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Floodplain Conveyance Zones and the Offsite Drainage Assessment Program

A Comprehensive Drainage Impact Analysis of Proposed Land Development Utilizing High-Resolution, 2D Hydraulic Models

By T. Stokka Brown Jr., MS, PE, CFM





 

Introduction

Over the past six decades, the Greater Baton Rouge area has been struck by two significant floods in 1983 and 2016, alongside numerous minor ones. These disasters have spurred public reactions of distress and agony, which later morphed into anger and worry. These sentiments stem from the perception that new land developments (herein referred to as “developments”) contribute to an increased flood risk. Although municipalities have established development codes and ordinances to regulate this risk, public confidence in the impact analyses carried out by developers' engineers — stating that each individual development will not increase the flood risk — has significantly diminished. However, the advent of comprehensive, watershed-wide two-dimensional (2D) hydraulic modeling offers municipalities a more effective way to demonstrate to the public that proposed developments will not increase flood risk across the watershed. This evaluation procedure is known as the Offsite Drainage Assessment (ODA).


The ODA serves as the mechanism through which the impacts of proposed developments on drainage are evaluated, using the municipality’s 2D hydraulic model of the existing system known as the Effective Hydraulic Model (EHM). This process is triggered when a proposed development falls within a Floodplain Conveyance Zone (FCZ). To ascertain the potential impact of the development, a range of simulated rainfall events — both with and without the proposed development — are used to analyze its influence on water surface elevations and flow across the watershed. In cases where potential impacts are detected, the developer is required to modify the proposed design. The design is revised and evaluated until it shows no adverse effects before construction approval is granted. The aim of ODA is not to stifle development but to ensure a thorough and rigorous analysis is conducted to mitigate any negative impacts, fostering the growth of flood-resilient communities.


Furthermore, as developments receive approval and proceed to construction, they are incorporated into the 2D model as part of the new EHM. This inclusion effectively creates a dynamic or 'living' model. This system has the capacity to assess both the impacts from individual developments and the cumulative effects of all developments since the initial EHM was created. This process enables municipalities to manage their floodplains more effectively and to progressively minimize the increase in flood risk over time.


Development Codes & Ordinances

Municipalities have traditionally implemented development codes and ordinances to prevent or at least mitigate the potential flood risk posed by new developments. These codes and ordinances are primarily housed within the Floodplain Management and Stormwater Management Plan sections. They incorporate elements like a Drainage Impact Study (DIS) and Floodplain Fill Mitigation. A DIS typically necessitates the development to detain, and in certain cases, over-detain a 10%, 4%, or occasionally, a 1% annual exceedance probability (AEP) rain event. These AEP events are commonly referred to as the 10-year, 25-year, and 100-year rain events, respectively. This control measure is put in place to manage additional rainfall runoff caused by an increase in the impervious surface area. It also necessitates no increase in peak water surface elevations (or "no-impact") for streams passing through the development site. The Floodplain Fill Mitigation requires that any fill in the floodplain is counterbalanced with an equivalent volume of cut to maintain a steady storage capacity for the rainfall runoff volume. Although debates persist regarding the return period design storm an internal drainage system and detention ponds should be designed for, our primary focus here is on the no-impact assessment.


Both municipalities and the engineering community have adhered to industry standards in conducting no impact analyses. However, advances in technology now provide more accessible tools for a more precise simulation of existing hydraulic conditions and the potential impacts of developments. This is especially relevant to the irregular, shallow sloping watersheds typical to the majority of Louisiana. In these unique watersheds, water can flow in various directions contingent on the intensity of the rain event. It is vital to capture these multi-directional flow paths to accurately simulate the existing system. Without a precise understanding or simulation of the present system, it becomes virtually impossible to assess the potential impact of a proposed development. In this context, two-dimensional (2D) and combined one- and two-dimensional (1D/2D) hydraulic models emerge as the most suitable for simulating these complex systems. They are therefore the best tools for performing no impact analyses for proposed development.


In a significant development in February 2016, the US Army Corps of Engineers' (USACE) Hydrologic Engineering Center (HEC), free to the public, released version 5.0 of their River Analysis System (RAS). This version included the capacity to perform 2D and combined 1D/2D unsteady flow modeling.* HEC has since launched additional versions of RAS, with the latest being 6.4.1, each of which further enhanced the program's capabilities.


