U.3.Airmy Gaast. Eng.Res-Ctr. CETA CETA 81-6 A Method to Forecast Sedimentation Rates Resulting from the Settlement of Suspended Solids Within Semienclosed Harbors by Craig H. Everts COASTAL ENGINEERING TECHNICAL AID NO. 81-6 JUNE 1981 wy ge i 2, al rae ox JOG VEC fe, o® G4 Approved for public release; distribution unlimited. U.S. ARMY, CORPS OF ENGINEERS COASTAL ENGINEERING RESEARCH CENTER Kingman Building Fort Belvoir, Va. 22060 > or republication of any of this material Reprint Army Coastal shall give appropriate credit to the U.S. Engineering Research Center. Limited free distribution within the United States of single copies of this publication has been made by this Center. Additional copies are available from: Nattonal Technical Information Service ATTN: Operations Diviston 5285 Port Royal Road Springfield, Virginia 22161 The findings in this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. Demcu AAPA TIP O 0301 0089795 5 UNCLASSIFIED SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered) READ INSTRUCTIONS REPORT DOCUMENTATION PAGE 1. REPORT NUMBER 2. GOVT ACCESSION NO.| 3. RECIPIENT'S CATALOG NUMBER CETA 81-6 4. TITLE (and Subtitle) 5. TYPE OF REPORT & PERIOD COVERED A METHOD TO FORECAST SEDIMENTATION RATES Berens oe ce Ga RESULTING FROM THE SETTLEMENT OF SUSPENDED ps Suey SOLIDS WITHIN SEMIENCLOSED HARBORS PERFORMING ORG. REPORT NUMBER 6. 7. AUTHOR(s) 8. CONTRACT OR GRANT NUMBER(s) Craig He Everts 10. PROGRAM ELEMENT, PROJECT, TASK 9. PERFORMING ORGANIZATION NAME AND ADDRESS AREA & WORK UNIT NUMBERS Department of the Army Coastal Engineering Research Center (CEREN-GE) Kingman Building, Fort Belvoir, Virginia 22060 11. CONTROLLING OFFICE NAME AND ADDRESS Department of the Army Coastal Engineering Research Center Kingman Building, Fort Belvoir, Virginia 22060 14. MONITORING AGENCY NAME & ADDRESS(if different from Controlling Office) D31235 12. REPORT DATE 13. NUMBER OF PAGES 15. SECURITY CLASS. (of this report) UNCLASSIFIED DECL ASSIFICATION/ DOWNGRADING SCHEDULE 15a, 16. DISTRIBUTION STATEMENT (of this Report) Approved for public release; distribution unlimited. DISTRIBUTION STATEMENT (of the abstract entered in Block 20, if different from Report) - SUPPLEMENTARY NOTES KEY WORDS (Continue on reverse side if necessary and identify by block number) Alaska Sedimentation Currents Semienclosed harors Dillingham Harbor Suspended sediment Mathematical model Tides ABSTRACT (Continue on reverse side if necesaary and identify by block number) When a semienclosed harbor is planned for an area where sediments may enter the harbor in suspension, it is desirable to forecast the rate at which those sediments will be deposited. A method to make such a forecast is presented in this report. The harbor shoaling rate (sediment accretion) is the dependent variable. The method is applicable to situations where the harbor is almost totally enclosed; bedload transport is negligible; deposition is nearly uniform throughout the harbor; sediment will not be resuspended (once deposited); and tide or river stage rise causes currents which move water and suspended sediment into the harbor. DD FORM 147 EDITION OF T NOV 65 IS OBSOLETE 1 yan 73 1473 UNCLASSIFIED SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered) ST — eae TT VaR Men >G 9 feed Dan BS Yet md ntuhi.:$ Jo + SPREE ae ai. ret Seca viygh She i, a an ge 2h Mtb Pee a is ere sa a : matt pra Coo i wh iy Mai) Latur ae rc 9 im wane ? " TT SOR ALT agers ie,” iy ; 4 f | Ai re iy es CE Na Mt , SEE cee! ee aps ven 2 re r i syed nal reer» cx Gaiepens ituhide rhendine. wie ei i pris ay tines FRRY () NO ‘vals opidna'd uf Lae F “ul 7 tg ae Kone ee Tapneenn | a, raat ee oe wen) ay: Bias ai Reps Birt oS! RMR A Me Dia eet ro Speers’ eae i Ried a be ie a ‘a ls ne oe ; } ee PREFACE This report presents a method for estimating the sedimentation rate in a semienclosed harbor before construction. Sedimentation is assumed to result when suspended solids carried into the harbor during a rising tide or rising river stage settle out before they can be removed from the harbor as the water