§ 3.0. Drainage design requirements
Adoption.
This ordinance is hereby adopted and shall be known as the "Official Drainage and Flood Control Ordinance" of the Town of Little Elm.
Purpose and Scope.
The purpose of policies and design standards set forth herein is to insure adequate stormwater drainage and flood control within the Town of Little Elm. The policies and standards herein are intended to protect public health and safety, to prevent property damage due to flooding, to equitably distribute the cost of necessary drainage improvements, and to minimize the maintenance cost of drainage facilities constructed. Any development or improvement of property affecting storm drainage and flood control in the Town of Little Elm is subject to the provisions of this ordinance.
A.
The development of land shall consider all drainage elements normally used in the engineering standards of design.
B.
Stormwater design shall consider the maximum results from a 100-year frequency storm. The release rate of stormwater shall be such that the flows of runoff water after the land development is completed shall not exceed the flows existing before land development is started. In cases where the land considered for development is adjacent to Lake Lewisville and a direct discharge system can be directed into the lake area, then the increased runoff may be allowed to flow without consideration of detention systems.
C.
Where existing development is in place upstream of the land area under consideration for development, provisions shall be made within the engineering design to provide capacities for the potential full flows which may be created by the upstream properties based on the result of a 100-year rainfall event.
D.
All standards of the corps of engineers established around Lake Lewisville will be followed. The developer or his designee shall certify that all standards of the corps of engineers around Lake Lewisville have been met.
Standard Provisions for Storm Drainage.
A.
All construction for storm drainage in the development or improvement of real property within the Town of Little Elm and its ETJ shall conform to the following standards and requirements:
B.
Storm sewer inlets shall be provided along paved streets at such intervals as are necessary to limit the depth of flow as follows: SPREAD OF WATER
Table 1: Allowable Storm Water Spread
Type of Street Allowable Speed Major thoroughfare One traffic lane in each direction to remain open
Collector street One moving traffic lane to remain open
Residential street Maximum depth for design storm as shown on
C.
Section VI herein (figure 2) is curb deep.
D.
A closed storm sewer system shall be required to accommodate a run-off exceeding the street capacity, as provided above, up to and including the design capacity of a 72-inch concrete pipe.
E.
An open channel may be permitted to accommodate run-off exceeding the design capacity of a 72-inch pipe, as provided below:
1.
Channels draining an area with a "CA" factor (coefficient of run-off and drainage area, as used in the hereinafter described "rational formula") of less than 600 shall be concrete lined to the design depth, plus six-inch freeboard except that a closed system as provided above may be used. A 20 foot wide access easement shall be provided along at least one side parallel to channel.
2.
Channels draining an area with a "CA" factor between 600 and 1,000 shall be improved to a capacity of the 100-year design discharge by excavation, straightening and realignment, as required, and also the construction of a concrete lined channel having a width of not less than the bottom width with concrete lined to a depth of at least two feet. Earthen side slopes shall be no steeper than 4:1, horizontal to vertical, and shall be sodded to prevent erosion.
3.
The setback for the building line shall be based on a minimum of five to one setback so that as a minimum of 20 foot of width access and maintenance strip is provided parallel and adjacent to each side of the top bank of all drainage channels in this section.
F.
In lieu of the improvements of a channel draining an area with a "CA" factor in excess of 1,000, the town council may elect to accept the dedication of all land within the natural 100-year floodplain of the existing drainage channel as a permanent right-of-way.
G.
That the criteria for drainage improvements as hereinabove set forth in paragraphs A. through D. of this section shall be applicable to publicly owned lands solely at the discretion of the Town of Little Elm.
H.
Excavation, fill and grading operations within the town limits shall be undertaken only after a proper permit has been obtained from the town.
I.
Responsibility of Owner or Developer for Storm Drainage.
1.
The owner or developer of property to be developed or used shall be responsible for all storm drainage flowing through or abutting such property. This responsibility includes the drainage directed to that property by prior development as well as the drainage naturally flowing through the property by reason of topography. It is the intent of this ordinance that provision be made for storm drainage in accordance with section III above, at such time as any property effected is proposed for development use.
2.
Where the improvement or construction of a storm drainage facility is required along a property line common to two or more owners, the owner hereafter proposing development or use of his property, shall be responsible for the required improvements at the time of development, including the dedication of all necessary right-of-way or easements, to accommodate the improvements.
