FEMA Floodplain Revision Robbinsdale Park Area Reconstruction City Project No. 15-2253 / CIP No. 50421.3-1B FEMA Case No. 19-08-0857P ROBBINSDALE PARK Background Who is FEMA? Federal Emergency Management Agency (FEMA) Establishes floodplain maps Determines regulations and flood insurance policies What is a Floodplain? The FEMA regulated floodplain is defined as the water surface boundary that is created during the 100-year storm event. What is the 100-year storm event? A storm intensity large enough to occur 1 time in 100 years on average. This 100-year storm may also be called the 1% storm (1/100-years = 1%). This means in any given year we have a 1% chance of witnessing this intensity of storm. This does not mean once this 1% storm occurs it will not be seen for another 100 years. FEMA also defines a water surface boundary for the 500-year event too. What is the 500-year storm event? Just like the 100-year storm, it is a storm intensity large enough to occur 1 time in 500 years on average. The 500-year storm event may also be called the 0.2% storm (1/500-years = 0.2%). The higher the probability of a storm occurring equals a lower amount of rainfall witnessed. 100-yr (1%) has less rainfall then the 500-yr (0.2%) What Type of Floodplains Exist? FEMA has many types of floodplain designations but ultimately, they can be broken down into two (2) main types. 1.) Studied – survey and engineering data determines floodplain boundary in-depth analysis completed The amount of water flowing down the drainage as well as the anticipated height of the water is calculated. 2.) Unstudied – a “best guess” of floodplain boundary is determined no in-depth analysis completed No amount of flow or anticipated water heights have been calculated. The intention of the Robbinsdale Park reconstruction project was to study the unstudied portion of floodplain running generally from the Fairmont Blvd. and E. Anaconda Rd. intersection to the intersection of Hawthorne Ave. and E. Meade St. This portion of floodplain to be studied is part of a drainage area referred to as the Hawthorne Drainage Basin Pre-Project FEMA Floodplain 100-yr Studied Meade. St. Hawthorne Ave. Hawthorne 100-yr Unstudied Grandview Dr. Grandview 500-yr Unstudied What is a Drainage Basin? Stormwater follows gravity (from higher elevations to lower elevations) A drainage basin is the total contributing area that flows to a single point (everything uphill) Think of a drainage basin as a very large pool with a deep end and a shallow end. If you were to fill the pool with water, the water would find its way downhill and ultimately end at the deep end. Thankfully, our drainage basin “pool” has outlets so water doesn’t build up and flood everyone in the basin. Typically, the larger the drainage basin size, the larger the stormwater flows seen at the lowest point. 1.2 mi2 How Big is the Hawthorne Drainage Basin? 774 acres Saint Patrick St. Meade St. Monument Health Hospital Ave. Hawthorne Highway 16 Highway Skyline/Tower Road Intersection How Much Water is There? About 1.0 mi2 drains into Robbinsdale Park from E. Fairlane Dr. That equals About 777 cubic feet per second (cfs) of stormwater during the 100-year storm event What does 777 cfs of water look like? Fire hydrants flow at about 1,000 gallons per minute (gpm) 1,000 gpm equals 2.2 cfs 777 cfs equals 353 fire hoses That’s a wall water 30’ wide and 26’ high every second How do we reduce the amount of water and limit flooding? Detention Cells Detention Cells What is a Detention Cell? It’s simply a pond that holds stormwater and slowly releases it downstream. Think of it as a supersized bathtub with a slow drain. Increasing the size of the pond allows for less water to be discharged downstream. Benefits: Slows down stormwater to a manageable rate Minimizes flooding downstream of pond Cleans stormwater (sediment and pollution) Many Detention Cells Are Throughout 8 in just the Hawthorne Rapid City Drainage Basin Saint Patrick St. Meade St. Hawthorne Ave. Hawthorne Highway 16 Highway Purpose of Robbinsdale Park Reconstruction Project Pre-Construction Conditions Prior to reconstruction, the two detention cells in Robbinsdale Park were significantly undersized. Most notably, the largest cell (southern cell) contained only 31% of the volume of the newly reconstructed cell. Trees had grown in the existing cell and further limited the volume capacity as well as created maintenance issues for the City. Pre-Construction Conditions Cont. Prior to reconstruction of Robbinsdale Park, downstream drainage improvements along Hawthorne Ave. were construction around 2002. Improvements included the installation of an 8’x4’ box culvert to parallel the existing 48” diameter storm sewer. Though additional stormwater can be taken underground with the new storm sewer, the detention cells in Robbinsdale Park were undersized and released too much water for this new stormwater infrastructure during large rain events. The newly reconstructed detention cells at Robbinsdale Park are designed to hold back as much stormwater as possible so that the downstream storm sewer along Hawthorne Ave. is not overwhelmed. This results in less stormwater needing to be placed above ground on the street and minimizes the chances of flooding occurring on properties downstream of Robbinsdale Park. Changes to the Southern Robbinsdale Detention Cell Pre-Construction Mid-Construction Near Final Post-Construction Southern Cell – Pre-Construction: Approx. 