Services
Investigation of Hydrodynamic Modifications to Storm Water Culverts Flowing to Alisos Creek
Introduction
The project, Rancho las Lomas (RLL), consisting of approximately 20 acres is located directly west of and adjacent to Santiago Canyon road about 1000 feet northerly of the junction of Santiago Canyon Road and Live Oak Canyon Road in Orange County, California. It is traversed from north to south by Santiago Canyon Road and by Alisos Creek, which meanders through the project as a natural drainage course in which several culverts and bridges have been constructed for road crossings. Also, an earth levee approximately 10 feet high and 750 feet long has been constructed between the east side of the natural drainage course and Santiago Canyon Road.
The drainage area consists of 678.5 acres and varies from elevation 2200 at the north end and to elevation 1113 at the south end as shown on the El Toro and Santiago Peak U.S.G.S. Quadrangle Maps used in the report update. At present, the drainage area has undergone residential development to the east modifying its natural state, which consisted of chaparral, open brush, and some live oak trees. This new development has modified the flow characteristics to 2 drainage structures crossing Santiago Canyon Road from the east and discharging into Alisos Creek, which flows north to south through the RLL. These characteristics vary from a report prepared November 11, 1989 by Carlat Engineering which calculated the on-site Q100 flows in Alisos Creek considering the off-site watershed in its ultimate developed state of single family residential, open space and recreational open space as shown on the Community Profile 59-A considered a part of this report. It is our contention that the modifications made at the inlets of these structures has modified their original engineered characteristics causing damage downstream of their point of connection and on the Rancho las Lomas site at the discharge points (see Photos 1 & 2 herein). We have made numerous requests of the County and RBF & Associates to obtain the hydrology report which had to be prepared in order for the project to be approved for construction. These critical documents do not appear to be available from any source. As a result, we have had to analyze the engineering working drawings prepared by RBF and backed into our conclusions. Every opportunity was given the County of Orange as well as RBF & Associates to defend their design but they have opted not to comment nor provide evidence that proper design engineering took place to protect existing development downstream.
The Orange County Hydrology Manual, Section A – Introduction, Paragraph A.2. Hydrologic Protection Levels, states, “ It is the goal of the agency to provide 100-year return frequency flood protection for all habitable structures and other non-flood proof structures. Consequently, all drainage plans must demonstrate this 100-year flood protection criteria”. The documents prepared by RBF are in violations of this section.
Orange County Subdivision Code Section 7-9-282 Drainage & Erosion Control, paragraph b., states, “In the event a subdivider unnaturally concentrates or diverts surface water running onto adjacent land, the map shall illustrate the manner in which water will be accepted and disposed of.” This paragraph applies to the Tentative Map submittal and approval process. This implies that discharge points would have to be addressed. They are not referenced nor addressed for improvements on the working documents. They modified the hydrodynamic loading of the 2 conveyances yet did not address potential discharge problems downstream affecting Alisos Creek.
County of Orange Grading & Excavation Code Sub article II. Drainage and Terracing, Section 7-1-833, states, “Drainage facilities and terracing shall conform to the provisions of Sub article II of the Grading Manual unless otherwise approved by the building office and delineated on the approved grading plan.” Downstream maintenance & erosion control were not addressed on the documents and we find the plans to be in non-compliance with the Code.
We intend to present evidence that Q25 flow velocities at the discharge of 2 structures have been violated due to connecting each structure to upstream closed piped systems and created downstream damage due to the conveyances becoming systems operating under pressure.
Observations, Conclusions and Recommendations
1. The property to the east of Rancho las Lomas (RLL) is depicted on Drainage Area Maps A & B (attached) and labeled as Sub areas C1, C2, D1, D2 and partial Sub areas C3 & D3.
2. Watershed Sub areas C1-3 in their natural state drained to the existing 4’x4’ RCB which conveyed flow to Alisos Creek under Santiago Canyon Road. In the event that the inlet structure became blocked, flow would then release to the south at elevation 1169.2 along the swale on the east side of Santiago Canyon Road and collect at the inlet of the 48” CMP. Should both culvert crossings be blocked then flow would release to the south at elevation 1136.2 and continue flowing along the east side of Santiago Canyon Road continuing to seek its natural course of Alisos Creek.