Stormwater Management Plan Updates Post August 2016

In the aftermath of the devastating flood in August 2016, several affected municipalities embarked on the process of revising their Drainage or Stormwater Master Plans. These updated plans involved a comprehensive evaluation of their existing drainage systems, identification of deficiencies, and development of mitigation strategies and projects to bolster these systems. Hydraulic models have always been a crucial component of this analysis, given their capacity to simulate the response of both existing and improved systems under various conditions, such as design and historic storm events.


With the advent of the 2D capabilities of HEC-RAS, drainage systems are now being developed using 2D hydraulic models. These sophisticated models enable us to capture benefits in the form of lower water surface elevations and shorter inundation periods than the existing to improved models. Additionally, these hydraulic benefits can be converted into monetary benefits using software like the Hydrologic Engineering Center’s (HEC) Flood Impact Assessment (FIA) or similar tools. These tools utilize water surface elevations, asset elevations, and depth damage curves to quantify benefits.


Offsite Drainage Assessment

With a comprehensive 2D hydraulic model of their surface drainage system in hand, municipalities are now equipped with a tool that serves a multitude of purposes. While these models are invaluable for evaluating the benefits of proposed improvements, they also offer a powerful means of assessing the potential impacts of proposed development. They can be employed for the Offsite Drainage Assessment (ODA) and the Floodplain Conveyance Zone (FCZ) with little to no additional costs. This multipurpose application of the models not only enhances the understanding of flood risks but also ensures cost-effectiveness in managing and mitigating these risks.


It is worth noting that these 2D hydraulic models are primarily designed to evaluate the capacity of a proposed development to convey water through the site from offsite sources, rather than assessing how effectively a proposed development manages water falling directly on the site, such as through surface and subsurface drainage routing and detention or retention areas. This distinct focus is what gives the process its name: Offsite Drainage Assessment or ODA. However, the evaluation of the onsite drainage system for adherence to local codes and ordinances remains an essential step, conducted in tandem with the ODA. The ODA furnishes a more extensive analysis to evaluate potential flood risk impacts from proposed developments to neighboring properties and the entire watershed.


Pioneering the creation of the FCZ and ODA, CSRS, LLC worked with the City of Central to first adopt them in April 2021 following the completion of their Drainage Master Plan. East Baton Rouge (EBR) Parish followed suit adopting the FCZ and ODA in April 2023 as part of their Stormwater Master Plan. For the EBR Stormwater Master Plan, CSRS, LLC lead the development of policy change (codes/ordinances) recommendations including the FCZ and ODA as a subconsultant to HNTB, Inc. Both the City of Central and EBR Parish now mandate certain developments within a Floodplain Conveyance Zone (FCZ) to undergo the Offsite Drainage Assessment (ODA), overseen by the City or Parish, respectively.


Floodplain Conveyance Zones

The FEMA Flood Insurance Rate Maps (FIRMs) currently serve as the regulatory tool for dictating where fill can be placed within the regulatory floodway. To ensure that the fill does not lead to an increase or "no-rise" in the 1% annual exceedance probability (AEP) also known as the 100-year flood elevations, a comprehensive hydraulic analysis is necessitated. Nevertheless, in regions like East Baton Rouge Parish and the City of Central, many channels and streams lack regulatory floodways as defined by FEMA. To fill this gap, we introduced a mapping tool known as the Floodplain Conveyance Zone (FCZ).


The FCZ expands upon the identification of areas critical for stormwater conveyance and the ODA helps to safeguard their capacity. The FCZs accurately pinpoint virtually all areas essential for stormwater conveyance across the modeled area. The ultimate aim of the FCZ is to delineate the sections of a stream or other watercourse and the surrounding land areas that are vital for stormwater conveyance. Figure 1 shows the final FCZs in an area of East Baton Rouge Parish that were developed as part of the EBR Stormwater Master Plan Policy Change Recommendations.**


Figure 1: EBR FCZ Online Map

(https://experience.arcgis.com/experience/fde8aa3162ea4518b429a7d183f72d46)


Methodology

These conveyance areas are quantified through a combination of depth and velocity, where depth is the height of water above the ground in a specific area, and velocity is the rate at which it flows across the surface. The combined product of depth and velocity, known as the "depth-velocity product," forms a unit flow rate that helps quantify conveyance. This approach builds upon existing studies*** that advocate for developing floodway-like zones using combined depth and velocity results.


