Loading...
HomeMy WebLinkAbout; Bristol Cove Drainage Area Study; Bristol Cove Drainage Area Study; 1973-06-01BRISTOL COVE DRAINAGE AREA STUDY June, 1973 SHUIRMAN • SIMPSON r irr CONSULTING CIVIL ENGINEERS NEWPORT CENTER BUILDING 359 San Miguel Drive Newport Beach, Celifornis 9266O BRISTOL COVE DRAINAGE AREA STUDY I. Purpose of Report Substantial quantities of silt are deposited during rainstorms in the northerly portion of the improved channel at Bristol Cove at the outlet of the 60" diameter storm drain pipe. The deposition of the silt causes the formation of a small bar or delta that renders the northerly portion of the channel unusable. The periodic removal of the deposited silt is expensive because it is all or partially under water necessitating the use of a drag line. The purpose of this report is therefore as follows: 1. Advise as to the scope of the problem. 2. Comment on alternative solutions to the problem. 3. Provide cost estimates for the alternative solutions. 4. Recommend a solution for review and consideration. II. Field Investigations Two separate field trips were made to observe the condition in the channel at the outlet of the 60" storm drain and to search for sources of erosion within the watershed that were contributing to the problem. On the second trip we were accompanied by Mr. Hunter Cook, City Engineer of the City of Carlsbad. O - 1 - On our first trip we were not able to observe the extent of the silt in the channel because of the high tide stage. However, on the second trip the tide was out and the delta formed by the silt was quite apparent. No measure- ments were taken, but from observation it is estimated that the quantity of silt in the channel at the present time is in excess of 5,000 cubic yards. The extent of the delta or bar renders the northerly 100 feet or so of the channel unusable for boat storage. The entire watershed is overlain with a highly erod- able marine terrace deposit. Practically all erosion observed of any appreciable amount is located on presently undeveloped property. The principal locations of erosion within the watershed are shown on the enclosed map and are designated as A, B, C, and D. These are locations where it is obvious that substantial erosion has occurred within the last few years. In addition to these point sources of erosion there is", general overall surface erosion throughout the watershed. However, the general surface erosion that contributes most to the problem in Bristol Cove is attribut- able to the steeper slopes of the undeveloped areas in the southeasterly portion of the watershed. Another source of silt from erosion is the unimproved channel on the westerly side of Park Drive northerly of Adams Street. However, /^\ except in the vicinity of Point C, the total amount,of erosion for this reach of unimproved channel does not appear to be substantial. III. .Principal Points of Erosion A general description of principal points of erosion is as follows: Point A: The erosion at this location is at the head of a natural channel. This is a natural process where major erosion normally occurs in the uppermost portions of a canyon where the slopes are steepest. There does not appear to be any recent man-made improvements that are contributing factors. Point B: This again, like Point A, is erosion occurring at the head end of a natural channel. However, the slope of this channel is substantially less than the channel in which Point A is located. Point B is located immediately downstream from the outlet^of a storm drain, and the water discharging from this storm drain at a high rate of velocity is probably the major cause of this erosion. Point C; This erosion is occurring in the natural channel on the" westerly side of Park Drive immediately downstream from a culvert that crosses beneath a private driveway. Again, as at Point B, the increased velocity from the culvert discharge is a major contributing factor. The erosion at this point, unlike that at Points A and B, if allowed to continue will undermine the existing culvert causing a washout of the drive- way and could also progress to the point where Park Drive could be undermined. Point D: The erosion at this point is occurring in a natural channel at a point in the change of grade of the flow line of the channel. Upstream from Point B, the slope of the channel is relatively flat as it traverses a grassy meadow. Downstream from Point D, the channel is substantially steeper. The channel through the grassy meadow is apparently stable under the velocities occurring, while the channel:downstream is not stable when subjected to the increased velocities. If no corrective measures are taken at this location, the erosion will continue to progress upstream into the grassy meadow. There are no man-made improvements in the immediate vicinity of this erosion that are contributing factors. However, the new subdivisions and other improvements upstream have undoubtedly increased the storm flow in