level falls. The work was carried out under the coastal engineering research program of the U.S. Army Coastal Engineering Research Center (CERC). The report was prepared by Craig H. Everts, Chief, Engineering Geology Branch, under the general supervision of NE. Parker, Chief, Engineering Development Division. Comments on this publication are invited. Approved for publication in accordance with Public Law 166, 79th Congress, approved 31 July 1945, as supplemented by Public Law 172, 88th Congress, approved 7 November 1963. yy TED E. BISHOP Colonel, Corps of Engineers Commander and Director CONTENTS CONVERSION FACTORS, U.S. CUSTOMARY TO METRIC (SI).cccccccccccceccceee SYMBOLS AND DI EVEPUN aeli@ NiStorevehotolelckelolelelclellololelelelalelclclelelclclelelolelclclelolsisloleletelalelsiclele ENTRODUGT LON s cic cc ccc «cle c cc cle cle 010 010 0 © 0 s)0 0100/0 0160 60 010 010 00s 00 00 cc eo closes © HARBOR CHARACTERS TehCSloreteleiolelelelolelelalelelelafelcieleleleielelelolelelclolelelcrolelclelelolefelelelclolels RROGEDURHeretelelolclelsicleletctclolctoleiolclolelclelelelelclelelclelelelellelelclelclelclelelcisl cle) clelslelolclclelclelelere 1. Data REqGuireMeENtSeccecccccccccccccccccccccccvcccccescvcccccccoce Do SYol itary OOO OOOO UD OODOOUOOUUOO UO OOOO OOOUOUUOUUUUOOOOUOOOOOOOOUOOG EXAMP LE 12340) VU IOISICICICICIOIOICIOIOICIOIOICICICICICICIOICIOICIOIIOCICICICICECICIOIOICIOICIC INC NCICICICNCNCHCNCNC NCIC SUMMARY AND CON GIEWSMON Srerctetatetotehelcleloleleicicielalolololciclclclelelclciclclelclclelalciciolclelelcletelele lo Byalloation ot Weitln@clooo00000000000000000000000000000000000000000 2. Settling Velocity Distribution... ccccccccccccrccccsccccscccccces 36 Sediment Mass Carried Into Hanah Olsenetetelciciclehelclohelelclalclelalelelelalclelaleteleterele 4. Effects of Navigation Chanmelecccccccccccccccccccccccccccsccccce Die Winter SaqlimMaMEAelOmMe oq nnd 0 00 no 0b 0D DODD DDDDOOD DDD OOOO DO000D000000 LITERATURE CIUDAD 6 OO ODO COO D OOOO O ODDO OOODDDODONDOOOODODDODODODDODDODDODN0NDN TABLE Part of sediment deposited in harbor DABSILBG OOO DODD ODDDDDD0D0DDDDD00000N000 FIGURES Definition sketch of an enclosed harbOreccceccccccccrcccccccccccccccccs Sediment-concentration correction coefficient. .ccccccccccccccccccsecccce Part of sediment, P, deposited in the harbor basin as a function of tidal range or river rise in the basin, as, and particle size... Graph to correct P when sill elevation iS not ZerOcccccececccccevcece Correction factor, Kp» for time variations in the suspended concentration of sediment entering the baSinecccececececccccccccccccce Location map of Dillingham Harbor, Alaskacccccccccccccccccvccccccsccccce Composite settling velocity and size-frequency histogram of suspended sediment obtained at Dillingham Harbor, Alaska..cccccccccoes Page 5 6 16 10 ll 12 12 14 15 CONVERSION FACTORS, UeS. CUSTOMARY TO METRIC (SI) UNITS OF MEASUREMENT U.S. customary units of measurement used in this report can be converted to metric (SI) units as follows: Multiply by To obtain inches 254 millimeters 2254 centimeters square inches 62452 Square centimeters cubic inches 16.39 cubic centimeters feet 30.48 centimeters 0.3048 meters square feet 0.0929 square meters cubic feet 0.0283 cubic meters yards 0.9144 meters square yards 0.836 square meters cubic yards 0.7646 cubic meters miles 1.6093 kilometers square miles 259.0 hectares knots 1.852 kilometers per hour acres 0.4047 hectares foot-pounds 1.3558 newton meters an apres ilsOLOW sz 1072 kilograms per square centimeter ounces 28-35 grams pounds 453.6 grams 0.4536 kilograms ton, long 1.0160 metric tons ton, short 0.9072 metric tons degrees (angle) 0.01745 radians Fahrenheit degrees 5/9 Celsius degrees or Kelvins! 