EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 10, 107-124 (1985) HOLDERNESS COAST EROSION AND THE SIGNIFICANCE OF ORDS ADA W. PRINGLE (nee PHILLIPS) Department of Geography, University of Laneaster. Laneaster LA1 4YR. U.K. Received 13 October 1983 Revised 30 June 1984 ABSTRACT A sample ord, a low section of beach characteristic of the Holderness coast, is examined as it moved southwards between 1977 and 1983 and its significant role in coast erosion is demonstrated. The reduction in beach level at the cliff foot by up to 3.9m enabled most HWN tides to reach it, as compared with only some HWS tides along the inter-ord beach, and the volume of till eroded from the cliffs increased by eight times to an annual mean of 72m3m-’. The Holderness tills are shown to be composed of 31 per cent sand and coarser sediment which is the sediment range of the beaches. Where an ord is sited the massive injection of beach sediment goes to form the ord’s most prominent constructional feature, the lower beach ridge which extends southwards from the centre of the ord. Analysis of 12 months’ observer wave data collected at Withernsea in 1969/70indicates that a net southward sand movement of l44000m~occurred. Comparison of this rate with beach sediment input rates along the whole coast backed by till cliffs suggests a sediment deficit at the northern end and a surplus towards the southern end. This conclusion is supported by an overall increase in beach sediment volume southwards from Barmston. Within this longshore sediment transport system, the ords migrate southwards from their point of origin in the Barmston-Skipsea area, without losing their identity until reaching the tip of Spurn Head. KEY WORDS Rhythmic beach feature Till cliff erosion Beach sediment input Wave analysis Longshore sediment movement INTRODUCTION The 60 km long Holderness coast, lying between the chalk promontory of Flamborough Head in the north and the sand and shingle spit of Spurn Head in the south takes the form of a zeta-cume (see Figure 1). The coast is backed by Pleistocene glacial till cliffs, which are renowned for their very rapid rate of erosion, calculated by Valentin (1954) as 1.2myr-’. This provides sand and coarser sediment for the beach, whilst silt and clay is carried away in suspension by waves and currents. The form of the beach and its response to varying wind, wave and tidal conditions was considered in an earlier paper (Pringle, 1981). It was observed that the most rapid rate of erosion occurred at the positions of low sections of beach, which form irregularly-spa& rhythmic features along the coast between Barmston in the north, and Spurn Head in the south. These features known locally as ‘ords’ migrate southwards along this coast, moving their associated zones of maximum cliff erosion with them. The characteristics of an ord are shown in Figure 2. The relationship of ords to previously described rhythmic coastal features found on a variety of coasts throughout the world (Komar, 1976) was examined earlier (Pringle, 1981) and it was concluded that the only similar feature was the low sections between ‘beach pads’ identified on St. Joseph Peninsula, Florida, U.S.A. (Entsminger, 1977). The absence of sand bars below Low Water Mark along the Holderness coast and the general absence of rip currents do not suggest the presence of cellular flows with which many other rhythmic features are associated; nor does it seem likely that ords are related to edge wave formation because of their wide, irregular spacing. 0 1 97-933 7/85/020 1 07- 1 8$0 1.80 0 1985 by John Wiley & Sons, Ltd. 108 A. W.PRINGLE Figure 1. Location map This paper reports on the study of a selected, sample ord as it migrated along the southern part of the Holderness coast between 1977 and 1983. Its movement has been monitored since 1969, when it was centred 250m south of Withernsea and Scott (1976) studied its development as it migrated from Hollym to Holmpton between 1973 and 1976. It has subsequently continued its southerly movement to lie south of Dimlington ORDS AND COASTAL EROSION 109 1 Steep, rapidly eroding till cliff 2 Lower angled, more stable cliff 3 Upper beach of coarse sand and pebbles 4 Till shore platform with armoured mud balls 5 Water-filled channel 6 Lower beach sand ridge 7 Lower beach, sand with surface water Figure 2. The characteristic features of a Holderness ord Highland in April 1983. This represents an overall movement of 7 km, or on average 0.5 kmyr- between 1969 and 1983. MONITORING