C.1 - Drainage Policy.

C.1.1

General.

A.

This manual represents the application of accepted principles of stormwater drainage engineering and is a working supplement to basic information obtainable from standard drainage handbooks and other publications on drainage design. The policy statements of this section provide the underlying principles by which all drainage facilities shall be designed. The application of the policy is facilitated by the technical criteria contained in the remainder of the manual. Drainage design and construction shall comply with this manual and Appendices F and G of this Appendix. In the event of any conflict between this Appendix C and Appendices F and G, the most stringent provision shall apply.

(Ord. No. 2018-21, § 2, 4-17-18)

C.1.2

City of Copperas Cove Drainage Policy.

A.

Application. The city's drainage policy shall govern the planning and design of drainage infrastructure within the corporate limits of the city and within all areas subject to its extra territorial jurisdiction, as required. Definitions, formulae, criteria, procedures and data in this article have been developed to support this policy. If any condition requiring some additional measure of protection is identified during design or construction, the design engineer shall make provisions within the design. All plans must be signed and sealed by a professional engineer registered in the State of Texas.

B.

General.

1.

Stormwater runoff peak flow rates for the 25-year and 100-year frequency storms shall not cause increased adverse inundation of any building or roadway surface.

2.

Street curbs, gutters, inlets and storm sewers shall be designed to intercept, contain and transport all runoff from the 10-year frequency storm, without overtopping the curb.

3.

In addition to subsection (2) above, the public drainage system shall be designed to convey those flows from greater than the 10-year frequency storm up to and including the 100-year frequency storm within defined public rights-of-way or drainage easements.

4.

When stormwater detention is provided, stormwater runoff peak flow rates shall not be increased at any point of discharge for the 25-year storm frequency and 100-year storm frequency events.

C.

Drainage flow in streets.

1.

No concentrated point discharges directly into streets will be allowed unless approved by the city engineer.

2.

No lowering of the standard height of street crown shall be allowed for the purposes of obtaining additional hydraulic capacity.

D.

Drainage system.

1.

Construction plans for proposed reinforced concrete box culverts, bridges and related structures may be adaptations of the current Texas Department of Transportation (TxDOT) Standards.

2.

For bridges and culverts in residential streets, runoff from the 100-year frequency flow shall not produce a headwater elevation at the roadway greater than either six (6) inches above the roadway crown elevation or any top of upstream curb elevation, whichever is lower.

3.

For bridges and culverts in streets other than a residential street, runoff from the 100-year frequency storm shall not produce a headwater elevation at the roadway greater than three (3) inches above the roadway crown elevation or three (3) inches above any top of upstream curb elevation, whichever is lower.

4.

All drainage facilities (including, but not limited to, headwalls, open channels, storm sewers, area inlets, and detention, retention and water quality controls and their appurtenances) shall comply with the following requirements, unless otherwise noted in this section.

a.

Storm sewer inlets and gutter transitions shall be designed to avoid future driveways and to avoid conflicts with standard water and wastewater service locations. No utilities shall be allowed to cross through a storm sewer inlet or culvert. No utilities shall be allowed to cross under a storm sewer inlet.

b.

Drainage channels and detention ponds that are to be maintained by the public (city) shall be contained within drainage easements. Adequate room for access shall be provided for drainage channels and detention ponds. Ramps no steeper than five (5) feet horizontal to one (1) feet vertical shall be provided to allow access to drainage channels and detention ponds. The minimum bottom width for any channel with vegetative side slopes shall be five (5) feet.

c.

Rip-rap for slope protection or velocity dissipation shall be formed concrete dissipators. Mortared rock or stone shall be allowed with a minimum of 12-inch diameter rock or stone.

d.

Storm drains between lots (crossing blocks) shall be avoided as much as possible. When unavoidable, such drains shall be underground storm drains, located entirely on one (1) lot, laid along an alignment that retains the conduit within the dedicated drainage easement. Storm drains along rear of residential lots (through back yards) shall not be permitted, unless approved in writing by City Engineer and Public Works Director, such approval not to be granted unless running storm drain through the rear of a residential lot is due to topography and storm drain is placed underground. Easements shall be a minimum of fifteen (15) feet in width or one and one-half (1½) times the depth of the storm drain, whichever is greater. Fences may cross easements with underground facilities, but may not run parallel. Fences may not cross or run parallel within drainage easements designed for surface flow.

e.