1T>. obtain Celsius (C) temperature readings from Fahrenheit (F) readings, use formula: C = (5/9) (F -32). To obtain Kelvin (K) readings, use formula: K = (5/9) (F -32) + 273.15. SYMBOLS AND DEFINITIONS plan area of basin plan area of basin at sill elevation tidal amplitude or maximum river stage variation suspended-sediment concentration initial suspended concentration which enters harbor mean suspended-sediment concentration during the interval 0 < t <¢ 0/2 particle size in millimeters sediment-concentration correction coefficient used to calculate M sediment-—concentration correction coefficient used to calculate P correction factor for suspended-sediment concentration which varies linearly with time total mass of sediment which enters a harbor total mass deposited in basin mass of sediment per unit volume of deposited material part of M deposited in harbor, with Zee O and k, = 0 part of M deposited in harbor when Ze # O and ign = © part of M deposited in harbor when z, # O and Ka i O shoaling rate on bottom of harbor basin (not sidewalls) tidal period or duration of a complete rise and fall of a river hydrograph time in hours past previous time of low water unit volume of sediment deposited in basin particle settling velocity sill elevation sidewall slope of harbor basin water surface elevation bulk density of sediment deposit in basin density of sediment particles water density A METHOD TO FORECAST SEDIMENTATION RATES RESULTING FROM THE SETTLEMENT OF SUSPENDED SOLIDS WITHIN SEMIENCLOSED HARBORS by Craig H. Everts I. INTRODUCTION Some harbors experience sedimentation largely as a result of the fallout of suspended material. This report presents a method to forecast the sedimentation rate caused when waters laden with suspended solids enter a semienclosed harbor during a rising tide, or rising river stage, and settle out before they can be removed in suspension as the water level subsequently declines. The method can be used in planning harbor maintenance expenses before construction in an area where significant quantities of sediment are transported in suspension. Geometric characteristics of the proposed harbor are considered; consequently, the method may also be used to evaluate design tradeoffs (i-ee., sedimentation rate versus harbor size, project depth, and channel characteristics). II. HARBOR CHARACTERISTICS The general case of an enclosed harbor basin connected by a channel to navigable waters is considered (Fig. 1). The basin and channel may be of any size and shape as long as the rise and fall of the water surface inside the basin is nearly in phase with and of the same amplitude as that outside the basin. Sidewalls may be sloping or vertical. A sill in the navigation channel may be at any elevation, including the elevation of the channel bottom; iee.-, no sill, the most common case for enclosed harbors. A sill is used in high tidal range areas to reduce excavation costs when constructing the basin. It provides flotation for vessels in the basin at low tide stages, but restricts navigation in and out of the basin to times of higher tidal elevations. Harbors in Alaska with such sills are called “half-tide” harbors. Navigation Estuary Channe/ No Sill Condition Figure 1. Definition sketch of an enclosed harbor. Three major components of a half-tide harbor are illustrated: basin, sill, and navigation channel. The half-tide harbor is the most general case of an enclosed harbor; in most situations a sill will be absent. When the sill is absent, and where the basin bottom elevation is above MLLW, the basin will be dry at or near the time of low tide. a, = MHHW tide elevation; Ab = plan area of basin; n = water surface elevation; Be sill elevation; a = sidewall slope. To achieve reasonably accurate results in using the method, engineering judgment is required to assure the following conditions will exist at and near) the harbor: (a) Sediment will enter the basin in suspension; bedload transport will be negligible. (b) Water will enter or will be discharged from the basin through the channel as the water surface rises or falls in nearby navigable waters. (c) At any given time the flow in the channel will be unidirec- tional or nonexistent; i.e., the volume of water that will enter the basin as the water surface rises will be equal to the volume of the basin above the sill. If no sill exists, ZS basin bottom eleva- tion; if the sill elevation or basin bottom elevation