11 acre-feet of storage Southern Cell – Post-Construction: Approx. 35.5 acre-feet of storage Parts of the Southern Robbinsdale Detention Cell Impact Structures These structures lower the velocity of stormwater entering the detention cell. This helps minimize erosion in the detention pond. The structures works by creating a sudden change in direction of the stormwater. Parts of the Southern Robbinsdale Detention Cell Outlet Structure This structure controls how much water is released from this detention cell. As water fills the detention cell, different sized openings regulate the total flow of water. Water Quality The bottom 3.5’ of the detention cell is designed for a very slow release of water. This slow release allows sediment and other pollutants to settle out of the water and not be released downstream into Rapid Creek. Parts of the Northern Robbinsdale Detention Cell The northern detention cell is significantly smaller than the southern cell. The purpose of the detention cell is to regulate stormwater discharging from the subdivision directly west of Robbinsdale Park and to act as additional volume for any stormwater that the southern detention pond cannot handle. The northern pond is equipped with two outlet structures to regulate stormwater flows. The largest outlet structure is found near the eastern boundary of the detention cell and acts as a junction for underground stormwater infrastructure. This junction connects the storm sewer of the southern and northern ponds to the City’s existing stormwater infrastructure. Changes to the Floodplain FEMA Process to Change Floodplain The Robbinsdale Park Reconstruction project has gone through the FEMA LOMR process and has been approved. LOMR stands for Letter of Map Revision Approval of a LOMR by FEMA officially changes the floodplain outline as well as the flows and depths of a floodplain. The LOMR process for this project included the following: Revision of hydrologic model (how much stormwater) Revision of hydraulic model (where and how the stormwater moves) This new hydraulic model incorporates all the stormwater improvements within Robbinsdale Park as well as existing downstream and upstream infrastructure. Where to Find Floodplain Maps and Changes FEMA has established a website to search and look at current floodplains for the USA. www.fema.gov/national-flood-hazard-layer- nfhl Click the “NFHL Viewer” link on the page Please note that the changes to the floodplain around Robbinsdale Park will not be officially changed until October 5th, 2020. Until the October date, the current “unstudied” floodplain is still effective. Changes to Floodplain Boundaries Full floodplain changes Pre-Construction Floodplain Post-Construction Floodplain have been included on this slide. The next sixteen (16) slides are zoomed areas of interest to show the changes in more detail. Changes to Floodplain – Area 1 Changes to Floodplain – Area 2 Changes to Floodplain – Area 3 Changes to Floodplain – Area 4 Changes to Floodplain – Area 5 Changes to Floodplain – Area 6 Changes to Floodplain – Area 7 Changes to Floodplain – Area 8 Changes to Floodplain – Area 9 Changes to Floodplain – Area 10 Changes to Floodplain – Area 11 Changes to Floodplain – Area 12 Changes to Floodplain – Area 13 Changes to Floodplain – Area 14 Changes to Floodplain – Area 15 Changes to Floodplain – Area 16 May 18th, 2018 Storm Unexpected Testing of Detention Cells In drainage engineering, many assumptions need to be made before finalizing a design. One of the largest contributors to the design is determining what storm intensity to plan for. For the Robbinsdale Park Reconstruction Project, the detention cells were designed for the 100-year storm event (1% chance). As discussed earlier, this means in a given year there is only a 1% chance of this magnitude storm occurring. The reconstruction within Robbinsdale Park was completed on Monday, May 14th, 2018. Four days later, a high magnitude storm occurred over the Robbinsdale area of Rapid City that put the detention cells’ design to the test. From the conditions witnessed during the May 18th storm, the engineering design assumptions for the Robbinsdale detention cells can be checked. The remaining portion of this presentation focuses on what occurred on May 18th as well as how the Robbinsdale Park Reconstruction Project significantly reduced the potential for flooding downstream of the Park.