All bends, wyes and pipe size changes in storm sewers shall be prefabricated or shall occur at manholes/junction boxes. All alignment changes of forty-five (45) degrees or more shall occur at a manhole or junction box.

f.

Bedding of storm sewer shall be to six (6) inches above the top of pipe or to current public works standards (whichever is greater).

g.

Storm drains shall be reinforced concrete pipe (RCP), ASTM C76, minimum Class III, and minimum eighteen-inch diameter, with a minimum of two (2) feet of cover in unpaved areas and a minimum of one and five-tenths (1.5) feet of cover from bottom of the subgrade in paved areas. The engineer shall provide design information to the engineering department as appropriate to demonstrate that class of pipe used is sufficient for the loading conditions in situations where minimum cover cannot be met. Higher strength pipes shall be used where loadings warrant such.

h.

The use of high density polyethylene (HDPE) shall be allowed only if approved by the city engineer. Its use shall be limited to unpaved areas outside of city streets. All cross street storm drainage conduit shall be reinforced concrete pipe (RCP). All outfall structures shall be constructed of reinforced concrete and the connection with the outfall structure shall be accomplished using RCP. A transition fitting from HDPE to RCP shall be made upstream of the outfall structure.

i.

Junction boxes and manholes shall be reinforced concrete. Junction boxes in lieu of manholes shall be provided where any pipe opening exceeds thirty-seven (37) inches in diameter and where the distance from the outside surfaces of any two (2) pipes entering a manhole is less than one (1) foot, measured along the inside of the manhole.

j.

Channels.

(1)

Concrete Channels. Concrete channels shall fully contain design flows and facilitate self-cleaning.

(2)

Vegetated Channels. All slopes shall be hydro-mulched, sodded or seeded with approved grass, grass mixtures or ground cover suitable to the area and season in which they are applied. Seeded side slopes and bottoms shall be lined with erosion protection matting, unless vegetation is established. All earthen channels must have vegetation eighty-five (85) percent established, with no bare spots greater than ten (10) square feet, prior to acceptance by the City of Copperas Cove. If vegetation cannot be adequately established prior to the desired acceptance date, up to three (3) months additional grow-in time may be granted by the city engineer. Such an extension must be requested in writing with details of the efforts to be taken to ensure adequate vegetation will be established within three (3) months. Extension requests must also be accompanied by an irrevocable line of credit, surety, or maintenance bond equal to one hundred (100) percent of the cost to fully sod the entire area to be vegetated. This guarantee shall be separate from any other required maintenance bonds.

(3)

Major streams shall not be modified without consent of applicable state and federal agencies and authorization from the city engineer.

k.

Discharge from storm sewer outfalls shall not cause channel, bluff, or stream bank erosion. If the storm drain discharges to an open drainage facility (as determined by the city), the applicant must show acceptable nonerosive conveyance to that drainage facility, appropriate energy dissipation at the outfall and a stable headwall. No outfalls shall be allowed to discharge on the slope of the receiving channel without proper erosion control protection in the receiving channel.

l.

If the development is located such that there is considerable drainage from potentially developable upstream areas, the developer may request participation by the city for the cost of over sizing of elements of the overall drainage system. The city shall consider these requests on a case by case basis. Final determination of any cost sharing will be determined by the city council through a development agreement.

E.

Computations.

1.

Computations to support all drainage designs shall be submitted to the appropriate city departments for review. The computations shall be in such form as to allow for timely and consistent review and also to be made a part of the permanent city record for future reference. Computation shall include the impact of the proposed development to the downstream properties adjacent to the drainage resulting from the 100-year event. All computations submitted shall be certified by a professional engineer licensed in the State of Texas. The engineer shall provide the report to the city in both hard copy and a scanned electronic PDF file with the proper seal, signature and date.

2.

Determination of runoff. Numerous methods of rainfall-runoff computation are available on which the design of storm drainage and flood control systems may be based. The rational method shall be an acceptable means of computing runoff for drainage areas of two hundred (200) acres or less when designing streets, storm drainage systems, channels and culverts. When the drainage area exceeds two hundred (200) acres in size, the National Resource Conservation Service (NRCS) (formerly the Soil Conservation Service) hydrologic methods (available in TR-20, TR-55 or HEC) should be used.