is below MLLW, B= WMO = 0. (d) Water in the basin will be trapped below sill elevation. When a sediment particle settles below that elevation, even though it has not yet reached bottom, the particle is considered deposited. If no sill exists, bottom scour is assumed to be nonexistent. (e) Shoaling rates and the sedimentation processes will be uniform throughout the basin; no back eddies or other areas of preferred sedi- mentation will exist within the basin. III. PROCEDURE A mathematical model has been developed in integral form to predict sedimentation rates in enclosed harbors (Everts, 197/b). For many general situations the model has been solved numerically, and the results are pre- sented in this report. 1. Data Requirements. Information required to use the model includes (a) tidal or river hydro- graphs at the harbor site, (b) concentration of suspended sediment that will be carried into the harbor, (c) settling characteristics .of the suspended sediment, and (d) proposed harbor geometry. ae Tidal or River Hydrograph. The volume of inflow water to the basin is obtained using the tidal or river hydrograph. In this procedure, a simple cosine function is assumed to approximate the rise and fall of the water sur- face; ieee, a. 27t n=— {1 - cos — (1) 2 T where n = water surface elevation Ba = tidal amplitude or maximum river stage variation T = tidal period or duration of a complete rise and fall of a river hydrograph t = time past low water be Concentration of Suspended Sediment. Sediment is assumed to enter the harbor at a uniform concentration, c, through the water column. This vertical uniformity may result from mixing in the channel even where outside waters are stratified. Mixing will usually increase as the ratio of the tidal prism to channel cross section increases. In many cases the concentration of entering sediment will vary with time in the tidal cycle. Since this variation will affect the time and distance the sediment has to settle to be deposited, time dependence must be con- sidered. When a linear change is assumed, and the concentration at the beginning of the floodtide cycle, Cis the concentration averaged over the floodtide cycle, ¢, and the period T, of the complete tidal cycle are known, a simple relationship is used to account for concentration change with time during a flooding cycle sequence eo) Nee ee (2) where k, is the correction factor used for suspended-sediment concentration which varies linearly with time. The values of Cy and ec are obtained from analyses of water samples collected over one or more rising water sequences near the harbor site. ce Settling Characteristics of Suspended Solids. The rate at which suspended particles settle is a critical factor when determining whether they will be deposited or removed while still in suspension as the outside water surface drops and fluid leaves the harbor. Because a mass of sediment par- ticles in nature displays a wide range of settling velocities, a settling velocity distribution must be considered. For each particle settling velocity, Vs> there is a corresponding particle size, given as _ sphere diameter, d., in millimeters in which the sphere has a density, On 2.7 grams per cubic centimeter. Caution is recommended when using a grain-size distribution instead of a settling velocity distribution. Grain diameter is often determined in the laboratory as the effective diameter based on Stokes’ Law using distilled water. However, particle aggregation in the fluid in which the sediment was collected (often saline) may increase the settling velocity in nature. Therefore, settling velocity distribution should be measured in the ambient fluid at the normal fluid temperature expected in the harbor. d. Harbor Geometry. The plan area of the basin at sill elevation, Ay» the slope of the basin sidewall, a, and the elevation of the sill, Zeeeeice required parameters (Fig. 1). These geometric characteristics of the harbor are used to determine the volume of water and the mass of suspended sediment which will enter the harbor during each floodtide cycle. The parameters are also needed to determine the vertical distance through which a suspended- sediment particle must fall to be deposited and the area over which the sediment will be deposited. 