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HomeMy WebLinkAboutLETTER - Irrigation Ditch Findings PE Signed - Centennial Townhomes Area 1150 Hollipark Dr. Idaho Falls, Idaho 83401 208.681.8590 June 18, 2021 City of Rexburg Re: Centennial Townhomes and Downstream Ditch Removal Attn: Keith Davidson Connect Engineering (CE) was contracted in 2018 to design the civil engineering aspects for the Centennial Townhomes project. The property that was to be developed as part of this project contained an existing irrigation ditch that previously served the subject property and downstream landowners with irrigation water. This ditch at the time design began was clearly not being utilized by the owner of the Centennial Townhomes project and or downstream landowners. The ditch had not been repaired, cleaned and or maintained for some time. There were areas that the ditch bank no longer would allow the ditch to carry water downstream with out the water running out of the ditch, the culvert crossing Seventh North (7th N) the inlet side had been smashed and would restrict any flow and areas of the ditch had been silted in restricting flow. At that time CE began contacting the downstream landowners to discuss the status of the ditch and the possibility of removing the ditch. Each landowner confirmed that the ditch had not been utilized, cleaned or needed by them for some time. Each respective landowner confirmed that they would allow the ditch upstream to be removed or abandoned with the knowledge that would remove their ability to receive irrigation water to their property. At that time CE did not get signatures from each landowner but as part of this request has gone back and received signatures from each landowner. See Exhibit A. Prior to construction of the Centennial Townhomes project in 2019 CE met with Dale Jepson from Teton Island Feeder Canal Company to confirm that the existing ditch was not considered a drain ditch for the Irrigation Company. CE was told at that time the Irrigation Company did not control the ditch where it is considered a service ditch for the landowners but also did not consider it a drain ditch or never has considered it a drain ditch. It was asked if we needed to attend a board meeting to discuss the request of removing the ditch and CE was told that was not necessary due to the fact that the Irrigation Company did not control the irrigation ditch. At that time CE reached out to the upstream ditch users to notify them that from the Centennial Townhomes property south each of the landowners were granting permission for the irrigation ditch to be removed and or abandoned on their respective properties. The question was raised concerning the ditch being considered a drainage path for excess water from the upstream users. CE responded with the conversation with Dale Jepson at Teton Island Feeder Canal Company not considering the ditch a drainage ditch to which they didn’t agree and responded with what happens to the excess water of the ditch going through leaking headgates and or rotation of ditch users and open headgates. CE discussed with those users according to state statute 42-916 that each user of the irrigation ditch should exercise “good husbandry” of the water, but it is understood that some tail water through a leaking headgate could be expected. As part of the Centennial Townhomes project the irrigation ditch was removed and or abandoned from Stonebridge Street west. From that location east to the last headgate structure there is approximately 475 linear feet of ditch that has remained and was/is planned to remain as part of the Stonebridge Townhomes project that is east of the Stonebridge Street that can be utilized to store and or accommodate the potential for leaking headgates. CE was notified mid-summer of July 2019 during construction that there was a potential issue of flooding that took place by an upstream ditch user. CE met Lee Munns, a farmer leasing the property just north of the Centennial Townhomes project, on site to discuss what took place. CE was told that an upstream ditch user had finished using the water and had not notified the other ditch users that they were done with the water. Where Lee is the last user on the ditch and at the location where the last headgate remains closed, the water entered the property he was leasing during the night and made it to the end of the field on the west end with standing water. At that time, he went and closed the headgate at the main canal to relieve pressure on the ditch where the other users were not expecting water. At that time CE sent a reminding letter to the water user upstream that the ditch downstream had been abandoned and each user of the ditch needs to coordinate by state statute to avoid issues like these from taking place. This is the only event in the past three watering seasons that CE has been notified of. It was brought to the attention of CE in May of 2021 that an additional adjacent landowner on the northeast side of Stonebridge Townhomes had a concern that if the ditch was to overflow the banks that their home would be potentially flooded. CE hired a land surveyor, with permission of that landowner, to provide a topographic survey of their land and to establish a lowest opening and or finish floor elevation of the home that had potential for water to enter the residence. CE received that information and has evaluated the results of that survey. The lowest potential area near the home that potentially would allow water to enter the residence is at an elevation of 4875.41. The