3.
Where a property owner proposed development or use of only a portion of his property, provision for storm drainage in accordance with section III above shall only be required in that portion of the property proposed for immediate development or use, except as construction or improvements of a drainage facility outside that designated portion of the property is deemed essential to the development or use of that designated portion.
4.
The owner or owners shall dedicate to the town the required drainage easements. Determination of minimum easement required shall be made by the town engineer.
5.
In the event that an owner or developer desires to impound stormwater by excavation, filling or construction of a dam within a property, thereby creating a lake, pond, or lagoon as a part of the planned development of that property, the standard provisions for storm drainage as established in section III of this ordinance shall not be applicable, provided:
6.
That an engineering plan for such construction, accompanied by complete drainage design information, prepared by a registered professional engineer, shall have been approved by the Town of Little Elm;
7.
That the owner or developer shall have agreed to retain under private ownership the lake, pond, or lagoon constructed, and to assume full responsibility for the protection of the general public from any health or safety hazards related to the lake, pond, or lagoon constructed;
8.
That the owner or developer shall have agreed to assume full responsibility for the maintenance of the lake, pond, or lagoon constructed;
9.
That the obligations herein shall run with the land and shall be a continuing obligation of the owner or owners of such land;
10.
That the requirements of the Texas Water Code pertaining to impoundment of surface water are complied with including the design construction and safety of the impounding structure;
11.
On any existing structure, the owner will furnish a study proposed by a professional engineer for the town for approval.
J.
Responsibility and Participation of the Town in Storm Drainage Improvements.
1.
The town may, in its sound discretion, participate in the cost of such improvements as are required by this ordinance in an amount not to exceed 20 percent.
2.
The town may, in its sound discretion, participate in any project of improvements by the levy of a special assessment against the lands to be enhanced in value by such improvements.
3.
The town may acquire drainage easement necessary for such improvements by acceptance of dedication, purchase, or condemnation.
4.
The town shall, upon acquisition of the drainage easement and the completion of improvements as hereinabove provided, assume full responsibility for the maintenance of such drainage facilities.
K.
Engineering Design. The design of all storm drainage facilities shall be in accordance with the following criteria:
A.
Design frequency:
Table 2: Design Frequency
Drainage Facility Storm Frequency Closed storm sewer systems 10 r. with emergency 100-year overflow Closed storm sewer system at street low point or sag 25 r. with emergency 100-year overflow Culverts and bridges 100-year Concrete lined channels 50 r. with emergency 100-year overflow Earthen channels 100-year L.
Computation of stormwater run-off shall be by the "rational method", which is based on the principle that the maximum rate of run-off from a given drainage area for an assumed rainfall intensity occurs when all parts of the area are contributing to the flow at the point of discharge. The formula for calculation of run-off by the "rational method" is:
Q = CIA, where
Q = The maximum rate of discharge, expressed in cubic feet per second.
C = A run-off coefficient which varies with the topography, land use and moisture content of the soil at the time the run-off producing rainfall occurs. This run-off coefficient shall be based on the ultimate use of the land as recommended by the master plan for the Town of Little Elm and shall be selected from table I herein on the basis of the used shown on land use and zoning map of the comprehensive zoning ordinance for the Town of Little Elm, if an area has had a change of zoning to give the area land use for which the "C" in table II is higher than use shown on land use and zoning maps, the higher "C" factor shall be used.
M.
For drainage areas where the use of the rational method does not provide reliable data, the use of a unit hydrograph flow determination shall be made. The use of a unit Hydrograph calculation will be based upon standard and accepted engineering principles normally used in the profession subject to the approval of the town engineer.
I = Rainfall intensity in inches per hour from the applicable curves of figure 1. time of concentration or duration of rainfall for use in figure 1 shall be calculated by data shown in table 2.
A = The drainage area, expressed in acres, contribution to the runoff at the point in question. Calculation of the drainage area shall be made from a topographic map, a copy of which shall be submitted with the engineering plans for approval.
N.
Computation of runoff shall be based on a fully developed drainage area, or watershed, in accordance with the land use projected in the then current comprehensive land use plan for the Town of Little Elm.