3.

Detention pond storage determination. A flow routing analysis using detailed hydrographs must be applied for all detention pond designs. The NRCS hydrologic methods (available in TR-20, TR-55, HEC-1, HEC RAS and the Hydrologic Engineering Center (HEC)) hydrologic methods may be used for areas of two hundred (200) acres or more. Use of the modified rational method is limited to drainage areas less than two hundred (200) acres.

F.

Stormwater detention.

1.

Existing condition peak flows generated from the 25-year and 100-year frequency storm events shall not be increased. The peak flows from the proposed site development for a 25-year storm and a 100-year storm, shall be detained in on-site stormwater detention basins with release rates equal to, or less than the flows generated in the site's existing condition. Detention ponds must also be designed such that the 100-year storm will pass without overtopping the structure. The engineer shall design the system to safely discharge the 100-year storm without damage to the downstream property or the detention pond's embankment.

2.

Detention ponds shall be designed with adequate area around the perimeter for access and maintenance. The said area shall be a minimum of seven (7) feet wide for ponds with depths of five (5) feet or less (back slopes included) and a minimum of ten (10) feet wide for ponds over five (5) feet deep or with back slopes in excess of five (5) feet high. The said area shall not slope more than sixteen (16) percent (1:6 slope). Exceptions to this access area requirement can be approved by the City Engineer and Public Works Director in writing for privately maintained detention ponds that have at least one access point.

3.

The city engineer shall have the authority to waive the requirement for onsite detention, provided that at least one (1) of the following conditions are met:

a.

The development is adjacent to a defined water course that has sufficient capacity to convey the site's post-developed peak discharge from the 100-year storm event without creating an adverse impact on any other properties. The discharge in the water course shall be determined by using the 100-year storm event with the post-developed site and the remainder of the watershed in an ultimate build-out state.

b.

The development is located such that onsite detention may worsen downstream conditions of the watershed. In such cases, the engineer shall demonstrate that conveyance or a combination of detention and conveyance will provide a safer downstream condition.

G.

Floodplain management.

1.

General.

a.

Drainage areas within the subdivision boundary delineated by FEMA on the FIRM as the effective special flood hazard areas for the base flood must be shown on the subdivision plat.

b.

For the purposes of floodplain management, any concentrated flow within a watershed that has a drainage area of three hundred twenty (320) acres or greater, or any special flood hazard areas defined by FEMA, shall be defined as a floodplain. Floodplains with drainage areas between 320 and 640 acres that are not currently mapped as FEMA effective floodplains will not require a submission to FEMA to define the unmapped regulatory floodplain. Development in any defined floodplain must comply with the requirements of this subsection I.

c.

All new construction and substantial improvements of buildings (structures) shall have the lowest floor (including basement) two (2) feet above the base flood, based upon the current condition of all properties contributing to the point of consideration. If existing stormwater detention is to be included in the analysis, the topographical information used for the analysis must meet the [necessary] requirements.

d.

All floodplains shall be computed utilizing the computer software and methodologies outlined in the drainage criteria manual.

e.

If land development activities are proposed which will result in flood hazard boundary delineations different from those depicted on the current flood insurance rate map (FIRM) issued by the Federal Emergency Management Agency (FEMA), the applicant for a development permit shall obtain a conditional/letter of map revision (CLOMR/LOMR) from FEMA.

f.

All floodplain delineations for FIRM revisions shall be based upon cross sections created from methodology outlined in the latest revision of FEMA Policy Standards for Flood Risk Analysis and Mapping.

2.

Federal Emergency Management Agency.

a.

The Federal Emergency Management Agency (FEMA) maintains flood insurance rate maps (FIRMs) that depict floodplain and floodway boundaries. The floodplain and floodway boundaries depicted on FIRMs are based on existing conditions of development in the contributing area.

b.

FEMA reviews and approves or denies all revisions or amendments to FIRMs. FEMA revises or amends FIRMs by approval of a letter of map amendment (LOMA) or letter of map revision (LOMR). FEMA establishes the process and fees necessary for review of an application for LOMA or LOMR.

c.