2 Solution. Three steps required to determine the shoaling rate are: (a) predict the total mass of sediment that will enter the harbor; (b) predict the part of that mass which will be deposited; and (c) predict the shoaling rate (increase in bottom elevation) which will result. ae Step 1--Sediment Mass Entering Harbor. During a rising tide, or ris-— ing river stage, the mass of sediment carried into the harbor is dependent upon the concentration of entering suspended sediment, the characteristics of the tidal or river hydrograph, and the geometry of the proposed harbor. For a harbor basin with vertical sidewalls and no sill (2, = 0; Fig- 1), and into which the entering concentration of suspended sediment is constant (cz = @) (k. = 0; eq- 2), the total mass of sediment, M, which enters during a floodtide cycle is M= Ga, A (3) where A is the surface area of the basin. When a sill is considered (Z, > 0), but the entering concentration is constant through time (k, = 0) and the sidewalls are vertical, the mass which enters is M=ec A(a, - Z,) (4) With a sill and vertical sidewalls, but when the concentration of inflowing sediment varies with time (k, # 0), the mass which enters is Me € a, AK (5) where K is the sediment-concentration correction coefficient. Values of K are given in Figure 2. Sill Elevation Sediment -Concentration Correction Coefficient (K ) 0.4 #O3 0.2 0.1 0 -O! -0.2 -0.3 -0.4 Figure 2. Sediment-concentration correction coefficient, K, as a function of a linear, time-dependent change in suspended-sediment concentra-— tion, k, (see eqe 2) and sill elevation, Zoo where Zz, is some part of the maximum water surface elevation, a,- This figure, used in calculating the sediment mass which will enter a harbor, provides a correction procedure for situations where the entering suspended-sediment concentration varies with time or when the sill elevation is above MLLW. 10 Equations (3), (4), and (5) are applicable for basins with vertical sidewalls. If the sidewalls are sloping, the average plan area should be calculated for the half-full elevation above the sill. The actual sediment mass input may be slightly larger than that calculated using equation (5) if k, > 0, and slightly smaller if k, < 0 (the difference is small compared to other uncertainties in using this nailed) be Step 2--Sediment Mass Deposited in Harbor. Of the total sediment mass, M, carried into the harbor (eqs. 3, 4, or 5 of step 1), the part deposited is primarily dependent on the settling characteristics of the sediment, the distance the sediment has to settle to be deposited, the time interval during which the suspended material settles, and the elevation of the sill. Figure 3 shows the part of the sediment deposited, P, as a function of tidal range, a,, and particle size, d,- For harbors influenced by tide behavior as compared to floods of longer duration, the settling characteristics of disaggregated particle sizes finer than d, = 0.001 millimeter are such that very little material will be deposited in the harbor. ROE Gs 2 0.03 millimeter nearly all the sediment which enters the harbor will be deposited. The P values shown in Figure 3 are for Z, = O and k. = 0 (eq-e 2) conditions. Corrections must be made for these parameters when they are not zeroe — T= 12.4hr ——=— 7224.8 hr Particle Size, dg in mm TSE fost NAN KN CNG S A NSTI TS Th NSCINIs feces Se SL i coe 7 0.6 0.4 0.2 Part of Sediment Mass Deposited, P 0.1 0.2 a: oman. 2 3). 4 5 10 15 Tidal Amplitude , a, (m) Figure 3. Part of sediment, P, deposited in the harbor basin as a function of tidal range or river rise in the basin, as, and particle size. Sill elevation is zero. The sediment concentration entering the harbor is nonvarying with time; iee., k, = 0 (see eq- 2)- Sediment size is related to settling velocity for quartz spheres in freshwater at 20° Celsius. ll When the sill elevation, Z,, is not zero, Figure 4 is used to correct the P value. 