Background – HCFCD Poor Farm & Kilmarnock Regional Study
·In 2002, the Harris County Flood Control District launched a feasibility study titled “Poor Farm & Kilmarnock Regional Study” (PFKRS), with engineering consultant Claunch & Miller.
·The study was a collaborative effort between the Flood Control District and the cities of Bellaire, Houston, Southside Place and West University Place to evaluate the feasibility of improvements to Poor Farm Ditch and Kilmarnock Ditch to reduce the risk of flood damages in the area.
·Claunch & Miller, Inc. was supported by a Technical Study Group, established by the Flood Control District. In November 2003, the Study Group reached the following recommendations:
ØPoor Farm Ditch -
§Expand the capacity of Poor Farm Ditch between University and Bellaire boulevards
§Replace the Bellaire Boulevard Bridge
§Provide 43 acre-feet of mitigation for Poor Farm Ditch
ØKilmarnock Ditch -
§Replace the N. Braeswood/Beechnut Bridge
§Providing 21 acre-feet of mitigation for Kilmarnock Ditch
§Remove and/or replace the existingoutfallculverts (two 72-inch pipe culverts) into Brays Bayou
·Claunch & Miller completed the report in June 2004. The estimated total cost for all improvements was $11.3 million.
·The recommendations were presented to the public at several meetings, including a public meeting held on February 5, 2004.
·In concluding the PFKRS in March 2004, the study group members requested that the Flood Control District, among other items, consider securing mitigation for the projects a top priority.
·Actions taken by the Flood Control District since 2007 include:
ØMeyer Stormwater Detention Basin Mitigation Purchase:The District purchased 39 acre-feet in the 17-acre Meyer Stormwater Detention Basin (D500-08-00) for $3 million from the City of Houston through an interlocal agreement dated March 20, 2007. The detention facility is on Brays Bayou and is located northeast of Loop 610 and south of North Braeswood Blvd. Construction was completed in September 2009.
ØBellaire Boulevard Bridge Replacement Project:Through an interlocal agreement initially approved in November 2006 and amended in March 2009, the Flood Control District contributed $1,804,500 to the City of Southside Place for the Bellaire Boulevard Bridge replacement project at Poor Farm Ditch. Project work included associated paving, storm sewers, sidewalks and waterlines. The Flood Control District also provided the required 10.3 acre-feet of mitigation by reserving that volume as part of the District's capacity in the Meyer Stormwater Detention Basin. Bridge construction was completed in 2010.
ØPoor Farm Ditch Conveyance Improvements: Project Development Stage:In July 2007, the Flood Control District began a more detailed study of how to address Poor Farm Ditch’s aging infrastructure and stormwater carrying capacity. The goal was to identify the most effective plan for replacing Poor Farm Ditch under existing conditions, which include limited access andright-of-wayin a dense, residential area. The Flood Control District has been working with the city of Southside Place to determine Poor Farm Ditch Conveyance Improvements alternative that meets the requirements of both parties and is conducting analyses on proposed alternatives.The current Flood Control District CIP has $375,000 allocated for Poor Farm Ditch project design. It is important to note that an alternative has not yet been selected, and a construction project is not funded at this time.
Current Situation - City of West University Place/Bellaire Storm Sewer Project and Restrictor Removal and the Kilmarnock Ditch Outfall Analysis
·In early 2010, the three-year, $8 million College Street storm sewer system improvement project at Bellaire Boulevard and Poor Farm Ditch was completed.
·The Flood Control District determined that the project required a total of 13.5 acre-feet of stormwater detention mitigation when the (future) Poor Farm Ditch project was finished.
·Because the city of West University Place did not have the required mitigation in place, a “restrictor” was installed by the Flood Control District at the confluence of Poor Farm Ditch and Brays Bayou.
·In May 2011, the Flood Control District entered into an interlocal agreement with the City of West University Place to sell 4.4 acre-feet of detention volume in the Meyer Stormwater Detention Basin for $403,700. This allowed for partial removal of the restrictor in the College Street storm sewer outfall. The partial restrictor remained in place because the city still needed 9.1 acre-feet to mitigate the additional stormwater runoff.
