would trigger the need to comply with the community’s land use regulations. This value isrevised each year based on the percentage of change in the R S Means Building Construction CostIndex for the Seattle area.

Nonconforming Structure Thresholds:Homes that are legal nonconforming structures are governed by Chapter 17.60 of the Clyde HillMunicipal Code and may stay as they are without the need to meet the current requirements of Buildingand Zoning Codes unless:

1. They are required to be abated by the Uniform Code for the Abatement of DangerousBuildings or other regulations adopted by the City.

2. They are substantially damaged or destroyed by fire, explosion, act of God, act of a publicenemy, or other hazard, or

3. They are substantially demolished or remodeled above the substantial remodeled thresholdvalue.

A building or structure will be considered substantially demolished or remodeled when thevalue of any such remodeling within a 24-month period exceeds the threshold amount.Each year that amount will be increased or decreased by the percentage of change in theR S Means Building Construction Cost Index for the Seattle area.




Exceeding Threshold Requirements:When one of the above triggering events occurs, the building or structure and land upon which thebuilding or structure was located shall become subject to the current Building and Zoning Coderegulations for that area. Therefore, when a project is triggered by one of these thresholds, the applicanthas the following responsibilities:

1. Zoning & Building Code Responsibilities:The proposed work and the remaining structures on the property must be brought intoagreement with the City’s Zoning Regulations contained in Title 17 of the Municipal Code.This includes bringing the existing structure(s) into agreement with the current zoningregulations such as setbacks, height restrictions, structural lot coverage, impervious surfacecoverage and the number of accessory structures on the property.

The noncompliant structures and property are brought into agreement with the community’svision of zoning and property usage, which maintains equity and property values throughoutthe community.

2. Drainage Responsibilities:A substantial redevelopment will also require the property to come into agreement with theCity’s storm drainage regulations contained in Title 15 of the Municipal Code. The scope ofthe proposed project means that addressing drainage is an expected responsibility for aproject of that magnitude. This responsibility will help to control or eliminate storm waterproblems with adjoining neighborhood and public properties and prevent future problems bycontrolling discharges into the public system thereby protecting the capacity of the existinginfrastructure and help by phasing out “infiltration” systems that act as a leading cause ofdamage in the form of springs and subsurface erosion.

A significant construction project triggers the responsibility and the need to address drainage

by providing on-site storm water detention systems and hooking those private storm drainsystems to the City system.




12” Min

12” Min

Discharge Pipe (4” Min)

Catch Basin, Type 1(Inside Dimension —

22” x 26” )

Frame & GrateGrade

TYPE 1 — OIL/WATER SEPARATOR(Figure 1)

(Mandatory for all new or re-built driveways when a stormwater detent ion system is not required)

Ad justment Brick




DETENTION STORAGE PIPE TABLE(Table 1)

A B C D

Storage Pipe Size (Inches)Storage Pipe Length (Feet)

12" Dia. 15" Dia. 18" Dia. 24" Dia.

TotalImpervious

Area(Square

Feet)

StorageVolumeRequired

(CubicFeet)

OrificeDiameter(Inches)

Length Length Length Length

750 19 ½ 36 23 16 91000 37 ½ 47 30 21 121500 55 ½ 71 45 31 18

2000 749/16

94 60 42 242500 92 5/8 118 75 52 293000 110 11/16 141 90 62 353500 129 ¾ 165 104 73 414000 147 ¾ 188 119 83 474500 165 13/16 212 134 93 535000 184 7/8 235 149 104 595500 202 7/8 259 164 114 646000 220 15/16 283 179 125 70




STORAGE FACILITY PLAN & PROFILE

(Figure 2)




Flow Control Manhole

(Figure 3)

Notes:

• Frame and grate shall be set directly over the ladder and offset so that the overflow pipe shall bevisible at the edge of the access opening

• Elevations (_______) of component parts shall be provided by the Applicant

• The Flow Control Manhole shall be a standard Type II Catch Basin. Ladder runs shall beuniformly spaced 12” to 16 1/2 “ vertically

• All steel pipe and parts shall be galvanized

• The storage pipe shall generally have a minimum of 2 feet of cover




FILTER FABRIC

6" MIN

2% MIN

20% MAX

P IPE OUTSIDE

DIAMETER

12" MIN 36"

2' MIN 20% MAX GALVANIZED BOLTS

2"X12" P RESSURETREATED GRADE

BOARD

4"X 4"S UPPORTP OST

4" OR 6" P ERFORATEDP IPE

CLEAN (<=5% FINES )3/4" - 1.5" WASHED

Emergency Overflow Trench(Figure 4)

NOTES:

1. This trench shall be constructed so as to prevent point discharge and/or erosion.2. Trenches may be placed no closer than 50 feet to another.3. Trench and grade board must be level, aligned to follow contours of site.4. Grade board support post spacing as required by soil conditions.

storm water drainage guidelines jan08

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