Table 3: Design Criteria
Land Use from Master Plan Value of "C"
(Run-off Coefficient)Flat 0% to 1% Park areas—No developable land 0.2 Park and school land tract 0.3 Single-family residential 0.4 Duplex 0.45 Multiple family 0.5 Local business 0.6 Central business 0.75 Commercial 0.75 Industrial 0.7 Rolling - 1% to 3½% Park areas—No developable land 0.3 Park and school land tract 0.4 Single-family residential 0.5 Duplex 0.55 Multiple family 0.6 Local business 0.7 Central business 0.8 Commercial 0.8 Industrial 0.75 Rough - 3.5% and over Park areas—No developable land 0.35 Park and school land tract 0.45 Single-family residential 0.6 Duplex 0.65 Multiple family 0.7 Local business 0.75 Central business 0.85 Commercial 0.85 Industrial 0.8 Table 4: Average Velocity for Use in Determining Time of Concentration
Description of
Water Course
0% to 3% V in f.p.s. 4% to 7% V in f.p.s. 8% to 11% V in f.p.s. Over 12% V in f.p.s. Surface drainage 5 10 15 18 Channels Determine V by Manning's Formula Storm sewers Determine V by Manning's Formula f.p.s. = feet per second
P.
Average velocity of the runoff for calculating time of concentration or duration of rainfall for use in figure 1. These average velocities in this table shall be used unless the designer shows calculation of velocity by streets and/or storm sewers.
Q.
Using the average velocities from this table the designer shall calculate the time of concentration by the following formula unless more data are shown on the plans for calculating time of concentration.
T = "Inlet Time" + D/(V*60)
Where:
T = Time of concentration in minutes for use in figure 1.
D = Distance in feet from point of concentration to upper end of drainage area under consideration.
V = Velocity in feet per second from this table or velocity calculated by designer by streets and/or storm sewers. "Inlet Time" = 5 minutes from property zoned for multiple family, local business, central business, commercial, or industrial.
"Inlet Time" = 10 minutes for property zoned for parks, schools, single family residential, and duplex.
R.
The capacity of storm sewers, culverts, bridges, and open channels shall be determined by the use of the Manning's Formula, and the design of the facility derived accordingly. The Manning's Formula is:
Q = 1.486 R 2/3 S 1/2 * A
Where:
Q = Discharge, expressed in cubic feet per second.
n = Roughness coefficient, based on condition and type of conduit lining.
S = Slope of hydraulic gradient, expressed in feet of vertical rise per foot of horizontal distance.
R = Hydraulic radius (area of flow divided by the wetted perimeter).
A = Cross sectional area of flow in conduit, expressed in square feet.
S.
The construction of all concrete improvements shall be in accordance with the standards set forth in the Paving and Drainage Specifications handbook of the Town of Little Elm.
A.
The frequency chart in years (figure 1), the inlet capacity for low point inlets (figure 2), the drainage capacity road sections (figure 3), are hereby adopted as a part of this ordinance.
T.
Complete engineering plans for storm drainage facilities shall be prepared by a professional engineer, registered in the State of Texas, and experienced in civil engineering work. The total cost for such engineering plans and specifications shall be borne by the owner or the developer and shall be furnished to the town engineer for their review.
U.
In any development or improvement of property, the owner may be required to provide at their expense a preliminary drainage study for the total area to be ultimately developed. This study shall be submitted to the town engineer prior to approval of the engineering plans for the total development, or any increment thereof.
Construction in Areas Subject to Flooding.
A.
No building or structure shall hereafter be erected or relocated within the 100-year floodplains of the major drainageways, unless the finished floor elevation of such a building or structure is a minimum of two feet above the high water elevation calculated for the runoff from a rainfall having a design frequency of 100 years. In all other areas subject to flooding, the finished floor elevation shall be a minimum of the high water elevation calculated for the runoff from a rainfall having a design frequency of 50 years and shall not flood within the 100-year frequency. The owner or developer shall furnish, at their expense, to the town engineer sufficient engineering design information to confirm that the minimum floor elevation proposed is as required by this paragraph.
B.
No building or structure shall be erected or relocated within a floodplain of drainage channel which will obstruct the natural flow of water within that floodplain or channel.
C.
No excavation, filling, or construction of embankment, or landscaping shall be permitted within a floodplain or channel which will obstruct the natural flow of the water within that floodplain or channel, unless sufficient engineering design information is furnished to the town engineer in order that they may determine that same will not adversely affect flow characteristics within that floodplain or channel, resulting in damage to that or any other property nearby.