0 0.2 0.4 0.6 0.8 1.0 Figure 4. Graph to correct P (see Fig. 3) when sill elevation is not zeroe P, is part of the sediment mass deposited in the basin when sill elevation is included; k, = 0 (see eq. 2). If the concentration of suspended sediment entering the basin is time—- varying (i-e., k, # 0), an additional correction, K,, must be made (Fig. 5) when 0.2 < P, < 0.8-~ The part of the sediment which is deposited in the harbor, P.,, when the sill elevation and time-variable inflowing sediment concentration are considered, is (6) Kp) 0.05 Figure 5. Correction factor, Kp» for time variations in the suspended concentration of sediment entering the basin (used with eq. 6). 12 The total mass deposited in the basin, M4, is ce Step 3--Shoaling Rate in Harbor. It may be desirable to predict the rate of bottom elevation rise caused by sediment deposition. The sediment mass (step 2) must then be converted to sediment volume, and specifically to the volume of the water-sediment mixture which forms the deposit in the harbor. That part of a unit volume of deposited material, Voq> With a density of Pgs which is composed of sediment of density Ps, is — Pol Pw Vsa = Qe = Pre (8) where 0 <¢ Ved ¢ 1.0 or p,, is the density of water. The mass of sediment per unit volume of deposited material, m,, is my = Ps Veq > and the shoaling rate, S,, in the basin (not sidewalls) is Soe (10) = 1 rR a The assumption is that sediment which would be deposited on the basin side- walls moves downslope and is deposited on the basin bottom (A, = area of basin at sill elevation). Thus, during deposition, the basin sidewall slopes are assumed to remain constant and in their original position. IV. EXAMPLE PROBLEM Dillingham Harbor is an enclosed half-tide harbor located off Nushagak Bay, about 550 kilometers southwest of Anchorage, Alaska (Fig. 6). Sedimenta- tion has been a major maintenance problem requiring almost continuous dredging during the summer months. In addition to the high cost, the dredging inter- feres to some extent with navigation by fishing boats and commercial barges. Flow into and out of the basin is caused by tidal fluctuations in Nushagak Baye Water entering during floodtide carries large quantities of suspended materials, some of which settle below sill elevation before ebb tidal waters can remove them as the tide lowers. GIVEN: Data obtained in the vicinity of Dillingham Harbor in 1969, 1973, and 1974 (Everts, 1976b), are: (a) Tidal Characteristics. Mean tidal range: a, = 6 meters Tidal period: T = 12.4 hours (b) Suspended-Sediment Characteristics. Suspended-sediment concentration at t = 0: G = 0.8 kilo- gram per cubic meter (800 milligrams per liter) Suspended-sediment concentration at t = T/2: c(T/2) = 0.2 kilogram per cubic meter (200 milligrams per liter) 13 Dillingham 500m —=— Scondinovion Creek ll }Fowbanks Contours in Meters above MLLW Dillingham | Anchorage 4 Bristol Bay Q >. Meters of” ‘ o_o] 0 100 Wood River a ee Nushogak : Boy =a Sy ec Channel IX Kilometers 3 [—__ — __ o_o) 0 [O20 SO a) Bristol Bay a ee Nushogak Boy Figure 6. Location map of Dillingham Harbor, Alaska. Sediment-size distribution (see Fig. 7) Sediment density: 9, = 2./ grams per cubic centimeter Water density: p, = 1.0 gram per cubic centimeter Bulk density of sediment in basin: De = 1.5 grams per cubic centimeter (estimate from other sources; eeg-, see Everts (1976a, 1976b) and Everts and Moore (1976)) (ce) Harbor Characteristics. Sill elevation: ha 1.5 meters Plan area: A = 35,000 square meters (at the half-full basin elevation a, + Z,/2) Plan area: Ay (z,) = 21,500 square meters (at the sill elevation of 1.5 meters) FIND: Estimate the mass of sediment deposited in Dillingham Harbor each tidal cycle; estimate the shoaling rate (bottom elevation increase). 14 0.5 oO ow Frequency, f 0.2 0 — 0.125 0.063 0.03! 0.016 0.008 0.004 0.002 0.00! 