3. Beazer Homes Tract #11919 has been developed entirely within Watershed Sub areas C1-3, although a future roadway connection is shown on the drawings indicating a continuation into Watershed Areas D1-3. The plans developed by RBF split Watershed Sub areas C1-3 into 2 separate systems. The off-site flow is collected detained and released to a closed culvert system and connected to the south 48” RCP that is also now expected to collect and convey runoff from Watershed Sub areas D1-3. The on-site drainage for Watershed Sub areas C1-3 are collected in the street system and conveyed to the north 4x4 RCB. Without the original hydrology report, we would have to perform exhaustive reverse engineering of the hydrology for Tract #11919. The lack of this report makes it impossible to compare the changes this development has caused to our assumed Q100 flow as developed in the Carlat documents without great expense to the owners of RLL. Should this be necessary are capable and very willing to perform this service. I still feel that the necessary documents to make a proper comparison are available somewhere.
4. We decided to review changes to the hydrodynamics of the 2 culverts collecting storm flow and directing this flow to Alisos Creek. The data we used was the Q25 flows indicated on the RBF documents for Tract #11919. We built a computer model/data base of both storm collection systems shown on the drawings referenced above (calculations attached – pages 25-53). We then used the same total flow data from the RBF drawings and applied the Q25 flows to both conduit crossings given their original and as-built criteria. It was discovered that both conduits are now flowing at increased velocities, in feet per second, 122% and 134% higher than would have been possible when first installed or left in their natural state (see following comparison table).
5. RBF made an attempt to increase the friction “n” factor in the 4x4 RCB by proposing “roughness blocks” to be installed on 8’ centers for a portion of the culvert yet I am afraid that these will do little to effect discharge velocity. An acceleration due to a constricted nozzle effect may be the result without proper maintenance to assure the system does not clog and silt up causing a reduced pipe size condition rather that a roughage condition. No modifications were made to the 48” CMP other than changing it to a pipe flowing under pressure.
6. The RBF drawings ended design engineering and construction at the upstream (inlet) of the two crossings. There is no indication of any remedial work to be performed at the discharge end of the culverts. At the minimum, we would have expected to see a type of outlet stabilization in conjunction with some type of energy dissipating structure and transitional spillway to merge with Alisos Creek. As Photos 1 & 2 show, nothing of any benefit from an engineering standpoint was constructed. The concrete slope protection installed around the 4x4 RCB’s outlet has done little if anything other than provide the main watercourse of Alisos Creek with a structure to scour and undermine over time. In addition, considering the attention to aesthetic detail the owners of RLL pay to their property, I find the two discharge structures to be unacceptable from a landscape architectural view. They are both quite unsightly and the 48” CMP looks as if it was ignored completely. I find this unusual for a company of RBF’s reputation in the county.
7. Now without the proper original documents, it is hard to say if the Q100 flows have been increased over the assumptions made in the Carlat reports and contained herein. I am not even going to hazard a guess right now other than to say I feel that the Q100 flows between the outlet of the 4x4 RCB and the 48” CMP have been reduced in Alisos Creek which will help with the engineering criteria for the restroom and brides bridges at the Chapel area in RLL. It is the increased flow of combining a future developed Watershed Sub area D1-3 with the off-site flows from Watershed Sub areas C1-3 through the single 48” CMP. These combined flows could cause severe channel damage to Alisos Creek and the first downstream bridge crossing Alisos Creek on the RLL property during a Q100 storm event. Further study is required to target and mitigate these potential problems I am very afraid will occur.
Note: Comparison made using Q25 data obtained from RBF documents.
Photo #1 - Discharge Structure to Alisos Creek from Drainage Sub areas C1, C2 & C3 – 4’x4’ RCB - See Maps A & B
Photo #2 - Discharge Structure to Alisos Creek from Drainage Sub areas D1, D2 & D3 – 48” CMP - See Maps A & B
DRAINAGE AREA MAP ‘A'
DRAINAGE AREA CONCENTRATION POINT MAP ‘B’