D.
Building or Structure Setback Requirement. No building or structure shall hereafter be constructed, reconstructed, or relocated within 20 feet of any open drainage channel. It is the intent of this section to insure that an unobstructed width of at least 20 feet is maintained between the top side slope of any such drainage channel and any building or structure.
E.
Construction over Drainage Facilities. No building or structure shall hereafter be constructed, reconstructed, or relocated over or across any storm drainage facility, unless specifically approved by the town engineer pursuant to that criteria established in paragraphs A and B of this section.
F.
Construction of buildings or structures may be permitted over or across closed drainage systems, to include concrete box culverts or reinforced concrete pipe. Engineering plans and specifications shall be prepared by a professional engineer registered in the State of Texas and experienced in civil engineering work. The total cost for such engineering plans and specifications shall be borne by the owner or the developer and shall be furnished to the town engineer for their review and approval.
G.
Construction of pedestrian bridges or small vehicular crossings may be permitted by the town engineer. Engineering plans and specifications shall be prepared by a professional engineer registered in the State of Texas and experienced in civil engineering work. The total cost for such engineering plans and specifications shall be borne by the developer and shall be furnished to the town engineer for their review and approval.
Storage; General.
Stormwater management programs aimed at controlling increased stream water runoff generated by development are a top priority in urban watershed planning. More frequent flooding, increased rates and volumes of stormwater runoff, increased stream channel erosion and degradation, increased sedimentation, and increased water pollution are all problems intensified by increased stormwater runoff resulting from development. Storage of excess urban storm runoff is one of the most promising methods available in preventing urban flood damage.
1.
Storm runoff storage with reduced release rates can hold downstream flood flows to within the safe conveyance capacity of the storm sewer and stream system. In most cases it can be shown that storage is more economical than increasing downstream conveyance capacity. Storage facilities should be planned and designed to assure an effective and efficient operation and maintenance program.
2.
Retention and detention are two generalized types of storm runoff storage used to control flooding. Retention storage refers to storm runoff collected and stored for a significant period and released or used after storm runoff has ended. Retention storage usually consists of "wet reservoirs" which often have agricultural, recreational, and/or aesthetic value. Detention storage consists of reducing the rate of runoff for a short period of time to reduce peak flows by controlling the discharge through an outlet structure and by extending the period of runoff.
3.
Skillful use of the procedures outlined in this section will provide reasonable solutions to drainage policy requirements. Individual and community experience in the use of these techniques will evolve into a methodology which will allow urban growth without the attendant drainage problems.
Storage Classification. Storage can be classified by location as follows:
a.
Rainfall storage. The storage of water near the point of rainfall occurrence or before storm runoff accumulates significantly. Rainfall storage is usually accomplished by roof top ponding, parking lot ponding, property line swale ponding, and small on-site ponds.
b.
Runoff storage. Water stored after storm runoff has accumulated significantly and begun to flow in a drainage system. Runoff storage is usually accomplished by off stream storage, channel storage, or on-stream storage.
Rainfall Storage.
a.
Rooftops. The building codes require roof load design for rain and snow. The design load may be converted to equivalent water depth in inches which can be safely contained on flat roofs. The maximum storage allowed for design purposes should not exceed this depth unless a building is designed to withstand a greater roof load. The depth of water can be controlled by proper sizing of downspouts and by constructing scuppers through the parapet walls.
b.
Another method of achieving rooftop ponding involves placing loose gravel windrows or dikes a few inches in heights so that storm runoff is trapped as it moves toward the drainage outlet. Individual wedge-shaped ponds form behind the small dikes and slowly drain as the stormwater filters through the gravel. This solution is of limited value, however, as there is no way to assure its permanence.
c.
The possibility of overflows exist and will occur quite often during major storms. Overflow drains should be used to protect against roof overloading. Periodic inspection and proper maintenance will reduce the possibility and the hazards of overflows.
d.
Special attention must be given to the water tightness of the roof to assure that no leakage occurs due to the accumulation of water.
Parking Lots.
A.
Considerable area in urban development is occupied by parking lots. If planned correctly, these paved areas can provide adequate detention with minimum inconvenience to the public and without functional interference.
B.
