Drainage Improvement Plan

A floodplain is an area of land adjacent to a stream or river which stretches from the banks of the channel outward to higher elevations. &nbsp;The flat topography and complex channel network of Scott causes floodplains to stretch far from channels and cover large swaths of the City. The Federal Emergency Management Administration (FEMA) defines flood zones based on the 100-year floodplain.

The City of Scott is located within the upstream portion of two major drainage basins, Coulee Ile des Cannes and Coulee Mine. These two basins cover a large portion of central and western Lafayette Parish and both ultimately discharge into the Vermilion River. The Coulee Ile des Cannes watershed is the larger of the two covering approximately 53 miles (33,550 acres) with the Coulee Mine watershed covering approximately 16 miles (10,344 acres).

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<li>Zone A – BFE has not been determined; hydraulic analyses have not been performed<li>Zone AE – BFE has been determined</li><li>Zone AH – Flood depths of 1 to 3 feet (usually areas of ponding); BFE has been determined.</li></li>

<li>Zone X500 or Zone X (shaded) – Areas of 0.2% annual chance flood hazard<li>Floodway – The channel of a watercourse plus any adjacent floodplain areas that must be kept free of encroachment so that the 1% Chance Event can be carried without substantial increases in flood heights (currently defined as 1 foot, but subject to regulatory changes); located within Zone AE</li></li>

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The current effective Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRM) place approximately 65% of the City in a special flood hazard area (SFHA) as defined below. This is a reduction from 73% from the previous maps dated January 19, 1996 and January 20, 1999.

The City of Scott is generally flat with the channels running from north to south. The peak elevation seen is 45 feet and the minimum elevation is 22 feet, this makes for an average slope of less than 1%. As a result, stormwater often ponds where it falls and drains as shallow and typically slow-moving runoff across large areas known as sheet flow runoff. This leads to localized flooding conditions and occasionally interbasin exchange when stormwater flows between drainage basins.

An extensive and quantitative evaluation of the current state of the existing drainage system within the City of Scott was completed by means of a Watershed Report Card. This report card is based on the American Society of Civil Engineers (ASCE) Infrastructure Report Card and consists of an assessment of both the condition and hydraulic capacity for drainage features within the city limits. The condition of drainage features was quantified through a series of field visits and inspections to establish a detailed inventory with numerical rating, while the capacity of drainage structures was evaluated using existing hydraulic and hydrologic models only. All channels in the City of Scott do not have existing models at this time, but it has been recommended that the City moves forward with that task. The capacity of both the channels and structures were evaluated based on a 2 and 10 year storm event. The Drainage Improvement Plan has laid out a prioritization plan that includes channel improvements and crossing replacements to improve these grades in the future.

<li> Capacity: evaluate the existing structure's ability to convey the design storm under current and future conditions based on LADOTD standards and parish ordinances.<li>Condition: assess the existing physical condition of the structure as it related to conveying flow. Structural integrity was not assessed.</li></li>

<a href="http://www.lafayettela.gov/Pages/Index.aspx" target="_blank" rel="noopener noreferrer">Lafayette Consolidated Government</a> – Many of the channels running through Scott, including both Coulee Ile des Cannes and Coulee Mine extend beyond the City’s limits necessitating the collaboration of local governments to execute projects on drainage features that span political boundaries.

Hydraulic Structures

Channel_Network_Scott

Scott City Limits

Proposed Channels

Channel Improvements

Channel Network 

Proposed Features

Proposed Feature

Ile des Cannes Basin

Coulee Mine Basin

Channel Network 2 Year Grading

Hydraulic Structures 10 Year Grade

Channel Network

Louisiana Department of Transportation and Development – the <a href="http://wwwsp.dotd.la.gov/Pages/default.aspx" target="_blank" rel="noopener noreferrer">LADOTD </a>owns several significant roads and the underlying drainage features that run through the City of Scott requiring coordination to ensure those features are properly maintained.

<li>Program: Interest in installation of a flood gauge system.</li>

Hydraulic Structures (Bridges &amp; Culverts)

Beyond identification of the channel network and inventory of hydraulic structures, it is equally important to take stock of the existing ordinances and policies relating to drainage and how they are applied. A comprehensive understanding of the City’s ordinances allows Scott to anticipate areas of regulatory shortcomings as they prepare for continued growth and development. This understanding also enables Scott to target ordinances which can qualify it for improved standing in federal programs such as the Federal Emergency Management Agency’s (FEMA) Community Rating System (CRS). Below are existing and possible future ordinances and policies. 

