1.0 INTRODUCTION 1.1 OBJECTIVE This report, prepared for submittal to TxDOT, analyzes existing and proposed detention facilities draining into the TxDOT US90A storm sewer system. The results of the detailed analysis show that the facilities will mitigate the increased runoff from the Stafford Tract future development and would not negatively impact the receiving storm sewer system. 1.2 LOCATION AND DESCRIPTION The tract is located in Stafford, Texas, and bounded by Kirkwood Drive to the west, US90A/Southern Pacific Railroad corridor to the south, Greenbriar Drive on the north, and Houston Shell & Concrete site on the east. Existing drainage systems and proposed developments were analyzed in accordance with the Fort Bend County Drainage Criteria. The TxDOT drainage area map and hydraulic data sheet were used to identify drainage elements, boundaries, and parameters. Stafford Run Ditch was assumed for ultimate capacity. Maximum 100-year release rate is 63.12 cfs. Detention analysis for the 100-year storm runoff conditions using Small Watershed (Malcolm s) Method and routing using ICPR3 modeling are illustrated for two cases: 1) drainage area of 75.5 acres, and 2) drainage area allowable for the existing pond. 1.2.1 Background A Drainage Impact Analysis, dated April 2002, was performed by PBS&J on behalf of TxDOT, which identified the design conditions of the storm sewer system along the proposed US90A construction between Kirkwood Drive and Promenade Blvd. to the east. The maximum release rate that TxDOT allows to be discharged into the US90A system from the project area is 63.12 cfs. TxDOT is only concerned about the actual release rate and not the specific drainage area. 1.2.2 Review of Flood Plain The project area is located entirely in Zone X (Un-shaded), FIRM No. 48157C0140K, effective April 20, 2000 and 48157C0255J, effective January 3, 1977, not in the flood plain. 1
2.0 ANALYSIS Analysis of existing and proposed conditions performed to determine the 100-year storm runoff conditions. 2.1 EXISTING CONDITIONS The project area includes an existing pond that serves as a retention facility where the mass quantity of stormwater runoff is stored and discharged to the Stafford Run Ditch through a controlled flow outlet. The pond has apparently maintained a historic water level approximately 6 to 8 feet below the surrounding natural ground. The existing drainage pattern is from north to south. Natural ground elevations vary from approximately 80.0 to 82.5 feet at vicinity of the existing pond. 2.2 PROPOSED CONDITIONS All excess runoff from the project area will drain to the proposed detention pond and convey to the TxDOT US90A storm sewer system and Stafford Run Ditch, which has been constructed to ultimate capacity. Case 1 proposed detention pond must be sized to handle the 100-year developed flows of 75.5 acres and maximum release rate of 63.12 cfs. The existing pond will be expanded to provide the effective detention. The natural ground elevations in the vicinity will be approximately 82.0 feet after construction of the expansion where fill will be deposited adjacent to the pond. Case 2 investigates the allowable drainage area for the existing pond that provides approximately 31 acre-feet of storage below a 100-year ponding level of 81.3 feet. The allowable release rate will be based on the drainage area ratio of Case 1. 2.3 DETENTION POND ANALYSES 2.3.1 Proposed Detention Pond (Case 1) The onsite detention requirements assumed the entire 75.5 acres tract would be designed to drain to the existing pond on the Steppe tract with modifications to the pond necessary to meet the TxDOT, Fort Bend County, and City of Stafford design criteria. 2.3.1.1 Hydrology The 100-year 24-hour storm was used to size the detention pond. Peak flows for the existing and proposed conditions were computed based on the Rational Method. The Small Watershed (Malcom s) Method was used to create pattern hydrographs. The design hydrographs represent runoff volumes that are consistent with the design excess rainfall totals. The design hydrographs were transposed to the ICPR3 computer program for the reservoir routing results based on the proposed detention designs. The following criteria compute the Malcom's Method Hydrograph: Existing Conditions Proposed Conditions Drainage Area (Acres) = 75.5 75.5 Excess rainfall volume (in) = 9.75 11.05 Peak Flow Rate (cfs) = 63.12 252.47 Computed Tp (hrs) = 8.46 2.40 Time Increment (hrs) = 0.25 0.25 2