There are two general methods of stormwater detention that can be utilized on parking areas. One form involves the storage of runoff in depressions constructed near drains or outfall structures. The second method of stormwater detention on parking lots consists of using the paved parking areas to channel stormwater to grassed or gravel filled areas for maximum infiltration and decrease in overland flow.
Recreation Areas.
A.
Recreation areas, such as sports fields, generally have a substantial area of grass cover which often has a high infiltration rate. Storm runoff from such fields is minimal. A secondary use of such recreation fields can be made by providing for limited detention storage of runoff from adjacent areas.
B.
Since these areas are not used during periods of precipitation, detention ponding should not seriously impede their primary use. To minimize after effects, the recreation area should be designed so that it will thoroughly drain. In addition, the vegetation used on the area should be tolerant of periodic inundation and wetness.
Property Line Swales.
A.
Subdivision planning and layout requires adequate surface drainage away from buildings. This is obtained by sloping the finished grade in all directions away from the building. The layout often calls for a swale to be located along the back property line, which then drains longitudinally through the block. Where this is used, the design shall include a concrete flume section with baffles to slow the flow of water yet maintain constant point for discharge.
B.
Temporary ponding facilities along the rear lot line may include small controlled discharges along the back property line or, if the subsoil conditions are favorable, several inches of rainfall could be expected to percolate into the ground during and after a storm.
C.
Prior to planning for property line swale ponding, the engineer should determine that saturation of the subsoil will not have an adverse effect on building foundations. In cases where significant subsoil problems exist, water should not be ponded or percolated into the ground.
Combinations.
In many instances, one on-site detention method cannot conveniently or economically satisfy the required or needed amount of stormwater storage. Limitations in storage capacities, site development conditions, soils limitations and other related constraints may require that more than one method be utilized. For example, roof top, parking lot, and surface pond storage might all be required to compensate for the increase in runoff due to development of a particular site. Whatever combinations are suitable may be incorporated into a site development plan.
Runoff Storage.
A.
Off-stream Storage. The storage of water in depressed open areas, in reservoirs, and on low lying recreation fields to which storm peak flows are routed in usually termed off-stream storage. It is usually characterized by a side channel spillway or overflow from the main channel.
B.
Channel Storage. Although all channels inherently store water, channels can be made to attenuate runoff by altering their hydraulic characteristics in a way that will reduce peak flows. Side channels that run essentially parallel to the main stream channel are also a means of temporarily storing water during excessive rainfall events.
C.
On-stream Storage. The construction of an embankment across a channel so that a storage pond is formed represents on-stream storage. Spillway considerations are important to pass large floods exceeding the storage capacity. Properly protected roadway embankments are well suited for this purpose.
D.
Multipurpose Use.
a.
Rainfall storage and runoff storage systems are most practical when designed for multi-purpose use. For maximum land use efficiency the design engineer must consult early with the planning department and the parks and recreation department of the town.
b.
Lower cost recreation. Making use of detention storage areas as parks and greenbelts, ballfields and playing fields, satisfies two needs and reduces the cost of each. The mini-park concept, where small recreational areas are provided in neighborhoods within safe walking distances for children, can be a typical joint effort of the drainage design engineer and the planning and recreational staffs of the town. Provisions must be made for maintenance of such areas.
c.
Multi-use criteria. The multiple use of storage areas is a field of endeavor where the fervent disciplines should develop desirable and acceptable criteria fitting the needs of the local community. It is with storage, including both detention and retention storage, that an important potential exists for the reduction of flooding, drainage costs, and to some degree recreation costs.
Hydraulic Design Criteria.
a.
General. Hydraulic and hydrologic design criteria provide the guidelines for design and construction of stormwater storage facilities. These criteria are a necessary part of a stormwater management program needed to protect low-lying areas by preventing unacceptable increases in runoff rates as urban development progresses. The engineer should be familiar with the drainage policy of this ordinance before considering the feasibility of a storm runoff detention facility. Existing land contours of the property should be a consideration in developing the drainage plan. In many instances, storage can be achieved economically by blocking the overland flow of stormwater runoff with various land forms, curbs, walls, terraces, and other means. The amounts to designing a drainage system that will minimize the reduction in the critical time of concentration as the property is developed. A drainage project located in a watershed that has a master plan may involve additional considerations which require review. Consultation with the town is essential to insure compliance with the overall watershed master plan.
b.
