Dirt fill

A dirt pile on River Road in Denham Springs

New subdivisions continue to cause issues for the parish council.

Five will appear on the planning commission's agenda Wednesday night, one of which is a new development coming to the corner of Dunn Road and Lockhart Road, north of Denham Springs in Parish Council District 3.

According to the councilman from that area, Maurice 'Scooter' Keen, the preliminary plat shows that the developer is trying use roughly 1/3 an acre per home, on a 70 acre site with six total acres of retention.

Residents in the area are discouraged by the traffic and drainage implications of a new development there, causing enough concern for the citizens of Summer Run - just north of the new development - to call a meeting.

That meeting was panned as an attempt to push a political agenda, while also using a public space for free.

Keen said, however, that the meeting and concerns don't currently matter much. Not that he wouldn't like to change the ordinances that govern residential developments, he added, but right now the council's options are limited.

If a developer requests no waivers, and meets the requirements of a traffic and drainage impact study, then there is very little the council can do to stop them. According to parish council attorney Chris Moody, the council already took a loss in that department after getting sued by a developer several years ago and losing.

Moody also stated that, even if the requirements are not met for a subdivision, the council has very little control what comes after they approve a preliminary plat. In most cases, the parish's administration would handle any specific issues that result in permit problems, study problems, or waivers.

That piece of information was discovered during the Premier Concrete issue, when the attorney informed the council that the administration would normally handle the permitting issue after it had occurred.

There are construction requirements in subdivisions currently in the council's ordinances. Generally, waivers are requested for those constraints, including for setbacks to try and help squeeze more homes into a subdivision. But sometimes, waivers are requested for drainage and traffic which both have an impact study.

Keen said that he intends to focus on a waiver requested by the new subdivision at Dunn and Lockhart, which is focused on right-of-way and drainage.

Drainage has it's own impact study, outlined within Chapter 125 of the parish's code of ordinances, section 24.

Initially, the ordinance calls for designs to follow the parish's master drainage plan, should one exist. The council diverted money to such a plan in the 2020 budget, which was introduced at the end of November and will come up for a vote at the council's first meeting of December.

However, one does not currently exist, so the developers are guided to follow the outline of the ordinance.

The ordinance begins by saying subdivisions must contain subsurface drainage, or exposed ditch drainage - they cannot have both, and if a developer plans to switch or move to subsurface drainage it must be approved by the parish president's office.

Next, developers must protect current natural and man-made drainage implements in the area by providing sufficient right-of-way for the appropriate authority to come for maintenance.

A USGS contour map will also be provided to the developer to take nearby watersheds into account, which may also need to be developed to avoid negative effects on nearby watersheds and individuals who live within them.

Finally, the drainage impact study must be submitted, but for the following exceptions:

  1. Development in which the area of impervious surface does not exceed twenty (20%) percent of the development area at the point of discharge from the site. The total impervious area shall include all buildings, driveways, sidewalks, streets, parking lots, lakes, ponds, etc. All undeveloped open space, common area, etc., must be clearly identified.
  2. Additions or modifications to existing developments which result in no more than a ten (10%) percent increase in existing impervious area and which have existing public storm drainage facilities designed to accommodate runoff from the existing site.
  3. The proposed development results in no more than a ten (10%) percent increase in the ten (10) year pre-development peak discharge at the point of discharge from the development site.
  4. The site is located within existing developed areas which are served by a network of public storm drainage facilities which were designed to accommodate runoff from the development site.

If a development does not meet any of the above requirements for exemption, it must follow the outline below:

(a)Site location and description. The drainage impact study shall comply with the following minimum requirements:

(1)Location. Describe location of subject property located by township and range; identify adjacent developments, major drainage outfalls, streets, highways, lot and block page number; and provide a vicinity map.

(2)Description. Describe the predominate existing land use and future land use in project watershed using the latest data available. Describe the proposed development, soil types, vegetative cover, watershed slopes and provide an estimate of percent of impervious area for pre- and post-development conditions.

(b)Watershed map.

(1)The watershed map should show the location of the project, drainage boundaries and acreage, existing channels, ditches, natural drains, proposed major drainage structures, channel realignment cross section locations and contours.