THE SAMPLE ORD Field surveys were carried out in April and October each year from 1977 to 1983 to measure winter and summer changes respectively, and during the first three years, two additional surveys were made during the summer. The position of the ord was surveyed relative to a datum point at Old Hive, and within the ord five cross-profiles and one linking longitudinal profile were surveyed by automatic level and related to Ordnance Datum. The most northerly and southerly cross-profiles demarcated the limits of the ord along the coast and the remaining three cross-profiles were positioned within its central part. Height variations were thus recorded throughout the ord together with the extent and form of the main elements: the upper beach towards both ends, and the till platform and lower beach sand bar which characterize the central section. A plan of the ord was constructed from the survey data and further observations, which enabled area measurement to be made (see Figure 3). Cliff top erosion measurements were taken in April 1978 and 1979 and at six-month intervals thereafter. The datum points established for these were spaced at closer intervals than those of the Holderness Borough Council from which additional data was obtained annually each autumn. From these cliff top measurements volumes of till eroded from the cliffs were calculated, and, taking the composition of the till into account, volumes of sand and coarser sediment supplied to the beach could be estimated. As it was not possible to install a wave recorder for this project, wave measurements could be taken only at survey times from the coast although this data can be compared with daily observer wave measurements taken for 12 months in 1969/70. Three-hourly wind data was obtained throughout the whole period 1977 to 1983 from the Meteorological Office for Spurn Head. MOVEMENT OF THE ORD ALONG THE COAST The average rate of movement of 0.5 km southwards per year masks considerable variation as shown in Table I. The figures were calculated from the mid-point position of the exposure of till shore platform at or 11.4.8 NWI I SE IIIII 11111111I,,IIII I11111 11 (1111 111[1111,#, 750 1ooo 1250 1500 1750 2000 2250 2500 2750 3OoO A Upper beach sand and shingle Lower beach sand ridge Lower beach sand with surface water ? 1 Till shore platform Till shore platform wlth patches 1.4.8 of upper beach sedment Upper beach sediment wlth surface water m Water A to E Cross profiles NW SE Ill 1111111111111,1,111,,~~~~~1 20oo 2250 2500 2750 3ooo 3250 3500 B metres southeast of OM Hive Datum Figure 3. Ord plans. A: Under high wave energy, northerly storm conditions, with a tidal surge on 11.04.82. B Under low wave energy conditions on 01.04.83 ORDS AND COASTAL EROSION 111 Table I. Rate of movement of Ord Per 05yr Per year Date (m) (m) 4.4.77 - 29.10.77 668 S 7.4.78 68 S 736 S 21.10.78 245 S 14.4.79 175 S 420 S 2 1.10.79 360 S 16.4.80 355 N 5s 25.10.80 197 S 18.4.8 1 368 S 565 S 30.10.8 1 287 S 1 1.4.82 190 S 417 S 23.10.82 395 s 1.4.83 380 S 715 S Total 1977-1983 2978 S Annual mean 496 S close to the cliff foot. The half-yearly rates of movement varied between 668 m southwards and 355 m northwards. The latter was the only northward movement recorded and it was produced when the ord lengthened in both directions. The annual rates varied between 5 m southwards (this including the northward six-monthly record) and 775m southwards. It is noteworthy that the mean annual movement of 496m southwards for the six years 1977 to 1983 is almost the same as that already noted for the period 1969 to 1983. CHANGING FEATURES WITHIN THE ORD Height variations The height of the beach or exposed till shore-platform controls the degree of exposure of the cliff foot to direct wave attack and is therefore very important when considering varying rates of cliff erosion. Table 11 shows variations in cliff-foot height between the ord’s northern and southern ends, where cross-profile A and E respectively were surveyed, and its lower central section surveyed by cross-profiles B, C,and D. The 19 surveys carried out between 1977 and 1983 show that the mean heights of the northern and southern ends were 3.91 m and 3.37 m OD respectively (ranging between extremes of 2.61 m and 5.37 m at the north end, and 0.97 m and 5.25 m at the south end). Within the ord the mean height was 2.56 m OD (ranging between extremes of 0.90m and 4.18m). The height range within individual surveys varied between 1.06m and 3.87m, with a mean of 2.20 m.