A sensitivity analysis was conducted to explore how different combinations of depth and velocity affect the dimensions of the FCZ. This analysis was carried out for a predominantly rural and urban watershed separately, with the objective of identifying a combination that could be universally applied to both, thus ensuring consistency across the Parish.


The process used to evaluate the "effectiveness" of the depth and velocity parameter combinations is depicted in Figure 2. Initially, a set of parameters was selected to define an FCZ boundary. Following this, a model geometry was crafted to represent a complete build-out of the watershed. In this model, areas outside the FCZ were raised to one foot above the peak water surface elevation for a 1% AEP rain event, as illustrated in Figure 3. Finally, a simulation of a 1% AEP rain event was run using this new "build-out" model. The peak water surface elevations resulting from this model were then compared to those in the existing conditions model. This comparison offered insights into the "effectiveness" of the parameter combination and guided the selection of combinations for further testing.


Figure 2: Sensitivity Analysis Process


Figure 3: Build Out Scenario Development Process



A variety of depth-velocity product values from the 1% AEP event were evaluated, but they generated relatively narrow zones that were significantly sensitive to localized model instability. As a result, depth-based and velocity-based zones were created independently and combined into a final zone based on their overlapping areas. An additional zone was defined for regions where flood depth was exceptionally large ("large depth-based zones"), irrespective of velocity. A unique consideration was made for the Bayou Fountain-Bayou Manchac area due to its exceptionally flat topography, where a slightly lower velocity threshold was applied. The tolerances used to generate the final FCZ for EBR Parish are outlined in Table 1. For the City of Central, a simple depth-based approach with a tolerance of 0.5 feet from the 1% AEP event was used.

Component Zone

Model Output Tolerance Used

Depth-based

​0.5 feet

Velocity-based

​0.5 feet per second (0.25 for Bayou Fountain-Bayou Manchac)

Large Depth-based

4 feet

Final Conveyance Zone

Incorporates all 3 above

Table 1: EBR Parish 1% AEP Model Output Tolerances for Floodplain Conveyance Zones



The 2D hydraulic model generated the output used to establish the “raw” FCZ. The "raw" FCZ output was refined through a combination of automated geoprocessing tools used to clip, fill, and smooth the areas. Features that didn't represent conveyance, such as detention ponds and roadways, were removed along with minor offshoots of the main conveyance features.


For EBR Parish, FCZ on undeveloped land was classified as Type 1 conveyance zones, and FCZ across existing subdivisions was classified as Type 2 conveyance zones. In some upland areas in the northern-most part of the Parish, the model was unable to produce reliable velocity results due to the lower mesh resolution representing the conveyance features. When reliable velocity data were not available, the depth-based boundary was used to define an approximate zone. This FCZ was categorized as Type 3 conveyance zones. The City of Central FCZ was not divided into types.


Figure 4: Examples of Edits Made to EBR Parish FCZ


Uses and Limitations

The FCZ is designed to serve as a tool for both municipalities and developers. It is recommended that the municipality makes the FCZ map accessible through an online service, enabling developers to verify whether a prospective property is located within an FCZ. Both the City of Central and EBR Parish publish their FCZ on their respective online GIS sites. As enhancements are incorporated into the models over time, it is crucial to ensure corresponding updates to the FCZ.


The FCZ map is not a substitute for hydraulic modeling and does not represent all stormwater conveyance areas. It underscores those areas of conveyance as determined by the level of detail employed by the models used for their creation. Therefore, this level of detail should be taken into consideration when utilizing the map product.