0.0005 d, (mm) 18.1 3.5 0.88 0.22 0.055 0.014 0.0034 vs (mm/s) Figure 7. Composite settling velocity and size-frequency histogram of suspended sediment obtained at Dillingham Harbor, Alaska, May to September 1974. Histogram shows mean frequency per v, or d, unit shown on the abscissa. Vv, = particle settling velocity; al. = particle diameter in millimeters. SOLUTION: (a) Step 1--Determine the Sediment Mass that Enters the Basin Each Tidal Cycle. The mean concentration of suspended sediment, ¢€, that enters the basin during the floodtide cycle is c = ——— = ——— = 0.5 kilogram per cubic meter (11) The value of ky, the correction factor for a time-varying concentration of sediment entering the basin (from eq. 2), is eee bibl saaeny - = -0.19 per hour (12) 12.4 The sill elevation, z,, is as ASO Sy pence: (13) where aa = 6 meters. 15 The sediment-concentration correction coefficient is K = 0.65 when these values are applied to Figure 2. Using equations (5) and (6), the total mass of sediment calculated to be carried into the harbor is (23) + Ay mM = ¢|/—*2/ __| a x = (0.5)(35,000)(6)(0.65) = 68,250 kilograms (14) 2 per tidal cycle (b) Step 2--Find the Sediment Mass Deposited in the Harbor. Since the sill elevation of Dillingham Harbor is not zero, and the concentration of sediment entering the harbor varies through the floodtide (inflow) cycle, corrections must be made for those factors when using Figure 3. However, the settling velocity frequency distribution must first be calculated. This is done for each size-interval shown in Figure 7. The frequency value, f, references that part of the sediment mass with a settling velocity in a single size interval (see Table). Table. Part of sediment deposited in harbor basin. For each sediment-size interval the P value (Fig. 3) is corrected for a sill elevation of a,/4 (Fige 4), and further corrected for the time- varying concentration of sediment which enters the harbor (Fig. 5; eq. 6). These corrections are shown in the Table. The sum of Pats values shows 7/2 percent of the sediment that enters the basin will be deposited. The mass of sediment deposited, M, (using eq- 9), is 49,000 kilograms per tidal cycle. (c) Step 3--Find the Shoaling Rate in the Harbor. From equation (8) the value of V,q is 0-29, and using equation (9), m, is 790 kilograms per cubic meter. The summer season shoaling rate, S is 0.0030 meter per r? tidal cycle (eq. 10). 16 Ve SUMMARY AND CONCLUSIONS 1. Evaluation of Method. The sediment mass deposited and the shoaling rate (estimated by the method described in this report) resulted in values 12 percent less than those meas- ured in Dillingham Harbor during summer (ice-free) conditions. The method was also tested using shoaling rates and other data collected at and near a large sedimentation tank at Anchorage, Alaska (Everts, 19/6a)- The predicted shoal- ing rate at Anchorage was 10 percent less than measured in the tank. In both cases the model predictions were less than that which was actually measured; for a conservative application of the model it is recommended that the Ma value (total mass deposited; eq. 9) be increased by 15 percent. 2. Settling Velocity Distribution. This crucial factor may vary through time in a rising water cycle, and by time of year. Importantly, it may be highly dependent on aggregation of dis-—- crete particles in saline waters. Thus, when compiling the settling velocity distribution diagram, the velocities used should be those which actually may be expected to occur at the harbor site. This means particle settling behav- ior should be measured in the fluid where it was obtained. Aggregation can occur in glacial-source sediments with low clay percentages such as found in Alaskan estuaries (Everts, 1976c), as well as in sediment suspensions com- prised of a high percentage of clay minerals. 3. Sediment Mass Carried Into Harbor. The part of the sediment which enters the harbor and is subsequently deposited will probably be near constant from one tidal cycle to another, even though the total quantity which enters may vary widely (Everts and Moore, 1976; Everts, 1976a, 1976b). Everts (19/76b), for example, reported that the quantity carried into the basin at Dillingham Harbor varied by a factor of three between tidal cycles, but that the part deposited was near constant at 81 percent (range: 76 to 84.2 percent for five tidal cycles). Thus, to accu- rately predict M,, a number of tidal cycles or floodflow stages during a river rise must be sampled to obtain representative c¢ and Cy values. 