·In 2013, the Flood Control District performed an in-depth technical analysis on the mitigation requirement for Kilmarnock Ditch. Results of the analysis indicated that the original estimate in the Poor Farm & Kilmarnock Regional Study was conservative, and that the District could sell 9.1 acre-feet of capacity in the Meyer Stormwater Detention Basin to the City of West University Place for its mitigation requirements and still have sufficient capacity in the Meyer Basin to mitigate Kilmarnock Ditch and/or other projects that are determined to be needed in this area.The reason for the difference:The Poor Farm & Kilmarnock Regional Study was a feasibility study, and thus the analyses conducted within that framework were high level and minus the detail the Flood Control District conducts in its project development stage. Details of the analysis:
ØIn August 2013, the Flood Control District made the decision to perform additional technical analysis and study options for providing mitigation for a proposed Kilmarnock Ditch project. A factor in this decision was the city of West University's request to purchase capacity in the Meyer Stormwater Detention Basin in order to provide mitigation for its College Street storm sewer system improvement project. The following points were also considered in that decision-making process:
1.Kilmarnock Ditch has an adequate cross section.
2.The proposed improvements to Kilmarnock Ditch are to replace culvert crossings only.
3.Hydraulic analysis indicates that Kilmarnock Ditch's peak flow gets to Brays Bayou, albeit high head loss and overtopping occurs.
4.The small watershed technique used to estimate flow mitigation in the Poor Farm & Kilmarnock Regional Study is conservative.
5.The proposed Kilmarnock Ditch Conveyance Improvements project is not included in the Flood Control District's 5-year Capital Improvements Program (CIP).
ØIn November 2013, the Flood Control District documented findings of the Kilmarnock Ditch Outfall Analysis:
·The goal of the analysis was to determine whether overtopping occurs at the Kilmarnock Ditch outfall into Brays Bayou.
·A field survey was conducted to determine the elevations and sizes of the structures and the cross-section of the Kilmarnock Ditch.
·Survey data indicated that the top bank elevation of Kilmarnock Ditch varies from 48 to 50 feet along the left and right side of the channel. The elevation of the channel varies from 35 feet at upstream end to 30 feet at the downstream end. The channel depth varies from 13 feet to 18 feet.
·When flood water in Kilmarnock Ditch exceeds approximately 45 feet, it flows over the banks and into Brays Bayou.
·The analysis revealed that the effective peak flow is much higher than the peak flow calculated in the PFKRS feasibility study.
·The 1 percent exceedance probability event flow (2,020 cfs) overtopped the high bank elevation of Brays Bayou at the outfall. Kilmarnock Ditch's peak flow runs through the existing two 72-inch outfall pipes and over the north bank of Brays Bayou. The flow that can be conveyed without overtopping the north bank of Brays Bayou close to the outfall is approximately 1,200 cfs.
·Analysis Results:
oIt was determined that the Braeswood/Beechnut Street culvert has sufficient capacity to convey the 1 percent exceedance probability event flow.
oThe runoff conveyed by D114-00-00, which runs parallel to Kilmarnock Ditch and captures stormwater from most of the drainage area, discharges into Kilmarnock Ditch downstream of the Braeswood/Beechnut Street.
oThe two 72-inch outfall culverts do not appear to be a significant restrictor of flow since the flow overtops the high bank elevation of Brays Bayou at the outfall and gets to Brays Bayou.
oThe outfall culverts also minimize the backflow from Brays Bayou when the bayou is high.
oIncreasing the capacity of the two outfall culverts may have a negative impact on Kilmarnock Ditch's stormwater conveyance because it could allow additional back flow from Brays Bayou into Kilmarnock and increase potential flooding of the structures adjacent to the banks of Kilmarnock.
·Analysis Conclusion:The results of the analysis do not support the recommendation stated in the PFKRS report.The study did not take into account the overtopping flow at the outfall location and the Beechnut/North Braeswood crossing has sufficient capacity for the proposed ultimate conditions flow stated in the report.
·Based on findings in the Kilmarnock Ditch Outfall Analysis, the Flood Control District agreed to sell 9.1 acre-feet of stormwater storage capacity in the Meyer Stormwater Detention Basin to the City of West University Place at a cost of $91,750 per acre foot, for a total of $834,925. At its January 29, 2014 meeting, Harris County Commissioners Court approved the sale of capacity, and the agreement between the Flood Control District and the city.
·Under the agreement, the Flood Control District agreed to allow the city to remove (at its own expense) the existing restrictor at Poor Farm Ditch. The District also had to approve and supervise the restrictor removal.
·The City of West University Place removed the restrictor in February 2014.