(2)Contours may be taken from the latest U.S.G.S. seven point five (7.5) minute quadrangle map or better.

(3)The watershed map must be at least one (1") inch equals five hundred (500') feet scale or less.

(4)The pre-development and post-development ten (10)-year and one hundred (100) year runoff rate and water surface must be shown at all entrance and exit points of the development. Twenty-five (25) year runoff rate and water surface may be required by review engineer.

(c)Hydrologic design.

(1)The drainage impact analysis shall indicate existing condition peak ten (10) year and one hundred (100) year flow rates at the development entry and exit points. Twenty-five (25) year runoff flow rates may be required by review engineer.

(2)The drainage impact analysis shall indicate future condition peak ten (10)-year and one-hundred (100) year flow rates at the development entry and exit points. Twenty-five (25) year flow rates may be required by review engineer.

(d)Hydraulic capacities.

(1)On-site capacity. Indicate capacity of any existing drainage outfall facility (ditch, canal, culvert, bridge, etc.) within the proposed development site and required type size, and capacity of any proposed outfall facilities as defined in this Section.(2)Off-site capacity. Determine capacity of existing downstream outfall facilities (ditches, canals, culverts, bridges, etc.) that will be utilized to convey flow from the downstream limits of the proposed development. An inventory of downstream structures including size, type, invert elevation, and cover topping elevation should be made. Channel cross sections at upstream and downstream limits of the proposed development at structure locations and at intermediate canal locations shall be required to adequately define existing channel capacities.

(e)Special site conditions. Special conditions which may exist at the proposed development site should be clearly identified, including, but not limited to, such items as:

(1)Special flood hazard areas (FIRM Zones A and AE).

(2)Regulatory floodway (if applicable).

(3)Churches.

(4)Schools.

(5)Cemeteries.

(6)Landfills and hazardous waste sites.

(7)Parks.

(f)Study conclusions and recommendations. Study should clearly identify the results and conclusions of the analysis and provide recommendations of any required action so that no adverse impact is experienced by surrounding properties.

(g)Design and construction criteria.

(1)Design shall be in accordance with LA DOTD hydraulics manual. The runoff used shall be computed using the soil conservation service (S.C.S.) or rational method (Q=ACI) as described in the manual for all drainage areas (off-site and on-site) and any drainage channels affected by the development.

(2)Complete hydraulic calculations shall be prepared and sealed by a professional civil engineer and submitted along with the construction plans.

(3)Open canals shall have side slopes of three (3) to one (1) if not lined with concrete. Slope grades of one and one-half (1 1/2) to one (1) may be used if concrete lining is utilized.(h)Erosive soils. Many subdivisions are developed in areas of the Parish where erosive soils exists. All ditch side slopes shall be stabilized by fertilizing and seeding and erosion hay blankets installed per manufacturer and as approved by review engineer and Planning Director.

(1)The following servitude criteria shall be required for each ditch, canal, and storm sewer; however where applicable, local drainage districts reserve the right to review and request modifications as necessary to facilitate future maintenance of proposed ditches, canals and storm sewer systems, in addition, (with the approval from local drainage districts if applicable) the review engineer may allow variations based on sound engineering practices:

a. Storm sewers and swale ditches: fifteen (15') foot minimum servitude.

b. Ditches with a top width up to fifteen (15') feet: Width of ditch plus a minimum of fifteen (15') feet from the top of bank on one (1) side.

c. Canals with top widths greater than fifteen (15') feet: Width of canal plus a minimum of fifteen (15') feet on each side.

d. Canals with bottom widths greater than fifteen (15') feet and a top width of less than forty (40') feet: Width of canal plus a minimum of fifteen (15') feet from the top of bank on one (1) side and twenty-five (25') feet on the other.

e. Canals with a top width greater than forty (40') feet: twenty-five (25') feet from the top of bank on both sides. When a proposed ditch must discharge into a major unlined canal, the O/D/S shall be required to enclose the ditch, under the necessary strip of the major canal in an adequate size bituminous-coated metal pipe. The pipe shall be an appropriate length to provide a fifteen (15') foot-wide level surface to traverse ditch and extend four (4') feet into the canal beyond the side slope, and shall discharge into rip rap that extends a minimum of five (5') feet into the bottom of the canal. Rip rap shall be constructed immediately after conduit is installed.