Design factors and procedures. Before proceeding with engineering design of an on-site storage facility, the physical and technical factors should be identified and the basic design procedures established. A discussion of design factors and procedures follow.
(1)
The drainage plans for a particular storage facility should include all property lines, topography, area, location, and all other items necessary to meet policy requirements outlined in the "Drainage Policy for the Town".
(2)
Care must be taken to locate, layout, and design the storage facility in a manner to insure safety and ease of maintenance.
c.
The rate of inflow to the storage facility (inflow hydrographs) and all hydrologic considerations must assume ultimate development of the site's contributing area. Several inflow hydrographs should be prepared to examine their effect on downstream flooding for the various design storm frequencies. The number of inflow hydrographs to be provided may vary but should always adequately describe the range of flows expected for the design storm frequencies. These various design storm inflow hydrographs and an acceptable flood routing procedure will be necessary in sizing the outlet works for the storage facility. An emergency spillway should be provided to pass runoff that exceeds the design capacity of the detention facility.
d.
The maximum allowable release rate from a storage facility is selected after careful review of runoff rates for all pertinent storms. As a minimum, the pertinent design storm frequencies should include the 10, 25, and 100-year events. However, specific site conditions may require that additional consideration be given a particular area. The town engineering department should be contacted to identify special design conditions. The maximum allowable peak flow released from a particular site is selected to comply with the policy section of this manual. In all cases, the maximum release rate should be calculated using the hydraulic gradient created when the storage area is being utilized at full capacity.
e.
Key items to consider in determining the maximum allowable release rate are: a) the peak flow rates for the various design storm frequencies prior to development, b) the capacity of the downstream stormwater channel or storm sewer, c) compliance with a master plan for the watershed (if one exists), d) problems, if any, that are created onsite by the storage of stormwater runoff, e) potential problems created downstream when the onsite storage capacity is fully utilized and the excess runoff flows overland into downstream areas.
f.
The stormwater storage volume required is given by the maximum difference, at any time, between cumulative total inflow volume and cumulative outflow volume measured from the beginning of inflow for a 25-year storm. The maximum allowable release rate is not to be exceeded by the outflow peak discharge.
g.
The outfall or outlet structures should be designed on the basis of inlet or outlet control, whichever is applicable. The structures should be capable of safely and properly passing the flow range of design storm frequencies including the 100-year storm without causing downstream flooding or exceeding the maximum allowable release rate. Outlet works must also pass all of the runoff from the 100-year design storm within a reasonable length of time or a permit may be required from the Texas Commission on Environmental Quality (TCEQ). Wet ponds will in many cases also require a permit from the TCEQ.
h.
Special attention should be given to the provision of an emergency or overflow spillway which would pass excess flows greater than those of the 25-year design storm and overflows caused by clogging of the principal outlets. Downstream watershed considerations such as the potential loss of life and damage to property due to overtopping or failure of the structure and the storage facilities, area and capacity dictate the emergency spillway design storm. It is advisable to contact the state agencies such as the Texas Water Development Board (TWDB) and the TCEQ when planning a storage facility in order to meet their criteria.
i.
Surfaces shall be faced with embedded rock or aggregate masonry. Bare concrete shall only be permitted with the express written permission of the town council. Side slopes should be no steeper than 4:1 where feasible for reasons of public safety and maintenance.
H.
Hydraulic Design Methods.
a.
General. The two basic methods suggested for predicting the volume of runoff with time and the peak flow rate are the rational method and the standard method. The standard method is best suited for storage analysis of urban runoff but for relatively small drainage areas some form of the rational method if generally acceptable.
b.
Rational method storage analysis. When designing storage facilities for an area smaller than 400 acres, it is often justifiable to use the rational method to compute the design inflow hydrographs. This method relies heavily upon personal judgment but it is relatively simple and can give acceptable results if proper procedures are followed. There are several different techniques of employing the rational method that may be used.
(1)
One of the rational method techniques has been utilized by the federal aviation agency in designing airport drainage facilities. The procedure is presented in "Airport Drainage", prepared by the Federal Aviation Agency (FAA, 1966). The FAA technique is basically a graphical procedure which represents the cumulative storm runoff volume and the cumulative volume released from the storage facility through an outlet structure as functions of time. The maximum difference between the two volume curves represents the required storage volume of the storage basin. The procedure will consistently result in under designed storage facilities if a constant release rate is assumed from time zero. This is caused by sizing the outlet pipe using the Manning Equation and not recognizing the stage-discharge relationship. A reservoir routing technique unitizing the stage-discharge relationship of the storage facility should be used.