4. Effects of Navigation Channel. Part of the material that enters a harbor may be sediment that was deposited in the navigation channel at a low tide stage and was resuspended during the next rising tide (Everts, 1976b). The amount entering from this source is in addition to that which enters directly from the estuary and which would be obtained from samples collected before the harbor was constructed. The suspended-sediment data presented in the example problem, however, were collected in the channel at the sill at Dillingham Harbor. Compared with other samples collected in the estuary, apparently 60 percent of the sediment carried into the harbor originated in the estuary, and 40 percent came from resuspension within the entrance channel. This channel source should obviously be considered. Cost savings can be achieved when the entrance channel is designed as short as possible (Everts, 197/a). 17 5. Winter Sedimentation. Summer sedimentation rates will not prevail during ice-cover conditions. The submerged ice volume reduces the tidal discharge volume of water and sus-— pended sediment to the basin and that reduction can be estimated. Similarly, a reduction in the amount of water and suspended sediment in the channel will reduce the sediment mass inflow from that source. Because sediment size, water viscosity, and flocculation affect the settling velocity of suspended material in the basin, these factors must be considered on at least a seasonal basis. At Dillingham, median particle size varied from 0.014 millimeter in summer to 0.005 millimeter in winter. The effect of salt on the settling rate of clay minerals becomes distin- guishable at seawater concentrations of about 1 to 2 parts per thousand. Flocculation, and hence the settling rate of the flocs, increase to seawater concentrations of 10 to 15 parts per thousand. Suspended sediment in water samples with a salt content of 0.5 part per thousand at Dillingham during the summer did not appear to flocculate. However, when the winter salinities reached 3 parts per thousand, flocculation might have occurred. Temperature affects water viscosity and hence the fall velocity of the sediment particles. For example, at 0° Celsius winter water temperature, the fall velocity of an 0.008-millimeter particle is about 70 percent the velocity it would be at 10° Celsius. This must be considered when using Figure 3 (see Fig. 7 of example). 18 LITERATURE CITED EVERTS, C.H., "Shoaling Rate Prediction Using a Sedimentation Tank,” Proceedings of Ocean Engineering IIIT, 1976a (also Reprint 1-76, U.S. Army, Corps of Engineers, Coastal Engineering Research Center, Fort Belvoir, Va., NTIS AO29 725). EVERTS, C.H., “Half-Tide Harbor Sedimentation in Alaska," Assessment of the Aretite Marine Environment: Selected Toptes, DeWe Hood, and D.C. Burrell, eds., Imstitute of Marine Science, University of Alaska, Fairbanks, Alaska, 1976b. EVERTS, C.He, “Sediment Discharge by Glacier-Fed Rivers in Alaska,” Proceedings of the Symposium on Inland Waterways for Navigation, Flood Control, and Water Diverstons, 1976c. EVERTS, C.H., “Self-Maintaining Navigation for Certain Enclosed Harbors,” Proceedings of the Fourth Internattonal Conference on Port and Ocean Engineering Under Arctte Conditions, Ste Johns, Newfoundland, Canada, Vol. 1, 1977a, pp- 382-393. EVERTS, C.H., “Sedimentation Rate Prediction for Inclosed Harbors," U.S. Army, Corps of Engineers, Coastal Engineering Research Center, Fort Belvoir, Va., unpublished, 197/7b. EVERTS, C.H., and MOORE, H.E., “Shoaling Rates and Related Data from Knik Arm Near Anchorage, Alaska,” TP 76-1, U.S. Army, Corps of Engineers, Coastal Engineering Research Center, Fort Belvoir, Vae, Mar. 1976. 19 ‘ yi, ae Pere oe ad ' pe { esas! 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