<li>Policy: Proposing "zero net fill" for all areas within the City Limits.</li>

A drainage system can be designed and built to the highest specifications, but without a robust maintenance and inspection program it will quickly fall into disrepair and face a reduced ability to function as designed. A well-defined operating procedure that includes distinct responsibilities, specific tasks and standards, and a system of documentation is essential for maintenance of large drainage system. A Standard Operating Procedure (SOP), a formal maintenance plan, has the added benefit of increasing the City’s Community Rating System (CRS) score which can result in reduced flood insurance premiums in the City. The Drainage System Maintenance SOP serves five primary functions:

Approximately 50 people attended and the outcome of voting for Policy, Program and Projects is as follows:

In addition to topography, runoff is also a function of soil type and land cover within the drainage area. Not all precipitation that falls onto a watershed exits through the primary outlet. Some is absorbed by the soil which reduces the rate of stormwater runoff. Soils are classified into hydrologic soil groups (HSG) based on the rate of infiltration and the soil’s corresponding runoff potential. The four hydrologic soil groups are A, B, C, and D. Of these groups, A possesses the highest infiltration rates and smallest runoff potential and D the lowest infiltration rates and greatest runoff potential. According to the Natural Resources Conservation Services (NRCS) Soil Survey Geographic (SSURGO) data the majority of the soil located within the City of Scott falls into Group C or D with a small percentage classified as Group B. Soils classified as Group C are sandy clay loam and those classified as Group D clay loam, silty clay loam, sandy clay, silty clay or clay. Both groups demonstrate low to very low infiltration rates with high potential for runoff.

The City held a town hall meeting for public input on September 12, 2019. The meeting consisted of several stations listed below.

<li>55 Bridges<li>90 Culverts</li><li>32 Miles of Channel</li><li>105 Miles of Roadside Ditches</li><li>25 Miles of Subsurface Drainage</li></li>

<a href="https://www.cityofscott.org/" target="_blank" rel="noopener noreferrer">City of Scott Public Works</a> – The City’s Public Works Department under the purview of the newly created position of Drainage Superintendent will be tasked with the day-to-day implementation of the maintenance and inspection procedures.

<ul><li>After defining the system, the next step is to collect the inventory of the drainage infrastructure within the City.<li>The inventory was evaluated utilizing this Evaluation Data Sheet.</li><li>The drainage system inventory includes a total of 145 hydraulic structures in or adjacent to the City limits.</li><li>They range from culverts 36 inches in diameter to bridges over 100 feet in span.</li></ul></li>

<li>Zone X or Zone X (unshaded) – Areas determined to be outside of the 0.2% annual chance flood hazard</li>

The following agencies were identified as key stakeholders for the Drainage Improvement Plan:

Homes and structures located in Zone X are subject to a reduced flood risk but not zero risk.

<ul><li>Condition: assess existing condition of commercial and residential ponds including vegetation, silt deposits, outfall condition, freeboard, and debris/trash.</ul></li>

Hydraulic Structures 2 Year Grade

<li>Projects: Construction of new detention ponds and improvement of existing ponds.</li>

Story Map Shortlist

Land use and land cover also play a role in infiltration rates. As land is cultivated and/or developed the natural infiltration rates are reduced. Land use and land cover data (NOAA 2010) shows that land located within the City of Scott is predominantly variations of cultivated, developed, and pasture grassland. The change in land use values between previous NOAA datasets (2006 and 2010) indicates a trend towards increased development with a reduction in cultivated and undeveloped land. There are no current indications that trend will slow down. The development that is occurring within the City of Scott is resulting in areas that were once exposed soil becoming pavement and rooftops, consequentially producing a reduction in stormwater infiltration and an increase in stormwater runoff.

Both the Coulee Ile des Cannes and Coulee Mine channel networks discharge into the Vermilion River south of the City’s limits. The Vermilion River is a tidal river meaning its flowrate and water surface level are influenced by tides. Its complex geography and geometry can also cause the river to reverse course during extreme rain events. The combination of these factors results in both Coulee Ile des Cannes and Coulee Mine experiencing extended periods of elevated tailwater conditions. The tailwater-dependent nature of these watercourses is ultimately beyond the control of the City of Scott and requiring solutions that go beyond the maintenance and improvement of the existing drainage system. In this effort, the Drainage Improvement Plan includes a series of possible capital improvement projects including both regional detention and other stormwater diversion projects.

The Drainage Improvement Plan is more than just a set of standards and practices for the City to follow. The Drainage Improvement Plan at its core is an agreement of the community to hold itself responsible for combating flooding and drainage problems in all its forms. To function at its peak potential there must support from all stakeholders. Because of this, it is crucial that both the public and local government each play a role in the Plan’s development and implementation.

Perhaps the most significant stakeholder group is the citizens of the City of Scott. The buy-in of the citizens will fuel the drive of local officials to continue the work of the Drainage Improvement Plan. Their knowledge and experience dealing with drainage issues in their community provides irreplaceable guidance in the shaping of the plan.

Detention basins or ponds are excavated areas that are designed to fill with stormwater during or immediately following rain events. The outfall from the pond acts to limit the flow of water, enabling the pond to fill and detain the stormwater as it slowly discharges into a channel. This act of slowing down of stormwater reduces the likelihood that channels within the drainage system become filled beyond capacity and overtop their banks. Large detention basins can be constructed as multi-purpose facilities that include recreational features such as parks or walking trails. This allows the land to serve a function even when not in use for stormwater. Detention basins can also be designed to permanently hold water, these are known as wet ponds. These basins function in the same manner but are able to serve as water features, be stocked for fishing, and act as a habitat for birds and wildlife.