(2)
It is recommended that in most instances the contributing drainage area not be more than 25 acres when using this technique.
(3)
A second technique that employs the rational method is referred to as the modified rational method analysis. This technique manipulates the Rational Method to reflect the fact that storms with durations greater than the normal time of concentration for a basin will result in a larger volume of runoff even though the pear [peak] discharge is reduced. Even though rainfall intensities and resulting peak discharges associated with longer duration storms are less than those for short duration storms, the inflow may still be considerably greater than the outflow thus requiring more storage than in the case of shorter, higher intensity storms.
(4)
This approach becomes more valid on progressively smaller basins. The technique should, therefore, be limited to relatively small areas such as parking lots, roof tops, or other upstream facilities with contributing areas less than 25 acres.
I.
Detention Facilities.
a.
Intent. The Town of Little Elm encourages the use of detention basins based on the multi-faceted benefits of the community outlined below:
b.
Control of flood discharges. The intent of detention basin (storage) design shall be to reduce flood discharges for the ultimate watershed development conditions without increasing peak discharges above the peak discharges for undeveloped watershed conditions. This storage method serves as a key strategy for the stormwater management plan.
c.
Environment. As development occurs, there is a loss of wildlife and bird life habitat. It is possible to create a natural micro-environment around ponds that can offset this loss of habitat. These "natural" environmental pockets in conjunction with natural open space areas can help maintain the rural setting of the town. Detention ponds also benefit downstream water quality by allowing the sedimentation of pollutants.
d.
Recreation. Detention basins offer many recreational opportunities in an urban setting; for example, a normally dry detention basin could be used for athletic fields or picnic areas, while the banks of the pond offer valuable park space.
e.
Aesthetics. As an integral part of the Town of Little Elm, a detention basin needs to blend into the landscape and into the community. Simple yet inexpensive measures, such as gentle side slopes, planting of trees and shrubs, and other landscaping features can transform the detention facility into an attractive amenity for the neighborhood.
f.
Requirements.
(1)
Detailed engineering studies of the entire basin shall be required to evaluate the timing of hydrographs from regional and on-site facilities, as well as backwater calculations to determine the effect of the detention/retention facilities on upstream water surface elevations.
(2)
All detention facility designs shall be performed by a professional engineer registered in the State of Texas and shall meet the requirements of Permanent Rule 31, Texas Administration Code (TAC) Chapter 299, and other requirements, as applicable.
(3)
The minimum amount of storage volume of the detention basin shall be that volume required to reduce runoff rate to the undeveloped condition rate. Dedicated detention basins shall also include an additional one foot of freeboard and two feet of sediment storage. The volume of runoff storage for drainage areas greater than 160 acres shall be compared using unit hydrograph procedures. Acceptable unit hydrograph procedures include the Soil Conservation Service Dimension less Unit Hydrograph and Snyder's Unit Hydrograph. Manual methods or use of the computer programs TR20, HEC-1, and NUDALLAS are allowed for runoff hydrograph computation and flood routings. The criteria contained in this section shall serve as the minimum requirements for detention basin design. For drainage areas less than 160 acres, the above methods are recommended; however, an approximate routing method may be used subject to approval of the town engineer.
(4)
The maximum allowable release rate from any site shall be equal to the 100-year peak runoff rate prior to site development. The maximum design release rate may be less.
(5)
Where stormwater management improvements are required in the natural floodway or a permanent greenbelt or open space area, they should be incorporated into existing lakes, or they should be designed in as small an area as feasible to create natural contours. Note, however, that detention basins or retention ponds shall be a minimum of one acre in size to allow for proper maintenance, side slopes and outlet work operation.
(6)
Retention/detention ponds shall resemble natural ponds; in addition:
1.
The pond should expand gradually from the inlet towards the outlet, insuring that there are no "dead zones". That is, water entering the pond gradually spread out and uniformly displaces the water already present in the pond.
2.
The length-to-width ratio should be three to one or greater, to provide a long flow path.
3.
The average permanent pond depth should be greater than five feet, where possible.
4.