f. All drainage servitudes shall be labeled as drainage servitudes and shall be restricted to drainage uses only. No other structures shall be allowed within the drainage servitude (i.e., telephone junction boxes, cable junction boxes, power poles and/or junction boxes, owners minutes structures). The purpose of this is to ensure proper access for maintenance of the servitude by the drainage district.

g. No utilities shall place their services within the drainage servitude (i.e., above ground or buried cables, pipes, valves etc.)

h. Where a servitude lies between any two (2) lots or parcels of ground, a fifteen (15) minutes forty-five (45) degree chamfer will be placed on both sides of the servitude at its intersection with the back-of-lot servitude. This allows ease of access for drainage district equipment to turn the corner without going outside the servitude.

(i)Subdivision drainage shall be designed in accordance with one (1) of the following three (3) options:

(1)Open ditch subdivision. A subdivision that will be designed and built with open ditches. Installation of any subsurface drainage (other than a driveway culvert) is prohibited in any subdivision designed for open ditches. Driveway culvert pipe shall be designed and shown on the drainage layout map. Enclosure of open ditches in previously developed subdivisions shall follow the ordinance in place at the time of approval unless altered by ordinance. All sellers of any lot/parcel within an open ditch subdivision shall make the buyer beware that any subsurface drainage will not be allowed to be added (other than one (1) driveway culvert per lot or parcel). The following statement must be placed on the bill of sale: "BUYER BEWARE: Installation of any subsurface drainage (other than a driveway culvert) is prohibited in this subdivision designed for open ditches."

(2)Open ditch subdivision with design for subsurface. A subdivision designed for subsurface drainage and built as an open ditch subdivision. Should the subdivision be initially built as an open ditch subdivision, then any future installation of subsurface drainage shall be in accordance to the drainage plans provided in the construction plans.

(3)Subsurface drainage subdivision. A subdivision that will be designed and built for subsurface drainage.

(4)Outlet ditches. Outlet ditches (minor and major) located between lots shall be piped their entire length.

All design criteria of drainage, whether open or closed system, shall meet sound engineering practices and principles. The review engineer will have the option to ask for any outfall ditch (the term "outfall ditch" means a ditch that connects to the roadside ditch and outfalls at another location) to be enclosed for the entire length of the outfall.

(j)Detention basin. Whenever a detention basin is utilized by the O/D/S's engineer to minimize downstream flooding, the design shall address, at a minimum, the following:

(1)Detention basin shall be designed to detain flows so as not to increase in downstream runoff more than ten (10%) percent for a ten (10) year pre-development storm.

(2)Detention basin shall be checked for the twenty-five (25) and one hundred (100) year frequency to ensure that adequate capacity is provided in the basin and at the outlet to prevent flooding of upstream and downstream developments.

(3)Adequate land must be reserved for maintenance of detention pond.

(4)Detention basins may be wet (lakes or ponds) or dry.a.Wet detention basins shorelines and control structures shall be privately owned and maintained. The basin must have minimum side slopes of three to one (3 to 1) and must have a minimum fifteen (15') feet wide access along the entire perimeter for maintenance. Both the construction plans and final plat for development shall include a note which states that the proposed detention basin, shoreline and control structure shall be privately owned and maintained. Storm drainage pipe inverts must be designed to be above the normal water surface elevation of the basin, unless the review engineer approves variations.

b.Dry detention basins shall be privately owned and maintained as part of the development drainage system. The basin must have minimum side slopes of three to one (3 to 1) and must have a minimum fifteen (15') feet-wide access along the entire perimeter for maintenance. Storm drainage pipe inverts must be designed to be above the normal water surface elevation of the basin, unless the review engineer approves variations. The basin bottom shall be designed and compacted to allow for proper maintenance with mowing machines and other equipment

.(5)The O/D/S may propose off-site improvements to downstream facilities to minimize the impact of the development, subject to approval of the review engineer.

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