Channel Network 10 Year Grading

<li>Official Drainage Map that includes the channels within the City Limits and specifies the classification of each channel as either major, intermediate, or minor in which impacts both the inspection and maintenance activities. Hydraulic structures are not included on the official drainage map.<li>Primary responsibility of the inspection and maintenance program is placed on a newly created position of Drainage Superintendent.</li><li>Timeline and specific manners in which the drainage system is to be inspected and maintained.</li></li>

As development continues within the City of Scott and additional land is converted from undeveloped to developed, the soil can absorb less rainwater resulting in an increase in runoff. Detention basins are one option to mitigate this increase. Individual developments often include detention basins to collect stormwater from only within the development. In some cases, a more efficient use of land it to construct larger ponds known as regional detention ponds to store stormwater from a greater swath of area. This streamlines the inspection and maintenance of detention facilities within the City. The Drainage Improvement Plan has identified possible locations for large-scale regional detention ponds as shown in the map below. No feasibility studies were conducted as part of this plan.

Included below is a brief summary of what the SOP enforces.

Drainage System Maintenance &amp; Inspection

<li>Capacity: evaluate the existing channel network's ability to support current demands as well as future demands considering population growth and intensified rainfall events.<li>Condition: assess the existing physical condition of the channel network including vegetation, erosion, silt deposits, and debris/trash.</li></li>

Special Flood Hazard Areas (SFHAs) are areas identified by FEMA as subject to inundation by the 1% Chance Event also known as the base flood and sometimes referred to as the “100-year flood.” The Base Flood Elevation (BFE) is the maximum water surface level expected to occur during a 1% Chance Event. Areas with elevations below the BFE are designated as SFHAs by FEMA. The three SFHA designations that exist within the Scott city limits are:

<li>32 miles of channels<li>15,000 acres draining</li><li>Discharges into the Vermilion River</li><li>Composed of the Coulee Ile Des Cannes and Coulee Mine networks</li><li>Coulee Ile Des Cannes and tributaries consisting of 25 miles</li><li>Coulee Mine and tributaries consisting of 8 miles</li></li>

<li><a href="https://www.fema.gov/national-flood-insurance-program" rel="noopener noreferrer" target="_blank">National Flood Insurance Program (NFIP)</a> – federally administered flood insurance program<li><a href="https://www.fema.gov/national-flood-insurance-program-community-rating-system" rel="noopener noreferrer" target="_blank">Community Rating System (CRS)</a> – national program created by FEMA that credits communities for going above and beyond minimum requirements for participation in NFIP</li><li><a href="https://www.fema.gov/base-flood-elevation" rel="noopener noreferrer" target="_blank">Base Flood Elevation (BFE)</a> – elevation of surface water resulting from a base flood; also known as a 100-year flood water elevation</li><li><a href="https://www.fema.gov/flood-insurance-rate-map-firm" rel="noopener noreferrer" target="_blank">Flood Insurance Rate Map (FIRM)</a> – official map of a community on which FEMA has delineated the Special Flood Hazard Areas (SFHAs), the Base Flood Elevations (BFEs) and the risk premium zones applicable to the community</li></li>

<li><a href="https://library.municode.com/la/scott/codes/code_of_ordinances?nodeId=MUCO_CH18FLPR_ARTIINGE_S18-2DE" rel="noopener noreferrer" target="_blank">Freeboard</a> - all new or substantially improved structures within all designated Special Flood Hazard Areas (SFHA) to be constructed with the minimum lowest floor elevation one foot above the Base Flood Elevation (BFE)<li>Improper Drainage Structures – the City is authorized to replace or improve existing improper&nbsp;drainage&nbsp;structures at the cost of the property owner</li><li><a href="https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html" rel="noopener noreferrer" target="_blank">Rainfall Data</a> – development drainage studies are required to use the most recent version of the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 rainfall data</li><li><a href="https://library.municode.com/la/scott/codes/code_of_ordinances?nodeId=MUCO_CH18FLPR_ARTIVDEDRDE_S18-101FLDRPRST" rel="noopener noreferrer" target="_blank">Zero Net Fil</a>l - developments that fill or modify a designated SFHA must mitigate that development activity volumetrically</li><li>Post Development Runoff – residential and commercial developments larger than 2.5 acres reduce stormwater runoff by 15 percent from the predevelopment runoff rate</li><li><a href="https://www.fema.gov/floodway" rel="noopener noreferrer" target="_blank">Floodway</a> – development, fill, or an obstruction within a floodway requires an approved no-rise certification demonstrating they will cause no adverse flood effects</li></li>

<li>Requirements for reporting and documentation are set. All inspectors will complete an inspection report with the channel or structure inspected and details regarding the nature of the inspection and any issues discovered and if follow up maintenance activities are recommended. Includes a compliant / inquiry form available online for the general public to report concerns.&nbsp;</li>