FEMA reviews the impact of proposed site developments and offers or denies conditional assurance that a FIRM may be changed by the proposed development. FEMA offers this assurance by a conditional letter of map amendment (CLOMA) or conditional letter of map revision (CLOMR). The CLOMA or CLOMR is a conditional statement that the FIRM may be changed if:

(1)

The development is constructed as proposed in the CLOMA/CLOMR application, and if

(2)

A complete LOMA/LOMR is submitted after construction of the proposed development.

3.

Coordination of City of Copperas Cove and FEMA floodplain delineations.

a.

General requirements.

(1)

Any proposed development in the floodplain will require a detailed Hydrologic & Hydraulic study (completed to FEMA standards) that will be presented to the City or City's Consultant for review.

b.

If the floodplain depicted on the FIRM is required to be changed due to updated analysis of the floodplain under existing conditions, then the following requirements are applicable:

(1)

Prior to approval of a final plat with revised floodplain delineation included, the applicant must provide to the city evidence of a FEMA approved CLOMR/CLOMA or LOMR/LOMA.

(2)

With prior approval from the City Engineer, the applicant may provide an engineering study, in lieu of a FEMA-approved CLOMR/CLOMA, demonstrating compliance with Copperas Cove Ordinances that is sealed by a Texas Professional Engineer and accepted by the City Engineer. Engineering study must meet the requirements of a CLOMR/CLOMA submission to FEMA. Additional information may be requested from the developer to support submission. Submission of LOMR/LOMA to FEMA will still be required. In the event that FEMA rejects the LOMR/LOMA, the applicant shall be required to make the revisions FEMA requires to receive LOMR/LOMA approval.

(3)

Prior to issuance of building permits on lots within the current FEMA FIRM floodplain, the applicant must provide to the city evidence of final acceptance by FEMA of the LOMR/LOMA submitted under 1. above. The requirement for this subsection (3)b.2. shall be depicted on the plat as a plat note.

c.

If the floodplain depicted on the FIRM is required to be changed due to land development activities that alter existing conditions, then the following requirements are applicable:

(1)

Prior to approval of final plat, the applicant must provide to the city evidence of receipt by FEMA of an application for a CLOMR.

(2)

With prior approval from the City Engineer, the applicant may provide an engineering study, in lieu of a FEMA-approved CLOMR/CLOMA, demonstrating compliance with Copperas Cove Ordinances that is sealed by a Texas Professional Engineer and accepted by the City Engineer. Engineering study must meet the requirements of a CLOMR submission to FEMA. Additional information may be requested from the developer to support submission. Submission of LOMR to FEMA will still be required. In the event that FEMA rejects the LOMR/LOMA, the applicant shall be required to make the revisions FEMA requires to receive LOMR/LOMA approval.

(3)

Prior to recordation of a final plat, the applicant must provide to the city evidence of approval (either FEMA or City Engineer) of the CLOMR submitted under 1. above.

(4)

If the final plat is approved before it is determined that a CLOMR is necessary or desired, then prior to release of subdivision construction plans, the applicant must provide to the city a letter of acknowledgement by FEMA of receipt of a complete application for a CLOMR or have CLOMR submitted to the City Engineer for review (as outlined in 1. above).

(5)

Prior to issuance of building permits on affected lots, the applicant must provide to the city evidence of final acceptance by FEMA or the City Engineer of the CLOMR and a letter of acknowledgement by FEMA of a complete application for a LOMR. The requirement for this subsection (3)c.4. shall be depicted on the plat as a plat note.

(6)

Prior to issuance of a certificate of occupancy on any development in the floodplain, the applicant must provide to the city evidence of a final acceptance by FEMA of a LOMR/LOMA. The requirement for this subsection (3)c.5. shall be depicted on the plat as a plat note.

d.

LOMR-F requirements.

(1)

Applicability. A LOMR-F shall only be allowed under the following circumstances:

(a)

For any existing fill that was in place during the FEMA effective floodplain study; or

(b)

For an existing single isolated lot.

(c)

All others shall require a Conditional Letter of Map Revision Based on Fill (CLOMR-F). A CLOMR-F shall be prepared and submitted to the City and FEMA in accordance to FEMA's submission requirements. Prior to allowing a LOMR-F on any development in the floodplain, the applicant must provide to the City evidence of a final acceptance by FEMA of the CLOMR-F.

