§ 35-32. Lakes and dams.

(a)

In the event that an owner desires to modify an existing pond or lake or desires to impound stormwater by filling or constructing an above-ground dam, thereby creating a lake, pond, lagoon, or basin as part of the planned development of that property, the criteria listed below shall be met before city approval of the impoundment can be given. Ponds or lakes created by excavation of a channel area without erecting a dam above natural ground elevation or instream, low water check dams are also subject to the criteria listed below, with the exception of spillway capacity requirements. The city engineer has the final authority to determine the design criteria for a proposed dam, check dam, or excavated lake. The requirements of the State of Texas must also be met for the construction of dams, lakes, and other impoundments.

(b)

The design criteria for a dam is dependent on the size and hazard classification of the dam. The size and hazard classification will be based on Chapter 12 of the Texas Water Code and will be determined by the city engineer based on information furnished by the owner. The following criteria will be used to classify a dam:

(1)

Size. The classification for size is based on the height of the dam and storage capacity, whichever gives the larger size category. Height is defined as the distance between the top of the dam (minus the freeboard) and the existing streambed at the downstream toe. Storage is defined as the maximum water volume impounded at the top of the dam (minus the freeboard). The following categories in table 35-32A must be used:

Table 35-32A - Impoundment
Category	Storage (Acre-feet)	Height (feet)
Minor	<100	<10
Small	≥100 and <1,000	≥10 and <40
Intermediate	≥1,000 and <50,000	≥40 and <100
Large	≥50,000	≥100

 

(2)

Hazard potential. The hazard potential for a dam is based on the potential for loss of human life and property damage downstream from a dam in the event of failure. The following categories in table 35-32B must be used:

Table 35-32B Hazard Potential Classification
	Loss of Life	Economic Loss
Category	Extent of Development	Extent of Development
Low	None expected (no permanent structures for human habitation)	Minimal (undeveloped to occasional structures or agriculture)
Significant	Possible, but not expected (No urban developments and no more than a small number of inhabitable structures)	Appreciable ( notable agriculture, industry, or commercial development)
High	Expected (urban development or large number of inhabitable structures)	Excessive (extensive public, industrial, or agricultural development)

 

(3)

Spillway design flood. The classification of a dam based on the above criteria will be used to determine the Spillway Design Flood (SDF). The total capacity of a dam structure, including principal and emergency spillways, shall be adequate to pass the SDF without exceeding the top dam elevation at a minimum. The SDFs required for various dam classifications are shown in table 35-32C.

Table 35-32C Spillway Design Flood
Hazard	Size	SDF
Low	Minor	100 - year
	Small	1/4 PMF
	Intermediate	1/4 PMF to 1/2 PMF
	Large	PMF
Significant	Minor	1/4 PMF to 1/2 PMF
	Small	1/2 PMF TO PMF
	Intermediate	PMF
High	Small	PMF
	Intermediate	PMF
	Large	PMF

 

(c)

In all cases, the minimum principal spillway design capacity is the 100-year design flood. In certain cases, a dam breach analysis may be required to determine the proper classification of the structure. For all structures requiring a spillway design flood equal to the PMF, a dam breach analysis is required to determine the downstream consequences of a failure. All dams designed for a SDF of 1/2 PMF or less shall be constructed with a minimum freeboard of two feet above the SDF elevation.

(1)

Additional design requirements.

a.

An engineering plan for such construction, accompanied by complete drainage design information and sealed by a registered professional engineer, shall be submitted to the city engineer.

b.

The spillway and any emergency overflow areas shall be located so that flood waters will not inundate any buildings, roadways, or other structures.

c.

All Federal, State, and local laws and regulations pertaining to impoundment of surface water must be followed, including the design construction and safety of the impounding structure. Copies of any Federal, State, and local governmental or agency permits issued for the proposed impoundments shall be submitted to the city engineer.

d.

Any existing structure, which is included in the project area shall be improved to comply with the applicable Federal, State, and local governmental safety requirements for structures.

e.

Before removing, enlarging, or altering any existing lake, the Owner will furnish a study of the effects of the alteration upon flooding conditions both upstream and downstream. The study shall be prepared by a registered professional engineer and submitted to the city for approval prior to making the proposed alteration. Compensatory storage shall be provided in a manner such that equal or comparable flood retention capacity is maintained.

f.

Any improvements to existing dams or lakes or construction of new impoundments shall be made at the expense of the owner, prior to acceptance of the adjacent street, utilities, and drainage improvements under the City's Subdivision Ordinance, as amended.

(2)

Maintenance and Liability Criteria.

a.

The owner shall agree to retain private ownership of the lake, pond, lagoon, or basin constructed and to assume full responsibility for the protection of the general public from any health or safety hazards related to the lake, pond, lagoon, or basin constructed. A private drainage easement shall be dedicated to the City for the improvement and shall meet the requirements of this ordinance.

b.

The owner shall agree to assume full responsibility for the maintenance of the lake, pond, lagoon, or basin constructed. The owner shall keep the city engineer advised of the current person responsible for this maintenance.

(Ord. No. 2017-10008, § II, 10-26-17)