2.3.1.2 Detention Pond Area and Volume One detention pond is proposed to mitigate the developed flows. The effective initial stage of detention pond is assumed to be the outfall flowline of 73.94 feet. Surface area and volume versus elevation relationship for the detention pond is shown in the following table. Proposed Detention Pond Stage-Area-Volume Relationship 70. 72. 73.9 Elevation (ft, msl) 74.5 75.5 76.5 77.5 78.5 79.5 80.5 81.2 82.0 5 0 4 0.0 1.2 Pond Area (ac.) 6.09 6.12 6.19 6.27 6.34 6.42 6.50 6.58 6.63 6.69 7 4 0.0 0.9 11.5 17.6 23.9 30.2 36.5 43.0 49.5 54.2 59.5 Volume 8.09 0 8 1 7 0 0 9 5 8 1 4 Effective Volume (acft)* 0 0 0 1 9 5 9 1 5 0.0 0.0 15.8 22.1 28.4 34.9 41.4 46.1 51.4 0.00 3.42 9.57 * Effective Volume based on initial stage at elevation 73.94 feet The proposed pond will provide a maximum of 51.45 acre-feet at the top of berm elevation of 82.0 feet. 2.3.1.3 Outfall Structure The proposed detention pond will discharge into the TxDOT proposed US90A storm sewer system with flow rates consistent with the TxDOT plans and drainage solutions matching the system capacity. The proposed outfall is one 36-inch pipe at flowline 73.94 feet. Emergency overflow swale would be at elevation 81.2 feet with 8 feet bottom width and 8 to 1 side slopes. 2.3.1.4 Routing Results (Proposed Pond) 3
Detention storage was determined by routing the proposed 100-year hydrograph through the detention pond using ICPR model. The parameters and results of the 100-year storm are tabulated in the following table. The plot of hydrographs and computer modeling results are attached. Drainage Area (ac) Peak Inflow (cfs) Proposed Maximum Detention Provided Existing Developed Stage (ft) Storage Used Outflow (cfs) Berm El. (ft) Vol. 75.50 63.12 252.47 81.16 45.85 62.98 82.00 51.45 0.68 The maximum 100-year ponding level is 81.16 feet and freeboard of 0.84 feet provided. Rate 2.3.2 Drainage Area Allowable for Existing Pond (Case 2) 2.3.2.1 Existing Pond Area and Volume The surface area and volume versus elevation relationship is shown in the following table. Detention Pond Stage-Area-Volume Relationship Elevation (ft, msl) 70.5 72.0 73.94 74.5 75.5 76.5 77.5 78.5 79.5 80.5 81.3 Pond Area (ac.) 0.07 1.24 3.98 4.00 4.07 4.13 4.20 4.27 4.34 4.40 4.46 Volume 0.00 0.98 6.04 8.28 12.31 16.41 20.58 24.81 29.11 33.48 37.03 Effective Volume * 0.00 0.00 0.00 2.23 6.27 10.37 14.53 18.77 23.07 27.44 30.99 * Effective Volume based on initial stage at elevation 73.94 feet The existing pond will provide a maximum of 30.99 acre-feet of detention at an elevation of 81.3 feet. 2.3.2.2 Outfall Structure The existing pond will discharge into the TxDOT proposed US90A storm sewer system with flow rates consistent with the TxDOT plans and drainage solutions matching the system capacity. The allowable release rate for 75.5 acres is 63.12 cfs, therefore the proportion release for 51.5 acres (from trail and error) is 43.1 cfs using drainage area ratio. The proposed outfall would be one 36-inch pipe restricted to 30-inch opening at flowline of 73.94 feet. Overflow Sheet flow occurs at natural ground elevation of 81.3 feet during extreme storm events. 2.3.2.3 Hydrology The analysis uses the 100-year 24-hour storm and a trial and error method. For a drainage area of 51.5 acres, the peak flows for the existing and proposed conditions are 43.1 cfs (allowable release rate) and 172.2 cfs (inflow by drainage area ratio of 75.5 acres), respectively. The Small Watershed (Malcom s) Method was used to create pattern hydrographs. The design hydrographs represent runoff volumes that are consistent with the design excess rainfall totals. The design hydrographs were transposed to the ICPR3 computer program for the reservoir 4