A ten to 20-foot wide shallow bench shall be provided along the shores of the permanent pond for safety and to encourage the development of bottom growth in these areas. This vegetation will enhance the biologic treatment characteristics of the pond and also enhance the "natural" appearance of the pond.
5.
If several ponds are required, they should vary in size and shape, and be separated by tree groves, vegetated berms, or similar screening of a natural appearance.
6.
Landscape materials should enhance wildlife habitat where feasible.
7.
Where slope erosion protection is needed for the side slopes of a pond, rock or geotextiles are required as approved by the town engineer. Exposed concrete [sic]
g.
Detention facilities of less than ten acre-feet of flood storage shall have outlet structures designed for the 100-year frequency storm. Facilities greater than ten acre-feet of flood storage shall have outlet structures designed for the ten-year and 100-year frequency storms. The release rates shall be such that the flows after land development are no larger than the flows before land development.
h.
Outlets should be provided with a trash rack to prevent plugging with debris and to provide safety to the public. Due to unique design considerations, the town engineer may approve alternative outlet design.
i.
All federal, state and county laws pertaining to the impoundment of surface water shall have been compiled with, including the design, construction and safety of the impounding structure. Copies of any federal, state or county permits issued for the proposed impoundments shall be submitted to the town engineer.
j.
All detention/retention facilities shall include an additional one foot of freeboard and two feet of sediment in determining the design flood elevation.
k.
Provisions (such as emergency spillways or reinforcing of embankments) shall be made for the occurrence of overtopping or failure of the outlet structure. Under no circumstances should the emergency overflow have a direct path to any buildings or other structures used for human occupancy, commerce, or industry. Details of these provisions shall be supplied to the town engineer for approval.
l.
The flood storage volume of detention/retention facilities shall be designed to empty in a maximum of 12 hours if said facilities are to be considered an erosion/pollution control facility.
m.
Consideration should be given to the stocking of retention ponds with fish to control algae growth.
n.
Inlet structures shall be designed to both dissipate the flow energy and drop the inflow elevation to below that of the pond's water surface or basin's flow line.
J.
Duties of the Town and Appeal to the Council. The interpretation of the criteria herein above set forth in sections III, IV, V, VI, VII, VIII, and IX, of this ordinance shall be made by the town engineer. Any person adversely effected by the decision of the town engineer may, within ten days from the date of such decision, file an appeal in writing to the town council. Such appeal shall be filed with the town engineer and the town secretary and shall define the specific areas of controversy. It shall be the duty of the town council to hear any such appeal within 30 days from the date it is received by the town engineer and town secretary.
K.
Building Permits to be Withheld. No building permit shall be issued, nor certificate of occupancy approved for any construction, reconstruction, or development upon any land where such construction, reconstruction or development is not in conformity with the requirements and intent of this ordinance. Anyone who violates any of the terms and provisions of this ordinance shall be denied a building permit, until the violation is corrected.
L.
Repealing Clause. All provisions of other ordinances conflicting with the provisions hereof are hereby repealed. All other ordinances and provisions of such ordinances expressly in conflict with the provisions hereof shall remain in full force and effect.
M.
Maintenance Guarantee. The contractor shall guarantee the work which he does against defective workmanship and materials for a period of one year from the date of final acceptance for the work by the owner.
a.
Where defective workmanship and/or materials are discovered requiring repairs to be made under this guaranty, all such repair work shall be done by the contractor at this own expense within five days after written notice of such defect has been given to him by the owner. Should the contractor fail to repair leaks or correct such defective workmanship and/or materials within five days after being notified the owner may make the necessary repairs and charge the contractor with the actual cost of all labor and materials required.
b.
The contractor shall arrange to have his faithful performance bond run for a period of one year after the date of completion of the construction work to cover his guaranty as set forth above.
Severability. If any section, paragraph, clause, phrase or provision of this ordinance shall be adjudged invalid or held unconstitutional, the same shall not affect the validity of this ordinance as a whole or any part or provision thereof, other than the part so decided to [be] invalid or unconstitutional; not shall such unconstitutionality or invalidity have any effect on any other ordinances or provisions of ordinances of the Town of Little Elm.
Penalty. That any person who shall violate the provisions of this ordinance shall be deemed guilty of a misdemeanor and upon conviction therefore shall be subject to a fine not to exceed $2,000.00.
(Ord. No. 1444, § 2(Att. A), 3-6-2018)