top bank of the irrigation ditch directly to the west of the home is at an elevation of 4877.35 and the bottom of the irrigation ditch is at an elevation of 4875.09. The flow line of the irrigation ditch is 0.32 feet lower than the lowest point for potential flooding into the residence. The lowest elevation near the home and on the property was observed to be 4873.77 which is approximately fifteen (15) west of the westerly corner of the residence. The northern most portion of the property is at an elevation of 4874.83. The elevation near the southern property line is at an elevation of 4874.57. The adjacent property to the south on the north end is at an elevation of 4875.28 at the lowest point, which is approximately at the southwest end of the concerned property owner. This is 0.13 feet lower than the lowest potential elevation where water could enter the residence. With the existing elevations of the property and adjacent property the home currently has potential for water issues with out considering the future project to the south and or the recently completed Centennial Townhomes project. The lowest potential opening for water issue is as mentioned slightly higher than the south property, but the existing land elevations to the west do continue to get lower to the west. With the natural elevation of the land falling to the South West this allows for potential water to flow away from the residence and most likely not create a backwater situation towards the residence and allow for water to escape and flow south and west from the home eventually reaching the recently constructed Stonebridge Street. The water would then flow again south and west and eventually reach the constructed storm retention basin for the Centennial Townhomes project. With the subject residence and the elevation that it was built, there has been a potential for issues concerning the ability for water and or irrigation water to enter the residence since it was built due to the surrounding elevations of the adjacent properties, which are all flood irrigated properties. As mentioned above with the surrounding properties with the natural slope in a southwest direction does help the situation. I believe, unless a ditch user does not exercise care of the water rights they have the home does not pose a high risk of flooding. Connect Engineering has also completed the following calculations in response to the concern of an adjacent property owner to the Centennial Townhomes project located in Rexburg, Idaho specifically for the volume of water able to be accounted for with leaking headgates or tail water in the subject ditch. As mentioned above there is 475 linear feet of irrigation ditch that remains intact from Stonebridge Street to the east that was planned to retain the tail waters. From the United States Department of Agriculture Natural Resource Conservation Service Web Soils Survey the soils in the area of this concern are identified as Annis Silty Clay Loam which is identified to have an ability to transmit water (Ksat) of moderately high of 0.20 to 0.60 in/hr. For our calculations CE used 0.4 in/hr. With a ditch cross section of 475 linear feet and approximately 1.50 feet wide at the bottom yields 712.50 square feet of surface area to allow drainage. At 0.40 in/hr or 0.033ft/hr this allows for 23.75 cubic feet/hour to percolated into the ground. CE assumed that the headgate at the end of the ditch if left open 1 inch with a flow of 4 feet per second would yield 0.50 cubic feet/second or 30 cubic feet/hour of tail water. This is just slightly over the percolation rate of the 475 linear feet of ditch. If the ditch bottom were to be saturated and not allow for the full 23.75 cubic feet/hour percolation, then the volume of water that could be stored in that 475 linear feet of ditch would be as follows. 475 linear feet x 6 square feet = 2,850 cubic feet. The allowable storage time with zero percolation would be 2,850 cubic feet/ 30 cubic feet/hour which is equal to 95 hours which would allow for adequate time for any ditch user to make corrections to potential issues before the storage capacity was reached. CE also looked at the potential for the last headgate if it was to remain open. With 2,850 cubic feet of storage and flowing 4 cubic feet/second at the same ditch cross section of 6 square feet, this would allow for 1.98 hours of storage with no percolation (6 square feet x 4 feet/ second = 24 cubic feet/second or 1,440 cubic feet/hour. 2,850 cubic feet of storage/1,440 cubic feet/hour = 1.98 hours). This would also allow ditch users the ability to make necessary corrections prior to flooding downstream. If adjustments were not made the water would exit the westerly end of the ditch near Stonebridge Street and flow down the city street eventually reaching the storm water storage pond for Centennial Townhomes. Based on the above observations and calculations it is of my opinion the property located at 547 E 7th North in Rexburg, Idaho has a minimal, if any chance of flooding due to the construction of the Centennial Townhomes project and removal of the irrigation ditch that had been abandoned some time prior to that construction of that project. This opinion is based on the above-described elevations, calculations and the assumption that the service ditch users follow the requirements of utilizing flood irrigation per Idaho State Statutes and exercise good practice/good husbandry of the irrigation waters. Respectfully Submitted Connect Engineering Blake Jolley, P.E. (Idaho, Montana, Utah, Wyoming)