Example Offsite Drainage Assessment

Let's take a closer look at an illustrative example of an Offsite Drainage Assessment, specifically situated in the City of Central. The location of a proposed development is illustrated in Figure 5, demarcated in black with a hatched interior, with the Floodplain Conveyance Zone (FCZ) highlighted in blue. This site is hydraulically complex, with three points of inflow and three points of outflow as indicated by the arrows. In the absence of a 2D model, this area has traditionally been simplified using a single channel running down the main stream with cross-sections spanning the entire floodplain. However, this one-dimensional (1D) setup failed to capture the flow of water across the site from west to east. We will delve deeper into this topic shortly.


Figure 5: Proposed Development with Floodplain Conveyance Zones


The existing and proposed digital terrain models (DTMs), which play a crucial role in current and proposed model simulations, are showcased in Figure 6. The elevation scale ranges from white and grey for higher points, through red, orange, and yellow, down to green and blue for lower areas. The proposed DTM is constructed based on information supplied by the developer. The following details are required as part of the permit application process:

  • Drainage Impact Study

  • Description of the proposed development including:

    • Drawing showing horizontal and vertical extents of the fill area, ponds, roadways, drainage ways, and drainage reroutes, and

    • Size, type, length, and invert elevations of drainage structures.


Figure 6: Existing and Proposed Digital Terrain Models


A crucial first step involves updating the Effective Hydraulic Model (EHM) to incorporate key drainage features, ensuring that it accurately reflects the current hydraulic conditions in the area. This results in the creation of the Pre-Development Hydraulic Model, or Pre-Model. Subsequently, this Pre-Model is updated using the information provided by the developer to represent the proposed development, forming the Post-Development Hydraulic Model (Post-Model).


Both Pre- and Post-Models are run for the 10%, 4%, and 1% 24-hour duration annual exceedance probability (AEP) rain events. Figure 7 provides a glimpse of the resulting peak water depths and flow paths during the 1% AEP rain event as predicted by both the Pre- and Post-Models. To create an Impact Assessment Map, the peak water surface elevations derived from the Pre-Model are subtracted from the corresponding elevations in the Post-Model for each AEP rain event, thereby mapping out the differences.


Figure 7: Existing and Proposed Conditions 1% AEP Event Resultant Peak Water Depth and Flow Paths


Figure 8 illustrates the Impact Assessment Maps from the first and second iterations of the proposed development. Any values greater than zero, represented in red, indicate an increase in peak water surface elevations from the Pre-Model to the Post-Model. Conversely, values less than zero, shown in green, represent a decrease in peak water surface elevations from the Pre-Model to the Post-Model.


Ultimately, a detailed memorandum is produced. This essential document encompasses all pertinent information, including data supplied by the developer, the methodology applied in developing both the Pre-Model and Post-Model, and the insights gleaned from the ensuing analysis.


Figure 8: First and Second Impact Assessments during 1% AEP Event


The first round of analysis revealed that the proposed development was impeding flow across the site, an issue previously mentioned. If it were not for the 2D models and the Offsite Drainage Assessment (ODA), this initial design could have easily sailed through conventional regulations. The likely outcome would have been escalated flood risk in areas north and west of the development. Fortunately, with the ODA in place, the results of the preliminary assessment were communicated to both the City and the developer. This allowed for the models' insights to be leveraged in designing the development more effectively, considering the need for adequate flow across the site.


The second analysis iteration allowed more flow across the site, which subsequently increased the flood risk to the east. After several more iterations, it was determined that providing the developer with a copy of the Effective Hydraulic Model would be the most efficient means to assess variations in the design. As of the drafting of this article, the developer continues to work on the first version of their design, now equipped with the Effective Hydraulic Model.


Conclusion

The graphic results demonstrated here—particularly striking in full color—provide local authorities and the public with a clearer perspective of the possible impacts from proposed developments, subsequently fostering a stronger sense of trust. This case underscores the necessity for a more thorough evaluation of proposed development using the most sophisticated technology at hand. By introducing the Offsite Drainage Assessment (ODA) today, municipalities can facilitate more rigorous assessments of future development, thereby fostering communities that are safer from flooding.



References:

* https://www.hec.usace.army.mil/software/hec-ras/documentation/HEC-RAS%205.0.1%20Release%20Notes.pdf

** https://stormwater.brla.gov/main-report/

*** https://floodsciencecenter.org/event/ctp-webinar-using-a-depth-x-velocity-product-in-conjunction-with-floodways/

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