this channel which results in a greater rate of erosion. IV. Theoretical Quantities of Silt In order to determine the magnitude of the erosion that might be occurring within the watershed, calculations were made based on the methods described in "Debris Dams and Basins" prepared by the Los Angeles County Flood Control District. The amount of debris and silt that can be expected varies widely - 4 - from one watershed to another depending upon soil types, slopes and amount and type of development. The area of the watershed tributary to Bristol Cove is approximately 0.7 square miles. Based on these calculations, it appears that the average annual quantity of debris and silt could be approximately 19,000 cubic yards. A debris basin designed to intercept this estimated total quantity of debris and silt, if it has surface area of one acre, would need to have an overall average depth of 12 feet. V. Corrective Measures A. Erosion at Points A and B Tentative Maps 72-18 and 72-28 have been filed with the City of Carlsbad. The erosion at Points A and B is included within these proposed subdivisions, and the grading and storm drains proposed within these developments will essentially eliminate all of the erosion that is occurring at these points. However, the grading and drainage systems for these proposed developments should be carefully reviewed to make certain that new points of erosion are not created. For instance, if any storm drains are discharged into the channel downstream from Point A, the City should require that properly designed energy dissipators are placed on the outlets in order to minimize erosion in the downstream channel. Also, steps should be taken to require that adequate interim desilting O basins be installed downstream to intercept any silt from grading operations that are performed during the winter or before adequate planting can be established on the constructed slopes. If these subdivisions are constructed to generally accepted standards, then for all intents and purposes there will be very little erosion occurring in the watershed easterly of Park Drive that will contribute to the problem at Bristol Cove. These:two subdivisions, however, will cause an increase in the quantity and concentration of storm runoff. This increased flow and concentration will manifest itself in additional erosion in the vicinity of Point C unless protec- v_y tive devices are constructed. B. Point_C The erosion that is occurring at Point C can be corrected in one of two ways. One or more check dams can be constructed downstream of the existing culvert outlet, or the Master Plan underground storm drain could be constructed along Park Drive from Adams Street to Hillside Drive. .The Master Plan storm drain that is required to handle the design flows will need to be 48" in diameter. This drain could either be located along the existing channel or within the present or future right-of-way of Park Drive. The flow line of the O — 6 — existing channel is located approximately 60 to 80 feet westerly of the easterly right-of-way line of Park Drive. A storm drain of this importance should be located entirely within the future right-of-way of Park Drive. It is our understanding that the City contemplates a widening and improvement of Park Drive sometime in the future. This improvement would probably include the construction of the parallel storm drain. The 48" storm drain from the upstream end of the 72" pipe at Adams Street to the downstream end of the 60" pipe under Park Drive southerly of ,Hillside Drive would have a length of approximately 930 feet. It is esti- mated that the cost of this drain will be approximately $50/000. This storm drain should not be constructed unless the future improvement of Park Drive is considered in the design. The alternative to the construction of the underground storm drain would be the installation of one or more combina- tion check dams and desilting basins downstream of Point C. The nature of these check dams could vary from structures constructed of concrete to more temporary devices constructed of riprap or timber. The type of structure should be consist- ent with the life expectancy which, in this case, will be until the underground storm drain is constructed. Plate I is a sketch showing a suggested check dam and desilting basin that could be constructed downstream of - 7 - Point C. The basin as shown would have a capacity of approxi- mately 6,000 C.Y. The material for constructing the dam should be obtained on-site within the proposed basin area. This would increase the capacity to about 9,000 C.Y. The estimated cost of the basin is $10,000. Although the basin is located on private property, it may be possible to obtain permission from the owner for a temporary permit for use of the area until ;such time as development occurs. This basin would halt the continuing erosion as well as virtually eliminate the -silting problem in the channel. C. Point D The only practical solution for