routing results based on the existing pond. The following criteria compute the Malcom's Method Hydrograph: Existing Conditions Proposed Conditions Drainage Area (Acres) = 51.5 51.5 Excess rainfall volume (in) = 9.75 11.05 Peak Flow Rate (cfs) = 43.1 172.2 Computed Tp (hrs) = 8.46 2.40 Time Increment (hrs) = 0.25 0.25 2.3.2.4 Routing Results (Existing Pond) Routing of the 100-year hydrograph through existing pond uses ICPR model. The parameters and results are tabulated in the following table. The plot of hydrographs and computer modeling results are attached. There is not freeboard provided under Case 2. Drainage Area (ac) Peak Inflow (cfs) Maximum Detention Provided Existing Developed Stage (ft) Storage Used Outflow (cfs) Berm (N.G.) El. (ft) Vol. 51.50 43.1 172.2 81.3 30.99 43.0 81.30 30.99 0.60 The allowable drainage area is approximately 51.5 acres. Outfall pipe size is 36-inch restricted to 30-inch opening. The maximum 100-year release rate is 43.0 cfs. Rate 5
3.0 ICPR MODELING The analysis was completed using the ICPR3 modeling program. The ICPR3 program dynamically routes storm water through open channels and/or closed conduits and can simulate a variety of complex conveyance systems, including looped systems. As opposed to HEC-2, which cannot do sub-critical and super-critical flow computations concurrently, and only models one-dimensional flow, ICPR3 uses conservation of mass and the momentum equation to simulate unsteady hydraulic behavior. This means that bi-directional flow can be modeled and the tailwater elevation at an outfall is considered in the calculations. In its pond routing routine, HEC-1 cannot consider the effects of tailwater. Therefore, for analyzing a series of ponds, connected by culverts and controlled by weirs, ICPR3 provides a more realworld model. ICPR3 uses a link/node concept to idealize real-world systems. This concept requires that the drainage system be simplified into a network of nodes, or junctions, and links, or reaches. A node is a discrete location in the drainage system where conservation of mass or continuity is maintained. Nodes are placed upstream and downstream of any structure (e.g., weirs, culverts, and gates). All ponds or lakes are specified as nodes. In the ICPR3 model, a node is placed at each point where a stage calculation is needed. Runoff hydrographs generated by the HEC-1 model were assigned to nodes. Links are connections between nodes and are used to transfer or convey water through the system. The entire system of nodes and links forms the nodal network and serves as the computation framework for ICPR3. 3.1 Pond Modeling The ponds are modeled as level pools. This implies that the water surface within the storage area is flat, and rises or falls uniformly across the surface. A single node was used to model each pond in the system using a stage-area relationship to describe the water holding capacity of the pond. 3.2 Major Surface Water Inflow Points All inflows are assigned to nodes (point loading). ICPR3 includes three internal methods for generating storm water runoff hydrographs: the SCS unit hydrograph method, the Santa Barbara urban hydrograph method, and a kinematic overland flow method. 3.3 Links Channels, Pipes, and Weirs In ICPR3 links, the flow direction can be set to both, positive only, or no flow. Both is the normal setting and allows for flow reversals to occur. The positive flow option allows flow only in the direction indicated by designating a from node and a to node. ICPR3 automatically determines the flow regime (sub-critical or super-critical) in pipes and channels at every computational time increment. The program then applies the appropriate outlet or inlet condition specified. The both option was used for all channels, pipes, and weirs. 6
4.0 CONCLUSION The proposed development will have no adverse impact to the existing drainage system. 7