(2)

In order to facilitate specific circumstances, which will require the approval of the City Engineer and the City Floodplain Administrator, the City will allow the submission of a LOMR-F in lieu of a LOMA/LOMR under the following conditions:

(a)

Fill placed in the floodplain must meet the requirements for ASCE 24 for stability of fill;

(b)

No existing structures adversely impacted;

(c)

Maximum surface area increase of 1% over the existing floodplain as shown on the effective FIRM;

(d)

No greater than 1-foot rise in the base flood elevation;

(e)

Volume of fill located within the floodplain must be offset by an equivalent or greater cut in the floodplain. This is not the total volume of fill placed within the floodplain limits, but the volume of fill that is within the floodplain and is located between the existing ground and the base flood elevation. There must be a zero or negative net fill in the floodplain, also known as compensatory volume.

(3)

A detailed H&H study may be required to demonstrate compliance with the above requirements. The H&H study must be in a similar format that is acceptable to FEMA.

e.

The applicant shall bear the cost of engineering services required to develop the application, respond to review comments, and obtain final approval of LOMRs and CLOMRs. The applicant shall bear the cost of any fees associated with review and disposition of LOMRs and CLOMRs that are established by FEMA.

(1)

The fee for City Engineer review of a CLOMR shall match current FEMA fees.

4.

Drainage Easement Requirements for Floodplain Development.

a.

Floodplain development that adversely affects area(s) outside of the proposed development, that would not require an easement, would be restricted to each of the following:

(1)

No existing structures affected;

(2)

Maximum surface area increase of 1% over the existing floodplain on each affected parcel or, if owned by a single landowner, contiguous parcels; and

(3)

No increase in flood elevations that exceeds a 1-foot rise.

b.

An increase in the floodplain outside of the developed property and in excess of the easement requirements listed above would require a drainage easement. This is based on the following:

(1)

Compensation to the current landowner for loss of potential future development of property.

(2)

Restricts development in the drainage easement on the affected property.

H.

Lot grading.

1.

All site developments must provide a site grading and drainage plan that includes drainage computations, detention of runoff (if required) and a detailed site grading plan that does not adversely affect adjacent lots, property or downstream property.

2.

The grading or drainage plan shall include arrows indicating the direction of runoff for each lot. Where practical, all lots shall be graded from rear to front at which point the drainage shall be intercepted by the street. If rear-to-front grading cannot be met due to land slope, topography or existing trees, alternate grading plans may be utilized. In these instances it shall be demonstrated to the satisfaction of the city engineer that grading from front to rear would be more reasonably adaptable to the existing topography. All lots that fall into this second category shall be identified on the final plat by a listing table.

3.

Finished floor elevations shall be shown for all lots adjacent to or encroaching upon the FEMA designated 100-year flood plain. Finished floor elevations shall be a minimum of two (2) feet above the base flood elevations.

4.

Lot to lot drainage is prohibited except in residential developments where one (1) lot may drain onto one (1) adjacent lot to the rear. Residential lots may not drain from side to side unless directly adjacent to a city maintained facility (right-of-way or easement). The cumulative stormwater runoff on any single residential lot may not exceed the cumulative stormwater runoff generated from a total of two (2) residential lots.

5.

The applicant for a building permit for a developed lot that is graded from front to rear shall prepare a detailed site grading plan that includes elevations for all corners of the subject lot, all corners of the downstream lot, the finished floor slab elevation, final contours, swales, and any modifications to side yard or rear yard fencing to facilitate removal of runoff from the subject lot. The site grading plan for a subdivision must be sealed, signed and dated by a professional engineer licensed in the State of Texas for a development.

6.

Easements must be a minimum of fifteen (15) feet wide or one and one-half (1½) times the depth of any buried pipe, whichever is greater. All easements must be located entirely on one (1) lot.

I.

Erosion control. Such measures must be installed prior to city acceptance and must be maintained until a certificate of occupancy is issued on the property. Such installations shall comply with current TCEQ requirements. Additional measures may be required during and after construction if during subsequent runoff events erosion or sediment damage is documented as a violation of TCEQ regulations or city ordinance by city staff.

(Ord. No. 2016-61, § 1(Exh. A), 10-13-16)