correcting the erosion that is occurring at Point D would be the construc- tion of a combination check dam and desilting basin. The construction of the Master Plan storm drain at this location would not be practical until development occurs because the location of the underground storm drain would need to be compatible with the contemplated development. Plate II is a sketch showing a suggested check dam and desilting basin.that could be constructed downstream of Point D. The basin as shown would have a capacity of approximately 2,700 C.Y. If the material for constructing the dam is obtained from within the basin area, then the - 8 - capacity could be increased, to approximately 5,200 C.Y. The estimated cost of the basin is $10,000. The construction of the basin will effectively halt the erosion that is occurring at this location and will decrease the quantity of silt reaching the channel at Bristol Cove. The erosion that is occurring at Point D is on private property, and therefore the construction of any erosion control devices would need to have the concur- rence of the property owner. However, since the erosion is causing serious damage to the property, the owner should be cooperative. Whether or not the City can require the private property owner to construct and pay for the necessary erosion control devices has not been explored. D. Existing 60" Bypass Along Westerly Side of Channel The 60" bypass which was constructed at the time of the channel improvement apparently is presently inoperative due to siltation. If upstream erosion were controlled to the point to where the silt load was substantially reduced at the upstream end of this pipe, then it might be possible to bypass some of the flow through this pipe that is presently discharg- ing into the northerly end of the channel. However, this pipe, even if free of siltation, has a limited capacity. At low tide this pipe can handle approximately 170 cfs and at O mean high tide, approximately 105 cfs. The design flow is approximately 343 cfs. Since the design flow is so much larger than the capacity of this pipe, it does not appear feasible to make use of this facility in correcting the silt- ing problem that is occurring at'the northerly end of the channel. E. Debris Basin at Northerly End of the Channel The possibility of constructing a permanent debris basin in the vicinity of the outlet of the 60" pipe at the northerly end of the channel was investigated. It does not appear feasible to construct this debris basin immediately downstream of the existing outlet within the confines of the channel. In order that equipment could work within the debris ^^ 'basin in a relatively dry condition, the floor of the basin would need to be at or above mean sea level. Since the bottom of the channel is designed to be at an elevation of -9, the outer walls of the basin would need to be 17 or 18 feet high. In addition to the extremely high cost of con- structing a basin at this location, a large portion of the existing channel would also be obstructed. Enclosed in this report are sketches on Plates III and IV showing a plan and section for a basin constructed adjacent to the channel in Lots 42 and 43. If these lots are still controlled by the developer, a temporary or permanent - 10 - basin could be constructed at this location at a cost substan- tially less than at a location within the channel. The basin, as shown, would have a capacity of 800 cubic yards and would cost approximately $40,000. The basin construction at the location shown is expensive because of the restricted site that necessitates the use of concrete retaining walls around the perimeter. A less restrictive site could result in a substantial cost reduction. The locations suggested at Points C and D result in less expensive basin construction and substantially greater capacity. If another location cannot be obtained for the construction of a desilting basin, then it may be more economi- cal to continue periodically removing the silt from the channel. A basin constructed with a concrete floor, as suggested in the sketch, could be cleaned periodically by loading the material into trucks with a front end loader.. The cost of loading and transporting the material to a dis- posal area will cost from $1.00 to $1.50 per cubic yard, depending upon the length of haul. A portion of this cost might be recoverable from the disposal site. VI. Recommendations . 1. Construct temporary check dams at Point D. 2. Construct temporary check dams at Point C. - 11 - o o 3. Impose requirements on the development of Tenta- tive Maps 72-18 and 72-28 to insure that downstream erosion is prevented. 4. Delay the construction of the debris basin at the northerly end of the channel until one year after Recommenda- tions 1, 2 and 3 above have been put into effect. After this time, consider the construction of the desilting basin if warranted by the quantity of siltation that is still occurring. ******* - 12 o o o o OKz Q HU